Week 4 - Muscle Structure Flashcards
How many skeletal muscles make up the human body
Human body contains up over 600 skeletal muscles. Which makes up 40-50% of body mass
Functions of skeletal muscle
- Force production for locomotion and breathing
- Force production for postural support
- Heat production during cold stress
- Acts as an endocrine organ
What are the muscle actions?
Flexors (decrease angle at the joint)
Extensors (increase angle at the joint)
Attached to bones by tendons (origin end(fixed), insertion end moves))
Connective tissue surrounding skeletal muscle
Epimysium - surrounds entire muscle
Perimysium - surrounds fascicles
Endomysium - surrounds muscle fibers
basement membrane - just below endomysium
Sarcolemma - muscle cell membrane
What are myofibrils
Contain contractile proteins, actin - thin filament, myosin - thick filament
What does the sarcomere have
Includes Z line, M line, H zone, A band and I band
What the role of sarcoplasmic reticulum
(ON SHEET)
Storage cites for calcium
Terminal cistercae
Whats the role of transverse tubules
(ON SHEET)
Extend from sarcolemma to sarcoplasmic reticulum
Whats the role of satelite cells
(ON SHEET)
Play a key role in muscle repair. During muscle growth satelite cells increase the number of nuclei in mature muscel fibres
What is the myonuclear domain
(ON SHEET)
Volume of sarcoplasm surrounding each nucleus. Each nucleus can support a limited myonuclear domain.
More myonuclei allows for greater protein synthesis
Whats the impact of increased or decreased myonuclei
(ON SHEET)
Increased myonuclei -> muscle hypertrophy
Decreased myonuclei -> muscle atrophy
What is the neuromuscular junction
Junction between motor neuron and muscle fiber
Whats the motor end plate
Pocket formed around motor neuron by sarcolemma
Whats the neuromuscular cleft
Short gap between neuron and muscle fiber
What is acetylcholine (ACh)
(ON SHEET)
It is a neurotransmitter released from the motor neuron.
Causes an end plate potential (EPP)
Depolarization of muscle fiber
Signal for muscle contraction
What is the sliding filament model
(ON SHEET)
It is a model of muscle contraction. Muscle shortening occurs due to the movement of the actin filament over the myosin filament.
Reduction in the distance between Z lines of the sarcomere
What is the cross bridge formation in the sliding filament model
(ON SHEET)
Actin and myosin form to create a power stroke
What is muscle fatigue
A decline in muscle power output
What leads to a decline in muscle power output
Decrease in muscle force production at the cross-bridge level
Decrease in shortening velocity
Why are the causes of fatigue multifactorial
The cause of muscle fatigue depends on the exercise intensity that produce fatigue
What are the steps of excitation-contraction coupling
(ON SHEET)
- Signal from motor nerve fiber 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 depolarization.
- Depolarization opens calcium ion channels from sarcoplasmic reticulum and terminal cisterna of sarcoplasmic reticulum
- Calcium ions 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.
6-8. Energized 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 fiber is repolarised.
- Calcium is pumped from the cytosol into the sarcoplasmic reticulum
What are the main characteristics of moderate intensity exercise
Exercise intensity relative to lactate threshold - < lactate threshold
% maximal heart rate - 50 to 75%
%Vo2 max - <60%
Subjet perception of exercise intensity - light to somewhat hard
What are the main characteristics of heavy intensity exercise
Exercise intensity relative to lactate threshold - >lactate threshold
%maximal HR - 76 to 85%
%Vo2 max - 60 to 75%
Subject perception of exercise intensity - Hard
What are the main characteristics of very heavy intensity exercise
Exercise intensity relative to lactate threshold - >lactate threshold
%maximla heart rate - 86 to 100%
% Vo2 max - 76 to 100%
Subject perception of exercise intensity - Very hard
What are the main characteristics of severe intensity exercise
Exercise intensity relative to lactate threshold- > lactate threshold
%maximal heart rate - 100%
%Vo2 max >100%
Subject perception of exercise intensity - all out exercise
What are some of the possible causes of fatigue due to heavy intensity exercise
(ON SHEET)
Decreased Ca2+ release from sarcoplasmic reticulum
Accumulation of metabolites that inhibit myofilament sensitivity to Ca2+
What are some of the possible causes of fatigue due to moderate intensity exercise
(ON SHEET)
Increased radical production
Glycogen depletion
What often causes EAMS
Likely caused by hyperactive motor neurons in spinal cord
High intensity exercise can alter muscle spindle and golgi tendon organ function
What are some strategies that can be used to alleviate EAMS
Passive stretching often relieves this type of muscle cramp
Possible that activating ion channels in mouth/throat could send inhibitory signals to spinal cord, and thus inhibit overactive motor neurons
What are the 3 main characteristics important to muscle fiber type function
Oxidative capacity
Type of myosin isoform expressed
Abundance of contractile protein within the fibre
What are some of the main characteristics of type 2x fast fibers
Number of mitochondria - Low
Resistance to fatigue - Low
Predominant energy system - Anaerobic
ATPase activity - Highest
Vmax - Highest
Efficiency - Low
Specific Tension - High
What are the key characteristics of Type 2a fast fibers
Number of mitochondria - High/moderate
Resistance to fatigue - High/moderate
Predominant energy system - Combination
ATPase activity - High
Vmax - High
Efficiency - Moderate
Specific Tension - High
What are the key characteristics of type 1 slow fibers
Number of mitochondria - High
Resistance to fatigue - High
Predominant energy system - Aerobic
ATPase activity - Low
Vmax - Low
Efficiency - High
Specific Tension - Moderate
How is the shortening speed different between fiber types
Speed of shortening is greater in fast fibres
SR releases CA2+ at faster rate
Higher ATPase activity
Whats the effects of motor unit amount and speed for force productio
More motor units = greater force
fast motor units = greater force
How are muscle fibres innervated
Eahc mature fibre is innervated by a single axon
What are the characteristics of motoneurons that supply larger faster motor units
Larger cell bodies
Larger diameter axons
Greater number of axonal branches
Sparse afferent innervation
More complex and extensive motor end plate of neuromuscular junction
What is the Henneman Size Principle
Consistent pattern of recruitment. small = low force, large = high force
Progressive recruitment of units, force increased in a step-wise manner
Whats the size principle of motor units
Slow motor units have easily excited motor neurones
Fast motor units have higher threshold motor neurones - harder to excite
What is 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 fibers
Maximum velocity of shortening is greatest at the lowest force
What is the muscle force-power relationship
At any given velocity of movement, the
peak power generated is greater in a
muscle with a higher percentage of fast-
twitch fibers.
Aging and muscle loss on muscle function
Age-related muscle loss-sarcopenia
10% muscle mass lost between age 25 to 50 years
Additional 40% lost between age 50 to 80 years
Also a loss of fast fibers and gain in slow fibers
resistance training can delay age-related muscle loss
Diabetes and muscle function
Disease related muscle loss-cachexia
Associated with progressive loss of muscle mass
Adds to the age-related loss of muscle mass
Aerobic and resistance training are protective
Cancer and muscle loss
50% of cancer patietns suffer cachexia
Results in weakness, accounts for 20% deaths in cancer patients
Regular exercise and nutrition therapy may counteract cachexia
Muscular dystrophy and muscle function
Hereditary defects in muscle protein
Results in loss of muscle fibers and weakness
Duchene muscular dystrophy is most common in childhood
