Nervous Coordination- Neurones Flashcards
What is the neurone like (in terms of charge) in resting state?
The outside of the membrane is positively charged compared to the inside
Why is the neurones charge more positive on the outside in resting state?
Because there are more positive ions outside the cell than inside so the membrane is polarised
What does the membrane being polarised mean?
There’s a difference in charge (potential difference) across it
What is the potential difference across the membrane when at rest?
-70 mV which is resting potential
What is the resting potential created and maintained by?
Sodium-potassium pumps and potassium ion channel in a neurone’s membrane
What is the role of the sodium-potassium pumps with sodium ions?
Move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they can’t diffuse back in which creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside
What is the role of the sodium-potassium pumps with potassium ions?
Move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels
What is the significance of the sodium-potassium pumps?
Makes the outside of the cell positively charged compared to the inside
How do sodium ions move in sodium-potassium pumps?
Use active transport to move three sodium ions out of the neurone for every two potassium ions moved in- ATP is needed
How do potassium ions move in sodium-potassium pumps?
Allow facilitated diffusion of potassium ions out of the neurone down their concentration gradient
What does a stimulus trigger?
Other ion channels called sodium ion channels to open
What is triggered if the stimulus is big enough?
A rapid change in potential difference known as an action potential
What is step 1 in changes in potential difference during an action potential?- stimulus
Excites the neurones cell membrane, causing sodium ion channels to open- the membrane become more permeable to sodium so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient- makes the inside of the neurone less negative
What is step 2 in changes in potential difference during an action potential?- depolarisation
If the potential difference reaches the threshold (-55 mV) then more sodium ion channels open and sodium ions diffuse rapidly into the neurone
What is step 3 in changes in potential difference during an action potential?- repolarisation
At a potential difference of around +30 mV the sodium ion channels close and potassium ion channels open- membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient and starts to get the membrane back to its resting potential
What is step 4 in changes in potential difference during an action potential?- hyperpolarisation
Potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone- potential difference becomes more negative than the resting potential
What is step 5 in changes in potential difference during an action potential?- resting potential
Ion channels are reset- sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane’s excited by another stimulus
Why can’t the neurone cell membrane be excited again straight away after an action potential?
Because the ion channels are recovering and they can’t be made to open- sodium ion channels are closed during hyperpolarisation (refractory period)
When do some of the sodium ions diffuse sideways and what does this cause?
When an action potential happens which causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part causing a wave of depolarisation to travel along the neurone
Why does the wave of depolarisation move away from the parts of the membrane during the refractory period?
Because these parts can’t fire an action potential
What is happening during a refractory period?
Ion channels are recovering and can’t be opened
What does the refractory period act as?
A time delay between one action potential and the next
What are the three things an refractory period means?
- 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
What does the all-or-nothing principle mean?
Once 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 is not reached?
An action potential won’t fire
What effect will a bigger stimulus have?
Cause action potentials to fire more frequently
What does it mean that a neurone is myelinated?
They have a myelin sheath
What is a myelin sheath?
An electrical insulator
What cells is the sheath made up of in the peripheral nervous system?
Schwann cells
What are between the Schwann cells?
Nodes of Ranvier
What are concentrated at nodes?
Sodium ion channels
Where does depolarisation happen in a myelinated neurone?
Only at nodes of Ranvier
What is saltatory conduction?
Impulse ‘jumps’ between nodes
Why is conduction slower in an unmyelinated neurone?
The impulse has to travel along whole length of axon membrane
What are the three factors that affect conduction of action potentials?
Myelination, axon diameters and temperature
How does axon diameter affect conduction of action potentials?
A bigger diameter means an action potential is conducted quicker along axons
How does temperature affect conduction of action potentials?
Speed of conduction increases as temperature increases
