Neuroanatomy An Illustrated Colour Text, 4 ed.

Chapter 4. Autonomic nervous system

 

The term ‘autonomic nervous system’ is used to describe those nerve cells located within both the central and peripheral nervous systems that are concerned with the innervation and control of visceral organs, smooth muscle and secretory glands. The principal function of the autonomic nervous system is broadly described as maintenance, or homeostasis, of the internal environment. This is achieved by regulation of cardiovascular, respiratory, digestive, excretory and thermoregulatory mechanisms, which occurs automatically and with relatively little volitional control.

Autonomic afferent and efferent nerve fibres enter and leave the CNS through spinal and cranial nerves. Within the spinal cord and brain stem they establish interconnections through which autonomic reflexes are mediated. Afferent fibres also establish connections with ascending neurones through which conscious awareness of visceral functions is achieved. Changes in the internal and external environment, and emotional factors, profoundly influence autonomic activity, most notably via descending connections from the hypothalamus. Autonomic efferent neurones differ from those of the somatic nervous system in that, in the autonomic nervous system, there is a sequence of two neurones between the CNS and the innervated structure (Fig. 4.1). The synaptic connection between the two neurones is located peripherally in an autonomic ganglion. The first neurone is, thus, called the preganglionic neurone and its cell body is located in the spinal cord or brain stem. The second neurone is referred to as the postganglionic neurone and its cell body is located peripherally in an autonomic ganglion.

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Figure 4.1 Basic organisation of the autonomic nervous system into preganglionic and postganglionic neurones. In the sympathetic division (A), ganglia constitute the sympathetic chain, lying near the vertebral column. Preganglionic neurones, therefore, have relatively short axons, while postganglionic fibres are relatively long. In the parasympathetic division (B), ganglia are located close to the target organ. Consequently, preganglionic axons are long and postganglionic axons are short.

The efferent neurones of the autonomic nervous system fall into two distinct anatomical and functional divisions or systems: namely, sympathetic and parasympathetic. Many, although not all, structures that receive autonomic fibres are dually innervated by both sympathetic and parasympathetic systems. These exercise antagonistic effects upon the innervated structure (Table 4.1).

Table 4.1 Functions of the autonomic nervous system

Structure

Sympathetic effect

Parasympathetic effect

Iris of eye

Dilates pupil

Constricts pupil

Ciliary muscle of eye

Relaxes

Contracts

Salivary glands

Reduces secretion

Increases secretion

Lacrimal gland

Reduces secretion

Increases secretion

Heart

Increases rate and force of contraction

Decreases rate and force of contraction

Bronchi

Dilates

Constricts

Gastrointestinal tract

Decreases motility

Increases motility

Sweat glands

Increases secretion

 

Erector pili muscles

Contracts

 

Sympathetic division

Preganglionic sympathetic neurones are located exclusively in the thoracic and upper two or three lumbar segments of the spinal cord (Fig. 4.2). They lie in the lateral horn of the spinal grey matter, which is, therefore, only present at these levels (see Figs 8.88.9). Preganglionic axons leave the cord in the ventral nerve roots and join the spinal nerve. Postganglionic sympathetic neurones have their cell bodies in one of two locations: either the sympathetic chain of ganglia that lies alongside the vertebral column, or the plexuses (coeliac, superior mesenteric, inferior mesenteric) that surround the main branches of the abdominal aorta. In order to reach any of these locations, preganglionic axons in the spinal nerve enter the sympathetic chain. Ganglia of the sympathetic chain are linked to those spinal nerves which contain sympathetic outflow by two small nerves, the rami communicantes(Fig. 4.3). Preganglionic fibres pass into the chain via the white ramus communicantes, so called because the constituent fibres are myelinated.

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Figure 4.2 Organisation of the sympathetic division of the autonomic nervous system.

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Figure 4.3 Relationships between a typical thoracic spinal nerve and the sympathetic chain.

Those fibres concerned with innervation of structures in the head and thorax terminate in synaptic contact with postganglionic cell bodies in the sympathetic chain. The postganglionic fibres return to the spinal nerve via the grey ramus communicantes, so called because the fibres are unmyelinated. Those preganglionic fibres concerned with innervation of pelvic and abdominal viscera pass uninterrupted through the sympathetic chain and travel to the plexuses where their corresponding postganglionic cells are located. The neurotransmitter released by preganglionic sympathetic neurones is acetylcholine. The transmitter of postganglionic sympathetic cells is generally noradrenaline (norepinephrine), although the cells innervating sweat glands are cholinergic. The adrenal medulla is an exceptional organ in that it is directly innervated by preganglionic sympathetic neurones.

The effects of sympathetic nervous system activity are most apparent under conditions of stress, excitement or fear and are classically referred to as the ‘fight or flight’ response. The heart rate and blood pressure are increased. The bronchi are dilated to increase airflow to and from the lungs. Vasodilatation in skeletal muscles allows the increased blood flow required for energetic activity, while gastrointestinal blood flow and motility are decreased. Metabolic changes occur, such as an increase in blood glucose, to support high energy utilisation and sweating occurs to increase heat loss.

imageLesions of the sympathetic nervous system

Primary autonomic failure is a chronic degenerative disease of the nervous system and leads to fainting through failure to control the heart rate and blood pressure (postural syncope), incontinence of urine and bowel, and impotence.

Horner’s syndrome refers to drooping of the eyelid (ptosis) and constriction of the pupil (miosis) caused by damage to the sympathetic nerves to the levator palpebrae superioris and the radial (pupillodilator) fibres of the iris. The sympathetic nerves may be damaged as they descend in the brain stem (by stroke or tumour) and the spinal cord (by an expanded cavity or syrinx), as they emerge in the first thoracic nerve root (by tumour of the apical lung) and as they ascend in the sympathetic plexus around the carotid arteries (by swelling of the arteries in a migrainous attack).

Parasympathetic division

Preganglionic parasympathetic neurones are located in the brain stem and the spinal cord (Fig. 4.4). Within the brain stem, such cells lie in cranial nerve nuclei associated with the oculomotor, facial, glossopharyngeal and vagus nerves and provide innervation for structures of the head, thorax and abdomen. The parasympathetic components of these nerves are described in Chapter 10. Within the cord, preganglionic parasympathetic neurones lie in the second, third and fourth sacral segments and provide innervation for pelvic viscera. The cell bodies of postganglionic parasympathetic neurones lie in ganglia that are located close to the structure which they innervate. Within the alimentary canal, these neurones contribute to the myenteric (Auerbach’s) and submucosal (Meissner’s) plexuses.

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Figure 4.4 Organisation of the parasympathetic division of the autonomic nervous system.

These plexuses are also referred to as the ‘enteric nervous system’. Such a concept has arisen because the plexuses additionally contain afferent neurones and interneurones. The rich local interconnections of these cells are capable of sustaining the motility of the gastrointestinal tract in the absence of input from the CNS. Furthermore, these cells cannot simply be equated with the parasympathetic nervous system as traditionally defined, since they are known also to receive synaptic input from postganglionic sympathetic neurones. The neurotransmitter released by both preganglionic and postganglionic parasympathetic neurones is acetylcholine.