skeletal muscle Flashcards
explain cardiac muscle features
• Striated: generate force
• Contracts rhythmically
• Innervated by ANS
strong quick continuous involuntary
Explain skeletal muscle features
• Striated: generate force
• Contracts only when stimulated
• Innervated by somatic motor neurons
strong quick discontinuous, voluntary contraction
Explain smooth muscle features
• Non-striated: for pressure loads and maintain organ dimensions
• Contracts continuously
• Innervated by ANS
weak slow involuntary contraction
Explain Gross features of skeletal muscle
- Epimysium surrounds whole muscle
- Perimysium surrounds muscle fascicles
- Endomysium surrounds a muscle fiber
explain the many features of a muscle fiber
a single cell with multiple nuclei, organelles and contractile structures.
– Several myoblasts fuse to form a single muscle fiber
– Satellite cells are stem-like (repair) cells that survive into adulthood.
• Sarcolemma is the cell membrane around a single muscle fiber
• T-tubules are infoldings of sarcolemma
•sarcoplasmic reticulum stores and releases Ca++
Sarcoplamic reticulum (SR) consists of two regions
- Sarcotubules store Ca++ bound to calsequestrin.
2. Terminal cisternae contact T-tubules
what is a Triad ?
Triad is where action potentials trigger release of Ca++ from terminal SR
what are myofibrils?
• A myofibril is a chain of fused sarcomeres that runs the length of the muscle
what is a sarcomere?
Part of the cytoskeleton containing thick (myosin) and thin (actin) myofilaments
• Joined to each other by Z-lines
• Surrounded by SR
describe the sarcomere cytoskeleton
it provides stability for contractile elements
• Z-lines connects sarcomeres and their alignment makes muscle “Striated”.
these 2 proteins form a flexible filamentous network that surrounds the myofibrils.
• account for 15% of the total protein in the myofibril.
Titin (aka connectin) and nebulin
-titin is an elastic component in muscle which can stretch under tension
what 3 components make up the thin filaments known as myofilaments.
• Actin: globular proteins (G actin) polymerized into doubled strands (F actin)
• Tropomyosin: cover myosin binding sites
• Troponin: a regulatory protein bound to
tropomyosin; activated by Ca++
what role do the thick filaments known as myosin play.
– Tails form thick portion of myofibril
– Head and neck form cross bridges that bind to actin.
describe why skeletal (and cardiac) muscle is striated
- Striations are due to regularity of sarcomere structure within muscle fibers
- Z lines align with each other
- Z lines form the edges of sarcomeres, with I and A bands indicating different sectors of myofibrils
What are motor neurons?
- cell bodies in ventral horn project axons out through ventral root to muscle
What is the neuromuscular junction?
-synapse formed by motor neuron on skeletal muscle
Describe structure and functions of neuromuscular junction
- motor neuron axons branch and synapse onto muscle end-plate
- junctional folds on muscle cell lined with Ach receptors
Why do motor neurons release ACh onto neuromuscular junctions?
- ACh increases permeability of Na/K channel, creating EPP
What is the function of EPP (End-plate potential)?
- Large depolarization (like EPSP, but greater)
- EPP is sufficient amplitude to exceed threshold for AP (one axonal AP can initiate muscle AP)
Muscle action potential
EPP’s depolarize neighboring voltage-gated Na channels that generate muscle action potentials.
Describe first step in muscle excitation
Begins with Ca release from SR:
- AP spreads along T-tuble
- AP current activates DHP receptors
- when activated, DHP does not permit Ca flux into cell.
- It does conduct voltage to the RyR
- Ryanodine receptor (RyR) releases Ca from SR terminal cisternae.
Describe contraction during AP of muscle excitation
- Ca released from terminal cisternae of SR
- Ca binds to troponin to initiate contraction
Describe relaxation after AP
- Ca sequestered into longitudinal sarcotubules of SR via Ca-ATP-ase.
- Ca then translocated back to terminal cisternae for subsequent release.
What are the two primary cofactors that facilitate contraction?
Calcium (permission) and ATP (motivation)
What is the function of Ca in muscle contraction permission?
- Ca is released from SR and binds to troponin.
- Troponin alters tropomyosin to expose actin’s binding site for myosin.
What is the function of ATP in muscle contraction motivation?
- ATP binds to myosin head and hydrolyzes to ADP and P
- ADP + P enables myosin head to bind to actin
What are the 4 stages of the sliding filament mechanism of contraction?
- Crossbridge attachment
- Working stroke
- Detachment
- ATP hydrolysis
What conditions are required for actin to bind to myosin?
The myosin head needs to face up to ATP
What event triggers bending of the myosin head?
Working stroke
What conditions are necessary for muscle to relax?
As ATP attaches to the myosin head, the cross bridge detaches. Detachment is necessary for relaxation.
What does myosin type II head do?
- In skeletal muscle, myosin type II heads bind individually and do not step one after the other.
- second head interacts with active head to orient and stabilize structure
What causes myosin head to restore its initial position?
As ATP is split in ADP and P, cocking of the myosin head occurs.
Sarcomere contraction
The net effect of myofibril contraction is the approach of two actin molecules toward center, shortening sarcomere.
What happens during continued contraction?
- The two actin ends approach each other actually to the point of overlapping.
- Contraction can occur as long as there is some overlap between actin and myosin.
Length-tension relationship
Total muscle tension is the sum of active and passive tensions
Passive tension in length-tension relationship
Resistance of CT in muscle tissue (elastic titin fibers)
Active tension (isomeric) in length-tension relationship
Active tension is proportional to number of cross bridges, maximal level L0. Stretching muscle to L0 improves contractility.
Why does stretching muscle to L0 improve contractility?
- Optimal overlap between actin and myosin
- Tension on actin-myosin increases Ca binding forming more cross bridges. Beyond the optimal length, Ca binding diminishes.
- When the muscle is very contracted and actins overlap, stretching it will readjust the myofibrils to a more optimal overlap for maximal contraction force.
Velocity-tension relationship
- The velocity of isotonic muscle contraction decreases with increased loads.
- Increased loads slows the rate of cross bridge cycling
Is appreciable power produced at no load in velocity-tension relationship?
No, at no load, velocity is maximal and no appreciable power is produced.
Is power generated at excess load in velocity-tension relationship?
No, at excess load, muscle cannot contract, generating no power.
Where is power maximal in velocity-tension relationship?
Power is maximal between two extremes of load
Motor unit
Group of muscle fibers innervated by branches of single motor neuron
What are small motor units in muscles used for?
Adapted for fine and precise motor control
What are large motor units used for?
Adapted for greater force
Twitch
Twitch is a movement of a single motor unit caused by a single action potential
- can be summed to generate tetanus
What determines duration of twitch?
Level of myoplasmic Ca
Spasm (cramp)
sudden involuntary contraction generated by continuous CNS activity or muscle dysfunction
Wave (twitch) summation
Increasing frequency of motor neuron AP’s causes twitches to run together, diminishing the time for Ca sequestration and subsequent relaxation.
Pathological Tetanus (lockjaw)
Summation of twitches without relaxation produced by neurotoxin produced by anaerobic bacterium.
Energetics of Muscle
ATP is used to convert chemical energy into mechanical energy.
What are the three metabolic sources for ATP?
1) Direct phosphorylation via creatine (quickest)
2) Anaerobic glycolysis - requires glucose
3) Aerobic oxidative phosphorylation of fatty acids - requires O2
Where is myoglobin found?
In slower contracting oxidative types of muscles.
- It has higher affinity for O2 and serves as O2 reservoir in skeletal and cardiac muscle
- With lower O2 levels, it releases O2 for ATP in mitochondria
on a long enough timeline the survival rate for everyone drops to ?
zero
