Excitable tissue, Muscle lecture: one Flashcards
Describe in generic terms skeletal muscle (lecture one slide 3):
(what controls it and appearance)
- Under voluntary control
- Striated
- Multiple PERIPHERAL nuclei
- SINGLE ling cylindrical cells
Describe in generic terms cardiac muscle (lecture one slide 4):
(what controls it and appearance)
- Myogenic (electrical potential originates from heart itself) (Involuntary control)
- striated
- BRANCHED CELLS (ONLY 1-3 CENTRAL nuclei)
- Connected via INTERCALATED DISCS
Describe in generic terms smooth muscle (lecture one slide 5):
(what controls it and appearance)
- Involuntary
- Found in the wall of internal organs (gut, blood vessels etc)
- Spindle shaped, uninucleated cells
- Not striated
What are myocytes composed of?
A myocyte is an individual muscle fibre
- Myocytes contain myofibrils
- Myofibrils contain highly organised contractile filaments
How do muscles attach to bone?
Tendons
How big can a myocyte get to?
35cm and 0.1cm wide
What are the thin and thick filaments of myocytes?
Thin = Actin filament Thicc = Myosin filament
Describe the microscopic structure of the myofibril referring to thick filament, thin filament, H zone, Z disc, M line
- Thick filaments: run the entire length of an A band
- Thin filaments: run the length of the I band and partway into the A band
- Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
- H zone: lighter mid-region where filaments do not overlap
- M line: line of protein myomesin that holds adjacent thick filaments together
What membrane envelopes a muscle fibre?
Sarcolemma membrane envelopes collections of myofibrils
What are t tubules?
•T-tubules - deep invaginations continuous with the sarcolemma (cell membrane) and circle each sarcomere at each of the junctions of the A and I bands. Allows action potentials to be carried deep within the muscle cell (contain the calcium channels for Ca to enter the cell and cause Ca induced Ca release from the SR)
Whats the SR?
•Sarcoplasma reticulum (SR) – The calcium storage site. The terminal cisternae of the SR lie close to the T- tubules.
What pumps Ca back into the SR?
Ca ATPase pumps i.e SERCA
Describe the structure of the thick filament:
- Composition
- Structure
- function
- polarity
- protein anchor
- Composed of Myosin
- Each myosin has 2 subunits each with a globular head and a tail, the two tails intertwine to form a helix
- The heads have a binding site for actin
- The head is an enzyme that hydrolyses ATP (an ATPase)
- Arranged in a polarised fashion: i.e. with the myosin heads projection away from the M-line.
- Titin anchors the thick filament to the Z-line
Describe the structure of thin filaments:
- Composition
- Structure
- Regulatory proteins
- Composed primarily of globular actin proteins
- The filaments are composed of a double stranded helical actin chain (polymers).
- Troponin and tropomyosin are regulatory proteins associated with actin in skeletal and cardiac muscle
What regulates troponin?
Cytoplasmic [intracellular] Ca levels
What does tropomyosin do?
Interacts with the myosin binding site
What is the sliding filament theory of contraction?
The sarcomere shortens as the thin filaments are pulled over the thick filaments:
• The Z-line is pulled toward the M-line
• The I band and H zone become narrower
What are the four major steps of the cross bridge cycle?
- Cross bridge formation
- Power stroke
- Detatchment
- Energisation of the myosin head
Describe step one of the cross bridge cycle; Cross bridge formation
- Myosin binds to the actin binding site to form a cross bridge
BUT
This can only occur in the presence of Ca which binds to troponin, causing a conformational change of tropomyosin exposing the myosin binding site
Describe step two the power stroke of the cross bridge cycle:
• ADP is released
• The myosin head rotates
to its low energy state (about 45° to the actin) pulling with it the thin filament
• The result is shortening of the sarcomere.
Describe step three (detatchment) of the cross bridge cycle:
- A new ATP molecule binds to the myosin
- The actin-myosin bind is weakened and the myosin detaches
- (Note: No ATP = no detachment)
Describe step four (energisation of the myosin head) of the cross bridge cycle:
- Myosin head hydrolyzes the ATP to ADP + Pi (NB this is the ADP released in the subsequent step two)
- The myosin head moves back to its “high energy (cocked)” confirmation (about 90° to the actin)
Why is Ca soooo important in the cross bridge cycle?
- Ca provides the“on” switch for cross-bridge cycle to begin.
- When Ca binds troponin the tropomyosin moves to expose the myosin binding sites on actin
- The cross-bridge cycle will continue as long as calcium levels remain above the critical threshold (0.001-0.01 mM)
How is Ca regulated?
- In skeletal muscle opening of calcium channels in the SR allows the movement of calcium ions into the cytosol.
- Active transport pumps (Ca2+ ATPase (SERCA) are constantly moving Ca2+ from the cytoplasm back into the sarcoplasmic reticulum (SR)
