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Neuron Structure &
Signal Propagation

Master the anatomy of neurons and the electrochemical mechanisms behind action potentials, resting membrane potential, and synaptic transmission.

MCAT Biology

What Is a Neuron?

A neuron is the fundamental unit of the nervous system: a specialized cell that receives, processes, and transmits electrochemical signals. The human brain contains roughly \( 8.6 \times 10^{10} \) neurons, each forming thousands of connections.

Anatomy of a Neuron

Dendrites Soma Nucleus Hillock Axon Myelin Sheath Node of Ranvier Axon Terminals (Synaptic Boutons) Signal Direction →

Key Structural Components

Dendrites
Branched extensions that receive signals from other neurons via receptors
Soma
Cell body containing the nucleus: integrates incoming signals
Axon
Long projection that carries the action potential away from the soma
Myelin
Insulating lipid sheath produced by Schwann cells (PNS) or oligodendrocytes (CNS)

Resting Membrane Potential

At rest, a neuron maintains a voltage difference across its membrane of approximately \( -70 \text{ mV} \). The inside is negative relative to the outside.

1
\(\text{Na}^+/\text{K}^+\) ATPase pump: moves \( 3\,\text{Na}^+ \) out and \( 2\,\text{K}^+ \) in per ATP hydrolyzed
2
\(\text{K}^+\) leak channels: allow \(\text{K}^+\) to flow out down its concentration gradient
3
Large anions: proteins and organic phosphates trapped inside create a net negative interior

The Action Potential

+40 0 -55 -70 -90 mV Time (ms) Threshold Rest Resting Depolarization Repolarization Hyperpolarization Return to rest

Phases of the Action Potential

1
Stimulus reaches threshold (\( -55 \text{ mV} \)): voltage-gated \(\text{Na}^+\) channels open rapidly
2
Depolarization: \(\text{Na}^+\) rushes in, membrane potential shoots to \( +40 \text{ mV} \)
3
Repolarization: \(\text{Na}^+\) channels inactivate, voltage-gated \(\text{K}^+\) channels open, \(\text{K}^+\) exits
4
Hyperpolarization: \(\text{K}^+\) channels are slow to close, overshooting to \( \approx -90 \text{ mV} \)
5
Return to rest: \(\text{Na}^+/\text{K}^+\) ATPase restores \( -70 \text{ mV} \) resting potential

All-or-None & Refractory Periods

An action potential either fires at full amplitude or not at all: there is no partial firing. Signal intensity is encoded by frequency of action potentials, not amplitude.

Absolute Refractory
\(\text{Na}^+\) channels are inactivated: no stimulus, no matter how strong, can trigger another action potential
Relative Refractory
Some \(\text{Na}^+\) channels have reset: a stronger-than-normal stimulus can trigger a new action potential

Saltatory Conduction

Myelin Node Node Node Node "Jumps" between nodes Signal direction →

Myelinated vs Unmyelinated

Myelinated Unmyelinated
Conduction type Saltatory Continuous
Speed Up to 120 m/s 0.5 - 2 m/s
Insulation Schwann cells (PNS) / Oligodendrocytes (CNS) None
Energy cost Lower (fewer ions leak) Higher (continuous ion flow)
Examples Motor neurons, sensory neurons C-fibers (pain, temperature)

Synaptic Transmission

Presynaptic Neuron Vesicles
\(\text{Ca}^{2+}\)
Synaptic Cleft NT release Postsynaptic Neuron Receptors → EPSP (excitatory) or IPSP (inhibitory) Neurotransmitter Synaptic Vesicle

Steps of Synaptic Signaling

1
Action potential arrives at the axon terminal (presynaptic neuron)
2
Voltage-gated \(\text{Ca}^{2+}\) channels open: \(\text{Ca}^{2+}\) floods into the terminal
3
Vesicle fusion (exocytosis): \(\text{Ca}^{2+}\) triggers vesicles to merge with the membrane, releasing neurotransmitter into the synaptic cleft
4
Neurotransmitter binds receptors on the postsynaptic membrane, opening ligand-gated ion channels
5
Signal termination: enzymatic degradation, reuptake, or diffusion clears the neurotransmitter

EPSP vs IPSP

EPSP (Excitatory)
\(\text{Na}^+\) or \(\text{Ca}^{2+}\) enters postsynaptic cell, causing depolarization: membrane potential moves toward threshold
IPSP (Inhibitory)
\(\text{Cl}^-\) enters or \(\text{K}^+\) exits, causing hyperpolarization: membrane potential moves away from threshold

How to Trace a Signal

1
Identify the stimulus: what triggers the signal? (sensory input, neurotransmitter, etc.)
2
Check the resting state: \( V_m \approx -70 \text{ mV} \), mostly due to \(\text{K}^+\) leak and \(\text{Na}^+/\text{K}^+\) pump
3
Determine if threshold is reached: sum all EPSPs and IPSPs at the axon hillock
4
Follow the action potential: depolarization → repolarization → hyperpolarization → rest
5
Track the synapse: \(\text{Ca}^{2+}\) entry → vesicle fusion → NT release → receptor binding → postsynaptic response

Example: Ion Flow During Depolarization

Scenario: A neuron receives a sufficiently strong stimulus. Describe the ion movements during depolarization.

Example: Nernst Equation for \(\text{K}^+\)

Problem: If \([\text{K}^+]_{\text{in}} = 140 \text{ mM}\) and \([\text{K}^+]_{\text{out}} = 5 \text{ mM}\), calculate the equilibrium potential for \(\text{K}^+\) at 37 \(^\circ\)C.

Common Mistakes

✕ Confusing resting with equilibrium
The resting potential (\( -70 \text{ mV} \)) is a steady state maintained by pumps, not a true equilibrium. \( E_K \approx -90 \text{ mV} \) is the equilibrium for \(\text{K}^+\) alone.
✕ Thinking AP amplitude varies
Action potentials are all-or-none. A stronger stimulus does not produce a bigger AP: it produces more frequent APs.
✕ Wrong myelinating cell
Schwann cells myelinate the PNS. Oligodendrocytes myelinate the CNS. Each Schwann cell wraps one axon segment; each oligodendrocyte wraps multiple axons.

Summary: Key Concepts

Concept Key Detail Value / Ion
Resting potential \(\text{K}^+\) leak + pump \( -70 \text{ mV} \)
Threshold Voltage-gated \(\text{Na}^+\) opens \( -55 \text{ mV} \)
Peak depolarization \(\text{Na}^+\) influx \( +40 \text{ mV} \)
Repolarization \(\text{K}^+\) efflux \(\text{K}^+\) channels open
Hyperpolarization \(\text{K}^+\) overshoot \( \approx -90 \text{ mV} \)
Synaptic transmission \(\text{Ca}^{2+}\)-triggered exocytosis NT release

Major Neurotransmitters

Acetylcholine
Neuromuscular junctions, parasympathetic NS. Degraded by acetylcholinesterase.
Glutamate
Main excitatory NT in CNS. Binds NMDA and AMPA receptors.
GABA
Main inhibitory NT in CNS. Opens \(\text{Cl}^-\) channels (IPSPs).
Dopamine
Reward pathways, motor control. Implicated in Parkinson's and schizophrenia.

Let's Test Your Understanding

You've learned about neuron structure, the action potential, synaptic transmission, and key neurotransmitters. The next 5 questions will check your comprehension of these concepts.

Question 1 of 5

During the depolarization phase of an action potential, which ion movement is primarily responsible for the rapid rise in membrane potential?

Question 2 of 5

The \(\text{Na}^+/\text{K}^+\) ATPase pump moves ions in which ratio per cycle?

Question 3 of 5

Which cell type is responsible for myelinating axons in the central nervous system?

Question 4 of 5

During synaptic transmission, the influx of which ion into the presynaptic terminal directly triggers neurotransmitter release?

Question 5 of 5

A neuron simultaneously receives 3 EPSPs of \( +10 \text{ mV} \) each and 2 IPSPs of \( -8 \text{ mV} \) each. Starting from \( -70 \text{ mV} \), what is the resulting membrane potential, and will the neuron fire?