A neuron is composed of dendrites, a cell body, and an axon.
The axon sends a wave of depolarization along its length, which is part of the high-speed network that sends impulses from one part of the body to another.
The wave of depolarization is primarily the movement of two positive ions (Na+ and K+) from one side of the axon’s cell membrane to the other.
The Neuron at Rest
When a neuron is at rest, the outside of the membrane of the neuron is positively charged compared to the inside.
This is the result of the uneven distribution of positively charged ions (cations) and negatively charged ions (anions).
Outside the cell there are high concentrations of sodium ions (Na+) and lower concentrations of potassium ions (K+). Chloride (Cl−) is the dominant anion exterior to the cell.
Inside the cell there is a high concentration of K+ , a lower concentration of Na+ , and the dominant anions are negatively charged proteins, amino acids, phosphates, and sulfates.
At rest, the membrane is 50 times more permeable to K+ than to Na+.
That is, while Na+ is moving into the cell, there is more K+ diffusing out of the cell. As this happens, the inside of the cell becomes increasingly negatively charged because the larger anions are trapped inside.
Although the increasing negative charge within the cell attracts both the Na+ and K+, this force is offset by the Na+/K+ pump, which is found in the cell membrane.
The Na+/K+ pump uses active transport to pull three Na+ cations from the inside of the cell to the outside. In exchange, two K+ cations are pulled from outside to inside the cell, thereby increasing the difference in charge.
The final result is a relatively negative charge inside the cell compared to the outside. At rest, the difference in charge is approximately –70 mV.
The All-or-none Principle
Sensory neurons can be stimulated by chemicals, light, heat, or the mechanical distortion of their membrane.
Motor neurons and the neurons of the central nervous system are usually stimulated by neurotransmitters, which are chemicals secreted by other neurons.
An axon is governed by the all-or-none principle.
If an axon is stimulated sufficiently (above the threshold), the axon will trigger an impulse down the length of the axon. The strength of the response is uniform along the entire length of the axon. Also the strength of response in a single neuron is independent of the strength of the stimulus.
Depolarization
When a neuron is sufficiently stimulated, a wave of depolarization is triggered.
When this occurs, the gates of the K+ channels close and the gates of the Na+ channels open.
Sodium ions move into the axon. This input of positively charged ions neutralizes the negative charge in the axon. This change in charge is called the action potential.
The depolarization of one part of the axon causes the gates of the neighbouring Na+ channels to open, and this depolarization continues along the length of the axon.
Repolarization
Any specific region of the axon is only depolarized for a split second.
Almost immediately after the sodium channels have opened to cause depolarization, the gates of the K+ channels re-open and potassium ions move out. The Na+ channels close at the same time.
This process, combined with rapid active transport of Na+ out of the axon by the Na+/K+ pump, re-establishes the polarity of that region of the axon.
The brief time between the triggering of an impulse along an axon and when it is available for the next impulse is called the refractory period.
For many neurons, the refractory period is approximately 0.001 s. The signal moves at about 2 m/s.
The speed of a wave of depolarization is increased by the addition of a fatty layer called the myelin sheath. This layer is formed by Schwann cells lined up along the length of the axon. Between each Schwann cell is a gap called the node of Ranvier, where the membrane of the axon is exposed.
A nerve impulse that travels along a myelinated neuron is able to jump from one node of Ranvier to the next. This ability speeds up the wave of depolarization to 120 m/s.
The Synapse
Neurons do not touch one another; there are tiny gaps between them. These gaps are called synapses.
The neuron that carries the wave of depolarization toward the synapse is called the presynaptic neuron.
The neuron that receives the stimulus is called the postsynaptic neuron. 
When a wave of depolarization reaches the end of a presynaptic axon, it triggers the opening of special calcium ion gates.
The calcium triggers the release by exocytosis of neurotransmitter molecules. The neurotransmitter is then released from specialized vacuoles called synaptic vesicles, which are produced in the bulb-like ends of the axon.
The neurotransmitter diffuses into the gap between the axon and dendrites of neighbouring postsynaptic neurons.
The dendrites have specialized receptor sites and the neurotransmitter attaches to these receptors and excites or inhibits the neuron.
The excitatory response involves the opening of sodium gates, which triggers a wave of depolarization.
The inhibitory response makes the post-synaptic neuron more negative on the inside in order to raise the threshold of stimulus. This process is usually accomplished by opening chloride channels to increase the concentration of these negative ions in the neuron.
Neurotransmitters can also stimulate or inhibit cells that are not neurons.
The neurotransmitter that enters the synapse and attaches to the postsynaptic receptors is broken down almost immediately by an enzyme released from the presynaptic neuron.
For example, the enzyme cholinesterase breaks down the neurotransmitter acetylcholine.
Acetylcholine is the primary neurotransmitter of both the somatic nervous system and the parasympathetic nervous system. Acetylcholine can have excitatory or inhibitory effects. This neurotransmitter stimulates skeletal muscles but inhibits cardiac muscles.
Noradrenaline, also called norepinephrine, is the primary neurotransmitter of the sympathetic nervous system.
The neurons of the brain involve a wide variety of neurotransmitters that have numerous functions.
Glutamate is a neurotransmitter of the cerebral cortex that accounts for 75 percent of all excitatory transmissions in the brain.
Gamma aminobutyric acid (GABA) is the most common inhibitory neurotransmitter of the brain.
Many of the brain’s neurotransmitters have multiple functions.
Dopamine elevates mood and controls skeletal muscles, while seratonin is involved in alertness, sleepiness, thermoregulation, and mood.
