text: Chapter 4
  sciatic nerves pass behind the buttocks and backs of the thighs before dividing to innervate parts of the calf
  and foot. The femoral nerve courses through the front of the hip and innervates the quadriceps muscle on the
  front of the thigh.
  4-37. Motor neurons regulate how much force is generated by each muscle fiber. If the motor neuron is
  severed, the signal from the brain cannot travel along the motor neuron to the muscle. No force will be
  generated. The muscle is said to be paralyzed. Any interruption of the signal along the nerve’s path from the
  brain to the muscle due to injury, fatigue, or lack of training will impact the amount of force generated. If the
  nerve is intact, signals can be sent at increasing speeds to excite the muscle fibers innervated by that nerve.
  Faster signaling causes more frequent muscle fiber contraction and more force to be generated. Some motor
  neurons innervate more muscle fibers than others. Those motor neurons involved in large movements or
  gross motor patterns may innervate a dozen muscle fibers. If a muscle produces complex, small movements
  or fine motor patterns, its motor neurons may contact several hundred fibers.
  NEUROENDOCRINE SYSTEM
  4-38. The neuroendocrine system coordinates the response to exercise across multiple organs and hormone
  systems. The neuro part of this system is the autonomic nervous system. The endocrine part is the collective
  name for glands that can release hormones into the bloodstream. Hormones are chemicals that can alter the
  way an organ functions. For example, the body releases epinephrine (adrenaline that affects the heart) and
  norepinephrine (affects the blood vessels) in response to physical stress.
  4-39. Like the peripheral nervous system, the autonomic nervous system also lies outside the central nervous
  system. As its names suggests, the autonomic system functions without conscious input to stimulate and
  control the muscle tissue in the heart, organs, glands, and blood vessels. It therefore impacts the respiratory
  system (lungs), cardiovascular system (the heart, blood vessels, and blood), and organs (kidneys, liver, and
  stomach), endocrine system, and lymphatic system. The autonomic system has two parts: the sympathetic
  and parasympathetic systems.
  4-40. The sympathetic system stimulates cardiac muscle to increase heart rate, whereas the parasympathetic
  slows it down. Both systems impact the same tissue at the same time to maintain homeostasis, but either can
  be turned up to cause a change in response. When the body reaches a resting state, the parasympathetic system
  is dominant. In this state, the Soldier can better digest calories more efficiently, repair muscle tissue, and
  sleep.
  4-41. When the sympathetic nervous system is stimulated by increased activity or threat (physical and
  psychological stress), it reacts with the fight or flight response. The sympathetic nerves signal—
  * The adrenal gland to release epinephrine and norepinephrine causing the arteries to the heart and
  muscles to dilate.
  * The liver to secrete glucose and fatty acids into the blood for quick energy.
  * The pupils to dilate.
  * An increase in sweat to reduce a raised body temperature.
  CARDIORESPIRATORY SYSTEM
  4-42. The cardiorespiratory system includes the heart, lungs, and blood vessels. This system takes oxygen
  from small air sacs in the lungs and delivers it through the blood to the muscles so that they can perform
  work. The amount of oxygen consumed or metabolized is a measure of how well the respiratory and
  cardiovascular systems are working during exercise—or how well they are trained. Oxygen is used in
  chemical reactions inside cells to produce energy. If oxygen is available, muscles can do more work.
  4-43. During prolonged exercise, the heart must pump more blood to supply and maintain the high levels of
  oxygen required by the working muscles. The frequency of heart beats is referred to as the heart rate. The
  frequency of heart beats at rest is called the resting heart rate which is normally about 70 beats per minute
  (bpm). The resting heart rate for Soldiers with high levels of physical readiness may be well below 70 bpm.
  Frequency of heart beats during exercise is called exercise heart rate. The exercise heart rate can more than
  double in young adults during maximal effort with a maximum approaching 200 bpm.