Autonomic nervous system and visceral afferents

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13 Autonomic nervous system and visceral afferents

Sympathetic Nervous System

The sympathetic system is so called because it acts in sympathy with the emotions. In association with rage or fear, the sympathetic system prepares the body for ‘fight or flight’: 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 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.)

Functional sympathectomy of the upper limb may be carried out by cutting the sympathetic chain below the stellate ganglion. This is not an anatomical sympathectomy because the ganglionic supply to the limb from the middle cervical and stellate ganglia remains intact. It is a functional one because the ganglionic neurons for the limb are deprived of tonic sympathetic drive. Horner’s syndrome is avoided by making the cut at the level of the second rib: the preganglionic fibers for the head and neck enter the stellate direct from the first thoracic spinal nerve.

Two 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) of the hands – usually an embarrassing affliction of teenage girls.

The sympathetic supply to the eye is considered further in Chapter 23.

The medulla of the adrenal gland is the homolog of a sympathetic ganglion, being derived from the neural crest. It receives a direct input from fibers 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. 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.

Neurotransmission in the Autonomic System

Junctional receptors

The physiological 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 13.5)

Two kinds of α adrenoceptor and two kinds of β adrenoceptor have been identified for norepinephrine:

3 Postjunctional β1 adrenoceptors increase pacemaker activity in the heart and increase the force of ventricular contraction (Box 13.1). In response to a severe fall of blood pressure, sympathetic activation of β1 receptors on the juxtaglomerular cells of the kidney causes secretion of renin. Renin initiates production of the powerful vasoconstrictor angiotensin II.