10.3: The Neural Impulse

Part 1: The Axon and Neural Impulse

The neural impulse occurs as follows:

• Step 1 – Neurons waiting to fire are said to be in resting potential and polarized, or having a negative charge inside the neuron and a positive charge outside.
• Step 2 – If adequately stimulated, the neuron experiences an action potential and becomes depolarized. When this occurs, voltage-gated ion channels open, allowing positively charged sodium ions (Na+) to enter. This shifts the polarity to positive on the inside and negative outside. Note that ions are charged particles found both inside and outside the neuron.
• Step 3 – Once the action potential passes from one segment of the axon to the next, the previous segment begins to repolarize. This occurs because the Na channels close and potassium (K) channels open. K+ has a positive charge, so the neuron becomes negative again on the inside and positive on the outside.
• Step 4 – After the neuron fires, it will not fire again no matter how much stimulation it receives. This is called the absolute refractory period. Think of it as the neuron ABSOLUTELY will not fire, no matter what.
• Step 5 – After a short time, the neuron can fire again, but needs greater than normal levels of stimulation to do so. This is called the relative refractory period.
• Step 6 – Please note that this process is cyclical. We started at resting potential in Step 1 and end at resting potential in Step 6.

Part 2: The Action Potential

Let’s look at the electrical portion of the process in another way and add some detail.

• Recall that a neuron is normally at resting potential and polarized. The charge inside is -70mV at rest.
• If it receives sufficient stimulation meaning that the polarity inside the neuron rises from -70 mV to -55mV defined as the threshold of excitation, the neuron will fire or send an electrical impulse down the length of the axon (the action potential or depolarization). It should be noted that it either hits -55mV and fires or it does not. This is the all-or-nothing principle. The threshold must be reached just like in our earlier discussion of absolute and difference thresholds.
• Once the electrical impulse has passed from one segment of the axon to the next, the neuron begins the process of resetting called repolarization.
• During repolarization, the neuron will not fire no matter how much stimulation it receives. This is called the absolute refractory period.
• The neuron next moves into the relative refractory period, meaning it can fire but needs greater than normal levels of stimulation. Notice how the line has dropped below -70mV. Hence, to reach -55mV and fire, it will need more than the normal gain of +15mV (-70 to -55 mV).
• And then we return to resting potential.

Ions are charged particles found both inside and outside the neuron. It is positively charged sodium (Na) ions that cause the neuron to depolarize and fire and positively charged potassium (K) ions that exit and return the neuron to a polarized state.

Part 3: The Synapse

The electrical portion of the neural impulse is just the start. The actual code passes from one neuron to another in a chemical form called a neurotransmitter. The point where this occurs is called the synapse. The synapse consists of three parts — the axon of the sending neuron; the space in between called the synaptic space, gap, or cleft; and the dendrite of the receiving neuron. Once the electrical impulse reaches the end of the axon, called the axon terminal, it stimulates synaptic vesicles or neurotransmitter sacs to release the neurotransmitter. Neurotransmitters will only bind to their specific receptor sites, much like a key will only fit into the lock it was designed for. You might say neurotransmitters are part of a lock-and-key system.

What happens to the neurotransmitters that do not bind to a receptor site? They might go through reuptake which is the process of the presynaptic neuron taking up excess neurotransmitters in the synaptic space for future use or enzymatic degradation when enzymes are used to destroy excess neurotransmitters in the synaptic space.

What exactly are some of the neurotransmitters which are so critical for neural transmission? See Table 10.1 for details.