13 Autonomic nervous system and visceral afferents
Sympathetic Nervous System
The sympathetic outflow from the nervous system is thoracolumbar, the preganglionic neurons being located in the lateral gray horn of the spinal cord at thoracic and upper two (or three) lumbar segmental levels. From these neurons, preganglionic fibers emerge in the corresponding anterior nerve roots and enter the paravertebral sympathetic chain. The fibers do one of four things (Figure 13.1):
Clinical Panel 13.1 Sympathetic interruption
Stellate block
Injection of local anesthetic around the stellate ganglion – stellate block – is a procedure used in order to test the effects of sympathetic interruption on blood flow to the hand. Both pre- and postganglionic fibers 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’s syndrome, which consists of a constricted pupil owing to unopposed action of the pupillary constrictor, and (c) ptosis (drooping) of the upper eyelid owing to paralysis of smooth muscle fibers contained in the levator muscle of the upper eyelid (Figure CP 13.1.1).
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.)
The sympathetic supply to the eye is considered further in Chapter 23.
The sympathetic system exerts tonic (continuous) constrictor activity on blood vessels in the limbs. In order 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 (Box 13.4).
The parasympathetic outflow from the CNS is craniosacral (Figure 13.2). Preganglionic fibers 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 fibers emerge in four cranial nerves (Figure 13.3):
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 13.4). The receptors on the ganglion cells are nicotinic, so named because the excitatory effect can be imitated by locally applied nicotine.
Junctional transmission
Postganglionic fibers of the sympathetic and parasympathetic systems form neuroeffector junctions with target tissues (Figure 13.4). Transmitter substances are liberated from innumerable varicosities strung along the course of the nerve fibers.
Junctional receptors
Sympathetic junctional receptors (adrenoceptors) (Figure 13.5)
Two kinds of α adrenoceptor and two kinds of β adrenoceptor have been identified for norepinephrine:
Box 13.1 Innervation of the heart
Experimental evidence indicates that the preganglionic parasympathic supply originates in neurons occupying the caudal ventrolateral medulla oblongata. The fibers descend within the trunk of the vagus and synapse within mural ganglia on the posterior walls of the atria and in the posterior atrioventricular groove (Figure Box 13.1.1). Postganglionic cholinergic fibers supply the same tissues as those of the sympathetic, although the direct supply to ventricles and coronary arteries is slight.
There is a high level of autonomic interaction where innervation is dense, notably within nodal tissue, in the modes shown in Figures 13.5 and 13.7. Many sympathetic nerve endings also release neuropeptide Y, which binds to a specific receptor on cholinergic terminals with adjuvant inhibitory effect on ACh release.