Electric Signalling

Describe the neuron membrane at rest

Cytosolic side of plasma is relatively -ve charged compared to extracellular side (Resting Membrane Potential).

What’s the Equilibrium Potential?

Potential Tendency of an Ion to move down its concentration gradient is equal to Membrane potential.

Function of the Sodium-Potassium ATPase pump?

An ion pump which breaks down ATP when in presence of internal Na⁺

What drives the Na-K pump?

The chemical energy (from ATP), drives the pump, exchanging internal Na⁺ for external K⁺. (maintaining RMP)

Na-K pump process

1. 3 Na+ & 1 ATP binds to the protein “pump”.

2. Hydrolysis of ATP then phosphorylates the pump protein, which changes its shape, P_i binds to the pump.

3. The shape change then releases Na+ outside the cell, enabling K+ to bind to the pump.

4. To return back to original shape, P_i is released, causing K+ to be released inside the cell & exposing the Na+ binding sites. (This cycle repeats).

AP

Breif Reversal of RMP, causing membrane to become more permeable to Na+ then K+.

What is an AP determined by?

influx of Na+ (depolarisation), Efflux of K+ (hyperpolarisation).

What is the AP threshold?

Vm when enough voltage-gated Na+ channels open.

All or Nothing Principle

Either Action Potential Threshold is reached, or not enough magnitude to trigger.

Absolute Refractory Period

An Action Potential cannot be elicited

Relative Refractory Period

When the membrane potential is hyperpolarised, only a stronger depolarising stimulus can elicit a 2nd AP.

What ensures the one-way propagation of an AP along an axon?

Na+ channels behind the WOD become inactivated and K+ channels open, preventing backward depolarisation.

How do local anaesthetics work at cellular level?

They block voltage-gated Na+ channels. preventing generation of propagation of Action Potentials.

Difference in AP propagation between Myelinated vs Unmyelinated nerve fibers.

Myelinated: AP moves via a saltatory conduction (series of jumps between Nodes of Ranvier), making propagation faster

Unmyelinated: Signal moves as a continuous WOD along membrane.

What steps occur in sequence along an axon segment during AP propagation

Depolarisation: Voltage-gated Na+ channels open as signal arrives.

Repolarisation: Na⁺ channels inactivate & K⁺ channels open (refractory state)

Resting State: Segment resets back to resting membrane potential.

What myelinates axons in CNS

Oligodendrocytes

What myelinates axons in PNS

Schwann cells

What are Nodes of Ranvier?

Regular gaps in myelin sheath along an axon

Why are Nodes of Ranvier significant for Signal Transmission?

The axon is in direct contact with ECF at these gaps, allowing for ion exchange and action potential regeneration during saltatory conduction.

What 2 primary factors affect the conduction velocity of an Action Potential?

Axon Diameter (Larger = less resistance = faster)

Myelination (Insulation enabling saltatory conduction = faster)

Why does a Larger axon diamter Increase AP conduction velocity?

It decreases resistance to local current flow along inside of axon

What is saltatory conduction, why is it also faster?

Process where an action potential jumps from node to node along a myelinated axon.

Because Voltage-gated Na+ channels are concentrated at nodes of Ranvier, allowing current to spread farther & faster.

Where are voltage-gated Na+ channels concentrated on a myelinated axon

At nodes of Ranvier

What is a synapse?

Junction between a axon terminal and a neuron.

Whats synaptic transmission?

Process by which neurones pass electrical/chemical information onto other neurons.

2 types of synaptic transmission

Chemical transmission (uses neurotransmitters)

Electrical Transmission (Uses gap junctions)

What triggers the release of ACh into synaptic cleft at the neuromuscular junction?

Influx of Ca²⁺ through voltage-gated Ca²⁺ channels (opened by depolarisation) triggers Ca²⁺-dependent exocytosis of ACh vesicles.

What occurs when ACh binds to its receptors on the Postsynaptic membrane?

Ligand-gated cation channels open, allowing Na+ influx & K+ efflux, which depolarises the postsynaprtic membrane & triggers an Action Potential.

What role does acetylcholinesterase (AChE) play at neuromuscular junction?

AChE breaks down ACh in the synaptic cleft to terminate the signal. The products are taken back to presynaptic cell (ACh & Vesicles are recycled).

Outline 7 steps of synaptic transmission at NMJ in order

1. Action potential arrives at axon terminal.

2. Depolarization opens voltage-gated Ca²⁺ channels.

3. Ca²⁺ enters and causes ACh vesicle exocytosis.

4. ACh diffuses across the synaptic cleft to bind receptors.

5. Cation channels open, depolarizing the postsynaptic membrane.

6. Postsynaptic action potential fires.

7. AChE breaks down ACh, stopping the signal and recycling components.

What are EPSPs (Excitatory Post-Synaptic Potentials) and how do excitatory neurotransmitters cause them

EPSPs are local depolarisations caused by excitatory neurotransmitters that increase the likelihood of firing an AP if threshold is reached.

What are IPSPs (Inhibitory Post-Synaptic Potentials) and how do inhibitroy neurotransmitters cause them?

IPSPs are hyperpolarisations caused by inhibitory neurotransmitters that decrease nerve activity & prevent generation of AP.

Give examples of Excitatory vs. Inhibitory Neurotransmitters

E → Glutamate, Acetylcholine

I → GABA (𝜸-aminobutyric acid)

What determines whether a postsynaptic neuron fires an AP?

Synaptic Integration (summation), the net balance of all EPSPs & IPSPs converge on the neurone at any given moment, _{if the threshold potential is reached, an AP will fire.}

What are the three primary types of synaptic contacts based on target site?

Axo-dendritic: Axon to dendrite

Axo-Somatic: Axon to cell body (soma)

Axo-axonic: Axon to another axon

Define an axo-dendritic synapse?

A synaptic connection formed between an axon terminal of a presynaptic neuron & the dendron of a postsynaptic cell.

Define an axo-somatic synapse?

A synaptic connection between an axon terminal presynaptic neuron & the soma of a postsynaptic cell.

Define an axo-axonic synapse

Synaptic Connection formed between axon terminal of a presynaptic neuron and the axon of another neuron.

What is an excitatory synapse?

Synapse where presynaptic activity increase the excitability of the postsynaptic neuron by causing depolarisation.

What are the 2 key electrical characteristics of EPSPs & IPSPs compared to action potentials?

1. They are graded in intensity

2. Outlast the APs which triggered them.

Why are EPSPs able to undergo temporal & spatial summation?

As they are graded & outlast the brief presynaptic AP that generated them, allowing multiple potentials to add together over time or space.

What is Spatial vs. Temporal Summation

Spatial:Occurs when several EPSPs arrive at axon hillock simultaneously

Temporal: Postsynaptic potentials created at the same synapse in rapid succession can be summed.