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MUSCULAR SYSTEM
• Produces movement
• Maintains posture
• Stabilizes joints
• Generates heat
• Vital for maintaining body temperature.
ROLES OF MUSCLES IN BODY
TYPES OF MUSCLE
•Skeletal – striated & voluntary
•Smooth – involuntary
•Cardiac - heart
• Muscle attached to the skeleton.
• Striated muscle.
• Contains more than one nuclei per cell
• Contraction is under voluntary control.
SKELETAL MUSCLE
• Muscle is made up of thousands of cylindrical muscle fibers.
• Run from origin to insertion.
• Bound together by connective tissue.
• Each fiber contains:
• Array of myofibrils that are stacked lengthwise.
• Run the entire length of the fiber
SKELETAL FIBER STRUCTURE
•Many mitochondria for energy.
•Extensive smooth endoplasmic reticulum
•Many nuclei: each muscle fiber develops from the fusion of many cells
SKELETAL FIBER STRUCTURE
CARDIAC MUSCLE• Striated
• Each cell contains sarcomeres with sliding filaments of actin and myosin.
• The myofibrils of each cell are branched.
• The branches interlock with adjacent fibers by adherens junctions.
•Enable heart to contract forcefully without ripping the fibers apart.
• The action potential that triggers the heartbeat is generated within the heart itself.
CARDIAC MUSCLE
• Action potential that drives contraction of the heart passes from fiber to fiber through gap junctions.
• All fibers contract in a synchronous wave
• Sweeps from atria down through ventricles and pumps blood out of the heart.
• Refractory period is longer than the period it takes for the muscle to contract (systole) and relax (diastole).
• Has richer supply of mitochondria than skeletal muscle.
CARDIAC MUSCLE
• Made of single, spindle-shaped cells.
• No visible striations
SMOOTH MUSCLE
• Cells contain thick (myosin) and thin (actin) filaments.
• Slide against each other to produce contraction of the cell.
• Does not depend on somatic motor neurons to be stimulated.
• Motor neurons (of the autonomic system) reach smooth muscle and can stimulate it — involuntary action based on stimulus response.
SMOOTH MUSCLE
Muscles are composed of many fibers that are arranged in bundles called FASCICLES
Individual muscles are separated by FASCIA, which also forms tendons (connects muscles to bones).
MUSCLE STRUCTURE
Myofibrils = individual muscle fibers --> made of myofilaments
MUSCLE FIBER MADE OF MYOFIBRILS
• Contains protein filaments
• ACTIN (thin) and MYOSIN (thick)
• These filaments overlap to form dark and light bands on the muscle fiber.
• A band = dark • thick (myosin)
• I band = light • thin (actin)
• In the middle of each I band are Z lines.
• Sarcomere is section of myofibril from one Z line to the other
MYOFIBRIL
MYOFIBRILS ARE A SERIES OF CONNECTED SARCOMERES
SARCOMERE
•Basic contracting unit of muscle.
• The entire array of thick and thin filaments between the Z disks is called a sarcomere.
• Z lines on either end.
• Contains one A-band and 21/2 I-bands
SARCOMERE STRUCTURE
• The striations are caused by alignment of bands.
• Most prominent: A and I bands and Z line.
• A band: 2 proteins overlap.
• I band: only the actin protein.
SARCOMERE STRUCTURE
• The H zone: Part of the A band where thick and thin filaments do not overlap.
• M line runs through center of the sarcomere.
• Provides a scaffold for the assembly of myosin molecules in the thick filament
SARCOMERE STRUCTURE
• When muscle contracts the actin filaments slide into the A band, overlapping with myosin.
• Z lines move closer together
• I band becomes shorter
• A band stays at the same length
IT IS IMPORTANT TO REMEMBER THE MUSCLE HIERARCHY
fascicles
myofibrils
myofilaments
actin
myosin
muscle
myofibrilsmyofilament
sarcomere
muscle fiber
epimysium
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