Neuron Structure
This interactive diagram covers the 13 key structures of a multipolar motor neuron (AP Biology Unit 5 — Nervous System). The main view shows the full neuron from dendrites through the myelinated axon to the axon terminals, with a synaptic inset revealing the ultrastructure of the chemical synapse.
Cell body (soma)
The metabolic center of the neuron, containing the nucleus and all organelles (rough ER / Nissl bodies, Golgi, mitochondria) that sustain the cell's enormous protein synthesis demand. All proteins used in axons and dendrites are made here.
Nucleus
Houses the neuronal genome and directs transcription of genes for ion channels, receptors, and neurotransmitter-synthesizing enzymes. The prominent nucleolus reflects the high ribosome demand needed to maintain a cell that may extend over 1 meter.
Dendrites
The primary input zone. Branching processes covered in dendritic spines receive neurotransmitter signals from thousands of presynaptic neurons, generating local graded potentials (EPSPs and IPSPs) that travel toward the soma. More branching = greater computational capacity.
Axon hillock
The spike initiation zone — a cone-shaped region at the soma-axon junction with the highest density of voltage-gated \(\ce{Na+}\) channels. All graded potentials from dendrites are summed here; if the net depolarization reaches threshold (~−55 mV), an action potential fires (all-or-nothing law).
Axon
The output cable of the neuron. Conducts action potentials from the axon hillock to the terminals without decrement (signal strength does not weaken with distance). Microtubule tracks inside the axoplasm serve as rails for bidirectional axonal transport of vesicles and organelles (kinesin for anterograde, dynein for retrograde).
Myelin sheath
A multilayered spiral wrap of Schwann cell (PNS) or oligodendrocyte (CNS) plasma membrane around the axon. The high-lipid composition (~70–80% lipid) creates excellent electrical insulation, enabling saltatory conduction — action potentials jump node to node, increasing speed up to 100× and reducing \(\ce{Na+}\) entry (and therefore ATP needed for the \(\ce{Na+}\)/\(\ce{K+}\)-ATPase).
Schwann cell
The myelinating glial cell of the peripheral nervous system. Each Schwann cell forms one internodal myelin segment by spiraling its plasma membrane around the axon. Schwann cells also support PNS nerve regeneration after injury — unlike oligodendrocytes in the CNS.
Node of Ranvier
A 1–2 µm gap between adjacent myelin segments where the axon is exposed and densely packed with voltage-gated \(\ce{Na+}\) and \(\ce{K+}\) channels. Saltatory conduction regenerates the action potential only at nodes, skipping the insulated internodal regions and greatly reducing ion flux per action potential.
Axon terminals (boutons)
Swollen presynaptic knobs at the ends of axon branches. Each bouton contains synaptic vesicles, abundant mitochondria (to power vesicle cycling), and voltage-gated \(\ce{Ca^2+}\) channels. When an action potential arrives, \(\ce{Ca^2+}\) influx triggers neurotransmitter exocytosis.
Synaptic vesicles
Small (~40–50 nm) membrane-bound organelles packed with neurotransmitter (e.g., acetylcholine, glutamate, GABA, dopamine). When \(\ce{Ca^2+}\) enters the terminal, SNARE proteins (synaptobrevin + syntaxin + SNAP-25) mediate vesicle fusion with the presynaptic membrane and release neurotransmitter into the cleft by exocytosis.
Synaptic cleft
The ~20–40 nm extracellular gap between the presynaptic terminal and the postsynaptic membrane. Neurotransmitter diffuses across this gap. The cleft contains degrading enzymes (e.g., acetylcholinesterase) and adhesion proteins (neuroligins, neurexins) that hold the synapse together. Many drugs act here — SSRIs block serotonin reuptake; organophosphates inhibit acetylcholinesterase.
Postsynaptic membrane
The region of the target cell membrane (dendrite, soma, or muscle) facing the cleft. Thickened by the postsynaptic density (PSD), a protein scaffold that anchors receptors. Neurotransmitter binding here either opens ion channels directly (ionotropic receptors) or activates G-protein cascades (metabotropic receptors).
Receptor proteins
Integral membrane proteins in the postsynaptic membrane that bind specific neurotransmitters with high affinity. Ionotropic receptors (AMPA, NMDA, nicotinic ACh) are ligand-gated ion channels — fast, direct response. Metabotropic receptors (GABA-B, muscarinic ACh, all GPCRs) produce slower but more diverse and amplified intracellular responses via second-messenger cascades.