The human nervous system serves a single, yet profoundly complex function: to exchange information with every part of the body and orchestrate context‑appropriate responses.
Unlike most organ systems, its inner workings are visible only under a microscope. While the brain and spinal cord can be appreciated grossly, microscopic detail reveals a level of elegance and intricacy that defies simple description.
Nervous tissue is one of the body's four primary tissue types—muscle, epithelial, connective, and nervous. Its functional unit is the neuron, or nerve cell.
Neurons share the basic eukaryotic architecture of nuclei, cytoplasm, and organelles, yet they are highly specialized and diverse, both compared to cells from other systems and among themselves.
The nervous system is traditionally split into the central nervous system (CNS), comprising the brain and spinal cord, and the peripheral nervous system (PNS), which includes all other components.
At the cellular level, the CNS and PNS are built from two main cell types: neurons, the active signal‑carrying cells, and glia, the supportive cells that maintain homeostasis, provide insulation, and shape the neural environment.
Functionally, the nervous system divides into the somatic (voluntary) and the autonomic (involuntary) systems. The autonomic branch further splits into the sympathetic and parasympathetic divisions, governing “fight‑or‑flight” and restorative processes, respectively.
Neurons are universally composed of four key structures: the cell body (soma), branching dendrites, a single axon, and multiple axon terminals.
Named from the Latin for “tree,” dendrites radiate from the soma to receive signals from other neurons. Axons, often long and slender, carry the integrated message away from the soma toward target cells.
In sensory neurons, the initial dendritic segment extends peripherally to the stimulus site, while a central axon projects toward the CNS. In motor neurons, the dendrite is typically located in the CNS, and the axon travels outward to muscles or glands.
Beyond these core parts, neurons possess specialized adaptations that accelerate electrical transmission.
The myelin sheath, a lipid‑rich insulating layer produced by Schwann cells (PNS) or oligodendrocytes (CNS), wraps around axons. Interspersed gaps—nodes of Ranvier—allow rapid saltatory conduction of action potentials.
Disruption of myelin underlies degenerative disorders such as multiple sclerosis, where demyelination impairs neural signaling.
Communication between neurons, and between neurons and target tissues, occurs at synapses. An action potential triggers the release of neurotransmitters from axon terminals into the synaptic cleft, where they bind receptors on postsynaptic dendrites.
Signal propagation is governed by the action potential, an all‑or‑nothing electrical event driven by the controlled flow of sodium (Na⁺) and potassium (K⁺) ions across the membrane.
The sodium–potassium ATPase maintains a higher Na⁺ concentration outside the cell and a higher K⁺ concentration inside, establishing a resting membrane potential of approximately –70 mV.
When a stimulus opens voltage‑gated Na⁺ channels, Na⁺ rushes in, depolarizing the membrane. Rapid closure of Na⁺ channels and opening of K⁺ channels then repolarize the membrane, resetting it for the next action potential.
In myelinated axons, action potentials leap from node to node, maintaining speed while conserving energy. Improper spacing of nodes can either slow conduction or cause the signal to decay prematurely.
Multiple sclerosis, affecting an estimated 2–3 million people worldwide, exemplifies the devastating impact of myelin loss. Although there is no definitive cure, disease management with corticosteroids and disease‑modifying therapies improves quality of life and slows progression.
