Structure and Function of the Nervous System

Core Concepts

Anatomy of a Neuron

A neuron has four functional zones: dendrites (branched receivers), the cell body (soma) (integrates signals), the axon (a long fiber conducting the outgoing signal), and axon terminals (endings that release neurotransmitter into the synapse). Many axons are wrapped in myelin, fatty insulation made by oligodendrocytes (CNS) and Schwann cells (PNS), interrupted at nodes of Ranvier; the signal jumps node to node — saltatory conduction — far faster than continuous conduction. Myelin loss in multiple sclerosis therefore slows or blocks signaling, producing widespread symptoms.

The Action Potential: How the Electrical Signal Works

Neurons maintain a resting membrane potential near −70 mV (inside negative), thanks largely to the sodium-potassium pump. When a stimulus depolarizes past threshold, voltage-gated sodium channels open, sodium rushes in, and the potential shoots toward positive — depolarization. Then sodium channels inactivate and voltage-gated potassium channels open; potassium leaves, restoring negativity — repolarization. During the refractory period the neuron cannot fire again immediately, keeping signals one-directional.

The action potential is all-or-none: below-threshold stimuli produce nothing; above-threshold stimuli produce the same-size spike. What varies with stimulus strength is frequency — spikes per second. Drugs that block voltage-gated sodium channels raise the threshold or prevent propagation — how local anesthetics dampen pain and anticonvulsants reduce seizures.

Synaptic Transmission: From Electrical to Chemical

At the axon terminal, the electrical signal triggers neurotransmitter release through a five-step lifecycle: synthesis (built from precursors), storage (packed into vesicles), release (calcium entry fuses vesicles with the membrane), receptor binding (opens channels or starts cascades on the next cell), and termination — via reuptake (transporters pump the chemical back in), enzymatic degradation (acetylcholinesterase breaks down ACh; MAO and COMT break down dopamine/norepinephrine), or diffusion away.

Every step is a drug target: botulinum toxin blocks ACh release; SSRIs block serotonin reuptake; acetylcholinesterase inhibitors block degradation — the step tells you whether a drug raises or lowers the signal.

Receptors: Fast Gates and Slow Switches

Neurotransmitters act through two broad families. Ionotropic receptors (ligand-gated ion channels) open directly — fast, millisecond effects (nicotinic ACh receptor). Metabotropic receptors (G-protein–coupled) trigger slower, longer intracellular cascades (muscarinic, dopamine, most adrenergic receptors). This predicts kinetics: ionotropic drugs act quickly; metabotropic drugs act gradually and persistently.

Organization of the CNS

The CNS (brain and spinal cord) processes and integrates: the cerebrum handles conscious thought, sensation, and voluntary movement; the diencephalon (thalamus — sensory relay; hypothalamus — autonomic control, temperature, appetite) regulates; the cerebellum coordinates balance; the brainstem houses vital centers for breathing and heart rate; the spinal cord carries tracts and generates reflexes (rapid responses wired in the cord). This map matters for drugs: agents depressing the medulla's respiratory center slow breathing (opioid risk); agents on the basal ganglia's dopamine system affect movement.

Organization of the PNS: Somatic and Autonomic

The PNS has two functional divisions:

• Somatic (voluntary): motor neurons release acetylcholine at the neuromuscular junction, acting on nicotinic receptors to contract skeletal muscle. Muscle relaxants block this signal; acetylcholinesterase inhibitors amplify it.
• Autonomic (involuntary): controls organs, glands, and blood vessels through two opposing branches. Sympathetic (fight-or-flight): faster heart, dilated pupils, bronchodilation; postganglionic fibers release norepinephrine onto adrenergic receptors (alpha and beta subtypes). Parasympathetic (rest-and-digest): slows the heart, stimulates digestion and salivation, constricts pupils; its fibers release acetylcholine onto muscarinic receptors. The enteric system (the gut's own network) is often counted as a third division.

The autonomic map (memorize): in both branches, the preganglionic fiber releases ACh (nicotinic receptors on the ganglion). In the sympathetic branch, the postganglionic fiber releases norepinephrine (adrenergic receptors on the target) — except sweat glands, which use ACh. In the parasympathetic branch, the postganglionic fiber releases ACh (muscarinic receptors on the target).

What the Autonomic Map Means for Drugs

Because the branches oppose each other, drug effects follow a pattern: a sympathomimetic (adrenergic agonist) mimics fight-or-flight — faster heart, higher blood pressure, dilated pupils, dry mouth. A sympatholytic (adrenergic antagonist) blocks those effects — beta-blockers slow the heart. A parasympathomimetic (cholinergic agonist) mimics rest-and-digest — slower heart, more salivation, constricted pupils. An anticholinergic blocks muscarinic receptors, producing the classic dry-and-fast profile — dry mouth, blurred near vision, constipation, urinary retention. All examples are class-level; verify specific drugs and doses against current references and prescriber orders.

Common Confusions