After reading this article you will learn about the nature of nerve impulse.
It is now known that nerve impulse is electro-chemical. Whenever some stimulus impinges on a receptor, this introduces certain charges which are electro-chemical.
These charges are conducted by the nerves to the nerve centre and again the motor nerves relay back another nerve impulse which results in some sort of action. In the conduction of these nerve impulses there are certain peculiar features.
First, this process functions on all-or-none principle. That is, each nerve fibre is either stimulated fully or not at all. The stronger the stimulus, the greater is the number of fibres stimulated. A strong stimulus like a loud sound thus activates more number of fibres than a weak sound or stimulus.
Secondly, whenever a nerve fibre is stimulated, subsequent stimulation involves a greater time-gap between stimulation and activation. That is, if a nerve fibre is stimulated there immediately ensues a stage of refraction. The nerve fibre becomes totally inactive for some, time and this is followed by a period of partial inactivity.
The first phase is called absolute refractory period and the second phase is called partial refractory period. Thirdly, there is always a time-gap before the nerve fibre responds to a stimulus. This period is called the latency period. During the period of absolute and partial refraction, the latency period increases.
However, from the point of view of the experiencing individual there is no break in the experiencing of the stimulus; there is a continuity of experience. This is because different nerve fibres are stimulated in volleys. Each nerve is made up of a large number of fibres and these fibres are stimulated at different times. During the refractory period of one set of fibres, another set of fibres is activated. This accounts for the continuity in experience and also the continuous functioning of the receptor and effector organs.
Whatever the intensity of the stimulus, a minimum time gap is always involved between the occurrence of a stimulus and a response. The minimum time gap depends on a number of factors like the nature of the stimulus, its complexity, intensity etc. The minimum time-gap between the occurrence of a stimulus and the initiation of a response is called the reaction-time.
It includes the latency period, the time taken for the conduction of the nerve impulse and also the time taken by the nerve centre to send back a message to the effector organ. The more complex the stimulus, the more complex will be the response and, therefore, the longer the reaction-time. Reaction-time, to a certain extent, decreases with the repeated experience of a particular stimulus but is never less than a certain minimum.
The measurement of reaction-time was an important development in the history of psychology. Experiments on reaction-time were stimulated by the works of Helmholtz, Donders, Kulpe, Baldwin and others. In fact reaction-time experiments were one of the earliest in the history of experimental psychology.
Today, we differentiate between different kinds of reaction-time like simple, discriminatory and choice reaction-time. In simple reaction-time there is a single stimulus acting, releasing a nerve impulse which is conducted to a nerve centre and there is a single response. In discriminatory reaction-time one of a few possible stimulations is present at that time and there is a response.
The person responds to one and not the other. In choice reaction-time the individual responds to different types of stimuli with a different response to each of the stimuli. Choice reactions take a longer time than discriminatory reactions which in turn take a longer time than simple reactions.