Page 1:

Functional Properties of Muscle Tissue

• Muscle tissue has four functional properties:

  • Contractility: Muscle cells contain myofilaments (actin and myosin) that generate contractile force.

  • Excitability: Nerve signals or other stimuli excite muscle cells, causing contraction.

  • Extensibility: Muscle tissue can be stretched.

  • Elasticity: Muscle tissue can recoil and return to its resting length after being stretched.

Types of Muscle Tissue

• Skeletal Muscle Tissue:

  • Located in skeletal muscles that attach to and move the skeleton.

  • Makes up 40% of body weight.

  • Striated muscle tissue with visible dark and light stripes.

  • Innervated by the voluntary division of the nervous system and subject to conscious control.

• Cardiac Muscle Tissue:

  • Located in the wall of the heart.

  • Striated muscle tissue.

  • Contraction is involuntary and can occur without nervous stimulation.

  • Regulated by the involuntary division of the nervous system.

• Smooth Muscle Tissue:

  • Found in the walls of hollow internal organs (excluding the heart).

  • Non-striated muscle tissue.

Functions of Muscle Tissue

• Produce movement:

  • Skeletal muscle attaches to the skeleton and moves the body.

  • Muscle in the walls of visceral organs produces movement by squeezing fluids and substances through hollow organs.

• Open and close body passageways:

  • Sphincter muscles function as valves, relaxing to allow passage of substances and contracting to close the passageway.

  • Muscle tissue around the mouth and eyes opens and closes body orifices.

• Maintain posture and stabilize joints:

  • Skeletal muscles contract continuously to maintain posture.

  • Muscle tone helps stabilize and strengthen synovial joints.

• Generate heat:

  • Muscle contraction produces heat, helping to maintain normal body temperature.

  • Excess heat generated during exercise stimulates sweating to cool the body down.

Page 2:

Structure and Organizational Levels of Skeletal Muscle

• Muscle (organ):

  • Consists of muscle cells, connective tissue wrappings, blood vessels, and nerve fibers.

  • Covered externally by the epimysium.

• Fascicle (a portion of the muscle):

  • A discrete bundle of muscle cells segregated from the rest of the muscle by a connective tissue sheath called perimysium.

• Muscle fiber (cell):

  • An elongated multinucleate cell with a striated appearance.

  • Surrounded by the endomysium.

• Myofibril (complex organelle containing myofilaments):

  • Rodlike contractile organelles that occupy most of the muscle cell volume.

  • Composed of sarcomeres arranged end to end.

• Sarcomere (a segment of a myofibril):

  • The contractile unit of muscle tissue.

  • Composed of thin (actin) and thick (myosin) filaments.

• Myofilament or Filament:

  • Thin filaments contain actin molecules and regulator proteins (troponin and tropomyosin).

  • Thick filaments contain bundled myosin molecules.

• Elastic filaments composed of titin molecules maintain the organization of the A band and provide for elastic recoil when muscle contraction ends.

Page 3:

• Structural similarities shared by all muscle tissue:

  • Muscle cells are elongated and referred to as muscle fibers.

  • Involuntary division of the nervous system innervates muscle tissue.

• Unique functional properties of muscle tissue:

  • Skeletal muscle is voluntary and responsible for body movement.

  • Cardiac muscle is involuntary and forms the walls of the heart.

  • Smooth muscle is involuntary and found in the walls of organs.

• Types of muscle tissue:

  • Striated muscle tissue: skeletal muscle and cardiac muscle.

  • Visceral muscle tissue: cardiac muscle and smooth muscle.

  • Muscle cells in smooth muscle tissue lack striations.

Page 3:

• Skeletal muscle anatomy:

  • Each muscle is an organ made of several kinds of tissue.

  • Besides skeletal muscle tissue, a muscle also contains connective tissue, blood vessels, and nerves.

• Connective tissue and fascicles:

  • Several sheaths of connective tissue hold the fibers of a skeletal muscle together.

  • Sheaths from external to internal: epimysium, perimysium, endomysium.

  • Connective tissue sheaths bind muscle fibers together and hold them in parallel alignment.

  • The sheaths merge to form the tendon, which joins skeletal muscles to bones.

Page 4:

• Blood supply to muscles:

  • Muscles have a rich blood supply due to the high demand for nutrients and oxygen during muscle contraction.

  • Capillaries in the endomysium form a network that stretches and contracts with the muscle fibers.

• Muscle attachments:

  • Muscle attachment is the location on a bone where a muscle connects.

  • Each skeletal muscle extends from one bone to another, crossing at least one movable joint.

  • Origin: attachment on the less movable bone.

  • Insertion: attachment on the more movable bone.

  • When the muscle contracts, its insertion is pulled toward its origin.

  • Functions of the origin and insertion may switch depending on body position and movement.

• Layers of connective tissue in skeletal muscle:

  1. Epimysium: outer layer of dense, irregular connective tissue surrounding the whole skeletal muscle.

  2. Perimysium: layer of fibrous connective tissue surrounding each fascicle (bundle of muscle fibers).

  3. Endomysium: fine sheath of loose connective tissue surrounding each muscle fiber within a fascicle.

• Nerves and blood vessels:

  • Each skeletal muscle is supplied by one nerve, one artery, and one or more veins.

  • Nerves and vessels enter or exit the muscle near the middle of its length.

  • Nerves and vessels branch repeatedly in the intramuscular connective tissue, serving individual muscle fibers.

Page 5:

Skeletal Muscle Fiber Structure

• Skeletal muscle fibers are long, cylindrical cells

• Each fiber is formed by the fusion of hundreds of embryonic cells, so they contain many nuclei

• Myofibrils are present in large numbers in the sarcoplasm and make up more than 80% of it

• Myofibrils are unbranched cylinders and contain myofilaments, the contractile proteins

• Sarcomeres are repeating segments within myofibrils and are the basic unit of contraction in skeletal muscle

• Sarcomeres contain thin (actin) filaments and thick (myosin) filaments

• Regulatory proteins, troponin and tropomyosin, control the interaction of actin with the thick filaments

• Thick filaments consist largely of myosin molecules and contain ATPase enzymes for muscle contraction

• The sarcomere structure explains the pattern of striations in skeletal muscle fibers

Page 6:

Microscopic Structure of Skeletal Muscle Tissue

• Skeletal muscle fibers are huge cells with a diameter of 109-100 µm

• Striations in skeletal muscle fibers result from the internal structure of myofibrils

• Myofibrils are composed of sarcomeres, which extend from one Z disc to the next

• Sarcomeres contain thin (actin) filaments and thick (myosin) filaments

• The A band is the dark band created by the full length of the thick filaments and the inner ends of the thin filaments

• The H zone is the central part of the A band where no thin filaments reach

• The M line is in the center of the sarcomere and overlaps the inner ends of the thin filaments

• Tendons and aponeuroses are fibrous connective tissues that attach muscles to bones

• Indirect attachments are more common and involve tendons or aponeuroses

• Many muscles span two or more joints and are called biarticular or multijoint muscles

Page 7:

• Skeletal muscle tissue consists of myofibrils, which are made up of sarcomeres

  • Sarcomeres contain thick and thin filaments

  • The A band contains thick filaments, while the I band contains thin filaments

  • The H zone holds the thick filaments together

  • The Z disc runs through the center of each I band

• Titin is a protein that holds the thick filaments in place and contributes to muscle elasticity

• The sarcoplasmic reticulum and T tubules are involved in muscle contraction

  • The sarcoplasmic reticulum stores calcium ions

  • T tubules conduct nerve-generated impulses to ensure all myofibrils contract simultaneously

Page 8:

• Muscle contraction involves concentric and eccentric contractions

• Concentric contraction occurs when the muscle shortens and does work

• The sliding filament mechanism explains concentric contraction

  • Calcium ions are released from the sarcoplasmic reticulum and bind to the troponin molecule

  • This exposes the binding sites on the actin filament for the myosin heads

  • The myosin heads attach to the actin filament and pull it towards the center of the sarcomere

• Eccentric contraction occurs when the muscle generates force while lengthening

Page 9:

Skeletal Muscle Tissue Contraction

• Contraction is essential for controlled movement and resistance to gravity.

• Eccentric contraction is less understood than concentric contraction.

• Both types of contraction occur during push-ups.

• Concentric contractions raise the torso off the floor.

• Eccentric contractions resist gravity and control the downward motion of the torso.

• Eccentric contraction occurs in movements that resist gravity.

Muscle Extension

• Muscle tissue is extensible and can be stretched back to its original length after contraction.

• Muscle fibers are stretched by movements that are opposite to the movements the muscle normally produces.

• When a muscle is stretched, the amount of overlap between thin and thick filaments decreases.

• I bands and H zones lengthen as Z discs move apart.

• There is no change in the width of A bands.

Muscle Fiber Length and Force of Contraction

• The optimal resting length for skeletal muscle fibers is the length that generates the greatest pulling force when the muscle is contracted.

• Optimal length occurs when a fiber is slightly stretched, allowing thin and thick filaments to overlap moderately.

• If a muscle fiber is stretched too much, the myosin heads have nothing to attach to and no pulling force can be generated.

• If sarcomeres are compressed, little further shortening can occur.

• Whole skeletal muscles have a range of optimal operational length from about 80% to 120% of their normal resting length.

• Muscle attachments tend to keep muscles within the optimal range.

Clinical Application: Delayed-Onset Muscle Soreness

• Soreness that begins 8-24 hours after an activity is called delayed-onset muscle soreness.

• It is caused by microscopic tears in the muscle fibers.

• Most common after eccentric exercise.

• Inflammatory response to tears results in swelling in the connective tissues surrounding the muscle fibers.

• Swelling compresses sensory nerve endings, causing soreness.

• Lasts 3-7 days.

• Low-level aerobic activity increases blood supply to the muscle and speeds recovery.

• Microscopic tears stimulate increased production of myofibrils and myofilaments, resulting in increased muscle strength.

Page 10:

Innervation of Skeletal Muscle

• Calcium ions are released from the sarcoplasmic reticulum and muscle contraction is initiated by nervous stimulation.

• Motor neurons innervate muscle fibers.

• Motor neurons have dendrites and an axon.

• Dendrites are receptive regions of the neuron.

• Axon is a long cell process that initiates and transmits nerve impulses.

Neuromuscular Junction

• Each muscle fiber is served by a nerve ending called a neuromuscular junction or motor end plate.

• Neuromuscular junction is the point where the nerve ending and fiber meet.

• Terminal boutons or axon terminals are enlargements at the end of the axonal process that store neurotransmitters.

• Neurotransmitter at the neuromuscular junction is acetylcholine.

• Acetylcholine diffuses across the synaptic cleft and binds to receptor molecules on the sarcolemma.

• This induces an impulse that initiates fiber contraction.

• The neuromuscular junction has unique features such as trough-like depressions in the sarcolemma and invaginations covered with a basal lamina.

• The basal lamina contains acetylcholinesterase, which breaks down acetylcholine after a single contraction to prevent additional twitches.

Page 11:

• Motor units consist of a motor neuron and the muscle fibers it innervates

  • Motor neurons extend from the spinal cord to the muscle

  • Each motor neuron branches into terminal boutons that form neuromuscular junctions with muscle fibers

• The number of muscle fibers in a motor unit can vary

  • Average number is 150, but can be as high as several hundred or as low as four

• Muscles that require fine control have fewer muscle fibers per motor unit

• Bulky, weight-bearing muscles have many muscle fibers per motor unit

• Stimulation of a single motor unit causes a weak contraction of the entire muscle

• Each individual muscle contains many motor units

• Recruitment is the addition of motor units to accomplish a movement

• Different muscles have different sizes of motor units

• Practice questions on histology lab

Page 12:

• Skeletal muscle fibers are categorized based on how they manufacture energy and how quickly they contract

• Three types of muscle fibers: slow oxidative fibers (SO), fast glycolytic fibers (FG), and fast oxidative fibers (FO)

• Each muscle contains a mixture of the three fiber types

• Training can transform muscle fiber types

• Weight training increases the diameter and strength of fast muscle fibers

• Slow oxidative fibers obtain energy from aerobic metabolic reactions

• Fast glycolytic fibers depend on anaerobic pathways to make ATP

• Fast oxidative fibers are intermediate in characteristics between slow oxidative and fast glycolytic fibers

• Rhabdomyolysis is a condition where myoglobin leaks from muscle and can cause kidney failure

Page 13:

• Skeletal muscle tissue characteristics:

  • Body location: attached to bones or skin

  • Cell shape and appearance: single, very long cylindrical, multinucleate cells with obvious striations

  • Connective tissue components: epimysium, perimysium, and endomysium

  • Presence of myofibrils composed of sarcomeres

• Muscle fibers do not increase in number by dividing mitotically, but rather increase in diameter by building more contractile proteins and myofilaments.

Page 14:

• Disorders of skeletal muscle tissue:

  • Muscular dystrophy: group of inherited muscle-destroying diseases, most common form is Duchenne muscular dystrophy

  • Myofascial pain syndrome: pain caused by tightened bands of muscle fibers, treated with nonsteroidal anti-inflammatory drugs and stretching

  • Fibromyalgia: chronic-pain syndrome of unknown cause, treated with antidepressants, exercise, and pain relievers

Page 15:

• Muscular dystrophy: inherited muscle-destroying disease, affects muscles in childhood, most serious form is Duchenne muscular dystrophy

• Myofascial pain syndrome: pain caused by tightened bands of muscle fibers, treated with nonsteroidal anti-inflammatory drugs and stretching

• Fibromyalgia: chronic-pain syndrome of unknown cause, treated with antidepressants, exercise, and pain relievers

• Skeletal muscle tissue develops from embryonic mesoderm cells called myoblasts, which fuse to form skeletal muscle fibers

• Changes in skeletal muscle with age

Skeletal Muscle Tissue (Page 16)

Formation and Growth of Skeletal Muscle Fibers

• Skeletal muscle fibers do not undergo mitosis after formation

• During childhood and adolescence, muscle fibers lengthen and thicken to keep up with body growth

• Satellite cells surround skeletal muscle fibers and fuse with them during youth to help them grow

• Satellite cells also play a role in repairing muscle injuries

• Severely damaged muscle tissue is primarily replaced by scar tissue

Difference in Strength between Men and Women

• On average, adult men have greater body strength than adult women

• Women's skeletal muscles make up 36% of body mass, compared to 42% in men

• Greater muscular development in men is primarily due to the effects of androgen hormones, not exercise

• Men's larger size also contributes to the strength difference

• Body strength per unit muscle mass is the same in both sexes

Effects of Aging on Skeletal Muscles

• Connective tissue in skeletal muscles increases and the number of muscle fibers decreases with age

• Muscles become more sinewy and body weight declines in many elderly people

• Loss of muscle leads to a decrease in muscular strength, usually by 50% by age 80

• This condition is called sarcopenia and can lead to serious falls in the elderly

• Factors contributing to sarcopenia include a sedentary lifestyle, inadequate nutrition, reduced synthesis of muscle proteins, and a reduction in the rate of muscle rebuilding by satellite cells

• Exercise, even in advanced age, can reverse sarcopenia by increasing the size of remaining muscle fibers

• Weight training in the elderly maintains muscle strength

Check Your Understanding

• Skeletal muscle fibers are multinucleated because they are formed by the fusion of myoblasts

• Older adults can prevent or reverse the effects of