Peripheral Nervous System

• The peripheral nervous system is the entire network of nerves located outside the CNS. It extends to the top of the head, throughout the body to the tips of the fingers and toes and to all parts of the skin. • The PNS enables communication between the CNS and all other parts of the body outside the brain and spinal cord

• Its two functions are: o To carry information to the CNS from the body's muscles, organs and glands (about the internal environment) and from the sensory organs (about the external environment). o To carry information from the CNS to the body's muscles, organs and glands.

• The somatic nervous system is a subdivision of the PNS. It carries sensory information to the CNS and motor information from the CNS. Sensory information is communicated along sensory neural pathways and motor information along motor neural pathways

• The sensory neural pathways (also called afferent pathways) contain sensory neurons that receive signals from the eyes, ears, nose, skin, tendons, joints, skeletal muscles, tongue and many other tissues and organs. • Sensory neurons (also called afferent neurons) are specialised nerve cells that receive and carry sensory messages. These neurons generally respond to a particular type of stimulation. o For example, some neurons generally respond only to light, but not other types of stimulation such as mechanical energy (sound), odours (smell) and pressure (touch).

• The motor neural pathways (also called efferent pathways) contain motor neurons that carry motor information for voluntary bodily movements • Motor neurons (also called efferent neurons) are specialised nerve cells that carry messages to skeletal muscles causing them to contract or relax. o Skeletal muscles are attached to our bones and respond to messages from the CNS to initiate, change or stop movement.

• Sensory neurons and motor neurons rarely ever connect directly. Interneurons (also called connecting neurons) perform the important function of making the connection between sensory and motor neurons which rarely ever connect directly. They are found only in the CNS and are the most abundant type of neurons in the brain. • Voluntary movement is controlled through the coordinated actions of both motor and sensory neurons

• The autonomic nervous system is a subdivision of the PNS that connects the CNS to the body's internal organs and glands providing feedback to the brain about their activities. The ANS is called 'autonomous' because many of the organs, glands and processes under its controlled are self-regulating and not usually under voluntary control.

• Unlike the somatic nervous system which is responsible for initiating skeletal muscle movement, the ANS regulates the activity of the visceral muscles, organs and glans. Thus the messages carried between the CNS and visceral muscles, organs and glands either increase or decrease their respective activities to meet varying demands placed on the body.

• The ANS is not completely self -regulating. There are a few responses that we can control at certain times such as blinking, heart beat and breathing rate. • Biofeedback a process by which a person receives information about the state of an internal bodily activity that normally occurs automatically and then uses thought processes to exert control over that activity through the use of a strategy in order to control the particular autonomic response.

• Most visceral muscles, organs and glands receive messages from two sets of neurons that produce opposite effects and come from two distinct divisions of the ANS: o The sympathetic nervous system; responsible for increasing the activity of most visceral muscles, organs and glands in times of vigorous activity, stress or threat. o The parasympathetic nervous system; responsible for decreasing the activity of most visceral muscles, organs and glands and restoring body functioning to its normal state. • While the parasympathetic and sympathetic nervous systems are both active at the same time, one system usually dominates the other at any given time.

• The sympathetic nervous system activates internal muscles, organs and glands to prepare the body for vigorous activity or to deal with a stressful or threatening situation. It enhances survival by providing an immediate response, in a split second, to any kind of emergency.

• Process: o When a threat is perceived neurons in the sympathetic nervous system target organs and glands to respond in the required way. o Glands that are activated include the two adrenal glands located just above the kidneys. The adrena medulla within each adrenal gland secreted the hormones adrenaline and noradrenaline into the bloodstream. o These hormones then circulate throughout the body, complementing and enhancing the effects of the sympathetic nervous system by activating various muscles, organs and other glans in preparation for dealing with the potential emergency. o The results can be seen in the table

• The parasympathetic nervous system keeps the system of the body functioning efficiently. In times of minimal stress and in the absence of threats it helps to maintain the internal body environment in a steady, balanced state of normal functioning.

• Generally has the effect of counterbalancing the activities of the sympathetic nervous system. It restores the body to a state of calm once the need for the sympathetic nervous system has passed. • The parasympathetic nervous system dominates the sympathetic nervous system most of the time. It is involved in routine everyday activities.

• Also involved in the elimination of wastes and protection of the visual system through the production of tears and through automatic pupil constriction in conditions of bright light. • When returning the body to a balanced state the parasympathetic system reduces heart and breathing rates, minimises the release of sugar and fats into the bloodstream and acts in a way that is opposite to the sympathetic nervous system. • Once the danger has passed, the parasympathetic nervous system takes over and the various bodily systems activated by the sympathetic nervous system would gradually begin to return to normal • The parasympathetic system takes longer to return the body to its normal state compared with the sympathetic nervous system's immediate activation. This is because of the lingering presence of hormones that are released when the sympathetic nervous system is activated.