Autonomic Nervous System

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.

References:

  1. Acevedo, E. O., Webb, H. E., & Huang, C. J. (2012). Cardiovascular health implications of combined mental and physical challenge. In E. O. Acevedo (Ed.), The Oxford handbook of exercise psychology (pp. 169–191). New York: Oxford University Press.
  2. Boutcher, S. H., & Hamer, M. (2006). Psychobiological reactivity, physical activity, and cardiovascular health. In E. O. Acevedo & P. Ekkekakis (Eds.), Psychobiology of physical activity (pp. 161–175). Champaign, IL: Human Kinetics.

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