Nerve impulses are described as an all-or-nothing response. The threshold value of a neurone is the level of stimulation that is needed to depolarise it and create an action potential. If a stimulus falls short, then no impulse is generated at all. If a stimulus exceeds the threshold value, then an action potential is generated, which is independent of how much the stimulus exceeded the threshold.
The brain can interpret the size of a stimulus by analysing the Frequency of impulses sent along a neurone, as a stronger stimuli will generate a higher frequency. The body can also have different neurones with different threshold values, allowing the body to gauge the intensity of a stimulus.
Any stimulus, of whatever strength, that is below the thresholdvalue will fail to generate an action potential- this is the nothing part. Any stimulus above the threshold value will succeed in generating an actino potential. It doesnot matter how much aove the threshold a stimulus is, it will still only generate one action potential - this is the 'all' part. How then can an orgnaism percieve the size of the stimulus? This is acheived in 2 ways
- by the number of impulses passing in a given time. The larger the stimulus, the more impulses that are given
- by having different neurones with different threshold values. The brain interprets the number and tpe of neurones that pass impulses as a result of a given stimulus and thereby determines its size.
Comments
No comments have yet been made