Anatomy and Physiology 2e · The Nervous System and Nervous Tissue
Nervous Tissue
On this page 9 sections
In 30 seconds
Nervous tissue is the tissue of communication, made of two cell families with very different jobs. Neurons are excitable cells that generate and conduct electrical signals — the functional units of the nervous system. Neuroglia (glial cells) are the supporting cast: they nourish neurons, insulate axons, defend the tissue, and maintain the chemical environment. Glia do not conduct signals, but without them neurons could not survive or signal properly.
Because neurons are the cells that carry information, most of this topic is about their structure and classification. But the glia matter enormously: many nervous system disorders involve glial dysfunction, and glial cells are the source of several types of brain tumors.
Why this matters
Nervous tissue is the substrate of every thought, sensation, and movement, so its cellular details are the foundation for the rest of the book. Clinically, myelin is a recurring theme: in multiple sclerosis (MS) the CNS is affected; in Guillain–Barré syndrome the PNS is affected. Understanding which cells make myelin where helps you predict the pattern of dysfunction. And because glia outnumber neurons and divide throughout life, most primary brain tumors arise from glial cells (gliomas), not from neurons, which are largely post-mitotic.
The college version
Core Concepts
Anatomy of a neuron
A typical neuron An excitable cell that generates and conducts electrical signals Full entry → has three functional regions.
- Cell body (soma): contains the nucleus and most organelles, including Nissl bodies (clusters of rough endoplasmic reticulum). It is the metabolic center of the cell.
- Dendrites: branched, tapering processes that receive signals from other neurons or sensory receptors. They are the "input zone."
- axon A single process that conducts signals away from the cell body Full entry →: a single long process that conducts electrical signals away from the cell body. The axon hillock, where the axon leaves the soma, is commonly the "trigger zone" where action potentials begin. The axon ends in branched axon terminals (synaptic bulbs) that release chemical messengers (neurotransmitters) onto the next cell.
Classification of neurons
By structure (number of processes):
- Multipolar neurons — one axon and many dendrites; the most common type (most motor neurons and interneurons).
- Bipolar neurons — one axon and one dendrite A branched process that receives signals Full entry →; found in special sensory pathways such as the retina and olfactory epithelium.
- Unipolar (pseudounipolar) neurons — a single process that splits into a peripheral branch (dendrite) and a central branch (axon); typical of sensory neurons in dorsal root ganglia.
By function:
- Sensory (afferent) neurons carry information toward the CNS.
- Motor (efferent) neurons carry commands away from the CNS to muscles and glands.
- Interneurons (association neurons) lie entirely within the CNS and connect sensory and motor pathways; they are the most numerous.
Neuroglia of the CNS
- Astrocytes — star-shaped cells that support neurons metabolically, regulate the extracellular environment, and contribute to the blood–brain barrier by wrapping brain capillaries. Commonly taught as the most abundant glial cell (verify against current texts).
- Oligodendrocytes — produce the myelin sheath Lipid-rich insulation around axons Full entry → in the CNS. One oligodendrocyte CNS glial cell that makes myelin Full entry → myelinates segments of many axons.
- microglia Immune cells of the CNS Full entry → — the immune cells of the CNS; they phagocytose debris and pathogens and respond to injury or infection.
- Ependymal cells — line the brain's ventricles and the spinal cord's central canal; ciliated ones help circulate cerebrospinal fluid (CSF).
Neuroglia of the PNS
- Schwann cells — produce myelin in the PNS. One Schwann cell PNS glial cell that makes myelin Full entry → wraps one segment of one axon. They also assist in axon regeneration after injury.
- Satellite cells — surround and support neuron cell bodies within ganglia, regulating their chemical environment.
Myelin and the myelin sheath
The myelin sheath is a lipid-rich insulating layer formed by the plasma membranes of oligodendrocytes (CNS) or Schwann cells (PNS) wrapped around the axon. It speeds up signal conduction and reduces the energy cost of signaling. Between adjacent myelin segments are bare gaps called nodes of Ranvier, where the axon membrane is exposed and ion exchange occurs — this arrangement lets signals "jump" from node to node (saltatory conduction, covered in The Function of Nervous Tissue).
Because myelin is pale, regions packed with myelinated axons look white (white matter), while regions rich in cell bodies and dendrites look gray (gray matter).
How It Works / Step-by-Step Process
Follow a signal through a simple sensory pathway:
- Reception: a sensory receptor (e.g., in the skin) stimulates the peripheral branch of a unipolar sensory neuron.
- Conduction: the signal travels along the axon — quickly if myelinated, jumping between nodes of Ranvier.
- Cell body relay: the signal passes the cell body, located in a dorsal root ganglion (a cluster of PNS cell bodies).
- Central arrival: the central branch enters the spinal cord and ends on an interneuron.
- Integration: the interneuron passes the signal to a motor neuron or up a tract to the brain.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Oligodendrocytes and Schwann cells | Both make myelin | Oligodendrocytes are CNS (one cell, many axons); Schwann cells are PNS (one cell, one axon segment) |
| Dendrites and axons | Both neuron processes | Dendrites receive input toward the cell body; axons conduct output away from it |
| Bipolar and unipolar neurons | Number of processes | Bipolar has one axon + one dendrite (special senses); unipolar has one split process (sensory ganglia) |
| Microglia and other glia | "Support" role | Microglia are immune defenders (phagocytosis), not metabolic supporters |
| Neurons and glia | Which outnumbers which | Glia are commonly taught to outnumber neurons — verify the ratio against current texts |
| Gray matter and white matter | Location vs. content | Gray = cell bodies/dendrites; white = myelinated axons; the arrangement is inverted between brain and spinal cord |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of a neuron as a tiny wire: a main office (cell body), lines coming in (dendrites that receive messages), and one long outgoing wire (the axon that sends messages). Around many wires is a coating like lamp-cord insulation — myelin — that makes messages travel much faster. The helper cells (glial cells) are like the workers who build the insulation, feed the wires, and clean up when something breaks.
Worked example
Imagine a patient with tingling and weakness in one leg, and a neurologist suspects spinal cord demyelination. The reasoning chain uses this topic directly: myelin in the CNS is made by oligodendrocytes; if oligodendrocyte function is lost (as in MS), conduction in CNS tracts slows or blocks, producing the symptoms. In contrast, a PNS problem such as Guillain–Barré syndrome involves Schwann cell myelination and typically affects peripheral nerves first. Knowing which glial cell makes myelin where connects tissue-level concepts to whole-body symptoms and explains why similar symptoms can point to different parts of the nervous system.
Key takeaways
- Neurons = signaling; neuroglia = support. Glia outnumber neurons and do not generate action potentials.
- One axon, many dendrites is the classic layout: dendrites receive, axon conducts away from the cell body.
- Structural classes: multipolar (most common), bipolar (special senses), unipolar/pseudounipolar (sensory neurons in ganglia).
- Functional classes: sensory (afferent), motor (efferent), interneurons (CNS connectors, most numerous).
- Myelin makers: oligodendrocytes in the CNS (one cell, many axons); Schwann cells in the PNS (one cell, one axon segment).
- Astrocytes build the blood–brain barrier partnership; microglia are the immune cells; ependymal cells line CSF spaces.
- Gray vs. white matter = cell bodies vs. myelinated axons — reversed between brain (gray outside) and spinal cord (gray inside).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the three structural classes of neurons and where each is typically found.
Show answer
Multipolar (one axon, many dendrites — most common; motor neurons, interneurons), bipolar (one axon, one dendrite — special senses), and unipolar/pseudounipolar (single split process — sensory neurons in PNS ganglia).
Which glial cell makes myelin in the CNS, and which makes it in the PNS?
Show answer
Oligodendrocytes in the CNS; Schwann cells in the PNS.
What is the functional difference between dendrites and an axon?
Show answer
Dendrites receive signals (input zone); the axon conducts signals away from the cell body (output).
What are the three functional classes of neurons?
Show answer
Sensory (afferent), motor (efferent), and interneurons (association).
Why does myelin speed up signal conduction, and what are the bare gaps between myelin segments called?
Show answer
Myelin insulates the axon and lets the signal jump between nodes of Ranvier (saltatory conduction) — faster and more energy-efficient than continuous conduction.
Which glial cells contribute to the blood–brain barrier, and which are the immune cells of the CNS?
Show answer
Astrocytes contribute to the blood–brain barrier; microglia are the immune cells of the CNS.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- neuron
- An excitable cell that generates and conducts electrical signals
- dendrite
- A branched process that receives signals
- axon
- A single process that conducts signals away from the cell body
- astrocyte
- A star-shaped CNS glial cell
- oligodendrocyte
- CNS glial cell that makes myelin
- Schwann cell
- PNS glial cell that makes myelin
- microglia
- Immune cells of the CNS
- ependymal cell
- Cell lining CSF spaces
- myelin sheath
- Lipid-rich insulation around axons
- node of Ranvier
- Bare gap between myelin segments
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.