Biological Basis of Behavior

Biological Basis of Behavior

Part 1 of 5 — Neurons & the Action Potential

Neuron Structure & Signal Flow

$\text{Dendrites} \to \text{Cell body (soma)} \to \text{Axon hillock} \to \text{Axon} \to \text{Axon terminals}$

• Dendrites receive input; the axon hillock sums inputs and initiates the action potential if threshold (~ −55 mV) is reached.
• Myelin (oligodendrocytes in CNS, Schwann cells in PNS) speeds conduction via saltatory conduction (jumping node to node).

Resting Membrane Potential (~ −70 mV)

Set by the Na⁺/K⁺-ATPase (pumps 3 Na⁺ out, 2 K⁺ in) plus K⁺ leak channels. The membrane is most permeable to K⁺ at rest, so $V_m$ sits near the K⁺ equilibrium potential.

The Action Potential — Phases

All-or-None & the Refractory Period

• All-or-none: above threshold, every AP has the same amplitude; intensity is coded by FREQUENCY, not size.
• Absolute refractory period: Na⁺ channels inactivated → no new AP possible (ensures one-way propagation).
• Relative refractory period: a stronger-than-normal stimulus can fire (during hyperpolarization).

Neurons & Action Potentials 🎯

Worked Examples — Action Potentials

Question: A local anesthetic (e.g., lidocaine) blocks voltage-gated Na⁺ channels in a sensory neuron. Predict the effect on action potentials and on pain sensation.

Solution:

1. Voltage-gated Na⁺ channels drive the depolarizing (rising) phase. Blocking them prevents the membrane from reaching the AP upstroke. ✓
2. No action potentials can propagate → the sensory signal (pain) never reaches the CNS → numbness. ✓

MCAT note: This is exactly how local anesthetics work — they silence the axon by blocking Na⁺ entry.

Question: Extracellular K⁺ is raised experimentally. Qualitatively, what happens to the resting membrane potential and excitability?

Solution:

1. Raising external K⁺ reduces the K⁺ concentration gradient, so K⁺ efflux at rest decreases.
2. The membrane DEPOLARIZES (moves toward less negative). ✓
3. Initially the neuron is closer to threshold (hyperexcitable), but sustained depolarization inactivates Na⁺ channels → eventual loss of excitability (depolarization block). ✓

Connection: This is why severe hyperkalemia is dangerous to cardiac and neural tissue.

Question: Why does an action potential propagate down the axon in only one direction rather than back toward the soma?

Solution:

1. Behind the advancing AP, Na⁺ channels are INACTIVATED (absolute refractory period). ✓
2. They cannot reopen until the membrane repolarizes, so the region just traversed cannot be re-excited.
3. Only the unexcited region ahead can fire → unidirectional propagation toward the terminals. ✓

Key idea: The refractory period enforces one-way, non-decremental signaling.

Key Takeaways — Part 1

• Signal flow: dendrites → soma → axon hillock (threshold ~ −55 mV) → axon → terminals.
• Resting potential ~ −70 mV set by Na⁺/K⁺-ATPase (3 Na⁺ out / 2 K⁺ in) and K⁺ leak.
• AP: Na⁺ IN (depolarize) → K⁺ OUT (repolarize) → hyperpolarize; all-or-none; intensity coded by FREQUENCY.
• Refractory period ensures one-way conduction; myelin enables fast saltatory conduction (lost in MS).

Biological Basis of Behavior

Part 2 of 5 — Synapses & Neurotransmitters

Synaptic Transmission Steps

$\text{AP at terminal} \to \text{voltage-gated } Ca^{2+} \text{ IN} \to \text{vesicle fusion} \to \text{NT release} \to \text{receptor binding} \to \text{postsynaptic potential}$

Biological Basis of Behavior

Part 3 of 5 — Brain Regions & Their Functions

Organizing the Brain: Hindbrain, Midbrain, Forebrain

Cortical Lobes

Biological Basis of Behavior

Part 4 of 5 — Endocrine System & Behavior + Stress (HPA Axis)

Hormones modulate behavior more slowly and diffusely than neurotransmitters, but with longer-lasting effects.

Nervous vs. Endocrine Signaling

Behaviorally Relevant Hormones

Biological Basis of Behavior

Part 5 of 5 — Nervous System Organization, Genetics & Review

Divisions of the Nervous System

$\text{Nervous system} \to \begin{cases} \textbf{CNS} & \text{brain + spinal cord} \\ \textbf{PNS} & \text{somatic + autonomic} \end{cases}$