MCAT topics → Nervous and Endocrine Systems → Neuron Structure (Dendrite, Axon, Myelin)

Neuron Structure (Dendrite, Axon, Myelin)

MCAT trap: Swaps oligodendrocytes (CNS) and Schwann cells (PNS) for myelin production. Oligodendrocytes myelinate CNS axons; Schwann cells myelinate PNS axons.

Neuron structure is one of those topics where students think they know it because the diagram looks simple, but the MCAT probes the functional logic behind each structure — not just naming parts. The cell body (soma) integrates signals, dendrites receive input, and the axon hillock is where the neuron decides whether to fire. The axon conducts the signal, myelin speeds it up, and the axon terminal releases neurotransmitter. Every part has a specific job, and the exam will ask you to reason about what happens when one part is disrupted.

The myelin story is where students lose the most points. The MCAT consistently tests whether you can distinguish oligodendrocytes (CNS) from Schwann cells (PNS), and this distinction goes beyond a memory trick — the two cell types differ structurally and functionally. Oligodendrocytes extend processes to wrap multiple axons simultaneously; Schwann cells each myelinate a single axon segment. Nodes of Ranvier aren't just gaps — they're clustered with voltage-gated Na+ channels, which is exactly why saltatory conduction works.

Where students get confused: they mix up the axon hillock with the axon terminal, they swap which glial cell belongs to which nervous system, and they incorrectly assign phagocytic immune functions to astrocytes instead of microglia. These aren't random errors — they come from pattern-matching without understanding the underlying logic. The fix is to understand *why* each structure is where it is and what it's designed to do.

Common misconceptions

Common mistake
Wrong: Oligodendrocytes myelinate PNS axons and Schwann cells myelinate CNS axons.
Right: Oligodendrocytes myelinate CNS axons; Schwann cells myelinate PNS axons.
The swap is the single most common error on this topic. A reliable way to remember: 'O' for oligodendrocyte, 'O' for 'only in the CNS.' Schwann cells are peripheral — they're found in the PNS and are also capable of regeneration after injury, which is why PNS damage is more recoverable than CNS damage. The MCAT will exploit this by describing a demyelinating disease and asking which cell type is affected; if it's MS (a CNS disease), the answer is oligodendrocytes.
Common mistake
Wrong: Action potentials are initiated at the axon terminal where neurotransmitter is released.
Right: Action potentials are initiated at the axon hillock, where the threshold is lowest due to the highest density of voltage-gated Na+ channels.
The axon terminal is where neurotransmitter is released — that's the output end of the neuron, not where the decision to fire is made. The axon hillock sits at the junction between the cell body and axon and has the highest density of voltage-gated Na+ channels of any neuronal region, which means it has the lowest threshold for generating an action potential. Think of it as the 'trigger zone' — all the graded potentials converging on the soma get evaluated here, and if the sum crosses threshold, the hillock fires first.
Common mistake
Gap: Misses that oligodendrocytes wrap multiple axons while Schwann cells wrap only one
One oligodendrocyte can myelinate multiple axons simultaneously, whereas each Schwann cell myelinates only one axon segment.
This distinction matters because it explains why CNS demyelination is more catastrophic — one oligodendrocyte loss can disrupt multiple axons at once. Each Schwann cell wraps around a single internode of a single axon, so PNS damage is more localized. On the MCAT, this can show up in passage-based questions about the scope of damage in CNS versus PNS injuries, or in questions about the structural differences between the two glial cell types.
Common mistake
Wrong: Astrocytes are the primary immune cells of the CNS that phagocytose pathogens and debris.
Right: Microglia are the resident immune cells of the CNS responsible for phagocytosis; astrocytes support the blood-brain barrier and ion homeostasis.
Astrocytes are support cells — they maintain the blood-brain barrier, regulate extracellular ion concentrations (especially K+), and recycle neurotransmitters. They are not immune cells. Microglia are the CNS's resident macrophages: they surveil the brain, phagocytose debris and pathogens, and mediate neuroinflammation. Confusing them likely comes from astrocytes being described as 'supporting' the CNS broadly, but 'support' means metabolic and structural help, not immune defense.
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What the exam tests

  1. Know the function of each neuron compartment — cell body integrates signals, dendrites receive input, axon hillock initiates action potentials, axon conducts them, and axon terminals release neurotransmitter — and be able to apply these in a passage describing a lesion or toxin targeting a specific region.
  2. Understand how myelin is formed and by which cell type depending on location — oligodendrocytes myelinate CNS axons (and can wrap multiple at once), while Schwann cells myelinate PNS axons (one segment per cell).
  3. Recognize nodes of Ranvier as unmyelinated gaps where voltage-gated Na+ channels concentrate, enabling saltatory conduction — the exam may ask why conduction velocity increases with myelination or what happens when nodes are disrupted.
  4. Distinguish the roles of the major glial cell types — astrocytes (blood-brain barrier, ion homeostasis), oligodendrocytes (CNS myelination), Schwann cells (PNS myelination), microglia (CNS immune surveillance and phagocytosis), and ependymal cells (CSF production and lining of ventricles).

Can you avoid these mistakes?

A toxin selectively blocks voltage-gated Na+ channels clustered at short, unmyelinated gaps along a myelinated axon. What specific structures are being targeted, and what happens to conduction velocity?
A patient is diagnosed with a demyelinating disease affecting only CNS white matter tracts. Which glial cell type is being destroyed, and how does this differ structurally from the myelin-forming cell of the PNS?
Graded potentials (EPSPs and IPSPs) arrive at the dendrites and soma of a neuron. At what anatomical location does summation get 'converted' into an all-or-none action potential, and why is this location specialized for that role?
Match each glial cell to its primary function: astrocyte, microglia, ependymal cell, oligodendrocyte, Schwann cell. Which one is responsible for CSF production? Which one would respond first to a bacterial infection in the brain parenchyma?

Related topics

Resting Membrane Potential and Ion Gradients Action Potential (Depolarization, Repolarization, Refractory Periods) Saltatory Conduction and Conduction Velocity Chemical and Electrical Synaptic Transmission

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