MSS: Muscle Structure and Adaptation Flashcards
List differences of fast and slow twitch muscle fibres.
Slow twitch fibres (type I myosin heavy chain):
- Fatigue resistant
- Moderate max force
- Oxidative metabolism - high myoglobin ‘red muscle’
- Many mitochondria
- Rich vascularization
- Small diameter
Fast twitch fibres (type II MHC):
- Fatigue rapidly
- High max force
- Glycolytic (and Oxidative) - low myoglobin
- Fewer mitochondria
- Sparser vascularization
- Larger diameter
Give examples of muscles where there are different fibre type compositions.
Lateral rectus (eye muscle) - Mainly type II. Eye movements are very intermittent and very fast/brief (saccades)
Gastrocnemius (powerful calf muscle) - mix of type I & II. Rely on this muscle for jumping, running. It needs to generate rapid force for running and jumping.
Soleus (calf) - Type I slow twitch. Muscles that support body weight tend to have a higher proportion of type I slow twitch fibres because they don’t fatigue as fast.
How does training affect fibre type composition?
In untrained individuals there is a roughly even mix of slow to fast fibres in most muscles ( varies from muscle to muscle). In trained athletes there is a shift.
- Long and middle distance runners: 60-70% slow twitch fibres - endurance athletics need muscles that can keep preforming at a given level for hours at a
time. Indicates slow fibres with their high oxidative metabolism capability. - Sprinters: 80% fast twitch fibres - want to develop max power as fast as possible and doesn’t have to be sustained for hours. So high proportion of fast twitch fibres which generate fast explosive contractions relying heavily of glycolysis and anaerobic metabolism.
Describe excitation-contraction coupling.
- Start with an action potential in a motor axon
- This triggers an action potential in a muscle fibre
- Action potential in a muscle fibre propagated along the sarcolemma, down into the muscle fibre via the sarcolemma along the t tubules
- This then triggers a pulse of calcium from the sarcoplasmic reticulum.
- Following calcium release there is the sliding filament mechanism activated
- This leads to sarcomere shortening and muscle fibre shortening - this can be recorded as a tension
Describe the motor unit.
- The motor unit consists of a single motor neuron and the set of muscle fibres within a muscle that it innervates
- Motor unit size ranges from around 10 to many 100s of muscle fibres
- A muscle may be innervated by 10s to 100s of motor neurones
- Size of motor neurone correlates with size of motor unit
- Muscle fibres of a motor unit are generally of the same type
Describe summation in fast and slow motor units
- Fast motor units need higher firing rates to generate tetanic forces than slow motor units
- Slow motor units are recruited first, followed by fast units for higher levels of force generation
- Slow motor units are smaller than fast motor units so recruited first
Describe skeletal muscle tone.
- Most muscles at rest exhibit low level of contractile activity sustained by low frequency firing in several of the motor units innovating that muscle
- Denervation (no action potential firing) leads to complete relaxation e.g. if you cut the nerve to the muscle
- So there is a resting level of tension known as muscle tone in most muscles even when they are not actively being used.
- This is driven by reflex arcs from muscle spindles (sectioning dorsal roots abolishes resting tone) which feed back to the spinal motor neurones and maintain a certain level of baseline activity.
How is smooth muscle tone different to skeletal muscle tone?
- Skeletal muscle tone is due to baseline firing activity in the motor units innervating that muscle.
- Smooth muscle depends on the level of depolarisation. Resting tone can be relaxed by hyper-polarisation of the smooth muscle, or increased by depolarisation of the smooth muscle.
What is myogenesis?
- Process of muscle formation
- Some mesodermal cells in the embryo are committed by local paracrine factors that induce expression of myogenic regulatory factors (MRFs) such as Myf5 and Myod for mesodermal cells to become myoblasts
- Myoblast proliferate under influence of growth factors (form ball of cells). Special signals cause myoblasts to stop dividing
- They exit the cell cycle and there is terminal differentiation via another MRF known as Myogenin
- Cells elongate and express structural muscle-specific proteins (actin and myosin). Myotubes form from the myoblasts.
- Myotubes align and fuse forming a syncytium becoming multinucleated muscle fibres by another MRF known as Mrf4.
- A portion of the cells undergoing myogenic commitment are held back. These are called Satellite cells (muscle stem cells) - these are important for regeneration and postnatal growth.
What is hypertrophy?
- After birth, there is an increase in muscle mass due to increase in fibre size (hypertrophy)
- Muscle Stem Cells called Satellite cells serve as a reservoir of undifferentiated muscle precursors, self renewing (can divide)
- Muscle fibre growth involves satellite cell proliferation and incorporation of satellite cell nuclei into muscle fibres. Increased nuclei allows increased protein synthesis and muscle fibre size (hypertrophy)
- Muscle fibres are multinucleated. Maintain cytoplasm: nuclei ratio - as more nuclei are added, more proteins are synthesised.
- So satellite cells can be active periodically e.g. during bouts of muscle growth or regeneration. Satellite cells return to quiescence when not needed (and can be stimulated e.g. resistance exercise stimulates satellite cell proliferation and hypertrophy)
What is hyperplasia?
- Increase in muscle mass due to formation of new muscle fibres
- Some evidence from animal models that hyperplasia does occur
- However uncertain if this does happen and uncertain how widely it happens - so main mechanism of muscle growth is probably hypertrophy
What is sarcopenia?
- Sarcopenia: reduction in muscle mass
- Normal part of ageing process (peak muscle mass around late 20s or early 30s and progressive reduction with age after this)
- Atrophy (degeneration) of muscle fibres (increase in muscle degradation over regeneration)
- May also be due to disease or immobilisation (hospitalisation etc), also a problem under zero gravity
- Associated with decreased activation of satellite cells to proliferate and generate new nuclei to enable myofibril hyperplasia) and recruitment
- Anabolic resistance - reduced protein synthesis in response to hormonal stimulation (e.g. insulin) or resistance exercise
How can you resist sarcopenia?
Resistance exercise and remaining active - requires load bearing exercise in a normal active life
Resistance exercise can be very important in slowing the process of sarcopenia in elderly people. Even people in their 90s can promote muscle hypertrophy.
There is a tendency for type I fibres to express type II phenotypes in ageing muscle.
