Biology Lesson7.6

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