Introduction to Muscle Tissue

Introduction to Muscle Tissue

• Muscle tissue is one of the four primary tissue types in the body.
• It consists of muscle fibers, which are elongate cells capable of contracting along their longitudinal axis.
• There are three types of muscle tissue: skeletal, cardiac, and smooth.
• Skeletal muscle moves the body by pulling on bones.
• Cardiac muscle pushes blood through the circulatory system.
• Smooth muscle pushes fluids and solids along the digestive tract and performs varied functions in other systems.
• Muscle tissues share four basic properties: excitability, contractibility, extensibility, and elasticity.
• Excitability is the ability to respond to stimulation. Skeletal muscles normally respond to stimulation by the nervous system. Cardiac and smooth muscles respond to the nervous system and circulating hormones.
• Contractibility is the ability to shorten actively and exert a pull or tension.
• Extensibility is the ability to continue to contract over a range of resting lengths.
• Elasticity is the ability of a muscle to rebound toward its original length after a contraction.

Functions of Skeletal Muscle

• Skeletal muscles are contractile organs attached to bones of the skeleton directly or indirectly.
• Functions of skeletal muscle include producing skeletal movement, maintaining posture and body position, supporting soft tissues, regulating entering and exiting of material and maintaining body temperature.

Gross Anatomy of Skeletal Muscles

• Greek words sarkos (“flesh”) and mys (“muscle”) are prefixes to terms of the muscular system.
• The perimysium divides the muscle into fascicles, which contain bundles of muscle fibers.
• The perimysium contains blood vessels and nerves that supply each fascicle.
• The endomysium is a network of reticular fibers that surrounds each skeletal muscle fiber.
• The endomysium binds muscle fibers to each other and supports capillaries.
• Satellite cells, which function in the repair of damaged tissue, lie between the endomysium and muscle fibers.
• Connective tissue fibers of the endomysium and perimysium are interwoven, and those of the perimysium blend into the epimysium.
• Collagen fibers of the epimysium, perimysium, and endomysium may converge to form a fibrous connection of the muscle to bone, skin, or another muscle.
• These connections form tendons (thick cords or cables) or aponeuroses (flattened sheets).

Microanatomy of Skeletal Muscle Fibers

• Skeletal muscle fibers (myocytes) differ from other cells.
• They are very large, with a diameter of 100 µm and a length exceeding 12-16 inches.
• The sarcoplasm (cytoplasm) contains hundreds to thousands of myofibrils.
• Each myofibril is a cylindrical structure 1–2 µm in diameter and as long as the entire cell.
• The sarcolemma is the cell membrane of a skeletal muscle fiber.
• The sarcoplasmic reticulum (SR) is a network of tubules, vesicles, and cisternae.
• The sarcoplasmic reticulum (SR) stores calcium ions needed for muscle contraction.
• Mitochondria and glycogen granules are scattered among the myofibrils.
• A typical skeletal muscle fiber has hundreds of mitochondria.
• The entire cell shortens when a myofibril contracts.
• Mitochondrial biogenesis is the process by which new mitochondria are formed in the cell.
• There are two types of skeletal muscle fibers: extrafusal and intrafusal.
• Extrafusal muscle fibers make up the large mass of skeletal muscle tissue that contract.
• Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors that detect the amount and rate of change in fiber length).

Myofibrils and Myofilaments

• Myofibrils consist of bundles of myofilaments, which are protein filaments primarily made of actin and myosin.
• Actin filaments are “thin filaments."
• Myosin filaments are “thick filaments."
• Myosin heads are known as cross-bridges, connecting thick and thin filaments during muscle contraction.
• Actin and myosin are organized in repeating hexagonal units called sarcomeres.

Sarcomere Organization

• Sarcomeres give the muscle a striated appearance.
• Thick and thin filaments within a myofibril are organized in sarcomeres.
• Myofibrils are arranged parallel to the long axis of the cell, with their sarcomeres lying side by side.
• Each myofibril consists of approximately 10,000 sarcomeres.
• Sarcomeres are the smallest functional units of the muscle fiber.
• Interactions between thick and thin filaments of sarcomeres cause skeletal muscle fiber contractions.
• The Z line delineates the ends of the sarcomere.
• Thick filaments lie in the center of the sarcomere called the M line.
• The H zone contains exclusively myosin filaments.
• The A band includes the M line, H band, and zone of overlap (thick and thin filaments).
• The region between the A band and the Z line is part of the I band, which contains only actin filaments.
• The zone of overlap is where the thin filaments pass between the thick filaments.
• Thin filaments (actin) also contain tropomyosin (covers active sites) and troponin (holds tropomyosin in place).
• Sarcopenia is age related loss of muscle mass and function.
• Muscle Hypertrophy is an increase in the amount of cell subunits known as the myofilaments.

Muscle Contraction Theory and Process

• The sliding filament theory explains the mechanism for muscle contraction.
• The trigger for contraction is the presence of calcium ions, and the contraction requires ATP.
• Sliding occurs when myosin heads of thick filaments bind to active sites on thin filaments.
• Skeletal muscle cells transport calcium ions into the terminal cisternae of the sarcoplasmic reticulum. Electrical events at the sarcolemmal surface cause a contraction by triggering the release of calcium ions from the terminal cisternae.
• The electrical impulse is distributed by the transverse tubules that extend deep into the sarcoplasm of the muscle fiber.
• When an electrical impulse travels along a nearby T tubule, the terminal cisternae become permeable to calcium ions.
• Calcium ions diffuse into the zone of overlap and bind to troponin.
• Troponin changes shape and alters the position of the tropomyosin strand, exposing active sites on the actin molecules.
• Cross-bridge binding then occurs, and the contraction begins.
• The change in calcium permeability at the terminal cisternae is temporary, so additional impulses must be conducted along the T tubules.
• When electrical stimulation ends, the sarcoplasmic reticulum will recapture the calcium ions.
• The troponin–tropomyosin complex covers the active sites, and the contraction ends.
• The binding and breakdown of ATP prepares the myosin head for binding to an active site on actin.
• Once a cross-bridge forms, the myosin head pivots and pulls the thin filament toward the center of the sarcomere.
• Another ATP must bind to the myosin head for it to detach and reload for another cycle.
• Muscle fibers will eventually stop contracting as they run out of ATP.
• Creatine is naturally produced in the human body and is transported in the blood for use by muscles.
• Creatine phosphate (phosphocreatine or PCr) helps to supply energy to cells, primarily muscle, by increasing the formation of ATP.

Neural Control of Muscle Fiber Contraction

• Chemicals released by the motor neuron at the neuromuscular junction alter the transmembrane potential of the sarcolemma.
• This change initiates contraction by triggering the release of calcium ions by the sarcoplasmic reticulum.
• Each skeletal muscle fiber is controlled by a motor neuron inside the central nervous system.
• The axon of the motor neuron extends to the neuromuscular junction of that muscle fiber.
• The expanded tip of the axon is called the synaptic terminal.
• The synaptic terminal contains mitochondria and synaptic vesicles filled with acetylcholine (ACh).
• Acetylcholine is a neurotransmitter, a chemical released by a neuron to communicate with another cell.
• The ACh released binds to receptor sites on the motor end plate, initiating a change in the local transmembrane potential.
• This change results in the generation of an electrical impulse (action potential) that sweeps over the surface of the sarcolemma and into each T tubule.
• The synaptic cleft separates the synaptic terminal from the motor end plate.
• The synaptic cleft contains acetylcholinesterase (AChE), which breaks down molecules of ACh.
• Action potentials will continue to be generated until acetylcholinesterase removes the bound ACh.
• Myotomes are groups of muscles that a single spinal nerve root innervates.
• Rigor mortis occurs after death when muscles run out of ATP and the sarcoplasmic riticulum(SR) is unable to remove calcium ions triggering ongoing contraction due to cross bridges not detaching

Muscle Recoil

• Muscles return to normal length after contraction via muscle recoil.
• This is due to epimysium, perimysium, and endomysium retracting.
• Muscle recoil is also aided by antagonists and gravity.

Muscle Contraction Summary

• At the neuromuscular junction (NMJ), ACh released by the synaptic terminal binds to receptors on the sarcolemma.
• The SR releases stored calcium ions, increasing the calcium concentration of the sarcoplasm in and around the sarcomeres.
• Calcium ions bind to troponin, producing a change in the orientation of the troponin–tropomyosin complex that exposes active sites on the thin (actin) filaments.
• Myosin cross-bridges form when myosin heads bind to active sites.
• Cycles of cross-bridge binding, pivoting, and detachment occur, powered by the hydrolysis of ATP.
• Action potential generation ceases as ACh is broken down by acetylcholinesterase (AChE).
• The SR reabsorbs calcium ions, and the concentration of calcium ions in the sarcoplasm declines.

Axial Musculature

• Axial musculature moves the head, vertebral column, and rib cage for breathing.

Muscles of Facial Expression

• Muscles of facial expression originate on the surface of the skull.
• They move the skin when they contract.
• These muscles are innervated by the seventh cranial nerve (facial nerve).
• The largest group of facial muscles is associated with the mouth.
• Smaller groups of muscles control movements of the eyebrows, eyelids, scalp, nose, and external ear.

Extra-ocular Muscles

• Six extra-ocular muscles originate on the surface of the orbit and control the position of each eye.
• The extra-ocular muscles are innervated by the third (oculomotor), fourth (trochlear), and sixth (abducens) cranial nerves.
• There are 4 rectus muscles: superior, inferior, medial, and lateral.
• Also, there are inferior and superior oblique muscles

Muscles of the Tongue

• The muscles of the tongue insert on the tongue and have names ending in -glossus.

Muscles of Mastication

• The muscles of mastication move the mandible at the temporomandibular joint.

Muscles of the Pharynx

• The paired pharyngeal muscles are important in the initiation of swallowing and innervated by the ninth (glossopharyngeal) and tenth (vagus) cranial nerves.

Anterior Muscles of the Neck

• The anterior muscles of the neck control the position of the larynx, depress the mandible, tense the floor of the mouth, and provide a stable foundation for muscles of the tongue and pharynx.
• Scalene muscles are of interest in class.

Muscles of the Vertebral Column

• Muscles of the back form three layers: superficial, intermediate, and deep.
• The superficial muscles include the trapezius and latissimus dorsi, which move the pectoral girdle and upper limb.
• The intermediate layer includes spinal extensors and lateral flexors which are "I Like Sphagetti": Iliocostalis, Longissimus, and Spinalis.
• The deep layer interconnects and stabilizes the vertebrae to produce extension or rotation of the vertebral column.
• Vertebral column muscles include many extensors but few flexors due to location of body weight.

Muscles of the Anterior Thorax and Abdominal Muscles

• Two classes of muscles are seen in the anterior torso area: Oblique and Rectus.
• Oblique muscles compress structures or rotate the vertebral column.
• Rectus muscles are flexors of the vertebral column.
• The external intercostal muscles aid in inhalation by pulling the ribs up and anteriorly.
• The internal intercostal muscles aid in forced expiration by forcing the ribs toward one another.
• Both sets of intercostal muscles are important in costal breathing.
• The abdomen consists basically of four muscles: the external and internal oblique muscles, the rectus abdominis muscle, and transversus abdominis muscles.
• The rectus abdominis extends from the xiphoid process to the pubic symphysis.
• It is divided longitudinally by the linea alba.
• Transverse tendinous inscriptions divide this muscle into four repeated segments.
• Diastasis recti is a gap between the two sides of the rectus abdominis muscle.
• Diastasis recti is caused by the stretching of the linea alba.
• The transversus abdominis muscles aid in forced expiration and offer support to the abdominal and posterolateral region.

Diaphragm

• The term diaphragm refers to any muscular sheet that forms a wall.
• The diaphragm partitions or separates the abdominopelvic and thoracic cavities.
• Its contraction increases the volume of the thoracic cavity to promote inspiration.
• Its relaxation decreases the volume to facilitate expiration.
• Hiatal Hernia mimics heart attacks.

Muscles of the Pelvic Floor

• Muscles of the pelvic floor extend from the sacrum and coccyx to the ischium and pubis.
• Three main functions of muscles of the pelvic floor include supporting pelvic organs, flexing the sacrum and coccyx, and controlling urination and defecation.
• Muscular sphincters surround structures and permit voluntary control of urination and defecation.