Neuromuscular Structure and Function Flashcards
what 2 ions are responsible for changes in membrane potential?
- Na+
- K+
what 2 processes control resting membrane potential?
- diffusion
- active transport
what has the greatest influence on membrane potential
active transport of intra-cellular K+ and extracellular Na+
what is an anion?
a negative ion, usually set up in electrodes.
Overall, at rest, is the inside of the cell more negative or more postive compared to the outside?
more negative that the outside
what is the value of the resting membrane potential of the inside of a neurone cell in mV?
- -70mV
what does pumps 3Na+ into the cell and 2K+ out called?
Na+,K+, -ATPase
in all cells, what 2 things does Na+ and K+ being constantly pumped across the cell membrane maintain?
maintains:
- high Na+ concentration in the ECF and low Na+ concentration in the ICF
- high K+ concentration in the ICF and low K+ concentration in the ECF
what allows for constant diffusion of Na+ into the cells?
Na+ channels that are always open (leaky)
what allows for constant diffusion of K+ out of the cell?
K+ channels that are always open (leaky)
what is the relationship between pumping of Na+,K+ -ATPase and diffusion of Na+ and K+ when the cell is at rest?
at rest, pumping of Na+,K+ -ATPase exactly equals the diffusion of Na+ and K+
outline the ion fluxes that occur during an action potential in 10 steps
- Na+/K+ -ATPase actively maintains membrane potential of -70mV when no stimulus occurs
- IF stimulus causes AP TO RESCH THRESHOLD:
- stimulus causes some Na+ ion channels to open allowing some Na+ ions into the axon amnd membrane potential is less negative as Na+ diffuse into axon, around -55mV, but not enough to cause depolarisation
- IF ACTION POTENTIAL CAUSED BY STIMULUS REACHES THRESHOLD:
- ALL Na+ ion channels open, so there is a rapid influx of Na+ ions
- when membrane potential reaches +40mV, ALL Na+ channels close
- at the same membrane potential of +40mV, ALL K+ ion channels open
- there is rapid efflux of K+ ions, so electrical charge inside axon falls as K+ ions move out of axon, so axon membrane starts to be repolarised back to original -70mV
- K+ ion channels remain open until membrane potential reaches -80/-90mV, which is called hyperpolarisation
- this causes a refractory period where the axon membrane cannot be stimulated to propagate another action potential
- the Na+/K+ -ATPase now pumps 3Na+ in and 2K+ out, allowing the membrane potential to go from hyper-polarised back to resting potential of -70mV
what does the refractory period affect in the motor unit?
affects motor unit firing rate
how do you propagate an action potential in different directions, and why would we do this?
- propagate action potential in different directions through external electrical stimulation
- do this to investigate if there is nerve disorders and potentially where they are located
what is a node of Ranvier?
area of an axon with NO myelin AND HIGH density of Na+/K+ -ATPase
what is the advantage/need of a myelin sheath and how does it do this?
increases conduction velocity by acting as an electrical insulator at places that are not nodes of Ranvier
what is a disease that can cause a lot of demyelination?
multiple sclerosis
what is conduction velocity
the speed of AP propagation
what is it called when action potentials “jump” from Node of ranvier to node of ranvier?
saltatory conduction
in a sensory neurone, is the direction of Action potential Afferent or efferent and why?
Afferent as action potential travels from efferent (distal to centre) to afferent (close to centre)
in a motor neurone, is the direction of action potential Afferent or efferent and why?
in a motor neurone, direction of AP is efferent as action potential travels from afferent (close to centre), to efferent (away from centre)
what are 2 mechanism by which motoneuron activation occurs?
- Ionotropic mechanism (Na+ and K+)
- Neuromodulation
what is motoneuron activation the same as?
same as motor neurone recruitment
what is ionotropic mechanism?
when ions generates synaptic currents to initiate an action potential
what is neuromodulation?
the use of G-proteins in a 2nd messeger model to indirectly open ion channels (eg: Na+ or Ca2+)
what does neuronmodulation allow?
allows for lowering of lowering of activation threshold
what are the 2 most important neuromodulators in the motor system?
- serotonin (5-HT)
- Norepinephrine/Noradrenaline (NE)
outline how an action potential can travel from the pre-synaptic membrane to the post-synaptic membrane in 6 steps
- Action potential arrives at axon terminal and depolarise the presynaptic membrane
- this causes voltage-gated Ca2+ channels to open, resulting in an influx of Ca2+ ions
- the Ca2+ ions promote fusion of ACh vesicles to the presynaptic membrane
- release of ACh into the synaptic cleft occurs.
- ACh bind to and activate nACh receptors, leading to depolarisation of the muscle fibre membrane via net influx of Na+ ions
- this propagates an action potential into the post-synaptic membrane
how many ACh molecules are required to activate nACH receptors?
2 molecules are required
what does the release of a single vesicle of ACh result in compared to when there is release of several ACh vesicles?
- release of single vesicle: results in miniature end-plate potential
- release of several vesicles: results in full end-plate potential
what proteins are found on the nicotinic ACh receptor?
- a (alpha)
- B (beta)
- 𝛄 (gamma)
outline how Ca2+ is released in the sarcoplasmic reticulum in 3 steps
- action potential propagates down sarcolemma
- transverse tubules (T-tubules) conduct Action potential into the cell’s interior
- this depolarises the sarcoplasmic reticulum, causing it to open Ca2+ release channels
what are t-tubules and where are they situated?
deep invaginations of the sarcolemma situated to the junction of the A-bands and I-bands
what is the 2 function of T-tubules?
- to provide a mechanism for changes in membrane potential to be communicated to the inners of the muscle fibre and
- increase surface area for action potential
in striated muscle, how are the t-tubules positioned compared to the sarcoplasmic reticulum and what does this mean?
- situated in close apposition with SR
- this means that when the membrane on the t-tubule is depolarised, it also triggers Ca2+ release from the SR
- this therefore means that t-tubules and SR can work together to allow Ca2+ concentration to rise in the sarcoplasm in the area it is needed
