1. When a nerve is subjected to a threshold stimulus it undergoes excitation or it is stimulated.
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There should stimulus is roughly constant for a given type of nerve fibre, but is higher in small fibres than in large.
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2. Normally nerve impulses pass in one direction only but they may be conducted in either direction too.
Law of forward conduction of James states that if a nerve is stimulated in the middle, the nerve impulses in the form of electrical changes are conducted to both the ends of the nerve fibre.
3. Generally impulses flowing in a nerve fibre do not spread with the neighbouring fibres because nerves or nerve fibres arc generally covered by an insulating nerve sheath called the medullary sheath.
4. After one impulse has passed along the nerve fibre, there is a refractory period during which the nerve fibre is neither excitable nor conductive.
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During this period a second stimulus, however, of large strength, remains ineffective. In this period the fibre recovers and after recovery it again responds to its maximum.
If an inadequate stimulus is followed by another one within a very brief period, say 0-5 ms, there may be a response, so that there can be summation of two inadequate stimuli.
The absolute refractory period is followed by a relatively refractory one, during which the threshold is increased.
5. The nerve impulses follow the “all or none law”, i.e., if any stimulus is an adequately effective one that is either of threshold strength or more, the nerve fibre responds to a great extent with an impulse of maximum intensity but if it is below threshold strength it is incapable to elicit any response whatsoever.
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Any stimulus bringing out a response in the nerve, the response cannot be increased further by increasing the strength of the stimulus.
6. The impulse takes a certain time to travel along the nerve fibre. The larger, the fibres the more rapidly they conduct, and medullated fibres conduct more rapidly than non-medullated.
Thus the speed or the velocity of nerve impulse differs in different animals. In frog it is about 30 metres per second at the room temperature and for the human motor and sensory nerves it is about 120 metres per second.
While crossing from one fibre to another there is some resistance at the synapse which breaks the speed of the impulse.
Some of the fast-moving animals have single long fibres called the giant-fibres— the impulses travel very fast through them.
7. In a nerve fibre the impulse is always conducted at a constant amplitude and velocity.
8. For regular conduction of nerve impulses a continuous supply of oxygen is required in order to liberate the energy and generally it is believed that nerves do not undergo fatigue, however, the continued stimulation in the absence of oxygen may result in certain disorders or irregularities.
9. Normally the nerve fibres are capable in sending enormous number of impulses.
10. The nerve impulses travel in a nerve in a quick sequence. Each nerve impulse, is of the same nature and is similar in all the nerves.
Registering of different sensations in the brain is not dependent on the nature of the impulse but on the area in which it is received.