Nervous System

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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.

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.


Brain

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.

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.

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