Anatomy and Physiology 2e · The Tissue Level of Organization
Nervous Tissue Mediates Perception and Response
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In 30 seconds
Nervous tissue is the body's communication system, built from two cell populations that work as a team: neurons, which carry electrical signals, and neuroglia (glial cells), which support, protect, and nourish them. Nervous tissue lets you perceive the world — light on your retina, pressure on your skin, the smell of coffee — and respond, by contracting a muscle, releasing a hormone, or remembering a face.
The functional path is easy to remember: a stimulus is detected by a sensory (afferent) neuron The cell that conducts electrical signals in the nervous system Full entry →, which carries the signal toward the central nervous system (brain and spinal cord); interneurons process the information; then a motor (efferent) neuron carries a signal away to an effector — a muscle or gland — that produces the response. A reflex arc Receptor → sensory neuron → integration center → motor neuron → effector Full entry → is the shortest version: stimulus in, response out, no conscious thought. Neurons are the wires; neuroglia are the insulation, scaffolding, and custodians that keep the wires working.
Why this matters
Every thought, sensation, and movement runs through nervous tissue, making this topic the foundation for the nervous system chapters ahead. The neuron's structure — dendrites receiving, cell body integrating, axon Single long fiber that conducts signals away from the cell body Full entry → conducting — explains how signals travel from fingertip to brain and back in a fraction of a second. Neuroglia explain a whole family of conditions: multiple sclerosis (MS) involves damage to the myelin that oligodendrocytes produce in the CNS, slowing or blocking conduction; meningitis is inflammation of the membranes around the CNS; many brain tumors arise from glial cells rather than neurons.
For healthcare students, the reflex arc is a daily concept: knee-jerk, pupillary light, and deep tendon reflexes all test nervous tissue function. Knowing which neuron carries information in which direction — afferent toward the CNS, efferent away from it — is among the most reliable exam distinctions in anatomy. Person-first language matters: a person "has multiple sclerosis," not "is an MS patient," and symptoms vary widely. (Disease descriptions are commonly-taught reference concepts; verify against current sources.)
The college version
Core Concepts
Neurons: the signal carriers
A neuron has three functional regions:
- Dendrites — branched extensions that receive signals and conduct them toward the cell body.
- Cell body (soma) — contains the nucleus and most organelles; integrates incoming signals.
- Axon — a single long fiber that conducts the signal away from the cell body to the axon terminals, which release neurotransmitters onto the next cell. Most axons are wrapped in a myelin sheath Fatty insulation around axons produced by glial cells Full entry →, a fatty insulation that speeds up conduction.
Neurons are classified functionally as sensory (afferent — toward the CNS), motor (efferent — away from the CNS to effectors), or interneurons (connecting neurons within the CNS), and structurally as multipolar (many dendrites, one axon — most neurons), bipolar (one dendrite Branched extension that receives signals and conducts them toward the cell body Full entry →, one axon — special senses like the retina), or unipolar (a single process that splits — most sensory neurons).
Neuroglia: the support team
Neuroglia outnumber neurons and do the housekeeping. In the CNS:
- Astrocytes — star-shaped; support neurons, form the blood-brain barrier, regulate the chemical environment.
- Oligodendrocytes — produce the myelin sheath around CNS axons (one cell wraps several axons).
- microglia Immune cells of the CNS that engulf debris and pathogens Full entry → — the immune cells of the brain; engulf debris and pathogens.
- Ependymal cells — line the fluid-filled cavities (ventricles) and help circulate cerebrospinal fluid.
In the PNS: Schwann cells make myelin around peripheral axons (one cell wraps one axon segment) and help regrow damaged nerve fibers; satellite cells support neuron cell bodies in ganglia.
The reflex arc: perception to response in one loop
A reflex is a rapid, predictable, involuntary response to a stimulus. The classic arc has five parts: receptor (sensory ending detects the stimulus) → sensory neuron (carries the signal to the spinal cord) → integration center (an interneuron Neuron that connects other neurons within the CNS Full entry → or direct synapse in the CNS) → motor neuron (carries the signal out) → effector (muscle or gland responds). The patellar reflex uses just two neurons and one synapse, so the knee kicks before your brain "decides" anything — the brain learns about it afterward. Reflex testing is fast and reliable precisely because it bypasses conscious processing.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Dendrite | Axon | Dendrites receive and conduct toward the cell body; the axon conducts away from it |
| Neuron | Nerve | A neuron is a single cell; a nerve is a bundle of many axons plus connective tissue in the PNS |
| Afferent | Efferent | Afferent carries sensory info toward the CNS; efferent carries motor commands away from it |
| Oligodendrocyte | Schwann cell | Both make myelin, but oligodendrocytes are in the CNS (one cell, many axons); Schwann cells are in the PNS (one cell, one axon segment) |
| Neurons | Neuroglia | Neurons conduct signals; neuroglia support, insulate, and defend — and can divide, which most mature neurons cannot |
| Interneuron | Motor neuron | Interneurons stay inside the CNS and connect other neurons; motor neurons exit the CNS to drive effectors |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Nervous tissue is like a city's phone-and-wire system. The neurons are the telephone wires that carry messages, and the glial cells are the poles, insulation, and repair crews that hold the wires up and keep them working. When you touch something hot, a wire (sensory neuron) carries the "hot!" message to the main office (spinal cord), and the office instantly sends a message back on another wire (motor neuron) telling your hand to pull away — before you even think about it.
Worked example
Your fingertip brushes a hot stove. Walk through the reflex arc:
- Receptor: sensory nerve endings in your fingertip detect the intense heat.
- Sensory neuron: the signal races along the afferent axon (wrapped in Schwann-cell myelin, which speeds it up) toward the spinal cord.
- Integration center: inside the spinal cord, the sensory neuron synapses on an interneuron, which immediately synapses on a motor neuron — no trip to the brain required.
- Motor neuron: the efferent signal travels out to the muscles of your arm.
- Effector: the arm muscles contract, jerking your hand away.
You feel the pain only after the hand has moved, because the pain signal takes the slower route up to the brain. That delay shows why reflexes exist: they buy speed by cutting out the "thinking" part. The same five-step logic traces any reflex — the knee jerk, blinking, or pulling your foot back from a sharp object.
Key takeaways
- Two cell types: neurons (signal conduction) and neuroglia (support). Know the four CNS glia — astrocytes, oligodendrocytes, microglia, ependymal cells — and the two PNS glia — Schwann cells, satellite cells.
- Signal direction: dendrites and cell body receive → axon conducts → axon terminals release neurotransmitters. "Dendrites deliver, axons announce."
- Functional classes: sensory (afferent, toward CNS), motor (efferent, away from CNS), interneurons (between neurons in the CNS).
- Structural classes: multipolar (most neurons), bipolar (special senses), unipolar (most sensory neurons).
- Myelin speeds conduction, made by oligodendrocytes (CNS) and Schwann cells (PNS); damage to myelin (as in MS) slows or blocks signals.
- Reflex arc = receptor → sensory neuron → integration center → motor neuron → effector. The knee-jerk reflex is the classic two-neuron example.
- Glial cells can divide; most adult neurons cannot — why brain and spinal cord injuries are so serious.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the three functional parts of a neuron and the direction of signal flow through each.
Show answer
Dendrites receive signals (toward the cell body), the cell body integrates them, and the axon conducts the signal away to the axon terminals.
What is the difference between afferent and efferent neurons?
Show answer
Afferent (sensory) neurons carry information toward the CNS; efferent (motor) neurons carry commands away from the CNS to effectors.
Name the four types of neuroglia in the CNS and one job for each.
Show answer
Astrocytes (support and blood-brain barrier), oligodendrocytes (myelin), microglia (immune defense/cleanup), ependymal cells (line ventricles, help circulate cerebrospinal fluid).
Put the five components of a reflex arc in order.
Show answer
Receptor → sensory neuron → integration center → motor neuron → effector.
Why does the knee-jerk reflex happen before you consciously "decide" to kick?
Show answer
The reflex arc is processed in the spinal cord via a two-neuron or three-neuron loop, bypassing conscious brain processing — so the response is faster than the perception of pain.
Which glial cells make myelin, and where does each type work?
Show answer
Oligodendrocytes make myelin in the CNS; Schwann cells make myelin in the PNS.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- neuron
- The cell that conducts electrical signals in the nervous system
- dendrite
- Branched extension that receives signals and conducts them toward the cell body
- axon
- Single long fiber that conducts signals away from the cell body
- myelin sheath
- Fatty insulation around axons produced by glial cells
- afferent / efferent
- Carrying toward the CNS / carrying away from the CNS
- interneuron
- Neuron that connects other neurons within the CNS
- astrocyte
- Star-shaped CNS glial cell that supports neurons and forms the blood-brain barrier
- oligodendrocyte
- CNS glial cell that produces myelin
- microglia
- Immune cells of the CNS that engulf debris and pathogens
- Schwann cell
- PNS glial cell that produces myelin around peripheral axons
- reflex arc
- Receptor → sensory neuron → integration center → motor neuron → effector
- neurotransmitter
- Chemical released at axon terminals that signals the next cell
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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