Study guidelines
- 1.
Resolve the paradox that, despite an outflow restricted to 14 or 15 ventral roots, all 31 spinal nerve trunks acquire sympathetic fibres.
- 2.
Appreciate that the sympathetic ganglia along the abdominal aorta are activated by preganglionic fibres, as is the adrenal medulla.
- 3.
Pay special attention to the autonomic innervation of the eye, discussed both here and in Chapter 23 .
- 4.
Appreciate that the four parasympathetic ganglia in the head are functionally similar to intramural ganglia elsewhere.
- 5.
Be aware that the pelvic ganglia are mixed autonomic ganglia.
- 6.
Realise that the preganglionic neurons of both divisions are cholinergic and that the target receptors in all of the autonomic ganglia are nicotinic.
- 7.
Note that at the tissue level, synapses are replaced by looser ‘junctions’ that permit diffusion of transmitter to outlying receptors.
- 8.
Focus on four kinds of junctional receptors of the sympathetic system and on four actions initiated by muscarinic receptors in the parasympathetic system.
- 9.
Learn from Clinical Panel 13.2 how pharmacologists intercept the recycling and degradation sequence at sympathetic nerve endings. The same principles apply to central nervous system (CNS) drug therapy, notably in psychiatric disorders.
- 10.
Follow Clinical Panel 13.3 to contrast the effects of cholinergic and anticholinergic drugs.
- 11.
Appreciate that visceral afferents utilise autonomic pathways to gain access to the nervous system. They are especially important in the context of thoracic and abdominal pain.
Components of the autonomic nervous system
The autonomic (‘self-regulating’) nervous system is distributed to the peripheral tissues and organs by way of outlying autonomic ganglia. Controlling centres in the hypothalamus and brainstem send central autonomic fibres to synapse upon preganglionic neurons located in the grey matter of the brainstem and spinal cord. From these neurons, preganglionic fibres (mostly myelinated) project out of the CNS to synapse upon multipolar neurons in the autonomic ganglia. Unmyelinated postganglionic fibres emerge and form terminal networks in the target tissues.
Both anatomically and functionally the autonomic system is composed of sympathetic and parasympathetic divisions, but it is fully integrated with motor activity and the neuroendocrine system. While for the most part it ‘functions’ at an unconscious or involuntary level, cortical and subcortical areas play an interactive role and are themselves influenced by its activity.
Sympathetic nervous system
The sympathetic system is so called because it acts in sympathy with the emotions. In association with rage or fear or in situations that pose no threat, the sympathetic system prepares the body for ‘fight or flight’ or for ‘rest and digest’, respectively. In the ‘fight or flight’ response the heart rate is increased, the pupils dilate, and the skin sweats. Blood is diverted from the skin and intestinal tract to the skeletal muscles, and the sphincters of the alimentary and urinary tracts are closed.
The sympathetic outflow from the nervous system is thoracolumbar . The preganglionic neurons are located in the lateral grey horn of the spinal cord at thoracic and the upper two (or three) lumbar segmental levels. From these neurons, preganglionic fibres emerge in the corresponding ventral nerve roots and enter the paravertebral sympathetic chain . The fibres do one of four things ( Figure 13.1 ):
- 1.
Some fibres synapse in the nearest ganglion. Postganglionic fibres enter spinal nerves T1 to L2 and supply blood vessels, sweat glands, and erector pili (hair-raising) muscles in the territory of these nerves.
- 2.
Some fibres ascend the sympathetic chain and synapse in the superior or middle cervical ganglion or in the stellate ganglion . (The stellate consists of the fused inferior cervical and first thoracic ganglia; it lies in front of the neck of the first rib.) Postganglionic fibres supply the head, neck, and upper limbs; also the heart. Of particular importance is the supply to the dilator muscle of the pupil ( Figure 13.2 , Clinical Panel 13.1 ).
Figure 13.2
Horner syndrome, patient’s right side. Note the moderate ptosis of the eyelid and the moderate miosis (pupillary constriction). The affected pupil reacts to light but recovers very slowly.
Clinical Panel 13.1
Sympathetic Interruption
Stellate block
Injection of local anaesthetic around the stellate ganglion— stellate block —is a procedure used to test the effects of sympathetic interruption on blood flow to the hand. Both preganglionic and postganglionic fibres are inactivated, producing sympathetic paralysis in the head and neck on that side, as well as in the upper limb. A successful stellate block is demonstrated by (a) a warm, dry hand; (b) Horner syndrome , which consists of a constricted pupil resulting from unopposed action of the pupillary constrictor; and (c) ptosis (drooping) of the upper eyelid secondary to paralysis of smooth muscle fibres contained in the levator muscle of the upper eyelid ( Figure 13.2 ).
Dominance of the right stellate ganglion in control of the heart rate is shown by the marked slowing of the pulse following a right, but not a left, stellate block. (See also Box 13.1 .)
Functional sympathectomy of the upper limb may be carried out by cutting the sympathetic chain below the stellate ganglion. This is not an anatomic sympathectomy because the ganglionic supply to the limb from the middle cervical and stellate ganglia remains intact. It is a functional sympathectomy because the ganglionic neurons for the limb are deprived of tonic sympathetic drive. Horner syndrome is avoided by making the cut at the level of the second rib: the preganglionic fibres for the head and neck enter the stellate direct from the first thoracic spinal nerve.
Two relative indications for interruption of the sympathetic supply to one or both upper limbs are painful blanching of the fingers in cold weather ( Raynaud phenomenon ) and hyperhidrosis (excessive sweating/perspiration), a condition that typically begins in adolescence and is localised to areas with high concentrations of sweat glands (hands, feet, groin).
The sympathetic supply to the eye is considered further in Chapter 23 .
Lumbar sympathectomy
In the past, to improve blood flow to the lower limb and to treat neuropathic pain, the upper end of the lumbar sympathetic chain was cut to interrupt the preganglionic nerve supply. The usual procedure was to remove the second and third lumbar sympathetic ganglia. However, in males bilateral lumbar sympathectomy can result in persistent, painful erections ( priapism ) because of interruption of a pathway that maintains the resting, flaccid state of the penis. Currently this procedure is rarely performed because there is little evidence that it is effective in the majority of patients. Renal sympathetic nerve ablation (performed via a catheter approach) is increasingly being used to manage hypertension that is resistant to conventional forms of therapy.
Suggested references
- Porter S.B., Murray P.M.: Raynaud phenomenon. J Hand Surg Am 2013; 38: pp. 375-378.
- Thorp A.A., Larsen R.N., Schlaich M.P.: Renal sympathetic nerve ablation for the management of resistant hypertension: an update. Curr Opin Nephrol Hy 2013; 22: pp. 607-614.
Some fibres descend to synapse in lumbar or sacral ganglia of the sympathetic chain. Postganglionic fibres enter the lumbosacral plexus for distribution to the blood vessels and skin of the lower limbs.
Some fibres traverse the chain and emerge as the (preganglionic) thoracic and lumbar splanchnic nerves . The thoracic splanchnic nerves (often simply called the splanchnic nerves) pass through the lower eight thoracic ganglia, pierce the diaphragm, and synapse within the abdomen in the coeliac and mesenteric prevertebral ganglia and in renal ganglia . Postganglionic fibres accompany branches of the aorta to reach the gastrointestinal tract, liver, pancreas, and kidneys. Lumbar splanchnic nerves pass through the upper three lumbar ganglia and meet in front of the bifurcation of the abdominal aorta. They enter the pelvis as the hypogastric nerves before ending in pelvic ganglia , from which the genitourinary tract is supplied.
The medulla of the adrenal gland is the homologue of a sympathetic ganglion, being derived from the neural crest. It receives a direct input from fibres of the thoracic splanchnic nerve of its own side (see later).
The sympathetic system exerts tonic (continuous) constrictor activity on blood vessels in the limbs. To improve the blood flow to the hands or feet, impulse traffic along the sympathetic system can be interrupted surgically ( Clinical Panel 13.1 ).
Parasympathetic nervous system
The parasympathetic system generally has the effect of counterbalancing the sympathetic system. It adapts the eyes for close-up viewing, slows the heart, promotes secretion of salivary and intestinal juices, and accelerates intestinal peristalsis. A notable instance of concerted sympathetic and parasympathetic activity occurs during sexual intercourse.
The parasympathetic outflow from the CNS is craniosacral ( Figure 13.3 ). Preganglionic fibres emerge from the brainstem in four cranial nerves—the oculomotor, facial, glossopharyngeal, and vagus —and from sacral segments of the spinal cord .
Cranial parasympathetic system
Preganglionic parasympathetic fibres emerge in four cranial nerves ( Figure 13.4 ):
- 1.
In the oculomotor nerve, to synapse in the ciliary ganglion . Postganglionic fibres innervate the sphincter of the pupil and the ciliary muscle. Both muscles act to produce the accommodation reflex .
- 2.
In the facial nerve, to synapse in the pterygopalatine ganglion , which innervates the lacrimal and nasal glands; and in the submandibular ganglion , which innervates the submandibular and sublingual glands.
- 3.
In the glossopharyngeal nerve, to synapse in the otic ganglion , which innervates the parotid gland.
- 4.
In the vagus nerve, to synapse in mural (‘on the wall’) or intramural (‘in the wall’) ganglia of heart, lungs, lower oesophagus, stomach, pancreas, gallbladder, small intestine, and ascending and transverse parts of the colon.
Sacral Parasympathetic System
The sacral segments of the spinal cord occupy the conus medullaris ( conus terminalis ) at the lower end of the spinal cord, behind the body of the first lumbar vertebra. From the lateral grey matter of segments S2, S3, and S4, preganglionic fibres descend in the cauda equina within ventral nerve roots. Upon emerging from the pelvic sacral foramina, the fibres separate out as the pelvic splanchnic nerves . Some fibres of the left and right pelvic splanchnic nerves synapse on ganglion cells in the wall of the distal colon and rectum. The rest synapse in pelvic ganglia , close to the pelvic sympathetic ganglia already mentioned. Postganglionic parasympathetic fibres supply the detrusor muscle of the bladder; also the tunica media of the internal pudendal artery and of its branches to the cavernous tissue of the penis/clitoris (see later).
Neurotransmission in the autonomic system
Ganglionic transmission
The preganglionic neurons of the sympathetic and parasympathetic systems are cholinergic : the neurons liberate acetylcholine (ACh) on to the ganglion cells at axodendritic synapses ( Figure 13.5 ). The receptors on the ganglion cells are nicotinic , so named because the excitatory effect can be imitated by locally applied nicotine.
Junctional transmission
Postganglionic fibres of the sympathetic and parasympathetic systems form neuroeffector junctions with target tissues ( Figure 13.5 ). Transmitter substances are liberated from innumerable varicosities strung along the course of the nerve fibres.
The chief transmitter at sympathetic neuroeffector junctions is norepinephrine (noradrenaline) , which is liberated from dense-cored vesicles. The postganglionic sympathetic system in general is described as adrenergic . An exception to the adrenergic rule is the cholinergic sympathetic supply to the eccrine sweat glands over the body surface.
The chief transmitter at parasympathetic neuroeffector junctions is ACh. The postganglionic parasympathetic system in general is cholinergic .
Junctional receptors
The physiologic effects of autonomic stimulation depend upon the nature of the postjunctional receptors inserted by target cells into their own plasma membranes. In addition, transmitter release is influenced by prejunctional receptors in the axolemmal membrane of the nerve terminals.
Sympathetic junctional receptors (adrenoceptors) ( Figure 13.6 )
Two kinds of α adrenoceptors and two kinds of β adrenoceptors have been identified for norepinephrine:
- 1.
Postjunctional α 1 adrenoceptors initiate contraction of smooth muscle in peripheral small arteries and large arterioles, the dilator pupillae, the sphincters of the alimentary tract and bladder neck, and the vas deferens.
- 2.
Prejunctional α 2 adrenoceptors are present on parasympathetic as well as on sympathetic terminals. They inhibit transmitter release in both cases. On sympathetic terminals they are called autoreceptors .
- 3.
Postjunctional β 1 adrenoceptors increase pacemaker activity in the heart and increase the force of ventricular contraction ( Figure 13.7 , Box 13.1 ). In response to a severe fall in blood pressure, sympathetic activation of β 1 receptors on the juxtaglomerular cells of the kidney causes secretion of renin. Renin initiates production of the powerful vasoconstrictor angiotensin II .
Figure 13.7
Disposition of mural cardiac parasympathetic ganglia. (The assistance of Professor Andrew J. Armour, Centre de Recherche de l’Hôpital du Sacre-Coeur de Montréal, Quebec, Canada, is gratefully appreciated.)
Box 13.1
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