Summation

As previously mentioned, a single action potential at a single synapse results in a graded potential only: an EPSP or an IPSP. Therefore, generation of an action potential in the postsynaptic neuron requires the addition or summation of a sufficient number of excitatory inputs to depolarize this neuron to threshold. Two types of summation may occur:

• Temporal summation

• Spatial summation

Temporal summation occurs when multiple EPSPs (or IPSPs) produced by a single presynaptic neuron in close sequence exert their effect on membrane potential of the postsynaptic neuron. For example, an action potential in the presynaptic neuron produces an EPSP and partial depolarization of the postsynaptic neuron (see Figure 5.2). While the postsynaptic neuron is still depolarized, a second action potential in the presynaptic neuron produces another EPSP in the postsynaptic neuron that adds to the first and further depolarizes this neuron.

As more EPSPs add together, the membrane depolarizes closer to threshold until an action potential is generated. Although temporal summation is illustrated in Figure 5.2 with the summation of relatively few EPSPs, in actuality, addition of up to 50 EPSPs may be necessary to reach threshold. Because a presynaptic neuron may generate up to 500 action potentials per

Action potential

Action potential

Figure 5.2 Temporal summation. Multiple excitatory postsynaptic potentials (EPSPs) produced by a single presynaptic neuron in close sequence may add together to depolarize the postsynaptic neuron to threshold and generate an action potential.

—Threshold

Figure 5.2 Temporal summation. Multiple excitatory postsynaptic potentials (EPSPs) produced by a single presynaptic neuron in close sequence may add together to depolarize the postsynaptic neuron to threshold and generate an action potential.

minute, temporal summation occurs quite readily. The strength of the signal to the postsynaptic neuron is therefore influenced by the frequency of nerve impulses generated by the presynaptic neuron.

Spatial summation occurs when multiple EPSPs (or IPSPs), produced by many presynaptic neurons, exert their effects on the membrane potential of the postsynaptic neuron simultaneously. For example, Figure 5.3 depicts a single postsynaptic neuron that is innervated by three presynaptic neurons. Inputs from presynaptic neurons A and B are excitatory and the input from presynaptic neuron C is inhibitory. Once again, single action potentials in neuron A or B produce individual EPSPs insufficient to depolarize the postsynaptic neuron to threshold. However, if EPSPs from neurons A and B are produced at the same time, the depolarizations add together and the membrane potential of the postsynaptic neuron reaches threshold, resulting in generation of an action potential. Inputs from neurons A (excitatory) and C (inhibitory) occurring simultaneously may, in effect, cancel each other out, resulting in no change in membrane potential of the postsynaptic neuron.

As with temporal summation, this example has been simplified to illustrate the concept clearly. In actuality, a large number of excitatory inputs from different presynaptic neurons are necessary to depolarize the postsyn-aptic neuron to threshold. Because a typical neuronal cell body receives thousands of presynaptic inputs, spatial summation also occurs quite readily. The number of presynaptic neurons that are active simultaneously therefore influences the strength of the signal to the postsynaptic neuron. Under normal physiological conditions, temporal summation and spatial summation may occur concurrently.

Figure 5.3 Spatial summation. Multiple excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs) produced by many presynaptic neurons simultaneously may add together to alter the membrane potential of the postsynaptic neuron. Sufficient excitatory input (A and B) will depolarize the membrane to threshold and generate an action potential. The simultaneous arrival of excitatory and inhibitory inputs (A and C) may cancel each other out so that the membrane potential does not change.

Figure 5.3 Spatial summation. Multiple excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs) produced by many presynaptic neurons simultaneously may add together to alter the membrane potential of the postsynaptic neuron. Sufficient excitatory input (A and B) will depolarize the membrane to threshold and generate an action potential. The simultaneous arrival of excitatory and inhibitory inputs (A and C) may cancel each other out so that the membrane potential does not change.

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.

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