Prof. J.P.N. Mishra
Autonomic nervous system (ANS) regulates the activities of smooth muscle, cardiac muscle, and certain glands. Structurally, the system consists of visceral efferent neurons organized into nerves, ganglia, and plexuses.
It usually operates without conscious control. It was named autonomic because physiologists thought it functioned with no control from the central nervous system, that is why autonomous or self-governing. But now it has been established that the autonomic system is neither structurally nor functionally independent of the central nervous system. It is regulated by centers in the brain, in particular by the cerebral cortex, hypothalamus, and medulla oblongata.
It is consists of two sub divisions: the sympathetic and the parasympathetic (Fig 1-19). Many organs innervated by the autonomic nervous system receive visceral efferent neurons from both components of the autonomic system-one set from the sympathetic division; impulses transmitted by the fibres of one division stimulate the organ to start or increase activity, whereas impulses from the other division decrease the organ's activity. Organs that receive impulses from both sympathetic and parasympathetic fibres are said to have dual innervations. Thus, autonomic innervations may be excitatory or inhibitory. In the somatic efferent nervous system, only one kind of motor neuron innervates a skeletal muscle and, moreover, innervations are always excitatory. When a somatic neuron stimulates a skeletal muscle, the muscle becomes active. When the neuron ceases to stimulate the muscle, contraction stops altogether.
Activities: Most visceral effectors have dual innervations; that is, they receive fibres from both the sympathetic and the parasympathetic divisions. In these cases, impulses from one division stimulate the organ's impulses from one division stimulate the organ's activities, whereas impulses from the other division inhibit the organ's activities.
The stimulating division may be either the sympathetic or the parasympathetic, depending on the organ. For example, sympathetic impulses increase heart activity, whereas parasympathetic impulses decrease it. On the other hand, parasympathetic impulses increase digestive activities, whereas sympathetic impulses inhibit them. The actions of the two systems are carefully integrated to help maintain homeostasis.
The parasympathetic part is basically concerned with activities that conserve and restore body energy during times of rest or recovery of the body. It is an energy conservation-restorative system. In normal conditions, parasympathetic impulses to the digestive glands and the smooth muscle of the gastrointestinal tract dominate over sympathetic impulses, enabling energy-supplying food can be digested and absorbed by the body.
The sympathetic division, is primarily realted processes involving the excitation as when one is in a condition of stress due to either physical or emotional stimuli. When the body is in homeostasis, the main function of the sympathetic division is to counteract the parasympathetic affects just enough to carry out normal processes requiring energy. During extreme stress, however, the sympathetic dominates the parasympathetic. When people are confronted with a stress condition, for example, their bodies become alert and they sometimes perform feats of unusual strength. Fear stimulates the sympathetic division as do a variety of other emotions and physical activities.
Activation of the sympathetic part brings into operation a series of physiological responses collectively called the fight-N-flight response. It produces the following effects.
- The pupils of the eyes dilate.
- The heart rate and force of contraction increase and blood pressure increases.
- The blood vessels of the skin and viscera constrict.
- The remainder of the blood vessels dilates. This causes a faster flow of blood into the dilated blood vessels of skeletal muscles, cardiac muscle, and lungs-organs involved in fighting off danger.
- Rapid and deeper breathing occurs and the bronchioles dilate to allow faster movement of air in and out of the lungs.,
- Blood sugar level rises as liver glycogen is converted to glucose to supply to body's additional energy needs.
- The medullae of the adrenal glands are stimulated to produce epinephrine and norepinephrine, hormones that intensify and prolong the sympathetic effects noted previously.
- Processes that are not essential for meeting the stress situation are inhibited. For example, muscular movements of the gastrointestinal tract and digestive secretions are slowed down or even stopped.
Control by Higher Centers: The autonomic nervous system, now, is not taken as a separate nervous system. Although little is known about the specific centers in the brain that regulate specific autonomic functions, it is known that axons from many parts of the central nervous system are connected to both the sympathetic and the parasympathetic divisions of the autonomic nervous system and thus exert considerable control over it. Few autonomic centers in the cerebral cortex are connected to autonomic centers of the thalamus, which in turn, are connected to the hypothalamus. In this hierarchy of command, the thalamus sorts incoming impulses before they reach the cerebral cortex. The cerebral cortex then turns over control and integration of visceral activities to the hypothalamus. It is at the level of the hypothalamus that the major control and integration of the autonomic nervous system is exerted and much of this control is by way of hypothalamic influence of centers in the medulla and spinal cord (Saladin, 2004).
The hypothalamus receives messages from some areas of the nervous system concerned with emotions, visceral functions, olfaction, gestation, as well as changes in temperature, osmolarity, and levels of various substances in blood. Hypothalamus is connected to both the sympathetic and the parasympathetic divisions of the autonomic nervous system by axons of few neurons. The posterior and lateral portions of the hypothalamus controls the sympathetic division. When these areas are stimulated, there is an increase in heart rate and force of beat, a rise in blood pressure due to vasoconstriction of blood vessels, an increase in the rate and depth of respiration, dilation of the pupils, and inhibition of the gastrointestinal tract. Contrary to that the anterior and medial portions of the hypothalamus controls the parasympathetic division. Following stimulation of these areas there is a decrease in heart rate, lowering of blood pressure, constriction of the pupils, and increased secretion and motility of the gastro-intestinal tract.
Control of the autonomic nervous system by the cerebral cortex occurs primarily during emotional stress. In extreme anxiety, which can result from either conscious or subconscious stimulation in the cerebral cortex, the cortex can stimulate the hypothalamus as part of the limbic system. This, in turn, stimulates the cardiac and vasomotor centers of the medulla, which increases heart rate and force of beat, and blood pressure. If the cortex is stimulated by hearing bad news or experiencing an extremely unpleasant sight, the stimulation causes vasodilatation of blood vessels, a lowering of blood pressure, and fainting (Saladin, 2004).
Evidence of even more direct control of visceral responses is provided by data gathered from studies of biofeedback and meditation. In the simplest terms, biofeedback is a process in which people get constant signals, or feedback, about various visceral biological functions such as blood pressure, heart rate, and muscle tension. By using special monitoring devices, they can control these visceral functions consciously to a limited degree.
Functions of the sympathetic division: The major function of the sympathetic division is related to emergency managements. When the homeostasis of the body is threatened-that is, when we are under physical or psychological stress-outgoing sympathetic signals increase greatly. In fact, one of the very first steps in the body's complex defense mechanism against stress is a sudden and marked increase in sympathetic activity This brings about a group of responses that all go on at the same time. Together they make the body ready to expend maximum energy and thus to engage in the maximum muscular exertion needed to deal with the perceived threat for example in running or fighting termed as fight -n-flight response. Some particularly important changes for maximum energy expenditure by skeletal muscles are faster, stronger heartbeat, dilated blood vessels in skeletal muscles, dilated bronchi, and increased blood sugar levels from stimulated glycogenosis (conversion of glycogen to glucose). Also, sympathetic impulses to the medulla of each adrenal gland stimulate its secretion of epinephrine and some norepinephrine. These hormones reinforce and prolong effects of the nor-epinephrine released by sympathetic postganglionic fibres. The fight-or-flight reaction is a normal response in times of stress. Without such a response, we might not be able to resist or retreat from something that actually threatens our well-being. However, chronic exposure to stress can lead to dysfunction of sympathetic effectors-and perhaps even to the dysfunction of the ANS itself (Thibodeau and patton, 1999).
Functions of the parasympathetic division: The parasympathetic division is the dominant controller of most autonomic effectors most of the time. Under quiet, no stressful conditions, more impulses reach autonomic effectors by cholinergic parasympathetic fibres than by adrenergic sympathetic fibres. If the sympathetic division dominates during times that require fight-or-flight, then the parasympathetic division dominates during the in-between times of "rest-and-repair."
