Chapter 6 Muscular System

Muscle is a Contractile Organ There are three types of muscular tissue Skeletal muscle, cardiac muscle, smooth muscle Skeletal muscle is part of the skeleto-muscular system Functions to move bones In general, each skeletal muscle has a Belly – the middle portion of the muscle Origin - an end that remains stationary when the organ shortens Insertion - an end that moves during contraction Knowing the origin and insertion of any skeletal muscle offers clues about its function Skeletal muscle is composed of numerous elongated muscle cells Spanning from origin to insertion, one nested inside another Muscle cells also are called muscle fibers Muscle cells must shorten, creating tension Connective tissue structurally supports skeletal muscle

Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate Striated—have visible banding Voluntary—subject to conscious control

Skeletal Muscle Most skeletal muscles work in pairs Synergistic pairs Work together to cause movement in the same direction i.e., muscles of the quadriceps or muscles of the hamstrings Antagonistic pairs Cause movement in opposite directions, opposite to one another i.e., biceps and triceps

Fascicle (wrapped by perimysium) Muscle fiber (cell) Blood vessel Perimysium Epimysium (wraps entire muscle) Fascicle (wrapped by perimysium) Endomysium (between fibers) Tendon Bone Figure 6.1

Connective Tissue Wrappings of Skeletal Muscle Endomysium—encloses a single muscle fiber Perimysium—wraps around a fascicle (bundle) of muscle fibers Epimysium—covers the entire skeletal muscle Fascia—on the outside of the epimysium

Structure of Skeletal Muscle

Microscopic Anatomy Sarcolemma Myofibril Nucleus Light (I) band Dark (A) band (a) Segment of a muscle fiber (cell)

Microscopic Anatomy Sarcolemma—specialized plasma membrane Myofibrils—long organelles inside muscle cell Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum Myofibrils are aligned to give distinct bands I band = light band Contains only thin filaments A band = dark band Contains the entire length of the thick filaments

Sarcomere: contractile unit of Muscle Thick filaments = myosin filaments Composed of the protein myosin Has ATPase enzymes Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap somewhat Thin filaments = actin filaments Composed of the protein actin Anchored to the Z disc

Neuromuscular Junctions Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron Neuromuscular junction Association site of axon terminal of the motor neuron and muscle

Neuromuscular Junction- Initiation of Contraction Neurotransmitter—chemical released by nerve upon arrival of nerve impulse in the axon terminal The neurotransmitter for skeletal muscle is acetylcholine (ACh)

Muscle Contraction Cycle- sliding filament theory

Energy for Muscle Contraction Initially, muscles use stored ATP for energy ATP bonds are broken to release energy Only 4–6 seconds worth of ATP is stored by muscles After this initial time, other pathways must be utilized to produce ATP

Creatine Phosphate Direct phosphorylation of ADP by creatine phosphate (CP) Muscle cells store CP CP is a high-energy molecule CP transfers a phosphate group to ADP, to regenerate ATP Produces: 1 ATP is created per CP molecule Duration: 15 seconds Oxygen use: none

Aerobic Respiration C6H12O6 + 6O2  6CO2 + 6H2O + 32 ATP Products: 32 ATP, carbon dioxide, water Occurs: in the mitochondria Requires continuous oxygen Slow

Anaerobic Glycolysis Pyruvic acid is converted to lactic acid Oxygen Use: No Produces: 2 ATP, lactic acid Occurs: cytoplasm Duration: 40 s Not as efficient, but is fast Huge amounts of glucose are needed Lactic acid produces muscle fatigue

There are Three Types of Muscle Cells Fast Twitch (or fast glycolytic) Anaerobic, fatigue quickly Provide a short burst of extreme energy and contraction power Thicker, contain fewer mitochondria, usually contain larger glycogen Responsible for hypertrophy Because short bursts of power come from these fibers, exercises that continuously require bursts of power will enlarge them Weight training puts demands on fast twitch fibers, resulting in the hypertrophy (muscle enlargement) we associate with body-building Intermediate (or fast oxidative-glycolytic)

Three types of muscle cells Slow Twitch (or slow oxidative) Appear red, have a large blood supply, have many mitochondria within their sarcolemma, and store an oxygen-carrying protein called myoglobin Are sometimes called nonfatiguing or aerobic cells Distance running and other aerobic sports stimulate these cells Efficiency and strength come not from increasing mass but from using oxygen more efficiently Although training can alter the functioning of both red (slow twitch) and white (fast twitch) fibers, it does not change their proportions The percentage of fast and slow twitch fibers is genetically determined