Exam 2 Ch. 9 Study Guide Questions Flashcards
Define contractility and elasticity.
Contractility - forceful muscle shortening
Elasticity - muscle recoils to resting length after stretched
State 3 functions for skeletal muscle.
Body movement, maintain posture, shivering to increase body temperature
What is a fascicle? What covers it?
Fascicle - bundle of muscle cells (aka muscle fibers) covered by connective tissue perimysium
Describe in detail the structure and function of the sarcomere and its subunits within a myofibril.
Sarcomere are repeating (in series) units in a myofibril.
Z discs/lines form outer boundary where actin attached.
Myosin anchored in center.
Myosin and actin myofilaments form crossbridges and pull Z discs closer together which shortens sarcomere.
Actin wrapped with tropomyosin and troponin subunits which inhibit actin/myosin crossbridges in the absence of calcium.
What is the mechanism by which a motor neuron stimulates a muscle fiber at the synapse (start with the action potential and end with sarcolemma excitation)? What happens in botulism?
How about snake venom bites?
Action potential is formed by local changes in Na and K permeability. Na channels open and allow Na to travel down concentration gradient into cell which causes depolarization such that membrane potential of -85 mV peaks at +20 mV. Now Na channels close and K channels open which allow K to travel down concentration gradient out of cell repolarizing membrane potential back to -85 mV. Each patch of membrane fires like dominoes falling (one patch activating next patch) down the length of neuron towards synapse (where neuron meets muscle cell). Na/K ATPase pumps return ions back to original location (active transport).
Once the terminal end of neuron is reached, the AP stimulates synaptic knob (presynapse) voltage-regulated Ca channels to open which allows Ca to travel down concentration gradient into synaptic knob. This activates the Golgi Apparatus to release NT acetylcholine via exocytosis into the synaptic cleft. ACH diffuses to sarcolemma/motor end plate to bind receptors opening Na channels which travels into motor end plate starting new local action potential (aka motor end plate potential or EPS - excitatory postsynaptic potential).
Botulism - ACH is not released by presynapse of motor neuron so process stops and muscle cannot respond, possibly fatal if diaphragm (breathing muscle).
Snake Venom - competes with ACH to bind receptors and will not open Na channels so no EPSP
Explain the process of excitation/contraction coupling from sarcolemma to contraction.
With 53 completed, sarcolemma EPS spreads to sarcolemma if strong enough will develop full AP. AP travels along surface of sarcolemma then enters transverse (T) tubule (down the rabbit hole) and activates voltage-gated calcium channels located within the sarcoplasmic reticulum (SR) where Ca is stored. Channels open and Ca exits SR and fills sarcoplasm. Ca binds receptors on inhibitory protein troponin which changes shape and moves tropomyosin inhibitory protein slightly. This exposes myosin
ratchet binding site and myosin forms crossbridges with actin = shortening = contraction.
Explain in detail the ratchet or sliding filament theory of muscle contraction identifying the actin/myosin crossbridge formation. What are the roles for ATP in 53, 54 and 55?
Crossbridge formation (end of 54). Energy stored in myosin head fuels powerstroke, old ADP and Pi released. New ATP attaches and myosin releases from actin. ATP hydrolyzed into ADP and Pi to cause myosin ratchet/head to release and reset. Repositions to next site, crossbridge formed. Cycle continues.
How is strength of contraction varied? What are the fixed periods of time in a twitch contraction? Can you recognize these periods on a timeline (graph)?
Strength of contraction - increase number of motor units (motor neuron and muscle cells it serves) recruited
Latent/lag period/phase - delay due to steps in excitation/ contraction coupling prior to crossbridge formation
Period of contraction - crossbridges increase to increase force
Period of relaxation - Ca storage via Ca pumps into SR is the rate limiting factor, then reduction of crossbridges
What is the length-tension relationship for skeletal muscle fibers? Why?
Preset length of muscle allows for max crossbridge overlap and max tension/force.
Preset too short - too much actin/myosin overlap “block themselves” to reduce crossbridges
Preset too long - overstretched reduces actin/ myosin overlap to reduce crossbridges
Compare slow (type I) vs fast twitch (type 2 or lI) fibers. Which fatigues faster? Which has more power?
Slow oxidative-Slow twitch (type I) - better blood supply, more 02 delivery, more myoglobin to bind 02 and more mitochondria so make more ATP (aerobic respiration); less fatigue, high endurance
Fast glycolytic-Fast twitch (type Il) - less blood supply, less 02 delivery, fewer myoglobin and mitochondria so make less ATP (use stored glycogen for glycolysis); fatigue faster so low endurance, bursts of power
