Neuron Structure and Function

Neuron Structure and Function

Neurons (nerve cells) are the fundamental units of the nervous system — specialised cells capable of generating and transmitting electrical signals called nerve impulses (action potentials). The human brain contains approximately 86 billion neurons, each forming thousands of connections with others. Understanding their structure and function is essential for understanding how the nervous system processes information and coordinates behaviour.

Types of Neuron

Three main types of neuron carry out different roles in the nervous system:

• Sensory (afferent) neurons: carry signals from sensory receptors (in the skin, eyes, ears, and other sense organs) towards the CNS. They have long dendrites and relatively short axons.
• Motor (efferent) neurons: carry signals from the CNS to muscles and glands (effectors), producing movement and secretion. They have short dendrites and very long axons.
• Relay neurons (interneurons): found exclusively in the CNS; they connect sensory and motor neurons and form the complex circuits that underlie information processing. They have numerous short dendrites and axons.

Neuron Structure

All neurons share a basic structural plan, though the proportions vary with type:

• Dendrites: branching extensions from the cell body that receive incoming signals from other neurons. The greater the number and branching of dendrites, the more inputs a neuron can receive.
• Cell body (soma): contains the nucleus and cellular machinery responsible for maintaining the cell's life functions. It integrates incoming signals from dendrites.
• Axon hillock: the junction between the cell body and the axon; the site at which the decision to fire an action potential is made — if the integrated input reaches a threshold, an action potential is initiated here.
• Axon: a long, single extension that carries the nerve impulse away from the cell body towards the axon terminals. In myelinated neurons, the axon can be up to a metre long (e.g. motor neurons reaching the foot).
• Myelin sheath: a fatty insulating layer produced by Schwann cells (in the PNS) that wraps around the axon. Myelin dramatically increases the speed of nerve impulse transmission by enabling saltatory conduction — the impulse jumps from node to node rather than travelling continuously.
• Nodes of Ranvier: small gaps in the myelin sheath where the axon membrane is exposed. The action potential regenerates at each node, allowing the impulse to jump rapidly between nodes (saltatory conduction), increasing transmission speed from ~1 m/s to over 100 m/s in myelinated fibres.
• Axon terminals (synaptic knobs): the branching endpoints of the axon that form synapses with the next neuron, muscle, or gland. They contain vesicles (membrane-bound sacs) filled with neurotransmitters, which are released when the action potential arrives.

The Action Potential

A nerve impulse is an action potential — a brief, rapid reversal of the electrical charge across the neuron's membrane. At rest, the inside of the neuron is negatively charged relative to the outside (resting potential, approximately -70 mV), maintained by the sodium-potassium pump. When stimulation causes the membrane potential to reach the threshold (approximately -55 mV), voltage-gated sodium channels open, allowing sodium ions (Na⁺) to rush into the cell — depolarisation, rapidly reversing the charge to approximately +40 mV. This triggers the opening of potassium channels, allowing potassium ions (K⁺) to flow out — repolarisation, restoring the negative charge. Briefly, the membrane overshoots below the resting potential (hyperpolarisation) before recovering.

The action potential obeys the all-or-nothing principle: either the threshold is reached and a full-sized action potential fires, or no action potential fires at all. The intensity of a stimulus is encoded not in the size of individual action potentials but in their frequency — stronger stimuli produce more action potentials per second.