How Information Crosses Between Cells

When the action potential has moved along the axon of nerve cell, it eventually reaches the end of the axon and needs to be transmitted to the next neuron. There is a gap between neurons and the action potential cannot cross that gap as an electrical event. It is like having an air gap between two wires and the electrical current stops at the gap. Obviously, there must be some means of bridging the gap or the information being transmitted would stop here. Transmission across the gap is a chemical process, not electrical.

The gap between the end of the axon of one neuron (axon terminal) and the dendrite or cell body of the next is called the synaptic cleft. The first neuron is the presynaptic neuron and the receiving neuron is called the postsynaptic neuron. The entire junction, cleft, axon terminal, and dendrite is referred to as a synapse. When the action potential reaches the end of the axon, calcium ions flood into the axon terminal. This influx causes vesicles filled with neurotransmitter chemicals to move to the surface of the axon terminal and fuse with the cell membrane. This action releases neurotransmitter molecules into the synaptic cleft. They diffuse across the gap and bind to receptors on the next neuron. These receptors have a three dimensional shape that is exactly complimentary to the shape of the neurotransmitter. When the neurotransmitter binds to the receptors, one of two responses will occur:

1. The receiving neuron will get closer to an action potential threshold. When sufficient neurotransmitter molecules bind, the receiving neuron will reach the threshold and an action potential can begin moving along the next axon to the next synapse. This kind of synapse is called excitatory.

2. The receiving neuron may get farther from the threshold for an action potential. This type of synapse is called inhibitory.

            Once the neuron responds to the presence of the neurotransmitter, it must be removed to clear the synapse for the next action potential to come down the presynaptic neuron. This is done by either having an enzyme in the cleft destroy the neuron transmitter (catalyze a chemical reaction that breaks down the neurotransmitter) or having the presynaptic neuron take up the neurotransmitter back into the axon terminal.  If this is not done, the neurotransmitter in an excitatory synapse would keep stimulating the postsynaptic neuron as soon as the refractory period ended—over and over and over. Many insect sprays kill by inhibiting the enzyme cholinesterase in the synapse. This prevents the neurotransmitter acetylcholine from being removed. Since it is found in neuromuscular junctions, the muscle is stimulated to contract repeatedly, causing the death of the insect. 

            Make sure you read about the different neurotransmitters in your text. Each one is used only in certain areas of the nervous system.   

Questions to check understanding.

a. What is the gap between the two neurons called?

b. Describe the sequence of events that allows the action potential to be chemically transmitted across the gap.

c. What are the chemical molecules involved in this process called? Name three molecules. Which is predominantly involved with voluntary muscle activity?