Medical brain notes 24

  Introduction        The autonomic nervous system is made up of nerves supplying the viscera (and blood vessels) along with the parts of the brain and spinal cord related to them. It is subdivided into two divisions,  sympathetic  and  parasympathetic . Both these divisions contain efferent as well as afferentfibres. The efferent fibres supply smooth muscle throughout the body. The influence may be either to cause contraction or relaxation. In a given situation, the sympathetic and parasympathetic nerves generally produce opposite effects. For example, sympathetic stimulation causes dilatation of the pupil, whereas parasympathetic stimulation causes constriction. In hollow viscera like the stomach or urinary bladder, parasympathetic stimulation produces movement and inhibits the sphincters. An opposite sympathetic effect is usually described. In the case of blood vessels, the influence on smooth muscle may result in vasoconstriction or ...

  THE SYMPATHETIC TRUNKS          The sympathetic trunks (right and left) are the most easily seen parts of the autonomic nervous system. They are placed on either side of the vertebral column. Above, they extend to the base of the skull; and below, to the coccyx. Each trunk bears a number of enlargements placed along its length. These are the  sympathetic ganglia . The number of ganglia is variable. Generally there are three (superior, middle and inferior) in the cervical region; eleven in the thoracic region; four in the  lumbar region; and four in the sacral region, so that in all there are twenty two or twenty three ganglia on each trunk. The inferior cervical ganglion and the first thoracic are often fused to form a large  stellate ganglion .          The sympathetic trunks are connected to the spinal nerves by a series of communicating branches or  rami communicantes . These ar...

  AUTONOMIC PLEXUSES          It has been mentioned above that visceral branches of sympathetic trunks help to form various plexuses in the thorax, the abdomen and the pelvis. In addition to sympathetic fibres these plexuses contain parasympathetic fibres derived either from the vagus nerve, or from pelvic splanchnic nerves . They also contain collections of neurons which are often referred to as ganglia. In the thorax there are the  superficial anddeep cardiac plexuses  in relation to theheart, and the  pulmonary plexuse s in relation to the lungs (Fig. 19.2). In the abdomen there is a prominent  coeliacganglion  on either side of the aorta. Thetwo ganglia are interconnected by numerous fibres that form the  coeliacplexus . This plexus is closely related tothe coeliac trunk and sends ramifications along its branches. Other plexuses (or ganglia) are related to the abdominal aorta, to the superior mesenteric and i...

  PREGANGLIONIC AND POSTGANGLIONIC NEURONS        The autonomic pathway, for innervation of smooth muscle or gland always consists of two neurons that synapse in a ganglion (Fig. 3.2). The first neuron carries the nerve impulse from the CNS to the ganglion and is called the  preganglionic neuron . The second neuron carries impulses from the ganglion to smooth muscle or gland and is called the  postganglionic neuron . Sympathetic Preganglionic Neurons         The cell bodies of sympathetic preganglionic neurons are located in the intermediolateral grey column of the spinal cord in the thoracic and upper two (or three) lumbar segments (Fig. 19.3). Fibres arising from these neurons constitute the  thoracolumbar outflow . Their axons leave the spinal cord through anterior nerve roots to reach the spinal nerves of the segments concerned. After a very short course in the ventral primary rami these fibres ente...

  SOME FURTHER DETAILS OF AUTONOMIC NEURONS         Having considered the basic plan of the autonomic nervous system, we may now examine some further details of interest about autonomic neurons. 1.                The neurotransmitter acetylcholine is liberated at the terminals of preganglionic neurons, both sympathetic and parasympathetic. Acetylcholine is also liberated at the terminals of parasympathetic postganglionic neurons. However, the neurotransmitter liberated at the terminals of sympathetic postganglionic neurons is (as a rule) noradrenalin or adrenalin. Cells of the adrenal medulla which receive terminals of preganglionic sympathetic neurons, and produce noradrenalin and adrenalin, may be regarded as modified sympathetic postganglionic neurons. It may be noted that cells of the sympathetic ganglia and of the adrenal medulla have a common embryological origin from the ...

  AFFERENT AUTONOMIC PATHWAYS          We have noted that sensory neurons related to the autonomic nervous system are general visceral afferent neurons; and that their arrangement is similar to that of afferent fibres in cerebrospinal nerves. The neurons concerned are located in spinal ganglia, or in sensory ganglia of cranial nerves. They carry impulses arising in viscera, and in blood vessels, to the central nervous system. They may be associated with the parasympathetic as well as the sympathetic systems. Accordingly, the cell bodies of the neurons in question may be located in one of the following situations. Afferents Related to the Cranial part of the Parasympathetic System         These are general visceral afferent fibres related to the glossopharyngeal and vagus nerves. The cell bodies of the neurons concerned are located in sensory ganglia related to the cranial nerve in question. Their central pro...

  Some Further Comments on Autonomic Afferents        Some important points that may be noted about autonomic afferents are as follows. a.                Autonomic afferents are necessary for various visceral reflexes. Most of these impulses are not consciously perceived. b.                Some normal visceral sensations that reach consciousness include those of hunger, nausea, distension of the urinary bladder or rectum, and sexual sensations. Sense of touch or pressure perceived by the tongue and pharynx, and the sensation of taste are also visceral sensations. c.                 Under pathological conditions visceral pain is perceived. This is produced by distension, by spasm of smooth muscle, or by anoxia. d.     ...

The Eyeball        The  sphincter pupillae  is supplied by parasympathetic nerves. The preganglionic neurons concerned are located in the Edinger Westphal nucleus. Their axons travel through the oculomotor nerve and terminate in the ciliary ganglion. Postganglionic neurons are located in this ganglion. Their axons supply the sphincter pupillae and the ciliaris muscle (Fig. 19.9).          The  dilator pupillae  is supplied by sympathetic nerves. The preganglionic neurons concerned are located in the intermediolateral grey column of the first thoracic segment of the spinal cord. Their axons emerge through the anterior nerve root of the first thoracic nerve to reach the stellate ganglion. They, however, pass through this ganglion without relay, and ascend in the sympathetic trunk to reach the superior cervical sympathetic ganglion. Postganglionic neurons are located in this ganglion. Their axons pass...