The central nervous system (CNS) is composed of the brain and spinal cord. The CNS receives sensory information from the peripheral nervous system and controls the body’s responses. The peripheral nervous system involves all of the nerves outside of the brain and spinal cord that carry messages to and from the CNS. The peripheral nervous system has two branches: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls skeletal muscle as well as external sensory organs such as the skin. This system is described as voluntary because the responses can be controlled consciously, although reflex reactions of skeletal muscles are an exception.
The autonomic nervous system is the branch of the peripheral nervous system that controls involuntary actions, such as pulmonary respiration (breathing), heart rate and the force of contraction of the heart, and vasoconstriction and dilation (widening and narrowing of blood vessels) that impact blood pressure. This system is further divided into two systems: the sympathetic system and the parasympathetic system. The sympathetic division of the autonomic nervous system regulates flight-or-fight responses. This division is responsible for tasks such as relaxing the bladder, increasing heart rate, and dilating eye pupils. The parasympathetic division of the autonomic nervous system supports homeostasis and conserves physical resources. This division performs tasks such as controlling the bladder, slowing down heart rate, and constricting eye pupils.
Like other nerves, those of the autonomic nervous system convey their messages to the appropriate end organs (blood vessels, viscera, etc.) by releasing transmitter substances to which the receptors of the target cells are responsive. The most important of these transmitters in the autonomic nervous system are acetylcholine and norepinephrine. In the parasympathetic system, acetylcholine is responsible for most of these transmissions between the afferent (leading toward the CNS) and efferent nerves (leading away from the CNS) and between the efferent nerve endings that innervate cells or organs. Acetylcholine also serves to transmit nerve-to-nerve messages in the afferent nerves and the brain centers of the sympathetic nervous system. Norepinephrine is released from sympathetic nerve endings to end organs or cells, except at the sweat glands where acetylcholine is released. Epinephrine is also released from the adrenal medulla in response to sympathetic activation.
The rate and strength of cardiac contractions is under the predominant control of the sympathetic nervous system. Sympathetic stimuli from cardiac nerves cause acceleration of the heart rate. Increases in heart rate are also complemented by simultaneous reduction in the parasympathetic stimuli via the vagus nerve, which is responsible for slowing the heart rate. Thus, heart rate responses are often used as indirect measures of sympathetic and parasympathetic activity (autonomic nervous system activation).
Situations such as emotional excitement, fear, apprehension, psychological distress, panic reactions, and fight-or-flight stimuli activate the sympathetic nervous system. In addition, the activation of the autonomic nervous system, often demonstrated in preparation for and during activity, has been an area of investigation in sport and exercise physiology. Noninvasive measures of autonomic function (heart rate, blood pressure, galvanic skin response, etc.) have been used to investigate the role of the peripheral nervous system in human performance, infer affect and infer changes in affect during and following physical activity, understand the effects of overtraining, and explain potential positive adaptations that occur in parallel with improvements in fitness. In summary, biofeedback studies have demonstrated a relationship between effective control of autonomic function and human performance, a number of affective states have been shown to correlate with autonomic responses, overtraining seems to be associated with parasympathetic dysfunction, and improvements in fitness are associated with enhanced parasympathetic tone and diminished sympathetic activity. More specifically, the changes in response to aerobic training seem to occur primarily due to an alteration in the balance of sympathetic and parasympathetic activity. With training, several adaptations, including enhanced parasympathetic tone and increased stroke volume, explain the reduction in resting heart rate. At rest the enhanced parasympathetic activity seen in trained individuals leads to a reduction in heart rate; however, when presented with a mental challenge, trained individuals demonstrate greater vagal withdrawal and possibly an enhanced sympathetic response compared with untrained individuals. This adaptation seems to be beneficial for cardiovascular health.
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