Graded potentials

Graded potentials are short-distance signals (see Table 4.1). They are local changes in membrane potential that occur at synapses where one neuron

Table 4.1 Distinguishing Features of Graded Potentials and Action Potentials

Graded potentials

Action potentials

Short-distance signals

Magnitude is stimulus dependent

Signal travels by local current flow

Magnitude of signal dissipates as it moves away from the site of stimulation

Initiated at synapses (where one neuron comes into contract with another)

Result in depolarization or hyperpolarization

Long-distance signals

Magnitude is constant (all-or-none phenomenon)

Signal travels by local current flow or by saltatory conduction

Magnitude of signal is maintained along entire length of neuron

Initiated at axon hillock

Depolarization only comes into contact with another neuron. The magnitude of these signals varies with the strength of the stimulus. As the intensity of the stimulus increases, the number of ions diffusing across the cell membrane increases and the magnitude of the change in the membrane potential increases. This change may be in either direction, so the membrane potential may become more or less negative compared to the resting membrane potential (see Figure 4.1).

Depolarization occurs when the membrane potential becomes less negative, moving toward zero. As will be discussed, depolarization makes the neuron more excitable. Hyperpolarization occurs when the membrane potential becomes more negative, moving away from zero. Hyperpolarization tends to make the neuron less excitable. Depolarization and hyperpolarization signals are transient or short-lived. Once the stimulus has been removed, the membrane potential returns to its resting state. Following

_ .Depolarization

Repolarization y

Hyperpolarization

_ Resting membrane potential

Figure 4.1 Types of changes in membrane potential. The resting membrane potential in a typical neuron is -70 mV. Movement of the membrane potential toward zero (less negative) is referred to as depolarization. The return of the membrane potential to its resting value is referred to as repolarization. Movement of the membrane potential further away from zero (more negative) is referred to as hyperpolarization.

depolarization, the membrane is said to undergo repolarization, returning to its resting potential.

The mechanism by which the signal is transmitted along the cell membrane is referred to as local current flow or the movement of positively charged ions. In the area of a stimulus causing a depolarization, the inside of the cell becomes positive (less negative) relative to the outside of the cell. Because opposite charges attract, the (+) charges in this area are attracted to and move toward the negative charges on the adjacent areas of the internal surface of the cell membrane. As a result, these adjacent areas become depolarized due to the presence of these (+) charges. This process continues and the electrical signal travels along the cell membrane away from the initial site of the stimulus; however, these graded or local potentials travel only short distances. The cell membrane is not well insulated and the current (positive charges) tends to drift away from the internal surface of the cell membrane. Consequently, as the signal travels along the membrane, the number of (+) charges causing the depolarization of the next region of membrane continually decreases and the magnitude of the depolarization therefore decreases. The further away from the initial site of stimulation, the smaller the magnitude of the signal is until it eventually dies out.

Essentials of Human Physiology

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