Neuropeptides

Neurons in the brain or spinal cord synthesize neuropeptides. These peptides act as neurotransmitters or as neuromodulators—substances that alter a neuron's response to a neurotransmitter or block the release of a neurotransmitter.

Among the neuropeptides are the enkephalins that occur throughout the brain and spinal cord. Each enkephalin molecule is a chain of five amino acids. Synthesis of enkephalins increases during periods of painful stress, and they bind to the same receptors in the brain (opiate receptors) as the narcotic morphine. Enkephalins relieve pain sensations, and probably have other functions as well.

Another morphinelike peptide, called beta endorphin, is found in the brain and cerebrospinal fluid. It acts longer than enkephalins and is a much more potent pain reliever (Clinical Application 10.4).

Substance P is a neuropeptide that consists of eleven amino acids and is widely distributed throughout

Dendrite

Dendrite

Substance Neurotransmitter

Synaptic vesicle / Vesicle releasing neurotransmitter

Synaptic knob Mitochondrion

Synaptic vesicle / Vesicle releasing neurotransmitter

Synaptic knob Mitochondrion

Synaptic Vessels

Axon membrane

Neurotransmitter substance

Polarized membrane

Depolarized membrane

Neurotransmitter receptors and ion channels

Axon membrane

Neurotransmitter substance

Polarized membrane

Depolarized membrane

Neurotransmitter receptors and ion channels

Cell body or dendrite of postsynaptic neuron

Figure 10.18

Mitochondrion

Synaptic vesicle

Synaptic cleft

Postsynaptic membrane

Mitochondrion

Synaptic vesicle

Synaptic cleft

Postsynaptic membrane

(a) When a nerve impulse reaches the synaptic knob at the end of an axon, (b) synaptic vesicles release a neurotransmitter substance that diffuses across the synaptic cleft. (c) A transmission electron micrograph of a synaptic knob filled with synaptic vesicles (37,500x).

Shier-Butler-Lewis: I III. Integration and I 10. Nervous System I: I I © The McGraw-Hill

Human Anatomy and Coordination Basic Structure and Companies, 2001

Physiology, Ninth Edition Function

Nucleus

Neuron-

cell body

Presynaptic knob

Presynaptic axon

Axon

Nucleus

Neuron-

cell body

Presynaptic knob

Presynaptic axon

Axon

Presynaptic Integration

Figure 10.19

The synaptic knobs of many axons may communicate with the cell body of a neuron.

the nervous system. It functions as a neurotransmitter (or perhaps as a neuromodulator) in the neurons that transmit pain impulses into the spinal cord and on to the brain. Enkephalins and endorphins may relieve pain by inhibiting the release of substance P from pain-transmitting neurons.

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