The Nervous Impulse Flashcards
The movement of ions (eg Na+ and K+) across the axon membrane is controlled in what ways?
The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it.
Intrinsic proteins contain ion channels, some of which have ‘gates’.
Some intrinsic proteins actively transport K+ into and Na+ out of the axon (Sodium potassium pump)
What is the neurone’s resting state like?
(When it’s not being stimulated) the outside of the membrane is more positively charged than the inside (more positive ions outside than inside).
There’s a higher conc of Na+ ions outside cell and a higher conc of K inside the cell.
So the membrane is polarised- there’s a difference in charge.
The voltage across the membrane when at rest is called the resting potential and its about 65 mV.
The establishment of the resting potentials ‘65 mV’ is due to what events….
Na-K pumps use active transport to move 3 Na+ out of the neurone for every 2 K+ that move in. ATP is needed for this.
K+ ion channels allow facilitated diffusion of K+ out of the neurone, down their conc gradient.
The Na+ ions cannot diffuse back in because the membrane isn’t permeable for them- a sodium electrochemical gradient thus forms.
The Na-K pumps also move K+ into the neurone.
When the cells at rest, most K+ channels are open so the membrane is permeable to K+ so some diffuse back out.
Even through positive ions are moving in and out- more are moved out than enter so the outside is more positively charged.
How is a generator potential created?
When a stimulus is detected, the cell membrane is excited and becomes more permeable allowing more ions to move in and out of the cell.
This alters the potential difference across the cell membrane and therefore produces a generator potential.
What is the sequence of events in the action potential?
Stimulus- this excites the neurone of the cell membrane, causing Na+ ion channels to open. The membrane becomes more permeable to Na, so they diffuse down the sodium ion electrochemical gradient, making the inside less negative.
Depolarisation- if the P.D reaches the threshold (-55 mV), more Na+ channels open- more diffuse into the neurone.
Repolarisation- at a potential difference of around + 30 mV the Na+ channels close and K+ channels open. The membrane is more permeable to K+ so K+ ions diffuse out of the neurone down a gradient.
Hyperpolarisation- K+ ion channels are slow to close so theres a slight ‘overshoot’ where too many K+ leave and the P.D becomes more negative than the resting potential (-70 mV).
Resting potential- ion channels are reset, Na/K pump returns to memrane to its resting potential by pumping Na+ out and K+ in, and maintains the resting potential until the membrane’s excited by another stimulus.
What is 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.
This delay makes sure that action potentials are unidirectional.
What three factors affect the speed of conduction of action potentials:
- Myelination- some neurones have a myelin sheath which is an electrical insulator. Depolarisation only happens at the nodes of Ranvier (where Na+ ions can get through). The neurone’s cytoplasm conducts enough electrical charge to depolarise the next node (saltorary conduction and = very fast).
- Axon diameter- wider = less resistance = quicker
- Temperature- hotter = faster diffusion = quicker.
Which two proteins in a neurone’s cell membrane are responsible for creating and maintaining the resting membrane potential?
Sodium-potassium pumps and potassium ion channels.
Following a stimulus, explain how the opening of sodium ion channels affects the potential difference across a neurone cell membrane.
Sodium ions diffuse into the neurone down the sodium ion electrochemical gradient.
This makes the inside of the neurone less negative and so decreases the potential difference across the membrane.
Describe and explain the movement of sodium ions if the potential difference across a neurone cell membrane reaches the threshold level.
More sodium ions diffuse into the neurone because more sodium ion channels open.
After an action potential, why can’t the neurone cell membrane be excited again straight away?
What two effects does this have on the conduction of action potentials along a neurone?
The ion channels are recovering and can’t be made to open.
It makes action potentials discrete/separate impulses. It makes action potentials uni-directional.
Explain how waves of depolarisation are produced.
During an action potential, 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 ions diffuse into that part.
This causes a wave of depolarisation.
Describe the structure of a myelinated neurone.
A myelinated neurone has a myelin sheath made up of Schwann cells.
Between the Schwann cells are tiny patches of bare membrane called nodes of Ranvier where sodium ion channels are concentrated.
How does conduction long a myelinated neurone differ compared to conduction along a non-myelinated neurone?
In a myelinated neurone depolarisation/action potentials only happen at nodes of Ranvier.
In a non-myelinated neurone, depolarisation/action potentials occur as a wave along the whole length of the axon membrane.
Give two factors, other than myelination, that affect the conduction of action potentials.
Axon diameter and temperature.
