Skeletal muscle physiology week 4 Flashcards
True or false: Muscle cells convert chemical energy in the form of ATP into mechanical energy-contraction and heat.
True.
The globular heads of the myosin molecules (with ATP and actin binding sites) are called ___ ___.
cross bridges
Describe what occurs during skeletal muscle contraction.
- When myosin is in a relaxed state (cocked), it is energized with a bound ADP and Pi. The myosin cross bridges are engergized and ready to bind to actin. When Ca2+ released from the sarcoplasmic reticulum binds to TnC, tropomyosin flexes off of actin exposing the myosin binding sites.
- Myosin binds to actin (cross bridge state), releases Pi, and the powerstroke occurs, bringing actin filament inward. ADP is released after the powerstroke.
- ATP (with Mg2+) comes in and binds to the myosin head and myosin is released from actin. Myosin is again in the cocked state.
Each time contraction occurs, myosin heads bind to an even more distal actin molecule, causing muscle shortening (H and I bands shorten therefore shortening the whole sarcomere, Z lines come closer together). Note that if there is no ATP available to bind to the myosin cross bridges, they remain bound to actin, referred to as a rigor complex.
please see slide 8 of notes
Describe what is involved in muscle relaxation as it pertains to Ca2+.
After an AP causes Ca2+ release channels in the SR to open, they will (provided a second AP does not arrive) close within ~10 ms. The SR membrane contains a high density of Ca2+ ATPase pumps that use ATP to pump Ca2+ from the cytosol back into the SR. The pump is stimulated by the increased cytosolic [Ca2+]. Once the SR Ca2+ release channel is closed, cytoplasmic Ca2+ concentration falls to its low resting value within 50-100 ms. Additionally, parvalbumin is involved in muscle relaxation through its ability to rapidly bind Ca2+ which is shortly returned to the SR. Parvalbumin is considerably more abundant in fast (white) fibers and contributes to their more rapid relaxation. In total, the duration of tension in skeletal muscle resulting from a single AP is typically about 50-100 ms. The duration depends on the the type of muscle fiber. Note that this is much longer than the 1-2 ms duration of the AP that stimulated contraction in the first place.
What is malignant hyperthermia (anesthetic hyperthermia)?
the SR Ca2+ release channel in skeletal muscle becomes stuck open after an AP
Where do the products of acetylcholinesterase go after its action?
Into the presynaptic neuron for synthesis of more ACh
Discuss the effects of black widow spider venom on the NMJ.
causes explosive release of ACh
Discuss the effects of Clostridium botulinum toxin
on the NMJ. What is this used for clinically?
blocks ACh release. Used in Botox
Discuss the effects of Lambert-Eaton myasthenic syndrome on the NMJ. What is one of the sx?
Auto-immune disease where antibodies against pre-synaptic Ca2+ channels are produced, resulting in diminshedCa2+ influx into the pre-synaptic terminal. This results in reduced ACh release. sx include muscle weakness.
Discuss the effects of curare on the NMJ.
curare competively (reversibly) binds ACh receptors
Discuss the effects of Myasthenia gravis on the NMJ. What is one of the sx?
Auto-immune disease where antibodies are produced against ACh receptors causing their endocytosis and destruction. Results in varying degrees of muscle weakness
Discuss the effects of organophosphates (certain pesticides, military nerve gases) on the NMJ.
irreversibly inhibit acetylcholinesterase
Discuss the effects of neostigmine on the NMJ.
temporarily inhibits acetylcholinesterase
True or false: At the junction btwn the A and I band (twice for each sarcomere) the sarcolemma invaginates ino the cell fto form a mesh of tubules that run perpendicular to the long axis of the fiber called T-tubules. These planar meshes of tubules run twn myofibrils and penetrate to the center of the muscle fiber. Because T-tubules are continuous with the surface membrane, an AP propagating along the surface of the fiber will also propagate into the T-tubule and thus to the center of the fiber.
True.
The process by which electrical excitation of the muscle fiber leads to contraction of the muscle fiber is known as what?
excitation-contraction coupling (EC) coupling