Peripheral nervous system

The PNS contains 43 pairs of nerves: 12 pairs of cranial nerves that connect with the brain and 31 pairs of spinal nerves that connect with the spinal cord. Cranial and spinal nerves directly supply the skeletal muscles. Functionally, the PNS has two major divisions: the sensory division and motor division.

Sensory division

The sensory division of the PNS carries sensory information toward the CNS. Sensory (afferent) neurons originate in such areas as:

• Blood and lymph vessels;
• Internal organs;
• Special sense organs(taste, touch, smell, hearing, vision);
• The skin;
• Muscles and tendons.

Sensory neurons in the PNS end either in the spinal cord or in the brain, and they continuously convey information to the CNS concerning the body’s constantly changing status. By relaying this information, these neurons allow the brain to sense what is going on in all parts of the body and in the immediate environment. Sensory neurons within the CNS carry the sensory input to appropriate areas, where the information can be processed and integrated with other incoming information.

The sensory division receives information from five primary types of receptors:

1)      Mechanoreceptors that respond to mechanical forces such as pressure, touch, vibrations, or stretch.

2)      Thermoreceptors that respond to changes in temperature.

3)      Nocireceptors that respond to painful stimuli.

4)      Photoreceptors that respond to electromagnetic radiation(light) to allow vision.

5)      Chemoreceptors that respond to chemical stimuli, such as from foods, odors, or changes in blood or tissue concentrations of substances such as oxygen, carbon dioxide, glucose and electrolytes

Several of these receptors are important in exercise and sport. Let’s consider just a few. Free nerve endings detect crude touch, pressure, pain, heat, and cold. Thus, they function as mechanoreceptors, nocireceptors, and thermoreceptors. These nerve endings are important for preventing injury during athletic performance. Special muscle and joint nerve endings are of many types and functions, and each type is sensitive to a specific stimulus. Here are some important examples:

·         Joint kinesthetic receptors located in the joint capsules are sensitive to joint angles and rates of change in these angles. Thus, they sense the position and any movement of the joints.

·         Muscle spindles sense muscle length and rate of change in length.

·         Golgi tendon organs detect the tension applied by a muscle to its tendon, providing information about the strength of muscle contraction.

Motor division

The CNS transmits information to various parts of the body through the motor, or efferent, division of the PNS. Once the CNS has processed the information it receives from the sensory division, it decides how the body should respond to that input. From the brain and spinal cord, intricate networks of neurons go out to all parts of the body, providing detailed instructions to the target areas – for our purposes, muscles.

Autonomic nervous system

The autonomic nervous system, often considered part of the motor division of the PNS, controls the body’s involuntarly internal functions. Some of these functions that are important to sport and activity include heart rate, blood pressure, blood distribution, and lung function.

The autonomic nervous system has two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These originate from different sections of the spinal cord and from the base of the brain. The effects of the two systems are often antagonistic, but the systems always function together.

Sympathetic nervous system

The sympathetic nervous system is sometimes called the fight-or-flight system: It prepares the body to face a crisis and sustains its function during that crisis. When  excited, the sympathetic nervous system produces a massive discharge throughout the body, preparing it for action. A sudden loud noise, a life-threatening situation, or those last few seconds before the start of an athletic competition are examples of circumstances in which this massive sympathetic discharge occurs. The effects of sympathetic stimulation are important to the athlete:

• Heart rate and strength of cardiac contraction increase.
• Coronary vessels dilate, increasing the blood supply to the heart muscle to meet its increased demands.
• Peripheral vasodilatation allows more blood to enter the active skeletal muscles.
• Vasoconstriction in most other tissues diverts blood away froom them and to the active muscles.
• Blood pressure increases, allowing better perfusion of the muscles and improving the return of venous blood to the heart.
• Bronchodilatation improves gas exchange.
• Metabolic rate increases, reflecting the body’s effort to meet the increased demands of physical activity.
• Mental activity increases, allowing better perception of sensory stimuli and more concentration on performance.
• Glucose is released from the liver into the blood as an energy source.
• Functions not directly needed are slowed(e.g. renal function, digestion), conserving energy so that it can be used for action.

These basic alterations in bodily function facilitate motor responses, demonstrating the importance of the autonomic nervous system in preparing the both for and sustaining it during acute stress or physical activity.

Parasympathetic nervous system

The parasympathetic nervous system is the body’s house-keeping system. It has a major role in carrying out such processes as digestion, urination, glandular secretion, and conservation of energy. This system is more active when one is calm and at rest. Its effects tend to oppose those of the sympathetic system. The parasympathetic division causes decreased heart rate, constriction of coronary vessels, and bronchoconstriction.

The various effects of the sympathetic and parasympathetic divisions of the autonomic nervous system are summarized in the table below.

Effects of the sympathetic and parasympathetic nervous system on various organs
Target organ or system	Sympathetic effects	Parasympathetic effects
Heart muscle	Increases rate and force of contraction	Decreases rate of contraction
Heart: coronary blood vessels	Cause vasodilatation	Cause vasoconstriction
Blood vessels	Increase blood pressure; cause vasoconstriction in abdominal viscera and skin to divert blood when necessary; cause vasodilatation in the skeletal muscles and heart during exercise	Little or no effect
Liver	Stimulates glucose release	No effect
Cellular metabolism	Increases metabolic rate	No effect
Adipose tissue	Stimulates lypolysis	No effect
Sweat glands	Increase sweating	No effect
Adrenal glands	Stimulate secretion of epinephrine and norepinephrine	No effect
Digestive system	Decreases activity of glands and muscles; constricts sphincters	Increases activity of glands and muscles; relaxes sphincters
Kidney	Causes vasoconstriction, decreases urine formation	No effect