Two-neuron system in the periphery

The ANS comprises two lower motor neurons in series compared with the single LMN in the somatic nervous system. The cell body of the first neuron is in the CNS and it synapses with the second neuron in a peripheral ganglion. The postsynaptic fibre then synapses with the target organ. Presynaptic axons are myelinated and postsynaptic are non-myelinated.

The location of the second neuron is system specific. For parasympathetic fibres, the ganglion is terminal or intramural; that is it lies close to, or within the wall of, the organ being innervated. Therefore it has a long presynaptic neuron and short postsynaptic neuron.

For sympathetic fibres, the ganglion is remote from the organ being innervated. These ganglia are located ventral, or near to the vertebral column in a prevertebral or paravertebral position, respectively. Thus, the sympathetic system has a shorter presynaptic neuron and longer postsynaptic neuron (Fig. 12.2).

Fig. 12.2 The two neurons that comprise the sympathetic and parasympathetic nervous systems and their neurotransmitters. The effect of each system on the smooth muscles of the mammalian iris is illustrated. ACh = acetylcholine, NAd = noradrenaline.

Note that as the presynaptic nerve can pass through a number of ganglia before synapsing, the terms ‘pre-ganglionic’ and ‘post-ganglionic’ can be misleading. Presynaptic and postsynaptic neurons are the preferred terms.

A summary of the key anatomical features of each system is given in Table 12.1.

Table 12.1 Summary of key anatomical features of each system

Visceral afferent system

See also Chapter 6.

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.

Sympathetic/thoracolumbar division

Presynaptic fibres originate in the intermediate (lateral) horn of the thoracolumbar spinal cord. Fibres may leave in the ventral root from their spinal cord segment of origin, or they may pass cranially, or caudally, a number of segments within the spinal cord before exiting it. The fibres exit the spinal cord along with the somatic motor neurons using the ventral roots of C8–L4/5 (up to L7) spinal nerves. The ventral roots fuse with the dorsal roots to form proper spinal nerve at the level of the intervertebral foramen (see Fig. 1.1). Lateral to the foramen, the proper spinal nerve splits into epaxial, hypaxial and ventral branches. The ventral branch forms the ramus communicans (ramus – L = branch). The ramus communicans conveys sympathetic efferent and visceral afferent fibres, between the spinal cord and the bilateral, sympathetic trunks that run ventrolaterally on both sides of the vertebral column (Fig. 12.3). The ramus communicans conveys both presynaptic (myelinated) sympathetic efferent fibres that are travelling to the trunk, and postsynaptic (unmyelinated) fibres that return from the trunk to rejoin the spinal nerves. This gives rise to the names of white and grey ramus communicans, respectively. These paravertebral ganglia and trunks are prominent in the thoracic region and extend into the lumbar region (see Fig. 12.4). In the caudal lumbar area, the trunks may fuse and continue caudally, ventral to the sacral and caudal vertebrae. In the abdominal region, nerves leave the paravertebral sympathetic trunks and connect to prevertebral ganglia located ventral to the vertebral column near the large abdominal arteries. Cranially, the sympathetic fibres continue into the cervical region, in conjunction with the vagus nerve forming the vagosympathetic trunk; this is located in the carotid sheath.

Fig. 12.3 Sympathetic efferent and visceral afferent fibres connecting between the thoracolumbar spinal cord and the sympathetic trunk.

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