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14.10: Nervous System (Summary)
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The
neuron
is the basic unit of the nervous system. It consists of a
cell body
with a nucleus, filaments known as
dendrites
and a long fibre known as the
axon
often surrounded by a
myelin sheath
.
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A
nerve
is a bundle of axons.
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Grey matter
in the brain and spinal cord consists mainly of brain cells while
white matte
r consists of masses of axons.
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Nerve Impulses
travel along axons.
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Adjacent neurons connect with each other at
synapses
.
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Reflexes
are automatic responses to stimuli. The path taken by nerve impulses involved in reflexes is a
reflex arc
. Most reflex arcs involve 3 neurons - a
sensory neuron
, a
relay neuron
and a
motor neuron
. A stimulus, a pin in the paw for example, initiates an impulse in the sensory neuron that passes via a synapse to the relay neuron situated in the spinal cord and then via another synapse to the motor neurone. This transmits the impulse to the muscle causing it to contract and remove the paw from the pin.
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The nervous system is divided into 2 parts: the
central nervous system
, consisting of the brain and spinal cord and the
peripheral nervous
system
consisting of nerves connected to the brain and spinal cord. The
autonomic nervous system
is considered to be part of the peripheral nervous system.
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The brain consists of three major regions: 1. the
fore brain
which includes the
cerebral hemispheres
(or
cerebrum
),
hypothalamus
and
pituitary gland
; 2. the
hindbrain
or
brain stem
containing the
medulla oblongata
and 3. the
cerebellum
.
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Protective membranes known as the
meninges
surround the brain and spinal cord.
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There are 12 pairs of cranial nerves that include the optic, olfactory, acoustic and
vagus
nerves.
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The
spinal cord
is a cable of nerve tissue surrounded by meninges passing from the brain to the end of the tail.
Spinal nerves
emerge by a
ventral
and
dorsal root
between each vertebra and connect the spinal cord with organs and muscles.
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The
autonomic nervous system
controls internal body functions not under conscious control. It is divided into 2 parts with 2 different functions: the
sympathetic nervous system
that is involved in the flight and fight response including increased heart rate, bronchial dilation, dilation of the pupil and decreased gut activity. The
parasympathetic nervous system
is associated with decreased heart rate, pupil constriction and increased gut activity.