Neurons are the fundamental units of the nervous system, specialized for transmitting information. Each neuron consists of four main components. The cell body or soma contains the nucleus and most organelles. Dendrites are branched extensions that receive signals from other neurons. The axon is a long projection that carries signals away from the cell body. Finally, synaptic terminals at the axon's end release chemical messengers called neurotransmitters.
The resting potential is the foundation of neural transmission. When a neuron is at rest, the inside of the cell is about 70 millivolts more negative than the outside. This electrical gradient is maintained by the sodium-potassium pump, which actively transports three sodium ions out for every two potassium ions it brings in. The membrane is more permeable to potassium than sodium, allowing some potassium to leak out, contributing to the negative charge inside.
An action potential is the electrical signal that neurons use to communicate. It begins when the membrane potential reaches the threshold of negative 55 millivolts. During depolarization, sodium channels open rapidly, causing the inside to become positive. Repolarization follows as potassium channels open and sodium channels close. A brief hyperpolarization occurs before the membrane returns to its resting potential. This process follows the all-or-nothing principle.
Once generated, action potentials must travel along the axon to reach their target. In myelinated axons, the myelin sheath acts as insulation, forcing the signal to jump from one node of Ranvier to the next. This saltatory conduction dramatically increases speed, reaching up to 120 meters per second. Unmyelinated axons conduct signals continuously along their length, resulting in much slower speeds of only 1 to 2 meters per second.
Synaptic transmission is how neurons communicate with each other. When an action potential reaches the presynaptic terminal, calcium channels open, allowing calcium ions to enter. This triggers vesicles containing neurotransmitters to fuse with the membrane and release their contents into the synaptic cleft. The neurotransmitters bind to specific receptors on the postsynaptic cell, causing ion channels to open or close. This creates a postsynaptic potential that may trigger a new action potential, continuing the signal transmission.