nerones Flashcards
what do neurone cell membranes become at rest
polarised
what happens to the outside of a neurone in its resting state
In a neurone’s resting state (when it’s not being stimulated), the outside
of the membrane is positively charged compared to the inside. This is
because there are more positive ions outside the cell than inside.
what does it mean if the membrane is polarised
So the membrane is polarised — there’s a difference in charge
(called a potential difference or voltage) across it.
what is the voltage across the membrane called when its at its resting state
resting potential — it’s a b o u t- 70 mV (millivolts).
what is the resting potential of a voltage across the membrane created and maintained by
The resting potential is created and maintained by sodium-potassium pumps
and potassium ion channels in a neurone’s membrane:
what do the sodium potassium pumps do
The sodium-potassium pumps move sodium ions out of the neurone,
but the membrane isn’t permeable to sodium ions, so they can’t
diffuse back in. This creates a sodium ion electrochemical gradient
(a concentration gradient of ions) because there are more positive
sodium ions outside the cell than inside
The sodium-potassium pumps also move potassium ions in to
the neurone, but the membrane is permeable to potassium ions
so they diffuse back out through potassium ion channels.
• This makes the outside of the cell positively charged compared to the inside
how do sodium potassium pumps do what they do
These pumps use active transport to move there sodium ions (Na out of the neurone for every two potassium ions (K +) moved in. A T P is needed to do this.
what are potassium ion channels
Th ese channels allow facilitated
diffusion of potassium ions
(K +) out of the neurone, down
their concentration gradient.
what happens when neurone cell membranes become stimulated
they become depolarised
what does a stimulus trigger
A stimulus triggers other ion channels, called sodium ion channels, to open. If the stimulus is big enough, it’ll
trigger a rapid change in potential difference
what are all the steps of action potential in order
- stimulus
- depolarisation
- repolarisation
- hyperpolarisation
- resting potential
what happens in the first step of action potential: stimulus
this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative
what happens in the second step of action potential: depolarisation
if the potential difference reaches the threshold (around -55 mV), more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.
what happens in the third step of action potential: repolarisation
.at a potential difference of around + 30 mV the sodium ion
channels close and potassium ion channels open. The membrane is more permeable
to potassium so potassium ions diffuse out of the neurone d own the potassium ion
concentration gradient. This starts to get the membrane back to its resting potential.
what happens in the fourth step of action potential : hyper polarisation
potassium ion channels are slow to close so there’s a slight
‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential
difference becomes more negative than the resting potential (i.e . less than - 70 mV).
what happens in the fifth step of action potential : resting potential
the ion channels are reset. The sodium-potassium pump returns the membrane
to its resting potential and maintains it until the membrane’s excite d by another stimulus
what cant the neurone cell membrane be after an action potential
After an action potential, the neurone cell membrane can’t be excited again straight away. This is because the ion
channels are recovering and they can’t be made to open — sodium ion channels are closed during repolarisation and
potassium ion channels are closed during hyperpolarisation. This period of recovery is called the refractory period.
what does the action potential move along
The Action Potential Moves Along the Neurone as a Wave of Depolarisation
what are all the steps of Action Potential Moving Along the Neurone as a Wave of Depolarisation
1) W hen an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
2) This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
3) This causes a wave of depolarisation to travel along the neurone.
4) The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an
action potential.
what does the refractory period produce
The Refractory Period Produces Discrete Impulses
during the refractory period what happens to the ion channels
During the refractory period, ion channels are recovering and can’t be opened
what does the refractory period act as
So the refractory period acts as a time delay between one action potential and the next.
This means that:
- action potentials don’t overlap, but pass along as discrete (separate) impulses.
- there’s a limit to the frequency at which the nerve impulses can be transmitted.
- action potentials are unidirectional (they only travel in one direction).
what happens once the threshold is reached
O n c e the threshold is reached, an action potential
will always fire with the same change in voltage,
no matter how big the stimulus is
what happens if the threshold isn’t reached
If the threshold isn’t reached, an action potential won’t fire. A
This is the all-or-nothing nature of action potential
what will a bigger stimulus not cause
A bigger stimulus won’t cause a bigger action potential,
but it w ill cause them to fire more frequently.
