GENERAL SENSORY INTENSITY MODULATION
In general impulses travel along an axon at a uniform speed and amplitude. This generalization also applies to a train of impulses. A train of impulses is a group of impulses evenly spaced that code for a specific stimulus. Generally a large depolarization event that exceeds threshold, causes the trigger region to fire multiple action potentials in response to a single stimulus (the one that caused the large depolarization). Thus, in general, a more intense stimulus causes a larger magnitude depolarization, which in turn causes an impulse train, as opposed to a single impulse. Similarly large stimuli will then cause trains in which impulse magnitude and pattern are conserved. In peripheral neurons, the effector response is increased with increased rate of impulse transmission. Thus a train of 10 impulses produces a smaller response than a train of 20 impulses. The relation ship between signal intensity and firing frequency is generally thought to be a linear relationship. The process by which more intense stimuli cause longer trains is one part of a process known as encoding. It is in this way that large depolarizations cause longer trains, and this is percieved as a more intense stimuli. Most neurons in some way use this method impulse patterning to differentiate between various magnitudes of stimuli. However some CNS neurons have shown a decrease in impulse rate when given larger stimulations.
It can now be easily deduced that the amount of time between stimuli is one of the limiting factors in neural stimulus interpretation. Due to the time required to propagate a given train, very fast repetitive stimuli may be interpreted as a single steady stimulus. For example light flashes at 50 flashes per second are generally interpreted as a single light (the basis behind movies in which pictures are flashed at the rate of 48 per second). To look at this phenomenon in more detail, we'll say that a given flash of intensity X, causes a train of 15 spikes per stimulation (remember the number of spikes in the train is a reflection of stimulus intensity, not rate). The time it takes the axon hillock to stimulate this train we will call T. If the flashes come at a rate of T or greater then as soon as one train has left the axon hillock, there is very little time before another is immediately started. Thus instead of multiple trains spaced out over various times, one gets what appears to be one continuous signal. Thus stimulations at rates faster than T are percieved as a single stimulus. Because of this encoding is considered a limit of nervous system fidelity.
ADAPTATION
Sensory neurons are subject to a phenomenon called adaptation. It is something that everyone has experienced. For instance a person walks into a room and notices that the temperature is warmer than usual (by a few degrees). A few minutes later that same person no longer realizes that the room is at a higher temperature. As the person became aclimated to the given temperature, his nervous system underwent the adaptation process. Now the room temperature is no more noticeable than any other neurolgical "noise". A more scientific explanation of
adaptation is as follows: A stimulus of constant intensity produces a sensation (our perception of the stimulus) which declines with time. This decline in sensation may be neurologically explained by the alteration of the sensory output, via spike pattern modulation. During constant stimulation impulse frequency decreases with time. The fact that the decrease in impulse frequency coincides with and individuals decrease perception of the stimuli lends strong support to the idea that impulse frequency codes for stimulus intensity. Adrian and Zotterman conducted adaptation experiments on frog muscle. They summarized their results in this single sentence "The impulses set up by a single end-organ occur with a regular rhythm at a frequency which increases with the load on the mucsle and decreases with the length of time for which the load has been applied." This statement is considered the central core of adaptation studies.
The term accomadation is often improperly used interchangeably with the term adaptation. Acommodation properly refers to one of two processes; the gradual increase in action potential threshold when the stimulus is increased slowly, or in the visual system, accomadation refers to the adjustment of the optical system to varying image distances (Partridge and Partridge, 1993). Once complete adaptation occurs, the point at which there is no perception of stimuli, the receptor will return to it's resting output. It will however begin to respond again when there is a change in the stimulus. The magnitude and speed of this change in stimulus will cause effect the neurons response time and output level accordingly.
Adaptation has many evolutionary as well as pratical applications. What it does is it limits the amount of processing needed to be done by the brain, allowing other more important stimuli to be percieved and acted upon. There are many examples in the sensory system which promote the idea that the stimulus itself is not necessarily the most important aspect of sensory perception, rather the changes in that stimulus are most important. This allows an organism to detect and respond to changes in our perception more rapidly by limiting the sensory noise. For exampel is it extremely important to notice every detail in the dark, or is more important to notice a change in that perception, of lets say, a quickly moving object. Obviously it is more important to notice the change in the stimulus as that object could be a predator, or even food; either way quick recognition of and response to a change in stimuli has definate life preserving benefits.
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