If nerve impulses reach synaptic knobs at rapid rates, their supplies of neurotransmitters may become exhausted.
Impulses cannot be transferred between the neurons involved until more neurotransmitter is synthesized.
Such a condition seems to occur during an epileptic seizure. In this situation, abnormal and excessive discharges of impulses originate from certain brain cells.
Some of these impulses reach skeletal muscle fibers and stimulate violent contractions.
In time, the synaptic knobs seem to run out of neurotransmitter substances, and the seizure subsides.
“A drug called Dilantin (diphenylhydantoin) is commonly used to treat epilepsy. It seems to have a stabilizing effect upon excitable neuron membranes, apparently acting to increase the effectiveness of the sodium active transport mechanism. Consequently, the sodium ions move from inside the neurons, and the thresholds of their membranes are stabilized against excessive stimulation”.
Other factors that affect synaptic transmission include various drugs and chemicals.
For example, botulin, a toxin produced by certain bacteria {Clostridium botulinum), is responsible for botulism, a severe form of food poisoning. This toxin prevents the release of acetylcholine from synaptic knobs, so that nerve impulses are not transmitted across synapses.
A victim of botulism may die from paralysis of the breathing muscles. Caffeine, which is found in coffee, tea, and cola drinks, stimulates activity in the nervous system. It does this by somehow lowering the thresholds for triggering nerve impulses at synapses.
Thus, when caffeine is present, certain neurons are more easily excited than usual.
A number of drugs apparently produce their special effects by interfering with the normal actions of neurotransmitters.
These include LSD, which seems to counteract the function of serotonin; cocaine, which enhances the effects of norepinephrine by preventing its normal inactivation; and amphetamine, which promotes the excessive release of dopamine.
Some of the antianxiety drugs, including diazepam (Valium), seem to produce their effects by increasing the effectiveness of the inhibitory transmitter GABA.
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