L10 Skeletal Muscle Flashcards
Skeletal muscle
Storage and voluntary
Innervation: somatic
Movement Metabolic regulation: think diabetes Heat production: think shivering Adaptable to diff loading conditions Many clinical conditions involved Changes in muscle have important implications for physical function
Muscle fibers
Large, multinucleated
Composed of bundles of myofibrils
Myofibrils
Consists of a bundle of parallel myofilaments
Myofilaments
Organized into contractile units called sarcomeres (smallest functional unit of skeletal muscle)
Two types of myofilaments
Myosin and action
Myofibrils banding patterns
Dark band- myosin
Light band - actin
A band
Thick filaments (myosin)
Thin filaments also present in parts of A band
I band
Thin filaments plus Z line
H band
Centered around M line
Contains myosin plus M line proteins
Z line
Anchors thin filaments
Defines the sarcomeres
M line
Supports and organize the myosin filaments
Composed of cytoskeletal proteins
Each fiber innervated by
One neuron at one location
One neuron can innervate
Many fibers ( motor unit)
Motor unit can be different sizes
Excitation-contraction coupling
- Action potential at NMJ
- Ca release
- Myosin and action interaction
- Muscle contraction (cross-bridge cycling)
- Ca removal
- Relaxation
- Action ptoential at NMJ
Action potential from motor neuron results in release of ACh
Opening of ACh receptors results in EPP
EPP depolarization the motor end-plate and intimates action potentials in the muscle sarcolemma
- Calcium release
Action potential travels through the entire sarcolemma and into the transverse (T) tubules (toward where Ca stores- sarcoplasmic reticulum)
T-tubules make contact with terminal cisternae of SR forming triads (SR, T, SR)
Muscles can contract
Steps in Ca release from SR
T-tubules contain slow activating voltage gated Ca sensors called dihydropyridine (DHP) receptors in skeletal muscle (not acting as receptor! )
SR membrane contains Ca release channels called ryanodine receptors (RYR)(channels open when receive signal from DHP)
Mechanical connection btw DHP AND RYR
Thick (myosin) filaments
Composed of two proteins twisted together forming a tail with two head (cross-bridge)
Two pairs of light chains (MLC, head)
One pair of heavy chains (MHC, tail)
Has ATPase activity
Thin (actin) filaments
Complex of three proteins
Actin- attachment site for myosin cross bridges (Monomers make helix)
Tropomyosin- reg protein that sterically inhibits binding of myosin to actin (rod shape)
Troponin - regularity protein composed of 3 subunit (globular molecules)
Subunits of troponin
TnC : TROPHONIN CALCIUM
TnI : troponin inhibitory
TnT: troponin tropomyosin
Role of Ca in regulation of myosin binding to actin
- Myosin and actin interaction
Ca binding to the troponin complex allows for physical repositioning of tropomyosin filament
Which exposes the myosin binding site of the action molecules
Sliding filament hypothesis
Contraction occurs by sliding of the thin filament past the thick filaments with actual lengths of the filaments remaining unchanged
Series of cyclic reactions between the myosin head and the actin filament makes filaments slide: cross bridges
Predict that contractile force produced by the muscle fiber is proportional to the number of cross-bridges formed
What makes filament slide?
Series of cyclic reactions between the myosin head and the actin filament : cross bridges
Predict that contractile force produced by the muscle fiber is proportional to the number of cross-bridges formed
In skeletal muscle where does 100% of the Ca come from for the purpose of contractility l?
Sarcoplasmic reticulum
None comes from extracellular space!
Step 4: muscle contraction (cross bridge cycling)
At rest-filaments are not interacting
Myosin binds to action making a cross bridge (myosin head is extended)
Head/cross bridge bends, pulling myofilament inward creating a power stroke
Cross bridge detached at end of power stroke and returns to original conformation
Cross bridge binds to a more distal action molecule and cycle repeats
Muscle contraction/cross bridge cycling requires
ATP on myosin head
ATP hydrolyzed by ATPase, ADP and Pi remain attached to myosin as stored energy
No Ca- no excitation, muscle fibers remain at rest, no cross bridge
Ca- excitation, removes inhibitory influence from actin and the two bind (cross bridge)
Phosphate removal allows power stroke, ADP released immediately after bending
FRESH ATP binds head and detachment is possible (if not rigor mortis)
ATP hydrolyzed and head goes back to extended position , restart cycle
Amount of force exerted by muscle is directly related to
The amount of cross bridges formed
Capacity of sarcomere to generate cross bridges is directly proportional to
The alignment btw action and myosin filaments
Needs to be at maximum capacity for interaction
Steps 5 and 6:
Calcium removal and relaxation
Ca removed from the ICF back into the SR by sarcoplasmic reticulum Ca ATPase (SERCA)
Pump that moves Ca against conc gradient from ICF to SR
