Muscles 1 Flashcards
Describe skeletal muscle (6)
- Striated
- Voluntary*
- Single long, wide, cylindrical cells
- Multiple nuclei*
- Attached to bones by tendons
- Responsible for movement
Describe cardiac muscle (6)
- Striated
- Branched*
- Involuntary
- 1 (sometimes 3) nuclei*
- Lots of mitochondria*
- All cells electrically connected via intercalated discs (gap junctions and desmosomes)*
What is voluntary control?
Under control of somatic nervous system (only skeletal)
What is involuntary control?
Under control of autonomic nervous system (cardiac, smooth)
What is a motor unit?
What is the other way cells can be connected in tissue?
- A group of muscle cells innervated by a single neuron
- Direct cellular contacts
What are actins? (2)
- Globular proteins that form THIN filaments
- Have accessory proteins (troponin, tropomyosin)
What is troponin? (2)
- Regulatory accessory protein on actin
- Has Ca binding site = allows movement to start
What is tropomyosin? (2)
- Regulatory accessory protein on actin
- Regulates myosin head binding to actin
What are myosins? (3)
- THICK filaments
- 2 subunits: globular head, tail (forms double helix)
- Head is enzyme that breaks down ATP
Describe calcium in muscle processes (3)
- Increase in Ca levels is ‘on switch’ for contraction
- Interacts with troponin (in actin regulated muscles), calmodulin (in myosin regulated muscles)
- Comes from inside cell stores (SR) and outside cell
How does the cross-bridge cycle work? (4 steps)
Calcium binds to troponin causing it to change shape, tropomyosin reveals actin binding sites
- Myosin head binds actin and forms cross-bridge (must be activated by ATP hydrolyses)
- Power stroke: ADP is released and myosin slides actin towards centre
- Cross-bridge detachment: ATP binds to myosin causing it to detach
- ATP hydrolysed, reactivate myosin head
Describe a muscle contraction
What happens to the sarcomere?
- Caused by single AP
- Activates of cross-bridge cycle
- Sarcomere contracts and thin filaments are pulled over thick
- Z-discs pulled towards M-line, I-band and H-zone narrow, A bands don’t change
What is the length-tension relationship? (2)
- Ability to develop tension depends on initial strength
- Overlap determines cross bridges attached during isometric form
What is passive tension?
Elastic elements stretched out and want to return to shape
What are the components of a skeletal muscle cell? (4)
- Sarcomere made up of fibrils surrounded by sarcolemma
- Fibrils made up of thick/thin filaments
- SR
- T-tubules
What is the sarcoplasmic reticulum? (3)
- Extensive network of membrane-bound compartment surrounding fibril
- Calcium store
- Skeletal > cardiac > smooth
What are the t-tubules? (4)
- On surface of sarcolemma
- Interior of tubules is exterior of cell (tubules are continuous with sarcolemma)
- Receives AP
- Between A and I bands for skeletal / at Z lines for cardiac / not present in smooth
What is a triad?
What is the function of the arrangement?
- 2 bulges of SR (terminal cisternae) against 1 t-tubule
- Close so t-tubule can signal SR easily (to open calcium stores)
What are the components of the sarcomere in skeletal muscle? (5)
- M line: thick filaments linked by accessory proteins
- H zone: thick only
- A band: all of thick, some thin
- I band: thin only
- Z disk: boundary of sarcomere - attachment site for thin
In the length-tension relationship of skeletal muscle…
What happens when muscle is stretched out? (4)
> 2.2um
- Less overlap between filaments
- Myosin can’t grab onto actin = reduced number of cross-bridges
- Less tension as active forces decline
In the length-tension relationship of skeletal muscle…
What happens when the muscle length is half or more? (3)
< 2.0um
- Filaments collide and interfere = reduces force developed
- Myosin heads can’t effectively grab onto actin (and move towards M line)
In the length-tension relationship of skeletal muscle…
What is maximal force/normal working range of the muscle?
What is total tension?
- 2.0-2.2um
- Total tension: sum of active tension, dependent on sarcomere length and passive tension
What does isometric mean?
Contracted but not shortened
What does isotonic mean?
Muscle shortened (L-T relo can’t be measured)
What is the neuromuscular junction?
Where the axon terminals of motor neuron meet the motor end plate on a skeletal muscle fibre
Describe muscle contraction at the neuromuscular junction in skeletal muscle (7 steps)
- AP travels to motor neuron axon terminal
- VG Ca+ channels open and Ca+ diffuses into terminal
- Ca+ causes synaptic vessels to release ACh via exocytosis
- ACh diffuses across synaptic cleft and binds to ACh receptors (contain LG cation channels)
- LG cation channels open
- Na+ enter and K+ exit muscle fibre (causes depolarisation)
- Threshold reached and AP propagates sarcolemma and t-tubules
(stops when ACh removed from cleft; diffusion or ACh enzyme)
Describe excitation-contraction coupling in skeletal muscle (3)
- Voltage sensor in t-tubule is directly linked to Ca+ release channel in SR
- When AP reaches t-tubule the sensor opens the channel so Ca+ can flow out of SR
- Contraction ends when Ca is pumped back into SR via Ca2+ATPase pump (troponin moves back over binding site)
What is the role of creatine phosphate in muscle metabolism? (3)
- For <15s it can act as an ATP store
- Creatine phosphate + ADP = creatine + ATP
- ANaerobic (no O2)
What is anaerobic glycolysis in terms of muscle metabolism? (2)
- Short exercise 10-30s (fast, inefficient)
- Build up of lactate and H+ limit to 120s
Describe aerobic metabolism (3)
- Efficient but slow (produces a lot)
- Requires O2 - need good blood supply
- Postural and endurance
What is a type 1 muscle fibre? (5)
- Slow twitch (posture, walking)
- Aerobic
- Small
- High mitochondria/myoglobin/blood supply
- Low glycolytic capacity
What is a type 2b muscle fibre? (5)
- Fast twitch (jumping, weights)
- Anaerobic glycolysis
- Large
- Low mitochondria/myoglobin/blood supply
- High glycolytic capacity
What affects regulation of force? (2)
- RATE of stimulation of individual motor units
- NUMBER of motor units recruited
What is temporal summation?
regulation of force
Increased FREQUENCY of stimulation (AP) during contraction
- Rapid stimulation = sustained contraction
- Less/slow = more, stronger contractions
Describe recruitment of motor units
regulation of force
Amount of force developed determines the number of motor units recruited
- Heavy = more
- Increases VOLTAGE
What is different about the contraction of cardiac muscle? (3)
- Longer AP and contraction
- AP has plateau phase due to Ca current
- Contract:relax = 1:3 (refil) a premature contraction will be small due to less refil
What is the process of contraction in cardiac muscle? (3 steps)
- Rapid depolarisation (Na moves into cell quickly)
- Plateau phase (opening of slow VG Ca channels)
- Repolarisation (K channel open, inward channels close)
What are the features of the t-tubule and SR in cardiac muscle? (4)
- T-tubules on Z discs
- Less extensive SR
- LTCC (L-type Ca channel) on t-tubule (reason for plateau) = Ca source
- RyR receptors on SR, associated with Ca channel (Ca binds RyR to release Ca)
Describe the initiation of the heart beat at the SA node (2)
- Electrical activity starts at SA node then spreads throughout cells
- Pacemaker cells are ‘leaky’ because they let Na into the cell so the RMP naturally increases to threshold
What regulates the frequency of of the heart beat? (nerve, neurotransmitter, effect)
(2)
- PARAsympathetic (vagus) nerves release ACh and DECREASE depolarisation/heart rate
- SYMpathetic nerves release noradrenaline and INcrease depolarisation/heart rate
Describe the calcium cycle in cardiac cells compared to that of skeletal muscle (4)
- AP spreads sarcolemma to t-tubules and triggers L-type Ca+ channels to open = AP plateau and lets Ca+ into cell
- There’s always Ca+ pumped out through Na/Ca exchanger (gradient established by Na/K)
- Ca+ binds to RyR and opens SR Ca+ channel to flood cell
- “Calcium induced calcium release”
= contraction occurs
Describe the location and structure of smooth muscle (9)
Airways, bladder, reproductive, blood vessels, eyes, gut
- Spindle shaped
- Single nucleus
- Visceral arrangement (sheet) = single unit / multiunit smooth muscle tissue (isolated, need more nerves)
- No t-tubules (caveolae instead)
- Dense bodies (act like Z lines)
- Gap junctions
- Intermediate filaments (still have thick and thin)
- Poorly developed SR
Explain how smooth muscle contraction is initiated (4)
- VG Ca channels (few Na channels)
- Ca binds to CALMODULIN (not troponin)
- Which activates MLCK(inase)
- Which acts on myosin heads
Describe how smooth muscle contractile force may be modulated (3)
- Relaxes when myosin light chain phosphotase dephosphorylates it
- Ca-ATPase reduces intracellular Ca
- Contraction is enzyme regulated (slow but efficient)
