Visceral afferent fibers

The autonomic nervous system consists of central connections, visceral afferent fibers, and visceral efferent fibers. The hypothalamus is where the principal integration of the entire autonomic nervous... 

All viscera receive dual innervation. Thus, in contrast to somatic input to the central nervous system, which has a single, usually spinal, destination, input to the central nervous system from organs in the thoracic cavity arrives at two locations (1) the brainstem nucleus tractus solitarii (vagal afferent input) and (2) the thoracic spinal cord. Furthermore, visceral afferent fibers are disproportionately fewer than somatic afferent fibers. Consequently, the low number of visceral afferents and greater intraspinal distribution lead to a loss of spatial discrimination, consistent with the diffuse, difficult-to-localize nature of visceral pain [1]. 

Referred pain most likely results from the convergence of visceral and somatic afferent fibers on the same second-order neurons in the dorsal horn of the spinal cord (see Figure 8.3). Therefore, the brain has no way of identifying the original source of the pain. Because superficial inputs normally predominate over visceral inputs, higher centers may incorrectly attribute the pain to the skin instead of the deeper tissue. 

The trigeminal nerve (V) is the only cranial nerve carrying general sensory fibers. It innervates the head, neck, meninges, and sinuses. The glossopharyngeal nerve (IX) and the vagus (X) carry visceral afferents, sensory input from the viscera such as nausea, but not pain. 

Stimulation of free nerve endings known as nociceptors is the first step leading to the sensation of pain. These receptors are found in both somatic and visceral structures and are activated by mechanical, thermal, and chemical factors. Release of bradykinins, K1, prostaglandins, histamine, leukotrienes, serotonin, and substance P may sensitize and/or activate nociceptors. Receptor activation leads to action potentials that are transmitted along afferent nerve fibers to the spinal cord. 

Beside the efferent innervation by the autonomic nervous system there are peripheral afferent sensory fibers which form the autonomic refiex arcs. They play a major role in the transmission of visceral sensations and are responsible for visceral refiexes, for example in the autonomic regulation of the blood pressure. 

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