Autonomic Nervous System

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Chapter 21 Autonomic Nervous System

The autonomic nervous system represents the visceral component of the nervous system. It consists of neurones located within both the central nervous system (CNS) and the peripheral nervous system (PNS) and is concerned with control of the internal environment through innervation of secretory glands and with cardiac and smooth muscle. The term ‘autonomic’ is a convenient rather than appropriate title, because the functional autonomy of this part of the nervous system is illusory. Rather, its functions are normally closely integrated with changes in somatic activities, although the anatomical bases for such interactions are not always clear.

Visceral afferent pathways resemble somatic afferent pathways. The cell bodies of origin are unipolar neurones located in cranial and dorsal root ganglia. Their peripheral processes are distributed through autonomic ganglia or plexuses, or possibly through somatic nerves, without interruption. Their central processes (axons) accompany somatic afferent fibres through cranial nerves or dorsal spinal roots into the CNS, where they establish connections that mediate autonomic reflexes and visceral sensation.

Visceral efferent pathways differ from their somatic equivalents, in that the former are interrupted by peripheral synapses; there is a sequence of at least two neurones between the CNS and the target structure (Fig. 21.1). These are referred to as preganglionic and postganglionic neurones. The somata of preganglionic neurones are located in the visceral efferent nuclei of the brain stem and in the lateral grey columns of the spinal cord. Their axons, which are usually finely myelinated, exit from the CNS in certain cranial and spinal nerves and then pass to peripheral ganglia, where they synapse with the postganglionic neurones. The axons of postganglionic neurones are usually non-myelinated. Postganglionic neurones are more numerous than preganglionic ones; one preganglionic neurone may synapse with 15 to 20 postganglionic neurones, which permits the wide diffusion of many autonomic effects.

Fig. 21.1 Efferent pathways of the autonomic nervous system. The parasympathetic pathways are represented by blue lines, and the sympathetic by red lines. The interrupted red lines indicate postganglionic rami to the cranial and spinal nerves.

(After Meyer, Gottlieb. 1914. Die Exper. Pharmacol. (Wien) 36, 260.)

The autonomic nervous system can be divided into three major parts: sympathetic, parasympathetic and enteric. These differ in organization and structure but are closely integrated functionally. Most but not all structures innervated by the autonomic nervous system receive both sympathetic and parasympathetic fibres, whereas the enteric nervous system is a network of neurones intrinsic to the wall of the gastrointestinal tract.

Two long-held assumptions about the sympathetic and parasympathetic nervous systems are that they are functionally antagonistic (because activation of their respective efferents has opposing actions on target structures) and that sympathetic reactions are mass responses, whereas parasympathetic reactions are usually localized. A more realistic notion is that these sets of neurones represent an integrated system for the coordinated neural regulation of visceral and homeostatic functions. Moreover, even though widespread activation of the sympathetic nervous system may occur (e.g. in association with fear or rage), it is now recognized that the sympathetic nervous system is also capable of discrete activation, and many different patterns of activation of sympathetic nerves throughout the body occur in response to a wide variety of stimuli. Thus, sympathetic activity may result in the general constriction of cutaneous arteries (increasing blood supply to the heart, muscles and brain), cardiac acceleration, increased blood pressure, contraction of sphincters and depression of peristalsis, all of which mobilize body energy stores to deal with increased activity. Parasympathetic activity results in cardiac slowing and an increase in intestinal glandular and peristaltic activities, which may be considered to conserve body energy stores.

Autonomic activity is not initiated or controlled solely by the reflex connections of general visceral afferent pathways, nor do impulses in these pathways necessarily activate general visceral efferents. For example, in many situations demanding general sympathetic activity, the initiator is somatic and typically arises from either the special senses or the skin. Rises in blood pressure and pupillary dilatation may result from the stimulation of somatic receptors in the skin and other tissues. Peripheral autonomic activity is integrated at higher levels in the brain stem and cerebrum, including various nuclei of the brain stem reticular formation, thalamus and hypothalamus; the limbic lobe and prefrontal neocortex; and the ascending and descending pathways that connect these regions.

The traditional concept of autonomic neurotransmission is that preganglionic neurones of both sympathetic and parasympathetic systems are cholinergic, as are postganglionic parasympathetic neurones, whereas those of the sympathetic nervous system are noradrenergic. The discovery of neurones that do not use either acetylcholine or noradrenaline (norepinephrine) as their primary transmitter, and the recognition of a multitude of substances in autonomic nerves that fulfill the criteria for a neurotransmitter or neuromodulator, have greatly complicated the neuropharmacological concepts of the autonomic nervous system. Thus, adenosine 5′-triphosphate (ATP), numerous peptides and nitric oxide have all been implicated in the mechanisms of cell signalling in the autonomic nervous system. The principal cotransmitters in sympathetic nerves are ATP and neuropeptide Y, vasoactive intestinal polypeptide (VIP) in parasympathetic nerves and ATP, VIP and substance P in enteric nerves.

Sympathetic Nervous System

The sympathetic trunks are two ganglionated nerve cords that extend from the cranial base to the coccyx. The ganglia are joined to spinal nerves by short connecting nerves called white and grey rami communicantes. Preganglionic axons join the trunk through the white rami communicantes, whereas postganglionic axons leave the trunk in the grey rami. In the neck, each sympathetic trunk lies posterior to the carotid sheath and anterior to the transverse processes of the cervical vertebrae. In the thorax, the trunks are anterior to the heads of the ribs; in the abdomen, they lie anterolateral to the bodies of the lumbar vertebrae; and in the pelvis, they are anterior to the sacrum and medial to the anterior sacral foramina. Anterior to the coccyx the two trunks meet in a single median, terminal ganglion. Cervical sympathetic ganglia are usually reduced to three by fusion. The internal carotid nerve, a continuation of the sympathetic trunk, issues from the cranial pole of the superior ganglion and accompanies the internal carotid artery through its canal into the cranial cavity. There are from 10 to 12 (usually 11) thoracic ganglia, 4 lumbar ganglia and 4 or 5 ganglia in the sacral region.

The cell bodies of preganglionic sympathetic neurones are located in the lateral horn of the spinal grey matter of all thoracic segments and the upper two or three lumbar segments (Fig. 21.2). Their axons are myelinated, with diameters of 1.5 to 4 µm. These leave the cord in corresponding ventral nerve roots and pass into the spinal nerves, but they soon leave in white rami communicantes to join the sympathetic trunk (Fig. 21.3). Neurones like those in the lateral grey column exist at other levels of the cord above and below the thoracolumbar outflow, and small numbers of their fibres leave in other ventral roots. Preganglionic sympathetic neurones release acetylcholine as their principal neurotransmitter.

Fig. 21.2 Transverse section through the thoracic spinal cord showing the lateral horn, where preganglionic sympathetic neurones are located.

(Enhanced by B. Crossman.)

Fig. 21.3 Relationship between spinal nerves and the sympathetic trunk. The somatic components of spinal nerve roots are illustrated in the upper part of the diagram, and the visceral components are shown in the lower part. Somatic and preganglionic sympathetic fibres are coloured red, somatic and visceral afferent fibres are coloured blue and postganglionic sympathetic fibres are coloured black.

On reaching the sympathetic trunk, preganglionic fibres may behave in one of several ways (see Fig. 21.3). They may synapse with neurones in the nearest ganglion or traverse the nearest ganglion and ascend or descend in the sympathetic chain to end in another ganglion. A preganglionic fibre may terminate in a single ganglion or, through collateral branches, synapse with neurones in several ganglia. Preganglionic fibres may traverse the nearest ganglion, ascend or descend and, without synapsing, emerge in one of the medially directed branches of the sympathetic trunk to synapse in the ganglia of autonomic plexuses (situated mainly in the midline, such as around the coeliac and mesenteric arteries). More than one preganglionic fibre may synapse with a single postganglionic neurone. Uniquely, the suprarenal gland is innervated directly by preganglionic sympathetic neurones that traverse the sympathetic trunk and coeliac ganglion without synapse.

The somata of sympathetic postganglionic neurones are located mostly in ganglia of the sympathetic trunk or ganglia in more peripheral plexuses. Therefore, the axons of postganglionic neurones are generally longer than those of preganglionic neurones; an exception is some of those that innervate pelvic viscera. The axons of ganglionic cells are non-myelinated. They are distributed to target organs in various ways. Those from a ganglion of the sympathetic trunk may return to the spinal nerve of preganglionic origin through a grey ramus communicans, which usually joins the nerve just proximal to the white ramus; they are then distributed through ventral and dorsal spinal rami to blood vessels, sweat glands, hairs and so forth in their zone of supply. Segmental areas vary in extent and overlap considerably. The extent of innervation of different effector systems (e.g. vasomotor, sudomotor) by a particular nerve may not be the same. Alternatively, postganglionic fibres may pass in a medial branch of a ganglion directly to particular viscera, or they may innervate adjacent blood vessels or pass along them externally to their peripheral distribution. They may ascend or descend before leaving the sympathetic trunk. Many fibres are distributed along arteries and ducts as plexuses to distant effectors.

The principal neurotransmitter released by postganglionic sympathetic neurones is noradrenaline. The sympathetic system has a much wider distribution than the parasympathetic one. It innervates all sweat glands, the arrector pili muscles, the muscular walls of many blood vessels, the heart, the lungs and respiratory tree, the abdominopelvic viscera, the oesophagus, the muscles of the iris and the non-striated muscle of the urogenital tract, eyelids and elsewhere.

Postganglionic sympathetic fibres that return to the spinal nerves are vasoconstrictor to blood vessels, secretomotor to sweat glands and motor to the arrector pili muscles within their dermatomes. Those that accompany the motor nerves to voluntary muscles are probably only dilatatory. Most if not all peripheral nerves contain postganglionic sympathetic fibres. Those reaching the viscera are concerned with general vasoconstriction, bronchial and bronchiolar dilatation, modification of glandular secretion, pupillary dilatation, inhibition of alimentary muscle contraction and the like. A single preganglionic fibre probably synapses with the postganglionic neurones in only one effector system, which means that effects such as sudomotor and vasomotor actions can be separate.

Cervical Sympathetic Trunk

The cervical sympathetic trunk (Figs 21.4, 21.5) lies on the prevertebral fascia behind the carotid sheath and contains three interconnected ganglia: the superior, middle and inferior (stellate or cervicothoracic). However, there may occasionally be two or four ganglia. The cervical sympathetic ganglia send grey rami communicantes to all the cervical spinal nerves but receive no white rami communicantes from them. Their spinal preganglionic fibres emerge in the white rami communicantes of the upper five thoracic spinal nerves (mainly the upper three) and ascend in the sympathetic trunk to synapse in the cervical ganglia. In their course, the grey rami communicantes may pierce longus capitis or scalenus anterior.

Fig. 21.4 Dissection showing the prevertebral region and the superior mediastinum. On the right, the costal elements of the upper six cervical vertebrae have been removed to expose the cervical part of the vertebral artery. On the left, most of the deep relations of the common carotid artery and the internal jugular vein are exposed.

Fig. 21.5 Dissection showing the course of the right vertebral and internal carotid arteries and some of their branches.

Superior Cervical Ganglion

The superior cervical ganglion is the largest of the three ganglia. It lies on the transverse processes of the second and third cervical vertebrae and is probably formed from four fused ganglia, judging by its grey rami to C1–4. The internal carotid artery within the carotid sheath is anterior, and longus capitis is posterior (see Fig. 21.4). The lower end of the ganglion is united by a connecting trunk to the middle cervical ganglion. Postganglionic branches are distributed in the internal carotid nerve, which ascends with the internal carotid artery into the carotid canal to enter the cranial cavity, and in lateral, medial and anterior branches. They supply vasoconstrictor and sudomotor nerves to the face and neck, dilator pupillae and smooth muscle in the eyelids and orbitalis.

Middle Cervical Ganglion

The middle cervical ganglion (Figs 21.6, 21.7) is the smallest of the three; it is occasionally absent, in which case it may be replaced by minute ganglia in the sympathetic trunk or fused with the superior ganglion. It is usually found at the level of the sixth cervical vertebra, anterior or just superior to the inferior thyroid artery, or it may adjoin the inferior cervical ganglion. It probably represents a coalescence of the ganglia of the fifth and sixth cervical segments, judging by its postganglionic rami, which join the fifth and sixth cervical spinal nerves (but sometimes also the fourth and seventh). It is connected to the inferior cervical ganglion by two or more very variable cords. The posterior cord usually splits to enclose the vertebral artery, whereas the anterior cord loops down anterior to, and then below, the first part of the subclavian artery, medial to the origin of its internal thoracic branch, and supplies rami to it. This loop, the ansa subclavia, is frequently multiple, lies in close contact with the cervical pleura and typically connects with the phrenic nerve and sometimes with the vagus.

Fig. 21.6 Middle and inferior cervical ganglia of the right side, viewed from the right. Note the proximity of the inferior cervical and first thoracic ganglia, which often fuse to form a cervicothoracic (stellate) ganglion.

Fig. 21.7 Middle and inferior cervical ganglia of the right side, anterior view. Part of the vertebral artery has been excised to show the inferior cervical ganglion.

The middle cervical ganglion gives off thyroid and cardiac branches. The thyroid branches accompany the inferior thyroid artery to the thyroid gland. They communicate with the superior cardiac, external laryngeal and recurrent laryngeal nerves and send branches to the parathyroid glands. Fibres to both glands are largely vasomotor, but some reach the secretory cells. The cardiac branch, the largest sympathetic cardiac nerve, arises either from the ganglion itself or, more often, from the sympathetic trunk cranial or caudal to it. On the right side it descends behind the common carotid artery, in front of or behind the subclavian artery, to the trachea, where it receives a few filaments from the recurrent laryngeal nerve before joining the right half of the deep (dorsal) part of the cardiac plexus. In the neck, it connects with the superior cardiac and recurrent laryngeal nerves. On the left side, the cardiac nerve enters the thorax between the left common carotid and subclavian arteries to join the left half of the deep (dorsal) part of the cardiac plexus. Fine branches from the middle cervical ganglion also pass to the trachea and oesophagus.

Inferior (Cervicothoracic Stellate) Ganglion

The inferior cervical (cervicothoracic stellate) ganglion is irregular in shape and much larger than the middle cervical ganglion (see Figs 21.6, 21.7). It is probably formed by a fusion of the lower two cervical and first thoracic segmental ganglia, sometimes including the second and even the third and fourth thoracic ganglia. The first thoracic ganglion may be separate, leaving an inferior cervical ganglion above it. The sympathetic trunk turns backward at the junction of the neck and thorax, so the long axis of the cervicothoracic ganglion becomes almost anteroposterior. The ganglion lies on or just lateral to the lateral border of longus colli between the base of the seventh cervical transverse process and the neck of the first rib (which are both posterior to it). The vertebral vessels are anterior, and the ganglion is separated from the posterior aspect of the cervical pleura inferiorly by the suprapleural membrane. The costocervical trunk of the subclavian artery branches near the lower pole of the ganglion, and the superior intercostal artery is lateral.

A small vertebral ganglion may be present on the sympathetic trunk anterior or anteromedial to the origin of the vertebral artery and directly above the subclavian artery. When present, it may provide the ansa subclavia and is also joined to the inferior cervical ganglion by fibres enclosing the vertebral artery. It is usually regarded as a detached part of the middle cervical or inferior cervical ganglion. Like the middle cervical ganglion, it may supply grey rami communicantes to the fourth and fifth cervical spinal nerves. The inferior cervical ganglion sends grey rami communicantes to the seventh and eighth cervical and first thoracic spinal nerves, and it gives off a cardiac branch, branches to nearby vessels and sometimes a branch to the vagus nerve.

The grey rami communicantes to the seventh cervical spinal nerve vary from one to five (two being the usual number). A third often ascends medial to the vertebral artery in front of the seventh cervical transverse process. It connects with the seventh cervical nerve and sends a filament upward through the sixth cervical transverse foramen, accompanied by the vertebral vessels, to join the sixth cervical spinal nerve as it emerges from the intervertebral foramen. An inconstant ramus may traverse the seventh cervical transverse foramen. The number of grey rami to the eighth cervical spinal nerve ranges from three to six.

The cardiac branch descends behind the subclavian artery and along the front of the trachea to the deep cardiac plexus. Behind the artery it connects with the recurrent laryngeal nerve and the cardiac branch of the middle cervical ganglion (the latter is often replaced by fine branches of the inferior cervical ganglion and ansa subclavia).

The branches to blood vessels form plexuses on the subclavian artery and its branches. The subclavian supply is derived from the inferior cervical ganglion and ansa subclavia and typically extends to the first part of the axillary artery, although a few fibres may extend farther. An extension of the subclavian plexus to the internal thoracic artery may be joined by a branch of the phrenic nerve. The vertebral plexus is derived mainly from a large branch of the inferior cervical ganglion that ascends behind the vertebral artery to the sixth transverse foramen. There it is reinforced by branches of the vertebral ganglion or the cervical sympathetic trunk that pass cranially on the ventral aspect of the artery. Deep rami communicantes from this plexus join the ventral rami of the upper five or six cervical spinal nerves. The plexus, which contains some neuronal cell bodies, continues into the skull along the vertebral and basilar arteries and their branches as far as the posterior cerebral artery, where it meets a plexus from the internal carotid artery. The plexus on the inferior thyroid artery reaches the thyroid gland and connects with the recurrent and external laryngeal nerves, the cardiac branch of the superior cervical ganglion and the common carotid plexus.

Horner’s Syndrome

Any condition or injury that destroys the sympathetic trunk ascending from the thorax through the neck into the face results in Horner’s syndrome (Fig. 21.8), characterized by a drooping eyelid (ptosis), sunken globe (enophthalmos), narrow palpebral fissure, contracted pupil (miosis), vasodilatation and lack of thermal sweating (anhidrosis) on the affected side. Classically, this occurs when a bronchial carcinoma invades the sympathetic trunk (see Ch. 18, Case 2). It also occurs as a complication of cervical sympathectomy or radical neck dissection.

Fig. 21.8 Horner’s syndrome.

CASE 1 Harlequin Syndrome

A 55-year-old woman presents with a 3-month history of exercise-induced anhidrosis and lack of flushing on the left side of the face, with normal autonomic responses on the right. Examination demonstrates a left Horner’s syndrome with miosis and ptosis. Provocative manoeuvres such as controlled exercise confirm loss of flushing on the left side of the face, as described historically, along with hemifacial anhidrosis. No other neurological abnormalities are found.

Discussion: Asymmetric facial flushing and sweating suggest a diagnosis of harlequin syndrome, with asymmetric involvement of vasomotor and sudomotor fibres. Because one entire side of the face is involved, the autonomic lesion must be below the bifurcation of the carotid artery, presumably a preganglionic sympathetic lesion below the superior cervical ganglion but distal to the stellate ganglion. It has been suggested that the lesion involves preganglionic fibres originating at T2 and T3. Occasionally, segmental autonomic dysfunction is observed in an upper extremity as well. The presence of the ocular features of Horner’s syndrome suggests involvement of the ciliary ganglia in at least some cases. A relationship to Ross syndrome (segmental anhidrosis, tonic pupils and hyporeflexia) has also been suggested.

Parasympathetic Nervous System

Preganglionic parasympathetic neurone cell bodies are located in certain cranial nerve nuclei of the brain stem (see Fig. 5.6) and in the grey matter of the second to fourth sacral segments of the spinal cord. Efferent fibres, which are myelinated, emerge from the CNS only in certain cranial nerves (oculomotor, facial, glossopharyngeal, vagus) and the second to fourth sacral spinal nerves. The preganglionic parasympathetic neurones are cholinergic.

The cell bodies of postganglionic parasympathetic neurones are mostly sited distant from the CNS, either in discrete ganglia located near the structures innervated or dispersed in the walls of viscera. In the cranial part of the parasympathetic system there are four small peripheral ganglia—ciliary, pterygopalatine, submandibular and otic—which are all described on a regional basis. These are solely efferent parasympathetic ganglia, unlike the trigeminal, facial, glossopharyngeal and vagal ganglia, all of which are concerned exclusively with afferent impulses and contain the cell bodies of sensory neurones. However, the cranial parasympathetic ganglia are traversed by afferent fibres, postganglionic sympathetic fibres and, in the case of the otic ganglion, branchial efferent fibres, but none of these are interrupted in the ganglia. Postganglionic parasympathetic fibres are usually non-myelinated and shorter than those in the sympathetic system, because the ganglia in which the former synapse are in or near the viscera they supply. In contrast to the sympathetic system, postganglionic parasympathetic neurones are cholinergic.

Oculomotor preganglionic parasympathetic fibres originate in the Edinger–Westphal nucleus of the midbrain and travel in the nerve along its branch to the inferior oblique, reaching the ciliary ganglion, where they synapse. Postganglionic fibres, which are thinly myelinated, travel in the short ciliary nerves that pierce the sclera to run forward in the perichoroidal space to the ciliary muscle and sphincter pupillae. Their activation mediates accommodation of the eye to near objects and pupillary constriction.

CASE 2 Parinaud’s Syndrome

A 13-year-old boy has complained of headaches for several months. On examination, he is found to have bilateral disc oedema (papilloedema), with paresis of up-gaze. His pupils fail to constrict in bright light but constrict normally with accommodation. He also has convergence retraction nystagmus. Magnetic resonance imaging reveals a tumour involving the dorsal midbrain (collicular plate) that is also responsible for obstructive hydrocephalus.

Discussion: This boy has classic Parinaud’s (dorsal midbrain) syndrome, with prominent light-near dissociation along with paresis of up-gaze convergence, retraction nystagmus and eyelid retraction. It is caused by lesions affecting the dorsal midbrain (tectum) in the region of the superior colliculi and involving the pretectal nuclei. Supranuclear fibres destined for the oculomotor nerve complex are spared. Pupils are midsize or enlarged. Light-near dissociation (characterized by a poor pupillary response (reflex) to light, but with preservation of pupillary constriction to a near target) usually results from bilateral midbrain lesions, but not necessarily. Responsible lesions include tumour (e.g. pinealoma), hydrocephalus or infarction. It is of interest that Argyll Robertson pupils, which are seen, for example, in cases of neurosyphilis, may also exhibit pupillary-near dissociation, but the Argyll Robertson pupil is typically very small and irregular, with reduced dilatation in the dark. Again, the supranuclear connection between the pretectum and the midbrain Edinger–Westphal nucleus is spared, so that the pupillary-near reflex is preserved. The so-called tonic pupil of Adie’s syndrome, with a lesion involving primarily the ciliary ganglia, similarly exhibits light-near dissociation.

The facial nerve contains preganglionic parasympathetic axons of neurones with their somata in the superior salivatory nucleus (see Ch. 10). The fibres emerge from the brain stem in the nervus intermedius, leave the main facial nerve trunk above the stylomastoid foramen and travel in the chorda tympani, which subsequently joins the lingual nerve (see Ch. 11). In this way, preganglionic fibres are conveyed to the submandibular ganglion, where they synapse on ganglionic neurones. Postganglionic fibres innervate the submandibular and sublingual salivary glands and are said to travel in the lingual nerve. Some preganglionic fibres may synapse around cells in the hilum of the submandibular gland. Stimulation of the chorda tympani dilates the arterioles in both glands, in addition to having a direct secretomotor effect. The facial nerve also contains efferent parasympathetic lacrimal secretomotor axons, which travel in its greater petrosal branch and then via the nerve of the pterygoid canal, to relay in the pterygopalatine ganglion. Postganglionic axons are thought to travel by the zygomatic nerve to the lacrimal gland and by ganglionic branches to the nasal and palatal glands.

The glossopharyngeal nerve contains preganglionic parasympathetic secretomotor fibres for the parotid gland. These originate in the inferior salivatory nucleus and travel in the glossopharyngeal nerve and its tympanic branch. They traverse the tympanic plexus and lesser petrosal nerve to reach the otic ganglion, where they synapse. Postganglionic fibres pass by communicating branches to the auriculotemporal nerve, which conveys them to the parotid gland. Stimulation of the lesser petrosal nerve produces vasodilator and secretomotor effects.

The vagal nucleus (dorsal motor nucleus of the vagus) in the medulla is a major source of preganglionic parasympathetic fibres. Efferent fibres travel in the vagus nerve and its pulmonary, cardiac, oesophageal, gastric, intestinal and other branches. They synapse in minute ganglia in the visceral walls. Cardiac branches, which act to slow the cardiac cycle, join the cardiac plexuses, and fibres relay in ganglia distributed over both atria. Pulmonary branches contain fibres that relay in ganglia of the pulmonary plexuses. They are motor in function to the circular non-striated muscle fibres of the bronchi and bronchioles and are bronchoconstrictor in function. With the exception of the pyloric sphincter, gastric branches are secretomotor and motor to the non-striated muscle of the stomach, which they inhibit. Intestinal branches have a corresponding action in the small intestine, caecum, vermiform appendix, ascending colon, right colic flexure and most of the transverse colon. They are secretomotor to the glands and motor to the intestinal muscular coats, but inhibitory to the ileocaecal sphincter. Their synaptic relays with postganglionic neurones are situated in the myenteric (Auerbach’s) and submucosal (Meissner’s) plexuses.

Pelvic splanchnic nerves to the pelvic viscera travel in anterior rami of the second, third and fourth sacral spinal nerves. These nerves unite with branches of the sympathetic pelvic plexuses. Minute ganglia occur at the points of union and in the visceral walls, and sacral preganglionic parasympathetic fibres relay synaptically in these ganglia. The pelvic splanchnic nerves are motor to the muscle of the rectum and bladder wall but inhibitory to the vesical sphincter. They supply vasodilator fibres to the erectile tissue of the penis and clitoris and are probably also vasodilator to the testes, ovaries, uterine tubes and uterus. Filaments from the pelvic splanchnic nerves ascend in the hypogastric plexus and are visceromotor to the sigmoid and descending colon, left colic flexure and terminal transverse colon.