What is the nervous system?
The function of the nervous system is to control most of the body functions, especially rapid functions like muscular contract and visceral events (e.g- defecation, micturition). Long term control of metabolism is by hormones produced by the endocrine system.
1. General Design of the Nervous System
2. Sensory and Motor Divisions
The nervous system has 2 main divisions:
1. The sensory division, which receives sensations (afferent).
2. The motor division, which sends out signals for various functions such as muscular contraction (efferent).
The integration and processing of incoming sensations are necessary for the proper motor response.
3. Somatic and Autonomic Divisions
The nervous system can also be subdivided into:
1. The somatic division —which deals with external body functions, e.g. sensations from the skin and contraction of voluntary skeletal muscles.
2. The autonomic or visceral division —which deals with sensations from internal organs (e.g. intestines) and contraction of smooth muscles.
4. Higher Functions
The storage of information is memory. Sensory impulses received by the brain from the sensory division are perceived and interpreted (integrated) with past memory and experiences. The interpreted sensation is added on and stored in memory.
5. Major Levels of Central Nervous System Function
Spinal Cord Level
(a) Transmission of sensory impulses from the body to the brain.
(b) Transmission of motor impulses from the brain to the body (muscles).
(c) Reflex action at different levels.
(d) Reflexes of the autonomic nervous system to maintain tone of blood vessels, intestines and other hollow tube-like organs.
The Lower Brain Level
It consists of the stem of the brain and contains all the vital centers, which function subconsciously to maintain life:
(a) Vasomotor center —for maintaining blood pressure.
(b) Respiratory center —for maintaining respiration.
(c) Cardiac center —for adjusting the heart rate.
(d) Temperature regulation center —to maintain optimum body temperature.
(e) Salivation, swallowing and feeding center.
(f) Vomiting center
(g) Centre for emotions, e.g. anger, excitement, sexual urge.
The Higher Brain Level
These higher functions require the integration of the actions of the nerves of the brain itself. The number of nerve cells which are required for each function, e.g. thought, may be more than many billions of nerve cells.
6. The Nerve Cell and Transmission of Impulse
1. Thee nerve cell has a long tail called the axon. Impulses from the nerve cell pass down the axon which comes in contact with either another nerve cell, muscle or organ.
Nervous System 81
2. The axon of a nerve cell contacts the dendrites or pointed spikes of the second nerve cell. This contact point is called a synapse or a nerve-nerve junction.
3. The axon of the first nerve cell does not actually touch the dendrite of the second nerve cell. is a small gap. The impulse in the axon of the first nerve cell cause release of the chemical neurotransmitter substance, acetyl-choline. Acetylcholine combines with a specific receptor on the dendrite of the second nerve cell. Changes in permeability and migration of calcium and sodium ions cause an action potential to be created in the second nerve cell thus causing excitation of the second nerve cell. The acetylcholine in the nerve-nerve gap, after having served its purpose, is the destroyed by the enzyme cholinesterase.
4. Although acetylcholine is the most important chemical transmitter, there are others:
(a) epinephrine, norepinephrine, dopamine, serotonin.
(b) gamma-amino-butyric-acid (GABA).
(c) peptides including hormones, encephalins, angiotensin II, bradykinin and many other substances.
7. Sensory Receptors
Special senss: Eyes —Vision
Speciai receptors: Carotid and aortic bodies
Blood O2 concentration
Brain—Blood CO2 concentration (medulla)
Brain—Blood glucose (hypothalamus)
Brain—Osmolarity of blood
8. Sensory Pathway
1. Sensations arise in the sensory receptors.
2. They excite the peripheral which are long dendrites of nerve cell in the spinal cord.
3. The axons of the nerve cells ascend (go up) in the spinal cord and reach the brain. They synapse in a part of the brain (thalamus).
4. From the thalamus they reach the cortex of the brain (surface nerve cells) where they are perceived, and their nature interpreted.
5. There are 2 types of sensations:
(a) those which are felt consciously, e.g. pain, touch.
(b) those which are subconscious, e.g. sense of position of the body.
6. The special senses like vision, hearing are in a different class.
1. This type of pain arises in the pain receptors in the skin. subcutaneous tissue, muscle, bone, etc.
2. The chemical substance which stimulates pain receptors are prostaglandins serotonin, histamine.
3. Many -analgesic (pain relieving) drugs e.g. aspirin, inhibit the action Of prostaglandins and relieve pain.
4. Pain is feit in a special part of the brain called the thalamus. At the thalamus the pain carrying nerve fibres synapse, chemical transmitter is the prostaglandin, and the sensation of pain is carried to the cortex.
5. The pain fell is appreciated accurately by the cortex by reference to memory and past experience. e.g. mild or severe, pricking, burning, pressure, etc.
1. Visceral pain arises from internal organs. Unlike somatic pain which is specific and localized, visceral pain is generalized and diffuse.
2. There are no special pain receptors in viscera. Pain results from:
(a) Ischemia—insufficient blood supply, e.g. heart pain.
(b) Chemical, e.g. peptic ulcer
(c) Spasm of smooth muscle, e.g. intestinal colic.
3. Visceral pain is sometimes felt as superficial or somatic pain. The site of such pain called “referred pain” is from the same segment of the spinal cord which receives nerve fibres from the visera, e.g. heart pain.
10. The Motor System
1. Motor impulses arise in the cells of the cerebal cortex (motor area) and travel downwards to reach the motor cells of the spinal cord
2. They synapse and the axons of the motor cells of the spinal cord, reach as peripheral nerves to muscles.
3. The motor cells of the spinal cord receive also the axons of other nerve cells in the brain. These nerve cells are concerned with co-ordination and tone. The cells for co-ordination and tone lie in the brain as clumps of cells (basal ganglia and cerebellum).
4. The movement of muscles is directed by impulses from the motor cortex, and co-ordinated by the basal ganglia and cerebellum.