3.6.2 Nervous coordination

The inside of the cell must be kept more negative than the outside.

Answer: The sodium-potassium pump actively transports $3Na^{+}$ out and $2K^{+}$ in, while the membrane is more permeable to $K^{+}$ diffusing back out than $Na^{+}$ diffusing in.

The resting potential is maintained by the sodium-potassium pump ($3Na^{+}$ out, $2K^{+}$ in) and the differential permeability of the membrane (highly permeable to $K^{+}$ leaving, impermeable to $Na^{+}$ entering).

The inside of the membrane quickly changes from negative to positive ($+40mV$).

Answer: Voltage-gated sodium ion channels open, and sodium ions rapidly diffuse into the axon.

Generation of an action potential involves depolarisation, caused by the opening of voltage-gated $Na^{+}$ channels and the influx of $Na^{+}$ down its electrochemical gradient.

The word 'saltatory' comes from the Latin 'saltare', meaning to jump or leap.

Answer: In myelinated neurones, local currents cannot cross the myelin sheath, so action potentials only occur at the nodes of Ranvier, 'jumping' from node to node.

The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses. Saltatory conduction occurs in myelinated axons, where the impulse jumps between nodes of Ranvier, drastically increasing the speed of conductance.

During this brief moment, the sodium channels cannot be opened again.

Answer: It ensures that action potentials are propagated in one direction only, produces discrete impulses, and limits the maximum frequency of impulses.

The refractory period is essential because it produces discrete (separate) impulses, limits the frequency of action potentials, and ensures unidirectional propagation of the impulse.

An action potential is like firing a gun; pulling the trigger harder doesn't make the bullet go faster or hit harder.

Answer: An action potential is generated, and it will be the exact same size (amplitude) regardless of how large the stimulus is.

The all-or-nothing principle states that an action potential is always the same size ($+40mV$) once the threshold is reached. A stronger stimulus increases the *frequency* of impulses, not their amplitude or speed.

An influx of a specific ion is required to trigger vesicle movement.

Answer: Voltage-gated calcium ion channels open, and calcium ions diffuse into the synaptic knob.

The detailed structure of a synapse and of a neuromuscular junction. Depolarisation of the presynaptic knob opens voltage-gated $Ca^{2+}$ channels. The influx of $Ca^{2+}$ causes synaptic vesicles to move and fuse with the presynaptic membrane.

Enzymes ending in '-ase' generally break things down.

Answer: It hydrolyses acetylcholine in the synaptic cleft to prevent continuous generation of action potentials in the postsynaptic neurone.

Acetylcholinesterase breaks down acetylcholine in the synaptic cleft. The products are reabsorbed by the presynaptic neurone. This prevents continuous depolarisation of the postsynaptic membrane.

Hyperpolarisation makes the inside of the cell even more negative (e.g., $-80mV$), making it harder to reach the threshold.

Answer: The neurotransmitter opens chloride or potassium channels, causing hyperpolarisation of the postsynaptic membrane.

Features of synapses include... inhibition. Inhibitory neurotransmitters cause $Cl^{-}$ channels to open (influx of negative ions) or $K^{+}$ channels to open (efflux of positive ions), causing hyperpolarisation, which takes the membrane further from the threshold.