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 3rd 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 3rd 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:
1st 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 1st order axons cross the midline immediately and synapse with the 2nd 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 1st 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 3rd order neurons carry the information to the thalamus and other midbrain nuclei.
Unconscious proprioceptor sensory information.
Below L2 Level.
Central axons of the 1st order neurons end by making synapses with posterior horn nuclei. The ascending fibers of the 2nd 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 1st 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 5th 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 2/3rd 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 3rd order neurons take that information to the sensory cerebral cortex.
B. Lower abdominal visceral parasympathetic:
The Hypogastric nerve.
The afferent fibers, of parasympathetic 1st 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 7th 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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