Muscle Physiology Flashcards
Approximately how many nerve fibers innervate each muscle fiber making up a muscle?
- usually it is just one!
- (about 2% have more than one nerve fibers)
What is the cell membrane of the muscle fiber called?
- the sarcolemma
Explain the structure of a muscle fiber
- each fiber contains hundreds to thousands of myofibrils
- each myofibril contains about 3000 thin atin filaments and 1500 thick myosin filaments
What makes up the light bands on imaging? The dark bands?
- light bands: AKA I bands; these contain only actin
- dark bands: AKA A bands; these contain actin and myosin
What is a sarcomere?
- the functional contractile unit of myofibrils
- 1 sarcomere is the area between two successive I bands (technically between two Z-discs, but I bands contain the Z-discs)
- sarcomeres make up the visible striations on each myofibril
What fluid lies between myofibrils? What is is rich in?
- the sarcoplasm fills the spaces between myofibrils
- it is rich in potassium, magnesium, and phosphate, and contains a massive number of mitochondria
- (note that this is the same fluid present in the sarcoplasmic reticulum)
Explain the general pathway of muscle contraction.
- an action potential travels along the motor nerve to the muscle fiber (ACh is released at the NMJ)
- ACh binds to post-synaptic receptors (ACh-gated ion channels), causing large amounts of Na+ inflow to generate an action potential in the muscle fiber
- this depolarization causes the sarcoplasmic reticulum to release large amounts of Ca2+, which results in contraction
- (Ca2+ is then pumped back into the SR within a fraction of a second!)
Which filaments move during contraction (ie which band(s) will shorten during contraction)?
- actin filaments move during contraction (they slide/pull along the myosin filaments)
- this means that during contraction, the I bands (those containing only actin) shorten, pulling the Z-discs closer together, resulting in a shortened sarcomere (contraction)
- A bands do NOT change length; H-zones will increase in length
Explain the structure of a myosin filament.
(note that muscle contraction involves type II myosin; type I myosin are involved in microvili and vesicular transport)
- each myosin filament is made up of about 200 myosin molecules
- each myosin molecule contains 2 heavy chains in a double helix formation and 4 light chains
- the 2 heads of each myosin molecule have intrinsic ATPase (ATP hydrolysis) activity
Explain the structure of an actin filament
- each actin filament is composed of actin molecules, tropomyosin, and troponin (troponin C isotype)
- actin molecules have 2 heavy chains in a double helix formation, and each contains several binding sites for ADP
- tropomyosin is wrapped around the the actin filament and covers the ADP binding sites during rest
- troponin complexes are attached to the tropomyosin and have a high affinity for Ca2+
What is the role of tropomyosin and troponin in muscle contraction?
- these structures inhibit contraction at rest because tropomyosin is covering the actin’s active sites involved in contraction
- in the presence of Ca2+, however, troponin undergoes a change and moves tropomyosin off of the active sites, allowing their interaction with myosin to result in contraction
Explain the interaction between actin and myosin during contraction.
- 1) at rest, myosin cross-bridges are bound to ATP and myosin’s ATPase activity generates ADP and Pi (myosin is “cocked”)
- 2) Ca2+ allows actin’s active site to be revealed, and myosin cross-bridges immediately bind
- 3) ADP an Pi are released, resulting in the myosin’s “power stroke” to cause contraction
- 4) a new ATP molecule binds to myosin, resulting in its release of actin and return to resting state
Explain the different conformations of the myosin head when bound to different products.
- bound to ATP: resting, not attached to actin
- bound to ADP and Pi: head is extended towards the actin filament (“cocked”) and will bind as soon as the actin’s active sites are opened
- nothing: myosin pulls the actin, resulting in contraction (“power stroke”)
- bound to ATP: releases actin, returns to rest
What is rigor mortis? Why does it occur?
- rigor mortis is a condition of intense muscle contraction seen in recently deceased patients
- it is caused by the lack of ATP needed to release the myosin heads from the actin filaments (relaxation can’t occur without the next binding of ATP)
What rephosphorylates the released ADP back into ATP during prolonged contraction?
- (note that the ATP reserves are VERY small)
- anaerobic 1st responders are phosphocreatine (degraded by creatine kinase to transfer phosphate to ADP) and glycogen (converted to pyruvate and made into lactate via LDH to regenerate NAD+ needed for more glycogen breakdown into pyruvate) (type II slow fibers main this)
- glycolysis and glycogenolysis (and oxidative phosphorylation) are then used; aerobic (type I slow fibers main this)
