The Nervous and Endocrine Systems

Neuron Structure and Nerve Impulse Conduction

The nervous system relies on specialized communication cells called neurons, which serve as its primary structural and functional units. These cells are uniquely adapted to generate and transmit electrical signals throughout the body. A standard neuron features a cell body, also known as the soma, which houses the nucleus and the standard cellular organelles required for ongoing metabolism and cellular maintenance. Extending outward from the soma are dendrites, which are branched cellular extensions designed to receive incoming chemical or electrical signals from other neurons and convey those signals inward toward the cell body.

Signals gathered by the dendrites and soma travel toward the axon, a long projection responsible for conducting electrical impulses away from the cell body toward other neurons or target cells. The axon connects to the cell body at a specialized, cone-shaped region termed the axon hillock. Action potentials are typically initiated in the axon initial segment immediately adjacent to the axon hillock. To ensure that electrical signals travel rapidly, many axons are wrapped in a lipid-rich insulating layer known as the myelin sheath. In the peripheral nervous system, this protective covering is produced by Schwann cells, whereas in the central nervous system, it is formed by oligodendrocytes.

The myelin sheath is interrupted at regular intervals by uninsulated gaps known as the nodes of Ranvier. During saltatory conduction, the electrical impulse does not travel smoothly across every micrometer of the axon membrane; instead, action potentials are regenerated at these exposed nodes of Ranvier, dramatically increasing the transmission speed along the axon. Think of the myelin sheath as insulation wrapped around a long copper wire, preventing signal leakage and allowing the electrical impulse to leap efficiently from one exposed gap to the next.

An inactive or resting neuron maintains an electrical charge separation across its membrane, known as a negative resting membrane potential. This negative interior charge exists primarily because the resting neuronal membrane is considerably more permeable to potassium than to sodium. The underlying concentration gradients are continuously established and preserved by the sodium-potassium pump, an active transport mechanism that expels three sodium ions out of the neuron for every two potassium ions it imports. When a stimulus prompts an action potential, voltage-gated sodium channels open rapidly, causing sodium ions to rush into the cell; this sudden influx of positive ions constitutes depolarization. Repolarization immediately follows as voltage-gated potassium channels open, allowing potassium ions to diffuse outward, restoring the cell's negative internal charge.

Once the action potential reaches the terminal end of the axon, electrical communication converts into chemical communication at a chemical synapse. The arrival of the action potential stimulates voltage-gated channels to facilitate a calcium influx. This influx of calcium triggers the exocytosis of neurotransmitters, releasing them from the presynaptic neuron into the synaptic cleft. The released neurotransmitters then diffuse across this narrow space to interact with target cells or adjacent neurons.