Chest Pain.
(Pain Pathways of the Heart).
PKGhatak, MD
The American Heart Association and its affiliates have been very successful in educating people on the significance of chest pain and its relation to an acute heart attack.
Heart muscles are not richly supplied with pain nerve fibers (called Afferent or sensory fibers) compared with the pericardium (fibrous covering of the heart). A wider area of the heart muscles has to be injured to elicit pain sensation. Even then, the pain is not felt in the heart but projected on the Central chest wall, neck, Lower jaw, and arms down to the fingers, upper abdomen, or on the back in between the shoulder blades.
This peculiarity of projection of visceral pain onto the skin is limited to organs innervated by the Vagus nerve.
In the early stage of fetal development, the human body was a segmented tube; each segment had its own pair of blood vessels and pair of nerves. In the later phase of development, as limbs and internal organs developed, that representation remained intact.
In the case of the heart, the location of pain felt on the skin is due to the fact that the heart developed from the pair of blood vessels from Cervical 7 to Thoracic 3 segments. The sensory nerve fibers of the heart join the vagus and sympathetic nerves to carry pain sensation to the brain.
In 1991 Dr. Armor described a new concept – Heart-Brain, consisting of 40,000 neurons situated in the heart and function as a mini brain. Nerve fibers from these neurons make direct connections with the amygdala, hypothalamus, thalamus and relay information to the cerebral cortex. The sensation from these neurons modifies effects of both the sympathetic and parasympathetic nervous system, and according to Dr. Armor, generates memory, emotion and modulates pain sensation.
Sensory nerves of the heart.
The Vagus nerve and nerves of the sympathetic nervous system carry Pain sensation to the brain.
The sensory nervous system of the heart is distinct. The nerve cells, nerve fibers, and ganglions are separate from the outflow tracts of both the parasympathetic (vagus) and sympathetic systems.
Vagus nerve.
The sensory fibers of the heart originated in the Nodose Ganglion of the Vagus, situated in the Jugular foramen. These fibers travel with the vagus nerve into the chest. Fibers destine to innervate the heart pass through the cardiac plexus without making any connection. The pain sensation from the heart is carried to the sensory nucleus of the vagus – Nucleus Tactus Soliterious, situated in the dorsomedial medulla. From there the 2nd order neuron carries the sensation to the thalamus. From the thalamus, the pain sensation reaches the cerebral cortex.
80 % of pain sensation is carried by a pair of the vagus nerve. The right vagus nerve innervates the sinoatrial node, atrioventricular node, and atrial walls. The left vagus carry innervates these structures and also from the rest of the heart. There is a considerable overlap of innervation of the heart from both vagus nerves.
Sympathetic innervation of the heart.
The neurons, that carry the pain sensation from the heat in the sympathetic nervous system, are situated in the Dorsal Ganglion from the 8th cervical to the 3rd thoracic segment. The axon of the nerve enters the spinal cord via the Dorsal root and makes a connection with the neuron situated in the dorsal horn of the spinal cord. The second order neuron carries the pain sensation upward to Thalamus. And from Thalamus the sensation caries to the cerebral cortex by the 3rd order neuron.
Nerve supply to the pericardium.
The pericardium has two sets of innervations.
The surface or parietal pericardium is supplied with neurons situated in cervical 3 to cervical 5 segments. The pain fibers travel via the Phrenic nerve.
The visceral pericardium is supplied by nerves that travel to the brain via both sympathetic and parasympathetic nerves.
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