The Nerve Impulse Flashcards
what is the resting potential?
-the potential difference that neurones have across their cell surface membrane
why is there a resting potential what causes this?
-the neurones are polarised due to the electrochemical gradient across the membrane
what does polarised mean?
-it is when the outside of the neurone is more positive relative to the inside
how are neurones polarised during the resting potential?
-this potential difference is because of there being an excess of positively charged sodium ions outside the membrane
how many millivolts is the polarised potential difference in neurones during the resting potential? (the outside is more positive to inside)
-70mV
what is one adaptation of the cell surface membrane of neurones to ensure the outside is more positive to the inside during the resting potential?
-the cell surface membrane is largely impermeable to the flow of sodium ions when not conduction an impulse
when a neurone is stimulated what happens to the cell surface membrane and what does this cause to happen?
-the membrane becomes permeable to sodium ions
-because the outside is more positive to the inside the ions move down the concentration gradient and into the neurone
how does depolarisation of the neurone occur during the action potential and what does this cause to open?
-as the membrane becomes permeable to the ions the potential difference across the membrane decreases
-when a threshold is reached a number of gated protein channels open which rapidly causes more ions to enter which leads to this depolaristation
what does the action potential result in?
-the inside of the neurone being more positive to the outside
at the peak of the action potential what is the membrane potential in millivolts? why?
+40 mV
-because the inside is more positive compared to the outside
what is the name of the phase that begins immediately after a peak of action potential is reached?
-the recovery phase
what happens during the recovery phase?
-positive ions both diffuse and are pumped out of the neurone which rapidly restores the resting potential
what happens to the neurone membrane during the recovery phase?
-the membrane becomes largely impermeable to sodium ions again
why can another impulse not occur during the recovery phase?
-gated ion channels are closed
-resting potential has not been fully restored
what is another name for the recovery phase?
-refractory period
what is hyperpolarisation?
-at the end of the recovery phase/ refractory period there is a slight overshoot meaning the membrane becomes slightly more negative than normal resting potential
what are the 2 functions of the refractory period?
-ensures nerves are propagated in one direction (unidirectional) which is important as axons can transmit impulses in both directions
-limits the number of action potentials to prevent overstimulation of nerves
what are two important principles of the refractory period?
-the threshold stimulus
-the all of nothing law
what does the threshold stimulus refer too?
- the level of stimulation a neurone requires before an action potential is reached e.g. a critical point of depolarisation must be reached
what does the all or nothing law refer to?
-when the threshold stimulus is reached the action potential either occur or it doesn’t
how do nerve impulses move along neurones?
-a wave of depolarisation moves rapidly along the neurone
-immediately behind this depolarised zone the neurone becomes repolarised
what is the purpose of the local circuits being set up in the neurone?
-when one part of the membrane becomes depolarised local circuits are set up with the area immediately adjacent to the depolarised zone
how does nerve propagation actually occur?
-positive ions from the depolarised zone pass along directly infront to the polarised zone
-immediately before the depolarised zone the region behind this become repolarised
-outside the membrane (where it is more positive to the inside) the ions move into the depolarised zone
-causing a wave of depolarisation which causes the impulse so move along the nerve fibers
what are the 2 factors that influence the speed of a nerve impulse?
-the presence/ absence of myelin sheath
-thickness of axon
what does the myelin sheath act as and what does it prevent?
-acts as an electrical insulator
-prevents depolarisation
what is saltatory conduction and why does the myelin sheath cause this? does this speed up or slow down impulse propagation?
-every 1-2mm the sheath is disrupted by gaps called nodes of Ranvier
-at these gaps depolarisation can occur
-this means that the local circuits can only form at the nodes which causes sections of the neurone to be bypassed
-so the impulses jump from gap to gap which causes the speed to incease?
in what way does the diameter affect the speed of an impulse? why is this?
-the thicker the axon the faster the impulse
-less leakage will occur of ions due to the thicker axon
why is leakage of ions due to a thin axon a problem?
-more leakage will occur
-which makes it more difficult to maintain the potential gradients needed for resting and action potentials to form
how does temperature affect the speed of an impulse?
-temperature affects the rate of diffusion of ions involved and so affects the speed of impulse propagation
-high temp = faster impulses
