MCAT Content / Nerve Cell / Excitatory And Inhibitory Nerve Fibers Summation Frequency Of Firing

Excitatory and inhibitory nerve fibers: summation, frequency of firing

Topic: Nerve Cell

Summation occurs when excitatory impulses add together to reach the threshold to fire a neuron; however, if inhibitory impulses received are higher than the excitatory, they cancel excitation and inhibit an action potential from occurring.

A neuron has many inputs from many presynaptic neurons, whether excitatory or inhibitory. If the neuron only receives excitatory impulses, it will generate an action potential. However, if the neuron receives as many inhibitory as excitatory impulses, the inhibition cancels out the excitation, and the nerve impulse will stop there. Multiple synapses must create excitatory potential at the same time for the neuron to be sufficiently depolarized to fire an action potential. If the first fire is subthreshold, the neuron may fire again before depolarization dies; hence, the frequent firing of action potential takes place and results in having a higher threshold than the first one.

Summation is the addition of these impulses at the axon hillock. Together, synaptic summation and the threshold for excitation act as a filter so that the system does not transmit random “noise” as necessary information. Spatial summation means that the effects of impulses received at different places on the neuron add up so that the neuron may fire when such impulses are received together, even if each impulse on its own would not be sufficient to cause firing. Temporal summation means that the effects of impulses received at the same place can add up if the impulses are received in close temporal succession. Thus, the neuron may fire when multiple impulses are received, even if each impulse on its own would not be sufficient to cause firing.

Practice Questions

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Key Points

• Simultaneous impulses may add together from different places on the neuron to reach the threshold of excitation during spatial summation.

• When individual impulses cannot reach the threshold of excitation on their own, they can add up at the same location on the neuron over a short time; this is known as temporal summation.

• The action potential of a neuron is fired only when the net change of excitatory and inhibitory impulses is non-zero.

• The neuron may fire when multiple impulses are received, even if each impulse on its own would not be sufficient to cause firing.

• If the first fire of action potential is subthreshold, the neuron may fire again before depolarization dies; hence, the frequent firing of action potential takes place and results in having a higher threshold than the first one.


Key Terms

temporal summation: the effect when impulses received at the same place on the neuron add up

spatial summation: the effect when simultaneous impulses received at different places on the neuron add up to fire the neuron

axon hillock: the specialized part of the soma of a neuron that is connected to the axon and where impulses are added together

excitatory: a neuron that triggers a positive change

inhibitory: a neuron that triggers a negative change

pre-synaptic: is the neuron conducting impulses towards the synapse

neurotransmitters: a type of chemical messenger which transmits signals across a chemical synapse

action potential: a short term change in the electrical potential that travels along a cell

summation: the addition of these impulses at the axon hillock



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