Can a signal travel backward up the axon?

No, it cannot. After a section of the axon fires an action potential, its ion channels briefly lock shut. This brief resting phase, called the refractory period, forces the electrical wave to only travel forward toward the axon terminals.

Are some action potentials stronger than others?

No, action potentials operate on an 'all-or-none' principle. Much like firing a gun, pulling the trigger harder does not make the bullet travel faster. To communicate a stronger sensation, like extreme pain versus a light touch, the neuron fires action potentials more frequently, not more powerfully.

What happens to the neurotransmitters after the signal is sent?

They cannot stay in the gap, or the next neuron would fire forever. They are quickly cleared away by being broken down by enzymes, floating away, or being sucked back up into the original neuron in a process called reuptake.

How does the nervous system send signals?

The nervous system sends signals through a two-step process: an electrical signal travels quickly down a nerve cell, and a chemical signal crosses the tiny gap to the next cell. Think of it like a relay race. A runner sprints down the track (the electrical part) and then hands off a baton to the next runner (the chemical part).<br><br>The cells that perform this relay are called neurons. Each neuron has branches called dendrites to receive signals, a long cable called an axon to carry the signal, and axon terminals to pass the signal onto the next cell. The entire process relies on carefully controlled changes in cell voltage and the release of specialized chemicals.

The Electrical Sprint: Action Potentials

When a neuron is resting, it maintains a negative electrical charge inside its membrane compared to the outside. When a signal is triggered, tiny channels in the membrane open, allowing positively charged sodium ions to rush in. This sudden change in charge is called an action potential. The electrical spike travels down the axon like a wave cheering at a sports stadium. Once the wave passes, the neuron actively pumps ions back to their starting places so it is ready to fire again.

The Chemical Handoff: The Synapse

Electrical signals cannot jump the empty space between neurons. This gap is called the synaptic cleft. When the action potential reaches the very end of the axon, it forces tiny sacs to burst open and release chemical messengers called neurotransmitters. These chemicals float across the gap and lock into specific receptors on the dendrites of the next neuron. If enough neurotransmitters bind to the receptors, they trigger a new action potential in the receiving cell, continuing the relay.

Where Students Slip Up: Mixing Up the Two Phases

A common mistake is forgetting where the electrical and chemical phases happen. Remember that electricity does not leap between cells in the human nervous system. Inside the neuron, the signal is purely electrical (the action potential moving down the axon). Between the neurons, the signal is purely chemical (neurotransmitters crossing the synapse).

Worked through

Trace the path of a nervous system signal from the moment you touch a hot stove to when the signal reaches the next nerve cell in your arm.

First, heat receptors in your finger's sensory neurons are triggered, opening sodium channels and starting an action potential. This electrical signal travels up the long axon of the sensory neuron toward your spinal cord. When the action potential reaches the axon terminal, it causes the release of neurotransmitters into the synaptic cleft. These chemical messengers drift across the gap and bind to receptors on the dendrites of the next neuron (an interneuron), sparking a new electrical signal there.

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Where this comes from: OpenStax Biology 2e: Chapter 35, The Nervous System · Khan Academy: Human Biology Unit, The Nervous System