A nerve cell membrane exhibits an all-or-none response to excitation: that is, t
ID: 99564 • Letter: A
Question
A nerve cell membrane exhibits an all-or-none response to excitation: that is, the magnitude of the response is independent of the magnitude of the stimulus once a threshold value is exceeded. a. Explain in your own words why this is so (think in biological/physiological terms). b. If every neuron exhibits an all-or-none response, how do you suppose the nervous system of an animal can distinguish different intensities of stimulation? How do you think your own nervous system can tell the difference between a warm iron and a hot iron or between a chamber orchestra and a rock band?Explanation / Answer
All or none principle was first discovered by Henry Pickering Bowditch to explain the contraction of the heart muscle and was found later to be applicable to the nerve fiber or the muscle fibers .
According to the principle the response of a nerve or a muscle fiber to a given stimulus is independent of the strength of the stimulus.The nerve fiber elicits a response only when the threshold potential or polarization occurs and the nerve fiber fires otherwise there is no response . Any stimulus that generates a low threshold strength does not elicit the response.
Whenever a neuron sends information downs the axon from the cyton or the cell body towards the synapse an action potential results which changes the polarization of the cell, however the intensity of the stimulus has no effect on determining the strength of the action potential.Once the threshold reaches the neuron fires and an action potential is transmitted.
Neurons transmit information through chemicals known as the neuro transmmiters which are either excitatory and conduct the impulse when they are said to be the presynaptic neurotransmitters and they carry the impulse towards the synapse or inhibitory which do not conduct the impulse eg. Acetylcholine.
The post synaptic neurotransmitters like GABA act as inhibitory neurotransmitters and carry impulse away from the synapse.
The action potential of a nerve cell depends on the polarization or the re polarization of the nerve cell and is controlled by the distribution of these two ions namely the Na+ and the K + ions and the unequal distribution of these ions causes the membrane potential. Generally the neurons have more concentration of the potassium ions within the nerve cell and more of sodium ions outside the cell . The movement of these ions into and out of the cell is brought about by the sodium-potassium pump by active transport.
The diffuspon of the ions is through special gated channels known as the SODIUM GATES and POTASSIUM GATES. In a resting nerve cell membrane the sodium gates are closed and some of the potassium gates are open as a result of which the sodium ions cannot enter the cell while some potassium ions can diffuse out.The diffusion of the neuro transmitters across the membrane creates polarization and depolarization of the nerve cell depending on which the impulse is transmitted.
B. The nervous system distinguishes different intensities of the stimuls due to the fact that not all the nerve cell have similar threshold levels and hence different specific neurons are stimulated to differewnt responses and this helps in differentiating the stimulus.
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