The Nervous System

Core Review

Neuron Structure

Neurons are specialized cells for rapid electrical and chemical signaling.

• Dendrites: Branching projections that receive signals from other neurons. Contain neurotransmitter receptors. Conduct graded potentials toward the cell body.
• Cell body (soma): Contains the nucleus and organelles. Integrates incoming signals. If the summed depolarization at the axon hillock reaches threshold, an action potential is initiated.
• Axon: A single, long projection that conducts action potentials away from the cell body toward the axon terminals.
• Myelin sheath: Insulating layers of Schwann cells (PNS) or oligodendrocytes (CNS) wrapped around the axon. Increases conduction velocity via saltatory conduction — the action potential "jumps" between nodes.
• Nodes of Ranvier: Gaps in the myelin sheath where voltage-gated Na⁺ and K⁺ channels are concentrated. Action potentials are regenerated at each node.
• Axon terminal (synaptic bouton): The end of the axon where neurotransmitters are stored in vesicles and released into the synapse.

The Resting Membrane Potential (−70 mV)

At rest, the inside of the neuron is negative relative to the outside. This is established and maintained by:

1. Na⁺/K⁺ ATPase: Pumps 3 Na⁺ out and 2 K⁺ in per ATP (electrogenic, net loss of 1 positive charge).
2. K⁺ leak channels: The membrane is much more permeable to K⁺ than Na⁺ at rest. K⁺ diffuses out down its concentration gradient, leaving behind negative anions, until the electrical gradient pulling K⁺ back in balances the chemical gradient. This equilibrium potential for K⁺ (EK ≈ −90 mV) is the primary driver of the resting potential.

The Action Potential

An action potential is a rapid, all-or-none depolarization that propagates along the axon without decay.

Phases:

1. Resting state: −70 mV. Voltage-gated Na⁺ and K⁺ channels are closed.
2. Depolarization: A stimulus opens some voltage-gated Na⁺ channels. If threshold (≈ −55 mV) is reached, a positive feedback loop: more Na⁺ channels open → more depolarization → more Na⁺ channels open. Membrane potential shoots toward ENa (+60 mV). Peak ≈ +30 to +40 mV.
3. Repolarization: Na⁺ channels inactivate (ball-and-chain mechanism). Voltage-gated K⁺ channels open (delayed response), allowing K⁺ to rush out, restoring the negative internal potential.
4. Hyperpolarization (undershoot): K⁺ channels are slow to close, so membrane potential briefly drops below resting (≈ −80 to −90 mV).
5. Return to resting: K⁺ channels close. Na⁺/K⁺ ATPase restores ion gradients.

Key properties:

• All-or-none: If threshold is reached, the full action potential fires. Below threshold, nothing. There is no "partial" action potential.
• Refractory periods: Absolute refractory (Na⁺ channels already inactivated — another action potential impossible) followed by relative refractory (some Na⁺ channels recovered, K⁺ channels still open — stronger stimulus needed).
• Propagation: Action potentials travel in ONE direction (toward axon terminals) because the region just behind is refractory.

The Synapse

The synapse is the junction between two neurons (or a neuron and a target cell).

Chemical Synaptic Transmission:

1. Action potential arrives at the presynaptic terminal.
2. Depolarization opens voltage-gated Ca²⁺ channels. Ca²⁺ influx is the trigger.
3. Ca²⁺ causes synaptic vesicles to fuse with the membrane (via SNARE proteins) and release neurotransmitters into the synaptic cleft by exocytosis.
4. Neurotransmitters diffuse across the cleft and bind to ligand-gated ion channels (ionotropic) or G-protein-coupled receptors (metabotropic) on the postsynaptic membrane.
5. Binding opens ion channels → postsynaptic potential (EPSP if depolarizing, IPSP if hyperpolarizing).
6. Neurotransmitter is removed: reuptake (presynaptic transporter), enzymatic degradation (e.g., acetylcholinesterase breaks down ACh), or diffusion.

Summation: Postsynaptic potentials are graded, not all-or-none. Multiple EPSPs can summate (temporal or spatial summation) to trigger an action potential in the postsynaptic neuron.

Organization of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord. Integration and processing.
• Peripheral Nervous System (PNS): All nerves outside the CNS.
  • Sensory (afferent) division: Carries signals from receptors to the CNS.
  • Motor (efferent) division: Carries signals from the CNS to effectors.
    • Somatic: Voluntary control of skeletal muscle (uses acetylcholine, always excitatory).
    • Autonomic: Involuntary control of smooth muscle, cardiac muscle, glands.

Autonomic Nervous System

Note: Sweat glands (sympathetic) use ACh as the postganglionic neurotransmitter — an exception.

Common Traps

• "Na⁺ causes repolarization": Wrong. Na⁺ influx causes depolarization. K⁺ efflux causes repolarization. Students reverse this frequently.
• "Hyperpolarization = resting state": Hyperpolarization is a transient undershoot below resting potential — it is distinct from and occurs AFTER repolarization.
• "Sympathetic always uses norepinephrine": The preganglionic sympathetic neuron uses ACh. Only the postganglionic uses NE. Also, sweat glands (sympathetic) use ACh at the target.
• "The action potential jumps across the synapse": It does not — it stops at the presynaptic terminal. Chemical transmission (neurotransmitters) takes over at the synapse.