Muscle Flashcards
What are the 3 types of muscle?
- Smooth - involuntary (ANS)
- Cardiac - involuntary (ANS and circulating
chemicals) - Skeletal - voluntary (SNS)
- attach to bones (tendons) —> movement
What are the 4 shapes of muscle fibres?
- Parallel —> straight
- Fusiform —> eye
- Triangular
- Pennate —> feather-like
- Unipennate
- Bipennate
- Multipennate
What is the structure of skeletal muscle? (5)
- Myofilaments
- Myofibril = bundle of myofilaments
- Myofibre = bundle of myofibrils (whole length)
- Fascicles = bundle of myofibres
- Muscle = bundle of fascicles
What are the 3 sheaths of connective tissue in skeletal muscles?
- Epimysium —> surrounds muscle
- Perimysium —> surrounds muscle fascicles
- Endomysium —> surrounds muscle fibre
What is the structure of a myofibre?
- bundle of myofibrils
- network of sarcoplasmic reticulum
- T-tubules tunnel through centre
- sarcoplasm (cytoplasm) —> myoglobin
—> mitochondria - sarcolemma (plasma membrane) —> capillaries
What is the structure of a myofibril?
Overlapping sarcomeres
- 1-2 μm diameter
- striated —> light and dark bands
What is the structure of a myofilament?
Overlapping arrangement of thick (myosin) and thin (actin) filaments
H-zone = only myosin
A-band = all myosin —> dark
M-line = myosin line
I-band = only actin —> light
Z-disc = actin line
- sarcomere = Z to Z
Why do muscles have a striated appearance?
Light —> I-band
- actin = thin filament
Dark —> A-band
- myosin = thick filament
What are the 2 proteins in muscle?
- Myosin - two globular heads
- single tail of 2 α-helices
- 1 filament = hundreds of tails - Actin - molecules twisted into helix
- surrounded by tropomyosin strands with
troponin complexes
What is the structure of myosin?
- 2 globular heads
- tail of 2 α-helices
- 1 filament = hundreds of tails
What is the structure of actin?
- Molecules twisted into helix
- Surrounded by tropomyosin strands with troponin
complexes
What is the sliding filament theory?
Muscle contraction:
- I-band shortens
- A-band same
- H-zone narrows/disappears
—> muscle fibre shortens
What are the 3 stages of muscle contraction?
- Initiation
- Activation
- Excitation contraction coupling
What are the 5 steps of muscle contraction initiation?
- Pre-synaptic neurone —> action potential opens
VGCCs - Ca2+ enters pre-synaptic terminal —> exocytosis of
NT vesicles - ACh diffuses across cleft —> binds to muscle ACh
receptors - Action potential induced in muscle —> AP spreads
along muscle fibre membrane surface - AP stops when ACh broken down by ACh esterase
What are the 4 steps of muscle contraction activation?
- Action potential to T-tubules —> sensed by DHP
(dihydropyridine) receptors on T-tubule membrane - DHP receptor shape change —> attached ryanodine
receptor on sarcoplasmic reticulum opens - Ca2+ out sarcoplasmic reticulum via ryanodine
receptor —> Ca2+ released into filaments - Sarcoplasmic reticulum continuously supplied with
Ca2+ via active transport (ATP-driven pump)
What are the steps of muscle excitation contraction coupling?
- Ca2+ binds to troponin —> moves tropomyosin —>
myosin binding sites on actin exposed - ATP in myosin head hydrolysed —> ‘charges’
- Myosin head binds to actin binding site
(crossbridge) —> ADP + Pi released - Myosin head pivots (power stroke) —> pulls actin
filament to centre of sarcomere (M line) - New ATP binds —> actin released —> ready to
repeat with next actin
What are the 2 types of motor neurones?
- Upper —> brain
- Lower —> brainstem
—> spinal cord
What are motor units?
Motor neurone + all muscle fibres in innervates (600)
- In humans - 420,000 motor neurones
- 250,000,000 skeletal muscle fibres
What are the 3 types of motor unit?
- Slow (S, type I) - low force, fatigue resistance
- Fast, fatigue resistant (FR, type IIA) - medium force
- Fast, fatiguable (FF, type IIB) - high force
- depend on nerve innovating fibre
What are the 4 differences between the properties of slow vs fast twitch motor units?
Slow:
1. Cell bodies —> smaller diameter
2. Smaller dendritic trees
3. Thinner axons
4. Slower conduction velocity
What are the characteristics of S muscle fibres? (4)
- High myoglobin
- Red
- High aerobic capacity
- Low anaerobic capacity
What are the characteristics of FR muscle fibres? (4)
- High myoglobin
- Pink
- Medium aerobic capacity
- High anaerobic capacity
What are the characteristics of FF muscle fibres? (4)
- Low myoglobin
- White
- Low aerobic capacity
- High anaerobic capacity
What are the 2 graphs used to explain the differences between S, FR and FF muscle?
- Twitch: force (g) v time
- Rate of fatigue: contraction (%) v time
What are the 2 mechanisms used by the brain to regulate muscle force?
- Recruitment = number of motor neurones —>
number of muscle fibres contracting- size principle —> smaller (slow) twitch units first
- more force needed —> more units recruited
(allows for fine control with low force)
- Rate coding = frequency of contraction of each unit
- slow units —> lower frequency
- frequency (firing rate) inc —> force from unit inc
- summation —> frequency so fast —> no break
between APs —> no muscle relaxation
What are neurotrophic factors?
Growth factors for motor neurones
- prevent neuronal death —> promote neurone
growth after injury
What are the 3 types of muscle contraction?
- Isometric —> hold still
- Concentric —> muscle shortens
- Eccentric —> muscle extends
What are the 3 examples of motor units changing type?
- Training : IIB —> IIA
- Severe deconditioning: I —> II
- Spinal cord injury: I —> II
How does aging affect muscles?
Muscle loss
- lose more type II —> larger proportion of type I —>
slower contraction times
