Week 4 - Skeletal Muscle Flashcards
Describe the functions of skeletal muscle
Force production for locomotion and breathing, postural support, heat production during cold stress, and acting as an endocrine organ
What are the types of connective tissue surrounding skeletal muscle
Epimysium (entire muscle)
Perimysium (fascicles)
Endomysium (muscle fibres)
Basement membrane (below endomysium)
Sarcolemma (muscle cell membrane)
Describe the microstructure of muscle fibres
Myofibrils (Actin & Myosin)
Sarcomere (Z line, M line, H zone, A band, I band)
Describe the role of satellite cells
Satellite cells play a key role in muscle growth and repair by increasing the number of nuclei in mature muscle fibres
Describe the event that occurs in the neuromuscular junction
- ACh released into the synaptic cleft
- ACh binds to nicotinic receptors on the motor endplate
- Cation-selective ion channels open in the motor endplate
- Local depolarisation of the motor endplate
- Depolarisation of muscle cell membrane
- Raised intracellular Ca2+ concentration
- Muscle contraction
What is the sliding filament model
Muscle shortening occurs due to the movement of actin filament over the myosin filament
Describe the contribution of energy in muscle contraction
Myosin ATPase breaks down ATP as fibre contracts.
Sources of ATP: PCr, Glycolysis, Oxidative phosphorylation
Describe the steps to Excitation-Contraction Coupling
- Signal from motor nerve fibre enters synaptic knob
- Synaptic vesicles release acetylcholine across synaptic cleft and to the Ach receptors on the sarcolemma of muscle fibre
- Release of Ach causes excitation of muscle fibre, which then travels to the T tubule and causes depolarisation
- Depolarisation opens calcium ion channels from sarcoplasmic reticulum and terminal cisterna of sarcoplasmic reticulum
- Calcium ion bind to troponin on actin molecule which causes a shift in the position of tropomyosin so that the myosin binding sites on actin are exposed. An ATP is used in this step
- Energised myosin cross-bridge binds to the active site on actin and pulls on the actin molecule to produce a back and forth movement
- Ach release stops, and the muscle fibre is repolarised
- Calcium is pumped from the cytosol into the sarcoplasmic reticulum
Define muscle fatigue
Fatigue = a decline in muscle power output
Occurs from decrease muscle force production and muscle shortening velocity
Describe the mechanisms of muscle fatigue in heavy exercise
Decreased calcium release from sarcoplasmic reticulum
Accumulation of metabolites (Pi, H+, free radicals) that inhibit myofilament sensitivity to calcium
Describe the mechanisms of muscle fatigue at moderate intensity
Increased radical production (modification of cross-bridge head)
Glycogen depletion
Accumulation of Pi + H+ does not contribute to fatigue during moderate intensity exercise
What causes exercise-associated muscle cramps
Hyperactive motor neurons in the spinal cord
High-intensity exercise can alter muscle spindle and golgi tendon organ function.
Increased excitatory activity of muscle spindles and reduced inhibitory effect of the golgi tendon organ
What are some strategies to alleviate EAMS
Passive stretching often relieves this type of muscle cramp
Possible that activating ion channels in the mouth could send inhibitory signals to the spinal cord, but inhibit overactive motor neurons
Define Isotonic, Isokinetic and Isometric
Isotonic: Muscle tension remains unchanged where muscle length decreases
Isokinetic: Muscle length decreases with constant velocity
Isometric: Muscle contracts but does not change in length
What are the 3 main types of muscle fibres
Type I (Slow)
Type IIa (Mixed)
Type IIx (Fast)
How are muscle fibres typed
Contractile properties
Muscle biopsy
Oxidative capacity
Immunohistochemical staining
Gel electrophoresis
What factors determine the speed of muscle contraction and relaxation
Muscle twitch
Contraction-calcium released from SR
Relaxation-reuptake of calcium into SR
Speed of shortening is greater in fast fibres
What factors determine the force regulation in muscle fibres
Number and types of motor units recruited
Muscle length
Firing rate of motor neurons
Contractile history of muscle
Define the term motor unit
Each mature muscle fibre is innervated by a single axon
What do motoneurons supplying larger faster motor units have
Larger cell bodies
Larger diameter axons
greater number of axonal branches
Sparse afferent innervation
More complex and extensive motor end plate of neuromuscular junction
Describe the muscle force-velocity relationship
At any absolute force exerted by the muscle, the speed of movement is greater in muscles with a higher percentage of fast-twitch fibres
At any given velocity of movement, the peak power generated is greater in a muscle with higher percentage of fast-twitch fibres
Maximum velocity of shortening is greatest at the lowest force
What occurs to strength and muscle power with aging
Strength is lost with age and appear greater for lower body
Muscle power lower in older group as it declines with age
What are the factors that affect declining muscle aging
A loss of muscle mass
Muscle quality is reduced (fat accumulation in muscle increases)
Neuromuscular alterations (Denervation and less motor units as age increases)
Define denervation and collateral reinnervation
Denervation: disconnect of muscle fibre to motor neuron
Collateral reinnervation: muscle fibre reconnects to an adjecent motor axon neuron
What are some excitation-coupling processes on muscle function other than older age
- Changes in excitation-contraction coupling (decrease release of calcum)
- Changes in sarcoplasmic reticulum function (decrease calcium uptake)
- Slowing of myosin molecule
- Reduce actomyosin cross bridges
Define Henneman Size principle
Consistent pattern of recruitment
Small motor units = low force
Large motor units = high force
Slow have easily excited motor neurons
Fast motor units have higher threshold (harder to excite)
Describe the crossbridge cycle
- ATP binds to myosin head.
- ATP hydrolysis into ADP + Pi but remain on myosin head.
- Pi dissociate from myosin; forms new cross bridge.
- Powerstroke occurs, shortening the muscle.
- ADP dissociates from myosin; awaits another ATP molecule
What are the muscle force velocity relationships in muscle fibres
- At any absolute force exerted by the muscle, the speed of the movement is greater in muscles with higher percentage of fast-twitch fibres
- Maximum velocity of shortening is greatest at the lowest force
- At any given velocity of movement, the peak power generated is greater in a muscle with a higher percentage of fast-twitch fibres
- The peak power increases with velocity up to movement speed of 200 to 300 degrees
What is needed for muscle contraction to continue, and what happens what it ceases
- Shortening continues as long as ATP is available and Ca2+ can bind onto troponin
- When neural activity ceases, Ca2+ is removed from SR by Ca2+, tropomyosin moves into active site and muscle relaxes
How does denervation and collateral re-innervation alter muscle characteristics
Increased co-expression of myosin isoforms
Decreased force output
Decreased velocity of contraction and thus power
Increases size of motor unit (less efficient)
Preserves from muscle mass but not all
Muscle characteristics of master athletes
Muscle deterioration not as severe but still present
