Muscle Tissue and Muscle Contraction

Skeletal Muscle Tissue

Skeletal Muscle cells

Long cells with multiple nuclei - the cells are as long as the muscle and are arranged in parallel

the cells are connected side by side with fibrous connective tissue

Organization of skeletal muscles

Epimysium – the dense irregular (fibrous) connective tissue arising from Tendons surrounding skeletal muscles

Fascicles – bundles of skeletal muscle cells within the skeletal muscle

Perimysium - the dense irregular (fibrous) connective tissue surrounding the fascicles

Endomysium - the irregular (fibrous) connective tissu surrounding individual skeletal muscle cells

Organization of individual muscle cells (muscle fibers)

Sarcolemma – the cell membrane of the muscle cell -

Motor End Plate - a specialized region of the sarcolemma for receiving signals from neurons

Thin filaments (the protein Actin) – contractile proteins

Thick filaments (the protein Myosin) – contractile proteins

Sarcoplasmic reticulum a structure similar to the endoplasmic reticulum surrounding bundles of contractile proteins

Contractile Proteins

Thin filaments

Actin Chain - wound strands of chains of actin molecules

Active Sites - binding sites on the actin molecules

Thick filaments

Myosin Chain - helical array of myosin molecules

Globular Heads (cross bridges) - enlarged end of myosin molecule that projects away from the center of the thick filament

the cross bridge acts as an ATPase, breaks down ATP, and stores the energy

Contraction cycle

generation of an action potential - the sarcolemma has the abilty to conduct an electrical impulse (excitable membrane). A massive change in membrane permeability leads to a depolarization that sweeps across the cell (the action potential.)

conduction of an action potential - the action potential is immediately conducted across the cell and travels down each of the transverse tubules to act on the sarcoplasmic reticulum

action on sarcoplasmic reticulum to release calcium - arrival of the action potential activates and opens calcium channels in the membrane of the sarcoplasmic reticulum, permitting release of the stored calcium

calcium binds to troponin molecule, moving the troponin-tropomyosin complex and exposing the active sites

myosin cross bridges bind to the active sitesusing the energy stored in the myosin molecule, the globular head pivots toward the center of the sarcomere (ADP and phosphate released)

cross bridges detach when the myosin head binds another ATP molecule

free myosin head breaks down the ATP, retaining the ADP and phosphate, and storing the energy

myosin cross bridges bind to the active sites and the cycle repeats until calcium concentration returns to normal (by way of active transport into the sarcoplasmic reticulum)

 

David G. Ward, Ph.D.  Last modified by wardd 23 May, 2006