Nervous Tissue
Neurons & Neuroglia
Explore the two cell types that form the foundation of the nervous system: the signal-carrying neurons and their essential support cells, the neuroglia.
Anatomy & Physiology
What Is Nervous Tissue?
Nervous tissue is one of the four primary tissue types in the human body. It forms the brain, spinal cord, and all peripheral nerves. Its two principal cell populations work together to receive, integrate, and transmit information throughout the body.
Neurons
Electrically excitable cells that generate and conduct nerve impulses (action potentials).
Neuroglia
Support cells that protect, insulate, and nourish neurons: they do not conduct impulses.
Why Nervous Tissue Matters
Every thought, sensation, movement, and reflex depends on nervous tissue. Understanding its structure explains how the body:
1
Detects stimuli: sensory neurons convert light, sound, and pressure into electrical signals.
2
Integrates information: interneurons in the CNS process and interpret incoming data.
3
Produces responses: motor neurons carry commands to muscles and glands.
Anatomy of a Neuron
Key Parts of the Neuron
Dendrites
Short, branching projections that receive incoming signals from other neurons or sensory receptors.
Cell Body (Soma)
Contains the nucleus and most organelles; the metabolic center of the neuron.
Axon
A single, long fiber that conducts action potentials away from the cell body toward other cells.
Myelin Sheath
Lipid-rich insulating layer that speeds up signal conduction via saltatory conduction.
Axon Terminals
Bulb-shaped endings that release neurotransmitters into the synaptic cleft.
Functional Classification of Neurons
Structural Classification of Neurons
Introduction to Neuroglia
Neuroglia (glial cells) are the non-excitable support cells of the nervous system. Unlike neurons, they can divide throughout life: this is why most brain tumors are gliomas, not neuromas.
CNS Glia
Astrocytes, oligodendrocytes, microglia, ependymal cells
PNS Glia
Schwann cells, satellite cells
CNS Neuroglia
Astrocytes
Star-shaped cells that form the blood-brain barrier, regulate ion/nutrient balance, and anchor neurons to capillaries.
Oligodendrocytes
Produce the myelin sheath in the CNS. One cell myelinates portions of multiple axons.
Microglia
Phagocytic immune cells that engulf pathogens and clear debris in the CNS.
Ependymal Cells
Line brain ventricles and the central canal; produce and circulate cerebrospinal fluid (CSF).
PNS Neuroglia
Schwann Cells
Produce the myelin sheath in the PNS. Each Schwann cell wraps one segment of a single axon.
Satellite Cells
Surround and cushion neuron cell bodies in ganglia; regulate the chemical environment.
Neurons vs Neuroglia
| Neurons | Neuroglia | |
|---|---|---|
| Excitability | Electrically excitable | Non-excitable |
| Cell Division | Cannot divide (mostly) | Can divide throughout life |
| Relative Number | \(\sim 86\) billion in brain | \(\sim 85\) billion (roughly 1:1) |
| Primary Function | Transmit nerve impulses | Support, protect, nourish |
| Longevity | Extremely long-lived | Shorter-lived, replaceable |
How to Identify Neuron Types
1
Count the processes. Examine how many extensions leave the cell body: multiple dendrites + one axon = multipolar; one of each = bipolar; single process = unipolar.
2
Determine direction of signal. Toward CNS = sensory (afferent); away from CNS = motor (efferent); entirely within CNS = interneuron.
3
Check for myelin. White matter = myelinated axons; gray matter = cell bodies, dendrites, and unmyelinated axons.
4
Identify the glial partner. Myelination in CNS = oligodendrocyte; myelination in PNS = Schwann cell.
Example: The Reflex Arc
Example: Myelination in CNS vs PNS
Example: The Synapse
Common Mistakes to Avoid
✕ Neurons Divide Like Other Cells
Most mature neurons cannot undergo mitosis. Neuroglia are the dividing cells of nervous tissue.
✕ Confusing Schwann Cells and Oligodendrocytes
Schwann cells myelinate in the PNS; oligodendrocytes myelinate in the CNS. One Schwann cell wraps one segment; one oligodendrocyte wraps many.
✕ Thinking Axons Receive Signals
Dendrites receive incoming signals. Axons carry signals away from the cell body toward the next cell.
Complete Neuroglia Summary
| Glial Cell | Location | Function |
|---|---|---|
| Astrocytes | CNS | Blood-brain barrier, ion balance |
| Oligodendrocytes | CNS | Myelin (wraps multiple axons) |
| Microglia | CNS | Immune defense, phagocytosis |
| Ependymal Cells | CNS | Produce and circulate CSF |
| Schwann Cells | PNS | Myelin (wraps one axon segment) |
| Satellite Cells | PNS | Support neuron cell bodies in ganglia |
Question 1 of 5
Which part of the neuron is primarily responsible for receiving signals from other neurons?
Dendrites are the short, branching processes that receive incoming signals and carry them toward the cell body. Axons conduct signals away from the cell body.
Question 2 of 5
Which glial cell produces the myelin sheath in the CNS?
Oligodendrocytes produce myelin in the CNS and can wrap segments of multiple axons. Schwann cells perform the equivalent role in the PNS, wrapping one segment of a single axon.
Question 3 of 5
A neuron that carries signals from the CNS to a muscle is classified as which functional type?
Motor (efferent) neurons carry signals away from the CNS to effectors such as muscles and glands. Remember: Efferent = Exiting the CNS.
Question 4 of 5
What is the primary reason that brain tumors are most commonly derived from neuroglia rather than neurons?
Tumors arise from uncontrolled cell division. Since most mature neurons cannot undergo mitosis, they rarely form tumors. Neuroglia retain the ability to divide, making gliomas the most common primary brain tumors.
Question 5 of 5
In the PNS, which glial cells surround and support neuron cell bodies within ganglia?
Satellite cells surround and cushion neuron cell bodies in PNS ganglia, regulating the chemical environment. Schwann cells, the other PNS glia, are responsible for myelination rather than cell body support.