Peripheral Nervous System

●     PNS

○     Receive and process information from sense organs, and to bring about responses to the information received.

○     Along with the endocrine system, it coordinates all our voluntary and involuntary actions.

○     What is the PNS?

■     Consists of nerve fibres, which carry information to and from the CNS, and groups of cell nerve bodies (ganglia), which lie outside the brain & spinal cord.

■     Nerve fibres arranged into nerves which arise from the brain and spinal cord.

■     Cranial Nerves (12 pairs)

■     12 pairs of nerves arising from the brain

■     Most are mixed nerves:

■     contain fibres that carry information to brain (sensory) & fibres that carry information away from brain (motor)

■     Few carry only sensory or only motor

■     Spinal Nerves (31 pairs)

■     31 pairs arising from the spinal cord

■     All mixed nerves & each joined to spinal cord by 2 roots:

■     Dorsal root: contains axons of sensory neurons that have cell body in small swelling known as dorsal root ganglion

■     Ventral root: contains axons of motor neurons that have their cell bodies in grey matter of spinal cord

●     Divisions of the PNS

○     The PNS can be divided into a number of divisions. We need to be able to compare the following divisions with one another:

○     Afferent Division

■     Sensory division - Fibres that carry impulses into the CNS

■     Somatic sensory neurons: Carried by sensory nerve cells from receptors in skin & around muscles & joints

■     Visceral sensory neurons: Sensory nerve cells that take impulses from internal organs

■     Motor Division - Fibres that carry impulses away from CNS

■     Somatic division: Takes impulses from the CNS to skeletal muscles

■     Autonomic division: Carries impulses from the CNS to heart muscle, involuntary muscle & glands

○     Efferent Division

■     Motor Division - Fibres that carry impulses away from CNS

■     Somatic division:
Takes impulses from the CNS to skeletal muscles

■     Autonomic division: Carries impulses from the CNS to heart muscle, involuntary muscle & glands

■     Responsible for control of the body’s internal environment & is involved in many mechanisms to keep internal environment constant

■     Usually operates without conscious control

■     Regulated by group of nerve cells in Medulla Oblongata, Hypothalamus & Cerebral Cortex

■     Auto means ‘self’, nomo means ‘govern’ = Autonomic System is self governing

■     ANS Control

■     Nerve fibres of ANS make up part of spinal nerves & part of some of cranial nerves

■     Carry impulses to heart muscle, internal organ muscles & glands

■     2 motor neurons involved in autonomic pathway.

■     One neuron has its cell body in the CNS, whilst the other has its cell body in a ganglion

■     1 motor neuron in somatic pathway

■     Parasympathetic vs Sympathetic Division

■     Parasympathetic: generally produces responses that maintain body during relatively quiet conditions. Nerve endings release acetylcholine

■     Sympathetic: produce responses that prepare body for strenuous physical activity. Fight-or-flight response. Nerve endings release noradrenaline

■     Under normal circumstances we are not aware of the activities of ANS

■     Right now your sympathetic & parasympathetic nerves are sending out impulses to the internal organs to maintain stability of body functions

■     Eg. Heart with only sympathetic input -  rate of 100 beats per minute, parasympathetic stimulation keeps it down to around 70 to 80 beats per minute

■     In threatening situation, balance between sympathetic & parasympathetic stimulation is changed, sympathetic becomes dominant

■     Fear, stress, anger, danger, competition all evoke fight or flight response or alarm reaction

■     Prepares body for increased activity