Muscle L2: Life cycle of the musculotendinous unit Flashcards
Embryonic development of skeletal muscle
- 0 to 8 weeks = embryo: initial development of each organ system 3-8 wks period of greatest sensitivity
- Week 3: trilaminar germ disc craniocaudal axis bilateral symmetry
- 9 to 38 weeks = fetus: period of functional maturation
Embryonic development of skeletal muscle: 0-8 weeks
0 to 8 weeks = embryo: initial development of each organ system 3-8 wks period of greatest sensitivity
Embryonic development of skeletal muscle: 3 weeks
Week 3: trilaminar germ disc craniocaudal axis bilateral symmetry
Embryonic development of skeletal muscle: 9-38 weeks
9 to 38 weeks = fetus: period of functional maturation
What is neurulation?
Development of the neural tube and neural crest cells
What week have somites been develop by?
They are developed by week 3
What do somites form?
Adjacent to neural tube, go on to form most of the vertebral column, skeletal muscle and dermis corresponding to spinal cord segments
______ forms from mesoderm
Notochord
What are somites?
- Division of the mesoderm
- Develops either side of the notochord, and developing neural tube
- Develops into the different parts of the musculoskeletal system
- Bilaterally paired
The somite differentiates to form ______tome (ultimately skin cells), _____tome (ultimately skeletal muscle cells) and _____tome (ultimately connective tissue / bone cells).
dermatome; myotome; sclerotomes
Cells of the myotome differentiate into _____ (muscle forming cells).
myoblasts
Myoblasts elongate and aggregate into ______.
bundles
Fuse longitudinally and form the multinucleated fibres (________).
myotubes
Satiated by _____ month of development
3rd
What is maturation?
The process of progression towards a mature state
As muscles grow in length during maturation, the number of sarcomeres _______, which occurs along length of muscle cell
increases
After birth (or by the end of the first year) muscle growth (cross sectional area) is due to ________ (increase in volume by enlargement of its cells, generally more myofibrils – not more cells)
hypertrophy
Increase in size of skeletal muscle is primarily through a growth in size of its _______ cells – ie net positive protein synthesis (more synthesis than breakdown)
component
As muscles grow in length during maturation, the number of sarcomeres _______.
increases
However, satellite cells (stem cells) that sit within ________ can differentiate into ______.
endomysium; myoblasts
_______ fuse to other muscle cells during growth and repair throughout the life-span
Myoblasts
So satellite cells are also important for _______ during maturation
hypertrophy
During muscle growth satellite cells (green) give rise to _______ muscle precursor cells, myoblasts (light pink). Myoblasts fuse with one-another to generate nascent ______.
mono-nucleated; myotubes
During later steps of development, myoblasts fuse with existing myotubes promoting ______.
muscle growth
Upon injury, satellite cells are activated and _____ divide generating a new pool of myoblasts. These myoblasts once again fuse with each other and to injured myotubes to promote _______.
asymmetrically; muscle regeneration
At birth, muscle strength is considered muscle _______.
tone
Clinically, muscle tone is described as…
General characteristics of a muscle
What is muscle tone?
“A slight constant tension of healthy muscles”
- Often related to muscle mechanical properties (e.g. muscle stiffness), and the muscles response to stimuli (e.g. internal or external perturbations).
Muscle tone is a product of both _____ (with input from the nervous system), and _____ (passive structures within the musculotendinous tissue), components
neural; non-neural
What are neural components of “muscle tone”?
- Reflex contraction – proprioceptive inputs Level of arousal Heightened central drive
- All relate to sum of excitatory and inhibitory input to the motor neuron pool
- With prolonged high excitatory drive to muscles (spasticity) > changes in connective tissue ECM > non neurally driven increases in muscle tone also (contractures).
What are non-neural components of “muscle tone”?
- A degree of muscle tone remains without muscle activation by neural contributors. Generated by inherent viscoelastic characteristic of the musculotendinous unit.
- Sarcomeres (i.e. properties of the proteins within the muscle)
What is dystrophin?
Dystrophin is protein located in the intercellular surface of muscle fibres.
What is the function of dystrophin?
Stabilises muscle fibres during contraction and relaxation by binding to other proteins within the ECM.
What is the function of titin?
Titin links myosin filaments to the z-disc within each sarcomere, and facilitates the return of the myosin filament to its initial position after stretch.
Contributes to passive force in muscle.
Connective tissue (i.e. epimysium, perimysium and endomysium) bundle sarcomeres, and form ______.
ligaments/tendons
Abnormal collagen formation (low tone) causes ______.
Down Syndrome
_________ – the thickening of connective tissue in CP > high tone.
Fibrosis
- Once developed, these changes are essentially irreversible
What is the Modified Ashworth scale (MAS)?
measures resistance during passive soft-tissue stretching and is used as a simple measure of spasticity.
________ is one of the most common features observed in children with motor disorders ~ 2-6% of the population
Atypical muscle tone
What is hypertonia? Give a clinical example of this?
- e.g. Cerebral palsy (neuromuscular condition), damage to the brain at or around birth.
- Altered neural drive to muscles.
- Higher drive.
- Associated with pain & impedes participation in everyday activities.
- Secondary alterations in muscle structure during development
- Increased collagen content in the connective tissue matrix the surrounds muscle fibres
- Long term contracture > muscle shortening > reduced joint range of motion > bone deformities tibial torsion and excessive femoral neck anteversion
- Reduced muscle volume during development (may be due to decreased loading/disuse/altered drive).
- Some children with CP also have hypotonia/low muscle tone in regions.
What is hypotonia? Give a clinical example of this?
- e.g. some genetic syndromes – Duchenne muscular dystrophy (1/3500 boys)
- Very little or no dystrophin protein is produced
- Dystrophin anchors actin to the muscle cell wall and ECM. No anchor – little force can be produced
- Muscle weakness, fatigue and wasting, muscle shortening
- Children with MD often develop enlarged calf muscles (called calf pseudohypertrophy) as muscle tissue is destroyed and replaced by fat
From birth, is considered muscle ____. From 2 years, is considered muscle ______.
tone; strength/volume
What is the process of a human muscle from infancy to adulthood?
Muscle volume _____(increases/decreases) with age.
Increases
- Steeper growth for boys than girls
What are 3 features of measure architecture?
- fibre length
- muscle volume and physiological cross sectional area
- pennation angle
What are 6 physiological characteristics that are important to produce force?
- Muscle architecture: fibre length, muscle volume and physiological cross sectional area, pennation angle
- Sarcomere length (overlap of actin and myosin)
- Fibre types (e.g % type 1-slow twitch; type II-fast twitch)
- Type of contraction: isometric, concentric, eccentric
- Number/discharge rate of active motor units: optimising neural drive
- Passive force (elastic components / connective tissue)
What are 2 factors that are influenced by physical activity?
- muscle volume and physiological cross sectional area
- Number/discharge rate of active motor units: optimising neural drive
What physiological factors that influence force are likely to be different between boys and girls in the tasks previously shown?
- Physical activity difference between girls and boys (8-10yrs)
- Children with same chronological age present with large variation in growth and development
What are some features of the secular trend for growth and strength in children and adolescents?
- Rise in height
- In Canada and US, there has been a dramatic rise in body mass during the last 40-50 years.
- Generally, there is a positive correlation between body mass/height and grip strength.
- There has been no increase in male grip strength during the past 40-50 years
- Increase height with increased age …. PHV earlier in girls
- 14yr boys
- 12 yr girls
- PHV- peak height velocity
What is the peak height velocity?
the period where maximum rate of growth occurs during adolescence.
What are differences in peak height velocity between girls and boys?
- Very little increase in strength or endurance after PHV reached in girls.
- Measures also begin to plateau in boys after PHV.
- Not all about PHV and hormones – physical activity in adolescence also important …
