Local Potential Changes

Neurons are excitable; that is, they can respond to changes in their surroundings. Some neurons, for example, detect changes in temperature, light, or pressure outside the body, whereas others respond to signals from inside the body, often from other neurons. In either case, such changes or stimuli usually affect the membrane potential in the region of the membrane exposed to the stimulus.

Typically, the environmental change affects the membrane potential by opening a gated ion channel. If, as a result, the membrane potential becomes more negative than the resting potential, the membrane is hyperpo-¬°arized. If the membrane becomes less negative (more positive) than the resting potential, the membrane is depolarized.

Local potential changes are graded. This means that the amount of change in the membrane potential is directly proportional to the intensity of the stimulation. For example, if the membrane is being depolarized, the greater the stimulus, the greater the depolarization. If neurons are depolarized sufficiently, the membrane potential reaches a level called the threshold (thresh'old) potential, so-called because events are set into motion that result in an action potential, the basis for the nerve impulse.

In many cases, a single depolarizing stimulus is not sufficient to bring the membrane potential to threshold. However, if another stimulus of the same type arrives before the effect of the first one subsides, the local potential change is greater. This additive phenomenon is called summation (sum-ma'shun). Through summation, several subthreshold potential changes may combine to reach threshold. At threshold, an action potential is produced in an axon.

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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