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The Nervous System

Chapter 4: The Nervous System

3 min read Updated Mar 26, 2026
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1. (4.1) The central nervous system (CNS) consists of:
D. CNS = brain + spinal cord. Everything else (cranial/spinal nerves, ganglia, enteric plexus) is peripheral. Damage in CNS (myelinated by oligodendrocytes) generally does not regenerate.
2. (4.1) The peripheral nervous system is subdivided into:
C. PNS = somatic (voluntary, skeletal muscle) + autonomic (involuntary: sympathetic, parasympathetic, enteric). Sensory vs motor is a different axis (afferent vs efferent).
3. (4.2) The axon of a neuron:
A. The axon conducts action potentials from the axon hillock toward the terminal. Dendrites receive input; the soma contains the nucleus.
4. (4.2) The axon terminal is characterized by:
B. The terminal bouton is packed with vesicles docked near the active zone, plus many mitochondria to fuel Ca²⁺ pumping and vesicle recycling.
5. (4.3) Oligodendrocytes function to:
D. One oligodendrocyte wraps segments of several CNS axons. In contrast, one Schwann cell myelinates a single PNS axon segment.
6. (4.3) Astrocytes are primarily responsible for:
A. Astrocyte end-feet surround brain capillaries, maintaining tight junctions, regulating extracellular K⁺/glutamate, and feeding neurons lactate. Microglia are the phagocytes of the CNS.
7. (4.4) The resting membrane potential is maintained primarily by:
C. The membrane is most permeable to K⁺ at rest (K⁺ leak channels), so the resting potential sits near EKE_{K}. The Na⁺/K⁺-ATPase maintains the gradients that keep this potential stable.
8. (4.4) The typical resting membrane potential of a neuron is approximately:
B. Inside is negative relative to outside at rest. Most neurons rest near -70 mV, close to the K⁺ equilibrium potential.
9. (4.5) The rapid depolarizing phase of an action potential is caused by:
A. Once threshold (~-55 mV) is reached, voltage-gated Na⁺ channels open and Na⁺ rushes in down its gradient, driving the membrane toward +30 mV.
10. (4.5) The absolute refractory period exists because:
D. After opening, Na⁺ channels enter an inactivated (ball-and-chain) state. They must re-polarize back to rest before they can reset to the closed state. No stimulus, however strong, can fire another AP in the meantime.
11. (4.6) Saltatory conduction refers to:
B. Latin "saltare" = to jump. Myelinated segments are electrically insulated; the AP regenerates only at the ion-channel-rich nodes, massively speeding conduction.
12. (4.6) Myelin increases conduction velocity by:
C. Myelin is a thick lipid wrapping that drops membrane capacitance and leakage, so an AP regenerated at one node can rapidly trigger the next.
13. (4.7) Neurotransmitter release from the presynaptic terminal requires:
D. Depolarization opens P/Q- or N-type Ca²⁺ channels at the terminal. Ca²⁺ binds synaptotagmin on vesicles, driving SNARE-mediated fusion and transmitter release into the synaptic cleft.
14. (4.7) An inhibitory postsynaptic potential (IPSP) often arises from:
A. Cl⁻ entry (or K⁺ exit) hyperpolarizes the membrane, moving it away from threshold. GABA and glycine are the dominant inhibitory transmitters.
15. (4.8) The cerebellum is primarily responsible for:
C. The cerebellum compares intended with actual movement and tunes motor output. Damage produces ataxia (uncoordinated gait, intention tremor).
16. (4.8) Broca's area, crucial for speech production, is located in the:
B. Broca's area is in the left inferior frontal gyrus. Wernicke's area (comprehension) sits in the left superior temporal gyrus. Broca's aphasia = nonfluent, effortful speech.
17. (4.9) The knee-jerk (patellar) reflex is an example of a:
A. Stretching the quadriceps activates muscle spindles; Ia afferents synapse directly onto α-motor neurons in the spinal cord - one synapse, quick reflex.
18. (4.9) Sensory (afferent) fibers of a spinal reflex arc enter the spinal cord via:
D. Sensory in through dorsal root; motor out through ventral root (the Bell-Magendie law). The dorsal root ganglion contains pseudounipolar sensory neuron cell bodies.
19. (4.10) Sympathetic nervous system activation typically causes:
C. "Fight or flight": ↑HR, ↑BP, bronchodilation, pupil dilation, ↓digestion, vascular constriction in skin/gut, dilation in skeletal muscle.
20. (4.10) The primary postganglionic neurotransmitter in the parasympathetic nervous system is:
B. Parasympathetic postganglionic fibers release ACh onto muscarinic receptors. Sympathetic postganglionic fibers (except those to sweat glands) release norepinephrine onto adrenergic receptors.
21. (4.11) The primary somatosensory cortex is located in the:
D. The postcentral gyrus processes touch, proprioception, and pain via the somatosensory homunculus. The precentral gyrus (frontal) is the motor cortex.
22. (4.11) The corticospinal tract carries:
C. The lateral corticospinal tract is the main descending motor pathway. Its name tells the direction: "cortico" (from cortex) → "spinal" (to cord).
23. (4.12) Auditory transduction in the inner ear occurs when:
A. Pressure waves in cochlear fluid deflect stereocilia of hair cells against the tectorial membrane. The tip-link channels open, depolarizing the cell and releasing glutamate onto the auditory nerve.
24. (4.12) Cone photoreceptors in the human retina are primarily responsible for:
B. Cones (three types: S, M, L) are concentrated in the fovea and support color + high-acuity vision. Rods (one type) dominate outside the fovea and handle dim-light vision.

You are sitting in a quiet room reading this sentence. In the time it took your eyes to scan that line, your nervous system fired millions of electrical signals - from your retinas to your visual cortex, through language-processing areas, into memory circuits, and back out to the tiny muscles controlling your eye movements. All of it happened in milliseconds. You did not have to think about any of it.

Now imagine touching a hot stove. Before your brain even registers pain, your hand has already pulled away. The spinal cord made that decision on its own, buying you precious fractions of a second. Your nervous system is not just fast - it is the fastest communication network in your body, operating on electricity and chemistry simultaneously.

Understanding the nervous system is not optional for the MCAT. It bridges biology, biochemistry, physics (electrical circuits, membrane potentials), and behavioral sciences (brain function, sensation, perception). A single MCAT passage about multiple sclerosis can test your knowledge of myelin, action potentials, ion channels, and clinical symptoms all at once. Master this chapter, and you will have the foundation to handle all of it.

Your Nervous System is an Electrical Grid

Think of your nervous system as a city’s electrical grid. The brain is the central power station - it generates commands, processes incoming data, and coordinates everything. The spinal cord is the main power line running from the station to every neighborhood. Nerves are the individual wires branching into homes and businesses.

Neurons are the actual wires carrying current. They are insulated with myelin (like rubber coating on copper wire) to keep signals fast and prevent short circuits. At every junction between wires, there is a tiny gap - the synapse - where the electrical signal gets converted to a chemical one, then back to electrical on the other side. It is like a relay switch at a substation.

The system has two modes: sympathetic (“emergency power” - everything cranks to maximum during a crisis) and parasympathetic (“energy-saving mode” - the grid powers down non-essentials and focuses on maintenance). Both run simultaneously; it is the balance that shifts.

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