Muscles (Function and Organization) Flashcards
Skeletal Muscle (description)
- Usually attached to bones
- Voluntary control
- Striated
Cardiac Muscle (description)
- Located on in the wall of the heart
- Muscle fibers joined together by intercalated discs
- Branched networks of fibers contract as a unit
- Self-exciting and rhythmic
- Involuntary
- Striated
Smooth Muscle (description)
- Located in the lining of hollow organs, blood vessels, and skin
- Shorter fibers (compared to skeletal muscle fibers)
- Single, centrally located nucleus
- Elongated with tapering ends
- Involuntary
- Not striated due to random organization of myofilaments
Structure of a Muscle
- Muscle (belly)
- Fascicles
- Muscle fibers
Fascia
Covers muscle (belly)
Epimysium
Covers muscle (belly)
Perimysium
Surrounds fascicles
Endomysium
Surround individual muscle cells/fibers
Structure of a Muscle Fiber
- Sarcolemma
- Myofibrils
- Actin and Myosin
- Sarcomeres
Sarcolemma
Cell membrane of a muscle cell
Myofibrils
Make up muscle fibers; fundamental role in muscle contractions
Sarcomeres
Structural and functional unit of myofibrils; distance between two Z lines
Thick Filaments
Composed of myosin; cross-bridges
Thin Filaments
Composed of actin
Motor Unit
All the muscle fibers controlled by a single motor neuron
Stimulus for Muscle Contraction
- Motor neuron stimulates the release of Acetylcholine (ACh), a neurotransmitter
- Occurs in the neuromuscular junction
- Triggers a chemical reaction in the muscle tissue (muscle impulse)
Contraction
- Muscle impulses cause the release of calcium ions into cytoplasm
- Calcium binds to the actin filaments which changes its shape and exposes the myosin binding sites
- Actin and myosin filaments bind via myosin cross-bridges
- Process continues as long as Ca+2 ions and ATP are present
- ATP is produced by cellular respiration
Contraction (Cross-Bridge Cycling)
- Myosin heads pull on actin filament toward center of sarcomere (= power stroke); ADP and phosphate are released from myosin head
- New ATP binds to myosin; myosin cross-bridge releases
- New ATP is split; myosin head goes back to original position and is ready to bind again
Contraction (Sliding Filament Model)
Sarcomeres shorten as myofilaments slide past one another; Z-lines move closer together
Relaxation
- Motor neuron stimulus to muscle stops
- Calcium moves back into sarcoplasmic reticulum
- ATP binds to myosin head to break cross-bridges
- In the absence of Ca+2 ions, actin returns to its resting shape and prevents myosin binding
- Muscle fibers relax and lengthen
Muscle Fatigue
Inability of muscles to contract and/or relax
Muscle Fatigue (causes)
- Decreased blood flow
- Ion imbalances across the sarcolemma
- Accumulation of lactic acid
Cramp
Sustained, involuntary muscle contraction; can result from fatigue
Heat Production
- By-product of cellular respiration
- Skeletal muscles are major sources of body heat
- Blood distributes heat throughout body
Skeletal Muscle (categorization)
- Origin
- Insertion
- Prime mover
- Synergists
- Antagonist
Origin
Immovable end of a skeletal muscle
Insertion
Movable end of a skeletal muscle
Prime Mover
Primarily responsible for movement of a skeletal muscle
Synergists
Skeletal muscles that assist the prime mover
Antagonist
Skeletal muscle that resists prime mover’s action and causes movement in the opposite direction
Life-Span Changes
- Myoglobin, ATP, and creatine phosphate (needed for ATP production) decline
- By age 80, half of muscle mass has atrophied
- Adipose cells and CT replacement muscle tissue
- Exercise helps to maintain muscle mass and function
