OSSF Action Potentials

Resting membrane potential depends on

Ion concentration gradients (chemical force), electrical forces, pumps, and membrane permeability (mainly K⁺ through leak channels)

Graded potential

A small change (under ~20 mV) in membrane potential away from rest; can be excitatory (depolarising) or inhibitory (hyperpolarising)

Size of a graded potential depends on

The intensity (strength) of the stimulus

Key difference: graded vs action potential spreading

Graded potentials SHRINK as they spread; action potentials do NOT

Action potential (AP)

A rapid, large but brief depolarisation that is propagated (spreads along the membrane)

Which cells have action potentials?

Neurons and all muscle cells (skeletal, smooth, cardiac)

All-or-none principle

Once threshold is reached, an AP fires fully; if threshold isn't reached, no AP fires at all

Threshold

The membrane voltage that must be reached to trigger an action potential

AP Phase 1: depolarisation

At threshold, voltage-gated Na⁺ channels open; Na⁺ rushes in; Vm rises toward E Na (~+60 mV)

Why is depolarisation positive feedback?

Depolarisation opens Na⁺ channels → more depolarisation → opens even more Na⁺ channels

Why doesn't the AP peak reach E Na?

Voltage-gated Na⁺ channels inactivate before Vm gets there (and K⁺ channels are opening)

AP Phase 2: repolarisation

Voltage-gated K⁺ channels open; K⁺ rushes out (pushed by its gradient and the now-positive inside); with Na⁺ channels inactivated, Vm falls

AP Phase 3: hyperpolarisation (undershoot)

K⁺ keeps leaving, so Vm becomes more negative than rest, moving toward E K; voltage-gated K⁺ channels close before reaching E K

What restores resting potential after an AP?

Leak channels

Role of the Na⁺/K⁺ pump during APs

Maintains the concentration gradients over many action potentials (not needed for each single AP)

Two gates of the voltage-gated Na⁺ channel

Activation gate and inactivation gate

Na⁺ activation gate

Closed at rest; opens FAST when the membrane reaches threshold

Na⁺ inactivation gate

Closes after a brief delay (time-dependent) at threshold; stays closed until the membrane returns to resting potential ('ball and chain')

Three states of the voltage-gated Na⁺ channel

Rest: closed but able to open → Threshold to peak: open → Peak back to rest: inactivated (closed, can't open)

Voltage-gated K⁺ channels

Closed at rest; opened by depolarisation but open more SLOWLY than Na⁺ channels; close slowly → cause the undershoot

Why does Na⁺ win when both Na⁺ and K⁺ channels are briefly open?

Na⁺ permeability is much greater at that moment

Absolute refractory period

NO action potential can be generated, however strong the stimulus; occurs during depolarisation and repolarisation

Cause of the absolute refractory period

Na⁺ inactivation gates are closed, so no Na⁺ can enter

Relative refractory period

An AP CAN fire, but only with a STRONGER stimulus; occurs during hyperpolarisation

Why do refractory periods matter?

They make APs travel one way and limit how fast a neuron can fire

Depolarisation block

Sustained depolarisation keeps Na⁺ inactivation gates closed → APs weaken then fail

Mild vs severe hyperkalaemia on excitability

Mild: closer to threshold → INCREASED excitability. Severe/sustained: Na⁺ channels inactivated → DECREASED excitability (depolarisation block)

Why can severe hyperkalaemia be fatal?

Depolarisation block in the heart → slowed conduction, bradycardia, arrhythmias, cardiac arrest

How do local anaesthetics work?

They block voltage-gated Na⁺ channels → no action potential → no pain signal

Similarities of APs across excitable cells

Very rapid depolarisation; all-or-none; roughly the same amplitude; amplitude does not depend on stimulus size

How is stimulus intensity coded?

By the FREQUENCY of action potentials, not their size (amplitude)

Differences of APs between cell types

Speed and complexity (shape) vary, and so does what they trigger

What does an AP trigger in a neuron / muscle / endocrine cell?

Neuron: chemical transmission. Muscle: contraction. Endocrine cell: secretion

Cardiac AP plateau

A long flat phase caused by Ca²⁺ influx balanced by K⁺ efflux

Neuron

A nerve cell that receives, integrates and transmits electrical signals

EPSP

Excitatory postsynaptic potential: a graded potential that DEPOLARISES the membrane

IPSP

Inhibitory postsynaptic potential: a graded potential that HYPERPOLARISES the membrane

Where are EPSPs and IPSPs summed?

At the AXON HILLOCK (trigger zone)

Why does the AP start at the axon hillock?

It has a high density of voltage-gated Na⁺ channels

Summation

Adding together graded potentials so that weak inputs can combine to reach threshold

Temporal summation

Rapid, repeated signals from the SAME input add together over TIME

Spatial summation

Signals from DIFFERENT inputs arriving at the same time add together

What decides whether a neuron fires?

The sum of all EPSPs and IPSPs at the axon hillock reaching threshold

How does an AP propagate along an axon?

Depolarisation opens Na⁺ channels in the next patch of membrane ahead; behind, K⁺ leaves and repolarises

Why does an AP travel in only one direction?

Na⁺ channels just behind the AP are inactivated (refractory)

Axolemma / axoplasm

Axolemma = axon's plasma membrane. Axoplasm = cytoplasm inside the axon

Myelin sheath

Tightly wrapped lipid bilayers plus proteins that insulate and support the axon

Conduction in unmyelinated axons

The AP travels continuously along the membrane (slower)

Nodes of Ranvier

Unmyelinated gaps between myelin segments, where voltage-gated Na⁺ channels cluster to regenerate the AP

Saltatory conduction

The AP 'jumps' from node to node in myelinated axons; much faster

How does myelin speed conduction?

Increases membrane resistance and decreases capacitance, so the depolarising wave decays less

Factors at the nodes that affect speed

Distance between nodes and the number of voltage-gated Na⁺ channels at each node

Range of conduction velocity

About 0.2 to 120 m/s

Two main factors that determine conduction velocity

Axon diameter and myelination

How does a larger axon diameter increase speed?

It decreases internal resistance to current flow along the axon

Fastest type of axon

Large-diameter AND myelinated