Nervous tissue forms the complex communication network that regulates and controls bodily functions and activities. This specialized tissue is the primary component of the nervous system, which is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord, while the PNS comprises the branching nerves that extend throughout the rest of the body. The fundamental function of nervous tissue is to rapidly detect stimuli, process information, and transmit signals to coordinate responses across vast distances. This high-speed communication allows for sensory input, muscle control, integration, and higher mental activity.
The Two Primary Cell Categories
Nervous tissue is characterized by the presence of two distinct and interdependent cell populations: neurons and glial cells. Neurons, often called nerve cells, are the excitable functional units responsible for generating and transmitting electrical and chemical signals. They are the cells that perform the computation and communication that defines the nervous system.
Glial cells, or neuroglia, are non-neuronal cells that provide a comprehensive support framework. While they do not produce electrical impulses themselves, their collective role is to maintain the health and optimal function of the neurons. This partnership is absolute, as neurons cannot perform their signaling tasks without the extensive maintenance and protection provided by their glial partners.
Neurons The Communicators
Neurons are specialized cells for receiving, integrating, and transmitting information over long distances. The three primary anatomical components of a neuron are the cell body, the dendrites, and the axon. The cell body, also known as the soma, contains the nucleus and the necessary organelles for cellular activity, such as protein synthesis.
Dendrites are highly branched, tree-like extensions that project from the soma and function as the neuron’s primary receiving antennas. They collect incoming signals from other neurons and relay this information toward the cell body. The axon is a single, typically long extension that conducts the signal away from the soma toward target cells.
The transmission of information along the axon occurs through an electrical impulse called an action potential. This is a rapid, all-or-nothing change in the voltage across the neuron’s membrane, which is initiated when the summation of incoming signals reaches a specific threshold. The action potential is generated by the swift, coordinated opening and closing of voltage-gated ion channels, primarily involving the influx of sodium ions and the subsequent efflux of potassium ions.
Once the electrical impulse reaches the end of the axon, it arrives at a specialized junction called the synapse. Here, the electrical signal is converted into a chemical signal through the release of signaling molecules called neurotransmitters. These chemicals diffuse across the tiny synaptic gap and bind to receptors on the target cell, which may be another neuron, a muscle cell, or a gland. Neurons are post-mitotic, meaning they do not divide, highlighting their irreplaceable role as the excitable units of the nervous system.
Glial Cells The Support System
Glial cells are smaller and far more numerous than neurons, and they perform a diverse array of functions that are essential for neuronal survival and signaling efficiency. These supportive cells are categorized based on their location in either the CNS or the PNS.
Astrocytes, found in the CNS, are star-shaped cells that provide physical and metabolic support to neurons. They help maintain the chemical environment, regulate the concentration of ions and neurotransmitters outside the neurons, and are instrumental in forming the blood-brain barrier.
Another type of CNS glia, the oligodendrocyte, is responsible for producing myelin, a fatty insulating sheath that wraps around axons. Myelin is not a continuous layer, but rather is segmented, allowing the electrical signal to jump between the unmyelinated gaps, which significantly increases the speed of impulse conduction. In the PNS, Schwann cells perform this same myelin-forming function, wrapping around the axons of peripheral nerves.
Microglia are the resident immune cells of the CNS, constantly surveying the brain and spinal cord environment for signs of damage or disease. They act as phagocytes, engulfing and removing cellular debris, dead neurons, and pathogens to protect the delicate neural tissue. Satellite cells are found in the PNS, where they surround the cell bodies of peripheral neurons and help regulate their external chemical environment, performing a role similar to astrocytes.