Nerve impulses and Action Potential

what is a nerve impulse?

message that travels along a nerve fibre (neuron)

What is the speed of nerve impulses?

transmission speed can vary dependant on if the fibre is myelinated or unmyelinated. how are the diameter of a fibre is (larger diameter = faster transmission) but nerve impulses are transmitted very quickly

why are nerve impulses transmitted very quickly?

this is so that the body can respond rapidly to change in the external or internal environments

a nerve impulse is an….

electrical charge

why is a nerve impulse described as electrochemical?

this is because a nerve impulse is a change in electrical voltage. this is brought about by changes in chemicals (specifically concentrations of ions inside and outside cell membrane of neuron)

changes in ion concentrations lead to…

change in electrical voltage (action potentials) which travel as a wave long the axon

2 positive ions…

repel

2 negative ions…

repel

a positive and negative ion…

attract

what happens to an electrical force when opposite charges are separated?

an electrical force tends to pull them together. this force can be measured and the strength increases as charges get closer or larger

what happens when positive and negative charges come together (in regards to energy)?

energy is released

when a positive and negative charges are separated, they have….

the potential to come together and release energy. (this is caed potential difference)

how is potential difference measured?

in volts or millivolts

what is the conversion of volts to millivolts?

1 V = 1000mV

at resting, what ion is highly concentrated in the extracellular fluid?

highly concentrated sodium and chlorine ions.

what are ions?

ions are substances dissolved in fluid around and inside cells

where does membrane potential occur?

in all body cells, but is particularly large in nerve and muscle cells

what is the resting membrane potential of an unstimulated neuron?

-70mV

what does the resting membrane potential mean?

this means that potential inside the membrane is 70mV less than the outside.

How much higher is the concentration of sodium ions in the extracellular fluid and why?

there is 10 times more sodium ions than inside the neuron because the cell membrane is only slightly permeable to sodium, due to the limited leakage Chanels. therfore this limits the amount of facilitated diffusion of sodium ions

How is there potential in a neuron?

the potential from a neuron is caused by differences in concentration of ions, causing there to be potential between the inside and outside the the cell

why is the concentration of chlorine ions so much higher outside the neuron?

this is because the cell membrane is highly permeable to chloride ions, allowing their diffusion into protein channels

is the extracellular fluid more positive or negative?

positive

what ions is the intracellular fluid highly concentrated in?

main positive ion is potassium and there is many negative ions coming from a variety of organic substances made by the cell.

Is the intracellular fluid more positive or negitive and why?

the intracellular fluid is more negative because of the high concentration of negative ions, this is because the cell membrane is impermeable to these ions

why is potassium more highly concentrated in the intracellular fluid and by how much?

the concentration of potassium ions is about 30 times increases inside the neuron than outside. this is because the neuron is highly permeable to potassium due to the increased leakage channels. therefore more diffusion of potassium than sodium into channels

can the ions such as potassium, sodium and chlorine move through the phospolipid bilayer directly?

no, therefore they move through protein channels

what are leakage channels?

channels that are open at all times to allow movement of ions (such as sodium and potassium)

what is a voltage gated channel?

are transmembrane proteins that act as ‘gates’ for specific coins, opening and closing in response to binding chemical messages (therefore called ligand(is when a molecule binds to another molecule)

when are voltage gated channels open?

only when the nuron is stimulated

what are some other names for voltage gated channels?

ligand- gated or mechanical gated channels

what type of voltage gated channels are there?

gated potassium or sodium channels

what is the sodium potassium pump?

carrier protein that enables sodium and potassium to move across the cell membrane

what does a sodium potassium pump require?

An ATP molecule

what are the steps in using a sodium potassium pump?

three sodium ions will be moved into the sodium potassium pump, and and ATP molecule will bind. the sodium will be transported outside of the cell, and the ATP will be broken up so that there is one phosphorus still attached. this changes the shape of the sodium potassium pump so that two potassium molecules are able to bind and are transported into the cell. when the last phosphorus molecule leaves the shape of the sodium potassium pump changes so that 3 sodium ions can be taken back out of the cell

why does a sodium potassium pump require ATP?

because this movement of ions is against the concentration gradient.

what do the channels of the cell membrane look like at resting?

the gaped sodium and potassium channels are closed. the inside of the cell is relatively negatively charged because of the sodium potassium pump that always remains active (except during depolarisation)

Why does ATP split in a sodium potassium pump?

the splitting of ATP (with one phosphate still bonded with one ADP released) provides energy change the shape of the channel so that another ion can bind this change of shape drives the ions already inside the channel out also

why is the membrane said to be polarised?

because of electrical difference in charge across the membrane. with the inside of the cell being more negative than outside

what contributes to the resting membrane potential being -70mV?

the location of ions, the permeability of the cell membrane, the sodium potassium pump (that creates a negitive net flow of positive ions out of the cell (two potassium ions in with every three out)

what is an action potential?

is the rapid depolarisation and repolarisation of a cell.

why does action potential happen?

if a stimulus to a neuorn is suffiencent, a signal will be passed along the neuron. this happens due to the opening and closing of the voltage gated channels

when does depolarisation occur?

when a neuron becomes stimulated. therefore there is a sudden increase in membrane potential. this happens when the level of stimulation exceeds 15mV or -55mV.

what is the threshold?

the threshold is -55mV, and a stimulus must exceed this membrane potential for an action potential to occur. “all or nothing response”

what is the process of depolarisation?

a neuron is stimulatd by a neurotransmitter or sensory receptor, some sodium channels are opened and more sodium moves into the cell. this makes the intracellular fluid less negative and it will increase the potential difference. this produces a movement off sodium ions that proceeds independantly from the the stimulus. the membrane becomes more positive than the outside reaching approximately 40mV.

is there any movemnt of potassium out of the cell during this period (during depolarisation)?

yes, however the inward movements of sodium ions is too great to be balanced by potassium outward movements?

what is teh process of repolarisation?

when sodium channels close stopping the influx of sodium ions but at hte same time the voltage gated channels open and a flow of potassium ions out of the cell. overall making the cell more negative again, which decreases the membrane potential and membrane is now polarised. this repolarisation stops when membrane potential is back to -70mV.

What is hyper polarisation?

is when the potassium channels stay open for longer than needed. this results in membrane dropping below the resting membrane potential and causing the membrane to be hyperpolarised.

getting back to resting potential ?

happens when the gated potassium channels close. resting potential os regained by sodium potassium pump that works sodium out of the cel and potassium back in to retain the resting potential of -70mV

what is the refractory period?

happens once sodium channels have opened during depolarisation the cells become inactivated and will not respond to any more stimulus. and therfore for a breif period after being stimulated, the membrane will not undergo another potential.

when does the refactory period last?

lasts from -55mV to when the membrane potential reaches -70mV

an action potential occurs in one section of the membrane however that causes…

an action potential in the adjacent membrane, this processes continues along the length of the neuron and it is called a nerve impulse

does an action potential travel along a nerve fibre?

no, it is the message or a nerve impulse.

what is the process of conduction along unmyelnated fibres?

in an unmyeliated nerve fibre, depolarisation of one area of the membrane causes movement of sodium ions into adjacent areas. this movement stimulates the opening of voltage gated sodium channels in the next part of the membrane. this process repeats itself along the whole length of the membrane so that action potential moves along the membrane away from the point of stimulation. if the stimulus is occurring in the middle of the fibre (this would be an unusual circumstance however it would) cause impulses to travel in both directions along the fibre away from the point of stimulation.

how is a nerve impulse prevented from going backwards?

by the refractory period, during this period another action potential cannot be generated at the point of fibre. so nerve impulses are unable to move in that direction

what is the transmission pathway of a nerve impulse of a myelinated firbre?

in a myelinated fibre, the myelin sheath insulates the nerve fibre form extracellular fluid, this does not occur in the nodes of ranvier as the myelin sheath is absent. where a fibre is surrounded by myelin the ions cannot flow between the inside and outside of the membrane, therfore an action potential cannot form. instead an action potential jumps from one node of ranvier to the next. this allows much faster nerve impulses

why is there no action potential onmyelinated areas on a membrane?

ions cannot flow through he inside and outside of then fatty material of the myelin. therefore depolarisation can only occur in membranes without a myelins sheath.

what is it called when an action potential jumps from each node of ranvier?

‘jumping conduction’ or called saltatory conduction

does the size of the impulse vary?

a nerve impulse that travels along a nerve fibre is always the same size. regardless of the size of the stimulus. a weak stimulus ( providing it exceeds the threshold) will produce the same action potential as a strong one

what is an all or nothing response?

a stimulus is either strong enough to trigger and impulse or it is not

How can we feel more pain from one stimulus that another if the size of a nerve impulse is the same?

a song stimulus causes depolarisation of more nerve fibres than a weak impulse and a strong stimulus produces more nerve impulses in a given time than a weak stumble