MCAT Biology and Biochemistry Review by Princeton Review

Author:Princeton Review [Review, The Princeton]
Language: eng
Format: epub
ISBN: 978-1-101-88240-5
Publisher: Random House Children's Books
Published: 2015-03-16T16:00:00+00:00

Summation

Once an action potential is initiated in a neuron, it will propagate to the end of the axon at a speed and magnitude of depolarization that do not vary from one action potential to another. The action potential is an “all-or-nothing” event. The key regulated step in the nervous system is whether or not a neuron will fire an action potential. Action potentials are initiated when the postsynaptic membrane reaches the threshold depolarization (about –50 mV) required to open voltage-gated sodium channels. The postsynaptic depolarization caused by the release of neurotransmitter by one action potential at one synapse is not generally sufficient to induce this degree of depolarization. A postsynaptic neuron has many different neurons with synapses leading to it, however, and each of these synapses can release neurotransmitter many times per second. The “decision” by a postsynaptic neuron whether to fire an action potential is determined by adding the effect of all of the synapses impinging on a neuron, both excitatory and inhibitory. This addition of stimuli is termed summation.

Excitatory neurotransmitters cause postsynaptic depolarization, or excitatory postsynaptic potentials (EPSPs), while inhibitory neurotransmitters cause inhibitory postsynaptic potentials (IPSPs). One form of summation is temporal summation, in which a presynaptic neuron fires action potentials so rapidly that the EPSPs or IPSPs pile up on top of each other. If they are EPSPs, the additive effect might be enough to reach the threshold depolarization required to start a postsynaptic action potential. If they are IPSPs, the postsynaptic cell will hyperpolarize, moving further and further away from threshold, effectively becoming inhibited. The other form of summation is spatial summation, in which the EPSPs and IPSPs from all of the synapses on the postsynaptic membrane are summed at a given moment in time. If the total of all EPSPs and IPSPs causes the postsynaptic membrane to reach the threshold voltage, an action potential will be fired.

• In which one of the following ways can a presynaptic neuron increase the intensity of signal it transmits?20

A) Increase the size of presynaptic action potentials

B) Increase the frequency of action potentials

C) Change the type of neurotransmitter it releases

D) Change the speed of action potential propagation

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