6.2.1 Nerve impulses Flashcards
(8 cards)
The structure of a myelinated motor neurone.
Motor neurones have:
A large cell body with a single axon (and many smaller dendrites) extending from it. The axon ending is called the axon terminal.
The cell body houses the nucleus (and other organelles), and lies within the CNS.
The dendrites are highly-branched, allowing them to synapse with the axon terminals of other neurones. Dendrites carry impulses towards the cell body, whereas axons carry impulses away from the cell body.
Schwann cells form a myelin sheath around the axon. Between the Schwann cells there are tiny gaps called nodes of Ranvier.
The establishment of a resting potential.
Sodium-potassium pump actively transports 3 sodium ions out for two potassium ions in, resulting in a higher concentration of potassium ions inside the neurone and higher concentration of sodium ions outside.
The voltage gated channels are all closed, but the potassium channels are leakier then the sodium channels, meaning the membrane is more permeable to potassium ions leaving than to sodium ions entering, which further helps to maintain this electrochemical gradient.
Na and K ions are both positively charged.
Fewer of these positive ions are now inside the neurone than outside.
So inside less positive, more negative, than the outside.
Difference in charge across the plasma membrane is ‘membrane potential’.
Resting membrane potential is -70mV.
Depolarisation.
Depolarisation = ‘loss of polarisation’
During resting potential the neurone membrane is polarised - the inside being more negative than the outside.
Facilitated diffusion of sodium ions into a neurone down their electrochemical gradient will depolarise the membrane.
Once depolarisation of a membrane has begun, voltage-gated sodium ion channels will propagate that depolarisation along the full length of an axon.
However to begin depolarisation in the first place, other types of sodium ion channels are required,:
(a) ligand-gated sodium ion channels on the post-synaptic membrane, which are opened by binding of a neurotransmitter
(b) sodium ion channels in a sensory neurone which are opened by specific stimuli, such as the stretch-mediated sodium ion channels in a Pacinian corpuscle.
Facilitated diffusion of positively charged sodium ions into the neurone raises the membrane potential above -70mV. This rising potential is called the generator potential. If the generator potential reaches -55mV (‘threshold’) an action potential is generated. If not, the sodium potassium pump will return the sodium ions to the outside and the resting potential will be established again.
Action potential.
If a generator potential exceeds the threshold potential (-55mV) an action potential is generated. Above -55mV voltage gated sodium ion channels open, and sodium ions rapidly diffuse in down their electrochemical gradient, taking the membrane potential to +40mV, At this voltage the voltage-gated sodium ion channels close and the voltage-gated potassium ion channels open, allowing potassium ions to diffuse out. This returns the membrane potential to negative (repolarisation), but with the ions in the opposite positions to the resting potential. The neurone is now in the refractory period, and no further action potential can be produced until the sodium potassium pump restores resting potential.
Repolarisation briefly takes the membrane potential even lower than resting potential: hyperpolarisation.
Action potentials are an example of positive feedback - the influx of sodium ions opens more voltage-gated sodium ion channels, leading to an influx of sodium ions.
The importance of the refractory period.
Action potentials are discrete (meaning separate), stopping them from merging into one another.
Action potentials can only travel ‘forward’ ie along membrane that has not just been depolarised.
There is a minimum time between action potentials occurring at any one place along a neurone, setting an upper limit on the frequency of impulse transmission.
Myelinated versus non-myelinated.
Myelination provides electrical insulation.
Depolarisation can only occur at nodes of Ranvier (which are unmyelinated)
So myelinated neurones impulses show saltatory conduction (jumps between nodes).
In non-myelinated neurones depolarisation occurs along entire length of the axon, so is slower.
Factors affecting the speed of conductance.
Myelinated axons show saltatory conduction.
Axons with a larger diameters have less resistance to the flow of ions.
Higher body temperatures mean faster diffusion of ions.
All or nothing principle.
Every generator potential that reaches threshold triggers an action potential, any generator potential that does not reach threshold fails.
The action potential is the same magnitude regardless of the size of the stimulus. Only the frequency of impulses can convey information about the size of a stimulus.
