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Nervous System
Organization & Function

Master the structural and functional divisions of the nervous system: from neurons to neural circuits, and from central processing to peripheral signaling.

MCAT Biological Foundations

What Is the Nervous System?

The nervous system is the body's primary communication and control network. It detects stimuli, integrates information, and coordinates responses: all within milliseconds. Unlike the endocrine system, which uses hormones for slow, sustained signaling, the nervous system provides rapid, precise, and targeted communication via electrical and chemical signals.

Two Major Divisions

Central Nervous System (CNS)
Composed of the brain and spinal cord. It serves as the integration and command center: processing sensory input, making decisions, and sending motor commands.
Peripheral Nervous System (PNS)
All nerves outside the brain and spinal cord. It carries sensory information to the CNS (afferent) and motor commands from the CNS (efferent).

The Neuron: The Functional Unit

Nucleus Cell Body (Soma) Dendrites Axon Myelin Sheath Node Axon Hillock Axon Terminals Signal Direction

Nervous System Hierarchy

Nervous System CNS (Brain + Spinal Cord) PNS (Peripheral Nerves) Sensory (Afferent) Division Motor (Efferent) Division Somatic (Voluntary) Autonomic Sympathetic Parasym.

The Action Potential

An action potential is a rapid, transient reversal of the membrane potential that propagates along the axon. It follows the all-or-none principle: once threshold is reached (approximately \( -55 \) mV), a full action potential fires regardless of stimulus intensity.

\(+40 \text{ mV}\)
\(0 \text{ mV}\)
\(-55 \text{ mV}\)
\(-70 \text{ mV}\)
\(-90 \text{ mV}\)
 Time (ms) Voltage (mV) Threshold Resting Depolar. Peak Repolar. Hyperpolar. Resting

Ion Channels & Resting Potential

The resting membrane potential (approximately \( -70 \) mV) is maintained by the \( \text{Na}^+/\text{K}^+ \) ATPase pump, which moves \( 3 \, \text{Na}^+ \) out and \( 2 \, \text{K}^+ \) in per ATP hydrolyzed. At rest, the membrane is most permeable to \( \text{K}^+ \) through leak channels.

1
Resting state (\( -70 \) mV): Voltage-gated \( \text{Na}^+ \) and \( \text{K}^+ \) channels are closed. Leak \( \text{K}^+ \) channels maintain potential.
2
Depolarization: Stimulus reaches threshold (\( -55 \) mV). Voltage-gated \( \text{Na}^+ \) channels open, \( \text{Na}^+ \) rushes in.
3
Repolarization: \( \text{Na}^+ \) channels inactivate, voltage-gated \( \text{K}^+ \) channels open, \( \text{K}^+ \) flows out.
4
Hyperpolarization: Excess \( \text{K}^+ \) outflow briefly overshoots to ~\( -90 \) mV before returning to rest.

Sympathetic vs Parasympathetic

Sympathetic Parasympathetic
Nickname "Fight or Flight" "Rest and Digest"
Heart Rate Increases Decreases
Pupils Dilates (mydriasis) Constricts (miosis)
Digestion Inhibits Stimulates
Bronchioles Dilates Constricts
Neurotransmitter (post) Norepinephrine Acetylcholine (ACh)
Origin Thoracolumbar Craniosacral
Preganglionic Fiber Short Long

The Synapse & Neurotransmission

Presynaptic Neuron Synaptic Vesicles (neurotransmitters)
\( \text{Ca}^{2+} \)
 Exocytosis Synaptic Cleft Postsynaptic Neuron Receptors Ligand-gated ion channels open EPSP or IPSP

EPSP vs IPSP: Postsynaptic Potentials

EPSP (Excitatory)
Depolarizes the membrane (makes it more positive). Opens \( \text{Na}^+ \) channels, bringing the cell closer to threshold. Makes firing more likely.
IPSP (Inhibitory)
Hyperpolarizes the membrane (makes it more negative). Opens \( \text{Cl}^- \) or \( \text{K}^+ \) channels, moving cell away from threshold. Makes firing less likely.

Saltatory Conduction & Myelin

Myelin sheaths (formed by Schwann cells in the PNS and oligodendrocytes in the CNS) insulate the axon. The action potential "jumps" between Nodes of Ranvier: gaps in the myelin where ion channels are concentrated. This is called saltatory conduction.

Schwann Cells
Myelinate PNS axons. One Schwann cell wraps around one segment of one axon.
Oligodendrocytes
Myelinate CNS axons. One oligodendrocyte can myelinate segments of multiple axons.

Key Neurotransmitters

Neurotransmitter Type Key Function
Acetylcholine (ACh) Excitatory / Inhibitory Muscle contraction, memory, parasympathetic
Dopamine Excitatory / Modulatory Reward, motivation, motor control
Serotonin (5-HT) Inhibitory / Modulatory Mood, sleep, appetite
GABA Inhibitory Major inhibitory NT in the CNS
Glutamate Excitatory Major excitatory NT in the CNS
Norepinephrine Excitatory Alertness, sympathetic response
Endorphins Inhibitory Pain modulation, natural analgesic

How to Trace a Neural Pathway

1
Identify the stimulus: What is being detected? (e.g., touch, heat, light)
2
Receptor type: Which sensory receptor is activated? (mechanoreceptor, thermoreceptor, etc.)
3
Afferent pathway: Sensory neurons carry the signal toward the CNS via spinal or cranial nerves.
4
Integration center: The CNS (brain or spinal cord) processes and interprets the signal.
5
Efferent pathway: Motor neurons carry the response command back to effectors.
6
Effector response: Muscles contract or glands secrete. Ask: somatic (voluntary) or autonomic (involuntary)?

Example: The Reflex Arc

Stimulus (Hot stove) Receptor (Pain receptor) Afferent Integration (Spinal cord) Interneuron Efferent Effector (Arm muscle) Pull away This reflex bypasses the brain: the spinal cord is the integration center. This allows a faster response time.

Example: Fight-or-Flight Activation

You encounter a bear on a hiking trail. Your body activates the sympathetic nervous system:

1
Visual cortex processes the threat and signals the hypothalamus.
2
Hypothalamus activates the sympathetic chain via preganglionic neurons (using ACh).
3
Postganglionic neurons release norepinephrine onto target organs.
4
Adrenal medulla is directly innervated and releases epinephrine into the bloodstream.

Common Mistakes

✕ Confusing Afferent and Efferent
Afferent = sensory (toward CNS). Efferent = motor (away from CNS). Remember: Afferent = Arrives, Efferent = Exits.
✕ Mixing Up Myelin Cells
Schwann cells = PNS only. Oligodendrocytes = CNS only. Don't swap them: this is a frequently tested distinction.
✕ Assuming All Autonomic = Sympathetic
The autonomic nervous system has two branches. Both are active at all times: it's a matter of which branch dominates.

Summary: Nervous System at a Glance

Concept Key Details MCAT Focus
CNS Brain + Spinal cord Integration center
PNS Cranial + Spinal nerves Afferent vs Efferent
Action Potential All-or-none, threshold \( -55 \) mV \( \text{Na}^+/\text{K}^+ \) channel sequence
Synapse Chemical or electrical NT release, receptor binding
Sympathetic Fight or flight, NE Thoracolumbar, short pre
Parasympathetic Rest and digest, ACh Craniosacral, long pre
Myelin Schwann (PNS), Oligo (CNS) Saltatory conduction

Question 1 of 5

Which of the following correctly describes the function of afferent neurons?

Question 2 of 5

During an action potential, which event occurs immediately after voltage-gated \( \text{Na}^+ \) channels inactivate?

Question 3 of 5

A patient has damage to their oligodendrocytes. Which of the following is the most likely consequence?

Question 4 of 5

Activation of the parasympathetic nervous system would most directly cause which of the following?

Question 5 of 5

A neuron simultaneously receives 3 EPSPs and 2 IPSPs from different presynaptic neurons. The net result at the axon hillock just barely reaches threshold. This is an example of: