The postsynaptic cell sums excitatory and inhibitory input

Individual neurons can "decide" whether or not to fire an action potential by summing excitatory and inhibitory postsynaptic potentials. This summation ability is the major mechanism by which the nervous system integrates information. Each neuron may receive a thousand or more synaptic inputs, but it has only one output: an action potential in a single axon. All the information contained in all the inputs a neuron receives is reduced to the rate at which that neuron generates nerve impulses in its axon.

For most neurons, the critical area for "decision making" is the axon hillock, the region of the cell body at the base of the axon (see Figure 44.2). The plasma membrane of the

o a axon hillock is not insulated by glial cells and has many voltage-gated channels. Excitatory and inhibitory postsynaptic potentials from synapses anywhere on the dendrites or the cell body spread to the axon hillock by local current flow. If the resulting combined potential depolarizes the axon hillock to threshold, the axon fires an action potential. Because post-synaptic potentials decrease in strength as they spread from the site of the synapse, all postsynaptic potentials do not have equal influences on the axon hillock. A synapse at the tip of a dendrite has less influence than a synapse on the cell body near the axon hillock.

Excitatory and inhibitory postsynaptic potentials can be summed over space or over time. Spatial summation adds up the simultaneous influences of synapses at different sites on the postsynaptic cell (Figure 44.15a). Temporal summation adds up postsynaptic potentials generated at the same site in a rapid sequence (Figure 44.15b).

All the neuron-to-neuron synapses that we have discussed up to this point are between the axon terminals of a presy-naptic cell and the cell body or dendrites of a postsynaptic cell. Synapses can also form between the axon terminals of one neuron and the axon terminals of another neuron. Such a synapse can modulate how much neurotransmitter the second neuron releases in response to action potentials traveling down

= 2 1 Excitatory = 3 J synapses

= 2 1 Excitatory = 3 J synapses

Dendrites lN

Spatial Summation

Neuron

Spatial summation occurs when several excitatory postsynaptic potentials (EPSPs) arrive at the axon hillock simultaneously.

Action potential

Temporal summation means

Spatial summation occurs when several excitatory postsynaptic potentials (EPSPs) arrive at the axon hillock simultaneously.

Action potential

Temporal summation means

Postsynaptic Cell Spatial Summation
Milliseconds -

Resting potential

44.15 The Postsynaptic Neuron Sums Information Individual neurons sum excitatory and inhibitory postsynaptic potentials over space (a) and time (b).When the sum of the potentials depolarizes the axon hillock to threshold, the neuron generates an action potential.

its axon. We refer to this mechanism of regulating synaptic strength as presynaptic excitation or presynaptic inhibition.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Heike Kirsch
    Does spatial summation occur on the cell body?
    8 years ago
  • kifle haile
    Is temporal summation inhibitory or excitatory?
    7 years ago
  • Angelika
    What are the inputs and outputs of plasma membrane?
    7 years ago
  • Magnus
    What are the inputs and outputs of the cell membrane?
    7 years ago

Post a comment