The autonomic nervous system

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8 The autonomic nervous system

Introduction

There are three components of the autonomic nervous system.

The sympathetic nervous system can function more generally with respect to its less precise influence on physiology as it mediates whole-body reactions involved in the ’fight or flight’ responses. Both the sympathetic and parasympathetic systems are tonically active to help maintain a stable internal environment in the face of changing external conditions which is best described as homeostasis.

Both the sympathetic and parasympathetic systems comprise preganglionic and postganglionic neurons. The cell bodies of preganglionic neurons in the sympathetic system are located in the intermediolateral cell column (IML) of the grey matter of the spinal cord between T1 and L2. The axons of these neurons exit the spinal cord via the ventral root with the motor neurons of the ventral horn. A branch known as the white rami communicans (myelinated) carries these fibres to the sympathetic chain ganglia where many of the preganglionic cells synapse with postganglionic cells. At cervical and lumbar levels, postganglionic sympathetic neurons form grey rami communicans (unmyelinated and slower conducting) that are distributed to vascular smooth muscle, piloerector muscle, and sweat glands via the spinal nerves and their branches. At cervical levels, some of the postganglionic neurons also project to the eye, blood vessels, and glands of the head and face via the carotid and vertebral arterial plexi. The cell bodies of parasympathetic preganglionic neurons are located in discrete nuclei at various levels of the brainstem and at the IML column of levels S2–4 in the spinal cord (vertebral level L1–2). In contrast to the sympathetic system, the preganglionic parasympathetic neurons are generally longer than the postganglionic neurons as they synapse in ganglia further from their origin and closer to the effector than the postganglionic neurons innervate.

Organisation of the autonomic nervous system

The autonomic nervous system comprises the major autonomous or non-volitional efferent outflow to all organs and tissues of the body with the exception of skeletal muscle. Anatomically, the autonomic outflow from the spinal cord to the end organ occurs through a chain of two neurons consisting of a pre- and postganglionic component. The preganglionic component neurons live in the grey matter of the spinal cord. The postganglionic component neurons vary in location with some living in the paraspinal or sympathetic ganglia, and others in ganglia distant from the cord, known as stellate ganglia. Although historically only the efferent connections were considered, all of the projections of the autonomic nervous system are reciprocal in nature and involve both afferent and efferent components. The autonomic system can be divided into three functionally and histologically distinct components: the parasympathetic, sympathetic, and enteric systems. All three systems are modulated by projections from the hypothalamus. Hypothalamic projections that originate mainly from the paraventricular and dorsal medial nuclei influence the parasympathetic and sympathetic divisions as well as the enteric division of the autonomic nervous system. These descending fibres initially travel in the medial forebrain bundle and then divide to travel in both the periaqueductal grey areas and the dorsal lateral areas of the brainstem and spinal cord. They finally terminate on the neurons of the parasympathetic preganglionic nuclei of the brainstem, the neurons in the intermediate grey areas of the sacral spinal cord, and the neurons in the intermediolateral cell column of the thoracolumbar spinal cord. Descending autonomic modulatory pathways also arise from the nucleus solitarius, noradrenergic nuclei of the locus ceruleus, raphe nuclei, and the pontomedullary reticular formation (PMRF).

The parasympathetic system communicates via both efferent and afferent projections within several cranial nerves including the oculomotor (CN III) nerve, the trigeminal (CNV) nerve, the facial (CNVII) nerve, the glossopharyngeal nerve, and the vagus (CN X) and accessory (CN XI) nerves (Fig. 8.1). The vagus nerve and sacral nerve roots compose the major output route of parasympathetic enteric system control (Furness & Costa 1980). Axons of the preganglionic nerves of the parasympathetic system tend to be long, myelinated, type II fibres and the postganglionic axons tend to be somewhat shorter, unmyelinated, C fibres (see Chapter 7). The cell bodies of parasympathetic preganglionic neurons are located in discrete nuclei at various levels of the brainstem as described above and in the intermediolateral cell column of levels S2–4 in the spinal cord or vertebral level L1–2. In contrast to the sympathetic system, the preganglionic parasympathetic neurons are generally longer than the postganglionic neurons as they synapse in ganglia that are further from their origin and closer to the effector than the postganglionic neurons innervate.

The neurotransmitter released both pre- and postsynaptically is acetylcholine. Cholinergic transmission can occur through G-protein coupled mechanisms via muscarinic receptors or through inotropic nicotinic receptors. The activity of ACh is terminated by the enzyme acetylcholinesterase, which is located in the synaptic clefts of cholinergic neurons. To date, seventeen different subtypes of nicotinic receptors and five different subtypes of muscarinic receptors have been identified (Nadler et al. 1999; Picciotto et al. 2000).

Cholinergic, nicotinic receptors are present on the postsynaptic neurons in the autonomic ganglia of both sympathetic and parasympathetic systems. Cholinergic, muscarinic receptors are present on the end organs of postsynaptic parasympathetic neurons (Fig. 8.2).

The neurological output from the parasympathetic system is the integrated end product of a complex interactive network of neurons spread throughout the mesencephalon, pons, and medulla. The outputs of the cranial nerve nuclei including the Edinger–Westphal nucleus, the nucleus tractus solitarius, the dorsal motor nucleus, and nucleus ambiguus are modulated via the mesencephalic reticular formation (MRF) and PMRF. This complex interactive network receives modulatory input from wide areas of the neuraxis including all areas of cortex, limbic system, hypothalamus, cerebellum, thalamus, vestibular nuclei, basal ganglia, and spinal cord (Walberg 1960; Angaut & Brodal 1967; Brodal 1969; Brown 1974; Webster 1978). The relationship of the parasympathetic outflow to the immune system has received very little study to date and as a consequence very little is known about the influence of the parasympathetic or the enteric system on immune function.

Supraspinal modulation of autonomic output

Monosynaptic connections between two structures suggest an important functional relationship between the two structures in question. Polysynaptic connections may be important as well but are not as well understood as monosynaptic connections. Monosynaptic connections have been demonstrated to exist between a variety of nuclei in the medulla, pons, diencephalon, and the preganglionic neurons of the IML (Smith & DeVito 1984; Natelson 1985). Nuclei with monosynaptic connections with the neurons of the IML include:

The hypothalamus is the only structure with direct monosynaptic connects to the nuclei of the brainstem and to the neurons of the IML. This suggests that the influence of the hypothalamus on autonomic function is substantial.

The projections from the cerebral cortex and their role in modulation of autonomic function are not well understood. However, existence of direct projections from the cortex to subcortical structures regulating autonomic function has been established (Cechetto & Saper 1990). Neurophysiological studies demonstrating autonomic changes with stimulation and inhibition of the areas of cortex also suggest a regulatory role. The following outlines the established areas of cortex and their projection areas:

The autonomic ganglion

The autonomic ganglion is the site at which the presynaptic neurons synapse on the postsynaptic neurons. The sympathetic ganglia are situated paraspinally in the sympathetic trunk or prespinally in the celiac and superior mesenteric ganglia. The parasympathetic ganglia are situated in close proximity to the target structures that they innervate. The autonomic ganglia consist of a collection of multipolar interneurons surrounded by a capsule of stellate cells and connective tissue.

Incoming and outgoing nerve bundles are attached to the ganglion (Fig. 8.3). The incoming bundles contain afferent fibres from the periphery returning to the spinal cord, preganglionic axons that synapse on the postganglionic neurons in the ganglion, preganglionic axons that pass through the ganglion giving off collateral axons to the interneurons as they do so, and descending axons from cholinergic neurons in the spinal cord that modulate the activity of the interneurons in the ganglion. The interneurons in the ganglion are referred to as small intensely fluorescent cells and they are thought to be dopaminergic in nature. The outgoing bundle contains postganglionic axons, and afferent fibres from the periphery entering the ganglion (Snell 2001). The presence of such a complex structure in the ganglion has led to the suspicion that the ganglion is not just a relay point but an integration station along the pathway of the autonomic projections.

Parasympathetic efferent projections

The oculomotor parasympathetic fibres commence in the midbrain. These fibres are the axon projections of neurons located in the Edinger–Westphal nucleus (EWN) or accessory oculomotor nuclei. The parasympathetic projections travel with the ipsilateral oculomotor nerve and exit with the nerve branch to the inferior oblique muscle and enter the ciliary ganglion where they synapse with the postganglionic neurons. The axons of the postganglionic neurons then exit the ganglion via the short ciliary nerves and supply the ciliary muscle and the sphincter pupillae. Activation of the postganglionic neurons causes contraction of both the ciliary muscle, resulting in relaxation of the lens, and the sphincter pupillae muscle, resulting in constriction of the pupil. These actions can be stimulated separately or simultaneously as in the accommodation reflex (Fig. 8.4).

Functionally, the Edinger–Westphal nucleus receives the majority of its input from the contralateral field of vision. This involves the stimulus of the ipsilateral temporal and contralateral nasal hemiretinas, which results in the constriction of the ipsilateral pupil. For example, a shining light from the right field of vision will stimulate the left nasal hemiretina and the right temporal hemiretina which project through the left optic tract to the left EWN. The left EWN stimulation results in constriction of the ipsilateral (left) pupil. Some fibres from the left optic tract also synapse on the right EWN, effectively resulting in constriction of both pupils. This constitutes the consensual pupil reflex. Comparison between the time to activation (TTA) and time to fatigue (TTF) in each pupil following stimulation from the contralateral field of vision can be used to estimate the central integrative state of the respective EWN. This, in addition to further evaluation of the oculomotor and trochlear function, can then be used to estimate the central integrative state (CIS) of the respective mesencephalon. In situations where the CIS of the EWN is healthy one would expect rapid TTA and normal TTF response times, relatively, equal in both pupils. In situations where the CIS of one EWN is undergoing transneural degeneration of relatively short duration, one would expect an extremely rapid TTA followed by a relatively short TTF in the ipsilateral eye when compared to the contralateral eye. In situations where the CIS of one EWN is such that transneural degeneration, long-standing in nature, is present then one would expect the pupil of the ipsilateral EWN to show an increased TTA and a decreased TTF in comparison with the contralateral eye. On prolonged stimulus a pupil in this condition will often fluctuate the pupil size between normal and partial constriction. This is referred to as hippus.

The parasympathetic efferent projections of the facial nerve involve motor axons to the submandibular gland and the lacrimal gland. The motor fibres project in two different pathways and to two different ganglia. The motor projections to the submandibular gland arise from neurons in the superior salivatory nucleus in the medulla. The axons of these neurons emerge from the brainstem in the nervous intermedius and join the facial nerve until the stylomastoid foramen where they separate as the chorda tympani, which traverse the tympanic cavity until they reach and join with the lingual nerve. They travel with the lingual nerve until they reach and synapse on the postganglionic neurons of the submandibular ganglion. The axons from these neurons project to the submandibular glands via the lingual nerve supplying the secretomotor fibres to the gland. Activation of the postganglionic neurons results in dilatation of the arterioles of the gland and increased production of saliva (Fig. 8.5).

The motor projections to the lacrimal gland travel in the greater petrosal nerve through the pterygoid canal and synapse on the neurons of the pterygopalatine ganglion. The axons of the neurons in the pterygopalatine ganglion project their axons with the zygomatic nerve to the lacrimal gland and form direct branches from the ganglion to the nose and palate.

The efferent projections of the glossopharyngeal nerve contain axons that are secretory motor to the parotid gland. The projections start in the neurons of the inferior salivatory nucleus of the medulla and travel in the glossopharyngeal nerve through the tympanic plexus where they separate and travel with the lesser petrosal nerve to synapse on the neurons in the otic ganglion. The axons of these neurons then travel in the auriculotemporal nerve to the parotid gland. Activation of the neurons of the otic ganglion produces vasodilation of the arterioles and increased saliva production in the gland.

The motor projections of the vagus nerve arise from the neurons of the dorsal motor nucleus and the nucleus ambiguus of the medulla. The cardiac branches are inhibitory, and in the heart they act to slow the rate of the heartbeat. The pulmonary branch is excitatory and in the lungs they act as a bronchoconstrictor as they cause the contraction of the non-striate muscles of the bronchi. The gastric branch is excitatory to the glands and muscles of the stomach but inhibitory to the pyloric sphincter. The intestinal branches, which arise from the postsynaptic neurons of the mesenteric plexus or Auerbach’s plexus and the plexus of the submucosa or Meissner’s plexus, are excitatory to the glands and muscles of the intestine, caecum, vermiform appendix, ascending colon, right colic flexure, and most of the transverse colon but inhibitory to the ileocaecal sphincter (Fig. 8.6).

The pelvic splanchnic nerves are composed of the anterior rami of the second, third, and fourth sacral spinal nerves. These nerves diverge, giving off several collateral branches to supply the pelvic viscera. Most of the projections merge with fibres of the sympathetic pelvic plexus and pass to ganglia located adjacent to their target structures, where they synapse with their postganglionic components.

Functionally, the CIS of the medulla can be estimated by examining the activities of the cranial nerves, which mediate the effector functions of end organs that can be measured. For example, a patient that presents with excessive watering of the eyes, increased salivation and nasal mucus production, difficulty in taking deep breaths, decreased heart rate, stomach pain, intestinal cramping, and frequent loose bowel movements may indicate an overactive medullary region. An underactivated medullary region may present with dry eyes, dry mouth, dry nasal cavities, increased heart rate, and constipation. This highlights the importance of conducting a thorough neurological examination of both the motor and visceral functions of the cranial nerves and relating the findings in a functional manner back to the neuraxial structures involved.

The sympathetic system enjoys a wide-ranging distribution to virtually every tissue of the body (Fig. 8.7). The presynaptic neurons live in a region of the grey matter of the spinal cord called the intermediomedial and intermediolateral cell columns located in lamina VII. Axons of these neurons exit the spinal cord via the ventral rami where they further divide to form the white rami communicantes. The fibres then follow one of several pathways (Fig. 8.8):