A neuron is the functional unit of the nervous system. It consists of a cell body containing a nucleus, and cytoplasm, the dendrites and the axon. The axon or the nerve fibre and dendrites are extensions of the cell body. The dendrites are short branched fibres while the axon is a single long and slender fibre. Dendrites carry impulses to the cell body while the axons carry impulses away from the cell body.
CLASSIFICATION OF NEURONES
Three classes of neurones are;
1. Sensory or afferent neurones.
2. Intermediate neurons.
3. Motor or efferent neurons.
FUNCTIONS OF NEURONS
Neurons are concerned with receiving and relaying nervous impulses in reflex and voluntary actions.
1. SENSORY NEURONS: These transmit impulses from the receptors such as the eyes, skin, heart, liver, etc. to the brain and spinal cord.
2. MOTOR NEURONS: These transmit impulses from the brain and spinal cord to effectors such as muscles and glands.
3. INTERMEDIATE NEURONS: This connects a sensory neuron with a motor neuron and thereby transmit impulses from the sensory neuron to the motor neuron.
TRANSMISSION OF NERVE IMPULSES
MECHNISM OF TRANSMISSION OF IMPULSES
The transmission of impulses along the fibre is characterized by changes in electrical potential. At rest, the outside of the nerve is positively charged and the inside is negatively charged. Chemically, the nerve fibres have potassium ions concentrated inside the fibre while sodium ions are concentrated more on the outside.
When the fibre is maintained, there is a change in the permeability of the cell membrane. Sodium ions enter the nerve while potassium move to the outside. The change permeability along the nerve or the change in electrical potential along the fibre causes the movement of electrical current along the fibre. In this way, the impulse is propagated or transmitted. The original resting stage of the nerves is restored as soon as the impulses pass.
TRANSMISSION OF IMPULSES ACROSS THE SYNAPSE
Transmission of impulses across the synapse occur by chemical means. When an impulse arrives at a synapse, a chemical substance acetylcholine is released. This diffuses across the gap and causes the excitation of the post-synaptic cell. Thus the impulses are transmitted across the synapse.
REFLEX AND VOLUNTARY ACTIONS
When the nerve endings of a sense organ is stimulated by touching a hot object per say, impulses are transmitted through the sensory nerve passing through the dorsal root into the spinal cord. In the spinal cord, the impulses are relayed via the synapse into the intermediate neuron through another synapse to the motor or effector neurone. This passes through the ventral root, to the effector organ which brings a response. In the above case mentioned, the muscles contract and relax. The object is quickly dropped and the hand is withdrawn.
In a simple reflex arc, the impulse follow the path as indicated below.
Receptor (Skin) ————– Sensory neuron ———— Muscle in intermediate.
Neuron (CNS) —————Effector or Motor neurone ————-Effector (Muscle)
Examples of reflex actions are sneezing, jerking of the knee, blinking and covering the eyes, banging of fits on the table.
A voluntary action is one which can be controlled at will and hence it’s controlled by the brain.
Reflex action contrasted with Voluntary action
Reflex action is controlled by the spinal cord while voluntary action is controlled by the fore brain
The reflex action mediates an unconscious response to stimuli while the voluntary action is controlled at will.
The response of reflex action is quick while that of the voluntary action is more delayed.
The path of nervous impulse in reflex action is shorter but that of voluntary action is longer.
In reflex action, its action is initiated by stimulation while that of voluntary action is without stimulation.
The conditioned reflex is a behaviour pattern which is gained through learning and once acquired, can be performed without thinking. The roles of conditioned reflex on behaviour is as follows;
Conditioned reflexes enable us to acquire skills in performing our daily activities.
The training of animals involves conditioning them to certain routines. An example is association. For example, dogs can learn to associate the sound of bell with food. Thus, dogs produce saliva each time the bell is rang.
Examples of conditioned reflexes.
Much of our actions and behaviours are conditioned reflexes. These include;
4. Riding a bicycle.
DIFFERENCES BETWEEN REFLEX ACTION AND CONDITIONED REFLEX
1. Reflex actions are inborn behaviour while conditioned reflex are learned behaviour.
2. Reflex action nervous pathway is simple while that of conditioned reflex pathway is comparatively complex.
3. Reflex action is controlled by the spinal cord but conditioned reflex is controlled by the brain.
4. Reflex action stimulus and response are related while in conditioned reflex, the stimulus and response need not be related.
5. In reflex action, its impulse involved cannot be controlled but in conditioned reflex, the impulse involved can be controlled.