Synaptic Transmission Notes Summary

A nerve impulse travels in both directions away from the point of stimulation.

Within a neuron, however, an impulse will usually travel from a dendrite to its cell body and then move along the axon to the end.

There it crosses a synapse and continues to a dendrite or cell body of another neuron. The process of crossing the gap at a synapse is called synaptic transmission.

The typical one-way transmission from axon to dendrite or cell body is due to the fact that axons usually have rounded synaptic knobs at their ends, which dendrites lack.

These knobs contain numerous membranous sacs, called synaptic vesicles, and when a nerve impulse reaches a knob, some of the vesicles respond by releasing a substance called a neurotransmitter.

The neurotransmitter diffuses across the synaptic cleft and reacts with the neuron membrane on the other side.

If a sufficient amount of neurotransmitter is released, the membrane is stimulated, and a nerve impulse is triggered. Neurotransmitters are usually destroyed through rapid decomposition by enzymes present in synaptic clefts or are somehow removed.

Such destruction or removal of the neurotransmitter is important in preventing a continuous stimulation of a neuron on the distal side of a synapse.

Acetylcholine is the neurotransmitter released by most axons outside the brain and spinal cord, and by some axons within these organs.

This is the same substance that is released from the ends of motor neurons at the motor end plates of muscle fibers. In both cases, it is decomposed by the action of the enzyme cholinesterase.

“Parkinson's disease, which is characterized by slowness of movement, difficulty in initiating voluntary muscle actions, and tremors, is thought to be caused by degeneration of certain neurons in the brain that synthesize a neurotransmitter, called dopamine. 

The resulting deficiency of this neurotransmitter interferes with normal nerve impulses, causing the symptoms. This condition is often treated with a drug called L-dopa, which can be converted to dopamine by cellular enzymes”.