All psychological processes are responses of individuals to stimulations. This fact is often represented by the formula S-O-R, where S represents the stimuli (plural form for stimulus), O is the organism and R is the organism’s response. The sequence is as follows: (1) a stimulus strikes a receptor; (2) the receptor stimulates the dendrites of a sensory neuron making a nerve impulse travel to the afferent or incoming fiber to the end brush; (3) the impulse crosses the synaptic connection or junction and stimulates the dendrites of a connector neuron in the spinal cord; (4) the impulse passes another synapse from the connector neuron to the dendrites of an efferent or outgoing neuron; (5) the impulse passes along the efferent fiber to its end brush; (6) an effector – a muscle or gland – responds.
There are number of known processes of neural circuits and networks. Among them are the following:
Synaptic Summation – The impulse reaching a synapse from a single fiber may not bridge the synapse but two or more impulses arriving within a very brief period may cause a response. For example, when one stimulus is applied to one area of the skin and will not produce a response, a second stimulus applied nearby and almost simultaneously will carry the impulse across the synapse. With two impulses arriving within 15 milliseconds of one another, there is a summation of effects which produces a response.
Alternative Nerve Pathways – A chain of efferent neuron, one connector neuron and one efferent neuron functions to carry a neural message. Hundreds of nerve fibers are involved in even the simplest sensory-motor arc. Each of the many incoming fibers can make connections with many efferent fibers. A proliferation of possible pathways from stimulus to response is responsible for a variety of responses to the same stimulus. Many of these responses will be present particularly if the connector neurons of the higher brain centers are involved.
Reverberation – The nervous system does not always simply transmit a message and lie idly expectant and ready for a next stimulus. Because of the complex arrangement of neuron in the central nervous system, in many cases, the incoming impulse travels over the involved neural network and comes back again to restimulate the neuron originally bringing the impulse. The circular networks make possible a reverberation, which may continue long after the initial stimulus is removed. This can cause a motor response to continue and persist for a long time.
Temporal Summation – Summation may occur if the second stimulus is applied within 15 milliseconds after the first. There is recruitment, a process similar to summation, when a repeated stimulus sets off a response which a single stimulus failed to do. Repetitive stimulation brings additional pathways into action.
Reciprocal Innervations – This refers to the automatic process whereby there is simultaneous excitation of a set of motor neurons and inhibition of another. In many physical movements, there are two opposing sets of muscles. For the arms and legs, for example, the extensor muscles must tighten when the flexor muscles relax. If the action is to bend, the opposite happens. Through the mechanism of reciprocal innervations, only one set of muscles is activated at one time. Adjustive movement becomes possible.
Irradiation – as the strength of a stimulus is increased, even in a stimulus to a reflex arc, there is an involvement of more sensory connecting and effect fibers; and therefore, a more extensive response. The number of muscles used is increased because more sensory fibers are activated.
Timing – It takes time for neural impulses to travel along a fiber and more time for a synaptic connection. The amount of time involved in a simple sensory-motor process increases with the distance the impulses have to travel, and more significantly, with the number of synapses between the stimulus and the response. Simple reflex actions are very rapid, while more complex responses require a longer time.
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