The functions of neuron are irritability and conduction. Irritability is the response of an organism to the stimulus. Conduction is the transmission of nerve impulse through the neurons of sensory and motor nerves. The external stimuli are light, temperature, chemicals, electricity, sound, taste etc. These are first received by sensory cells. Then transmitted to the central nervous system (Brain, spinal cord) through the sensory nerves. The neurons of brain decide the action that the organism has to take. These decessions are passed to the effectors, muscles and glands for proper action.
The stimuli never enter into neurons directly. These enter into first sensory cells and then into neurons through the dendrites. But the external stimuli are changed into nerve impulses before entering into neuron from sensory cells.
NERVE IMPULSE DEFINITION
The nerve impulse can be defined as the change in the physical and chemical nature of the neuron when the neuron is subjected to the external stimuli.
The transmission of impulses through the neuron is called propagation or conduction of nerve impulses. The impulse transmission is by electrical potential and wave like.
The neurons are two types based on presence and absence of myelin sheath. They are myelinated and non- myelinated neurons. The transmission is different in the myelinated and non- myelinated neurons. The impulse transmission is slow in the non myelinated neuron. Whereas the nerve impulse transmission is very fast in the myelinated neuron.
1. NERVE IMPULSE PROPAGATION IN NON MYELINA TED
The propagation of nerve impulse can be studied in four stages. These are
i. Membrane Potential ii. Depolarisation
iii. Action potential iv. Repolarisation.
i. MEMBRANE POTENTIAL:
It explains the potential charge of neuron when the neuron is in resting state. Hence membrane potential is also called resting potential. When the neuron is in resting state, the number of sodium ions out side the neuron is more when compared to the numberof potassium ions. So, there is a positive charge around the neuron. Where as the number of potassium ions in the axoplasm is more than the number of sodium ions. In the axoplasm, in addition to these ions, there are many negatively charged impermeable proteins. Their number is 70 more than the number of positive ions. So, when the neuron is resting state, the membrane potential in the axoplasm is - 70 m.v (m.v - milli volts).
in the resting stage of neuron, sodium ions move out from axoplasm. Where as potassium ions move from out side into the axoplasm. This movement of sodium and potassium is called 'Na - k' pump.
Whenever the stimulus touches to neuron, sodium ions move into the axoplasm. When 70 sodium ions enter into the axoplasm, the potentiality becomes 'O' m.v. This process is called depolarisation.
iii. ACTION POTENTIAL:
It explains the potentiality of the axoplasm when the nerve impulse is passing through the neuron. The movement of sodium ions flowing from out side into the axoplasm continues until the axoplasm gets + 30m.v. So, the action potential is + 30 m.v. In this stage impulse moves through the neuron. But the depolarisatin and action potential never occur through out the length of the neuron at one time. The action potential occurs wave like.
It explains the alteratin of action potentiality to membrane potentiality. In repolarisation, the +30 m.v action potential is changed to - 70 m.v. membrane potential. In this, the sodium ions 100 move out from axoplasm. Hence axoplasm gets - 70 m.v.
2. ALL OR NONE LAW It explains the speed of movement of impulse in the non-myelinated neuron. The minimum amount of stimulus required forthe transmission nerve impulse is called threshold stimulus. When the stimulus is below the threshold stimulus, impulse transmission does not take place. Even if the stimulus is more than the threshold stimulus, the speed of impulse transmission is always same if the stimulus is the threshold stimulus or more than the threshold stimulus.
3. NERVE IMPULSE TRANSMISSION IN THE MYELINATED NEURON
In the myelinated neuron, the axon is surrounded by the thick white impermeable myelin sheath. In this neuron, nerve impulse transmission is different. It is called saltatory conduction. As the axon is surrounded by myelin sheath, there is no flow of sodium and potassium ions across the myelin sheath. But at the Ranvier nodes, membrane potential, action potential, and repolarisation occurdue to the absence of myelin sheath. In the myelinated neuron, nerve impulse Jumps from one Ranvier node to the next node. So, the impulse transmission is very fast.
4. SYNAPTIC TRANSMISSION
Synapse is the space between the telo dendrites of one neuron and dendrites of succeeding neuron. The neuron in front of synapse is called Presynaptic neuron and the neuron behind the synapse is called post synaptic neuron. At synapse the nerve impulse transmission is by acetyle choline.
The tips of telodendrites of pre synaptic neuron contain acetyle choline secreting knobs. When the impulse reaches to the actyle choline knobs, the acetyle choline breaks from the knob and drops into the dendrite of post synaptic neuron. After the impulse passes the synapse, the acetyl choline is hydrolysed by an enzyme called choline esterase.