Unit 1 - Muscles Flashcards
what is excitation- contraction coupling?
process by which a muscle action potential triggers a contraction
how is an endplate potential generated?
- AP in motor axon depolarises nerve terminal opening voltage-gated calcium channels
- Influx of Ca2+ ions triggers fusion of synaptic vesicles with membrane - releasing acetylcholine into synaptic cleft
- Acetylcholine binds to the nicotinic receptors causing an endplate potential due to opening of non-selective cation channels
- When epp reaches threshold, sarcolemma generates AP propagates the entire length of muscle fibre
what is another term for end plate and what type of transmission occurs at this point?
neuromuscular junction
- neuromuscular transmission -fast synaptic signally from motor nerve to skeletal muscle
why can miniature endplate potentials be observed when recording?
small spontaneous depolarisations reflecting release of acetylcholine from vesicles
what term describes the type of contraction skeletal muscle produces?
neurogenic - originates from CNS
what theory is used to describe muscle contraction and explain it
sliding filament theory:
- AP spreads along T-tubules causing voltage-gated Ca2+ channels on sarcoplasmic reticulum to open
Ca2+ ions released into sarcoplasm and binds to troponin - causing conformational change which changes its position so displaces tropomyosin exposing myosin head binding site
Myosin head binds to site and dissociates when ATP binds to myosin head
Hydrolysis of ATP causes change in angle of myosin head so it moves to relative to actin - generating force on tendons to which it is attached (power stroke)
This then repeats so cross-bridges break and reform= cross-bridge cycling
what does the amount of force exerted by skeletal muscle depend on?
- cross-sectional area
- number of active fibres
- frequency of stimulation
what is:
- a muscle twitch
- twitch tension
- maximal stimulus?
muscle twitch= response of a muscle to a single stimulus
t-tension= force developed from stimulus
maximal = stimulus that activates all muscle fibres
what is:
- summation
- tetanus/fused tetanus?
summation = second stimulus arrives before a muscle has fully relaxed, the total tension increases compared to just a single stimulus
- If in quick succession, tension summates progressively = tetanus
- fused tetanus - tension develops smoothly (greater tension - more Ca2+ ions bind to troponin)
what are the properties of type 1 muscle fibres?
- contract/relax slowly
- Thin
- Rich in mitochondria/ oxygen-binding myoglobin
- Rely on oxidative metabolism of fats for energy supply
- resistant to fatigue due to continuous blood supply
what are the two types of type 2 fibres and give their properties
type 2a: fast oxidative-glycolytic fibres
Higher myosin ATPase activity than type 1
thin/rich in mitochondria and myoglobin/ good blood supply - resistant to fatigue
type 2b: fast glycolytic fibres
-large diameter
- develop tension rapidly because high levels of myosin ATPase activity, glycogen and glycolytic enzymes
- Easily fatigued - limited blood supply/ few mitochondria/ myoglobin - pale appearance
what is difference between isometric and isotonic contraction?
- isometric - load prevents shortening
- isotonic - shortens against constant load
why is cardiac tissue described as a functional syncytium?
The pacemaker cells of the SA node sets the heart rate by spreading the AP across the whole of the heart since myocytes are tightly bound
what is pacemaker potential? how does it work?
- slow depolarisation that precedes AP- slow activation of sodium current when membrane is repolarised
- It opposes K+ current so membrane potential becomes progressively less negative- activates calcium current which accelerates rate of depolarisation until AP generated (Ca2+ ions -produce reversal of charge)
- Repolarisation occurs when calcium permeability falls/ K+ permeability increases so membrane hyperpolarised activating another pacemaker potential (cycle repeated)
why can’t cardiac muscle be tetanised?
cardiac AP have long duration between 150-300 ms - therefore mechanical response occurs when depolarised - allows to relax fully
how does EC coupling occur in cardiac muscle?
- AP causes L-type voltage-gated calcium channels to open
- Ca2+ influx activates ryanodine receptors (calcium release channels) found on sarcoplasmic reticulum
- rapid release of Ca+ ions( = Ca2+ induced Ca2+ release)
- leads to contraction (same mechanism as skeletal)