NOTES: The Muscular System (Ch 8, part 2) Structure of a Skeletal Muscle Fiber

Remember, the muscular system consists of three types of muscle tissue: Smooth Cardiac Skeletal *We’ll be focusing on skeletal muscle*

STRUCTURE OF A SKELETAL MUSCLE: *Individual muscles are the organs of the muscular system. They contain skeletal muscle tissue, nervous tissue, blood, and other connective tissues.

Connective Tissue “Covering”: • FASCIA: -connective tissue that covers skeletal muscles -Separates individual skeletal muscles from adjacent muscles -Hold muscles in position -Projects beyond muscle to form the TENDON SHEATH -Fibers in tendon may intertwine with fibers in a bone’s periosteum, attaching muscle to bone!

CONNECTIVE TISSUES IN THE MUSCLE IN MORE DETAIL: -Attach muscles to bones or to other muscles -A network of connective tissue extends throughout the muscular system -Includes these tissues which all together form the tendon: ● Epimysium: closely surrounds a skeletal muscle (directly under the fascia) ● Perimysium: extends inward from the epimysium and separates the muscle tissue into small compartments called FASCICLES ● Endomysium: surrounds each individual muscle fiber within a fascicle & insulates them from each other

It’s demo time!

Sarcolemma- plasma membrane of a muscle fiber cell Skeletal muscle fibers have the same structure as most normal cells, although some structures have different names Sarcolemma- plasma membrane of a muscle fiber cell Sarcoplasm- cytoplasm Sarcoplasmic Reticulum- a network of membrane tubules and sacs Similar to the endoplasmic reticulum of other cells Continually pumps calcium ions from the sarcoplasm and stores it in sacs Same but different in that: Contain a lot of Mitochondria per fiber (cell) Have several nuclei per one fiber (cell)  multinucleated

Nucleus

Skeletal muscles also have some different structures than in normal cells Myofibrils- make up skeletal muscle fibers Myofilaments- fine fibers that make-up myofibrils Two types of myofilaments: thick and thin fibers Sarcomere- a segment of the myofibril between two Z lines. Also the smallest functioning contractile unit in a muscle Transverse tubules (T- tubules)- allows impulses to travel along the sarcolemma and deeper into the cell

Also part of a muscle fiber (muscle cell)… **TRANSVERSE TUBULES (T tubules) extend inward from the cell membrane and associate with the SARCOPLASMIC RETICULUM (whose membranes surround each myofibril)

Now that we’ve discussed the Anatomy of a Muscle, we need to examine the junction between a nerve and a skeletal muscle. This junction is what “tells” the muscle to contract.

Neuromuscular Junction: MOTOR NEURONS stimulate muscle fibers to contract  In response to a nerve impulse, the end of a motor neuron axon secretes a NEUROTRANSMITTER, which stimulates the muscle fiber to contract

Neuromuscular Junction: One MOTOR NEURON and the MUSCLE FIBERS associated with it constitute a MOTOR UNIT  All muscle fibers of a motor unit contract together!

Excitation and Contraction A motor neuron forms a synapse with a muscle fiber Synapse means “chemical junction” One motor neuron may stimulate many muscle fibers 1) A nerve impulse reaches the end of the motor neuron 2) Synaptic vesicles in the nerve release the neurotransmitter acetylcholine (ACh) 3) ACh diffuses across the synaptic cleft and binds to ACh receptors on the motor endplate of the muscle (part of sarcolemma in contact with neuron) ANIMATION

4) The stimulation of the ACh receptors causes a new impulse to be propagated along the sarcolemma (plasma membrane) and down T-tubules 5) This impulse causes calcium to be released from the sarcoplasmic reticulum 6) The release of calcium starts the sliding filament theory (the contraction of the muscle!) ANIMATION

Another animation!