Muscle Tissue

Skeletal muscle tissue and the Muscular System

Three types of muscle

• Skeletal – attached to bone

• Cardiac – found in the heart

• Smooth – lines hollow organs

Skeletal muscle functions

• Produce skeletal movement

• Maintain posture and body position

• Support soft tissues

• Guard entrances and exits

• Maintain body temperature

Anatomy of Skeletal Muscle

Organization of connective tissues

• Epimysium surrounds muscle

• Perimysium sheathes bundles of muscle fibers

• Epimysium and perimysium contain blood vessels and nerves

• Endomysium covers individual muscle fibers

• Tendons or aponeuroses attach muscle to bone or muscle

Skeletal muscle fibers

• Sarcolemma (cell membrane)

• Sarcoplasm (muscle cell cytoplasm)

• Sarcoplasmic reticulum (modified ER)

• T-tubules and myofibrils aid in contraction

• Sarcomeres – regular arrangement of myofibrils

Myofibrils

• Thick and thin filaments

• Organized regularly

Muscle Fiber

Thin filaments

• F-actin

• Nebulin

• Tropomyosin

• Covers active sites on G-actin

• Troponin

• Binds to G-actin and holds tropomyosin in place

Thick filaments

• Bundles of myosin fibers around titan core

• Myosin molecules have elongate tail, globular head

• Heads form cross-bridges during contraction

• Interactions between G-actin and myosin prevented by tropomyosin during rest

Sliding filament theory

• Explains the relationship between thick and thin filaments as contraction proceeds

• Cyclic process beginning with calcium release from SR

• Calcium binds to troponin

• Trponin moves, moving tropomyosin and exposing actin active site

• Myosin head forms cross bridge and bends toward H zone

• ATP allows release of cross bridge

The Contraction of Skeletal Muscle

Tension

• Created when muscles contract

• Series of steps that begin with excitation at the neuromuscular junction

• Calcium release

• Thick/thin filament interaction

• Muscle fiber contraction

• Tension

Control of skeletal muscle activity occurs at the neuromuscular junction

• Action potential arrives at synaptic terminal

• ACh released into synaptic cleft

• ACh binds to receptors on post-synaptic neuron

• Action potential in sarcolemma

Excitation/contraction coupling

• Action potential along T-tubule causes release of calcium from cisternae of SR

• Initiates contraction cycle

• Attachment

• Pivot

• Detachment

• Return

Relaxation

• Acetylcholinesterase breaks down ACh

• Limits the duration of contraction

Tension Production

Tension production by muscle fibers

• All or none principle

• Amount of tension depends on number of cross bridges formed

• Skeletal muscle contracts most forcefully over a narrow ranges of resting lengths

• Twitch

• Cycle of contraction, relaxation produced by a single stimulus

• Treppe

• Repeated stimulation after relaxation phase has been completed

Summation

• Repeated stimulation before relaxation phase has been completed

• Wave summation = one twitch is added to another

• Incomplete tetanus = muscle never relaxes completely

• Complete tetanus = relaxation phase is eleminated

Tension production by skeletal muscles

• Internal tension generated inside contracting muscle fibers

• External tension generated in extracellular fibers

• Motor units

• All the muscle fibers innervated by one neuron

• Precise control of movement determined by number and size of motor unit

• Muscle tone

• Stabilizes bones and joints

Tension production by skeletal muscles

• Internal tension generated inside contracting muscle fibers

• External tension generated in extracellular fibers

• Motor units

• All the muscle fibers innervated by one neuron

• Precise control of movement determined by number and size of motor unit

• Muscle tone

• Stabilizes bones and joints

Contractions

• Isometric

• Tension rises, length of muscle remains constant

• Isotonic

• Tension rises, length of muscle changes

• Resistance and speed of contraction inversely related

• Return to resting lengths due to elastic components, contraction of opposing muscle groups, gravity

Energy Use and Muscle Contraction

Muscle Contraction requires large amounts of energy

• Creatine phosphate releases stored energy to convert ADP to ATP

• Aerobic metabolism provides most ATP needed for contraction

• At peak activity, anaerobic glycolysis needed to generate ATP

Energy use and level of muscular activity

• Energy production and use patterns mirror muscle activity

• Fatigued muscle no longer contracts

• Build up of lactic acid

• Exhaustion of energy resources

Recovery period

• Begins immediately after activity ends

• Oxygen debt (excess post-exercise oxygen consumption)

• Amount of oxygen required during resting period to restore muscle to normal conditions

Muscle Performance

Types of skeletal muscle fibers

• Fast fibers

• Slow fibers

• Intermediate fibers

Fast fibers

• Large in diameter

• Contain densely packed myofibrils

• Large glycogen reserves

• Relatively few mitochondria

• Produce rapid, powerful contractions of short duration

Slow fibers

• Half the diameter of fast fibers

• Take three times as long to contract after stimulation

• Abundant mitochondria

• Extensive capillary supply

• High concentrations of myoglobin

• Can contract for long periods of time

Intermediate fibers

• Similar to fast fibers

• Greater resistance to fatigue

Muscle performance and the distribution of muscle fibers

• Pale muscles dominated by fast fibers are called white muscles

• Dark muscles dominated by slow fibers and myoglobin are called red muscles

• Training can lead to hypertrophy of stimulated muscle

Physical conditioning

• Anaerobic endurance

• Time over which muscular contractions are sustained by glycolysis and ATP/CP reserves

• Aerobic endurance

• Time over which muscle can continue to contract while supported by mitochondrial activities

Cardiac Muscle Tissue

Structural characteristics of cardiac muscle

• Located only in heart

• Cardiac muscle cells are small

• One centrally located nucleus

• Short broad T-tubules

• Dependent on aerobic metabolism

• Intercalated discs where membranes contact one another

Functional characteristics of cardiac muscle tissue

• Automaticity

• Contractions last longer than skeletal muscle

• Do not exhibit wave summation

• No tetanic contractions possible

Smooth Muscle Tissue

Structural characteristics of smooth muscle

• Nonstriated

• Lack sarcomeres

• Thin filaments anchored to dense bodies

• Involuntary

Functional characteristics of smooth muscle

• Contract when calcium ions interact with calmodulin

• Activates myosin light chain kinase

• Functions over a wide range of lengths

• Plasticity

• Multi-unit smooth muscle cells are innervated by more than one motor neuron

• Visceral smooth muscle cells are not always innervated by motor neurons

• Neurons that innervate smooth muscle are not under voluntary control