Chapter 10: Skeletal Muscle Tissue

Basic Terminology: "myo" "mys", "sarco"

"Myo—" or "Mys—" mean muscle o Myofilaments = microfilaments containing contractile protein "Sarco—" means flesh (refers to muscle) o Sarcolemma = plasma membrane of muscle cells; flesh sheath o Sarcoplasm = cytoplasm of muscle cells o Sarcoplasmic reticulum = endoplasmic reticulum, specialized for the storage of calcium

Unique Feature of the Neuromuscular Junction:

• Each terminal bouton lies in a trough-like depression of the sarcolemma • This has its own invaginations covered with a basal lamina. • This basal lamina contains the enzyme acetylcholinesterase, which breaks down acetylcholine immediately after the neurotransmitter signals a single contraction. • This ensures that each nerve impulse to the muscle fiber produces just ONE twitch of the fiber, preventing undesirable additional twitches that would result if acetylcholine were to linger in the synaptic cleft.

Muscular Dystrophy (disorders of skeletal muscles)

group of inherited muscle-destroying diseases that generally appear in childhood; affected muscles enlarge with fat and connective tissue, while the muscle fibers degenerate.

motor unit

motor neuron + all the muscle fibers it innervates When a motor neuron fires, all skeletal muscle fibers in the motor unit contract together. **Muscles that require very fine motor control have very few muscle fibers per motor unit (e.g. muscles moving the fingers and eyes) • Muscles in a single motor unit are spread throughout the muscle • As a result, stimulation of a single motor unit causes a weak contraction of the entire muscle **Muscles whose movements less precise have many muscle fibers per motor unit (e.g. hip muscles)

Fibromyalgia (disorders of skeletal muscles)

mysterious chronic syndrome of unknown cause • Symptoms include severe musculoskeletal pain, fatigue, and sleep abnormalities and headache. Affects 2% of the population (mostly women) • Most common sites of pain are lower back or neck • To be diagnosed, pain must be present in at least 11/18 standardized points spread widely over the body • Treated with antidepressants, exercise and pain relievers

3 categories for THE SPEED of skeletal muscle contraction

speed of contraction (depends on how quickly a fiber breaks down ATP to gain energy needed for contraction; most muscles in the body contain all 3 fiber types, but the proportions differ from one muscle to another) slow oxidative fibers (SO) fast glycolytic fibers (FG) fast oxidative fibers (FO)

terminal boutons

"axon terminals"—cluster of enlargements at the end of axonal processes that stores chemical messenger molecules/neurotransmitters. They are separated from the sarcolemma of the muscle fiber by a space called the synaptic cleft. They contain vesicles that release a neurotransmitter when a nerve impulse reaches the terminals. The neurotransmitter diffuses across the synaptic cleft and binds to receptor molecules on the sarcolemma, where it induces an impulse that initiates fiber contraction.

neuromuscular junction

"motor end plate"; it is the point at which the nerve ending and fiber meet. Each muscle fiber in a skeleton is served by a nerve ending, which signals the fiber to contract.

2 Types of Tubules

1) Sarcoplasmic reticulum (SR) • Elaborate smooth endoplasmic reticulum whose interconnecting tubules surround each myofibril • Most SR tubules run longitudinally along the myofibril. • Other SR tubules (terminal cisterns) form larger, perpendicular cross-channels over the junction between each A band in a myofibril and its adjacent I bands (A-I junction) 2) T tubules • "Transverse tubules" • Deep invaginations of the sarcolemma that run between each pair of terminal cisterns • The complex of the T tubule flanked by 2 terminal cisterns at the A-I junction is called a triad T-tubules are continuations of the sarcolemma, so they conduct each impulse to the deepest regions of the muscle fiber. This ensures that the deep-lying myofibrils contract at the same time as the superficial ones. - Impulses traveling down the T-tubules stimulate the release of calcium from the terminal cistern. - Calcium diffuses through the cytosol to the thin filaments and triggers muscle contraction. After contraction, calcium is pumped back into the sarcoplasmic reticulum for storage.

2 Mechanisms of Contraction

1) concentric - shortening muscle 2) eccentric - lengthening muscle

3 Types of Muscle Tissue

1. Skeletal Muscle Tissue—located in skeletal muscles: discrete organs that attach to and move the skeleton; makes up 40% of body weight; STRIATED muscle: has dark and light stripes extending transversely across its muscle cell; elongated cylindrical skeletal muscle cells are called MUSCLE FIBERS; they're innervated by VOLUNTARY division of nervous system and is subject to conscious control. 2. Cardiac Muscle Tissue—occurs only in the wall of the heart; they're STRIATED muscle and the contraction is INVOLUNTARY. 3. Smooth Muscle Tissue—found in the walls of hallow internal organs other than the heart, such as the stomach, urinary bladder, blood vessels, and respiratory tubes. The muscle cells of smooth muscles LACK STRIATIONS and are elongated and referred to as muscle fibers. They are innervated INVOLUNTARILY. **Cardiac and Smooth muscle tissues are collectively called visceral muscle meaning that they both occur in the visceral organs and are innervated by the involuntary division of the nervous system.

4 Functional Properties of Muscle Tissue (CEEE)

Contractility a. Muscles contain myofilaments—a specific type of microfilament that is responsible for the shortening of muscle cells. There are 2 Types that contain the protein either actin or myosin. Excitability a. Nerve signals or stimuli excite muscle cells and cause electrical impulses to travel along the cells' plasma membrane b. This initiates contraction in muscle cells Extensibility a. Can be stretched i. Contraction of one skeletal muscle stretches the opposing muscle b. Muscular wall of a hallow organ is stretched by the substances contained within that organ (e.g. food in digestive tract or urine in the urinary bladder) Elasticity a. After it's stretched, the tissue recoils passively and resumes its resting length

Direct/Fleshy vs. Indirect attachments

Direct or fleshy attachments • Attaching strands of connective tissue are so short that muscle fascicles themselves appear to attach directly to the bone Indirect attachments • Connective tissue extends beyond the end of the muscle fibers to form a cordlike tendon or a flat sheet called an aponeurosis • More common than direct attachments • **Raised bone markings are usually present where tendons meet bones; the markings include: tubercles, trochanters and crests • Raphe- when tendons attach to skin or cartilage or to sheets of fascia or to a steam of fibrous connective tissue (raphe), instead of bone

3 Connective Tissues and Fascicles (external to internal): Epi, Peri, Endo

Epimysium a. Outer layer of dense, irregular connective tissue that surrounds the entire skeletal muscle b. "Outside the muscle" c. Sometimes it blends with deep fascia that lies between neighboring muscles Perimysium a. Muscle fibers are separated into groups within each skeletal muscle; this looks like a bundle of sticks tied together and is called a fascicle. b. The perimysium is a fibrous connective tissue that surrounds each fascicle c. "Around the muscle" Endomysium a. "Within the muscle" b. Within the fascicles, each muscle fiber is surrounded by this endomysium which is a fine sheath of loose connective tissue consisting mostly of reticular fibers

"recruitment" (motor units)

Each individual muscle contains many motor units. o Recruitment- the addition of motor units to accomplish a movement o If a small force is required, a small number of motor units are stimulated o As more force is needed, additional motor units are recruited

A, M, I, Z.... bands!

MAKE THIS FLASHCARD: • Dark bands created by full length of thick filaments • Inner ends of thin filaments overlap the thick filaments (region is called the A band) • The central part of the A-band where no thin filaments reach is the H-zone. The M-line is in the center of the H-zone, which contains rods that hold the thick filaments together • Regions on either side of the A band only contain thin filaments are called I bands o A = dark bands are in the A bands (anisotropic) o I = light bands are in the I bands (isotropic)

motor neurons neuron

Motor Neuron- nerve cells that innervate muscle fibers o A neuron has cell processes that extend from the cell body: dendrites are receptive regions of a neuron; an axon is a long, singular cell process that initiates and transmits nerve impulses

Myoblasts

Myoblasts- embryonic mesoderm cells where all muscle tissues develop from; fuse to form skeletal muscle fibers. → This fusion is the reason skeletal muscle fibers are multi-nucleated. Muscle fibers begin to make thick and thin myofilaments and gain ability to contract. Skeletal muscles are contracting by Week 7, when the embryo is only about 2cm long. → Nerves grow into muscle masses from spinal cord, bringing skeletal muscles under control of nervous system. → Skeletal muscle fibers undergo mitosis. During childhood and adolescents, the cells length and thicken to keep up with the growing body.

optimal AND non-optimal muscle resting lengths for skeletal muscles

OPTIMAL: the length that will generate the MOST pulling force when the muscle is contracted. o Length occurs when a fiber is slightly stretched, so that its thin and thick filaments overlap to only a moderate extent o The myosin heads can move and pull along the whole length of the thin filaments. NONOPTIMAL: o If a muscle fiber is stretched so much that the thick and thin filaments don't overlap at all, the myosin heads have nothing to attach to, and therefore NO pulling force can be generated. o OR if the sarcomeres are so compressed that the thick filaments are touching the Z-discs, little further shortening can occur

Nerves and Blood Vessels

One nerve, one artery and one or more veins supply skeletal muscles: all of these enter or exit the muscle near the middle of its length. There is a high demand for rich blood supply to muscles because the contracting muscle fibers need nutrients and oxygen. Capillaries in the endomysium form a network; long capillaries are wavy when the muscle fibers contract and stretched straight when the muscle extends.

Muscle Attachments: Origin & Insertion

Origin- attachment of muscle on the less movable bone Insertion- attachment of muscle on the more movable bone • When the muscle contracts, its insertion is pulled toward its origin; the functions of the origin and insertion could switch depending on body position and the movement the produced when the muscle contracts • Attach to the origins and insertions via strong fibrous connective tissues that extend into the fibrous periosteum of the bone

The Skeletal Muscle Fiber

Skeletal muscle fibers are long, cylindrical cells. They're HUGE cells (diameter is 10-100 micrometers), and their length is from several to dozens (in short muscles) to centimeters (in long muscles). These cells are multi-nucleated, and the nuclei are on the periphery of each fiber, just deep to the sarcolemma.

innervation of skeletal muscles: what happens?

The release of calcium ions (Ca+ ions) from the sarcoplasmic reticulum and the subsequent contraction of skeletal muscle is initiated by nervous stimulation.

2 ways skeletal muscle fibers are categorized

Types of skeletal muscle fibers are categorized by (1) how they manufacture energy in the form of ATP, and (2) how quickly they contract.

Titan: what is it? what are its 2 basic functions?

WHAT IS IT? -- spring-like molecule in sarcomeres that resists overstretching; found in elastic filaments and extend from the Z-disc to the thick filament and run within the thick filament to attach to the M line. 2 Basic functions of TITAN o 1) Holds the thick filament in place in the sarcomere, which maintains the organization of the A band o 2) Unfolds when the muscle is stretched and then refolds when the stretching force is released, contributing to muscle elasticity • It strongly resists excessive stretching that tries to pull sarcomere apart

Strength of Skeletal Muscles: Men vs. Women

Women's skeletal muscles make up 36% of body mass Men's skeletal muscles make up 42% of body mass • Due to effects of androgen hormones on skeletal muscle (testosterone) • With strenuous muscle exercise, enlargement of muscles is greater in men than women, again because the influence of male sex hormones

what happens to muscles as humans age

o Amount of connective tissue in skeletal muscles increases o Number of muscle fibers decreases o Muscles become stringier/sinewy o Body weight declines o Loss of muscle leads to decrease in muscular strength (50% by age 80) • This condition is called sarcopenia: "flesh wasting" • Cause is because reduction in rate at which aging satellite cells can rebuild muscle • Can be reversed with exercise • Weight training in elderly increases size of remaining fibers, thus maintaining muscle strength

Myofibrils

o Create striations because its internal structure is long, rod-shaped organelles o Un-branched cylinders present in large numbers, making up 80% of the sarcoplasm o Specialized contractile organelles unique to muscle tissue o Contain myofilaments • Myofilaments- contractile proteins (actin or myosin) responsible for the shortening of muscle cells o In a fiber separated from one another by other components of the sarcoplasm, like mitochondria and glycosomes—both of which supply energy for muscle contraction o Composed of repeating segments called sarcomeres

Eccentric contraction

o Muscle generates force while lengthening o It's essential for controlled movement and resistance to gravity ----> Whenever muscles act as a break they're contracting eccentrically o Ex. During the down portion of push-ups, the pectoralis muscles of the chest contract eccentrically by resisting gravity and controlling the downward motion of the torso. o More examples: going down stairs, running downhill, landing from a jump

Concentric contraction

o Muscle generates force while shortening o In skeletal muscles, it's explained by the sliding filament mechanism: • Contraction results as the myosin heads of the thick filaments attach to the thin filaments at both ends of the sarcomere and pull the thin filament toward the center of the sarcomere by swiveling inward. • After the myosin head pivots at it's "hinge," it lets go, returns to its original position, binds to the thin filament farther along its length and pivots again. • (Swivel and grab and slides the filament along) • It's initiated by the release of calcium ions from the sarcoplasmic reticulum and the binding of those ions to the thin filaments. • Process is powered by ATP o Contraction affects the striation patterns of skeletal muscle: • In a fully relaxed sarcomere the thin filaments partially overlap the thick filaments. • When the muscle is stimulated to contract, the action of the thick filaments forcefully pulls the 2 Z-discs closer together, causing each sarcomere to shorten. As the Z-discs move closer together, the I-bands shorten and the H-zones disappear completely. • When the muscle is stretched rather than contracted, the overlap between the thin and thick filaments decreases: the I-bands and H-zones lengthen as the Z-discs move apart. There is NO CHANGE in the width of the A-bands.

2 categories for HOW SKEL. MUSCLE FIBERS ARE MANUFACTURED

o Oxidative fibers—predominately produce ATP aerobically (using oxygen) o Glycolytic fibers—make ATP anaerobically (without oxygen) via glycolysis

Myofascial Pain Syndrome (disorders of skeletal muscles)

pain caused by tightened bands of muscle fibers that twitch when the skin over them is touched. • Sensitive areas are called trigger points • Mostly associated with overused or strained postural muscles • Pain is felt from a distance from trigger point in places called reference zones • Affects ages 30-60 • Treated with non-steroidal anti-inflammatory drugs and by stretching affected muscles (massage).

fast oxidative fibers (FO)

• Intermediate in many characteristics • Speed of contraction • Like FG, they contract quickly • Like SO, they are oxygen-dependent and have HIGH myoglobin content, a large number of mitochondria, and a rich supply of capillaries • Depend on aerobic metabolism (with oxygen), so... • Fatigue resistant, but less so than SO

Muscle Extension

• Muscle tissue is extensible and muscle fibers are stretched (extended) back to their original length after they contract • What's responsible for this stretching? o A skeletal muscle (and its contained fibers) are stretched by a movement that is the opposite of the movement the muscle normally produces. • Ex. A muscle that normally abducts the arm at the shoulder is stretched by adducting the arm at this joint

slow oxidative fibers (SO)

• Obtain energy from aerobic metabolic reactions (with oxygen) • Large amount of mitochondria, the sites of aerobic metabolism • Rich supply of capillaries • Thin RED fibers • Abundant content of myoglobin—an oxygen-binding pigment in sarcoplasm • Contract slowly • Resistant to fatigue • Prolonged contractions • Generate LOW power • Ex. Lower back to maintain posture

fast glycolytic fibers (FG)

• Pale because they have little myoglobin • 2x the diameter of SO/slow oxidative fibers • Contain more myofilaments and thus generate MORE power • Depend on anaerobic pathways (without oxygen) to make ATP • Fewer mitochondria • Contract rapidly • Tire quickly • Ex. Muscles of upper limbs, which lift heavy objects for brief periods

Functions of the Muscle Tissue

• Produce movement (skeletal muscles attach to bones and move the body by moving the bones; muscles in the walls of visceral organs produce movement by squeezing fluids and other substances through these hollow organs) • Open and close body passageways (Sphincter muscles encircle many body openings and function as valves: relaxing to allow passage of a substance, contracting to close passageway. This function is especially apparent in the digestive and urinary tracts.) • Maintain posture and stabilize joints (skeletal muscles continuously contract to maintain posture; muscle tone is the constant low-level contraction of muscles and helps stabilize and strengthen synovial joints) • Generate heat (muscle contraction produces heat, which plays a vital role in maintaining normal body temperature: e.g. exercising means sweating to cool us down)

Sarcomeres

• Sarcomeres- basic unit of contraction in skeletal muscle o Boundaries at 2 ends of each sarcomere are called Z discs or Z lines o Attached to the Z-lines are many fine myofilaments called thin (actin) filaments (which primarily consist of actin). o In the center of the sarcomere and overlapping the inner ends of the thin filament is a cylindrical bundle of thick (myosin) filaments (consisting mostly of myosin molecules, but also ATPase enzymes that split ATP to release the energy required for muscle contraction) • Both ends of a thick filament are studded with knobs called myosin heads o The structure of a sarcomere explains the patterns of striations in skeletal muscle fibers

Satellite cells (in muscles)

• Skeletal muscle fibers are surrounded by satellite cells—immature cells that resemble undifferentiated myoblasts. During youth, satellite cells fuse into existing muscle fibers to help them grow. o After injuries to muscles, satellite cells proliferate in damaged muscle tissue and start producing proteins to repair injury o Regeneration capacity of skeletal muscle tissue isn't complete, and several damaged tissue is replaced by scar tissue