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Lesson 7.6
Remember a muscle cell is called a muscle fiber
Muscles have three layers of wrappings: 1. Endomysium – wraps an individual muscle fiber
2.Perimysium wraps many muscle fibers forming bundles called fascicles
3. Epimysium wraps the fasciles to form the muscle.
These wrappings taper past the belly and form the tendon.
Structure of muscle cell
1. Have many nuclei (skeletal)
2. Many mitochondria
3. Has endoplasmic reticulum called sarcoplasmic reticulum
Structure of muscle cell
1. Have many nuclei (skeletal)
2. Many mitochondria
3. Has endoplasmic reticulum called sarcoplasmic reticulum
4. Contains myofibrils which are the contracting unit of the cell
5. Has a Cell membrane
Myofibrils:Run the length of the cell
Each cell contains dozens to hundreds myofibrils
Composed of smaller units called sarcomeres
Muscle fiber/cell
Nucleus
Cell membrane
sarcomere
Sarcomere:Composed of overlapping protein fibers• myosin – a thick protein fiber•actin – a thin protein fiber
overlapping of actin and myosin makes muscle look striated
Thick Myosin protein filament
Thin Actin protein filament
How does a muscle contract
Nervous system sends a message by the motor neuron to the muscle fibers stimulating them to contract.
(Note: nerve and muscle do not touch each other.)
Neuromuscular junction – place where motor neuron and muscle fiber meet (but don’t touch).
Neurotransmitters – chemicals that carry a nerve impulse across the neuromuscular junction stimulating myofibrils to contract.
Note: A muscle fi ber is either contracted or
relaxed
This is known as the all-or-none principle of muscle
contracti on.
Muscle contraction control:
oOne motor neuron branches and controls a few to hundreds of muscle fibers.
oMotor neuron called a motor unit when branches and includes the muscle fibers it connects with.
Dexterity vs. strength
oDexterity - motor unit only branches a few times.
ex. Your eyes when you read or thread a needle
Dexterity vs. strengthoDexterity
oStrength - motor unit will branch stimulating hundreds of muscle fibers at the same time.
ex. Lifting a heavy box