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IB DP Biology — C: Interaction and Interdependence | SL | C2.2 Neural Signalling
📋 What You Will Learn
- Identify the structural components of a synapse and explain how neurotransmitters are released and received
- Explain the mechanisms of long-term potentiation (LTP) and long-term depression (LTD) as the cellular basis of learning and memory
- Describe how the brain integrates excitatory and inhibitory signals to produce coordinated responses
- Analyse experimental data on synaptic transmission, including the effects of drugs and toxins on neural signalling
- Evaluate the relationship between synaptic plasticity and higher brain functions such as learning, memory, and addiction
🔍 Guiding Question How does the structure of the nervous system allow for the integration of information and the generation of coordinated responses — and how does experience itself change the structure of the brain?
🌍 Why This Matters Every time you learn a new skill — a language, a chord on a guitar, a route through a city — your brain physically rewires itself. Synaptic connections strengthen, weaken, and are pruned in response to experience. This is not metaphor; it is measurable molecular biology. Understanding synaptic plasticity explains not only how we learn, but why addiction is so difficult to break, why antidepressants take weeks to work, and how stroke patients can recover lost function through rehabilitation.
The Synapse: A Junction With a Purpose
Here is the problem the nervous system had to solve: neurons are electrically isolated from one another. An action potential racing down an axon cannot simply leap across a gap to the next cell — it needs a chemical messenger to carry the signal forward. The synapse is the elegant solution. It converts an electrical signal into a chemical one, releases that chemical into a tiny cleft, and allows the next cell to convert it back into an electrical signal. This conversion is not just a relay — it is a decision point.
📖 Key Definition — Synapse A synapse is a junction between two neurons (or between a neuron and an effector cell) where a signal is transmitted from the presynaptic cell to the postsynaptic cell via the release of a chemical neurotransmitter into the synaptic cleft.
Structural Components of a Chemical Synapse
The presynaptic terminal (also called the synaptic knob or bouton) contains mitochondria and synaptic vesicles loaded with neurotransmitter molecules. Mitochondria are essential here — releasing neurotransmitter is an energy-demanding process requiring ATP for vesicle recycling and active transport. The synaptic cleft is the narrow extracellular gap (approximately 20–40 nm wide) separating the two cells. The postsynaptic membrane is studded with receptor proteins that are specific to the neurotransmitter being released.
Synaptic Transmission: Step by Step
An action potential arrives at the presynaptic terminal and depolarises the membrane. This depolarisation opens voltage-gated calcium ion (Ca²⁺) channels in the presynaptic membrane. Calcium ions flood into the terminal down their electrochemical gradient — this is the trigger for vesicle fusion.
The influx of Ca²⁺ causes synaptic vesicles to fuse with the presynaptic membrane by exocytosis, releasing neurotransmitter molecules into the synaptic cleft. This process is called exocytosis. Neurotransmitter molecules diffuse across the narrow cleft — a journey of only nanometres — and bind to specific receptor proteins on the postsynaptic membrane.
📖 Key Definition — Neurotransmitter A neurotransmitter is a chemical messenger released from the presynaptic terminal into the synaptic cleft that binds to specific receptors on the postsynaptic membrane, generating either an excitatory or inhibitory response in the postsynaptic cell.
Binding of neurotransmitter to its receptor causes ion channels in the postsynaptic membrane to open. If the channels allow Na⁺ to enter, the postsynaptic membrane depolarises — this is an excitatory postsynaptic potential (EPSP). If Cl⁻ enters or K⁺ exits, the membrane hyperpolarises — this is an inhibitory postsynaptic potential (IPSP). Whether