Extragonadal Germinal Cell Tumors

 

Primary Extragonadal Germinal Cell Tumors

          PKGhatak, MD

Primary extragonadal germinal cell tumors.

The germinal cell produces sperm or ovum. Under normal circumstances, the germinal cells are present only in the gonads (Testicles and Ovaries).

In a rare embryonic mishap, the germinal cells are present in the Pineal gland, Mediastinum and Retroperitoneal areas. Germinal cells in these locations may turn into tumors, both benign and malignant tumors.

A short review of the origin of germinal cells is essential in understanding these aberrant locations of the germinal cells.

Gonads and germinal cells have separate lines of origin. Gonads are mesodermal tissues, whereas germinal cells originate in the Yolk Sac of the developing embryo. At 5 weeks the germinal cells leave the yolk sac and migrate to the developing fetus, and travel along the Allantois behind the hindgut, behind the peritoneum and move all the way to the developing neural tube which later develops into the brain. Most of the germinal cells migrate to the genital ridge and lodge in the gonadal tissue, the rest of the germinal cells disappear.

The allantois is a narrow tube through which placental blood vessels run to and from the fetus and placenta. The fetus and extra fetal tissues (yolk sac) lie submerged in amniotic fluid in the amnion sac, the membrane of the sac is called the coelomic epithelium.

Due to developmental errors, a few germinal cells remain in the brain, mediastinum and retroperitoneal tissues. Tumors that develop at these locations are called extragonadal (non-gonadal) germinal cell tumors.

Histopathological Types.

Germ cell tumors in unusual locations may be benign or malignant. The benign tumors are called teratomas.

Teratoma.

The germ cells are endowed with the power to produce any or all cell lines and are progenitors of the Totipotent stem cells. In teratomas this characteristic is maintained, as a result, the teratomas contain hair, teeth, nails, sweat glands and other tissues. Tumors are generally multicyclic. In males, the tumor though benign by the pathological criteria but behaves like a malignant tumor. Teratomas are diverse in histology and also vary in biological behavior. In women, teratomas are benign multicyclic tumors containing hairs, teeth, nails, glands, bone and cartilage. In males, teratomas are benign looking but may behave like malignant.

Malignant germinal tumors.

The tumors are mixed cell types but one cell line dominates and is named accordingly. The usual varieties are Embryonal cell carcinoma, Choriocarcinoma. Yolk cell Carcinoma, and Seminoma.

Location of tumors.

Germinal cell tumors are midline tumors. In the brain, the usual site is the pineal gland and occasionally appears in the Pituitary gland. In the mediastinum, they are in the anterior mediastinum in between the lungs and behind the thymus gland. The abdominal site is usually in the sacrum behind the hindgut and peritoneum.

Pineal gland germinal cell tumors.

These tumors are generally malignant. Symptoms are of three categories. A growing tumor increases cerebrospinal fluid pressure and produces headaches, nausea, vomiting, and the 6^th cranial nerve palsy. Local infiltration of tumor-cerebellar dysfunction affects balance and walking. The hormone of the pineal gland is Melatonin. Disruption of melatonin production results in sleep rhythm change, and difficulty in falling asleep.

The malignant tumors secrete Chorionic gonadotropin and Alpha fetoprotein. The blood levels of these two are elevated in nearly all cases and in 1/3 of cases the CSF levels are also high. It is well known that in raised CSF pressure situation the spinal tap is contraindicated; in this circumstance, the CSF is obtained by the 4^th ventricle puncture. Blood chorionic gonadotropin is not specific for gonadal cell tumors, it is elevated in pregnancy and pregnancy related complications and in menopause. Alpha fetoprotein is also elevated in hepatic cell carcinoma and neonatal hepatitis. High chorionic gonadotropin and alpha fetoprotein in addition to an MRI of the brain suggestive of a pineal growth, in non-pregnant women, is as good as biopsy confirmed pineal germinal cell tumors.

In gonadal cell tumors of the brain, matters little, at the time of initial diagnosis, whether the tumor is benign or not. Biopsy of pineal gland tumors is reserved for tumor recurrence. At that time cell types help to direct more specific chemotherapy agents and additional surgery.

Mediastinal germinal cell tumors.

Germinal cell tumors are the second most mediastinal malignant tumors in childhood. Teratomas do not elevate blood fetoprotein and chorionic gonadotropin.

Chest x-ray detects tumors, and most patients are asymptomatic at the time of diagnosis. Biopsy of the tumor can be safely performed by the retrosternal approach. At times teratomas are detected in the thymus gland, rather than behind it.

Symptoms vary from asymptomatic to obstructive symptoms of trachea-bronchi and blood vessels of the mediastinum. Treatment is surgery, and chemotherapy and radiation are added if the tumors are malignant. In certain circumstances, chemotherapy and radiation are followed by surgery.

Ovarian germinal cell tumors are mostly benign cystic. The testicular germinal cell tumors are generally malignant seminoma and non-seminoma, appear in equal frequency. Testicular malignant tumors do not secrete chorionic gonadotropin and alpha fetoprotein.

Retroperitoneal germinal cell tumors are generally benign teratomas. And carry the best prognosis of the three locations.

Extragonadal germinal cell tumors are rare. Tumors are seen in children and young adults. Except for the sacral location, most of the tumors are either malignant or potentially malignant. Intracranial germinal cell tumors pose a diagnostic challenge but blood and CSF markers along with MRI images are virtually diagnostic.

The prognosis of sacral tumors is the best. Mediastinal seminomas have a better prognosis than Non seminomas. Overall, the 5- year survival rate is between 40 to 90 %.

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Hearing in Old Age.

 Hearing in Old Age

PKGhatak, MD

If one has passed 65 years of age, and for the first time hears someone telling he/she is short of hearing. The person feels a bit startled and annoyed. But the same person did not feel the same way when had to get a pair of reading glasses. Dealing with the deficit in hearing is different due to social stigmata. And wearing a hearing aid is a no-no.

The Latin term of old age hearing deficiency is Presbycusis. In the USA, one in three are over the age of 65 yrs. and nearly one in two over 75 yrs. has difficulty in hearing.

Persistent exposure to loud noise damages the delicate hearing organ and results in selective deafness for the frequency of sound overexposed to and once damaged it fails to grow back. In normal conversation the sound waves are like ripples on a slow flowing stream, noise in the work environment is like winds in winter storms and the noise that hits the ears in live concerts is like tsunami waves. The delicate hearing organs are not designed for mega loudspeakers and music synthesizers and the sound receptors simply wither away.

A short account of the hearing should make this clear.

The receptors of sound are called Organ of Corti. The Organ of Corti is located in the inner ear called Cochlea because of its resemblance to a snail. Inside the bony cochlea, all along the entire length, has another canal made of the membrane. Both the bony and membranous canals are filled with fluids, however, the fluid of the two canals is different in composition. The fluid of the bony canal is called Perilymph and has a high concentration of Sodium, the fluid in the membranous canal is called Endolymph which has higher Potassium levels.

Organ of Corti.

On the base layer of the membranous canal called the Basement Membrane, several groups of Hair Cells are situated on it. The hair cells are arranged in 4 layers of cells and bunched together in groups. Nerve fiber is attached to the lower pole of each hair cell. The tall hair cells have hair like projections on the top which are embedded in the top layer of the membranous canal. When the basement membrane moves up and down, the hair bends and stretches. This movement triggers the opening of pores of the hair cells and allows Sodium from the outer canal to enter the hair cells, triggering an electrical impulse that the nerve fibers carry to the hearing center in the brain.

The hair cells at the beginning of the canal respond to high frequency sounds and the hair cells on the far end of the spiral canal respond to low frequency sounds. And in between the hair cells respond to sound from high to low frequencies.

The three outer layers of hair cells receive signals from the brain and function as sound amplifiers to the outgoing impulse to the brain like transistors in radios.

The tension of the basement membrane has regional variation based on the thickness and composition of tissues. The region of the high tension area of the basement membrane vibrates to high frequency sounds and the low frequency sounds move the low tension area.

Functions of the other parts of the ear.

The outer ear, the pinna, collects sound waves, concentrates the sound waves and sends them down the ear canal. The eardrum vibrates and the vibration is transmitted to the stirrup-like tiny piece of bone attached to the inner side of the eardrum. The final of the three-piece of bone seats perfectly on the opening of the bony canal called the Oval Window and tissues around the bone make the joint airtight. Since the surface area of the oval window is 1/20th of the eardrum, the sound wave is magnified 20 times at the oval window. The waves are transmitted to the perilymph. The movement of the perilymph moves the endolymph in the membranous canal. As the endolymph moves, the basement membrane moves up and down generating nerve impulses. The 2 and 1/3 turns of the cochlea with progressive narrowing, amplify sounds further as the sound waves move to the far end of the cochlea.

Cause of hearing loss in elderly.

Like any organ, age takes its toll, but misuse and abuse accelerate the degenerative changes leading to loss of function. And in this modern age, humans are surrounded by air pollution, water pollution, light and sound pollution. The OSA regulations aim to protect the employees but the compliance is not universal and particularly true for companies with seasonal employees and mom and pop shops. And those weekend homeowners using lawnmowers, chain saws or leaf-blowers are likely to have a hearing deficit.

Genes are blamed for most maladies and so are presbycusis also. But the blame lies squarely on misuse.

The deficit due to presbycusis is detected on both ears. In an acquired illness deafness is unilateral.

The world around us.

It is interesting to survey the living world around us and note the evolution of hearing.

Plants.

Some claim plants can hear us talking to them, which makes plants grow healthier and faster. But plants have no nervous system at all. So, it is up to future scientists to find that truth.

The unicellular organism onward up to the worm:

The perception of sound in these organisms is through the surface in contact with the environment. The worms have nerve innervation of the segmental body, the nerve ends detect ground vibrations.

Insects:

The majority of insects can hear. An insect's so-called ear is an open tube, the opening of the tube is covered with overlapping cuticles. The location of sound receptors varies from one species to the other. It can be located in the abdomen, thorax, or head. Insects can hear a wide range of sounds.

Amphibians:

Salamander has primitive hearing organs inside the head and a tiny opening, one on each side admitting sound waves to the inner ear.  Frogs can hear in water and on land.

Fish:

Fish have the most elaborate hearing system. Fish have well developed inner ears on each side of the head. In addition, the lateral line of fish has nerves connected with receptors called cilia. Cilia move with the vibration of water. In some fish, the swim bladder has projections reaching the inner ears and act as sound receptors. The head of some bony fish acts as a receptor of wave movement. In some bony fish, the pectoral fine bones function as additional water vibration receptors. Fish, being underwater animals, hear only low frequency sounds.

Reptiles:

Snakes.  Snakes have no external or middle ears but have well developed internal ears. A special bone in the head connected with other bones by ligaments acts as the sound receiver. Snakes also feel ground vibrations with the body.

Lizards have the middle and inner ears but not the external ear. Ground burrowing lizards have a hearing system like snakes.

Birds:

Birds have ears, but not the pinna. The outside opening of the external canal is covered with special feathers without any burs. In vultures and condors, the external ear openings are easily visible. Songbirds have a wide range of hearing.

Mammals:

Whales and dolphins used to be land mammals, later they went back to the water. The pinna of aquatic mammals has disappeared, and the ear canals are filled with either wax or oil to prevent water entry but conduct water vibration well. Water being dense and not easily compressible, the sounds travel far and wide.

Dolphins can hear in water and out of water. In water, they hear through the vibration receptors of the lower jaw bone. When out of the water, the dolphins hear like land animals, the air enters the ears through two small openings on the side of the head. In addition, dolphins have an echolocation box on the head, a specialized receptor for ultrasonic sounds, and as the ultrasounds are received the dolphins reconstitute the ultrasound in the form of images of the prey and the immediate surroundings.

Whales:

Baleen whales (toothless) have two small external openings for the ear canal and the canal is filled with wax. The inner hearing organ is like land animals.

Toothed whales have no openings for their ears. They have a specialized structure on the lower jaw bone which acts as a receiver of sound waves. Whales can hear from very long distances but only low frequency sounds.

Land animals.

Dogs: Dog is the hearing champion among territorial mammals. A dog can move pinna in any direction by using some of the 30 muscles. A dog's hearing range is 20 to 40,000 cycles per second (Hertz); far beyond the human hearing range of 20 to 4,000 Hertz.

A look at the range of hearing of some animals.

Moth – up to 300,000 Hertz

Bullfrog - 50 – 4,000 Hertz

Owl – 200 – 12,000 Hertz

Songbird - 1000 – 8000 Hertz

Dolphin - 75 – 15,000 Hertz

Beluga whale -1000 -12,000 Hertz

Human - 64 – 23,000 Hertz

Dog - 20 – 45,000 Hertz.

The champion of hearing is the Moth.

Presbycusis comes with old age but by using ear covers in a loud noisy environment the hearing deficit can be delayed, or completely prevented.

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Brain Hemorrhage

Brain hemorrhage is also called Intracranial bleeding

PKGhatak, MD

When we say bleeding inside the head, it sounds like the bleeding has happened in the brain substance. Fortunately, that is less common than bleeding outside the brain proper but inside the skull.

This diagram is an exaggerated view of 3 covers of the brain called meninges.

If you can strip the scalp that covers the head, you see a bony box, the cranium or skull. A skull is made up of several bones, appears more like plates, joined together by narrow wave like suture lines or joints. Though called joints, these suture lines are tightly fitted with virtually no space in between. The bones also behave differently because skull bones have no periosteum which supplies new bone for fracture healing. A skull fracture may take 6 months to heal or may not heal at all. But fibrous tissues grow around the fractures hold the bone pieces together.

Epidural bleeding.

Moving further towards the brain the first layer of the brain covering is a tough membrane, like parchment paper, called Dura Mater. There is only a virtual space between the bone and dura mater known as Epidural space. Three pairs of arteries, one on each side from each pair nourish the dura mater. The middle one, the middle meningeal artery, is the dominant one. It runs along grooves inside of the bone. Fracture of bone also tears the artery. Blood sprats out from the artery in the epidural space and strip away dura mater from the bone. With each heartbeat, more blood accumulates. And bleeding will not stop until the artery is clamped.

Blood pushes soft brain tissues to the other side and finally pushes the brain downwards. At this point, without medical intervention, death is imminent.

Subdural bleeding.

Move further toward the brain from the dura mater. The second layer of the brain cover is the Arachnoid membrane.

Arachnoid means spider like. The arachnoid membrane has many long, loose fibers resembling spider legs. The virtual space between the dura mater and arachnoid membrane is known as Subdural space. Bleeding in this space is called Subdual bleeding or subdural hematoma. Many tiny veins traverse subdural space. Veins are thin-walled and subjected to easy rupture.

In old age the brain tissues shrink and move away from the bone, making the vein unsupported and liable to break with sudden shaking of the head or head moving back and forth after a sudden stop of an automobile or after a fall. Because bleeding from veins is slow and the accumulated blood inside the head increases the pressure that may be enough to stop venous bleeding. But that is only the first part.

The patient may not complain much initially or have some headaches. But after 3- 4 weeks, family members will notice the patient is having difficulty in walking, weakness of legs, or urinary incontinence. These new symptoms are due to the clotted blood drawing cerebrospinal fluid and enlarging. Early evacuation of clots is called for before neurological damage becomes permanent.

Subarachnoid bleeding.

Below the arachnoid membrane is the final covering of the brain called Pia Mater.

Pia means delicate, The word is derived from the Italian word mother. This layer is very vascular and is intimately attached to the brain tissue. If you again look at the diagram you will see medium sized blood vessels are going in and out of this Pia matter.

There are 2 pairs of arteries that bring blood to the brain, one pair from the sides and one pair from the back. The internal carotid artery, one on each side, enters the brain through a small opening of the skull. After entering the skull, it pierces the dura mater and arachnoid membrane and gives branches and then continues as the main artery of the brain called the Middle cerebral artery. Most of the subarachnoid bleeding occurs from one spot or another from the junction where the middle cerebral artery gives off branches. All these branches lie on the surface of the pia mater, underneath the arachnoid membrane.

Like epidural bleeding, subarachnoid bleeding is from a break in the arterial wall. And so blood is spurting out with each heartbeat and bleeding will not stop until the bleeding artery is clamped.

A special characteristic of subarachnoid bleeding is that the blood initially leaks only a small amount producing excruciating pain and if the patient arrives in ER in time, then a proper diagnosis and treatment can be initiated before the actual rupture of the arterial wall happens.

Unlike the previous two bleedings discussed above, subarachnoid bleeding is not just from one cause. A very brief discussion is presented here.

Common causes of subarachnoid bleeding.

By convention, subarachnoid bleeding is categorized into two groups- A. trauma and B. spontaneous bleeding.

A. Trauma.

When injury to the head produces damage to the brain tissues, the injury tears all three layers of meninges, and not only the arachnoid membrane. These types of injuries are very serious and properly treated only in trauma centers.

 B. Spontaneous arachnoid bleeding.

Spontaneous subarachnoid bleeding is of two categories.

1. Hereditary or gene mutation, 2. Acquired condition leading to bleeding.

1. Hereditary.

People are born with abnormal connective tissue that makes blood vessels wall liable to fray easily. However, not all the members are equally affected, those who have high BP develop bleeding. Some of the hereditary conditions are fibromuscular dysplasia, Marfan syndrome, Ehlers Danlos syndrome, and polycystic disease of the kidney. A few of them will be presented here.

 A. Berry aneurysm, also known as Saccular aneurysm.

A tiny bubble like outpouching develops at the junction of a branch from the main artery. The tiny aneurysm has a narrow neck. When one is present, more aneurysms may turn up later. If detected in time the aneurysmal sac can be sealed off by injecting endovascular coiling via a catheter threaded through the femoral artery into the sac of the aneurysm. The blood inside the aneurysm will clot around the coil and seal off the aneurysm.

B. Fusiform Aneurysm.

The fusiform aneurysm is less common, these aneurysms have no neck, unlike berry aneurysms. Also, structurally these are not outpouching but the weakness of the middle layer of the artery at certain points, which bulges out. The resultant aneurysms have a wide channel communicating with the artery and the outer covering is made up of thin adventitia. Coil embolization is not suitable, instead, a supporting wall structure called Pipeline Embolization Device (PED) is placed at the site by a catheter introduced via the femoral artery. PED provides the scaffolding upon which endothelium grows and repairs the weakness permanently. After placement of PED blood is prevented from entering the aneurysm and it shrinks and disappears.

C. Arteriovenous malformation (A-V malformation)

The malformed blood vessels are not strong enough to withstand high BP and are liable to rupture. Intracranial A-V malformation is usually seen in people who also have A-V malformation of the face and neck area.

 2. Acquired causes.

a. In today's world, Cocaine and Amphetamine addiction are the primary cause of spontaneous cerebral bleeding. Sickle cell anemia can obstruct arteries during a sickle cell crisis. Obstruction of blood flow weakens the arterial wall and causes bleeding.

 b. Mycotic aneurysm.

Infection of the cerebral arterial may happen if the infected emboli from heart valves are carried to the brain and lodged in arteries. The organisms likely to produce such emboli are Staph and Streptococci, Bacteroides, Clostridia, and fungi – Aspergilla and Mucormycosis from nasal sinuses infection.

 c. Vasculitis.

Vasculitis/arteritis also happens from noninfectious causes. Noninfectious vasculitis consists of many varieties and has various pathogenesis. Subarachnoid bleeding is only a possibility and is not common in every type of vasculitis. Those that may produce bleeding will be mentioned.

1. Giant Cell Arteritis. (GCA).

The other name of GCE is temporal arteritis. It is an immunological disorder of uncertain etiology. The hallmark of GCE is granuloma formation with Giant cells. The new blood vessels that grow in the granuloma may bleed and can cause subarachnoid hemorrhage,

2. Polyarteritis Nodosa.

Polyarteritis nodosa is characterized by the presence of localized inflammatory nodules in multiple systems, most easily detected on the skin of the lower legs; usual lesions are in the kidneys and respiratory tract. Though nervous systems are affected but are mostly in the peripheral nervous system and very rarely produce cerebral bleeding.

3. Recently, cerebral amyloid angiopathy is added to the list as an unusual and infrequent cause of cerebral infarction or intracerebral hemorrhage due to cerebral amyloid angiopathy.

Other systemic causes.

Many other conditions produce spontaneous subarachnoid bleeding. The following are a few examples.

a. Low platelet count from Dengue, Covid-19, and many other viral infections.

b. Platelet antibodies from autoimmune diseases and Intravascular coagulation use up platelets and may bleed in addition to blood clots. Antiplatelet drugs use is a risk factor

c. Anticoagulant medications use for medical reasons, including aspirin and cox inhibitors.

d. Leukemias. Chemotherapy, radiation therapy, and brain cancers.

 D. Strokes.

Hemorrhagic stroke.

Cerebral blood vessels, like coronary arteries, undergo plaque formation and rupture, producing hemorrhagic strokes. Fortunately, the incidence is falling from the wide knowledge gained from the press and social media.

Hypertension, diabetes, high cholesterol, obesity, cigarette smoking, sedimentary lifestyle and hereditary are risk factors for hemorrhagic stroke.

 Ischemic stroke is more common among all causes of stroke.

The diagnosis and treatment of subarachnoid bleeding have advanced greatly by the use of intracranial Doppler ultrasonography, subtraction angiography and MRI. Interventional radiologists have taken over the delicate operation of catheter placement inside the tiny blood bubbles and injecting coils or pipeline devices.

Intravenous Labetalol and Nifedipine are very useful drugs to lower high BP rapidly and safely. Prevention of pressure buildup inside the head following intracranial bleeding is treated by timely placing a drain in the ventricular cavity. Various monitoring devices and sharing patients' care with specially trained nurses have greatly improved the outcome of this potentially fatal disease.

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DREAM

 Dream

                                                        

PKGhatak, MD

The dream the mother of Siddhartha Gautama, queen Maya, had, is widely known all over the world. One night Queen Maya saw in her dream that a white elephant entering her abdomen by the side. Later on, she found out to be pregnant. After giving birth to Buddha, she passed away.

The dreams of Gilgamesh are also known to history enthusiasts. In his first dream, Gilgamesh saw a meteor fall on a field; his mother told him that meant he would have compassion. In his second dream, he found an axe on the street. That became a person and a constant companion in war.

 The religious texts of every nation have records of events foretold in dreams. Kings and emperors employed dream interpreters in their courts. That job was risky because many of them lost their heads when they misinterpreted their dreams.

In the nineteenth century, the most widely known dream interpreters were Sigmund Freud and Carl Jung. Their theories are subjects of study in today's universities but are largely discredited by evidence based medicine. 

 These Dreams Ushered in the Modern Era.

Albert Einstein dreamed that he was skiing downhill, going faster and faster till he was approaching the speed of light. Later he worked on the speed he witnessed in the dream and found the equation of the speed of light.

Srinivas Ramanujan, a mathematical genius, gave many breakthrough formulae in math. One that is most talked about is the formula of infinitive series for Pi. He said these formulae came to him in a series of dreams over a period of time and were gifts from God. 

Otto Loewi dreamed of the same experiment for two consecutive nights. After the second dream woke him up, he immediately went to his laboratory and that work led to the discovery of a chemical, acetylcholine, the transmitter of nerve impulses.

Niels Bohr, the father of quantum mechanics, in his dream saw the nucleus of an atom with spinning electrons going around it like planets go around the sun. He was quick to get up and write that down. Later that became the structure of the atom as we understand it today.

August Kekule saw in his dream that a snake was swallowing its own tail, soon the snake looked like a ring. That gave him the clue that organic chemicals also exist in ring forms. That led to his discovery of the benzene ring, the fundamental structural formula of most biochemicals.

Frederick Banting got instructions for an experiment on the pancreas in a dream. He experimented on laboratory dogs based on the instruction and subsequently discovered Insulin that saved numerous lives of diabetics, type I.

Finally, if you feel like dreaming, then sing along - I am dreaming of a white Christmas.

https://www.youtube.com/watch?v=Op6XUBTDKNU

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MEMORY

 Memory.

PKGhatak, MD

Barbara Streisand sings in her famous song the "Memory” --

"Midnight, not a sound from the pavement
Has the moon lost her memory?
She is smiling alone
In the lamplight, the withered leaves collect at my feet
And the wind begins to moan.............

All alone with the memory
Of my days in the sun
If you touch me, you'll understand what happiness is
Look, a new day has begun."

https://www.youtube.com/watch?v=MWoQW-b6Ph8

What is memory.

One medical dictionary says– “the mental faculty that enables one to retain and recall previous experienced sensations, impressions, information, and ideas.”

But this definition does say anything about how the mental faculty enables. Intensive research is going on currently to address that question.

It is easier to answer where memory is stored in the brain than how memory is formed. With the wide use of brain imaging tools, MRI has advanced research greatly at the macroscopic level. Functional MRI locates memory centers very preciously and the location is based on the nature of information, emotion associated with it and how long the memory is stored. The Hippocampus, Neocortex, Amygdala, Cerebellum, and prefrontal cortex serve as memory centers. Hippocampus is the initial place where memory is formed. The long term memory is shifted to the neocortex from the hippocampus, and when the process is complete the memory disappears from the Hippocampus. Emotional or fear provoking memories are generated with input from Amygdala. Body movement associated memories are stored in the cerebellum.

Memory formation.

Humans are capable of remembering a vast amount of information. If one has any doubt just watch TV the Jeopardy show, and you will be simply amazed.

You need not be surprised that the vast amount of written literature is accomplished with only 26 letters of the alphabet and a few punctuation marks and simple rules of grammar.

And the computer memory is written by just two inputs 0 and 1. Brain cells use Long Term Potentiation (LTP) and Long Term Depression (LPD) to record memory.

LTP and LTD.

Two types of effects are observed in memory formation. LTP is observed when information is strong and repeated. LTD happens when information is received once or only a few times and the memory lasts maybe a minute only. LTP and LTD are alphabets of memory. Synaptic plasticity and neurons are the punctuation and grammar of memory. Memory is formed from persistent changes in the strength of connections between the neurons. Different groups of neurons are responsible for different information and thoughts

Synaptic Plasticity.

Synaptic plasticity describes persistent changes in the strength of connections between the neurons producing structural changes both in the synapses and neurons and the creation of new synapses. And new neurons are formed in the hippocampus when information is repeated often.

Molecule of Memory.

The molecule that is universally present in all nucleated cells is called Calmodulin.

Calcium ion (Ca++) is only selectively admitted inside the cells when a stimulus arrives at the synaptic junction or neuron. Inside the cells, Ca++ combines with calmodulin to form Calcium / Calmodulin dependent protein kinase (CaMKII).

CaMKII is widely used in many important cell functions. Inside the neurons, the CaMKII exists as isomers; and for memory generation, four isomers of CaMKII are involved. Each isomer functions independently and only in one aspect of memory formation.

Calmodulin acts as a messenger carrying information from the cell cytoplasm to the nucleus of the cell. Kinase is an enzyme that breaks down high energy ATP (adenosine triphosphate) and the released energy that is utilized in chemical reactions.

CaMKII by repeated interaction with the nucleus induces gene variations and results in new mRNA generation. CaMKII activates down the line Kinases for chemical reactions. Many other molecules have also been identified that play various roles in storing and retrieving information from neurons.

Types of memory.

Memories develop in two phases. Short term and Long-lasting memory.

The short term memory-

In the initial phase, the learning of short term memory the brain retains the information only for a brief period of time. According to Meffert of Johns Hopkins, the brain only makes transient changes of the memory molecules already exist and minutes later the molecule is recycled or destroyed.

Long lasting memory.

Meffert thinks the brain cells not only make changes in the molecules already present but also change the gene expressions – turning on and turning off genes. Neurons produce new proteins, increase the rate of production, or shut down lines of proteins.

The strengthening of synaptic connections, forming new connections, and enlargement of neuron partnerships are also part of this process.

Memories are Explicit or Declarative, and Implicit or Non-declarative.

Explicit memories deal with facts, events, people, places and objects. The center is located in Hippocampus. Implicit memories are motor skills and perceptual. The cerebellum is involved. The reflex action in invertebrates is equivalent to the explicit memory of vertebrates.

Taken from Richard F. Thompson and Jeansok J. Kim. PNSA, Nov.26,1996.

Episodic memories.

Telving called another type of memory-the Episodic Memory. Episodic memory allows us to replay old experiences in our own mind resulting in refinement, making associations with similar experiences that result in enhancement of that experience. If emotions are generated during the replay, then the Amygdala stores that memory. In a recent publication, Dr. Reddy from France says in human "Time Cells" which are present in the hippocampus, fire repeatedly and link time, place, person of an event as the hippocampus is developing the memory. That is why we remember not only the event but when and under the circumstance - like "what were you doing when you heard that President Kennedy was shot"

Memory development, retention and recall are subjects of serious study at present. There are many more molecules, enzymes and cell groups involved in many aspects of memory. The increasing incidence of dementia in the elderly has produced a strain on the finances of medical care and personnel. A clear understanding of different aspects of memory is urgently needed so that the fundamental pathology of dementia will be understood and properly addressed.

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SLEEP

 Sleep

PKGhatak, MD

Sleep is a state of the body during which most of the bodily functions are suspended, consciousness and voluntary actions are absent but readily reversible to external stimuli.

Sleep is a compulsory phase of daily occurrence for all animals - even whales sleep in water; and sharks who must move in order to force water through the gills to meet oxygen requirements, undertake a kind of restfulness, a phase equivalent to sleep. Young dolphins are seen sleeping while swimming but remaining in bodily contact with others within the pods. Plant physiologists have observed sleep like state in plants. Developing human fetuses also sleep in the mother's womb. Newborns sleep most of the time with short periods of wakefulness which gradually increases as the child advances in age. Adults sleep 6 to 8 hrs. a day, the elderly require less sleep and octogenarians experience short periods of awakening during nocturnal sleep.

People who work shift duty jobs have difficulties in falling asleep after switching over and most of them develop hypertension, diabetes, obesity, metabolic syndrome, ulcers, increased irritability and behavioral changes. Night duty workers have a 10 % reduction in longevity compared with the workers doing the same job during the daytime.

Sleep deprivation causes increased Cortisone release, increased Ghrelin secretion, decreased Leptin and increased hunger and appetite.

The circadian rhythm controls home release; at the start of darkness, Melatonin is released from the pineal gland which prepares the body to fall asleep. During sleep cortisol level falls, antidiuretic hormone release is stopped. Pituitary Growth hormone is released. Cytokines -tissue necrosis factor (TNF), interleukin 1(IL1) are released and help tissue repair at night; and also, TNF and IL 6 levels increase that activate the liver cell regeneration.

Long term memory development takes place during sleep, and growth in children accelerates during sleep. The BP, heart rate, respiration decrease, voluntary muscles cease to function temporarily but cardiac and smooth muscles continue to work. As we begin to wake up, all the above changes are revered. Increased cortisol and increased BP may trigger a heart attack in patients with coronary artery disease.

Dreams during sleep are a natural occurrence, many remember the dream on waking up, but most others fail to recall. Scientists believe rewiring neurons in the Neocortex and Hippocampus during sleep generates dreams. Many believe octopus dreams during sleep as evident by the color changes.

Young children develop nightmares during dreams which experts think is a benign phase of brain development. Sleepwalking at a young age is also a benign condition but in adults, it is not normal.

Sleep apnea in an obese person is a risk factor for hypertension, cardiac arrhythmia, and heart attacks. This condition is due to the decreased tone of muscles of the glottis and tongue falling backward obstructing the entry of air into the laryngopharynx. Snoring during sleep is a similar condition but airways still remain partially open and breathing does not stop.

In any discussion on sleep, REM (rapid eye movement) and Non-REM (NREM) are discussed. In adults, NREM sleep takes place about 70% of sleep time. Scientists have divided NREM into 3 phases based on the frequency of the brain waves. Phase 3 of NREM is deep sleep and most of the bodily functions are lowest during this period but at the same time tissue repairs, healing and rejuvenation of body and mind take place here. The changeover from one stage to the next does not necessarily follow any particular order. In babies REM happens 50% of sleep time, In younger adults NREM takes place at the beginning of sleep followed by REM about 90 minutes later. Initial REM sleep lasts only 10 minutes and increases progressively and at the end may last 60 minutes. Heart rate, BP and respiration increase during this phase and dreams happen during REM sleep. Reinforcement of memory happens in REM sleep. The brain switches REM and NREM in some order initially but in older individuals no particular pattern is recognizable. In old age REM and NREM switch back and forth in short order and even at the onset of sleep no order is observed.

Narcolepsy and Restless Leg Syndrome are neurological disorders and are not discussed here.

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Folic Acid

 Folic Acid

PKGhatak, MD

Folic acid is a water soluble yellow crystalline substance. It is a chemical made of Pteridine, p-aminobenate and Glutamic acid. Glutamic acid may be present as monoglutamate but usually exists in polyglutamine forms, containing anywhere from 2 to 7 glutamic acid residues. About 30 different derivatives of folic acid are present in nature. In the plasma, the Folic Acid (FA) is pteroylmonoglutamic acid, but within the cells, folic acid exists in polyglutamic form. In the blood, the majority of folic acid is present in RBC. The normal Plasma FA level in adults is 3 to 17 ng/ml. RBC folate level resembles body storage and deficiency appears 90 days before low plasma FA.

Other names.

Citrovorum factor. An essential growth factor for bacteria Leuconostoc citrovorum was detected and named citrovorum factor and later was identified to be folic acid but the name remains. At one time FA was called vitamin B9.

Folic acid however cannot enter the cells as such, until it is reduced to Tetrahydrofolate (THF) by the enzyme THF Reductase, B12 methyl coenzyme and NADH acting as H donors. Once inside the cells, lysosomal enzymes convert it into polyglutamate and then combine with substrates to form Folic acid coenzymes.

Folic acid coenzymes are single C-atom transfer and utilization agents. One [C-atom] maybe 1. Formyl group [-CHO], 2. Formate [- H COOH] , 3. Hydroxymethyl [– CH2.OH]. These three groups are metabolically interchangeable. THF coenzymes combine with one of the above one atom C and form and play a very important role in DNA, RNA and phospholipids synthesis.

Dietary source.

Source - fruits, leafy vegetables, field mushrooms, liver, and fortified food items.

Daily requirement is 400 micrograms for adults and for pregnant women 600 mcg/day. A FA dose of 5000 mcg/day to pregnant women is harmful to the developing child, and increased incidence in respiratory allergy, insulin resistance and delayed psychomotor development are seen in these children.

The Folic acid storage in the liver is good for 3 months of use and the total amount is 5 mg.

Absorption of FA.

Polyglutamic FA in food is converted to monoglutamate in the intestinal mucosa, then FA is absorbed in the upper small intestinal epithelial brushed border by a transported protein. Renal tubular cells reabsorb FA from glomerular filtrate.

Transport of FA in and out of cells requires conversion of FA to polyglutamate and monoglutamate and vice versa. And each phase requires a specific transporter protein.

Actions of FA coenzymes are necessary for methylation. Folic acid is required in the formation of choline, Serine, Glycine, Methionine. and histidine.

Purine and thymine are used in DNA synthesis.

FA coenzymes convert amino acid histidine to glutamic acid through an intermediate step where foraminmonoglutamic acid is formed. This is the basis of the Figlu test to detect folic acid deficiency. This test is just of historical importance, the blood FA test has replaced Figlu test.

The function of FA.

It is an essential vitamin for all cells of the body for cell growth, maturation, and repair. FA is utilized for DNA, RNA, mRNA and phospholipid synthesis. It is necessary for the methylation of fatty acids, metabolism of several amino acids. Growth and development of the fetus. 

FA deficiency.

Deficiency. Alcoholism, dietary habits, overcooked vegetables in a large volume of water.

Drug interference- anticonvulsants, oral contraceptives, Methotrexate. Trimethoprim-sulfamethoxazole and Sulfadiazine. Aminopterin (MTX) is a permanent folic acid antagonist and is used in the treatment of Leukemia and immunosuppressant, used as an adjunct in order to reduce steroid dose.

The medical condition leading to deficiency - pregnancy, hemolytic anemia, exfoliative dermatitis, GI mucosal atrophy, psoriasis.

Effects of FA deficiency.

The tissues which turn over rapidly exhibit the earliest changes in the development of the nucleus but the cytoplasm continues to develop normally. Blood cells- RBC, WBC and Platelets are not only decreased in number but also deformed. Since RBCs have no nucleus, the nuclear abnormality is clearly visible in peripheral WBC. CBC. Anemia. Most RBCs are large and come in various thicknesses and sizes giving an increase in RWD, PVC, and a low HCT. Leukopenia and polysegmented neutrophils and decreased Platelet count. Erythroblasts in peripheral blood. 

Neurological.

Changes are noted in a fetus and newborn, commonly spina bifida and anencephaly.

Folic acid deficiency is common in pregnancy due to increased demand by the fetus which cannot be met by diet alone. WHO recommends fortification of rice, cereals, flour, pasta and bread with folic acid. It plays a vital role in the synthesis of the nucleus of all cells.

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Vitamin B12

 

Vitamin B 12

PKGhatak, MD

Vitamin B12 is a water soluble vitamin, a chemical compound containing a cobalt atom. Vitamin B12 is essential for humans and must be supplied in food. Liver, meat, milk, egg, herring and mackerel fish are the chief source of B12; vegetables are nearly devoid of B12. Stomach acid and pepsin release B12 from the food, B12 then combines with a glycoprotein. B12 is absorbed in the terminal ileum. The liver is the principal store of B12. Breast milk contains enough B12 to meet a developing child's requirements. The daily requirement for adults is 3 micrograms, for infants 0.3 micrograms and for children the amount varies according to age. Adults have about 3 year reserve of B12 in the liver.

Chemistry:

B12 was originally obtained from the liver by Dr. Castleman in 1948. The isolated compound was light sensitive but when combined with a cyanide group, B12 becomes a stable crystalline form - cyanocobalamin. The central part of the molecule is four pyrrole rings surrounding a single six valent cobalt atom. This ring is called the Corrin ring system. The corrin ring is synthesized from delta aminolaevulinic acid by a process where the methyl group is supplied by methionine (amino acid). The central cobalt atom is attached to the N molecule of each of the four pyrrole rings. The 5th valent of Cobalt is attached to the N molecule

 of 5,6-dimethyl benzimidazole ribose phosphate and the 6th valent of the cobalt is attached to a variable group, R - cyano or 5-deoxyadenosyl or a methyl group. These forms are interchangeable with the cells. The R binding compound gives the type of B12 that will be formed eg- cyanocobalamin and adenosylcobalamin. In food, B12 presents as Adenosyl B12, Hydoxyadenosyl B12, Methyl B12, Cyano B12 and Sulphito B12, of which the first two forms are commonly found.

In nature, B12 is also present as hydroxocobalamin, a red color compound and nitrocobalamin. Methylcobalamin and  Hydroxycobalamin are therapeutically active and last longer in the body. B12 in plasma is bound to plasma proteins in variable amounts. The intracellular B12 level in the mitochondria is dependent on the amount of free B12 present in the serum. 

Physiology:

Acid and pepsin in the stomach release B12 from the food. The intrinsic factor (IF) of Castle, a glycoprotein, secreted by the parietal cells of the stomach, combines with B12 in the duodenum and prevents B12 from digestion by GI enzymes and keeps it safe from gut bacteria using it. As B12 reaches the terminal ilium, the receptors present on the surface epithelial cell of the ilium bind with it, then release B12 from the IF. Only free B12 is available for absorption. About 1 to 2 % B12 is absorbed by simple diffusion, the rest is ferried across the cells by an active process and carried by the Transporter I protein. The Transporter II protein carries B12 to the liver, bone marrow and other tissues. The proportion of B12 absorbed from food is limited by the amount of available IF. As a result, only about 20% is absorbed; that amount generally satisfies the daily 3 micrograms of B12 requirement. In macrocytic anemia or pernicious anemia, 1500 to 3000 micrograms of B12 are prescribed and 1 to 2% absorption by diffusion meets the therapeutic needs. The only source of B12 for the herbivorous animal is that produced by gut bacteria. 

B12 inside the cells.

B12 with the attached transport protein enters the cells. The lysozymes free the B12. The B12 then is taken up by mitochondria. Here B12 first converted to Cobalamin II alamin. Then cobalamin II alamin is converted to adenosylcobalamin and methylcobalamin. Hydroxycobalamin is also present in the cytoplasm. 

The action of coenzymes:

Methylcobalamin is the cofactor of methionine synthase. Methionine synthase converts Homocysteine to Methionine. In another reaction methionine synthase transfer the methyl group from 5 methyltetrahydrofolate (THF) to homocysteine. THF is the active form of vitamin Folic Acid. THF is essential in DNA synthesis.

The adenosylcobalamin is the cofactor of methylmalonyl CoA mutase (CoA means coenzyme A). Methylmalonyl CoA mutase converts methylmalonyl CoA to Succinyl CoA in the Krebs tricarboxylic acid cycle.  Energy supplying amino acid metabolism requires to pass through succinyl CoA step. Similarly, the fatty acid metabolism is also dependent on this methyl B12 coenzyme.

The function of B12:

B12 coenzymes are required for the synthesis of 1. Purine Nucleotides, 2. metabolism of several amino acids and fatty acids 3. the normal growth and development of cells.

Myelin sheath of nerve fibers.

In the initial steps of myelin sheath synthesis, the B12 coenzyme produces methylmalonyl-CoA from branched amino acids threonine, isoleucine and valine. In B12 deficiency malonyl CoA is reduced and methylmalonyl-CoA is substituted. This results in defective myelin and early breakdown of the myelin sheath.

Causes of B12 deficiency.

Dietary.

 B12 deficiency is expected in strict vegetarian Hindus in India. In the west, vegans are similarly affected.

Diseases of the stomach.

Acid suppression by pantoprazole over a prolonged period, atrophic gastritis, partial gastrectomy,

Intrinsic factor (IF) deficiency.

Congenital absence of IF.

Acquired autoimmune disease producing antibodies to the parietal cells or antibodies to IF.

Diseases of the terminal ileum.

Crohn's disease. Surgical removal of terminal ilium, Celiac disease. Tropical sprue. Malabsorption syndrome. HIV infection, Frequent bacterial enteritis. Change of gut bacterial population.

Intestinal parasite. Diphyllobothrium latum, a fish water fluke, infestation of the gut.

Diseases associated with Megaloblastic anemia.

Type 1 diabetes mellitus, Graves disease, Hypothyroidism, Addition's disease. Parkinson syndrome.

Risk factors. North European ancestry. Alcoholism.

Diagnosis of B12 deficiency:

1. Blood test.

B12 deficiency is easily detected by obtaining blood B12 levels.

2. Associated abnormalities of blood picture.

CBC. Any or all of these features may be present. Anemia, Macrocytic RBCs, 

Increased RWD (Red blood cell width distribution), Ovalocytes, Leukopenia, and hypersegmented neutrophils. Thrombocytopenia, pancytopenia. Increased blood LDH, Bilirubin. and AST. Decreased Haptoglobin, and increased serum Methylmalonic acid and Homocysteine.

Some neurological symptoms are suspected to be from the B12 deficiency but the serum B12 levels may be normal. The B12 deficiency is confirmed by finding a blood methylmalonic acid over 1000 mcg/ml and high homocysteine levels.

Diagnosis of Pernicious Anemia.  

1. Presence of megaloblastic anemia and 2. deficiency of Intrinsic factor.

Intrinsic factor deficiency may be due to congenital or hereditary.  Acquired causes are due to the presence of antibodies. Antibodies are of two types. 1. Complement fixing antibodies to parietal cells and, 2. antibodies against the Intrinsic factor.

Symptoms of B12 deficiency.

Weakness, glossitis, anemia.

Neurological.

Some or none of the following symptoms are usual findings.

Peripheral neuritis, Depression, dementia, Psychosis, Cerebellar ataxia, Subacute combined degeneration of the spinal cord. Optic atrophy. Cranial nerve nephropathy.

In general, Megaloblastic anemia is common in B12 deficiency, neurological manifestations are variable and occasionally appear before hematological changes.

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Thalamus and Outline of Sensory System.

 Thalamus and Outline of the Sensory System

PKGhatak, MD

Thalamus:

Thalamus is a collection of neurons in the midbrain; in fact, there are two in number- the thalami. Thalami are placed close together in the middle of the midbrain, separated by the Third Ventricle containing CSF (cerebrospinal fluid). They are the mirror image of each other. Thalamus is the initial sensory receiving center of all the incoming information from the body, namely 1. the somatic sensation from the skin - touch, pain, cold and hot sensations. 2. Proprioceptor sensation of the body - positions of limbs, head in relation with each other. 3. visceral sensation from chest, abdomen and pelvis - the sense of bloating, cramps, pain and discomfort. 4. special senses from eyes, ears, tongue - visual, auditory, taste but not the smell sensation.

Thalamus relays the sensory input received from different parts of the body to specialized areas of the cerebral cortex for detailed analysis, at the same time receives communications from the cerebral cortex. The interplay of these back and fourth information between the thalamus and cerebral cortex determines the final sensory appreciation, the body's response to sensory information, memory formation and association. Thalamus also generates local sleep patterns - namely sleep and wake rhythm during the non-rapid eye movement sleep.

To achieve these functions, the thalamus is extensively connected with all major nuclei of the midbrain - hypothalamus, amygdala, hippocampus, etc., and cerebellum, medulla oblongata and cerebral cortex.

There are 6 functional groups of thalamic nuclei. These are 1. Anterior thalamic nuclei- facing the forehead. 2. Lateral thalamic nuclei- facing towards the ear of the same side. 3. Medial nuclei - facing each other across the midline. 4. Paraventricular-located next to the 3^rd ventricle. 5. Interthalamic - making the connection between the two thalami. 6. Reticular a net like nuclei is located at the bottom of the thalamus.

Anterior thalamic nuclei: Afferent fibers relay here from Limbic strictures and the medial mammary nucleus. Efferent fibers from the thalamus connect to the Cingulate gyrus, parahippocampal nuclei, perforated substance and limbic nuclei.

Function: Emotion, memory, alertness.

Lateral thalamic nuclei: Receives afferent fibers from Globus pallidus, substantial nigra. Efferent connection to the frontal lobe of the cerebral cortex, and Interthalamic nuclei.

Function. Planning and initiating movement.

Ventral thalamic nuclei are the Ventrolateral and Ventromedial groups of nuclei.

Ventromedial: receives input from the contralateral cerebellum, the ipsilateral motor cortex.

Efferent output to the primary motor cortex and premotor cortex.

Function. Motor response and integration of sensory and motor functions.

Venteroposterior:

Afferent input from lateral spinothalamic and dorsal spinothalamic tracts. These are the main sensory receiving nuclei of the thalamus. Also, fibers from medial lemniscus and trigeminal thalamic sensory fibers end here.

Efferent output to the sensory parietal cortex, insular cortex.

Function. The initial sensation of pain, temperature, touch, discriminating touch, and conscious proprioception.

Other thalamic nuclei receive and communicate with various midbrain nuclei, different areas of the cerebral cortex and cerebellum.

Sensory system.

A general outline somatic sensory system consists of receptors, neurons, ascending tract in the spinal cord and hindbrain, and nerve centers.

 The Receptors are: -

Naked nerve ending. It is sensitive to pain stimuli. Ruffini endorgan records stretch sensation, deformities, warm temperature. The end bulb of Krouse detects cold, these are also present in the mucous membrane of eyes and mouth and genitalia. Meissner's corpuscles-an onion like layered receptor receives slow vibration from the skin and bones and also touch sensation. Merkel discs receive light touch and two-point discriminating tactile sensations sustained pressure and touch. Pacinian's corpuscles record deep pressures and fast vibrations. The hair follicle plexus is stimulated by the movement of hair. Muscle spindles record muscle contractions, cramps, and stretch sensation. Golgi tendon organ records the stretch produced in the tendons. Bulbous corpuscles are stimulated by the stretch of the capsule of joints.

A number of skin receptors are also present in joint capsules, tendons and bones.

Joint kinesthetic receptors. These are responsible for information generated from limbs and joints during movement.

Vestibular organs. Present in the middle ears, these receptors record position of the head on three dimensional axis.

 The Neurons: -

 The neuron with its long arm - afferent/axon, carries the impulse towards the central nerve center. The short arm of the neuron, called dendrons, makes a synaptic connection with the 2nd order neuron. The nerve bundles in the spinal cord take the impulses all the way up till they reach the medulla oblongata, then the fibers cross the midline to the contralateral thalamus, So, the thalamus of one side receives the sensory information from the opposite side of the body. The 3^rd order neuron of the thalamus carries the impulses to specific areas of the parietal sensory cortex of the brain located on the same side as the thalamus is located.

 The Pathways to the Thalamus:

The neurons for skin sensations are located in the dorsal root ganglion of the spinal nerves. The axons enter the spinal cord by the dorsal root. Thereafter the fibers make three ascending paths to the thalamus on the opposite side and end in the contralateral thalamus.

Pain, two-point discrimination touch, and vibratory sensation pathway.

Axion carrying the sensation turns upwards in the spinal cord along the posterior ascending column. Below the T6 level ascending fibers are placed in the medial position then fibers from each higher segment are placed lateral to the fibers already present – somatotopy organization. The fibers make synaptic connections with neurons called Nucleus Gracile located in the lower medulla. The second order neurons cross the midline - decussate in the medulla oblongata and ascend to the ventromedial nuclei of the contralateral Thalamus via medial lemniscus.

The axon above the T6 level to C1 similarly ascends up the spinal cord lateral to the Ascending tract of Gracile. Similar somatotopy organization is present in this tract. At the lower medulla and fibers make synaptic connections with a collection of neurons called Nucleus Cuneatus. Then follow the same path as the tract of Gracile. 

Cold / hot sensation and pain pathway:

1^st order neurons are dorsal root ganglions. The axons enter the spinal cord via the posterior root and move up or down 1 or 2 segments then make synapses with the second order neurons located in the posterior horn. The fibers cross the midline in front of the central spinal canal and then ascend upwards occupying a lateral position in the spinal cord known as the lateral spinothalamic tract. These fibers reach the ventromedial nuclei of the thalamus via the medial lemniscus and there they make synapses. The rest of the pathway is similar to tracts of Gracile and Cuneatus.

Crude touch and firm pressure pathway:

The layout is similar to the lateral spinothalamic tract except the 1^st order axons cross the midline immediately and synapse with the 2^nd order neurons. And that tract occupies a more anterior position in the spinal cord. The rest of the track follows the temperature path.

Proprioceptive sensory tract:

The 1^st order neurons are the dorsal root ganglions. The axons after entering the spinal cord take one of the three paths.

Those carrying proprioceptive destined to reach the conscious level follow the fibers of the somatic sensory fibers, traveling up along tracts of Gracile and Cuneatus. One big difference is that the proprioceptor fibers do not cross the midline and enter the ipsilateral cerebellum through the inferior cerebellar peduncle and make synaptic relay with the nuclei present in the vermis and para ventral nuclei of the cerebellum. The dendrons carry the information to the deep nuclei of the cerebellum, and the 3^rd order neurons carry the information to the thalamus and other midbrain nuclei.

Unconscious proprioceptor sensory information.

Below L2 Level.

Central axons of the 1^st order neurons end by making synapses with posterior horn nuclei. The ascending fibers of the 2^nd order neurons immediately cross the midline in front of the spinal channel. Then ascend to the midbrain at the junction of the midbrain and medulla, then cross the midline again and enter the cerebellum through the superior cerebellar peduncle. The spinal cord location of these fibers is called the Anterior spinothalamic tract.

Above the L2 level and up to the T1 level.

The axons of the 1^st order neuron terminate in the Clarke nucleus of the posterior horn of the same side of the spinal cord. The ascending axons of Clarke nuclei remain on the same side of the spinal cord and travel up as the posterior spinocerebellar tract. Then reaching the medulla enters the cerebellum through the inferior cerebellar peduncle.

Above T1 level.

The axons of 1st order neurons ascend with the Cuneatus tract and make synapses with accessory Cuneatus nuclei. The fibers enter the cerebellum through the inferior cerebellar peduncle.

 The Trigeminal nerve.

The skin of the forehead, face and lower jaw are innervated by the 5^th cranial nerve- The trigeminal nerve. The nucleus is known as the trigeminal ganglion located on the petrous process of the temporal bone. The second order neurons are located in a long structure extending from the upper cervical cord to the lower midbrain. The axons from these neurons cross the midline and join the medial lemniscus to the thalamus.

Visceral senses.

The general outlay of the sensory information reaching the brain is different. One reason is the viscera are not paired – one heart, one liver, one large gut and on the small intestine, etc. Only there are two lungs.

The second reason is that visceral sensations are carried both by the sympathetic and parasympathetic nerves nervous systems. The parasympathetic system carries the general sensation and the sympathetic system general and also the pain sensation. There is no hard and fast distinction of senses into ipsilateral or contralateral representation in the thalamus or in the cerebral cortex. In most cases, the thalamus and the cerebral cortex area receive visceralsensory information from both sides of the body.

Parasympathetic visceral sensory system:

The receptors: The receptors are present in all the layers of the small and large intestines. Glands and solid organs likewise have several receptors.

The parasympathetic sensory outlay is a bit different from the general sensory system, again are of two groups - A. all viscera located in the thorax, abdomen including the medial 2/3^rd of the transverse colon, B. the lateral 1/3 of transverse colon, sigmoid colon, rectum, anal canal, urinary bladder, urethra and organ of sex.

A. Upper abdominal visceral path of parasympathetic:

The Vagus nerve.

Sensory information from the thoracic viscera, abdominal viscera and medial 2/3 rd. of the transverse colon is carried by the sensory division of the vagus nerve to the Nucleus Nodosa of the vagus situated in the medulla on the ipsilateral side.  The second order neuron of the vagus carries the sensation upward in Tactus Soliterious to both Thalami. The 3^rd order neurons take that information to the sensory cerebral cortex.

B. Lower abdominal visceral parasympathetic:

 The Hypogastric nerve.

The afferent fibers, of parasympathetic 1^st order neurons, are located in the posterior horn of the T10 to S 2 segments of the spinal cord. These fibers entering the spinal cord make synapse with the second order neuron of the parasympathetic system. They travel upwards to the medulla along with somatic sensory fibers in the Gracile tract and the end of the fiber in the sensory dorsal nucleus of the vagus.

  The Inferior hypogastric nerve.

The nucleus is located in the dorsal root ganglion of S2 to S$ segments. The axons travel along with the pelvic and pudendal nerves to innervate the remaining pelvic organs and sex organs. The upward journey starts with the fibers of the Gracile and ends in the sensory nuclei of the Vagus in the medulla.

Sympathetic sensory system.

Receptors. Similar to parasympathetic sympathetic receptors are present from all layers of viscera and glands and in addition, unmedullated C-fibers carry pain sensation. The first-order neuron is located in the dorsal root ganglions of the entire spinal cord.

The pain of cutting, burning, crushing is not carried from the viscera but inflammation, necrosis, obstruction related pain is carried from the visceral wall by the undulated C-fibers of sympathetic nerves. On their onward journey to the spinal cord, the fibers accompany the sympathetic tract carrying efferent sympathetic stimuli.

 Sensation from eyes, lachrymal glands, nose. palate, and salivary glands.

 Sympathetic afferent fibers pass through the cervical sympathetic ganglia, then accompany the postganglionic sympathetic fibers. The fibers leave the sympathetic nerve and join the white rami to join the mixed spinal nerve and enter the spinal cord via white rami by the dorsal root. It makes a synaptic connection with the thoracolumbar sympathetic neurons. The second order neurons carry the sensation along with the somatic senses of the thalamus.

 The sensory sympathetic innervation of the heart, trachea bronchi.

 The dorsal root ganglions of C4 to T5 segments carry sensory information and fibers pass through the celiac ganglion. Then the upward path is the same as above.

Innervation of the stomach, liver, bile duct, gall bladder, spleen, kidneys, pancreas and small intestine.

The T6 to T11 segments supply sensory fibers and the fibers pass through the superior mesenteric ganglion.

 The rest of the GI organs and pelvic organs send sympathetic sensory information.

 The sensory afferent fibers originate inT12 to L2 segments. The fibers pass through the Inferior mesenteric ganglion.

 L3 to S5 segments innervate blood vessels of the perineum and inferior extremely.

Special senses:

Visual.

The visual receptors are rods and cones of the retina.

The right optic nerve carries visual input from the temporal half of the right eye and the nasal half of the left eye. The left optic nerve has fibers from the temporal half of the left eye and the nasal half of the right eye. The crossing of nasal fibers is known as optic chiasma. These fibers then sweep around the midbrain and make a synaptic connection in the lateral geniculate body of the ipsilateral side. The lateral geniculate body is a special section of the lateral dorsal thalamus. The visual sensation reaches the visual cortex by way of the optic radiation. The visual sensation of one side of the visual field is projected to the same side of the visual cortex but contains images from both retinae. This is because there is an overlap of image when placed close to the nose, each eye captures the image but at a different half of the retina.

Taste sensory pathway.

Receptors:

The tongue, soft palate and throat contain taste buds. These buds are distributed widely but some are more concentrated in one area than the rest. The tip of the tongue contains sweet sensory taste buds and the sides of the tongue contain salt and sour taste buds, the dorsal surface of the tongue for all taste buds, and the back of the tongue for the bitter. The anterior two thirds of the tongue is innervated by the tympanic branch of the 7^th cranial nerve, the Facial nerve. The distal 1/3rd of the tongue and throat by the Glossopharyngeal nerve, the 9th cranial nerve. The epiglottis taste buds are innervated by the Vagus nerve, the 10th cranial nerve. The neurons for the taste sensation of the 7th nerve are located in the Trigeminal nucleus, the neurons for the glossopharyngeal are present in the Petrosal ganglion and vagus sensory neurons are located in the medulla and is known as the Dorsal nucleus of the vagus. The dendrons of these nerves are carried in the tactus solitaries of the vagus. The ascending fibers from these ganglia cross the midline to the opposite side and reach the poster ventral nucleus of the thalamus by way of the Medial lemniscus. The thalamic nuclei relay the taste sensation to the Gustatory cerebral cortex and anterior insular cortex.

These three nerves also carry the general visceral sensation from the tongue, mouth, palate, epiglottis and throat in a separate group of fibers but follow the same path the rest of the way.

Auditory Pathways:

The receptors of sound are hair cells of the organ of Corti located in the cochlea of the middle ear. The 1st order neurons are cochlear neurons carrying the sound impulses to the Superior Olive nucleus of both sides but mostly to the opposite side. The second order neurons of the superior olive pass through the reticular formation of the midbrain and terminate in the lateral geniculate body of the same side. Like the medial geniculate body, the lateral geniculate body is a part of the thalamus and both bodies are located close together. From the lateral geniculate body, the last relay takes the sound sensation to the auditory center in the cerebral cortex. One side of the auditory cortex receives sounds from both ears.

Smell sensation.

Thalamus is not the initial nerve center for the smell.

Receptors: Nasal epithelium contains olfactory sensory cells. The axons pass through the cribriform plate of the ethmoid bone. These neurons relay information with the neurons of the olfactory bulb located above the cribriform plate. The information goes to the ipsilateral primary smell center by the fiber bundle called the olfactory tract. The olfactory tract splits into medial and lateral tracts, and the medial tact carrying the smell sensation terminates in the olfactory cortex. The lateral tact connects with the thalamus, amygdala, and other midbrain nuclei. 

The brain receives a condition of heart rate. BP, O2 saturation and respiration continuously by sensory input via paths describes under visceral sensation. The second set of information the brain receives is from the skin and bones and joints via the dorsal ascending column, spinothalamic tracts and spinocerebellar tracts. Integration of all the internal and external sensations and their reflex actions and cerebral motor actions are the basic survival requirement of the species.

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