The autonomic nervous system

Abstract

A large part of the nervous system is dedicated to the control of the internal viscera and their functions. Much of the activity of these organs is controlled reflexly at the brainstem level, e.g. the cardiovascular and respiratory centres (the vital centres) in the reticular formation of the medulla controlling cardiac and respiratory activity. There are also centres in the cerebrum, notably the hypothalamus in the diencephalon. Somatic and visceral functions are closely integrated at these higher levels; think of the effect that emotional factors or somatic stimulation can have on heart rate, blood pressure, and gastrointestinal activity when we are nervous or are in pain. The nerves involved in these activities are described as visceral sensory or visceral motor nerves because they control visceral function; this distinguishes them from somatic sensory nerves from peripheral receptors and somatic motor nerves controlling voluntary function. Visceral motor neurons innervate smooth muscle and secretory cells of the gastrointestinal and respiratory systems, the smooth and cardiac muscle of the cardiovascular system, the sweat glands and arrector pili muscles of the skin, and the muscles of the ciliary body and iris of the eyeball. In many cases, there is a dual supply from the sympathetic and parasympathetic divisions of the autonomic nervous system. In both divisions of the autonomic nervous system, there is a sequence of two neurons between the CNS and the effector organ which synapse in peripheral autonomic ganglia. The neurons from the CNS to the synapse in the ganglion are the preganglionic neurons and those from the ganglia to the effector organs are the postganglionic neurons. The enteric plexus is a third set of neurons interposed between the post-ganglionic neurons and the effector cells in the gastrointestinal tract. Figure 17.1 compares the general arrangement of the sympathetic and parasympathetic nervous system. The cell bodies of sympathetic visceral preganglionic motor neurons are located in the intermediolateral horns of the thoracic and upper lumbar segments of the spinal cord while those of the parasympathetic visceral preganglionic (secretomotor) neurons are in the nuclei of four of the cranial nerves and the sacral segments of the spinal cord.