7 May 2009

Analog Neural Potential Gradation in Marieb & Hoehn, Human Anatomy & Physiology

Corry Shores
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Marieb & Hoehn

Human Anatomy & Physiology

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential that can be either depolarizations or hyperpolarizations. These changes cause current flows that decrease in magnitude with distance. Graded potentials are called "graded" because their magnitude varies directly with stimulus strength. The stronger the stimulus, the more the voltage changes and the farther the current flows.

Graded potentials are triggered by some change (a stimulus) in the neuron's environment that causes gated ion channels to open. Graded potentials are given different names, depending on where they occur and the functions they perform. When the receptor of a sensory neuron is excited by some form of energy (heat, light, or other) the resulting graded potential is called a receptor potential or generator potential. When the stimulus is a neurotransmitter released by another neuron, the graded potential is called a postsynaptic potential, because the neurotransmitter is released into a fluid-filled gap called a synapse and influences the neuron beyond (post) the synapse. (401)

When neurotransmitter binds to the receptor protein, this receptor changes its three-dimensional shape. This in turn causes ion channels to open and creates graded potentials. (410)

Many receptors present on postsynaptic membranes at chemical synapses are specialized to open ion channels, thereby converting chemical signals to electrical signals. Unlike the voltage-gated ion channels responsible for action potentials, however, these chemically gated channels are relatively insensitive to changes in membrane potential. Consequently, channel opening at postsynaptic membranes cannot possibly become self-amplifying or self-generating. Instead, neurotransmitter receptors mediate graded potentials -- local changes in membrane potential that are graded according to the amount of neurotransmitter released at the time it remains in the area. (411)

For sensation to occur, a stimulus must excite a receptor.

The stimulus energy must be converted into the energy of a graded potential, a process called transduction. (494)

Marieb, Elaine N., & Katja Hoehn. Human Anatomy & Physiology. London: Pearson, 2007.

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