Nervous Tissue: Neurons and Neuroglia

Neuron structure (a closer look). As introduced in histology, a neuron has dendrites (receive incoming signals), a cell body/soma (contains the nucleus; integrates), and an axon (transmits the outgoing signal). The axon ends in axon terminals that release neurotransmitters to the next cell. Neurons are excitable (they generate electrical signals) and long-lived but mostly do not divide, so lost CNS neurons generally aren't replaced.

Three functional types of neurons:

• Sensory (afferent) neurons carry information from receptors toward the CNS (for example, signaling touch or temperature).
• Motor (efferent) neurons carry commands from the CNS out to muscles and glands.
• Interneurons (association neurons) lie entirely within the CNS and connect sensory and motor neurons, doing most of the integrating and decision-making. They are by far the most numerous.

A simple reflex illustrates all three: a sensory neuron detects a hot surface, interneurons in the spinal cord process it, and a motor neuron triggers withdrawal.

Neuroglia — essential support. Glial cells outnumber neurons and are vital. Recall the major types:

• CNS glia: astrocytes (support neurons, regulate the chemical environment, help form the blood–brain barrier), oligodendrocytes (produce myelin in the CNS), microglia (immune defense), and ependymal cells (line CNS cavities, help make and circulate cerebrospinal fluid).
• PNS glia: Schwann cells (produce myelin in the PNS) and satellite cells (support neuron cell bodies in ganglia).

Myelin and conduction speed. Myelin is a fatty, insulating sheath wrapped around many axons in segments, with small gaps between segments (nodes). Myelin dramatically speeds signal conduction because the impulse effectively "jumps" from gap to gap rather than traveling smoothly along the whole membrane (this saltatory conduction is detailed in the next section). Bundles of myelinated axons form the white matter of the CNS, while neuron cell bodies and unmyelinated fibers form the gray matter. Loss of myelin — as in multiple sclerosis (CNS) or certain neuropathies (PNS) — slows or blocks signals, causing weakness, numbness, and coordination problems.

How it works

Signal handling by neuron type:

Sensory neuron (receptor → CNS)
   → interneurons integrate within the CNS
   → motor neuron (CNS → muscle/gland)
Glia support throughout; myelin (oligodendrocytes/Schwann cells) speeds conduction

Comparisons

Common confusions

• Sensory vs motor vs interneuron. In (afferent), out (efferent), and within-CNS (interneuron).
• Gray vs white matter. Gray = cell bodies/unmyelinated; white = myelinated axons (white from myelin's fat).
• Oligodendrocyte vs Schwann cell. Same job (myelin) — CNS vs PNS.
• Neurons rarely divide — a reason CNS damage is often permanent.

Memory aids

• "Sensory = Senses in; Motor = Moves out; Inter = In-between (CNS)."
• White matter = "Wrapped (myelin) tracts."
• Astrocyte = "star that guards the brain's border."

Quick review

• A neuron has dendrites (receive), a cell body (integrate), and an axon (transmit); neurons are excitable and rarely divide.
• Three functional types: sensory (in), motor (out), interneurons (within CNS, most numerous).
• Neuroglia support neurons: astrocytes (barrier), oligodendrocytes/Schwann cells (myelin), microglia (immune), ependymal (CSF), satellite (PNS support).
• Myelin speeds conduction; its loss (e.g., MS) causes neurological deficits. White matter = myelinated; gray matter = cell bodies.