skeletal muscle Flashcards
Three major types of muscle
Skeletal muscle - voluntary, attached to bone.
Cardiac muscle - involuntary, not attached to bone. Specialized form of skeletal muscle.
Smooth muscle - involuntary, not attached to bone. Myocytes with a ‘fusiform’ shape.
DIFFERENCES IN CELL STRUCTURE, SHAPE AND ARRANGEMENT WITHIN EACH MUSCLE
TENDONS
Bundles of collagen fibres that attach muscle to bone
MUSCLE
skeletal muscle made from bundles of multinucleated muscle cells
MYOFIBRIL
each muscle cell formed form bundles of actin and myosin filaments organized into myofibrils
sarcoplasmic reticulum
Six actin filaments surround a single myosin filament
Sarcoplasmic reticulum homologous to endoplasmic reticulum in other cells: Ca2+ store in skeletal muscle
T-Tubules: propagates action potentials into the interior of the myofibril from the surface
The Sarcomere
The sarcomere is the functional unit of skeletal and cardiac muscle
Runs from Z line to the next Z line
Functional unit of skeletal muscle (Z disk/line to Z disk/line)
Thin filaments
Thin filamements are made from globular actin proteins (G-actin)
G-actin forms protein chains termed filament-actin (F-actin)
A Band
Thick filaments (myosin) found as a dark band at the centre of the sarcomere
I Band
Contains thin filaments (actin) that do not overlap with the thick filaments. Found either side of the A band with the Z disc in the centre.
M line/disc
proteins that link the central regions of the thick filaments
H Zone
light area found between the ends the thin filaments; found in the centre of the sarcomere.
Z line/disc
Network of proteins that anchor the thin filaments
The Sliding Filament Mechanism
Contraction means the development of force rather than shortening
Myofilaments remain the same length, however they overlap to a greater extent
CROSS BRIDGE CYCLING
RESTING MUSCLE:
- Energized myosin molecule bound to ADP and Pi
- Tropomyosin covers and masks the ‘myosin binding sites’ on the actin filament (prevents interaction between the two filaments)
- ACTIVATION OF CONTRACTION:
- Muscle stimulated and cytosolic calcium levels increase
- The calcium binds to the troponin complex
- Calcium-bound troponin moves troopomyosin and unmasks the myosin binding sites on actin
- Energized myosin molecule (+ADP) binds to actin
- Cross bridge formation causes the release of ADP and Pi and movement of the crossbridge
- TERMINATING CROSS BRIDGE:
- ATP binds myosin and breaks the actin-myosin cross bridge
- ATP converted to ADP and myosin returns to the energized position
- The Neuromuscular Junction (NMJ)
Myelin sheath that surrounds the motor neuron ends near the surface of the muscle fibre
Axon terminals contain vesicles that contain acetylcholine (Ach)
Region of the muscle fibre directly under the terminal portion of the axon is the motor end plate
Axon terminal + motor end plate = neuromuscular junction
Action potential in the motor neuron depolarizes the axon terminal leading to opening of a voltage dependent calcium channel
Intracellular calcium increase triggers the release of Ach containing vesicles
Ach diffuses to the motor end plate and activates the nicotinic Ach receptors (ionotropic receptors)
Depolarizes the motor end plate = end plate potential