Nervous tissue is the body’s specialized communication network, enabling rapid signaling and control throughout an organism. This tissue detects changes in the internal and external environment and coordinates an immediate response. Its role is to manage and regulate all body functions, from involuntary actions like digestion to complex processes such as thought, memory, and reasoning. It facilitates the body’s interaction with its surroundings and maintains a stable internal state.
The Functional Unit: Neurons
The core element of nervous tissue is the neuron, a unique cell that specializes in generating and transmitting electrochemical signals across vast distances. Each neuron possesses a distinct structure that supports its communicative function, beginning with the cell body, or soma, which contains the nucleus and the cell’s metabolic machinery. Branching out from the soma are the dendrites, which function like antennae, receiving incoming messages from thousands of other neurons. These tree-like extensions are the primary receivers of information, channeling signals toward the cell body for processing.
The processed signal then travels down a single, long extension called the axon, which is designed to conduct the electrical impulse away from the soma. This electrical signal, known as an action potential, is a rapid, temporary shift in the electrical charge across the neuron’s membrane. The speed and reliability of this transmission are paramount for swift responses to stimuli. The axon terminates at a structure called the synapse, where the electrical signal is converted into a chemical message.
At the synapse, the neuron releases chemical messengers called neurotransmitters into a tiny gap between cells. These molecules diffuse across the synaptic gap and bind to specific receptors on the receiving neuron’s dendrites or cell body. This chemical exchange allows the signal to jump from one cell to the next. This system of electrical generation and chemical release is the mechanism by which all sensory input, motor commands, and cognitive functions are carried out.
The Supporting Structure: Glial Cells
While neurons are the conductors of information, the system’s survival and efficiency depend on the second main cell type, collectively called neuroglia or glial cells. These cells do not transmit nerve impulses themselves but provide a necessary framework for the neurons, offering physical support, nourishment, and protection. Glial cells are generally smaller than neurons, yet they often outnumber them.
One of the most recognized functions of glia is insulation, which is performed by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). These cells wrap a fatty layer called the myelin sheath around the axons of many neurons. This lipid-rich covering acts like electrical insulation, dramatically increasing the speed at which the action potential can travel down the axon. Without this insulation, complex, rapid motor movements and sensory processing would not be possible.
Astrocytes, named for their star-like shape, are highly abundant and perform various homeostatic functions. These cells regulate the chemical environment surrounding neurons, managing the concentration of ions and neurotransmitters. Astrocytes are also instrumental in forming the blood-brain barrier, which controls which substances can pass from the bloodstream into the brain tissue. Furthermore, microglia act as the immune defense cells of the nervous system, monitoring the environment for damage or pathogens and clearing cellular debris.
Organization Across the Body
Nervous tissue is organized into two large divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS serves as the primary processing center, consisting of the brain and the spinal cord, which are protected by bone and specialized membranes. The PNS encompasses all nervous tissue outside the brain and spinal cord, connecting the CNS to every other part of the body.
Within the CNS, the tissue is distinctly categorized into two types based on its appearance and composition: gray matter and white matter. Gray matter is primarily composed of neuron cell bodies, dendrites, and neuroglia, and it is the region where information processing and integration take place. In the brain, gray matter forms the outer layer, while in the spinal cord, it is concentrated in the central, butterfly-shaped core.
White matter, conversely, gets its lighter color from the high concentration of myelinated axons. These bundled axons form tracts that serve as relay cables, transmitting signals rapidly between different gray matter areas and connecting the CNS to the PNS.