The Central nervous system consists of the brain, the spinal cord, and
the body’s nerve network. This complex system is based on one kind
of cell the neurons. The brain, the mass of tissue inside the head,
has the greatest number of these cells, most of which are in it’s
outer part, the cerebrum. Below the cerebrum is the cerebellum and the
brain stem, which is linked to the spinal cord.
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The Brain: The brain has been compared to a giant telephone
exchange or to a computer. It functions as both, handling incoming and
out going calls, and making decisions, as diverse as whether to laugh
or cry and whether the temperature of the body should be higher or
lower, on the basis of information fed into it.
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Cerebrum: The brain’s most obvious external features are two
soft hemispheres, which make up the cerebrum. These hemispheres make
up 70% of the whole brain and nervous system. They are “mirror
images” of each other, and each is chiefly concerned with the
movements and sensations of only one side of the body. Sensations on
the right side of the body and the control of the muscles on that side
are functions of the left hemisphere, and vice versa. It consists of 2
layers: (1) outer cortex or grey matter - which is the decision maker
of the brain, (2) Inner layer of white matter - made up of nerve
fibers.
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Cerebellum: The cerebellum functions below the level of
consciousness. It is concerned with balance, and is the center for the
co-ordination of complex muscular movements
Brain Stem: Links the spinal cord to the brain. They lie below
the cerebral hemispheres.
The Spinal Cord: The spinal cord is the body’s main nerve
trunk-a cylinder of nerve tissue 18 inches long about as thick as a
man’s little finger. It runs down the back from the medulla
oblongata, at the base of the brain. It is enclosed in a set of 3
membranes, similar to those surrounding the brain. Between the layers
of membranes, Cerebro-spinal fluid acts as a cushion, to protect the
cord from damage.
Nerve Fibres: The spinal cord is a column of nervous tissue,
which is spread throughout the body; they carry impulses to and from
the brain. Nerve fibres from the brain and spinal cord are bundled
together to form 12 pairs of cranial nerves, connected to the brain
and 31 pairs of spinal nerves
Spinal Nerves:
1) CERVICAL NERVES - (8 pairs) serve mainly the arms.
2) THORACIC NERVES - (12 pairs) lead to the sternum, internal organs
and muscles of the chest.
3) LUMBAR NERVES - (5 pairs) serve the abdominal wall and legs.
4) SACRAL & COCCYGEAL NERVES - (6 pairs) lead mainly to the legs.
Cranial Nerves:
The brain has links with the sense organs and the muscles of the head
by means of 12 pairs of cranial nerves
1) OLFACTORY: - sense of smell
2) OPTIC: - sense of sight balance
3) OCULOMOTOR: - Focusing, regulating the size of the pupil, balance
4) TROCHLEAR: - movement of the eyeball.
5) TRIGEMINAL: - Chewing, sensation from the face
6) ABDUCENT: - movement of eye, sense of taste
7) FACIAL: - movements of facial expression
8) VESTIBULOCOCHLEAR: - maintenance of balance, sense of hearing
9) GLOSSOPHARYNGEAL: - secretion of saliva, sense of taste, movement
of pharynx
10) VAGUS: - movement and secretion
11) ACCESSORY: - movement of the head, shoulders, pharynx and larynx
12) HYPOGLOSSAL: - movement of tongue.
Autonomic Nervous System: The autonomic system controls glands,
such as the salivary glands, and the internal organs-the bladder,
heart, intestines, liver, lungs and sexual organs. Nearly all the
actions of the autonomic system are outside voluntary control eg. You
cannot normally “will” your heart to beat faster; but if you are
given a fight, your pulse involuntarily speeds up.
The autonomic division of the nervous system consists of two opposing
parts,
>>The sympathetic;
>>The parasympathetic; which operate below the level of
consciousness
>>The sympathetic nerves: - Through the sympathetic nerves, the
brain mobilizes the body for action to meet possible danger.
1) IRIS- Changes size, when someone is frightened or angry, the brain
stimulates the sympathetic nerves to their part of the eye, causing
the pupils to open wide.
2) SALIVARY GLANDS- produces less saliva, so that the mouth goes dry.
3) LUNGS & WINDPIPE- are affected under stress; breathing becomes
faster, so that the body gets more oxygen.
4) HEART- pumps faster, during times of fear & anger. Normally you
are unaware of the beating of the heart, but its increased activity in
times of excitement raises the blood pressure, pumping more blood to
supply energy for muscles.
5) ADRENAL GLANDS- at the top of the kidneys secrete the hormone
adrenaline, which prepares the body to fight or run away
6) LIVER-releases glucose under emotional stress, providing extra
energy for muscles.
7) STOMACH & INTESTINES- have their blood diverted to the heart,
CNS and muscles, so that they can operate under stress. The wave like
movements of the intestinal walls stop, and the various sphincters
close. ·
Parasympathetic Nerves: - are concerned with restoring the body to
peaceful activity after an emergency
1) Heart - slows down & the blood pressure falls after the danger
is over.
2) Bladder - can be contracted and it’s sphincter may open, causing
urination.
