Neurons, Action Potentials, and Synapses

Neurons are cells that carry information as electrical signals called nerve impulses (action potentials), and they pass signals to other cells across junctions called synapses using chemical messengers called neurotransmitters. In brief: a neuron has a cell body, dendrites that receive signals and an axon that carries the impulse away. There are three types: sensory neurons, relay neurons (interneurons) and motor neurons. An impulse travels along an axon as a wave of ion movement, and at the synapse the signal becomes chemical.

Understanding neurons explains how we sense, think and move. This guide describes neuron structure and types, how a resting neuron and an action potential work, how an impulse travels faster in myelinated axons, what happens at a synapse, a worked example with speeds and times and the usual mistakes.

What is the structure of a neuron?

A neuron is a specialized cell with three main parts.

• Cell body (soma): contains the nucleus and most organelles.
• Dendrites: short, branching extensions that receive signals from other neurons and carry them toward the cell body.
• Axon: a long fiber that carries the impulse away from the cell body. It can be more than a meter long, as in the neurons that go from the spinal cord to the foot. Many axons are covered by a myelin sheath, a fatty insulating layer made by glial cells (Schwann cells in the PNS), with small gaps between the segments called nodes of Ranvier.

At the end of the axon are branches with swellings called axon terminals, which contain vesicles filled with neurotransmitter.

What are the types of neurons?

• Sensory neurons carry impulses from receptors (such as those in skin or eyes) to the CNS.
• Interneurons (relay neurons) are found within the CNS and connect sensory and motor neurons. They make up most of the neurons in the brain.
• Motor neurons carry impulses from the CNS to effectors, which are muscles or glands.

How does a nerve impulse travel?

The resting potential

When a neuron is not sending a signal, the inside of the axon is negatively charged relative to the outside, at about −70 millivolts (mV). This is the resting potential. It is maintained by the sodium-potassium pump, which moves 3 Na⁺ out for every 2 K⁺ in, and by channels that let K⁺ leak out more easily than Na⁺ leaks in.

The action potential

When a stimulus is strong enough to push the membrane potential to the threshold, about −55 mV, voltage-gated sodium channels open. This causes an action potential, which has these phases:

1. Depolarization: Na⁺ ions rush in, and the inside becomes positive, up to about +40 mV.
2. Repolarization: sodium channels close, and potassium channels open. K⁺ flows out, and the potential falls back.
3. Hyperpolarization and recovery: the potential briefly drops below resting level, and the pump restores the original ion distribution.

The action potential follows the “all-or-nothing” principle: if the threshold is reached, the impulse has the same size every time, and below it, nothing happens. A stronger stimulus does not make a larger impulse, but produces more impulses per second. After an impulse, there is a short refractory period of a millisecond or two when the neuron cannot fire again, so impulses travel in one direction.

The depolarization at one point triggers depolarization in the next section of the membrane, so the impulse moves along the axon, like a row of falling dominoes, without getting weaker.

What affects the speed of an impulse?

• Myelination: in myelinated axons, the impulse jumps from one node of Ranvier to the next, called saltatory conduction, which is much faster. Speeds reach 100 to 120 meters per second. In unmyelinated axons, speeds are about 0.5 to 2 meters per second.
• Axon diameter: wider axons conduct faster. Squid giant axons, which are up to 1 millimeter in diameter, work without myelin.
• Temperature: impulses are faster at higher temperatures.

Multiple sclerosis is a disease in which the immune system damages myelin, slowing or blocking nerve signals.

How does a synapse work?

A synapse is the junction between two neurons, or between a neuron and an effector. A tiny gap, the synaptic cleft, about 20 nanometers wide, separates the cells. The impulse cannot jump across this gap, so the message is passed by chemicals.

1. The impulse arrives at the axon terminal and opens voltage-gated calcium channels. Ca²⁺ enters.
2. Calcium causes vesicles to fuse with the membrane and release neurotransmitter, such as acetylcholine, into the cleft by exocytosis.
3. The neurotransmitter diffuses across the cleft and binds to receptors on the membrane of the next cell.
4. The receptors open ion channels. If enough channels open to reach the threshold, a new action potential starts.
5. The neurotransmitter is removed by enzymes, such as acetylcholinesterase, or taken back up into the terminal. This ends the signal and allows it to be reset.

Synapses transmit in one direction only, because only the presynaptic terminal has vesicles. Some neurotransmitters excite the next cell, while others (such as GABA) inhibit it. Many drugs and poisons act at synapses: nicotine mimics acetylcholine, and nerve gases block acetylcholinesterase.

Neuron types compared

Worked example: reaction times and speeds

A person touches a hot pan. The sensory neuron runs from the finger to the spinal cord, a distance of about 0.8 m, and the motor neuron runs from the spinal cord back to the arm muscles, about 0.6 m. Assume myelinated fibers with a speed of 100 m/s, and that each synapse adds 0.5 ms (0.0005 s). There are two synapses in a simple reflex arc.

1. Conduction time: total distance = 0.8 + 0.6 = 1.4 m. Time = 1.4 ÷ 100 = 0.014 s = 14 ms.
2. Synaptic delay: 2 × 0.5 ms = 1 ms.
3. Total: 14 + 1 = 15 ms, or 0.015 s.

The reflex begins about 15 milliseconds after the touch, much faster than a voluntary reaction, which takes around 200 milliseconds. If the same path used unmyelinated fibers at 1 m/s, the conduction time would be 1.4 s, long enough to cause a severe burn. The comparison shows the importance of myelination. Real reflexes also include the time to excite receptors and muscles, so the actual response takes a little longer.

How were nerve impulses discovered?

Luigi Galvani showed in the 1780s that electricity can make a frog’s leg muscle twitch. In the 1930s John Young discovered the giant axon of the squid. Alan Hodgkin and Andrew Huxley used it in the late 1940s and early 1950s to measure the changes in ion movement during an action potential, for which they shared the 1963 Nobel Prize in Physiology or Medicine with John Eccles, who studied synapses. Otto Loewi showed in 1921 that nerves release chemical messengers, using the hearts of frogs.

Which mistakes are common?

Mistake 1: Saying that a nerve impulse is an electric current like that in a wire

It is a wave of changes in ion movement across the membrane, much slower than electricity in a wire.

Mistake 2: Thinking a stronger stimulus makes a bigger impulse

All impulses have the same size. A stronger stimulus makes more impulses per second.

Mistake 3: Saying impulses cross the synapse as electricity

In most synapses the signal is carried by neurotransmitter molecules that diffuse across the gap.

Mistake 4: Confusing myelin with the axon itself

Myelin is the insulating layer made by other cells that wraps the axon.