Membrane Potential and Neuromuscular Junction

resting membrane potential

voltage difference across membrane at rest typically about -70 mV with inside negative relative to outside

why resting membrane potential is negative

membrane is more permeable to K+ so K+ leaks out leaving negative charge behind plus non diffusable intracellular anions remain

role of Na+/K+ ATPase in RMP

pumps Na+ out and K+ in against gradients maintaining ionic differences and contributing to negativity

three forces acting on membrane

diffusion electrical gradient and active transport determine ion movement and membrane potential

action potential definition

sudden reversal of membrane polarity due to stimulus causing rapid depolarization then repolarization

graded potential definition

localized change in membrane potential confined to a small region and varies with stimulus strength

difference between action potential and graded potential

action potential is all or none and propagates graded potential is local and varies with stimulus

threshold in action potential

level of depolarization required to open many voltage gated Na+ channels and trigger full action potential

depolarization mechanism

opening of voltage gated Na+ channels causing rapid Na+ influx making inside positive

repolarization mechanism

inactivation of Na+ channels and opening of K+ channels causing K+ efflux restoring negativity

recovery phase of action potential

closure of excess K+ channels and return to resting permeability restoring resting membrane potential

peak membrane potential in action potential

approximately +35 mV due to Na+ influx

types of ion channels

leakage voltage gated ligand gated and mechanically gated

ligand gated channels example

acetylcholine receptor at neuromuscular junction

neuromuscular junction definition

synapse between motor neuron terminal and muscle fiber for transmission of action potential

components of NMJ

presynaptic terminal synaptic cleft postsynaptic motor end plate with nicotinic receptors

motor unit definition

one motor neuron and all muscle fibers it innervates

neurotransmitter at NMJ

acetylcholine

ACh synthesis

produced in presynaptic terminal using choline acetyl transferase

ACh release trigger

arrival of action potential causes Ca2+ influx leading to vesicle exocytosis

role of Ca2+ in NMJ

initiates fusion of synaptic vesicles and release of acetylcholine

postsynaptic event at NMJ

ACh binds nicotinic receptor opening Na+ channels causing Na+ influx

end plate potential definition

graded depolarization of muscle membrane due to Na+ influx after ACh binding

how action potential is generated in muscle

end plate potential reaches threshold then voltage gated Na+ channels open generating action potential

termination of ACh action

acetylcholinesterase degrades ACh and choline is recycled

excitation contraction coupling

process where muscle fiber depolarization leads to contraction

function of somatic nervous system at NMJ

controls voluntary muscle contraction via motor neurons

myasthenia gravis mechanism

autoantibodies destroy acetylcholine receptors reducing neuromuscular transmission causing weakness

effect of botulinum toxin

blocks acetylcholine release by inhibiting vesicle exocytosis

effect of hemicholinium

blocks choline uptake preventing acetylcholine synthesis

effect of vesamicol

blocks acetylcholine storage in vesicles preventing release

key ion responsible for resting membrane potential changes

K+ due to high membrane permeability

sequence of NMJ transmission

action potential arrives Ca2+ influx acetylcholine release receptor binding Na+ influx end plate potential muscle action potential

importance of Na+ influx at NMJ

critical step that depolarizes membrane to trigger action potential

excitable tissue definition

tissues like nerve and muscle that respond to stimuli by electrical changes

muscle response to excitation

contraction

nerve response to excitation

transmission and integration of signals

mechanically gated channel definition

channels that open in response to physical deformation such as stretch

motor end plate

functional region of muscle membrane rich in acetylcholine receptors

key step generating action potential at NMJ

Na+ influx through ligand gated channels

effect of blocking Na+ channels

prevents depolarization and action potential generation

mechanism of local anaesthetics

block voltage gated Na+ channels preventing nerve impulse transmission

effect of hyperkalaemia on membrane

increases membrane excitability by reducing resting potential difference

reason K+ changes strongly affect membrane potential

membrane is highly permeable to K+ at rest