Movement of impulses
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How is an electrical impulse transmitted by a neurone?
- Through the movement of positively charged ions (cations) through the membrane of the cell
- This changes the potential difference across it
What is the resting potential?
- The potential difference across the membrane of a neurone when it is not transmitting an impulse
- It has been measured at approximately -70mV
How is resting potential achieved?
A carrier protein in the neurone membrane called the sodium/potassium pump actively transports 3Na+ ions out of the cell for every 2 K+ ions in
What does the sodium/potassium pump create?
- A concentration gradient of ions
- A higher concentration gradient of Na+ outside of the cell and a higher concentration of K+ ions inside the cell
Other than the sodium/potassium pump, how else do ions move through the membrane during resting potential?
- A small number of ions move in via facilitated diffusion through specific channel proteins
- There are more channel proteins specific to K+ ions than they are for Na+ ions
What is the net result of ion movements during resting potential?
- There are more positive ions outside the neurone than inside
- This leads to a resting potential of -70mV
What is an action potential?
A reduction of the potential difference across the membrane of an axon
How is an action potential initiated?
- The neurone is first stimulated by a receptor
- Sodium ion channels in the membrane open allowing sodium ions to diffuse into the axon down a concentration gradient
What is the result of the sodium ion channels opening?
- There is a higher concentration of positive ions inside the axon than the outside
- The axon is said to be depolarised
What is the approximate voltage of an action potential?
How does the action potential stimulate further change?
It causes ion channels to open in the next part of the membrane causing the action potential to move along the axon in a wave of depolarisation
What requirements must be met in order for an action potential to be sent?
The threshold value must be exceeded
What is the generator potential
The response of receptors to a stimulus causing sodium channels to open resulting in the potential difference across the membrane becoming less negative
What does the size of the generator potential depend on?
The strength of the stimulus
What occurs when the generator potential exceeds threshold value?
- Large numbers of voltage gated sodium ion channels open in the membrane of the sensory neurone
- An action potential of the same size is generated regardless of how large the stimulus is
What occurs if the generator potential fails to exceed threshold value?
The sodium ion channels remain closed
What is the name given to the relationship between generator potential and threshold value?
The all or nothing principle
How is information about the size of the stimulus conveyed?
Via the frequency of impulses
What must happen once the membrane has been depolarised?
It must be depolarised back to resting potential to allow further action potentials to be conveyed
How is the resting potential restored?
- Sodium ion channels are are closed and potassium ion channels are opened in the membrane of the axon
- This allows potassium ions to diffuse out of the axon down a concentration gradient
What is the result of the sodium ion channel closure?
More positive ions are outside the axon than inside and the potential difference across the membrane becomes more negative
What is the refractory period?
- For a short time after repolarisation, the sodium ion channels remain closed
- During this time the axon cannot be depolarised and no impulse can pass
Why is the refractory period important?
Each action potential is kept discrete with no overlap
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