THE MUSCULAR SYSTEM: SKELETAL MUSCLE TISSUE AND MUSCLE ORGANIZATION

THE MUSCULAR SYSTEM: SKELETAL MUSCLE TISSUE AND MUSCLE ORGANIZATION

Skeletal Muscles Attach to bones Produce skeletal movement (voluntary) Maintain posture Support soft tissues Regulate entrances to the body Maintain body temperature

Properties of Skeletal Muscles Electrical excitability -ability to respond to stimuli by producing electrical signals such as action potentials -two types of stimuli: 1. autorhythmic electrical signals 2. chemical stimuli Contractility -ability to contract when stimulated by an AP -isometric contraction: tension develops, length doesn t change -isotonic contraction: tension develops, muscle shortens Extensibility -ability to stretch without being damaged -allows contraction even when stretched Elasticity -ability to return to its original length and shape

Classification According to arrangement of fibers and fascicles Parallel muscles Parallel to long axis of muscle Convergent muscles Fibers converge on common attachment site Pennate muscles One or more tendons run through body of muscle Unipennate, bipennate, multipennate Circular muscles Fibers concentrically arranged

Origin remains stationary Origins and Insertions Typically proximal to insertion Insertion moves Muscles identified by Origin Insertion Primary action Classified as Prime mover (agonist) Synergist Antagonist

Muscle Names Yield clues to muscle orientation, location or function Biceps brachii (two heads, arm) Vastus femoris (large, femur) Orbicularis oculi (circular, eye) Rectus abdominus (erect, abdomen)

Axial Musculature Arises from and inserts on the axial skeleton Positions the head and spinal column Moves the rib cage, assisting in breathing Axial musculature Originates and inserts on axial skeleton Appendicular musculature Stabilizes or moves components of the appendicular skeleton

Gross Anatomy muscles are really groups of fascicles the fascicles are groups of muscle fibers = considered to be an individual muscle cell

the muscle fiber is made up of protein filaments = myofibrils each myofibril is comprised of repeating units = sarcomeres

Gross Anatomy muscle is wrapped in a protective fascia -fascia = sheet of fibrous connective tissue that supports and surrounds muscle or organs a superficial fascia separates muscle from the overlying skin -also known as the subcutaneous layer -made up of areolar tissue and adipose tissue -provides support for blood vessel and nerves -the adipose tissue stores most of the body s triglycerides and provides insulation muscles with similar functions are grouped and held together by layers of deep fascia -dense irregular connective tissue -allow free movement of muscles, carries nerves, BVs

three layers of connective tissue extend from the deep fascial layer Epimysium Perimysium Endomysium these layers further strengthen and protect muscle outermost layer = epimysium encircles the entire muscle next layer = perimysium surrounds groups of 10 to 100 individual muscle fibers separates them into bundles = fascicles give meat its grain because the fascicles are visible both epimysium and perimysium are dense irregular connective tissue penetrating the fasicles and separating them into individual muscle fibers = endomysium (areolar connective tissue)

all three of these connective tissue layers extend beyond the muscle and attaches it to other structures -called a tendon = cord of regular dense CT that attaches a muscle to the periosteum of bone when the CT extends as a broad flat sheet = aponeurosis generally muscles are supplied with one artery and two veins they accompany the nerve nerves that induce muscle contraction = somatic motor neurons (part of the somatic division of the PNS) communication between muscle and these neurons Neuromuscular junction (NMJ)

Microanatomy of Skeletal Muscle Fibers New terminology Cell membrane = sarcolemma Cytoplasm = sarcoplasm Internal membrane system = sarcoplasmic reticulum Large, multinucleated cells embryonic development stem cells (satellite cells) differentiate into immature myoblasts which begin to make the proteins of the myofilament These myoblasts mature into myocytes Multiple myocytes fuse to form the muscle cell (muscle fiber) once fused, these muscle cells lose the ability of undergo mitosis number of muscle cells predetermined before birth But satellite cells can repair damaged/dying muscle cells throughout adulthood

Transverse tubules Invaginations of sarcolemma Carry electrical impulses Myofibrils within sarcoplasm skeleton of protein filaments (myofilaments) organized as Sarcomeres Myofilaments form the myofibrils Thin filaments (actin, troponin, tropomyosin) Thick filaments (myosin) Muscle Cell Anatomy

Microanatomy of Skeletal Muscle Fibers muscle fibers are bound by a plasma membrane = sarcolemma thousands of tiny invaginations in this sarcolemma called T or transverse tubules - tunnel in toward the center of the cell -T tubules are open to the outside of the fiber - filled with interstitial fluid - action potentials generated in the neuron travel along the sarcolemma and the T tubules - allows for the even and quick spread of an action potential deep into the cell the cytoplasm is called a sarcoplasm -substantial amounts of glycogen - can be broken into glucose -contains myoglobin - binds oxygen needed for muscle ATP production

Microanatomy of Skeletal Muscle Fibers contractile elements of the myofibrils = myofilaments -2 microns in diameter -comprised of primarily actin or myosin -give the muscle its striated appearance Fibers also have a system of fluid-filled membranes = sarcoplasmic reticulum -encircles each myofibril -similar to the ER -have dilated end sacs = terminal cisterns -stores calcium when at rest - releases it during contraction -release is triggered by an AP

The Proteins of Muscle Myofibrils are built of 3 kinds of protein contractile proteins myosin and actin regulatory proteins which turn contraction on & off troponin and tropomyosin structural proteins which provide proper alignment, elasticity and extensibility titin, myomesin, nebulin, actinin and dystrophin Dystrophin connects sarcomere to sarcolemma -transmits tension along muscle Actinin part of Z-line Titin connects myosin to Z-line and M-line Role in recovery after being stretched Nebulin forms core of the actin chain/thin filament

Types of Muscle Fibers Fast fibers = glycolytic Slow fibers = oxidative Fibers of one motor unit all the same type Percentage of fast versus slow fibers is genetically determined Proportions vary with the usual action of the muscle - neck, back and leg muscles have a higher proportion of postural, slow oxidative fibers - shoulder and arm muscles have a higher proportion of fast glycolytic fibers

Fast Fibers Large in diameter Contain densely packed myofibrils Large glycogen reserves Fast oxidative-glycolytic (fast-twitch A) red in color (lots of mitochondria, myoglobin & blood vessels) split ATP at very fast rate; used for walking and sprinting Fast glycolytic (fast-twitch B) white in color (few mitochondria & BV, low myoglobin) anaerobic movements for short duration; used for weight-lifting

Slow fibers Half the diameter of fast fibers Three times longer to contract Continue to contract for long periods of time e.g. marathon runners

Atrophy wasting away of muscles caused by disuse (disuse atrophy) or severing of the nerve supply (denervation atrophy) the transition to connective tissue can not be reversed Hypertrophy increase in the diameter of muscle fibers resulting from very forceful, repetitive muscular activity and an increase in myofibrils, SR & mitochondria

Muscle Metabolism Production of ATP: -contraction requires huge amounts of ATP -muscle fibers produce ATP three ways: 1. Creatine phosphate 2. Aerobic metabolism 3. Anaerobic metabolism

Creatine Phosphate Muscle fibers at rest produce more ATP then they need for resting metabolism Excess ATP within resting muscle used to form creatine phosphate By the enzyme creatine kinase Creatine phosphate: 3-6 times more plentiful than ATP within muscle Its quick breakdown provides energy for creation of ATP Sustains maximal contraction for 15 sec (used for 100 meter dash). Athletes tried creatine supplementation gain muscle mass but shut down bodies own synthesis (safety?)

M line Sarcomere Structure sarcomere = regions of myosin (thick myofilament) and actin (thin myofilament) bounded by the Z line (actinin) actin filaments project out from Z line myosin filaments lie in center of sarcomere - overlap with actin and connect via cross-bridges myosin only region = H zone myosin filaments are held in place by the M line proteins. actin only region = I band length of myosin filaments = A band contraction = sliding filament theory -actin and myosin myofilaments slide over each other and sarcomere shortens

Contraction: The Sliding Filament Theory Contraction: Active process Elongation is passive Amount of tension produced is proportional to degree of overlap of thick and thin filaments SF Theory: Explains how a muscle fiber exerts tension Four step process Active sites on actin Crossbridge formation Cycle of attach, pivot, detach, return Troponin and tropomyosin control contraction

Contraction: The Sliding Filament Theory Actin filament has a myosin binding site This site is covered up by troponin and tropomyosin in relaxed muscle Removal of troponin/tropomyosin is required for contraction myosin thick myofilament is a bundle of myosin molecules -each myosin protein has a globular head with a site to bind and breakdown ATP (ATPase site) and to bind actin (actin binding site)

The Events in Muscle Contraction

Muscle Contraction: A summary ACh released from synaptic vesicles Binding of ACh to motor end plate Generation of electrical impulse in sarcolemma Conduction of impulse along T-tubules Release of Calcium ions by SR - binds to troponin Exposure of active sites on actin Cross-bridge formation and contraction

The Neuromuscular Junction end of neuron (synaptic terminal or axon bulb) is in very close association with the muscle fiber distance between the bulb and the folded sarcolemma = synaptic cleft nerve impulse leads to release of neurotransmitter (acetylcholine) N.T. binds to receptors on myofibril surface binding leads to influx of sodium, potassium ions (via channels) eventual release of calcium by sarcoplasmic recticulum = contraction Acetylcholinesterase breaks down ACh Limits duration of contraction

Each skeletal fiber has only ONE NMJ MU = Somatic neuron + all the skeletal muscle fibers it innervates Number and size indicate precision of muscle control Muscle twitch Single momentary contraction Response to a single stimulus All-or-none theory Either contracts completely or not at all Motor Units Motor units in a whole muscle fire asynchronously some fibers are active others are relaxed delays muscle fatigue so contraction can be sustained Muscle fibers of different motor units are intermingled so that net distribution of force applied to the tendon remains constant even when individual muscle groups cycle between contraction and relaxation.

Axial muscles organized into four groups Muscles of the head and neck Muscles of the vertebral column Oblique and rectus muscles Muscles of the pelvic floor

Muscles of the Vertebral Column Longus capitus Longus colli Rotate and flex the neck Quadratus lumborum muscles Flex the spine Depress the ribs

The Oblique and Rectus Muscles

The Diaphragm

Muscles of the Pelvic Floor

Appendicular musculature is responsible for Stabilizing pectoral girdle Stabilizing pelvic girdle Moving upper and lower limbs Four groups of muscles Muscles that position the pectoral girdle Muscles that move the arm Muscles that move the forearm and hand Muscles that move the hand and fingers

Extrinsic Muscles that Move the Hand and Fingers

Muscles that Move the Thigh