Nervous Tissue

Two cell classes. Nervous tissue is built from neurons, which do the actual signaling, and neuroglia ("nerve glue"), which support, protect, and nourish neurons. Neuroglia are far more numerous than neurons and are essential — without them, neurons cannot function.

Neuron structure. A typical neuron has three regions, matching its job of receiving, integrating, and sending signals:

• The cell body (soma) contains the nucleus and organelles; it is the neuron's metabolic center.
• Dendrites are branching extensions that receive signals from other neurons and funnel them toward the cell body. More dendrites means more incoming connections.
• The axon is a single, often long projection that sends the signal away toward its target (another neuron, a muscle, or a gland). Some axons run over a meter (for example, from the spinal cord to the foot).

Many axons are wrapped in myelin, a fatty insulating sheath that dramatically speeds signal conduction — signals effectively jump between gaps in the myelin (the process detailed in the nervous system unit). Loss of myelin, as in multiple sclerosis, slows or blocks signaling.

Neuroglia by location. Glial cells differ in the central nervous system (CNS: brain and spinal cord) and peripheral nervous system (PNS: nerves elsewhere). You don't need every detail now, but the major players include:

• Astrocytes (CNS): the most abundant glia; support neurons, regulate the chemical environment, and help form the blood–brain barrier.
• Oligodendrocytes (CNS) and Schwann cells (PNS): produce myelin (oligodendrocytes in the CNS, Schwann cells in the PNS).
• Microglia (CNS): immune defense cells that remove debris and pathogens.
• Ependymal cells (CNS): line fluid-filled spaces and help circulate cerebrospinal fluid.
• Satellite cells (PNS): support neuron cell bodies in ganglia.

Fast signals, poor repair. Neurons are highly specialized and mostly do not divide (recall they largely rest in G0). This helps preserve learned connections but means that destroyed neurons are generally not replaced, so damage to the brain or spinal cord tends to be permanent. Peripheral axons can sometimes regrow slowly if the cell body survives (aided by Schwann cells), but CNS regeneration is very limited.

How it works

Signal flow through a neuron:

Dendrites receive a signal
   → Cell body integrates it
   → Axon carries the signal away
   → Signal passes to the next cell (neuron, muscle, or gland)

Myelin around the axon speeds this transmission.

Comparisons

Common confusions

• Dendrite vs axon. Dendrites receive (into the cell body); the axon sends (away). "D for delivery in, A for away."
• Neuron vs neuroglia. Neurons signal; glia support. Both are needed.
• Myelin isn't the axon. It's an insulating wrap made by other cells (oligodendrocytes/Schwann cells).
• Oligodendrocyte vs Schwann cell. Same job (myelin) in different places: CNS vs PNS.

Memory aids

• "Dendrites = Deliver signals IN; Axon = Away."
• Schwann = "PNS wraps" ; Oligodendrocytes = "CNS wraps."
• Astrocytes = "star-shaped support + barrier."

Quick review

• Nervous tissue = neurons (signal) + neuroglia (support).
• A neuron has dendrites (receive), a cell body (integrate), and an axon (send); myelin speeds conduction.
• Glia include astrocytes (support/barrier), oligodendrocytes/Schwann cells (myelin), microglia (immune), ependymal (CSF), satellite (PNS support).
• Neurons largely don't divide, so CNS damage (stroke, spinal cord injury) is usually permanent.