13.4 Nervous transmission Flashcards
whats action potential
the change in voltage across a cell membrane
potential difference is usually around +40mV
Electrical impulse that travels down the axon triggering the release of neurotransmitters
whens a neurone polarised
when its polarised
whats the resting potential
the difference in potential difference across a membrane, usually around -70mV the outside of the axon is more positively charged than inside the axon and the membrane is said to be polarised
What determines the resting potential
the distribution of Na+ and K+ ions
How does the sodium-potassium pump produce the resting potential
3 Na+ out for every 2 K+ that come in, this creates an imbalance of charge
How do K+ ion channels produce the resting potential
Some are open allowing K+ to diffuse out, increasing the charge imbalance
What is the period when the membrane is hyperpolarised called
the refractory period its after action potential
how is resting potential generated
actively transported 3 Na+ ions (out of axon) 2 K+ ion (in axon). outside of axon is more +ve.
more Na+ outside than inside the axon,
so some ions diffuse back into the axon down it’s electrochemical gradient through the few opened gated Na+ ion channels
Because there are more positive ions outside the axon than inside the axon, a p.d. of - 70 mV is created across the membrane
Events that occur during an action potential
- neurones resting potential = - 70 mV
- stimulus energy triggers some Na+ channels to open & Na+ diffuses into the axon, making the inside of the neurone more positive
- This change in charge causes more Na+ channels to open, allowing more Na+ to diffuse into the axon; this is an example of positive feedback
- When the potential difference reaches + 40 mV, the Na+ channels close and the K+ channels open
- K+ diffuse out the axon, reducing the charge, resulting in the inside of the axon becoming more negative than outside the axon
- Lots of K+ diffuses out of the axon, resulting in the inside of the axon becoming more negative than its normal resting state, this is known as hyperpolarisation
- The K+ channels close
- The sodium-potassium pump caused Na+ ions to move out the cell and K+ ions to move in; the axon returns to its resting potential as its repolarised
Propagation of action potentials along an axon
The initial stimulus triggers an action potential in the sensory receptor & the first region of the axon is depolarised
This stimulates the depolarisation of the next region of the membrane
The process continues along the length of the axon forming a wave of depolarisation
Once Na+ is inside the action, they diffuse along the axon down a concentration gradient, triggering the depolarisation of the next section (mexican wave idea)
whys the refractory period important
It prevents the propagation of an action potential backwards along the axon & ensures the action potential is unidirectional and do not overlap
whats the process of saltatory conduction
Depolarisation only occurs at the nodes of ranvier where no myelin is present
Therefore longer localised circuits arise between adjacent nodes
The action potential ‘jumps’ from node to node in the process of saltatory conduction
Why is saltatory conduction faster & better?
Ion channels take time to open & reducing the number of places the channels open increases the speed of transmission
More energy efficient over time as it reduces ATP used for repolarisation
Factors that affect action potential speed
- Myelination
- Axon diameter (less resistance to the flow of ions)
- Temperature (ions diffuse faster at higher temperatures)
whats the All-or-nothing principle
A certain level of stimulus, the threshold value, will always trigger a response
If this threshold is released an action potential will be created
The same sized action potential will be generated no matter the size of the stimulus but the stimulus size does affect the number of action potentials in a given amount of time
