Skeletal muscle Flashcards
Where are Skeletal muscle cells orginate?
- Mesodermal origin
What are mature cells in adult skeletal muscle called?
- Myotubes
- these are differentiated multinucleated cells formed by cytoplasmic fusion of immature mononuclteated myoblasts
What are the muscle fibres bounded by ?
- Plasma membrane - sarcolemma
- they have cytoplasm- sarcoplasma
What is the name for many muscle fibres grouped together?
- Fascicles
- the fascicles are the smallest unit of structure visible to the naked eye
- It is the ability if the fascicle to contract that determines the character of the muscle
What is the endomysium?
- Connective tissue that surrounds individual fibres
What is the perimysium?
- connective tissue that encloses the functional fascicular unit
What is the epimysium?
- The connective tissue that surrounds the muscle in its entirety
What is surrounding each myofibril ?
- A membranous sac = sarcoplasmic reticulum
- it serves as a repository for calcium which is released to stimulate contraction
What do T tubules do?
- They connect at a membranous junction with the sarcoplamic reticulum of each myofibril
- the function is to carry depolarization of the surface membrane deep inside the muscle fibre
How does the arteriole entry the muscle fibre?
- Perimysium - where the arterioles penetrate the sheath surrounding the fascicle
- the arterioles enter obliquely or at 90 degrees to the muscle fibres then run parallel
- terminal capillaries are associated with muscle fibre nuclei
what are the major contractile proteins in skeletal muscle?
- Actin
- Myosin
What is the grouped functional unit of actin/myosin filaments?
- Myofibril
Myofibrils are segmented into functional contractile units called what?
- Sarcomeres
- visible under electron microscope
- 2-2.5 microns in length
- 1 micron in diameter
- length varies with muscle activity but shows a variance along the length of the myofibril
- sarcomeres in the myotendinous junction tend to be shorter
What is the A band?
- Represents the Myosin filaments
- Anisotrophic on light microscope
- ***“AM”
What does the I band represent?
- Actin filaments in adjacent sarcomeres where there is no overlap with myosin filaments
- Isotrophic on light microscope
- **IA**
What does the H band represent?
- Myosin filament segment where there is no interdigitiating actin filaments
What do the M lines represent?
- Connections between adjacent mysoin filaments in their central region
- these are termed M band proteins
What do Z discs represent?
- Attachment of adjacent sarcomeres
- the disc lies between sarcomere
What is the arrangement of actin and myosin filaments?
- Hexagonal lattice in the centre of the sarcomere
- ie each myosin filament is bounded by 6 actin filaments
- this becomes more square towards the end of each sarcomere
What other important proteins have a role in maintaining the structure of the sarcolemma?
- Dystrophin ( absent in duchenne muscular dystrophy)
- as a group called structural proteins as they maintain the overall architecture of the sarcomere during contraction
What is actin?
- A globular protein (molecular weight 42 000)
- chief constituent of the thin filament of the sacromere
What other proteins constitute the thin filament of the sarcomere?
- Tropomyosin
-
Troponin subunits
- troponin C
- troponin T
- troponin I
What does tropmyosin do?
- It exends across seven actin subunits
- Blocks the bindings sites of the myosin head unit until unblocked by calcium binding to the troponin C subunit ( tropomysoin moves & mysoin- binding site exposed)
- the activated troponin C subunit counteracts the inhibitory effect of the troponin I subunit
- Troponin T assists toponin C binding to tropmyosin
What is the myosin structure?
- Made up of 6 distinct subunits
-
2 heavy chains
- S1 segment/cross bridge= articulate with actin
- S2 segment= forms flexible neck which moves to allow articulation of the S1 head segment
- 4 light chains - uncertain function
-
2 heavy chains
Describe the sliding filament contraction/ cross bridge theory by Huxley?
- in rush of Ca 2+
- Ca 2+ binds to Troponin C
- Activated troponin C displaces inhibitory Troponin I from site on tropomyosin/actin complex
- Troponin T assists
- Tropomyosin undergoes conformational change to allow myosin head enagagement
- Myosin head engages
- Forced generation occurs due to S1 head segement rotation
What is the force of a muscle is porportional to?
- Cross sectional area
- the more sarcomeres that are acting parallel to each other the higher the force generated
What is speed of muscle related to ?
- Length of the muscle
- Upon stimulating a muscle all sarcomeres contract at the same time.
- for long muscle there will be a greater change in length per unit time ie the greater the muscle velocity
- the sarcomeres act in series
What is the power of the muscle a product of ?
- Force x velocity
- short fat muscles produce high force but low max velocity
- long thin muscles produce low force but high max velocity
What is the name given to all muscles fibres innervated by the same motor neuron?
- Motor unit
Can you draw a neuromuscular junction?
Describe what happens at the Neuromuscular junction?
- Motor neurone, action potential propagates an influx of Ca2+ thru voltage- sensitive channels
- Increased concentrations of intracellular Ca2+ cause preformed vesicles of acetylcholine to fuse with presynaptic nerve membrane
- Acetylcholine in the synaptic cleft binds to the postsynaptic receptors on the sarcolemma
- Binding of acetylcholine depolarises the muscle fibre membrane
- Depolarisation is dependent on the amount of acetylcholine releases into the synaptic cleft
- Also dependent on the rate of release of acetylcholine into the cleft as it is broken down rapidly by cholinerases from the post synaptic membrane
What is the myotendinous junction?
- An area where insertion of every skeletal muscle fibre into its tendon occurs
- specific morophology
- shorter sarcomere lengths
- greater no of organelles per cell
- greater synthetic ability
- interdigitation of the cell membrane
- extracellular connective tissue
- high degree of membrane folding- increases resistance to stress by increasing the surface area and reducing the angle of the force vector applied
- net result junction very strong
Describe the different types of muscle?
-
Type 1 “Slow Red Ox”
- Slow fibres
- Large conc of myoglobin ( red in colour)
- Oxidative
- Very fatigue Reisistance
-
Type 2a
- Fast fibres
- Oxidative
- Glycolytic ( realtively white in colour)
- Fatigue resistant
-
Type 2b
- Fast fibres
- Glycolytic ( white in colour)
- Fatiguable
What is the single most important factor in fibre type expression?
- Pattern of activity imposed on the muscle
- ie atheletes who train for endurance can reach proportions of up to 80% of type 1 fibres in their muscles
What causes damage to muscle cells in truama?
How does this continue?
- Raised calcium concentrations
- Once cell damaged the process continues via a calcium-activated enzymes - Proteases and phospholipids
- also by free-radicals and oxidation of liberated free fatty acids
What are the modes of muscle injury?
- Muscle belly tear
- Muscle laceration
- Musculotendinous junction injury
- Ischaemic damage and compartment syndrome
- Denervation
- Crush injury and rhabdomyolysis
-
Malignant hyperpyrexia
- skeletal muscle reaction to halothane with prolonged muscle contraction -> metabolic distintegration of muscle
- K is released first so risk of MI, followed by myoglobin release and renal failure
- Delayed muscle soreness
- muscle soreness that develops 24-72hrs following intense exercise
What is the prognosis of a proximal belly tear?
- Worse the prognosis
- As more bullk is denervated
What do muscle lacerations result in ?
- Dense fibrous scars
- Myotubes regenerate across scar tissue in small no
- Partial lacerations have better functional outcomes cf complete belly lacerations
- complete lac- distal portion wastes rapidly.
- muscle regeneration can occur with a nerve supply but permanent muscle atrophy will develop if regeneration fails
- A complete lac in mid-substance can recover only 50% of previous force that was generated by the muscle
Where do complete tears normally occur?
- Myotendinous junction, with a segement of adjacent muscle avulsed when stretched to failure
Where do incomplete tears occur?
- Near the myotendinous junction in an area of relatively stiffer sarcomeres
Describe the EMG of skeletal muscle?
- is a recording of electrical activity in skeletal muscle
- Needle electrodes are inserted into muscle to be studied
- The electrode records any spontaneous activity within the muscle are rest and the signal given by firing of motor units when the muscle is activated
- normally no spontaneous activity at rest in a healthy muscle after the needle is inserted
- the patterns of motor activity ( motor unit action potentials) are related to the nerve supply to the muscle in question
What is the EMG findings of acute degenerative injury?
- Spontaneous activity of Sharp waves
What is the EMG findings of chronic degenerative injury?
- Fasiculations
What is seen on EMG with early re-innervation?
- Reduced amplitude motor action potentials
- Longer duration
- Poor recruitement
- the new sprouts conduct slowly , producing temporary dispersion ( prolonged MUAP) adn MUAP polyphasicity
What is seen on EMG with late re-innervation?
- Large ampitude, stable consistent firing Motor action potentials
- Good recruitement of units gives polyphasic signal
