Nervous tissue forms the master control and communication network of the human body. By rapidly transmitting electrical and chemical signals, it coordinates every activity from simple reflexes to complex abstract thought.
Nervous tissue consists of two principal cell types: neurons and neuroglia.
Neurons are the structural units of the nervous system. These excitable cells generate and transmit electrical signals across long distances. They typically possess a high metabolic rate and, in mature humans, largely lose their ability to divide.
Neuroglia, or glial cells, surround and wrap neurons. Historically viewed as mere passive glue, these smaller cells outnumber neurons significantly and actively support, insulate, and protect delicate neuronal networks.
A typical neuron features a cell body, branching dendrites, and a single axon.
Structurally, neurons are classified by the number of processes extending from their soma: multipolar neurons have many processes, bipolar neurons have two, and unipolar neurons have a single short process that divides into central and peripheral branches.
Neuroglia provide a supportive scaffolding for neurons. Their specific types and roles vary between the central nervous system (CNS) and the peripheral nervous system (PNS).
| Glial Cell Type | Location | Primary Function |
|---|---|---|
| Astrocytes | CNS | Form the blood-brain barrier, mop up leaked potassium ions, and recycle neurotransmitters |
| Microglia | CNS | Act as phagocytes, engulfing invading microorganisms and cellular debris |
| Ependymal Cells | CNS | Line cerebrospinal fluid-filled cavities and circulate the fluid using cilia |
| Oligodendrocytes | CNS | Form insulating myelin sheaths around multiple CNS nerve fibers |
| Schwann Cells | PNS | Form myelin sheaths around a single segment of a PNS nerve fiber |
| Satellite Cells | PNS | Surround neuron cell bodies in ganglia, regulating their chemical environment |
Myelin is a whitish, protein-lipoid sheath that wraps around long or large nerve fibers. This fatty covering protects the fiber, insulates it electrically, and dramatically increases the speed of nerve impulse transmission.
In the peripheral nervous system, myelination is performed by Schwann cells that wrap themselves concentrically around the axon. The gaps left between adjacent Schwann cells along the axon are called nodes of Ranvier. These uncovered gaps are crucial because electrical signals jump from node to node in a rapid process known as saltatory conduction.
Neurons are electrically excitable. When at rest, a neuron maintains a resting membrane potential, typically around -70 millivolts, meaning the inside of the cell is more negative than the outside. This charge difference is maintained by the sodium-potassium pump and leak channels.
When a strong enough stimulus arrives, it triggers an action potential—a brief reversal of membrane potential where the inside becomes temporarily positive.
Communication between neurons occurs at synapses, specialized junctions where an axon terminal of one neuron meets the dendrite or cell body of another.
When an action potential reaches the axon terminal, voltage-gated calcium channels open. Calcium influx causes synaptic vesicles to fuse with the membrane, releasing chemical messengers called neurotransmitters into the synaptic cleft. These molecules diffuse across the narrow gap and bind to specific receptors on the post-synaptic cell, triggering a new electrical change. Common neurotransmitters include acetylcholine, dopamine, and serotonin.
Damage to nervous tissue has vastly different outcomes depending on the anatomical location. Neurons in the central nervous system rarely regenerate due to growth-inhibiting proteins produced by glial scars and oligodendrocytes.
In contrast, peripheral nerve fibers can sometimes repair themselves if the cell body remains intact and the damage occurs outside the CNS. Schwann cells play a vital role here by forming a regeneration tube that guides the sprouting axon back toward its original target organ. However, precise functional recovery depends heavily on how cleanly the nerve was severed.