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
Sample Pages PDF ExtractThis is not the complete chapter Sample Pages PDF Extract
This is not the complete chapter