AQA A Level Biology復習筆記6.2.3 Nerve Impulses
Action Potentials
- Neurones?transmit?electrical impulses, which travel extremely quickly along the neurone?cell surface membrane?from one end of the neurone to the other
- Unlike a normal electric current, these impulses are?not?a flow of electrons
- These impulses, known as?action potentials, occur via very?brief changes in the distribution of electrical charge?across the cell surface membrane
- Action potentials are caused by the?rapid movement of sodium ions and potassium ions?across the membrane of the?axon
Resting potential
- In a?resting axon?(one that is?not transmitting impulses), the?inside?of the axon always has a slightly?negative electrical potential?compared to outside the axon
- This?potential difference?is usually about?-70mV?(ie. the inside of the axon has an electrical potential about 70mV?lower?than the outside)
- This is called the?resting potential
Action potentials
- There are?channel proteins?in the?axon membrane?that allow?sodium ions?or?potassium ions?to pass through
- When an?action potential?is stimulated (eg. by a receptor cell) in a neurone, the following steps occur:
- Sodium ion channels?in the axon membrane?open
- Sodium ions?pass?into the axon?down the?electrochemical gradient?(there is a greater concentration of sodium ions outside the axon than inside. The inside of the axon is negatively charged, attracting the positively charged sodium ions)
- This?reduces the potential difference?across the axon membrane as the?inside?of the axon becomes?less negative?– a process known as?depolarisation
- Depolarisation triggers?more channels?to open, allowing?more sodium ions?to enter and causing?more depolarisation
- This is an example of?positive feedback?(a small initial depolarisation leads to greater and greater levels of depolarisation)
- If the potential difference reaches around?-50mV?(known as the?threshold potential),?many more channels open?and?many more sodium ions enter?causing the inside of the axon to reach a potential of around?+30mV
- An action potential is generated
- The depolarisation of the membrane at the site of the first action potential causes?sodium ions to diffuse?to?along the?axon,?depolarising?the membrane in the next section of the axon and causing sodium ion voltage-gated channel proteins to open there
- This is known as?conduction?and is sped up by the presence of?Schwann cells
- This triggers the?production of another action potential?in this section of the axon membrane and the process continues
- In the body, this allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane
How an impulse is transmitted in one direction along the axon of a neurone
Repolarisation and the refractory period
- Very shortly (about 1 ms) after an action potential in a section of axon membrane is generated, all the?sodium ion voltage-gated channel proteins?in this section?close, stopping any further sodium ions diffusing into the axon
- Potassium ion voltage-gated channel proteins?in this section of axon membrane now?open, allowing the diffusion of potassium ions?out of the axon, down their concentration gradient
- This?returns the potential difference to normal?(about -70mV) – a process known as?repolarisation
- There is actually a short period of?hyperpolarisation. This is when the potential difference across this section of axon membrane briefly becomes?more negative than the normal resting potential
- The potassium ion voltage-gated channel proteins then?close?and the sodium ion channel proteins in this section of membrane?become responsive to depolarisation again
- Until this occurs, this section of the axon membrane is in a?period of recovery?and is?unresponsive
- This is known as the?refractory period
- The refractory period is very important as it ensures that ‘new’ action potentials are generated ahead (ie. further along the axon), rather than behind the original action potential
- This makes the action potentials?discrete?events and means the impulse can only?travel in one direction. This is essential for the successful and efficient transmission of nerve impulses along neurones.
The five stages of an action potential: stimulus, depolarisation, repolarisation, hyperpolarisation and return to resting state
Nerve Impulses
- When receptors (such as chemoreceptors) are stimulated, they are?depolarised
- If the stimulus is very?weak?or?below a certain threshold, the receptor cells won’t be sufficiently depolarised and the sensory neurone will not be activated to send impulses
- If the stimulus is strong enough to increase the?receptor potential above the threshold?potential?then the receptor will?stimulate the sensory neurone?to send impulses
- This is an example of the?all-or-nothing principle
- An impulse is only transmitted if the?initial stimulus is sufficient?to increase the membrane potential above a?threshold potential
- Rather than staying constant,?threshold levels in receptors often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones
The receptor potential increases as the strength of the stimulus increases. As the strength of stimulus increases beyond the threshold, the frequency (not amplitude) of impulses increases.
Exam Tip
Some receptors, like the chemoreceptors described above, are?specialised cells?that detect a?specific type of stimulus?and affect the sensory neurone’s electrical activity. Other receptors are just the?ends of the sensory neurones?(for example, many types of touch receptors).
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