BIOB30 Ch8 (266-273).docx

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Department
Biological Sciences
Course
BIOB34H3
Professor
Joanne Nash
Semester
Fall

Description
BIOB30 Chapter 8 (266-273) 1 Neurons: Cellular and Network Properties Action Potentials Are Conducted  Conduction – the high speed movement of an action potential along the axon  During an action potential, lost energy is constantly replenished  The depolarization of a section of axon causes positive current to spread through the cytoplasm in all directions by local current flow o At the same time, on the outside of the axon membrane, current flows back toward the depolarized section o The local current flow in the cytoplasm diminishes over distance as energy dissipates and would die out if not for the voltage-gated channels  Axon contains many voltage-gated Na+ channels o When depolarization reaches these, the open and allow more Na+ to enter reinforcing the depolarization o This mechanism initiates the positive feedback loop  Positive feedback loop o An above threshold graded potential enters the trigger zone o Depolarization opens voltage-gated Na+ channels o Na+ enters the axon and the initial segment of axon depolarizes o Positive charge from the depolarized trigger zone spreads to adjacent sections of membrane  It is repelled by Na+ that entered the cytoplasm and attracted by the negative charge of the resting membrane potential  The flow of local current toward the axon terminal begins conduction of the action potential o When the membrane distal to the trigger zone depolarizes, its Na+ channels open and Na+ enters o Positive feedback loop: depolarization of Na+ channels, Na+ enters, causing more depolarization and opening more Na+ channels in the adjacent membrane  This has begun  The continuous entry of Na+ ensures the signal wont diminish as the action potential propagates itself  In graded potentials, Na+ enters only at the point of the stimulus so they lose strength over distance o As each segment reaches the peak of action potential, its Na+ channels inactivate o During the falling phase, K+ channels are open allowing K+ to leave the cytoplasm o Then the K+ channels close and the membrane in that segment of axon returns to its resting potential o Positive charge from a depolarized segment of membrane may flow backward to the trigger zone but it has no effect  That section of axon is in the absolute refractory period so has inactivated Na+ channelsBIOB30 Chapter 8 (266-273) 2 Neurons: Cellular and Network Properties  What happens to current flow backward (i.e. retrograde current flow) from the trigger zone into the cell body? o Scientists thought it didn’t matter because there wasn’t many voltage- gated ion channels in the cell body o But cell bodies and dendrites do have voltage-gated ion channels so retrograde signals are able to influence and modify the next signal that reaches the cell Larger Neurons Conduct Action Potentials Faster  Two physical parameters influence the speed of an action potential conduction in a mammalian neuron: 1. The diameter of the axon 2. The resistance of the axon membrane to ion leakage out of the cell o Larger on both make for faster action potentials  Analogy: In a water pipe, water touching the walls encounter resistance o The moving water and the stationary walls cause friction o Water in the center of the pipe has no resistance and flows faster o So larger pipe means less water experiences friction  Some organisms use giant axons for rapid escape responses (e.g. squid, earthworms, fish) o Up to 1mm in diameter o These have been important in electrical signaling research because they are easily punctured with electrodes 
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