Neuromuscular junctions Flashcards
What is a neuromuscular junction?
- the point where a motor neurone meets skeletal muscle fibre
Compare a neuromuscular junction and a chollenergic synapse?
- have neurotransmitters transported by diffusion
- have receptors that bind to acetylcholine, leading to the influx of NA+ ions
- use a NA+/K+ pump to repolarise the membrane
- use acetylcholinesterase to breakdown acetylcholine
contrast a neuromuscular junction and a chollnergic synapse?
neuromuscular = only excitatory, only link neurones to muscles, only uses motor neurones, action potential ends there, acetylcholine binds to receptors in the sarcolemma
cholinergic = both excitatory and inhibitory, links neurones to neurones or neurones to other effectors, motor sensory and intermediate neurones may be involved, a new action potential may be produced, acetylcholine binds to receptors on the postsynpatic neurone
What is the process in which skeletal muscles contract?
- the sliding filament mechanism
What occurs during the sliding filament mechanism?
- I bands become narrower, Z lines get more closer together which shortens the sarcomeere
- the H zone gets narrower
- The A band staus the same width which shows that muscle contractions are due to the overlap of actin and myosin
Describe the process of muscle stimulation.
- An action potential reaches a neuromuscular junction and causes the opening of calcium ion protein channels
- calcium ions then diffuse into the presynaptic knob of the motor neurone
- causes the synaptic vescicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
- acetylcholine diffuses across the synaptic cleft and binds to receptors on the sarcolemma, causing the influx of Na+ ions and the depolarisation of the sarcolemma
Describe the process of muscle contraction (icl brace yourself)
- the action potential travels deep into the fibre through a system of t- tubules which are extensions of the sarcolemma and branch throughout the sarcoplasm
-the t - tubules are in contact with the sarcoplasmic recticulum which has actively transported calcium ions from the sarcoplasm, leading to a calcium ion concentration gradient - the action potential opens the calcium ion protein channels on the sarcoplasmic recticulum and calcium ions diffuse into the sarcoplasm down a concentration gradient
- the calcium ions bind to receptors on tropomyosin molecules, causing the tropomysin molecules that were blocking the binding sites on actin to pull away
- Now that the actin binding site is exposed, the myosin head that has ADP molecules attached can form a crossbridge and bind to the actin filament
- once attached to the actin filament, the myosin head changes shape, pulling the actin filament along and releasing a molecule of ADP, this is the power stroke
- An ATP molecule then binds to the myosin head, causing the crossbridge to break and detaching mysoin from the actin filament
- the calcium ions then active ATPase which hydrolyses ATP to ADP. The hydrolysis of ATP to ADP causes the myosin head to move to its original position ( recovery stroke)
- the cycle then repeats
How do the actin filaments move in opposing directions?
- the myosin molecules are joined tail to tail in opposite facing sets, this means the actin filaments that they bind to move in opposite directions
- this pulls actin in opposite directions, shortening the distance between the two z lines
What occurs during muscle relaxation?
- when nervous stimulation stops, calcium ions are actively transported back into the sarcoplasmic recticulum uisng energy from the hydrolysis of ATP
- this prevents the binding of myosin as tropomyosin is covering actin, therefore ceasing contraction
What are the three uses of ATP
- the recovery stroke of the mysoin head
- breaking the crossbridge between actin and myosin
- the reabsorption of calcium ions into the sarcoplasmic recticulum
How can ATP be generated?
- glycolysis generates ATP
- phosphocreatine = stored in muscle, source of phosphate
