C2.2

Neuron

A specialised cell that carries electrical impulses in the nervous system.

Cell body of a neuron

Contains the cytoplasm and nucleus.

Axon

A single long nerve fibre that conducts electrical impulses away from the cell body.

Dendrites

Multiple short fibres that conduct impulses toward the cell body.

Nerve impulse

An electrical signal transmitted along a neuron by movement of ions.

Electrical nature of nerve impulses

Caused by movement of positively charged ions across membranes.

Resting potential

The membrane potential of a neuron at rest, about −70 mV.

Membrane polarization

State where the inside of the neuron is negatively charged relative to outside.

Membrane potential

The voltage difference across a cell membrane.

Sodium-potassium pump

Uses ATP to pump 3 Na⁺ out and 2 K⁺ into a neuron.

Role of ATP in resting potential

Provides energy for active transport of Na⁺ and K⁺ ions.

Reason resting potential is negative

More K⁺ diffuses out than Na⁺ diffuses in and negatively charged proteins remain inside.

Action potential

A rapid reversal of membrane potential during a nerve impulse.

Depolarization

Loss of membrane polarization due to Na⁺ entering the neuron.

Repolarization

Return of membrane potential to negative due to K⁺ leaving the neuron.

Threshold potential

The membrane potential required to trigger an action potential.

Voltage-gated sodium channels

Open when threshold is reached, allowing rapid Na⁺ influx.

Voltage-gated potassium channels

Open after depolarization to allow K⁺ efflux.

All-or-nothing principle

Action potentials occur fully or not at all if threshold is reached.

Propagation of action potentials

Action potentials move along axons due to local currents.

Local currents

Diffusion of Na⁺ ions that depolarizes adjacent regions of the axon.

Direction of impulse propagation

Ensured by refractory period and channel inactivation.

Refractory period

Short time when a neuron cannot generate another action potential.

Hyperpolarization

Membrane potential becomes more negative than resting potential.

Myelinated axon

An axon insulated by a myelin sheath.

Myelin sheath

Lipid-rich insulation formed by Schwann cells.

Nodes of Ranvier

Gaps in myelin sheath where ion channels are concentrated.

Saltatory conduction

Action potentials jump from node to node in myelinated fibres.

Advantage of saltatory conduction

Increases speed of nerve impulse transmission.

Unmyelinated fibres

Conduct impulses more slowly than myelinated fibres.

Axon diameter and speed

Larger axon diameter increases conduction speed.

Giant squid axon

Large diameter unmyelinated axon used to increase impulse speed.

Positive correlation

As one variable increases, the other increases.

Negative correlation

As one variable increases, the other decreases.

Correlation coefficient (r)

Measures strength and direction of correlation.

Coefficient of determination (R²)

Shows proportion of variation explained by correlation.

Synapse

A junction between neurons or between a neuron and an effector.

Chemical synapse

Synapse that uses neurotransmitters to transmit signals.

Synaptic cleft

The gap between presynaptic and postsynaptic membranes.

Unidirectional transmission

Synapses allow impulses to pass in one direction only.

Presynaptic neuron

The neuron that releases neurotransmitters.

Postsynaptic neuron

The neuron that receives neurotransmitters.

Calcium ions in synapses

Enter presynaptic neuron and trigger neurotransmitter release.

Neurotransmitter

Chemical messenger released at synapses.

Acetylcholine (ACh)

A neurotransmitter used in many synapses including neuromuscular junctions.

Synaptic vesicles

Membrane-bound sacs containing neurotransmitters.

Exocytosis in synapses

Fusion of vesicles with membrane to release neurotransmitter.

Excitatory postsynaptic potential (EPSP)

Depolarization of postsynaptic membrane.

Ligand-gated sodium channels

Open when neurotransmitter binds, allowing Na⁺ entry.

Acetylcholinesterase

Enzyme that breaks down acetylcholine.

Inhibitory neurotransmitter

A neurotransmitter that prevents action potential generation.

Inhibitory postsynaptic potential (IPSP)

Hyperpolarization of postsynaptic membrane.

Hyperpolarization effect

Makes it harder to reach threshold potential.

Summation

Combined effect of multiple synaptic inputs.

Temporal summation

Repeated impulses from one synapse add together.

Spatial summation

Impulses from multiple synapses combine.

Exogenous chemicals

Chemicals from outside the body that affect synapses.

Neonicotinoids

Pesticides that block acetylcholine receptors in insects.

Effect of neonicotinoids

Block synaptic transmission causing paralysis.

Cocaine

Drug that blocks reuptake of neurotransmitters.

Effect of cocaine

Increases dopamine in synapses causing pleasure.

Pain receptors (nociceptors)

Sensory neurons with free nerve endings.

Stimuli detected by nociceptors

Heat, acids, and chemicals such as capsaicin.

TRP channels

Ion channels that open in response to damaging stimuli.

Pain perception

Action potentials travel to the brain where pain is perceived.

Emergent property

A characteristic arising from interactions between components.

Consciousness as emergent property

Results from interaction of neurons in the brain.