Week 8 - Skeletal Muscle Flashcards
Summaries structure of skeletal muscle large to small
Muscle belly
Epimysium (deep fascia)
Perimysium
Fasciculus
Endomysium (between fibers)
Single muscle fiber
Sarcolemma
Nuclei
Sarcoplasm
Myofibril
Describe structure of muscle fiber
Lots of myofibrils bound together with mitochondria in sarcolemma. Contains multiple nuclei. Has Light I bands and Dark A bands.
Describe the structure of a single a single sarcomere
Each myofibril is composed of thousands of sarcomeres joined in series and parallel to one another, they are the functional units of a muscle.
They have:
H Zone - Only Myosin
M line - Middle of myosin
Z line - Middle of actin
I band - Only actin
A band - all of myosin, may have some actin
Describe the structure of actin (thin filament)
Subunits in double helical strands.
Tropomyosin interacts with actin and covers binding sites where thick filament can bind. Very important.
Describe the structure of Myosin (thick filament)
Around 300 myosin molecules per thick filament head
Two sub units:
S1 - Globular head
S2 - Flexible region and tail
What is the sliding filament mechanism?
During contraction filaments slide past each other, each of the two filaments remain relatively unchanged in length despite changes in gross muscle length.
Proposed by Huxley in 1954
Describe the sarcoplasmic reticulum
Interconnecting tubules surrounding myofibrils
regulates intracellular levels of calcium
Stores calcium and releases on stimulation to allow contraction.
Describe the steps of the cross-bridge cycle
- AP arrives
- Calcium Ions released from SR
- Ca attaches to troponin
- Tropomyosin moves, uncovering binding sites
- Hydrolysis of ATP changes angle of the myosin head. CROSS-BRIDGE FORMED.
- Phosphate group released from myosin head chases angle of myosin head. Called a POWERSTROKE.
- Myosin head picks up another ATP and bond with actin is released.
- Repeats until Ca or ATP levels drop.
CYCLE REPEATS
Describe Length - Tension relationship
Filaments overlapped, not able to generate tension
Muscle stretches, no longer butting against Z line, able to produce tension to an optimal point.
If the fibre is lengthened further, less overlap between myosin and actin means less opportunity to develop tension
Describe the force-velocity relationship
Force during shortening < isometric force: Faster the movements the less time myosin heads have to attach to binding site.
Force during lengthening > isometric force: Compliant portion of myosin stretched further than during isometric force, forcible detachment of myosin heads with stretch.
Describe the three fiber types
Type 1 - Slow twitch oxidative, Myosin Heavy chain 1 (MHC), Slow mATPase activity, small cross sectional area, low max force, slow max shortening velocity, weak max power, red, 4.2 capillaries/fibre, fat energy source, oxidative, high mitochondrial density, high endurance capacity.
Type 2A - Fa
Describe the structure of a motor neuron
Motor neuron - nerve cell
Motor unit - a single motor neuron and all the fibers it innervates
Gross movements 2-3000 fibers per motor unit.
Fine movements 2 or 3 fibers per motor units.
Describe the structure of the neuromuscular junction
where motor neuron meets the muscle fiber
Motor end plate: pocket formed around motor neuron by sarcolemma.
Synaptic cleft: short gap between neuron and motor end plate.
Acetylcholine is released from the motor neuron resulting in depolarisation of motor end plate (muscle fiber).
Describe efferent and afferent neurons
Efferent - Neurons that send impulses from CNS to limbs and organs
Afferent - Neurons that carry impulses from sensory receptors or sense organs toward CNS.
What are the purpose of golgi tendon organs?
Senses tension in the tendon when muscle contracts. Has an inhibitive afferent neuron, When excessivley large forces are generated, feedback from GTO causes activation of muscles to decrease.
