Unit 8: Muscle Physiology Flashcards
During embryonic development, huge skeletal fibres are formed by fusion of what component?
Myoblasts
What are the contractile elements of skeletal muscle?
Myofibrils
*Consist of thick and thin filaments
What protein make up thick filaments?
Myosin
What proteins make up thin filaments?
Actin
What are the 2 man components of connective tissue that covers muscle?
Collagen & Elastin
What are the 3 layers of connective tissue?
- Epimysin
- Perimysin
- Endomysin
What is a dark (A band) ?
- Arranged of stacks of thick filaments along thin filaments that overlap the ends of the thick filaments.
- Contains an M line, a series of proteins that holds the thick filaments in place.
What is the H zone of an (A band)?
-Contain an H zone (middle ear of the A band- that has no thin filaments)
What is a light I Band?
-Consists of thin filaments that do not project into the A band.
What is a Z line?
–>The middle of each I band has a vertical Z line, and the area between the 2 Z lines is called a sacromere, the functional unit of skeletal muscle.
What are the 2 crucial sites needed for myosin-actin interaction?
1) Actin-myosin binding site
2) Myson ATPase site (ATP splitting)
What are the 3 thin filament proteins?
1) Actin
2) Tropomyosin
3) Troponin
Why can’t muscle contraction occur in a relaxed muscle?
-Tropomyosin and troponin block the binding site.
Basic muscle contraction process.
The binding of Troponin to Ca2+ which allows for the movement of tropomyosin, no longer blocking the binding site. This allows actin and myosin to bind, and the muscle can contract.
What is the sliding filament mechanism?
-The THIN FILAMENTS SLIDE INWARD, over the thick filaments, towards the A bands center during contraction.
Z lines are pulled closer together, and sacromeres shorten.
*All sacromere’s shorten at once, and the muscle fibre simultaneously shortens.
->Known as concentric contraction
*Thick filaments remain unchanged.
What are the mechanisms of a Power Stroke?
When actin and myosin make contact at a cross bridge, the bridge changes shape- bending inward towards the sacromere like the stroking of an oar.
- The power stroke pulls thin filaments inward.
- repeated cycles complete the shortening process.
How is an actin-myosin interaction stopped?
-At the end of the cross bridge, the myosin-actin link breaks. The cross bridge returns to its original shape, ready for another actin to bind the myosin head.
What is the term “excitation-contraction” for muscles?
excitation- generation of action potentials in the muscle fibre
contraction- actin-myosin binding that causes sacromeres to shorten
What are Transverse Tubules (T Tubules) ?
Carry action potentials generated throughout the muscle fiber
What is the Sarcoplasmic Reticulum?
-Segments are wrapped around each A/I band, and have sac like regions called LATERAL SACS.
What are Lateral Sacs?
-Store calcium, and a spread of an action potential down a T tubule leads to the release of calcium from the sarcoplasmic reticulum into the cytosol.
What is the role of Calcium in the contraction of muscles?
Ca2+ is what moves tropomyosin during cross bridge activity, allowing actin and myosin to bind.
How is the myosin ATPase site involvement in the myosin-actin cross bridge?
- The breakdown of ATP occurs before actin and myosin meet.
- Pi and ADP are released (ATP is used) when the myosin and actin interaction occurs, after calcium has been released.
- ->When Pi and ADP are released, the myosin ATPase site is free for attachment of another ATP molecule.
- This allows for the bending back of the cross bridge and the cycle can continue.
How does the cycle continue ?
The new bound ATP is split by ATPase, energizing the myosin cross bridge once again, and on binding with another actin, the cross bridge bend to make a power stroke, and so on.
Why does Rigor Mortis occur?
- Actin and Myosin once bound, are stuck in place… They can’t detach because their is no fresh ATP.
- ->The thick and thin filaments stay in place, causing muscle stiffness.
What is the Latent period of muscle contraction?
-The lag in between a skeletal muscle fibres action potential, and the contractile appartus’ response.
What is the Contraction Time of muscle contraction?
The time from contraction onset until peak tension occurs.
*Occurs until all Ca2+ is used up from the lateral sacs.
What is the Relaxation Time of muscle contraction?
The time from peak tension until relaxation is complete
What is the Refractory period of muscle contraction?
The period following contraction where excitation is impossible.
What is a twitch?
–>A single action potential in a muscle fibre that produces a weak, brief contraction.
How is gradation of a contraction determined?
- The number of muscle fibers contracting
2. The tension developed by each fibre.
What is Fatigue?
Inability of a muscle to maintain tension at a given level.
*Asynchronous recruitment of motor neurons can take place (switching off to give others a break- can only occur at submaximal contractions)
What is Twitch Summation?
- A muscle fiber undergoes an action potential, and is re stimulated with another action potential before it has completely relaxed from the first twitch.
- Twitches are added together.
What is Tetanus?
- A muscle fiber is stimulated so rapidly that it does not have a chance to relax at all between the 2 stimuli.
- 3-4x stronger than a regular contraction.
How is twitch summation maintained?
Availability of calcium
What is Optimal Muscle Length?
The length at which maximal force can be achieved on a subsequent tetanic contraction.
What are series-elastic components?
Stretchy spring like components
–>Connective tissue and tendons
What is an origin?
Stationary part of the muscle (doesn’t move)
What is an insertion?
Skeletal part of the muscle (moves)
What is an Isotonic Contraction?
Muscle fibre tension remains CONSTANT as the muscle fibres change length.
What is an Isometric Contraction?
The muscle fibre is PREVENTED FROM SHORTENING, so tension develops at constant muscle fiber length.
What is Dynamic Contraction?
The whole muscle changes length
What is Concentric Contraction?
Produces tension during a shortening motion (z-lines move closer together)
What is an Essentric Contraction?
Produces tension while strengthening, STRETCHES Z LINES.
What processes in Muscle Contraction Require ATP?
- Splitting of ATP
- Binding of fresh ATP
- Active Transport of Calcium back into the SR during muscle relaxation.
What is Creatine Phosphate?
High energy molecule!
produces ATP from pi!
*Catalyzed by creatine kinase
What is the advantage of a tissue abundant with myoglobin?
Higher affinity for tissue to carry oxygen, as well as other nutrients.
What is Muscle Fatigue?
When a muscle can no longer respond to stimulation with the same degree of contractile ability (during exercise)
What is Central Fatigue?
Occurs when the CNS no longer adequately activates the motor neurons supplying working muscle cells.
What is Neuromuscular Fatigue?
The inability of active motor neurons to synthesize Ach rapidly enough to sustain chemical transmission of action potentials from the motor neurons to the muscles.
Slow Twitch Fibres ?
Type 1
Fast Twitch Fibres?
Type 2
Slow Oxidative Fibers (Type 1)
- high myoglobin content
- produce energy by oxidative processes
Fast Oxidative Fibres (Type 2)
- High myoglobin content
- Produce energy by oxidative processes
Fast Glycolytic Fibres (Type 2b)
- Low myoglobin content
- Produce energy by anaerobic processes
What are Higher % of fast glycolytic good for?
-Sprinting
What are Higher % of slow-oxidative good for?
-Marathon running
What is Muscular Hypertrophy?
- An increase in the mass or girth or a muscle and can induced by a number of stimuli.
- MUST affect protein-pattern expression
True or False: Fast- glycolytic fibres can change diameter in response to short, regular shifts of high intensity exercise.
True.
–>Oxidative fibres CAN’T change size
What is muscle atrophy?
Decrease in muscle mass
Can occur in 2 ways
1. Disuse Atrophy: Occurs when a muscle is not used for a long period of time, even though the nerve supply is still intact.
2. Denervation Atrophy: Occurs after the nerve supply to a muscle is lost.
Control of Motor Movement:
1) Input from afferent neurons
2) Input from the primary motor cortex
3) Input from the brain stem as part of a multi neuronal motor system.
What are Muscle Spindles?
Distributed throughout the muscle fibres as the fleshy part of the muscle.
-Consists of intrafusal fibres surrounded by extrafusal fibres (contain myofibrils)
What are Intrafusal Fibres?
Consist of non-contractile elements with contractile elements on the ends.
What are Gamma Motor Neurons?
-Innervate a muscles spindle INTRAFUSAL FIBRES
What are Alpha Motor Neurons?
-Innervate a muscles spindles EXTRAFUSAL FIBRES
What are Primary (Annulospiral Endings?)
- Type of afferent sensory ending that is wrapped around the central portion of the intrafusal fibres.
- Detect changes in length and speed when strecthing occurs.
What are the Secondary (Flower-Spray) Endings?
- Clustered at the end of segments of many intrafusal fibres.
- ->Sensitive ONLY to changes in length during stretching.
What are muscle spindles mainly involved in?
Stretch Reflex
What are the mechanisms of “Knee Jerk” reflex?
Tapping of the patellar tendon passivley stretches the quadracep muscle, activating intrafusal spindle fibres.
–>Results in a knee jerk contraction
What are the Golgi Tendon Organs?
-Monitor changes in muscle tension
What are the 3 filaments in smooth muscle?
- Thick myosin filaments
- Thin actin filaments
- Filaments of intermediate size for support
What are smooth muscle cells lacking that is responsible for their unstriated look?
- ->Lack of sacromeres, therefore they lack Z lines
* Do contain dense bodies which contain similar protein constituents as Z lines.
Do smooth muscle cells have T tubules?
No!
If T tubules don’t exist in smooth muscle cells, how does myosin interact with actin?
- ->The myosin light chain MUST be Phosphorylated to interact with actin.
- ->Calcium binds Calmodulin, creating a complex that activates myosin kinase, which in turn phosphorylates myosin. (chemical change- not a physical change)
- *Calcium is removed at the end to allow for a slow contraction rate.
How are cardiac muscle fibres connected?
-Intercalated discs, which form a unique network through STRONG desmosomes and tight junctions.
How is Multiunit Smooth muscle stimulated?
- Consists of multiple discrete units that function independently of one another & must be stimulated separately by nerves to contract (neurogenic)
- ->Stimulated by the autonomic nervous system
What system innervates skeletal muscle?
The somatic nervous system
Where is most single unit smooth muscle tissue found?
Visceral tissue
- ->Found within the walls of hollow tissues and organs
- fibre’s become excited and contract as a single unit
- Electrically linked by gap junctions
What is it called when interconnected muscle cells function electrically and mechanically as a unit?
Functional Syncytium
What is special about single unit smooth muscle?
It is self excitable
What is a Pacemaker Potential?
-The membrane potential gradually depolarizes on its own because of passive ionic fluxes accompanying membrane permeability.
What is a Slow-Wave Potential?
- Gradually alternating hyperpolarizations and depolarizing swings in potential are caused by automatic cycling changes in the rate of sodium transported across the membrane.
- Spontaneously threshold causes action potentials
What is Myogenic activity?
- Once an action potential is generated by a self-excitable cell smooth muscle cell (single unit), it is contracted by the remaining nonpacemaker cells to contract in functional syncytium.
- ->Spreads via gap junctions WITHOUT nervous input
In smooth muscle, what does an increase in Ca2+ result in?
More cross bridge activity.
In skeletal muscle, what does an increase in Ca2+ result in?
*Only a certain amount of Ca2+ is needed to trigger an action potential, which permits ALL cross bridges to cycle.
What determines how sensitive smooth muscle cells are to neurotransmitters and hormones?
-The cells distribution of cholinergic and androgenic receptors.
What is the resting length for smooth muscle?
Much less than optimal length (Lo)
–>Can be stretched considerably before reaching its optimal length.
Which is faster? Smooth muscle or skeletal muscle contractions?
Skeletal muscle contractions
*ATPase splitting is much slower in smooth muscle
