Muscle anatomy and function Flashcards
Myofibril
A cylindrical organelle running the length of the muscle fibre, containing actin and myosin filaments.
Sarcomere
The contractile unit of the myofibril, divided into I, A and H bands.
What proteins does the myofibril contain?
Myosin
Actin
Actin
A thin, contractile protein filament found in muscle cells.
- They overlap the myosin filaments
- Actin and myosin together form sarcomeres (contractile unit of skeletal muscle)
Myosin
A thick contractile protein filament with protrusions known as myosin heads.
- Myosin and actin form sarcomeres (contractile unit of skeletal muscle)
What band does myosin appear on?
- It appears as a dark band
- ‘H’ band
(H zone disappears in the sarcomeres contracted state)
What band does actin appear on?
- It appears as a light band
- ‘I’ band
What is it at the end of each sarcomere?
Z line- actin is attatched to the z line
Sarcomeres are joined together lengthways at the z line.
What region is the middle of the sarcomere?
M line
What does the A band refer to?
The dark A band identifies where both actin and myosin exist
What is the cell membrane of muscle cells called?
Sarcolemma
What is the cytoplasm of muscle cells called?
Sarcoplasm
How are the transverse (T) tubules created and what do they do?
- Sarcolemma folds into the sarcoplasm
- Help to spread electrical impulses throughout the cell by passing message to sarcoplasmic reticulum to release calcium ions.
Sarcoplasmic reticulum
- They store calcium ions which causes contraction
- Each myofibril is enveloped in it
What is skeletal muscle fibre also known as?
Striated
How many actin filaments surround 1 myosin filament?
6
What is the name of the nerve that stimulates a muscle to contract by carrying nerve impulses?
Motor neurone which controls a number of muscle fibres
What is sliding filament theory?
- It explains the sequence of events leading up to skeletal muscle contraction.
What happens during muscle contraction?
The thin actin filaments slide inwards over the thicker myosin filaments causing a shortening of each sarcomere along the entire length of the myofibril.
What happens to the H zone during the sarcomeres contracted state?
It disappears
Whats happening at rest with the protein tropomysosin?
The protein tropomyosin (on the thin filament- actin) blocks the binding site as it covers the actin-myosin binding site and it is held in place by troponin.
What happens when the muscle fibre relaxes?
The myofilaments slide away from eachother and move further apart lengthening the sarcomere.
What is the region on actin where myosin binds called?
Actin-myosin binding site
Sliding filament theory (stages)
- Electrical impulse reaches the muscle fibre
- Calcium ions released from sarcoplasmic reticulum
- Calcium binds to troponin and tropomyosin uncovers it’s myosin binding sites on the actin filament.
- Cross bridge formation (myosin attatches to actin)
- ATP energy is released
- Cross bridge ‘rachet’ (actin is pulled over the myosin so the muscle shortens)
- Stimulation stops, calcium ions are reabsorbed into the sarcoplasmic reticulum causing relaxation of the muscle.
What is a motor unit?
A group of fibres and it’s motor neurone
What is the motor end plate?
The point at which the nerve reaches the muscle fibre
(transfer impulse from motor neurone to muscle fibre)
Components of a motor neurone?
- Dendrites
- Axon
- Nodes of ranvier
- Myelin sheath
Dendrites
Receives electrical impulse
Axon
Transmits the electrical impulse
Nodes of ranvier
Allows impulses to jump so increases their speed
Myelin sheath
Insulates and so speeds up the conduction
What is the nerve action potential?
- Following an impulse derived from the brain/CNS an impulse will travel along the motor neurone where the dendrites receives the impulse.
- Impulse will travel along the axon where it’s insulated by a myelin sheath and has nodes of ranvier which will speed up delivery of the impulse until it arrives at the motor end plate.
Motor neurone activity (stages)
- A nerve action potential/ impulse arrives at a muscle fibre travelling down a motor neurone and reaches the motor end plate.
- As the NAP reaches the motor end plate a neuro-chemical transmitter known as acetyl choline (Ach) transports the signal over the synpase where the receptor cells on the sarcolemma recives it.
- It is now known as muscle action potential as the impulse is now located in the myofibril.
- Spread of muscle action potential via the T tubules
- Muscle contraction
What is the ‘all or none law’?
If a motor impulse is of sufficient intensity the motor unit is stimulated and all the muscle fibres within it will contract to their maximum possible extent.
It essentially states that the individual muscle fibres within a motor unit will contract fully or not at all
In order to activate the muscle fibres a minimum amount of stimulation is needed (threshold)
If a stimulation equals or exceeds the threshold all the fibres within the motor unit will contract
What is total force output affected by?
The number of motor units recruited.
What is unit summation?
The bodies recruitment of motor units depends on how much total force it wishes to create.
(increased by recruiting more motor units)
How does the muscle vary it’s strength production?
The strength of the muscle contraction can be altered by recruiting more or less motor units.
Maximal contractions will recruit all motor units within a muscle but weaker contractions will only recruit a few motor units.
What is wave summation?
The ability to vary the total force of contraction within the motor unit by varying the speed and frequency of nerve impulses.
How can the force of contraction increase (wave summation)?
- Force of contraction increases if a second impulse arrives before the first twitch has had time to relax so force is likely to escalate as it builds on top of the first twitch before it’s had time to relax.
- Closer the impulses are the greater the wave summation
- Frequency with which the impulse arrives at the muscle fibre affects the contraction force.
What is spatial summation?
During sub-maximal, contraction, units will work together to share the force production across the muscle so reduce fatigue.
- It does it by activating different units at different times.
What are the types of skeletal muscle fibres?
- Type 1 slow twitch
- Type 2 fast twitch
- Type 2a (fast twitch)- fast oxidative glycolytic
- Type 2x (fast twitch)- fast twitch glycolytic fibres
Slow twitch fibres
- Contract more slowly
- Highly resistant to fatigue
- Favoured by endurance athletes
- More aerobic
- Type of muscle fibre that uses oxygen to produce energy and there associated with endurance based activities
Fast twitch fibres
- Contract more rapidly
- Generating greater forces
- More liable to fatigue
- More prevalent in sprinters and power athletes.
- More anaerobic
Characteristics of slow twitch fibres (type 1)
- Slow speed of contraction (contract at 20% speed of the fast)
- Very resistant to fatigue
- High aerobic capacity
- low anaerobic capacity
- High mitochondrial density
- High capillary density
- High myoglobin content
E.g. marathon
Characteristics of type 2a (fast oxidative glycolytic)
- Fast speed of contraction
- High anaerobic capacity
- More resistant to fatigue rather than type 2b
- Moderate mitochondrial density
- Moderate capillary density
- High pc store
- moderate aerobic capacity
E.g. 1500m
Characteristics of type 2b (fast twitch glycolytic fibres)
- Fast speed of contraction
- High anaerobic capacity
- Lower resistant to fatigue compared to type 2a
- Low mitochondrial density
- High pc store
- low capillary density
- low aerobic capacity
E.g shot putt
Muscle spindle
- Detects muscle movement (proprioceptor) - muscle spindle apparatus
- When it lengthens it causes contraction of a few fibres in the centre of the muscle (intrafusal fibres)
- Called a stretch reflex (in response to a lengthening in the muscle)
- An example would be in golf, where the back
swing stretches the triceps and deltoid immediately prior to their
concentric contraction.
What are the 3 phases of the stretch shortening cycle?
- Eccentric stage
- Amortization phase
- Concentric phase
Eccentric phase
- Pre-load
- Muscle spindle apparatus sends signal to brain as they have detected movement
Amortization phase
- Between eccentric and concentric phase
- Pause period
- Intrafusal muscles contract (stretch reflex)
Concentric phase
- Muscles shorten
What does a Z line mark?
The boundary of each sarcomere
