Action Potential and Nervous Impulse Flashcards
Define action potential
The change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell
What is the threshold?
to generate an AP an axon requires a stimulus of a certain minimum strength raising the MP to a certain level = the threshold
What is an axon hillock?
Last part of the nerve cell body, connects to axon
needs to depolarise in order for the entire axon to depolarise
What are the stages of an AP?
- resting membrane potential.
- depolarising stimulus.
- voltage-gates Na+ channels open, Na+ enter cell, voltage gated K+ channels being to open slowly. Membrane depolarises to threshold.
- rapid Na+ entry depolarises cell.
- once Na+ channels are open they are prone to inactivate (close). Na+ channels close and slower K+ channels open.
- repolarisation: K+ moves from cell to ECF.
- K+ channels remain open and additional K+ leaves cell, hyperpolarising it.
- voltage gated K+ channels close, some K+ enters cell through leak channels.
- cell returns to resting ion permeability and resting membrane potential
How can membrane current be measured?
Voltage-clamp = enables membrane currents to be measured at set MP. Decide on MP, measure current by measuring movement of different ions
Describe hyperpolarisation
more K channels open than at RMP, recovery = slowly K channel close
What is the refractory period?
aka hyperpolarisation. Refractory period – second action potential cannot occur here.
ARP = absolute refractory period – no matter how much stimulus no AP.
RRP = relative refractory period – strong stimulus should be able to initiate AP
Outline recovery
when Na open they are prone to close = inactivation – need to recover before they can open again – this is achieved by hyperpolarisation
What is the structure of a voltage gated Na+ channel?
1 peptide, 4 repeats. 1 α subunit. Pore region dictates what ion flows through the channel. 4th membrane spanning domain (voltage sensor) has a lot of AA+ve = creates voltage field – if you change the MP you change the voltage field = conformational change = pore open – flow of ions = channel susceptible to inactivation
What is a particle plug?
once the pore is open inactivation particle can enter and block = membrane has to hyperpolarise for particle to be removed
Describe local current theory
when Na+ flow in, +ve charges repel any other +ve charges and attract –ve charge = set up local current = injection of current = charge spread along axon = once you get depolarisation you get passive depolarisation of adjacent cell by pushing it to threshold
What is capacitance?
ability to store charge, rises with size, larger tissue can hold more electrons = more charge
What is myelination?
Axon through middle. Folding of schwann cell. Gaps = node of ranvier = high conc of Na+ channels found. Without myelin depolarisation spread wouldn’t go far enough to initiate next depolarisation.
What is saltatory conduction?
AP jumps node to node = much faster conduction velocity. Myelin good insulator – causing local circuit current to depolarise next node above threshold
Describe demyelination
AP arrives at damaged myelin – poor working local circuit = no depolarised to threshold = no AP e.g. multiple sclerosis (CNS) Landry-Guillain-Barre syndrome (peripheral NS)
What is action potential membrane resistance?
amount of ion channels open
lower resistance = more open = higher loss of local current = limiting the spread of local current effect
In terms of resistance and capacitance, what does the myelin sheath aim to do?
increase resistance = less channels open = lower loss of local current = increasing spread of local current effect
decrease capacitance = lower storage of charge = shorter to charge up = faster spread of local current
What are nodes of ranvier?
Folding of schwann cell myelinate an axon, the gaps = node of ranvier = high conc of Na+ channels found
What allows for an AP to be propagated?
local currents
Give an example of a disorder affecting the CNS
multiple sclerosis
all CNS nerves, demyelinating
Give an example of a disorder affecting the PNS
Landry-Guillain-Barre syndrome
autoimmune disorder affecting PNS = rapid-onset muscle weakness caused by the immune system damaging the PNS
What is conduction velocity?
propagation speed of impulse: larger diameter = higher conduction velocity
How many ions need to flow to generate an AP?
very little due to large diff in ion conc
What cells myelinate in the CNS?
oligodendrocyte
What cells myelinate in the PNS?
schwann cells
