Neurophysiology of nerve impulses

A change in membrane potential whose magnitude is proportional to the stimulus and that decreases with distance as it spreads through the cytoplasm.

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Post Merge: 11 years ago

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Upon being stimulated (by another neuron or as a special receptor) the dendrites of a neuron produce a graded potential. Stimulation can occur in many ways, including chemical stimulation (neurotransmitters, etc.), mechanical stimulation (certain pain receptors, hair receptor, etc.), light stimulation (photoreceptors) and a few other methods. Regardless of the method of stimulation the same general result is effected. Certain receptor protein ion channels on the dendrites are activated, and opened. This causes an influx (or efflux) of whatever ion the receptor channel was geared for. Thus it can cause either a depolarization (an excitatory response that may lead to an action potential) or hyperpolarization (an inhibitory response which makes it harder for an action potential to occur) depending on the ion let in (or out). The more receptor protein ion channels that are stimulated the stronger, or more intense the signal. A slight stimulation of a receptor protein ion channel will often open the channel and let in ions. However in order for a depolarization of threshold strength to reach the axon hillock, it generally requires multiple depolarizing receptor protein ion channels to open. Because of this, graded potentials are said to summate. In effect the total polarizing effect of the ion channels adds together. This is important in that one channel cannot stimulate an action potential, it takes multiple channels working in concert to depolarize a membrane enough to cause and action potential. Graded potentials (or receptor potentials when they occur in receptor cells) are short lived depolarizations or hyperpolarizations of an area of membrane. These changes cause local flows of current (current reflects the movement of ions) that decrease with distance. The magnitude of a graded potential is a direct reflection of the intensity or strength of the stimulus. The more intense the stimulus, the more ion channels that are opened, and the greater the voltage change (hyper or de- polarization) and the farther the current flows. Once stimulated polarization occurs as ions rush in. These ions accumulate very close to the stimulated area. From there the surplus of ions radiates out in all directions, polarizing adjoining membranes. As this polarization spreads like a wave it leaves behind it a wake of formerly polarized membrane that very quickly returns to resting membrane potential. As the wave of polarization ripples across the membrane it either summates (adds or subtracts) with other polarization waves or quickly looses intensity. This is in part due to the ever present leakage channels . Because the current dissapates quickly and is short lived graded, potentials can only act as signals over short distances. However their purpose is only to drive the axon hillock to threshold membrane potential, at which point an action potential will be generated.