interlace in a thick network called a plexus. The cervical plexus is located in the neck, and the brachial plexus is in the shoulder. In the pelvic region are the lumbar, sacral, and pudendal plexuses.
The autonomic nervous system, as its name implies, functions automatically. It helps to regulate the smooth muscles, cardiac muscle, digestive tube, blood vessels, sweat and digestive glands, and certain endocrine glands. It is not directly under the control of the brain but usually works in harmony with the nerves that are underthebrains control. The autonomic nervous system is divided into the sympathetic and parasympathetic systems (see table 3-1).
Numerous ganglia (nerve centers) located just outside the spinal cord, beside the vertebrae, are the basis of the sympathetic (thoracolumbar) system. These nerve centers connect with the thoracic and lumbar regions of the spinal cord and, through the spinal nerves, with the muscles, organs, and glands they affect.
Because one function of the sympathetic system is to increase the activity of the body to enable it to meet danger or undergo strenuous physical activity, it has been called the fight or flight nervous system. The sympathetic nerves, when stimulated, usually discharge as a unit, and the effects can be noticed especially under circumstances of fright or rage; for example, the heart beats faster, blood pressure rises, the spleen discharges red blood cells into the blood, the blood sugar level rises, the pupils dilate, and the peripheral blood vessels constrict. These changes prepare the body for a stressful situation.
The ganglia of the parasympathetic system are located in the midportion of the brain, the medulla oblongata, and the sacral regions. For this reason the parasympathetic system is sometimes called the craniosacral system. The ganglia in the midbrain and medulla oblongata send impulses out along cranial nerves (oculomotor, facial, glossopharyngeal, and vagus). The sacral ganglia stem from the second, third, and fourth sacral nerves.
The parasympathetic nerves do not all discharge at once. They aim more toward conserving and restoring energy. Their actions slow the heart beat, lower the blood pressure, stimulate gastrointestinal movements and secretion, aid absorption, constrict the pupils, dilate peripheral blood vessels, and empty the bladder and rectum. Overall they promote the autonomic restoration of body systems to normal functioning after sympathetic stimulation. The sympathetic and parasympathetic systems counterbalance each other to preserve a harmonious balance of body functions and activities.
The sensory system functions to inform areas of the cerebral cortex of changes that are taking place within the body or in the external environment. The special sensory receptors are designed to respond only to a special individual stimulus such as sound waves, light, taste, smell, pressure, heat, cold, pain, or touch. Positional changes, balance, hunger, and thirst sensations are also detected and passed on to the brain.
Odor is perceived upon stimulation of the receptor cells in the olfactory membrane of the nose. The olfactory receptors are very sensitive, but they are also easily fatigued. This explains why odors that are initially very noticeable are not sensed after a short time. Smell is not as well developed in man as in other mammals.
The taste buds are located in the tongue. The sensation of taste is limited to sour, sweet, bitter, and salty. Many foods and drinks tasted are actually smelled, and their taste depends upon their odor. This can be demonstrated by pinching the nose shut when eating onions. Sight can also affect taste. Several drops of green food coloring in a glass of milk will make it all but unpalatable, even though the true taste has not been affected.
The eye, the organ of sight, is a specialized structure for the reception of light. It is assisted in its function by accessory structures, such as the