MadSci Network: Neuroscience |
MAD Scientist Network June 28, 1999 How fast are nerve signals? Nerve conduction: Nerve cells when stimulated generate what is known as an Action Potential or Spike (visit simulations at http://pb010.anes.ucla.edu/ ). This Action Potential event, in mammalian nerve cells, lasts about 0.7ms (milliseconds). The Action Potential is a sharp local change in the voltage across the plasma membrane from an initial resting potential in the order of -55mV (millivolts) to a momentary potential across the membrane of +40 to +50mV. A change in the order of 100mV. This change in potential across the cell membrane is brought about by a sudden flow of positive Sodium ions into the cell and the flow of positive Potassium ions out of the cell. The change in the number of positive and negative ions inside versus outside the cell determines the voltage across the plasma membrane at any given time. After the Spike, the cell membrane falls into a refractory period (where no new Action Potentials are possible) that last in the order of 2ms. During this time the potential across the cell membrane is lower then the resting potential making it very difficult for the cell to generate a new Spike. The concentrations of ions are being restored to Resting Potential values during this period. As the potential across the plasma membrane changes at one point on the cell membrane, neighbouring areas of the membrane become excited. The original site goes back to its resting potential state, and the neighbouring sites generate an Action Potential. In all, these will produce a spreading pattern of reversals of the voltage across the neuron's cell membrane (spreading depolarization potential). The Action Potential thus propagates from its initial trigger site on the plasma membrane towards the neuron's axon. Propagation along the axon occurs only in one direction: Away from the neuron's nucleus. Axons are surrounded (or ensheathed) along their entire length by auxiliary nervous system cells known as Glia Cells. These Glia Cells surround the axons in essentially two ways. In one of them, Glia Cells fill the space between cris-crossing, or between parallel arranged axons (or nerve bundles). Axons in the gray matter are ensheathed this way. Conduction along the axons occurs by the spreading of a depolarization potential as discussed above. Nerve signals (or trains of Action Potentials) travel in these types of axons at conduction speeds ranging from 2 to 10 m/s, depending on the diameter of the axons ( thicker axons have faster propagation speeds). The second way Glia cells surround axons, is by wrapping each axon with a spiral layer of Glial cell membrane known as Myelin. Myelinated axons are located in the white matter areas of the brain and most of the nerve bundles or peripheral nerves that relay information either to our muscles, or from sensory cells associated with fast reflexes or fast volitive actions. The myelin wrapping works essentially as an effective insulator that allows for small exposures of the axons (known as Ranvier Nodes) where the reversal of voltage across the membrane takes place.The propagation of the nerve impulses occurs in this case in a saltatory fashion, where the voltage perturbation jumps from node to node. The length of the Myelin sheath between nodes can be as much as one or two millimetres long. Saltatory conduction, as is known, is very efficient and allows axons conduction speeds ranging from 10m/s up to 120m/s [36km/h to 432 km/h!] The auditory and visual systems have myelinated nerves that allow for a fast and reliable transmission of signals from the eye, or the cochlear sound receptors, to the bain. The distance covered are very small ( a few decimeters). Time delays for such scales, for large myelinated axons as the ones involved in these systems, will be around 5ms or less. The returning impulses from the brain will take at the most, another 10ms to reach the muscles of the hand to grasp the falling ruler. The rest of the delay in the response is due to processing delays at the sensory organs and in the brain's processing centers. References: Junge, D.(1976). Nerve and Muscle Excitation. Sinauer Associates, Inc. Sunderland, Massachusetts. Ritchie, J.M. (1984). Physiological Basis of Conduction in Myelinated Nerve Fibres. In Myelin. Morell, P., Ed. 2nd ed. Plenum Press. Newy York. Pp: 117 -146. Kuffler, S.W., Nichols, J.G., and Martin, A.R. (1984). From Neuron to Brain. Sinauer Associates, Inc. Sunderland, Massachusetts. Bezanilla, F. (1998). ELECTROPHYSIOLOGY and The Molecular Basis of Excitability. Simulation Programs. http://pb010.anes.ucla.edu/. [Last visited June 28, 1999].
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