Neurons and Glial Cells

Core Concepts

Anatomy of a neuron: structure follows function

Neurons are polarized — different regions do different jobs. Dendrites are short, branched extensions studded with receptors; they collect signals and carry them toward the cell body. The cell body (soma) houses the nucleus and performs housekeeping. The axon is a single long fiber conducting the signal away; the axon hillock, where the axon joins the soma, is where the signal is generated. The axon ends in branched axon terminals, which release chemical messengers onto the next cell.

Classifying neurons: shape and function

By structure: multipolar neurons (one axon, many dendrites) dominate the CNS and include all motor neurons; bipolar neurons (one axon, one dendrite) appear in specialized pathways such as the retina; unipolar (pseudounipolar) neurons have a single process that splits, one branch to the periphery and one to the CNS — the classic shape of sensory neurons.

By function: sensory (afferent) neurons carry information toward the CNS; motor (efferent) neurons carry commands away from the CNS to muscles or glands; interneurons — by far the most numerous — connect neurons within the CNS and do the processing.

Glial cells: the support team

• Astrocytes (CNS): the most abundant glia. They help form the blood-brain barrier by wrapping capillaries and signaling endothelial cells to form tight junctions, regulate ion balance, recycle neurotransmitters, and feed neurons.
• Oligodendrocytes (CNS): make myelin, each cell wrapping several axons at once.
• Microglia (CNS): resident immune cells that patrol for pathogens and debris and prune unused synapses.
• Ependymal cells (CNS): line the ventricles and help produce and circulate cerebrospinal fluid (CSF).
• Schwann cells (PNS): make myelin in the periphery, one segment per cell, and aid nerve regeneration.
• Satellite cells (PNS): support sensory neuron cell bodies in ganglia.

Myelin, white matter, and gray matter

Myelin speeds conduction. In the CNS, myelinated axon bundles look pale and are called white matter; regions dense in cell bodies and dendrites look darker and are gray matter. The brain has gray matter outside and white inside; the spinal cord is reversed. When the immune system attacks myelin, as in MS, conduction slows or stops, and symptoms appear in whatever pathways the lesions affect — vision, balance, or limb control.

The blood-brain barrier

This selective filter is formed by tight junctions between brain capillary endothelial cells, reinforced by astrocyte processes. It keeps many toxins, pathogens, and large molecules out of brain tissue while admitting oxygen, glucose, and other essentials. It is not absolute: lipid-soluble molecules (alcohol, many drugs) cross easily, which is why some medications act on the brain and others never reach it.

How It Works / Step-by-Step Process

Tracing a signal through cell types:

1. A receptor in the skin detects pressure and triggers a sensory neuron.
2. The sensory axon travels through a peripheral nerve to the spinal cord; its cell body sits in a dorsal root ganglion.
3. Inside the cord, it synapses on an interneuron, which processes the input.
4. The interneuron connects to a motor neuron, whose axon exits in a ventral root.
5. The motor neuron reaches a skeletal muscle and triggers contraction.
6. Along the way, Schwann cells keep peripheral signals fast and astrocytes stabilize the cord's environment.

Common Confusions