13.3 The Nervous Impulse Flashcards
What is ‘resting potential’?
What is it’s value?
In an neurone’s resting state (when its not being stimulated), the outside of the cell is more positive relative to the inside (as more positive ions are outside the cell than inside)
So the membrane is polarised - there’s a difference in charge.
The voltage across the membrane when its at rest is called the resting potential - its about -70 mV (millivolts)
Describe and explain how the resting potential across a neurone’s membrane is created and maintained
The resting potential is created and maintained by sodium-potassium pumps ad potassium ion channels in a neurone’s membrane:
- sodium-potassium pumps use active transport to move three sodium ions out of the neurone for every two potassium ions moved in
- potassium ion channels allows facilitated diffusion of potassium ions out of the neurone, down their conc gradient
1) 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 because there’s more positive ions on the outside of the cell than inside
2) the sodium-potassium pumps also move potassium ion into the cell
3) when the cell’s at rest, most potassium ion channels are open. This means the membrane is permeable to potassium ions, so some diffuse back out through the potassium ion channels.
As in total more positive ions move out the cell than enter, the outside of the cell is politely charged relative to the inside
Describe and explain how action potentials are generated
Explain the changes in p.d. Across a neurones cell membrane during an action potential
When a neurone is stimulated, sodium ion channels in the cell membrane open. If the stimulus is big enough, it’ll trigger a rapid change in potential difference. This causes the cell membrane to become depolarised. The sequence of events that happens is known as an action potential.
1) Stimulus - this excited the neurone 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. This makes the inside of the neurone less negative.
2) Depolarisation - if the p.d. Reaches the threshold (-55 mV), voltage-gated sodium ion channels open and more sodium ion diffuse into the cell. This is positive feedback.
3) Repolarisation - at a potential difference of around +30 mV the sodium ion channels close, and voltage-gated potassium ion channels open. The membrane is now more permeable to potassium ions, so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential. This is negative feedback.
4) Hyperpolarisation - potassium ion channels are slow to close so there’s a slight ‘overshoot’ (refractory period) where too many potassium ion diffuse out of the neurone. The p.d. Becomes more negative than the resting potential (less than -70 mV)
5) Resting potential - the ions channels reset. The sodium-potassium pump returns the membrane to it resting potential by pumping sodium ions out of the neurone and potassium ions in, and maintains the resting potential until the membrane’s excited by another stimulus
Describe and explain the refractory period
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 is called refractory period.
The refractory period acts as a time delay between one action potential and the next. This makes sure that action potentials don’t overlap but pass along as separate impulses.
The refractory period also makes sure action potentials only travel in one direction
Describe and explain waves of depolarisation
When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
This causes sodium ion channels in the next region of the neurone to open and sodium ion diffuse into that region
This causes a wave of depolarisation to travel along the neurone
Describe how bigger difference have higher frequency of impulses
Once the threshold is reached an action potential while always fire with the same change in voltage, not matter how big the stimulus is.
If the threshold isn’t reached, an action potential won’t fire. This is a part of the all-or-nothing nature of action potentials
A bigger stimulus won’t cause a bigger action potential, but it will cause them to fire more frequently.
So if the brain receives a high frequency of action potentials, it interprets this as a big stimulus and responds accordingly
What are the three factors that affect the speed of conduction of action potentials
1) Myelination
2) Axon diameter
3) Temperature
Describe and explain what myelination is
Some neurones are myelinated (sensory and motor)- they have a myelin sheath.
- the myelin sheath is an electrical insulator
- In the PNS made up of Schwann cells, which are wrapped around the axon (and/or dendron)
- between the Schwann cells are tiny patches of bare membrane called the nodes of Ranvier.
Sodium ion channels are concentrated at the nodes of Ranvier.
Action potentials are stimulated at the nodes of Ranvier
Describe and explain saltatory conduction
In a myelinated neurone, depolarisation only happens at the nodes of Ranvier (where sodium ions can get through the membrane)
The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse ‘jumps’ from node to node - this is saltatory conduction and its really fast
In a non-myelinated neurone, the impulse travels as a wave along the whole length of axon membrane (slower than saltatory conduction)
Explain how axon diameter affects speed of conduction
Action potentials are conducted a lot quicker along axons with bigger diameters because there’s less resistance to the flow of ions than in the cytoplasm of a smaller axon
With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker
Explain how temperature affects speed of conduction
The speed of conduction increases as temperature increases, because ions diffuse faster
But if temp exceeds around 40 degrees, the proteins begin to denature and the speed decreases
