Visceral afferent system
See also Chapter 6.
Key points
■ Visceral receptors are stimulated by pressure, stretch and chemical changes.
■ Input to the CNS is via cranial nerves (CNN VII, IX, X) and peripheral branches of autonomic and spinal nerves.
Input stimulates reflex activity and conscious perception.■ The solitary tract and its nucleus in the medulla oblongata receives input from cranial nerves and makes connections with the reticular formation for reflex function.
Receptors located in viscera throughout the body are sensitive to pressure, stretch and chemical changes. Most viscera are not sensitive to touch or cutting. Axons travel via local cranial nerves (CNN VII, IX, X), branches of sympathetic nerves and spinal nerves. Cell bodies are located in specific ganglia such as the geniculate ganglion of CN VII, the proximal and distal ganglia of CN X, and spinal ganglia.
Input via cranial nerves (CNN VII, IX, X) goes to the solitary tract and its nucleus in the medulla oblongata. The efferents from this nucleus go to reticular formation for reflex function (respiratory, cardiac, digestive, elimination). The solitarothalamic tract also conveys information to the thalamus and hence to the somatosensory cortex for conscious perception.
Input via segmental spinal nerves goes via the dorsal horn, synapses locally for reflex function, or enters the lateral funiculus (both ipsi- and contralateral) and travels cranially to the thalamus and somatosensory cortex.
Gut function can also occur due to local reflexes that do not involve the CNS. Some visceral afferents from the gut make local connections, in enteric plexi, with visceral motor nerves in the wall of the organ, causing local reflex activity. Most of the smooth muscle contractions such as segmentation, peristalsis and defecation, can occur in the denervated gut due to activity of local pacemakers found in the intestinal wall.