Nervous tissue
Revised 2025-10-16 9:40:46 AM EDT
Nervous system cells
At the heart of nervous tissue is the nerve cell (“neuron”) – a highly specialized cell that receives, processes, and sends information. Although neurons come in many shapes, you can understand any of them by learning three core parts.
Each terminal has a rounded synaptic knob filled with synaptic vesicles – microscopic packets of chemical messenger. The meeting place where one neuron communicates with another (or with a muscle or gland) is the synapse: the terminal sits opposite the receiving cell across a narrow space called the synaptic cleft. The membrane of the sending side is the presynaptic membrane; the receiving side is the postsynaptic membrane. When an electrical signal reaches the end of the axon, vesicles fuse with the presynaptic membrane and release neurotransmitter into the cleft. The transmitter diffuses across and binds receptors on the postsynaptic membrane, changing the activity of the target cell that the presynaptic neuron is sending a message to.
The electrical signal moving along the axon is the nerve impulse or action potential. You can picture it as a brief, self‑propagating spike of voltage that travels down the axon without fading. The phrase impulse conduction simply describes the way the action potential moves down the axon.
In many neurons, this process is dramatically sped up by the myelin sheath (sometimes shortened to “myelin”), a fatty insulating wrap that forms a sheath around the axon. Outside of the brain and spinal cord, myelin is produced by the Schwann cell. Each Schwann cell wraps a small segment of the axon and leaves tiny gaps between segments called the Nodes of Ranvier. The action potential traveling down the axon effectively “jumps” from node to node, which conserves energy and increases speed and reliability of the signal. Because the Schwann cell’s outermost layer remains after wrapping, the PNS also has a thin outer coat called the neurolemma; this layer is important for peripheral axon repair after injury.
A few more details will help you visualize synapses as living workspaces rather than abstract diagrams. Within each synaptic knob, synaptic vesicles are loaded with neurotransmitter by tiny pumps. When an action potential reaches the axon terminal, calcium channels open and calcium flows in to the axon terminal. This rise in calcium triggers vesicle fusion with the presynaptic membrane and the neurotransmitter within is pushed out of the axon terminal and into the synaptic cleft. The synaptic cleft is so narrow that molecules drift across in a fraction of a second to bind to receptors on the postsynaptic membrane.
Depending on the receptor, the effect may be excitatory (encouraging the next cell to fire its own action potential) or inhibitory (making firing less likely). You can think of these as “yes” and “no” votes for whether the target cell should do something. Afterward, enzymes break down some transmitter, and specific transporters recycle the rest back into the presynaptic terminal for future use. In sensory pathways, thousands of synapses are arranged in series and in parallel, creating richly tunable “dimmer switches” for perception and movement.
Gray matter and white matter
Knowing those parts of the neuron equips you to understand how nervous tissue is arranged inside the body. At a gross‑anatomy level, the nervous system is often described as two colors: gray matter and white matter. Although both are made of neurons and support cells, they have very different compositions and jobs.
Gray matter
Gray matter is where information is processed and exchanged. It contains neuron cell bodies, dendrites, many synapses, and short, unmyelinated segments of axons. In the brain, gray matter forms the thin outer shell known as the cerebral cortex and also appears as deeper clusters of cell bodies. In the spinal cord, gray matter forms an H‑shaped core, where incoming sensory information is integrated and outgoing motor commands are shaped. When you imagine perception, memory, and decision‑making, you are picturing the work of gray matter – the places where synapses change strength and new patterns are formed through learning and experience.
White matter
White matter is the wiring that connects distant regions. It appears pale because it is packed with bundles of myelinated axons; myelin’s fatty composition gives the tissue its characteristic whiteish color in contrast to the gray matter. White matter allows fast communication between gray matter regions so that what is decided in one place can influence another almost immediately. In the brain, white matter fills the interior of the brain to connect brain cells, even connecting the left and right hemispheres of the brain. In the spinal cord, white matter surrounds the gray core and carries ascending sensory information from the body and descending motor commands from the brain to the spinal levels.
Here we meet an important naming convention that locates you in the nervous system at a glance.