Provide anatomy and
physiology advice to clients
Organisation of the nervous
system
Functions
• The nervous system receives sensory
input from millions of sensors, it processes
input and decides on action. Action
include activating muscles which is called
motor output.
Components
• The Central Nervous System (CNS)
consists of the spinal cord and brain. In
this course we only briefly mention brain
activity.
• The Peripheral Nervous System (PNS) is
composed of all other nerves outside the
CNS and that includes our muscles, so
this must be a focus for our study.
Peripheral Nervous System (PNS)
• The PNS consists of sensory or afferent nerves
which conduct information from the sensory
receptors to the CNS. The sensory receptors are
located throughout the body. We will look at
these shortly.
• The other component of the PNS is the Motor or
efferent nerves. These carry information from
the brain to organs such as muscles. Each
motor neuron has its cell body located in the
brain stem or spinal cord grey matter and
projects its axon out to the muscle that it
controls.
The Motor Division
• The motor division consists of the somatic
nervous system which contain nerves
under our voluntary control and the
Autonomic Nervous System which contain
nerves controlled involuntary.
Autonomic Nervous System
• Autonomic Nervous System (ANS), which
consists of visceral motor nerve fibres. These
regulate the activity of smooth muscle, cardiac
muscle and glands. We only mention the
actions of the ANS in passing as these nerves
are part of the involuntary system. Fortunately
for us we don’t have to think about breathing or
controlling our heart rate as the ANS looks after
these things independently.
Sympathetic and Parasympathetic
Divisions
• The Autonomic Nervous System consists
of the Sympathetic division, which
stimulate organs to mobilise energy, and
the Parasympathetic System. This
influences organs to restore to equilibrium.
The Motor Neuron
• Where Efferent nerves join with the muscle that it
innervates, it loses its myelin sheath and divides into fine
terminal branches, fusing with the muscle at what is
called the end plate or neuromuscular junction. You can
see in this slide how the motor nerve has branched and
ends as end plates on muscle fibres. Motor end plates
are larger on fast twitch fibres than slow twitch fibres.
Look closely at the end plates and you will see swelling.
These swellings contain vesicles of acetyl choline and
release of these promotes the action potential being
propagated along the muscle fibre.
Muscular contraction
• Each muscle has at least one nerve attached to it and
where the nerve enters the muscle it branches into a
number of axon terminals which all join to a single fibre.
We saw this in our last session. A motor neuron and all
the muscle fibres that it innervates is called a motor unit.
Now when a motor neuron fires, all the fibres that
innervates respond by contracting. Large weight bearing
muscles have large motor units. This means that there
may be several hundred muscle fibres per motor unit.
Muscles that exert fine control such as moving the
fingers have small motor units with only a few fibres.
You can see a motor unit in this slide.
Sensory Receptors
• Throughout our body we have millions of
sensory receptors. It is their job to monitor our
environment and tell our brain when things have
changed. When we touch something hot for
instance our brain is very quickly alerted to this
change and makes us pull our hand away as a
protective mechanism. There are five types of
sensory receptors and each has a different
structure and role to play, but their job is always
monitoring information and telling the CNS about
changes.
Types of sensory receptors
• Mechanoreceptors generate nerve impulses when they
are deformed by mechanical forces such as touch,
pressure, vibration and stretch. They can be on the
outside of our body or on the inside. You might recall
that presence of food in the oesophagus changes its
shape and stimulates a mechanoreceptor to send a
message to the brain to start peristalsis. One on the
outside of us is called Meissner’s corpuscle and this
picks up very light touch. If you touch your skin very
lightly you can still feel the touch thanks to this receptor.
There are many of these just under the skin of your
fingers, toes and on non hairy parts of your body. Each
one can sense light touch for about 8mm all around it.
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Other Sensory Receptors
• Thermoreceptors are very sensitive to
temperature change. They are free dendritic
nerve endings and exist in most body tissue.
When temperature changes they send a
message to the brain. Photoreceptors are very
specialised receptors that respond to light
energy and are located in the retina of our eye.
There are millions of chemoreceptors right
throughout our body that constantly sense our
chemical composition and act if it changes from
equilibrium. Finally we have nociceptors in our
body that respond to painful stimuli.
Proprioceptors
• Of particular interest to exercise scientists are
the proprioceptors. These occur in skeletal
muscles, tendons, joints, ligament and other
connective tissue. They advise the brain about
our movement by monitoring the degree of
stretch in the organs that they occupy. Our
brain must continually be advised about our
muscle activity and proprioceptors send this
information. By constantly sensing movement
they tell our brain about our orientation in 3
dimensions.
Proprioceptors
• Proprioceptors include muscle spindles
found in skeletal muscle and are made of
modified skeletal muscle fibres. Another
proprioceptor is Golgi tendon organs.
These are found in tendons near to where
the muscle joins the tendon. They consist
of small bunches of tendon fibres in a
capsule. Around these are dendrites that
coil around the fibre. And joint
kinaesthetic receptors are located in joints.
Proprioceptor action
• Muscle spindles are found in all skeletal muscles
that are controlled by small motor units such as
the ones found in the hand. They have their
own sensory and motor nerve supply and
specialised muscle fibres that are called
intrafusal fibres. Their role is to respond to
changes in muscle length and tension. When
the muscle stretches it activates the sensory
receptors and sends signals to the spinal chord
and thence to efferent motor nerves that
innervate the muscle.
Detail of a muscle spindle
• These slides show detail of a muscle
spindle. You can see large nuclear bag
fibres. The bottom one is the part that
transmits signals via afferent fibres to the
spinal cord.
Activity
• Write a report advising clients about
different components of the nervous
system and proprioceptors.