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.