In the brain and spinal cord, some synaptic knobs release neurotransmitters that cause an increase in membrane permeability to sodium ions, and thus trigger nerve impulses. This action is said to be excitatory.
Substances of this type include serotonin, dopamine, and norepinephrine.
Other synaptic knobs release substances that decrease membrane permeability to sodium ions, thus causing the threshold of stimulation to be raised. This action is called inhibitory, for it lessens the chance that a nerve impulse will be transferred to an adjoining neuron.
Inhibitory substances include gamma-aminobutyric acid (GABA), which is an amino acid that is synthesized primarily in the brain and spinal cord, but is not incorporated into protein molecules. Another amino acid, glycine, seems to function as an inhibitor in some of the spinal cord synapses.
The synaptic knobs of a thousand or more neurons may communicate with the dendrites and cell body of a particular neuron.
Some of these knobs probably have an excitatory action, while others are likely to be inhibitory.
The effect on the neuron will depend on which knobs are activated from moment to moment.
In other words, if more excitatory than inhibitory knobs are functioning, the neuron's threshold may be exceeded, and a nerve impulse will be triggered to pass over its surface.
Conversely, if more inhibitory knobs are active, no impulse will be conducted.
0 comments:
Post a Comment