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The human central nervous system (CNS) consists of the brain and spinal cord, together housing roughly 100 billion neurons. Each neuron contains a cell body that orchestrates its function, dendrites that receive signals from other neurons, and a long axon that carries electrical impulses.
Electrical signals called nerve impulses travel along axons, enabling rapid communication within the nervous system.
Neurotransmission is the relay of signals from one neuron to another. When a neuron’s axon fires, the resulting nerve impulse reaches the dendrites of the next neuron. The impulse then initiates another action potential, propagating the message further along the chain. The speed of this transmission depends largely on the presence of myelin, an insulating sheath produced by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the CNS. Myelin wraps around the axon, leaving gaps known as nodes of Ranvier. This configuration allows impulses to jump between nodes—a process called saltatory conduction—boosting speed up to about 250 miles per hour.
All cells maintain a membrane potential, the voltage difference across their membrane. At rest, a neuron has a negative internal charge, largely due to a higher concentration of potassium (K⁺) ions inside the cell and sodium (Na⁺) and chloride (Cl⁻) ions outside. When a stimulus arrives, voltage‑gated Na⁺ channels open, letting Na⁺ flood in and depolarize the membrane. This depolarization constitutes the action potential, lasting only 1–2 milliseconds. Shortly afterward, voltage‑gated K⁺ channels reopen, restoring the negative charge (repolarization). This rapid cycle of depolarization and repolarization carries the electrical impulse along the axon.
At the axon terminal, the electrical signal is converted into a chemical one. Neurotransmitters are released into the synaptic cleft—the narrow space between neurons—and diffuse across to bind receptors on the dendrite of the postsynaptic neuron. Binding opens ion channels, altering the postsynaptic cell’s membrane potential and determining whether it becomes stimulated or inhibited. Afterward, neurotransmitters are either enzymatically degraded or reabsorbed (reuptake) for reuse. This chemical signaling extends beyond neurons, allowing the nervous system to coordinate muscles, glands, and other organs.
