Muscle Flashcards
Three types of muscle
- skeletal muscle
- cardiac muscle
- smooth muscle
Skeletal Muscle
used for posture and locomotion. This is the muscle that enables our arms and legs to contract, under our conscious control
-composed of bundles of long (up to 1 ft) thin cells called muscle fibers
Cardiac Muscle
responsible for the rhythmic contractions of the heart
Smooth Muscle
cuases involuntary contraction in blood vessels, gut, bronchi, and uterus.
Muscle fibers
- generated during development by the fusion of a large number of small precursor cells called myoblasts.
- each myoblast has a single nucleus, whereas the fiber is a mutlinucleated cell.
- -is striped
- mature muscle fibers are long thin cells with many nuclei.
- consist of cylindrical bundles called myofibrils
tendons
-attaches muscle to bone on both sides of a joint
What causes the striations within each myofibril
- caused by light I-bands and dark A-bands
- in the center of each light band is a dark like called the Z-line
Sarcomere
- the contractile unit of skeletal muscle.
- this unit is delineated by the I,A, and Z bands/line
- consists of two sets of parallel and partially overlapping protein filaments; thickfilaments extending from one of the A band to the other, and thin filaments attached to the Z lines and extending across the I band and part way into the A band.
Myofybril is a lattice of what?
thick and thin filaments
I-band
Mostly thin filaments
H-zone
Mostly thick filaments
A band
thick and thin filaments
Actin
- thin filaments consist of actin.
- Each actin filament is formed from two chains of globular actin subunits, twisted into a helix.
Myosin
- thick filaments are made myosin
- thick filaments –> alot = myosin bundles.
Sliding Filament Model
- muscle contraction occurs when the thin filaments slide over the thick filaments.
- Note that neither the thick or think filaments change in length.
- the thin filaments are pulled over the thick filaments by the myosin head groups, which repeatedly grab, pull and release the thing filaments,
- the reaction is driven by ATP hydrolysis
What does the contraction of sarcomeres do?
It shortens the entire myofibril
Length-tension relation
- if the filaments are too close or too far the maximal tension is low.
- the maximal tension is reached at a mid point where the filaments are the perfect distance apart.
The cross bridge cycle
-driven by ATP binding and hydrolysis by the myosin head groups.
myosin binds to action –> power stroke –> ADP + Pi release –> myosin dissociated –> ATP attaches –> ATP Hydrolysis to produce ADP + Pi –> back to start.
The number of fibers that are innervated by a single motor neuron
-can range from 10 (e.g. extraocular muscles) to 100 (muscles of the hand) to several thousand (large flexor and extensor muscles of leg).
Structure of neuromuscular junction
- Motor axon
- Active Zone
- ACh synaptic vesicles
- nACh receptors
- Basement Membrane (AChE)
How does Neuromuscular transmission work?
- Action potential in motor neuron
- Acetylcholine Release at presynaptic terminal
- Na+ influx through activated nicotinic acetylcholine receptors
- Endplate potential
- Fiber action potential fired
How does Ca released from the SR affect the Muscle fibers
- calcium released from the SR binds to tropin on the thin filaments.
- this causes a conformational change in tropinin, which in turn moves the associated tropomyosin molecule away from from the myosin binding site on actin, which can then bind the heads of the thick filaments.
Twitch
- contraction of muscle fiber in response to a single action potential
- lags behind the muscle action potential, because of the delays associated with excitation-contraction coupling.
What does the duration of the contraction reflect
-primarily, the time it takes for the Calcium conc. in the muscle to return to baseline.
Tension
-force generated by a muscle
Generation of muscle tension
Tension is exerted by a whole muscle is controlled by:
- recruitment
- Summation
*Skeletal muscle is adapted for large force generation over a narrow operating range
Recruitment
an increase in the number of active fibers
Summation
- additive effects of several closely spaced twitches.
- applies to individual muscle fibers
- Single action potentials in the motor neuron, spaced more than a few hundred milliseconds apart, cause a transient twitch of the muscle fiber
- if the action potentials are applied more rapidly, the twitches begin to add together
- Motor neurons typically fire in bursts, resulting in, sustained contraction of the muscle fiber.
Tetanus
-motor neurons typically fire in bursts, resulting in, sustained contraction of the muscle fiber.
What is an important part for increasing muscle tension
the recruitment of additional motor units
Creatines Roles in Skeletal Muscle Energy Metabolism
- there is a transfer of a P from creatine phosphate to ADP via Creatine Kinase
- this creates enough ATP for a few seconds of muscle activity
What sustains levels of ATP during prolonged muscle activity?
- glycolysis and oxidative phosphorylation.
- Glycogen in the muscle, glucose and fatty acids from the blood provide fuel.
Types of Skeletal Muscle Fibers
- Fast glycolytic Fibers
- Slow oxidative fibers
- Fast oxidative fibers
Fast glycolytic Fibers
- myosin with high ATPase activity
- no myoglobin (“white muscle”)
- for generation of larger force over short periods of time
Slow oxidative fibers
- myosin with low ATPase activity
- Myoglobin to facilitate oxygen transport from blood (“red muscle”)
- for generation of low levels of force over long periods of time
Fast Oxidative Fibers
-intermediate properties. “fast” myosin and oxidative metabolism
Muscle fatigue
- not completely understood
- failure of action potentials in muscle fibers
- Lactice acid build up, which reduces muscle pH, altering protein structure and function.
Diseases of the Motor Unit
- Neurogenic Disorders
- myopathic disorders
- Upper motor neuron
Neurogenic disorders
- due to changes in motor neuron cell bodies of axons
- clinical features include muscle atrophy, fasciculations, decreased muscle tone
Myopathic Disorders
- due to degeneration of muscle with little or no change in motor neurons
- characterized by muscle weakness, myotonie, myoglobinurea (heme-containing proteins in urine), and increase in sarcoplasmic enzymes in plasma.
Upper motor neuron diseases
-cause overactive reflexes, spasticity
Myopathies
Duchene Muscular Dystrophy
- inherited, X-linked disorder. Affects males only
- starts in legs, Progresses rapidly
- Caused by mutation in gene encoding dystrophin. 430 kDa protein. Critical component of muscle cytoskeleton
Smooth muscle
- actin and myosin in smooth muscle do not have the highly ordered structure of skeletal and cardiac muscle, this smooth muscle lacks striations.
- requires Ca, but involves different molecular mechanisms than contraction of skeletal or cardiac muscle.
