What Is the Difference Between a Graded Potential and Action Potential and How Does This Impact Impulse Transmission?
When the intensity of the stimulus is increased, the size of the action potential does not become larger. Rather, the frequency or the number of action potentials increases. In general, the greater the intensity of a stimulus, (whether it be a light stimulus to a photoreceptor, a mechanical stimulus to the skin, or a stretch to a muscle receptor) the greater the number of action potentials elicited. Similarly, for the motor system, the greater the number of action potentials in a motor neuron, the greater the intensity of the contraction of a muscle that is innervated by that motor neuron. Action potentials are of great importance to the functioning of the brain since they propagate information in the nervous system to the central nervous system and propagate commands initiated in the central nervous system to the periphery. Consequently, it is necessary to understand thoroughly their properties. To answer the questions of how action potentials are initiated and propagated, we need to record the potential between the inside and outside of nerve cells using intracellular recording techniques.The potential difference across a nerve cell membrane can be measured with a microelectrode whose tip is so small (about a micron) that it can penetrate the cell without producing any damage. When the electrode is in the bath (the extracellular medium) there is no potential recorded because the bath is isopotential. If the microelectrode is carefully inserted into the cell, there is a sharp change in potential. The reading of the voltmeter instantaneously changes from 0 mV, to reading a potential difference of -60 mV inside the cell with respect to the outside. The potential that is recorded when a living cell is impaled with a microelectrode is called the resting potential, and varies from cell to cell. Here it is shown to be -60 mV, but can range between -80 mV and -40 mV, depending on the particular type of nerve cell. In the absence of any stimulation, the resting potential is generally constant.
We show that this channel noise and the added non-linearity of Na+ channels destroy input information provided by graded generator potentials. Furthermore, action potentials themselves cause information loss due to their finite widths because the neuron is oblivious to the input that is arriving during an action potential. Consequently, neurons with high firing rates lose a large amount of the information in their inputs. The additional cost incurred by voltage-gated Na+ channels also means that action potentials can encode less information per unit energy, proving metabolically inefficient, and suggesting penalisation of high firing rates in the nervous system (Manwani A, Koch C, 1999).
In fact, nerve and muscle cells are physiologically similar, and there are even hybrid cells, such as in the heart, that have characteristics of both nerves and muscles. Some animals, like the infamous electric eel use muscles ganged so that their voltages add in order to create a shock great enough to stun prey. Propagation of a nerve impulse down a myelinated axon, from left to right. The signal travels very fast and without energy input in the myelinated regions, but it loses voltage. It is regenerated in the gaps. The signal moves faster than in unmyelinated axons and is insulated from signals in other nerves, limiting crosstalk.
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