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The PNS (Peripheral Nervous System)

The Peripheral Nervous System is outside of the brain and spinal

cord and includes nerves and ganglia (a group of nerve cells).

The PNS is divided into three nervous systems: (i) SNS (Somatic

— involved with skeletal muscle); (ii) ANS (Autonomic) and ENS

(Enteric intestinal).

How it Works

We  think   to  do  something   via our five    senses, our feelings    and 

kinesthetic sense (e.g. pick something up, sit down, cook a meal,

run for your life or kick that soccer ball). The brain receives our

messages and promptly

delivers them to the ap-

propriate set of nerves

through electrical im-

pulses. These impulse

messages move along

an axon inside a neuron

traveling from the brain

or spinal cord to a group

of skeletal muscles or

elsewhere in the body.

The impulses reach

their destinations and

stimulate the area. They

spread out across the

area they are servicing creating cellular electrical, chemical reac-

tions that stimulate the area to respond appropriately to the initial

stimulus (e.g. the skeletal muscle will contract or relax, the eye

will blink).

Motor Unit

The motor unit is the basic functional unit of the skeletal muscle.

It  consists    of  a   motor   nerve   and the group   of  muscle  fibers  it  

stimulates. Motor   units   can have    three   to  3,000   muscle  fibers. 

In the gastrocnemius there are 580 motor units and 1,030,

fibers! The connection  point   between the motor   nerve   and the 

individual  muscle  fiber   is  called  the neuromuscular   junction.   

All or None Principle

In  response    to  nervous stimulation,    a   muscle  fiber   will    contract    

to its maximal potential (under existing conditions) or not at all.

When    a   motor   unit    is  stimulated, all the muscle  fibers  within  that    

unit will contract.

Muscle Fiber Recruitment

When more muscle power is needed to carry out a task (e.g. lift

a heavy weight), the body can recruit more motor units and/or in-

crease the frequency of nervous stimulation to the existing units

CNS PNS

Spinal Cord

or Brain

Sensory Neurons

Motor Neurons

Receptor

Skeletal

Muscle

Chapter 4 Anatomy and Physiology of the Muscles

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This is not the complete chapter