Muscle cell physiology Flashcards
What are the three types of muscle cell?
Skeletal, cardiac, smooth
Morphology of skeletal muscle
Long and cylindrical
Morphology of cardiac muscle
Branched
Morphology of smooth muscle
Spindle/fusiform
Nuclei distribution of skeletal muscle
Multiple, located peripherally
Nuclei distribution of cardiac muscle
1/2 located centrally
Nuclei distribution of smooth muscle
One located centrally
Does skeletal muscle have a sarcomere?
Yes - striated pattern
Does cardiac muscle have a sarcomere?
Yes - striated pattern
Does smooth muscle have a sarcomere?
No
Does skeletal muscle have T-tubules?
Yes, forms triad with sarcoplasmic reticulum
Does cardiac muscle have T-tubules?
Yes, forms diad with sarcoplasmic reticulum
Does smooth muscle have T-tubules?
No - caveolae
Does skeletal muscle have electrical coupling of cells?
No
Does cardiac muscle have electrical coupling of cells?
Yes - intercalated disks with gap junctions
Does smooth muscle have electrical coupling of cells?
Yes - gap junctions
Can skeletal muscle regenerate?
Yes - satellite cells
Can cardiac muscle regenerate?
No
Can smooth muscle regenerate?
Yes
Does mitosis take place in skeletal muscle?
No
Does mitosis take place in cardiac muscle?
No
Does mitosis take place in smooth muscle?
Yes
Does skeletal muscle have extracellular calcium for contraction?
No
Does cardiac muscle have extracellular calcium for contraction?
Yes
Does smooth muscle have extracellular calcium for contraction?
Yes
Regulation of cross-bridge formation in skeletal muscle
Ca 2+ binds to troponin
Regulation of cross-bridge formation in cardiac muscle
Ca 2+ binds to troponin
Regulation of cross-bridge formation in smooth muscle
Ca2+ calmodulin activation of myosin kinase and phosphorylation of myosin
Control of contraction in skeletal muscle
Motor neurons
Control of contraction in cardiac muscle
Autonomic nerves - beta-adrenergenic agonists
Control of contraction in smooth muscle
Autonomic nerves - hormones
Is there summation of twitches by increased stimulus frequency in skeletal muscle?
Yes
Is there summation of twitches by increased stimulus frequency in cardiac muscle?
No
Is there summation of twitches by increased stimulus frequency in smooth muscle?
Yes
Does tension vary with filament overlap in skeletal muscle?
Yes
Does tension vary with filament overlap in cardiac muscle?
Yes
Does tension vary with filament overlap in cardiac muscle?
Yes
How long are muscle fibres?
Up to 30cm
Diameter of muscle fibres
10-100 micrometres
What are the smaller units of muscle fibres?
Myofibrils
What is the sarcolemma?
Specialised plasma membrane
What is the sarcoplasm?
Contains glycosomes and myoglobin
What are myofibrils?
Composed of overlapping thick filaments (myosin) and thin filaments (actin)
Why do myofibrils appear striated?
A bands are dark (thick and thin filaments)
I bands are light (thin only)
What is the sarcomere?
Repeating contractile unite between myofibrils
Extend between z-lines
Why are the nuclei of muscle fibres peripherally located?
The inside is full of myofibrils
From smallest to biggest, place fibre, myofibrils and fascicles
Fascicles - fibre - myofibrils
What are T-tubules?
Holes between myofibrils which go to centre and form triad with sarcoplasmic reticulum (t-tubule with SR either side)
Where is calcium stored in the SR?
Lateral sacks
Why are muscles able to generate lots of ATP?
Lots of myoglobin and mitochondria
How are muscles stimulated to contract?
Impulse travels along t-tubules and activates receptors on SR to release calcium into myofibrils
How many heads do thick filaments have and what do they do?
2 heads - one for ATP and another for actin
How many actin filaments do the thin filaments have and what are they?
Troponin and tropomyosin
What does tropomyosin do initially?
Blocks myosin binding site on actin
What are the 3 types of troponin?
Troponin C (calcium binds) Troponin I (inhibitory) Troponin T (binds to tropomyosin)
What happens to troponin C and I when calcium is released by the SR?
Calcium binds to troponin C which forces a change in troponin I to release tropomyosin
What is a myosin bridge?
Myosin head bound to actin
What is the cross-bridge lining?
Calcium dependent, ATP driven attachment, bending and release of myosin across bridges from myosin binding sites which moves actin filaments towards sarcomere
Describe excitation-contraction coupling
Electrical impulse conducted along sarcolemma from NMJ
T-tubules direct signal to deep within SR
Direct mechanical coupling between DHPR and RyR
Calcium released from lateral sacks of SR (via RyR) binds to troponin on actin filaments, imitating cross-bridge cycling and muscle contraction
Calcium driven back into SR by ATP-driven pumps
What does DHPR stand for?
Dihydropyridine receptor
What does RyR stand for?
Ryanodine receptor
Describe the four stages of muscle contraction
1) Myosin ATPase splits ATP into ADP and Pi but both remain bound to cross-bridges as stored energy
Myosin active site on actin concealed by tropomyosin and inhibited by actin binding to inhibitory region of troponin
2) Calcium released from SR upon excitation binds to troponin
Triggers conformational change in filament
Troponin I inhibitory region detaches from actin and tropomyosin moves away from myosin binding site
Myosin cross bridges attach to actin filaments
3) ADP and Pi release energy to generate power stroke
Myosin and actin filaments slide across each other and sarcomeres contract
Where is calcium stored?
Sacs in sarcoplasmic reticulum
What is a triad?
T-tubule surrounded by two layers of SR
What is the terminal cisternae?
Flat section of the SR
Which band is the dark band?
A
Which band is the light band?
I
What is the m-line?
Tails of the thick filament wrap around each other
Where are z-disks found?
End of sarcomere
How does information travel across the neuromuscular junction?
Reaches terminal bouton of motor neuron
Action potential opens voltage gated ion channels when calcium moves in (depolarisation)
Calcium moving in leads to vesicles of ACh fusing with membrane and releasing ACh into cleft
ACh binds to receptors on motor endplate (motor neurone directly innervates muscle fibres)
Nicotinic ACh receptor is the sodium channel gated by ACh
ACh binds to receptor and opens the pore, letting sodium in
Generates new action potential
More channels that open, more sodium moves in
Leads to EPP
How is an electrical impulse switched off?
ACh broken down by AChesterase to form choline and acetate
Choline re-uptaken by co-transport with sodium into motor neurone
Repackaged with acetyl to from CoA into vesicles to start again
What is the excitation-contraction coupling process?
Wave of depolarisation along sarcolemma as sodium channels open and sodium enters
Travels down T-tubules (invaginations of sarcolemma)
Opens voltage gated channels (L-type calcium channel) on DHPR
Voltage gated channel changes on depolarisation and opens RyR receptor
Calcium moves into sarcoplasm
Calcium binds to troponin C to initiate muscle contraction
What kind of molecule is RyR?
Ligand gated calcium channel
How do you switch off excitation-contraction coupling?
SERCA pumps calcium into SR using ATP
Calcium gets ‘mopped up’ by calsequetrin which overcomes repulsion between charges to hold calcium together
How are muscles stimulated to contract?
Calcium binds to troponin C
ATP broken into ADP and Pi which bind to myosin head
Myosin head is ‘cocked’
Calcium binding to TnC switches on thin filament
Conformational change to thin filament - Tm moves, switches filament on
Myosin croc-bridges form with binding site on actin
Conformational change occurs in myosin head to create power stroke
Releases ADP and Pi
ATP binds to myosin ATPase
Cross bridge (myosin head) detaches from actin
ATP hydrolysed to ADP and Pi - binds to myosin head
Myosin head is ‘re-cocked’
What happens to z-lines the muscles contract?
Move inwards
What happens to sarcomeres when muscles contract?
Shorten and thicken
What happens to A-bands and I-bands when muscles contract?
A-bands retain width
I-bands get thinner
How is ATP used in muscle contraction?
When bound to myosin, it is hydrolysed to ADP and Pi
Once bound, the ADP and Pi are released
Myosin has no ATP
ATP made in sarcoplasm and reattaches to myosin
How is energy supplied to the SERCA pump?
Creatine phosphate - immediate source of ATP and phosphate
Oxidative phosphorylation - main soured when O2 present
Glycolysis - main source when no O2
What are the two types of muscle fibres based on speed of contraction?
Slow fibres (type I) Fast fibres (type II)
What are slow oxidative fibres for?
Fatigue resistant, endurance
What are fast oxidative fibres for?
Sprinting, moderately fatigue resistant
What are fast glycolytic fibres used for?
Rapid powerful movements but fatigue quickly
What is a muscle spasm?
Sudden and involuntary transient muscle contraction
What is muscle cramp?
Involuntary, tetanic muscle contraction
What is muscle atrophy?
Wasting/decrease in size of muscle due to disease
What is muscle strain?
Muscle pain and inflammation caused by overuse
What is a muscle sprain?
Treating of ligaments around a joint due to excessive joint movement
Characteristics of slow oxidative fibres
Many mitochondria, dense capillary supply, high myoglobin content, thin, low myosin ATPase
Characteristics of fast glycolytic fibres
Few mitochondria, few capillaries, high glycogen content, large diameter, high myosin ATPase activity
Which stain is used to see muscles?
Myosin ATPase
