Nervous tissue is one of the four fundamental tissue types in the human body, alongside muscle, connective, and epithelial tissues. It stands out for its remarkable complexity and versatility.
The cells that compose nervous tissue are known as neurons—nerve cells that carry electrochemical signals—or, more colloquially, "nerves." These neurons are supported by a diverse group of cells called glial cells (or neuroglia), which provide essential structural, metabolic, and protective functions.
Neurons are the functional carriers of information, while glial cells act as the nervous system’s indispensable support network. Glia, literally Latin for "glue," are crucial for maintaining the integrity and performance of neural circuits.
Glial cells are found throughout the body, with the majority residing in the central nervous system (CNS)—the brain and spinal cord—and a smaller subset in the peripheral nervous system (PNS)—all neural tissue outside the CNS.
Key CNS glial subtypes include astrocytes, ependymal cells, oligodendrocytes, and microglia. In the PNS, the primary glial cells are Schwann cells and satellite cells.
Nervous tissue is unique because it is excitable and capable of generating and transmitting action potentials—brief electrical impulses that propagate along neurons.
Neurons communicate by releasing neurotransmitters across synapses, the tiny gaps between the axon terminal of one neuron and the dendrites or cell body of the next. This chemical signaling underpins everything from reflexes to complex cognition.
Functionally, the nervous system is divided into the somatic (voluntary) and autonomic (involuntary) branches, with motor neurons (efferent) transmitting commands to muscles and glands, sensory neurons (afferent) conveying environmental and internal information to the CNS, and interneurons serving as local relays.
The human brain contains an estimated 86 billion neurons, of which roughly 75 % are glial cells. This ratio underscores the importance of glia in supporting neuronal function.
Neurons share several key structures: dendrites, a cell body (soma), an axon, and axon terminals. Dendrites receive synaptic input; the soma houses the nucleus; the axon transmits action potentials; and axon terminals release neurotransmitters into the synaptic cleft.
Neurons can be categorized by morphology:
While neurons conduct electrical signals, glial cells do not transmit action potentials. Instead, they form regular junctions with neurons and other glia, facilitating support and communication without synaptic activity.
Glia possess a single process attached to their soma and retain the ability to divide—an essential feature given their constant exposure to mechanical and metabolic stress.
Astrocytes are star‑shaped cells that maintain the blood–brain barrier, regulate extracellular ion concentrations, and modulate synaptic activity via gliotransmitters. They exist in protoplasmic and fibrous forms and constitute a major component of the brain’s structural scaffold.
Ependymal cells line the ventricles and spinal cord, producing cerebrospinal fluid (CSF) that cushions neural tissue and facilitates waste clearance. They also play roles in neural regeneration and are arranged into a choroid plexus that exchanges substances between CSF and blood.
Oligodendrocytes generate the myelin sheath that insulates axons in the CNS, enabling rapid saltatory conduction of action potentials. Each oligodendrocyte can myelinate multiple axons, creating nodes of Ranvier where ion channels are concentrated.
Microglia serve as the brain’s resident immune cells, surveying the neural environment, phagocytosing debris, and sculpting synaptic connections during development. Aberrant microglial activation has been linked to neuroinflammatory processes in Alzheimer’s disease and other neurodegenerative disorders.
Satellite cells envelop neuron cell bodies within ganglia, regulating the chemical milieu and providing metabolic support. They are implicated in chronic pain pathways and help maintain the stability of peripheral sensory networks.
Schwann cells produce myelin in the PNS, wrapping a single segment of an axon between nodes of Ranvier. Unlike oligodendrocytes, each Schwann cell myelinates only one portion of a single axon, ensuring precise insulation and rapid signal propagation.
Related article: Where are Stem Cells Found?
