Toxicology Exam II Flashcards

1. axonic transmission
2. synaptic transmission

Impulse travels through a neuron. greatest distance traveled is along the axon. hence, "axonic"

Signal passes across the synapse separating the axon terminal of a presynaptic neuron from the post-synaptic membrane of another cell (vix., the membrane covering the surface of a dendrite of another neuron or another signal- receiving cell, e.g., in a muscle or a gland)

Axonic transmission results from changes in teh distribution of cations and anions across the plasma membrane. when a neuron is at rest the inside of the membrane has a slightly negative charge (-70mV) compared to the outside of the membrane. neurologists call such a neuron, or that portion, "polarized".

Na+ and K+ concentrations differ substantially between the inside and outside of the membrane. Na+ concentration is higher outside than inside and K+ concentration is higher inside than outside. Na+ and K+ cations leak via "leak" channels, but it does not leak very fast because if it did the Na/K exchange pumo would burn through huge amounts of ATP trying to maintain the proper concentration gradient

Both neurons and muscle cells use changes in cell membrane potential (i.e., the charge inside the plasma membrane compared to just outside the membrane) to receive, intergrate, and transmit nervous impulses

Changes in membrane potential result from changes in membrane permeability to one or more specific ions. for example, a bunch of Na+ channels within a particular patch of membrane may open at the same time, resulting in a huge rush in of Na+ driven by sodium concentration and electrostatic gradient

1. graded potential local change that gets weaker with distance
2. action potential: a long distance change that does not get weaker with the distance it travels

When "local currents"/graded potentials carry enough charge to the base of the axon to raise the potential of the inside fo the membrane to the "threshold voltage" (-55mV). action potentials carry nervous impulses along axons only.

Voltage gated Na+ channels open, allowing influx of Na+ which further raises (depolarizes) the membrane potential which causes still more Na+ channels to open and so on. the result is a positive feedback loop that fairly quickly (within <1ms) opens up essentially all the sodium channels at the base of the axon

The membrane potential has risen enough to reverse sign and reach a peak voltage usually somewhere between +30 and +40mV. neurologists routinely describe neurons in this state as "depolarized" even though a substantial difference in charge exists across the plasma membrane

You need a bunch of a special kind of ion pores: voltage-gated ion channels for Na+ and K+, and the Na kind are especially important. axons have a lot of these and dendrites have nearly none

By their own concentration gradient and by their attraction for the excess negative charge a little bit ahead of them on the inner surface of the membrane. in the next stretch on axon, voltage-gated sodium channels await the arrival of enough Na+ to raise the local membrane potental to the threshold level. once the threshold is passed, the positive-feedback loop ensures that a new avalanche of Na+ will raise the potential of that next segment of axon membrane up to peak voltage. the while process repears until high Na+ concentrations reach teh synaptic terminals at the end of the axon

K+ ions. to restore Na+-filled portion of an axon to the resting potential, just when the membrane potential of that portion of the axon reaches its peak (positive potential), pop open some voltage-regulated K+ channels. the Na+ channels begin to close shortly after the K+ channels begin to open. at about the time the membrane potential returns to the resting state (70mV), the K+ channels begin to close. this happens at nearly teh same time when the Na+ channels are fully closed. K+ channels gradually close over the next 1.5 to 4ms.

Dozens of different voltage-gated K+ channels exist in humans, so evolution has been able to engineer different nerves to have K+ cahnnels with different closing speeds