Neurones

Describe resting state

Not being stimulated, where the outside of the neurone is positively charged compared to the inside- membrane is polarised as there is a difference in voltage across it

How many millivolts is the membrane at when at rest (resting potential)

-70mv

How is a resting potential created and maintained

Sodium potassium pumps and potassium ion channels in the membrane

Why do sodium ions not move back into the neurone through the membrane once pumped out

The membrane isnt permeable to sodium ions so they cant diffuse back in

What does pumping sodium ions out do

Creates an electrochemical gradient because there are more positive sodium ions outside the cell than inside

What are the sodium potassium pump,

potassium ion channels and

sodium ion channels all types of

Transport protein

How do potassium ions move back out after being pumped into the neurone

Membrane is permeable to potassium ions so they diffuse back out through potassium ion channels

Whats the sequence of events for an action potential

Stimulus

Depolarisation

Repolarisation

Hyperpolarisation

Resting potential

What does a stimulus trigger

Excites the neurone cell membrane causing sodium ion channels to open. The membrane becomes more permeable to sodium ions so they diffuse into it down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.

Explain depolarisation

If the potential difference reaches the threshold (-55mV) more sodium ion channels open and more sodium ions diffuse rapidly into the neurone.

Explain repolarisation

At a potential difference of around +30mv the sodium channels close and potassium ion channels open. The membrane is more permeable to them so they diffuse out down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential

Explain hyperpolarisation

Potassium ions channels are slow to close so there is a slight ‘overshoot’ where too many potassium ions diffuse out. The potential difference becomes more negative than the resting potential (less than -70mV)

Explain how the membrane gets back to resting potential

The ion channels are reset. The sodium potassium pump returns the membrane to its resting potential and maintains it until the membrane is excited by another stimulus

Explain how a wave of depolarisation happens

Some of the sodium ions that enter the neurone diffuse sideways which causes sodium ion channels in the next region to open and diffuse into that part. The wave moves away from the parts of the membrane in the refractory period because these parts cant fire an action potential.

What does the refractory period do for a neurone

Makes sure action potentials dont overlap- discrete impulses.

Ensures action potentials are unidirectional.

What is the all or nothing nature

Once the threshold is reached, an action potential will always fire.

If the threshold isnt reached an action potential wont fire.

A bigger stimulus wont cause a bigger action potential but will cause them to fire more frequently

What are the three factors that affect the speed of conduction of action potentials

Myelination

Axon diameter

Temperature

Explain how myelination affects speed of conduction of an action potential

A myelin sheath coats SOME neurones.

It is an electrical insulator.

It is made of Schwann cells

Nodes of ranvier are the gaps that the impulse can ‘jump’ across. This is called saltatory conduction and is really fast.

Explain how axon diameter affects speed of conduction of action potentials

Action potentials are conducted quicker along axons with a bigger diameter because they are less resistant to the flow of ions. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker.

Explain how temperature affects the speed of conduction of an action potential

The speed of conduction increases as the temperature increases too, because ions diffuse faster. The speed only increases up to around 40 degrees calcius - after that the proteins begin to denature and the speed decreases.