Neurophysiology - Skeletal muscle Flashcards
What is the primary function of skeletal muscle
To reduce the distance between its site of origin and insertion, thereby producing movement
What are the other roles of skeletal muscle
- Maintenance of posture and joint stability
- Support soft tissues (abdo wall and pelvic floor)
- Sphinteric function in GI/Urinary system (provides voluntary control of swallowing, defaecation, micturition)
- Heat production
Differentiate:
Endomysium
Perimysium
Epimysium
Endomysium
- Layer of connective tissue surrounding each myocyte
Perimysium
- Bundles of ± 100 myocytes surrounded by perimysium are called FASCICLES
Epimysium
- Thick layer of connective tissue that encases entire muscle
this all converge together to form a tendon or aponeurosis which usually connects with muscle or bone
Describe the myocyte microscopic appearance
Length - may span entire length of muscle
Diameter - 50 um
Multinucleate
Striations (sleletal and cardiac)
What specialized cellular features do myocytes have in addition to the usual complement of: Golgi apparatus, mitochondria and ribosomes?
- Sarcoplasmic Reticulum (SR)
- -> modified ER that acts as an intracellular store for Ca++ which can rapidly release and sequester Ca++ - Transverse T - Tubules
- -> invaginations of muscle surface membrane (sarcolemma) capable of relaying APs deep into the myocyte interior - Myofibrils
- -> The contractile apparatus of the cell –> achored to sarcolemma at either end –> shortening when they contract - Myofilaments
- -> within the myofibrils are bundles of myofilaments containing contractile proteins actin and myosin - Glycogen stores
- -> Release glucose to provide energy for muscle contraction
What is a sarcomere
- Functional unit of skeletal muscle
- Interdigitating thick (myosin) and thin (actin) filaments
- Arranged in a regular repeating overlapping pattern giving an alternating sequence of dark and light bands
What are the key features of the sarcomere –> draw a diagram and label it
Z -disc: Located at either end of the sarcomere bisecting the I band
Thick and thin filaments - thin filaments are joined at the Z disc. Thick filaments are in the center interdigitating with the thin filaments
I band (isotropic) or light band - contains the portion of the thin filament that does not overlap with the thick filament
A band (anisotropic) or dark band - entire length of thick filament including regions that overlap the thin filament
H band (Heller) the part of the A band that contains only myosin
Describe the key features of thick and thin filaments
Thick
- Myosin = two globular heads + a tail
- binding sites for actin and ATP
- Each thick filament is surrounded by 6 thin filaments in approximately a hexagonal shape
Thin filaments
- Actin (contractile protein)
- Myosin binding site
- Tropomyosin (covers binding site preventing cross bridge formation)
- Troponin complex
- -> Troponin C (Contains Ca binding site –> hence ‘C’)
- -> Troponin I (Uncertain role)
- -> Troponin T(binds complext to tropomyosin hence ‘T’)
What is the function of Calcium in skeletal muscle contraction
It binds to troponin C which causes tropomyosin to roll out of the myosin binding site on the actin protein. Cross bridge formation can then proceed.
What is meant by excitation-contraction coupling. Describe this in skeletal muscle
Processes linking depolarisation to muscle contraction.
Excitation: AP –> via T-tubules to sarcoplasmic reticular Ca+ store –> Dihydropyridine receptor (DHPR) = modified V gated L-type Ca channel –> small amount of calcium enters myocyte –> Ryanodine receptor (RyR) opens –> massive release Ca++ into sarcoplasm x 2000 –> Ca binds Troponin C –> conformational change –> tropomyosin moves out of myosin binding site –> cross bridge formation –> contraction
Describe the pathophysiology of Malignant Hyperthermia and the mechanism of action for the treatment of it
MH –> genetic defect RyR –> triggered (sux/volatiles) –> Uncontrolled Ca++ release from SR –>
- Tetanic muscle contraction –> rhabdomyolysis
- Heat production
- ATP consumption (SR increased Ca++ sequestration)
- Hypermetabolic state
- -> Increase O2 consumption
- -> Increased CO2 production
- -> Metabolic acidosis
Treatment:
Dantrolene –> binds RyR inhibiting further Ca++ release
How has the mortality of malignant hyperthermia changed subsequent to introduction of dantrolene and greater awareness of the condition
80% mortality down to 2-3% mortality
How does skeletal muscle contract
Myosin binding site exposed (due to Ca+)
- Myosin binds ATP –> ADP + P –>bind to mysoin binding site forming a cross bridge.
- Energized Myosin flexes on its actin binding site moving the actin filament closer to the center of the sarcomere –> ADP + P now dissociate
- A fresh ATP binds and the process repeats
Describe muscle relaxation and mention the physiology behind the phenomenon of rigor mortis
Sarcoplasmic Reticulum Ca+ ATPase (SERCA) –> uses ATP to sequester Ca2+ back into the sarcoplasmic reticulum –> Tropomyosin block of myosin binding sites –> muscle relaxes and sarcomere returns to its original length.
Rapid decline in ATP after death –> insufficient ATP for SERCA –> calcium not sequestered –> muscles do not relax –> Rigor mortis
What are the three roles of ATP in skeletal muscle contraction
- Energizing myosin head
- Detachment myosin head from actin filament
- Muscle relaxation (SERCA)
