C2.2 Neural signallingIB Biology SL: Revision notes
Section 1
Neurons
Neurons are cells of the nervous system that carry electrical impulses. The cell body contains the nucleus and cytoplasm, and elongated nerve fibres of varying length project from it:
- an axon: a single long fibre;
- dendrites: multiple shorter fibres.
Impulses are conducted along these fibres.
Section 2
The resting potential
The sodium–potassium pump uses energy from ATP to pump three Na⁺ out and two K⁺ in (opposite directions). This establishes concentration gradients (Na⁺ high outside, K⁺ high inside).
The membrane is polarised: there is a membrane potential of about −70 mV (inside negative). Reasons it is negative: more positive ions pumped out than in; the membrane is more permeable to K⁺, which leaks out; and negatively charged proteins stay inside.
Section 3
Nerve impulses and their speed
A nerve impulse is an action potential propagated along a nerve fibre. It is electrical because it involves movement of positively charged ions.
Speed varies: giant axons of squid (up to about 500 µm wide) conduct faster than small non-myelinated fibres, because speed rises with diameter. Myelinated fibres conduct much faster than non-myelinated fibres of the same or even much larger diameter, because myelin insulates the fibre.
Section 4
Correlation, r and R²
A positive correlation: both variables increase together (e.g. conduction speed and axon diameter). A negative correlation: one increases as the other decreases (e.g. conduction speed and animal size).
The correlation coefficient (r) runs from −1 to +1; values close to ±1 mean a strong correlation. The coefficient of determination (R²) shows what proportion of the variation in the dependent variable is explained by the independent variable: R² = 0.81 means 81%.
Correlation does not prove causation, even with R² close to 1.
Section 5
Synapses and neurotransmitter release
Synapses are junctions between neurons, and between neurons and effector cells such as muscle fibres. A signal passes in only one direction, because neurotransmitter is released only from the presynaptic neuron and receptors are only on the postsynaptic membrane.
When an impulse depolarises the presynaptic membrane, calcium ions enter. Calcium acts as a signalling chemical inside the neuron, causing vesicles to release neurotransmitter into the synaptic cleft by exocytosis.
Section 6
Excitatory postsynaptic potentials
The neurotransmitter diffuses across the cleft and binds to transmembrane receptors on the postsynaptic membrane. With acetylcholine, the receptors open and positive ions (mainly Na⁺) enter, depolarising the postsynaptic membrane: an excitatory postsynaptic potential.
Acetylcholine is used at many types of synapse, including neuromuscular junctions.
Sequence to learn: depolarisation → Ca²⁺ in → exocytosis → diffusion → binding → Na⁺ in → depolarisation.
That's the notes covered.
Carry on to the next subtopic.