chapter 13.4 propagation of nervous impulses

what is the resting potential of a neuron

-70mV

what is the voltage of an action potential?

40mV

What is the threshhold potential for an action potential to occur

-55mV

what is a resting potential

the potential difference when a neuron isnt transmitting an impulse accross its membrane

state of a neuron during resting potential

the outside of the membrane is more positive compared to the inside of the membrane (axon), which is more negative

how is a resting potential established in a neuron

Active transport of Na+ ions and K+ ions across the membrane by the sodium-potassium pump

how do K+ and Na+ ions move across the bilayer

through channel proteins since they cannot diffuse through the bilayer

creation of a resting potential (6)

-Na+ actively transported out of the axon, K+ actively transported into the axon by an intrinsic protein called the sodium potassium pump
-but movement isnt equal: for every 3 Na+ moved out, 2 K+ moved in
-as a result, there are more sodium ions outside the membrane than inside the axon, and move K+ inside than outside.
-this causes a diffusion gradient, where Na+ wants to move in and K+ wants to move out
-however, most of the gated protein channels for Na+ are closed, so lots of K+ moves out, and little Na+ goes back in
-therefore there are many more positively charged ions outside the membrane than inside, creating a resting potential.

what happens when a stimulus is detected by a sensory receptor

stimulus is detected by a sensory receptor

the energy from the stimulus temporarily reverses the charges on the axon membrane, and the potential difference across the membrane rapidly changes (called depolarisation)

what is depolarisation

A change in potential difference from negative to positive

what is repolarisation

a change in potential difference from positive to negative

when does an action potential occur

potential occur

when protein channels in the axon membrane change shape as a result of the voltage which opens/closes voltage-gated ion channels

what are voltage gated ion channels

ion channels that open and close in response to changes in membrane potential

action potential graph (6)

1. Resting potential
2. Stimulus reaches threshold potential (-55mV)
3. Depolarization: Na+ channels open due to inital Na+ moving in, K+ channels closed
4. Na+ channels close, K+ channels open
5. Repolarization: K+ diffuses out which reduces the charge
6. hyperpolarisation: initially, lots of K+ diffuse out so resting potential gets lower than normal for a bit, then sodium potassium pump returns axon to resting potential

does the potassium sodium pump still work during depolarisation?

Yes

what is positive feedback

during the creation of an action potential, the change in charge from the first Na+ ions moving causes more Na+ channels to open, allowing more Na+ ions to move through, causing a cascade of Na+ channels opening

what is a nerve impulse

an action potential that starts at one end of the neurone and is propagated along the axon to the other end of the neurone

how does propagation occur

-initial stimulus causes a change in sensory receptor, which triggers an AP, so the first region of the axon membrane is depolarised.
-this acts as a stimulus for the depolarisation of the next region, forming a wave of depolarisation

This is because, the depolarisation (reversal of membrane charge) causes voltage gated sodium channels to open a little further away from original depolarisation.

What is a refractory period?

A period of time after an action potential where the axon cannot be excited again

voltage gated ion channels stay closed, so ions cannot diffuse across the membranes so there is no electrochemical gradient established

Why is the refractory period important? (3)

It prevents the propagation of an action potential backwards along the axon
- The refractory period makes sure action potentials are unidirectional.
- It also ensures that action potentials do not overlap and occur as discrete impulses.

Why are electrical impulses much faster in myelinated neurones than in non-myelinated (4)

- As depolarisation can only happen at nodes of Ranvier, where no myelin is present
- here, Na+ can pass through the protein channels
- longer localised 'circuits' therefore arise between adjacent nodes
- the action potential then 'jumps' from one node to another in a process known as saltatory conduction

What is saltatory conduction?

the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials.

Why is saltatory conduction beneficial compared to continuous conduction

- fewer ion gates are needed to send the action potential along the axon
- it is also more energy efficient as repolarisation uses ATP in the sodium-potassium pump, so less repolarisation needed saves energy

3 factors that affect velocity of an action potential

1. Myelination of the neuron
2. Axon diameter- the bigger the axon diameter, the faster the impulse as there is less resistance to flow of ions in the cytoplasm
3. Temperature- higher temp= faster action potential until about 40c, where protein channels denature

What is the all or nothing principle?

a muscle fibre will either respond fully to a stimulus if it reaches the threshold potential, or not respond at all if the stimulus is below the threshold. once the threshold is reached, a muscle fibre will respond maximally and will not produce a partial response

what is hyperpolarisation?

Hyperpolarization (Afterhyperpolarization): After repolarization, the voltage-gated potassium channels remain open a little longer than needed, causing the membrane potential to become even more negative than the resting membrane potential.

What is the difference between the refractory period and hyperpolarisation