L24 - Skeletal Muscle Flashcards
What are the three major types of muscle?
- skeletal
- cardiac
- smooth
What is skeletal muscle?
Muscle attached to the bone
- voluntary
What is cardiac muscle?
Muscle not attached to the bone
- specialised form of skeletal muscle
- involuntary
What is smooth muscle?
Muscle not attached to the bone
- myocytes with a fusiform shape
- involuntary
What are key characteristics of skeletal muscle?
- attached to bone
- composed of fibres
- posture and movement
- nervous control (voluntary)
What are the different skeletal muscle structures?
- muscle
- tendons
- myofibrils
What are muscles?
Skeletal muscle made from bundles of multinucleated muscle cells
What are tendons?
Bundles of collagen fibres that attach muscle to bone
What are myofibrils?
Muscle cell formed from bundles of actin and myosin filaments organised into myofibrils
How many actin filaments surround a single myosin filament?
6 actin filaments
What is the sarcoplasmic reticulum?
Homologous to the ER found in other cells
- wrapped around the myofibril
What does the SR act as in skeletal muscle?
Ca2+ store
What type of pattern is visible on skeletal muscle? What is it caused by?
Striated pattern
- Accumulation of protein at different points of the fibre
What are the functinal units of striated muscle?
- Z-disc (a-actinin)
- thin filament (actin, tropomyosin, troponin)
- M-band (myomesin)
- thick filament (myosin, titin)
What is the A band?
Thick filaments (myosin)
- dark band at the centre of the sarcomere
What is the I band?
Contains thin filaments (actin)
- don’t overlap with thick filaments
- either side of A band with the Z disc in the centre
What is the H zone?
Light area found between the ends of thin filaments
- the centre of the sarcomere
What is the M line?
Protein that link central regions of thick filaments
What is the Z disc?
Network of proteins that anchor the thin filaments
What is one sarcomere?
Z line to Z line
What are thick filaments like?
- 15nm in diameter
- hundres of motor proton myosin
- heavy and light chains
What is the tail of a myosin molecule formed of?
- 2 intertwined chains
- double globular head projecting from it at an angle (half left, half right)
= middle filament region (M-region)
What are thin filaments like?
- F-actins stranfs, coposed of string of globular G actin subunits
- has active site that binds to head of myosin molecule
- has tropomyosin that blocks active sites
- bound by Ca-binding troponin
- attached to Z disc
What does a-actinin do?
Bind and cross-link the ends of F-actin filaments from adjacent sarcomeres at the Z line
What is a-actinin-2?
Ubiquitous in all types of muscle fibres
What are the different parts of troponins?
- Troponin I (inhibitory)
- troponin C (Ca binding)
- troponin T (tropomyosin binding)
What is the sliding filament mechanism?
Contraction - development of force rather than shortening of muscle fibres
- myofilaments remain same length but overlap to a greater extent
What are the sequence of steps in the cross bridge cycling?
- resting muscle
- activation of contraction
- terminating cross bridge
What happens while the muscle is resting (in the cross bridge cycling)?
- myosin molecules bound to ADP and Pi
- tropomyosin covers ‘myosin binding sites’ on actin filaments
What happen in the activation of contraction (in the cross bridge cycling)?
- muscle stimulated, Ca levels inc
- Ca2+ bind to troponin complex
= moves tropomyosin, unmasks myosin binding sites - +ADP binds to actin
- cross bridge formation releases ADP and Pi = movement of crossbridge
What happens in the termination of the cross bridge (in the cross bridge cycling)?
- ATP binds myosin, breaks actin-myosin crossbridge
- ATP converted to ADP, myosin returns to energised position
What do skeletal muscle contractions require?
Synaptic input from motor neurons
What can a single motor neuron do?
Innervate multiple muscle fibres
- causin simultaneous contraction
What is the structure of the neuromuscular junction (NMJ)?
- myelin sheath ends near surface of muscle fibre
- axon terminal has vesicles containing Ach
- ## motor end plate direclty under terminal portion of axon
What happens at NMJs?
- AP depolarises the axon terminal, open VGCC
- releases Ach containing vesicles
- Ach diffuses to motor end plate, activates nAch receptors (ionotropic receptors)
- depolarises motor end plate = end plate potential
What are the different fibre types of skeletal muscle?
- fast twitch (glycolytic, explosive power)
- slow twitch (aerobic, fatigue resistant)
What is the speed of contraction determined by?
Myosin heavy chain isoform expressed by the fibre
What is the gene for speed?
Alpha actinin 3
- expressed in a subset of fast twitch muscles
- presence of absence of variant correlates with athletic performance
