Nature of Neuronal Signals

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Nature of Neuronal Signals

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 5 Current Lecture Nature of Neuronal Signals All living cells generate an electrical potential difference (Vm) across their plasma membranes due to an unequal distribution of ions across the membrane because of selective permeability and transporters like the sodium potassium pump. Neurons and muscle cells are unique in that their electrical properties are used to generate signals in the form of brief changes in membrane potential. There are two general types of signals: 1. graded/local signals. Graded because the size of the signal varies with the strength of the stimulus. And local because they are restricted to the point they originated from on the membrane. The amplitude exponentially decays with distance as it spreads away from the starting point, so it doesn’t get very far. 2. all-or-none/regenerative signals. All-or-none because once the stimulus brings the Vm up to the threshold value, it causes a very fast, positive peak called the action potential whose amplitude is generally the same. Regenerative because as it spreads, it regenerates itself continually. No matter how long or big the axon is, once you generate an AP, it goes all the way to the end of the cell, undiminished in amplitude, until it hits the end where it stops. Stimulus = anything done to the cell that provokes a response, such as release of neurotransmitters, exposure to light, etc. Going up = positive inside. Down = negative inside. Vm when one ion is permeable 2 important conditions behind the mechanisms of membrane potential: 1. There is an unequal distribution of a particular ion across the membrane. 2. The membrane of the cell has to be permeable to the ion (due to the presence of selective membrane ion channels) Left – Only K+ is permeable Potassium will diffuse outwards down its concentration gradient, leaving the inside of cell negative. Simultaneously, potassium also diffuses inwards down its electrical gradient, so you have a flux of potassium moving in two directions oppositely across the membrane. The rates of movement will continue until they reach equilibrium. At equilibrium, the flux of an ion down its concentration gradient will equal the flux of ions down its electrical gradient, and net movement will be zero. In membranes with only one ion permeable, equilibrium is achieved instantaneously. Right - Only Na+ is permeable

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