How does an impulse travel from one neuron to another?

Answer 1

Between neurons, there are two primary methods in which an impulse or action potential can reach another neuron, via a chemical and electrical synapse.

Chemical synapse

This involves a chemical messenger and the fact that both neurons are not physically connected, rather the chemical must diffuse across a synaptic cleft to pass on the message. Chemical synapses tend to be slower than electrical ones.

1. The process begins with a wave of electrochemical excitation called an action potential traveling along the membrane of the presynaptic cell, until it reaches the synapse.
2. The electrical depolarization of the membrane at the synapse causes channels to open that are permeable to calcium ions.
3. Calcium ions flow through the presynaptic membrane, rapidly increasing the calcium concentration in the interior.
4. The high calcium concentration activates a set of calcium-sensitive proteins attached to vesicles that contain a neurotransmitter chemical.
5. These proteins change shape, causing the membranes of some "docked" vesicles to fuse with the membrane of the presynaptic cell, thereby opening the vesicles and dumping their neurotransmitter contents into the synaptic cleft, the narrow space between the membranes of the pre- and post-synaptic cells.
6. The neurotransmitter diffuses within the cleft. Some of it escapes, but some of it binds to chemical receptor molecules located on the membrane of the postsynaptic cell.
7. The binding of neurotransmitter causes the receptor molecule to be activated in some way. Several types of activation are possible, as described in more detail below. In any case, this is the key step by which the synaptic process affects the behavior of the postsynaptic cell.
8. Due to thermal shaking, neurotransmitter molecules eventually break loose from the receptors and drift away.
9. The neurotransmitter is either reabsorbed by the presynaptic cell, and then repackaged for future release, or else it is broken down metabolically.

This type of synapse is often found in the muscles.

Electrical synapse (a.k.a. gap junctions)

This may also involve a chemical messenger however generally they are the charged ions directly from the action potential. Additionally, the connection between different neurons are physical and direct, resulting in a generally faster connection than a chemical synapse.

1. The process begins with a wave of electrochemical excitation called an action potential traveling along the membrane of the presynaptic cell, until it reaches the synapse.
2. Ions directly travel through the gap junctions and into the next neuron.

This type of synapse is often found in the heart.