An action potential is a wave of voltage. Synapses are chemistry between waves. Together: a nervous system.
Every sub-topic below feeds at least one of these questions.
How are electrical signals generated and moved within neurons?
How can neurons interact with other cells?
The required syllabus content for C2.2, in order. Each card is one lesson-sized checkpoint.
An axon is a long single fibre. Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres.
They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative.
Nerve impulses as action potentials that are propagated along nerve fibres
Variation in the speed of nerve impulses
Synapses as junctions between neurons and between neurons and effector cells
Release of neurotransmitters from a presynaptic membrane
Generation of an excitatory postsynaptic potential
Neurons have a cell body for the genetic and metabolic essentials, plus long fibres for sending and receiving signals.
Carry impulses from sense organs (eyes, ears, skin, etc.) to the central nervous system.
Live entirely within the CNS, connecting other neurons. The vast majority of neurons in the brain are interneurons.
Carry impulses from the CNS to muscles or glands, triggering action.
Three parts of every neuron:
At rest, the inside of a neuron is more negative than the outside. The Na⁺/K⁺ pump establishes this gradient — and uses ATP doing so.
The Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per cycle, using 1 ATP. Combined with large negative organic ions trapped inside, this produces a net negative interior at about −70 mV.
This resting potential is the platform from which action potentials are launched. Maintaining it costs substantial energy — neurons are some of the most metabolically active cells in the body.
A nerve impulse is an action potential propagated along a nerve fibre. It involves the flow of positively charged ions across the membrane.
The signal isn't a chemical or a pulse of light — it's an electrical wave caused by Na⁺ and K⁺ briefly flowing in and out of the axon. Because Na⁺ and K⁺ are charged, their movement changes the membrane potential — and that change propagates as a wave along the fibre.
Conduction speed varies dramatically — from 0.5 m/s in small unmyelinated fibres to 150 m/s in fat myelinated ones.
Axon diameter ↑ → conduction speed ↑ (positive correlation). The squid giant axon is up to 1 mm wide and conducts at ~25 m/s — making it indispensable to neuroscience pioneers who needed something big enough to record from.
Myelin sheaths insulate most of the axon, leaving small unmyelinated gaps called nodes of Ranvier. Action potentials jump from node to node. Speeds up to 150 m/s. Much more energy-efficient too.
Bigger animals tend to have slower nerve impulses if you don't correct for adaptations (negative correlation). But within an animal, larger axons and myelinated axons conduct faster (positive correlation with diameter, with myelination). Use correlation coefficients to quantify strength of these relationships, and R² to assess how much one variable explains the other.
Where two neurons meet — or a neuron meets a muscle or gland — the signal jumps across a tiny gap chemically. That's a synapse.
A typical (chemical) synapse has three parts:
Signals pass one way only across a synapse — vesicles are on the presynaptic side, receptors on the postsynaptic side. This directional asymmetry gives the nervous system its one-way circuits.
Action potential arrives → Ca²⁺ enters → vesicles fuse → neurotransmitter diffuses → postsynaptic channels open → next action potential triggered.
Released by motor neuron axon terminals; binds nicotinic acetylcholine receptors on the muscle fibre's sarcolemma; opens Na⁺ channels; muscle fibre depolarises and contracts. The named example in the IB.
An extra 9 sub-topics for HL — same syllabus, deeper mechanism.
Depolarization and repolarization during action potentials
Propagation of an action potential along a nerve fibre/axon as a result of local currents
Oscilloscope traces showing resting potentials and action potentials
Saltatory conduction in myelinated fibres to achieve faster impulses
Saltatory conduction in myelinated fibres to achieve faster impulses
Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials
Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron
Perception of pain by neurons with free nerve endings in the skin
Consciousness as a property that emerges from the interaction of individual neurons in the brain
An action potential is a tight sequence of voltage-gated channel openings and closings — Na⁺ in, then K⁺ out — that produces a sharp spike and recovery.
When Na⁺ floods into one patch of axon, it diffuses sideways and triggers the next patch's voltage-gated channels — propagating the action potential along the fibre.
The Na⁺ ions don't stay still after entering. They diffuse along the inside of the axon, creating a local current. That current depolarises the adjacent patch of membrane to threshold, opening its Na⁺ channels — propagating the action potential. Behind the wave, the previous patch is repolarising.
Plotting membrane potential vs time produces the classic spike — used in the original Hodgkin and Huxley experiments to crack the mechanism.
Phases visible on the trace:
Total duration: ~3.5 ms. Frequency of impulses can be measured by counting spikes per second.
In myelinated axons, action potentials skip from one node of Ranvier to the next — faster and more energy-efficient than continuous conduction.
Myelin (made by Schwann cells in the PNS, oligodendrocytes in the CNS) insulates most of the axon. Sodium-potassium pumps and voltage-gated channels are concentrated at nodes of Ranvier — short gaps between myelin segments.
Action potentials can only occur at the nodes. The signal effectively "jumps" from node to node — saltatory conduction (saltare = to leap). Much faster and uses far less ATP because only the nodes need to actively pump ions.
External chemicals can interfere with synaptic transmission. Two named examples — one a pesticide, one a recreational drug.
Similar shape to acetylcholine. Bind irreversibly to acetylcholine receptors in insects → block synaptic transmission → paralysis and death. Human receptors have a different shape → much less toxicity to humans. But they harm non-target insects like bees.
Dopamine transporters normally clear dopamine from synaptic clefts. Cocaine blocks these transporters. Dopamine builds up in synapses → continuous stimulation of reward-pathway neurons → euphoria → addiction.
Excitatory inputs push toward firing; inhibitory inputs push away. The neuron adds them all up — and either crosses threshold (fires) or doesn't.
Open channels for negative ions (Cl⁻) in the postsynaptic membrane. The postsynaptic cell becomes hyperpolarised — further from threshold. Action potential less likely. GABA and glycine are the major inhibitory neurotransmitters in the human brain.
A postsynaptic neuron has many synapses with many different presynaptic neurons. Some release excitatory neurotransmitters; some inhibitory. The postsynaptic neuron sums these inputs. If the net effect crosses the threshold potential, it fires; otherwise it doesn't. All-or-nothing — an action potential either happens or it doesn't.
Pain receptors in the skin are free nerve endings with channels that open in response to specific harmful stimuli.
Nociceptor channels open in response to:
When channels open, positive ions enter, threshold is reached, an action potential is generated. The impulse travels via sensory neurons to the brain, where pain is perceived.
No single neuron is conscious. But the interaction of billions of them produces something that is — the brain's most striking emergent property.
Emergent properties arise when components interact — the whole has properties not present in any part. A single H₂O molecule is not "wet"; only many of them together. A single neuron doesn't think; ~86 billion of them, interacting through trillions of synapses, somehow do.
Consciousness — awareness of self and surroundings — is the IB's named example of an emergent property in biology. How exactly the interaction of neurons produces it is the central question of cognitive neuroscience and philosophy of mind.
If you can't define one of these in a sentence, that's where to revise next.
“In what ways are biological systems regulated?”
“How is the structure of specialized cells related to function?”