MCAT Biology Nervous System Review

Understanding the nervous system is fundamental to medicine and a heavily tested area on the Biological and Biochemical Foundations of Living Systems (Bio/Biochem) section of the MCAT. Mastery of this topic is essential not just for scoring well but for grasping the physiological basis of behavior, disease, and pharmacology—skills critical for your future clinical practice.

Neuron Structure and Function

The neuron is the basic signaling unit of the nervous system. Its specialized structure is perfectly adapted for its function: to receive, integrate, and transmit information. Key anatomical components include the dendrites, which receive incoming signals; the cell body (soma), which contains the nucleus and integrates signals; the axon, which conducts the electrical signal; and the axon terminals, which release neurotransmitters. The myelin sheath, produced by oligodendrocytes in the CNS and Schwann cells in the PNS, insulates axons to dramatically increase the speed of signal propagation via saltatory conduction.

For the MCAT, you must be able to trace the direction of information flow: from dendrites to soma, down the axon, to the terminals. Experimental passages often involve toxins or genetic conditions that disrupt specific parts of the neuron. For example, a question might describe a demyelinating disease like multiple sclerosis and ask you to predict the effect on conduction velocity.

Action Potential Generation and Propagation

An action potential is a rapid, all-or-none electrical signal driven by changes in membrane permeability to ions. The resting membrane potential, typically around -70 mV, is maintained by the sodium-potassium pump (Na+/K+ ATPase) and the differential permeability of the membrane, favoring K+ leak channels.

The process follows a strict sequence:

1. Depolarization: A stimulus opens voltage-gated sodium channels. Na+ rushes in, making the interior more positive.
2. Repolarization: Sodium channels inactivate, and voltage-gated potassium channels open. K+ rushes out, restoring negative internal charge.
3. Hyperpolarization (Refractory Period): Potassium channels are slow to close, briefly making the cell more negative than resting potential. During the absolute refractory period, no new action potential can be initiated; during the relative refractory period, a stronger-than-usual stimulus is required.

The key to MCAT questions is understanding the ionic basis. Memorize that the equilibrium potential for Na+ is approximately +60 mV and for K+ is approximately -90 mV. The Nernst equation, $E_{ion}=\frac{61.5}{z}\log \frac{[ion{]}_{out}}{[ion{]}_{in}}$, is fair game for calculations. When given a voltage-graph, identify the phases based on ion flow.

Synaptic Transmission and Neurotransmitter Systems

Synaptic transmission converts an electrical signal (action potential) into a chemical signal and back to an electrical signal in the postsynaptic cell. At the chemical synapse, the action potential triggers voltage-gated calcium channels to open. Ca2+ influx causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. These neurotransmitters bind to ligand-gated ion channels or G-protein coupled receptors on the postsynaptic membrane, generating either an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP). Summation (temporal and spatial) of these potentials at the axon hillock determines if a new action potential fires.

Key MCAT neurotransmitter systems include:

• Acetylcholine (ACh): Used at neuromuscular junctions, in the parasympathetic system, and in memory. Degraded by acetylcholinesterase.
• Dopamine, Norepinephrine, Serotonin: Monoamines involved in mood, reward, and arousal. Often targeted by psychiatric medications.
• GABA and Glycine: Major inhibitory neurotransmitters.
• Glutamate: The major excitatory neurotransmitter.

Expect passage-based questions on drugs that agonize or antagonize these systems. For instance, a toxin that blocks acetylcholinesterase would lead to constant muscle stimulation.

Nervous System Organization: CNS, PNS, and ANS

The nervous system is organized hierarchically. The central nervous system (CNS) consists of the brain and spinal cord and is responsible for integration and command. The peripheral nervous system (PNS) includes all nerves outside the CNS and is subdivided into sensory (afferent) and motor (efferent) divisions.

The motor division is further split:

• Somatic Nervous System: Voluntary control of skeletal muscles.
• Autonomic Nervous System (ANS): Involuntary control of glands, cardiac muscle, and smooth muscle. The ANS has two primary, often antagonistic, divisions:
• Sympathetic Nervous System: "Fight or flight." Increases heart rate, dilates pupils, inhibits digestion. Uses norepinephrine as its primary postganglionic neurotransmitter.
• Parasympathetic Nervous System: "Rest and digest." Decreases heart rate, constricts pupils, stimulates digestion. Uses acetylcholine as its primary postganglionic neurotransmitter.

On the MCAT, you will need to predict the physiological outcome of activating one division over the other. The dual innervation of most organs by the ANS is a high-yield concept.

Autonomic Nervous System Pharmacology

Pharmacology questions are common. The effects of drugs can be predicted by knowing the receptors and neurotransmitters at each ANS synapse. The two main receptor classes are:

• Cholinergic Receptors: Bind ACh. Nicotinic receptors are ionotropic and found on all postganglionic neurons and skeletal muscle. Muscarinic receptors are metabotropic (GPCRs) found on target organs of the parasympathetic system.
• Adrenergic Receptors: Bind norepinephrine/epinephrine. These are all GPCRs (e.g., ${\alpha }_1$, ${\beta }_1$, ${\beta }_2$) with different effects on different tissues.

An agonist mimics a neurotransmitter; an antagonist blocks it. For example, atropine is a muscarinic antagonist that would block parasympathetic "rest and digest" signals, leading to dilated pupils and dry mouth—a classic side effect.

Common Pitfalls

1. Confusing Graded Potentials and Action Potentials: Graded potentials (like EPSPs/IPSPs) are variable in strength, decremental (they fade with distance), and occur in dendrites/cell bodies. Action potentials are all-or-none, non-decremental, and occur along axons. Mixing these properties is a classic trap.
2. Misidentifying ANS Effects: A common error is to think the sympathetic system is always excitatory and parasympathetic always inhibitory. While often true, the key is to think about the need of the moment. Sympathetic activity inhibits digestion (relaxes smooth muscle in gut) but excites the heart. Always reason from the "fight or flight" vs. "rest and digest" paradigm.
3. Incorrect Ion Flow During Action Potential Phases: Students often think repolarization is caused by Na+ exiting the cell. Remember: Depolarization = Na+ in. Repolarization = K+ out. Hyperpolarization = too much K+ out.
4. Overlooking the Synaptic Delay: Electrical signals travel quickly along axons, but the conversion to chemical signaling at the synapse introduces a brief, measurable delay. In experimental data, this delay is evidence of a chemical synapse (vs. a faster electrical synapse).

Summary

• Neurons are structurally specialized for directional communication: dendrites receive, the axon hillock integrates, the axon propagates via action potentials, and terminals release neurotransmitters.
• Action potentials are all-or-none, driven by sequential opening of voltage-gated Na+ and K+ channels, and propagate unidirectionally due to the refractory period.
• At the synapse, an electrical signal is converted to a chemical one. Neurotransmitters cause graded postsynaptic potentials (EPSPs/IPSPs) that summate to trigger a new action potential.
• The nervous system is organized into the CNS (brain/spinal cord) and PNS (sensory/motor). The motor PNS includes the somatic (voluntary) and autonomic (involuntary) systems, the latter with sympathetic ("fight or flight") and parasympathetic ("rest and digest") divisions.
• Pharmacology questions require knowing the neurotransmitters (ACh vs. NE) and receptor types (nicotinic, muscarinic, adrenergic) at each ANS synapse.
• For MCAT passages, focus on interpreting electrophysiology graphs by linking shape to ion flow, and use the "big picture" frameworks (like ANS antagonism) to predict experimental outcomes.