Skeletal Muscle Contraction Flashcards
1
Q
skeletal muscle
A
- 40% of the body – a big chunk of our body is made up of muscle weight
- Can only contract – this is all the skeletal muscles can do! This is why we have oppositional muscle groups
- A muscle can flex or extend a joint but it can’t do both
2
Q
microscopic anatomy of muscle
A
- Bundles (fascicles) of bundles (fibers)
- The fibers are bundles of myofibrils (muscle proteins)
- Individual cells contain myofibrils
- Sarcolemma surrounds muscle cell
- Myofibrils are contractile elements containing myofilaments (smaller proteins)
- Mitochondria (2% of the volume in skeletal whereas it is 25% in cardiac cells)
- Sarcoplasmic reticulum stores calcium
3
Q
Myofilaments
A
- Actin (thin) – represents the thin filaments
- Actin is a helical coil of actin subunits (light blue balls)
- Arranged in long polymers )2 long polymers are coiled around each other)
- Myosin (thick) – has head group with two hinges (one at the attachment of head group and one on the tail region)
- Tail region is coil of two proteins
- Many myosin subunits are put together to form thick filaments
- Troponin are the bright pink comp
4
Q
mechanism of contraction
A
- Begins with stimulation by nervous system in skeletal muscle
- Skeletal muscle doesn’t contract unless stimulated by a nerve cell
- Each fiber must be stimulated by a nerve though one nerve does not stimulate all fibers (why not?)
- “motor unit” – the individual motor nerve fibers and all the muscle fibers innervated by that nerve
- If you have a muscle with all its fibers that has a lot of motor units, you have a lot more options for how you contract the muscle
- Gives you a lot more dexterity
- The larger muscles that move the limbs tend to have larger motor units because you don’t need as refined movements
5
Q
Neuromuscular Junction
A
- Union between nervous and muscular systems
- Uses acetylcholine as NT
- Binding of receptor opens a sodium channel and may trigger action potential
- Sarcoplasmic reticulum releases its store of calcium
6
Q
sliding filament
A
- Sliding filaments – thick and thin filaments are going to slide over each other
- Myosin, each composed of 6 polypeptide chains (head region and tail region)
- Protein heads (“Cross bridges”) extend away from the body of the filament
- Successive heads offset by 120 degrees
7
Q
actin filaments
A
- Actin (thin) filaments made of 2 helically coiled F-actin strands
- F-actin strands made of G-actin which is attached to one ADP molecule which may act as a site of cross bridge binding
- One end attached to Z-disc while the other protrudes between thick filaments
8
Q
tropomyosin filaments
A
- Tropomyosin connect loosely within F-actin coils (composed of G actin subunits)
- Each tropomyosin molecule spirals around F-actin covering approximately 7 ADP sites
- Troponin made of 3 protein subunits
- TI binds actin
- TT binds tropomyosin
- TC binds calcium (calcium is in the SR waiting to be released when the muscle undergoes an action potential
9
Q
troponin/tropomyosin complexes
A
- Troponin/tropomyosin complexes inhibit binding of cross bridges by physically blocking binding sites that the myosin head groups want to bind to
- Calcium release inhibits blockade by removing complexes from binding sites
- Calcium binds to the TC subunit which pulls the myosin off
- Uncovering sites on actin exposes them to myosin binding and “walk-along”
10
Q
role of ATP
A
- Binds head of the cross bridge – hydrolyzed and causes the myosin head group to reset to an open posture
- That open myosin head group has the ADP and Pi still attached so that opening up of the head group is considered potential energy (storing energy from hydrolysis of ATP)
- When the myosin attaches to the thin filament, the energy is released
- Is cleaved by ATPase activity of the cross bridge to ADP + Pi which remain attached
- Energy stored by hydrolysis is released when myosin heads bind to actin
- ADP+Pi are released with conformational change and new ATP is bound resulting in release of actin site and return of head
11
Q
single fibers
A
- Increasing tension developed with stretch (maximum at approx. 110% of normal sarcomere width)
- Maximum tension is achieved when the thin filaments are pulled to the end of the thick filaments – generate the most force of contraction when it draws the thin filaments together (up to 10% of myosin width)
- Limited by the ends of the myosin filaments
- Shortening sarcomeres limits tension as z-lines contact myosin filaments
- No cross bridges at myosin centers
12
Q
whole muscle
A
- Similar tension/stretch relationships as single fibers
- Normal resting length very near length for maximal tension development
- Velocity of contraction inversely related to load – the greater the load, the lower the velocity
- Work = Load x Distance moved
13
Q
energy for contraction
A
- ATP required in large amounts
- Muscle sequester short supply
- ADP must be phosphorylated and returned to high energy ATP state
- Creatine phosphate (phosphocreatine) stored in muscle provides, fast but short-lived source of phosphate. Stored in levels about 5x greater than raw ATP
- It can phosphorylate very quickly but can’t keep up very long
- Glycogen: storage form of sugars in muscle which can be liberated and broken down to pyruvate for glycolytic cycling.
14
Q
oxidative phosphorylation
A
-Oxidative phosphorylation: dietary sugars broken down first through glycolysis and the Krebs cycle then combine with oxygen as part of an enzymatic cascade producing ATP as a by-product
15
Q
comparison of energy sources
A
- CP: 4M/min ATP produced for seconds
- Glycogen/lactic acid (anaerobic): 2.5M/min for minutes
- Oxidative phosphorylation: 1M/min as long as the food holds out!
- You can do this forever as you have glucose
16
Q
contraction of whole muscle
A
- Term denotes development of tension, separate from “shortening”
- Isometric = “same length.” Tension = Load
- Isotonic = Tension > load
- Lengthening contraction = Tension < Load
17
Q
muscle structure
A
- Fast Fibers: large fibers, very strong. Large SR = large, fast Ca ion release. Large supply of glycolytic enzymes. Oxidative metabolism less important, blood supply reduced, fewer mitochondria – can generate a great deal of strength but Fatigue quickly.
- Slow Fibers: smaller, more extensive blood supply, increased mitochondria, large myoglobin content, slow fatiguing.
- Much more dependent on circulating sugars
18
Q
spatial summation
A
- Size does matter: Size principle
- Smaller motor units contract first (like the low gear on a car), but with growth of signal strength, larger motor units are recruited (smaller motor neurons are more excitable)
- Smooth contraction ensured by alternating stimulation of various motor units
19
Q
temporal summation
A
- Alteration of frequency of stimulation
- At some level contractions fuse into state of sustained contraction referred to as “tetanus.”
- Nearly every movement involves tetany
20
Q
strength of contraction
A
- Can achieve values of 4kg/cm2 or 50psi
- Step like increase in strength of contraction after period of rest (staircase effect). May occur due to flooding of the sarcoplasm with calcium
21
Q
muscle tone
A
- Resting tension on the muscle
- Results from low baseline rate of nerve impulses and neurotransmitter release from the spinal cord and causes a trophic effect that maintains the bulk of musculature
- Low level of release of neurotransmitter which provides low level of muscle tone
22
Q
fatigue
A
- Increases in proportion to the loss of glycogen stores
- Transmission of nerve impulses at the NMJ also reduced after prolonged stimulation – due to inability to keep up with recycling of neurotransmitter
23
Q
lever system
A
- Muscles apply tension at insertion points on bone
- Work with joints to affect movement of the skeleton – muscles can only move the skeleton if the skeleton is flexible – always crosses a joint
- Force related to the distance of the attachment from the fulcrum, the length of the lever arm to be moved and the original position of the lever.
- Force estimated by: (cross sectional area) x50psi
24
Q
muscle alteration
A
- Almost continuous process
- Fiber number changes rare
- Fiber hypertrophy by addition of myofibrils
- New sarcomeres added with stretch
- Increase in metabolic enzyme and glycogen storage, blood supply with sustained aerobic activity
25
Q
denervation
A
- Muscle atrophy begins immediately
- Most fibers destroyed and replaced by fibrous or fatty tissues or the muscle just sort of deteriorates
- Neighboring motor units can sprout new branches to re-innervate muscle fibers resulting in “macromotor units.”
- Micromotor units have less dexterity than the original motor unit
26
Q
dystrophies
A
- Genetic disorders (> 30 defects known)
- Mixed muscle atrophy, hypertrophy and necrosis
- Muscle fibers replaced by fat and fibrotic material (psuedohypertrophy)
- Characterized by insidious, progressive weakness
27
Q
duchenne MD
A
- Heritable absence of dystrophin (protein required for muscle structure)
- Males, females have 50% chance of carrying/passing mutation
- Onset of progressive weakness leading to paralysis at 3-5 yr, most lose ability to walk by 12 yo
- Becker MD similar but less severe disorder of dysfunctional dystrophin
28
Q
adult MD
A
- Fascioscapulohumeral MD – slowly progressive disorder of face, arms, shoulder beginning in teens
- Myotonic – MC adult form characterized by cardiac abnormalities and cataracts, swan neck, drooping eyelids
29
Q
MD
A
- Mostly affects boys (rarely girls).
- Often brothers or male relatives have same problem.
- First signs appear around ages 3 to 5: the child may seem awkward or clumsy, or he begins to walk ‘tiptoe’ because he cannot put his feet flat. Runs strangely. Falls often.
- Problem gets steadily worse over the next several years.
- Muscle weakness first affects feet, fronts of thighs, hips, belly, shoulders, and elbows. Later, it affects hands, face, and neck muscles. “Walk up” from seated/lying position.
- Most children become unable to walk by age 10.
- May develop a severe curve of the spine.
- Heart and breathing muscles also get weak. Child usually dies before age 20 from heart failure or pneumonia.
