Muscle physiology Flashcards
What are the subdivisions of a muscle
Muscle - fascicles (bundles of muscle fibres) - fibres/cell - Myofibril (cylindrical element) - sarcomere - (end to end chain of repeating units) - Myofilaments - both thin and thick filaments
What is the Z-band
The length of one sarcomere
What happens to the Z band during contraction
Decreases in length
What is anchored to the Z line
Actin
What is the I band
Region of non overlapped actin - distance decreases during contraction
What is the A band
Length of the myosin filaments - remains the same during contraction - appears as a dark band
What is the H zone
Region of non overlapped myosin filaments
Distance decreases during muscle contraction
What is the M line
Bare zone
What is the role of titin
Anchored to the Z line and attached to myosin
Positions the myosin halfway between the z discs
Acts like a spring and prevents over contraction
What is the role of nebulin
Dictates the length of the actin filament
+ end of actin capped to prevent depolymerisation
What is the role of CapZ and alpha actinin
Make up the Z disc that actin is anchored to
What is the role of Tropomodulin
Caps the actin filament -end
What is a motor unit
The motor neuron and the muscle fibres it innervates - one neuron will innervate several fibres
How is fine control of contraction achieved
Comes from motor units that only innervate a small number of muscle fibres
What is the benefit of having more motor units
More of a graded contraction - known as spatial summation
What type of receptor is found at the neuromuscular junction
nAchRs - ligand gated ion channels
What is the skeletal muscle action potential similar to
Same as neuronal action potential
Short duration
Very negative resting potential
What channels lead to fast activation of the AP
Sodium channels
What can be said about the K channels that are open at rest and the ones that open to mediate repolarisation
They are different channels - slow activation of the K channels open at rest
What is the structure of the nAchR
Pentameric structure 2 alpha subunits 1 beta 1 gamma 1 delta
How many transmembrane domains does it have
4
How many Ach molecules need to bind before the receptor opens
2 - binding sites lie between the alpha and gamma subunits
What is Myasthenia Gravis
Most common neuromuscular junction disorder
Inappropriate antibodies to AchRs on the postsynaptic membrane
Igs bind to channel proteins and stop Ach binding
Weakness, tiredness, fatal if respiratory failure occurs
What is the effect of acetylcholinesterase inhibitors for MG treatment
Enhances Ach levels in the synaptic cleft
Prolongs Ach availability - allows more time for binding to receptors that are still functioning
What is the effect of corticosteroids for MG
Immunosuppressant - reduction of antibody levels - cyclosporin - reduce number for Ach
What is the effect of Immunoglobulins for MG
Antibody binds to injected Igs rather than AchR
What is the effect of plasmapheresis on MG
Filtration of the plasma - quick removal of antibodies
What is the effect of thymectomy on MG
Removal of thymus gland - removes ability of patients to secrete antibodies
What is a triad used in postsynaptic calcium handling
Consists of sarcoplasmic reticulum cisternae on either side of a t-tubule
What is the importance of T-tubules in muscle fibres
Create invaginations in the membrane so the action potentials run deeper within the membrane
Calcium influx is therefore close to the myosin and actin
What is the role of v gated Ca channels
L type channels in clusters of 4 allow calcium influx from the extracellular fluid
Ltype are mechanically coupled to ryanodine receptors - opening causes opening of ryanodine which releases of calcium stores in the sarcoplasmic reticulum
Calcium coming in feeds back to open more ryanodine receptors - Ca induced Ca release
