Muscles

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Lecture Series

Muscle Tissue

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INTRODUCTION • Motion results from alternating contraction (shortening) and relaxation of muscles; the skeletal system provides leverage and a supportive framework for this movement. • The scientific study of muscles is known as myology.

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Muscle Tissue

• Alternating contraction and relaxation of cells • Chemical energy changed into mechanical energy

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DEVELOPMENT OF MUSCLE • With few exceptions, muscles develop from mesoderm. • Skeletal muscles of the head and extremities develop from general mesoderm; the remainder of the skeletal muscles develop from mesoderm of somites Somite—one of a paired, block-like masses of mesoderm arranged segmentally alongside the neural tube of the embryo, forming the vertebral column and segmental musculature.

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Developmental Anatomy of the Muscular System

• Develops from mesoderm • Somite formation – blocks of mesoderm give rise to vertebrae and skeletal muscles of the back • Muscles of head & limbs develop from general mesoderm

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Fusion of Myoblasts into Muscle Fibers

• Mature muscle cells developed from 100 myoblasts that fuse together in the fetus. (multinucleated) • Mature muscle cells are not known to divide. • Muscle growth is a result of cellular enlargement (hypertrophy) & not cell division (hyperplasia) • Satellite cells retain the ability to regenerate new cells.

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OVERVIEW OF MUSCLE TISSUE • Types of Muscle Tissue • Skeletal muscle tissue is primarily attached to bones. It is striated and voluntary. • Cardiac muscle tissue forms the wall of the heart. It is striated and involuntary. • Smooth (visceral) muscle tissue is located in viscera. It is nonstraited (smooth) and involuntary.

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3 Types of Muscle Tissue

• Skeletal muscle – attaches to bone, skin or fascia – striated with light & dark bands visible with scope – voluntary control of contraction & relaxation – Voluntary movement—example--walking

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3 Types of Muscle Tissue • Cardiac muscle – striated in appearance – involuntary control – autorhythmic because of built in pacemaker – Heart—pumps the blood throughout the body

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3 Types of Muscle Tissue • Smooth muscle – attached to hair follicles in skin – in walls of hollow organs -- blood vessels & GI – nonstriated in appearance – Involuntary – Involuntary movement—examples—digesting food, blood pressure

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Functions of Muscle Tissue • Producing body movements • Stabilizing body positions • Regulating organ volumes – bands of smooth muscle called sphincters • Movement of substances within the body – blood, lymph, urine, air, food and fluids, sperm • Producing heat – involuntary contractions of skeletal muscle (shivering)

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Properties of Muscle Tissue • Excitability – respond to chemicals released from nerve cells • Conductivity – ability to propagate electrical signals over membrane • Contractility – ability to shorten and generate force • Extensibility – ability to be stretched without damaging the tissue • Elasticity – ability to return to original shape after being stretched

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SKELETAL MUSCLE TISSUE • Each skeletal muscle is a separate organ composed of cells called fibers.

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Skeletal Muscle -- Connective Tissue • Superficial fascia is loose connective tissue & fat underlying the skin • Deep fascia = dense irregular connective tissue around muscle • Connective tissue components of the muscle include

– epimysium = surrounds the whole muscle – perimysium = surrounds bundles (fascicles) of 10-100 muscle cells – endomysium = separates individual muscle cells • All these connective tissue layers extend beyond the muscle belly to form the tendon

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Connective Tissue Components

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Connective Tissue Components • Tendons and aponeuroses are extensions of connective tissue beyond muscle cells that attach muscle to bone or other muscle. • A tendon is a cord of dense connective tissue that attaches a muscle to the periosteum of a bone. • An aponeurosis is a tendon that extends as a broad, flat layer.

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Nerve and Blood Supply • Each skeletal muscle is supplied by a nerve, artery and two veins. • Each motor neuron supplies multiple muscle cells (neuromuscular junction) • Each muscle cell is supplied by one motor neuron terminal branch and is in contact with one or two capillaries. – nerve fibers & capillaries are found in the endomysium between individual cells

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Muscle Fiber or Myofibers

• Muscle cells are long, cylindrical & multinucleated • Sarcolemma = muscle cell membrane • Sarcoplasm filled with tiny threads called myofibrils & myoglobin (red-colored, oxygen-binding protein) Principles of Human Anatomy and Physiology, 11e

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Sarcolemma, T Tubules, and Sarcoplasm • Skeletal muscle consists of fibers (cells) covered by a sarcolemma . – The fibers contain T tubules and sarcoplasm – T tubules are tiny invaginations of the sarcolemma that quickly spread the muscle action potential to all parts of the muscle fiber. • Sarcoplasm is the muscle cell cytoplasm and contains a large amount of glycogen for energy production and myoglobin for oxygen storage.

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Transverse Tubules

• T (transverse) tubules are invaginations of the sarcolemma into the center of the cell – filled with extracellular fluid – carry muscle action potentials down into cell • Mitochondria lie in rows throughout the cell – near the muscle proteins that use ATP during contraction Principles of Human Anatomy and Physiology, 11e

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Myofibrils and Sarcoplasmic Reticulum • Each fiber contains myofibrils that consist of thin and thick filaments (myofilaments).

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Myofibrils & Myofilaments

• Muscle fibers are filled with threads called myofibrils separated by SR (sarcoplasmic reticulum) • The sarcoplasmic reticulum encircles each myofibril. It is similar to smooth endoplasmic reticulum in nonmuscle cells and in the relaxed muscle stores calcium ions. • Myofilaments (thick & thin filaments) are the contractile proteins of muscle Principles of Human Anatomy and Physiology, 11e

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Sarcoplasmic Reticulum (SR)

• System of tubular sacs similar to smooth ER in nonmuscle cells • Stores Ca+2 in a relaxed muscle • Release of Ca+2 triggers muscle contraction Principles of Human Anatomy and Physiology, 11e

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Filaments and the Sarcomere • Thick and thin filaments overlap each other in a pattern that creates striations (light I bands and dark A bands) • The I band region contains only thin filaments. • They are arranged in compartments called sarcomeres, separated by Z discs. • In the overlap region, six thin filaments surround each thick filament

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Sarcomere

• Exercise can result in torn sarcolemma, damaged myofibrils, and disrupted Z discs (Clinical Application). Summary • Whole muscle>>group of muscle cells unite to form a muscle fiber (one giant large cell) >>myofibrils >>consists of sarcomeres>>thin and thick filaments

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Thick & Thin Myofilaments

• Supporting proteins (M line, titin and Z disc help anchor the thick and thin filaments in place) Principles of Human Anatomy and Physiology, 11e

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Thick & Thin Myofilaments Overlap

Dark(A) & light(I) bands (electron microscope) Principles of Human Anatomy and Physiology, 11e

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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 and dystrophin

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The Proteins of Muscle -- Myosin

• Thick filaments are composed of myosin – each molecule resembles two golf clubs twisted together – myosin heads (cross bridges) extend toward the thin filaments • Held in place by the M line proteins.

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The Proteins of Muscle -- Actin

• Thin filaments are made of actin, troponin, & tropomyosin • The myosin-binding site on each actin molecule is covered by tropomyosin in relaxed muscle • The thin filaments are held in place by Z lines. From one Z line to the next is a sarcomere.

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The Motor Unit

• Motor unit = one somatic motor neuron & all the skeletal muscle cells (fibers) it stimulates (10 cells to 2,000 cells) – muscle fibers normally scattered throughout belly of muscle – One nerve cell supplies on average 150 muscle cells that all contract in unison. • Total strength of a contraction depends on how many motor units are activated & how large the motor units are

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Structures of NMJ Region • Synaptic end bulbs are swellings of axon terminals • End bulbs contain synaptic vesicles filled with acetylcholine (ACh) • Motor end plate membrane contains 30 million ACh receptors.

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Overview: From Start to Finish Basic Structures • Nerve ending • Neurotransmitter • Muscle membrane • Stored Ca+2 • ATP • Muscle proteins

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Excitation - Contraction Coupling

• All the steps that occur from the muscle action potential reaching the T tubule to contraction of the muscle fiber. Principles of Human Anatomy and Physiology, 11e

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Review • Motor neuron—A neuron that transmits impulses from the central nervous system to an effector. • Axon—A nerve fiber that conducts a nerve impulse away from neuron cell body. • Synaptic end bulb—Tiny enlargement at the end of an axon that secretes the neurotransmitter. • Motor end plate—Specialized portion of a muscle fiber membrane at the neuromuscular junction. • Synaptic vesicles—vesicles that contain the neurotransmitter.

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Review • Neurotransmitter—Chemical substance secreted by the terminal end of an axon that stimulates a muscle fiber contraction or an impulse in another neuron. • Muscle fiber (skeletal muscle)—muscle cell • Motor unit—A motor neuron and the muscle fibers associated with it.

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Skeletal Muscle • One end of a skeletal muscle is usually fastened to a relatively immovable or fixed part, and the other end is connected to a moveable part on the other side of a joint. • The immovable end is called the origin and the movable end is called the insertion. -biceps—origin in the shoulder -insertion passes the elbow and connects with the radius (of the forearm) • Prime mover—when the arm is lifted horizontally away from the side, a contracting deltoid muscle is responsible for this movement. Near-by muscles help hold the shoulder steady so that the prime mover can do its job—these muscles are called synergists (prime mover is an agonist) Principles of Human Anatomy and Physiology, 11e

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Skeletal • Antagonist—These muscles are capable of resisting a prime mover’s action and are responsible for movement n the opposite direction. The antagonist of the prime mover that rises the arm can lower the arm. • Action—contraction (action potential) • Myofibrils—the contractile organelles of skeletal muscle • Muscle fiber—cells • Fascia—A sheet or broad band of fibrous connective tissue that supports and surrounds muscles and other organs of the body. • Peristalsis-Muscular movements or contractions (wavelike). • Fascicle-A small bundle or cluster (nerve or muscle). Principles of Human Anatomy and Physiology, 11e

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Naming Muscles • Latin and Greek roots—mid-to late—1500’s anatomists • Muscles can be named according to the direction their fibers run, their size, where they are found in the body, what bones they attach to, what the muscles look like, where it is in relation to certain bones and their function in the body. Often, the name can have a combination of the above. • Direction//Relative size//Relative shape//Principal action// Number of origins//Structure near which a muscle is found

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CARDIAC MUSCLE TISSUE - Overview • Cardiac muscle tissue is found only in the wall of the heart. – Its fibers are arranged similarly to skeletal muscle fibers. – Cardiac muscle fibers connect to adjacent fibers by intercalated discs which contain desmosomes and gap junctions. – Cardiac muscle contractions last longer than the skeletal muscle contractions due to the prolonged delivery of calcium ions from the sarcoplasmic reticulum and the extracellular fluid. – Cardiac muscle fibers contract when stimulated by their own autorhythmic fibers. • This continuous, rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue. Principles of Human Anatomy and Physiology, 11e

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Anatomy of Cardiac Muscle

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Striated , short, quadrangular-shaped, branching fibers Single centrally located nucleus Cells connected by intercalated discs with gap junctions Same arrangement of thick & thin filaments as skeletal muscle

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Cardiac versus Skeletal Muscle • More sarcoplasm and mitochondria • Intercalate—to insert between//intercalated disc(k)— an irregular transverse thickening in cardiac muscle. • Larger transverse tubules located at Z discs, rather than at A-l band junctions • Less well-developed SR • Limited intracellular Ca+2 reserves – more Ca+2 enters cell from extracellular fluid during contraction • Prolonged delivery of Ca+2 to sarcoplasm, produces a contraction that last 10 -15 times longer than in skeletal muscle Principles of Human Anatomy and Physiology, 11e

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Appearance of Cardiac Muscle

• Striated muscle containing thick & thin filaments • T tubules located at Z discs & less SR Principles of Human Anatomy and Physiology, 11e

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Physiology of Cardiac Muscle

• Autorhythmic cells – contract without stimulation • Contracts 75 times per min & needs lots of O2 • Larger mitochondria generate ATP aerobically • Extended contraction is possible due to slow Ca+2 delivery – Ca+2 channels to the extracellular fluid stay open

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Two Types of Smooth Muscle • Visceral (single-unit) – in the walls of hollow viscera & small BV – autorhythmic – gap junctions cause fibers to contract in unison • Multiunit – individual fibers with own motor neuron ending – found in large arteries, large airways, arrector pili muscles, iris & ciliary body

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SMOOTH MUSCLE • Smooth muscle tissue is nonstriated and involuntary and is classified into two types: visceral (single unit) smooth muscle and multiunit smooth muscle. – Visceral (single unit) smooth muscle is found in the walls of hollow viscera and small blood vessels; the fibers are arranged in a network and function as a “single unit.” – Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye. The fibers operate singly rather than as a unit.

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Microscopic Anatomy of the Smooth Muscle • Sarcoplasm of smooth muscle fibers contains both thick and thin filaments which are not organized into sarcomeres. • Smooth muscle fibers contain intermediate filaments which are attached to dense bodies.

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Microscopic Anatomy of Smooth Muscle

• Small, involuntary muscle cell -- tapering at ends • Single, oval, centrally located nucleus • Lack T tubules & have little SR for Ca+2 storage Principles of Human Anatomy and Physiology, 11e

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Microscopic Anatomy of Smooth Muscle • Thick & thin myofilaments not orderly arranged so lacks sarcomeres • Sliding of thick & thin filaments generates tension • Transferred to intermediate filaments & dense bodies attached to sarcolemma • Muscle fiber contracts and twists into a helix as it shortens -relaxes by untwisting

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Physiology of Smooth Muscle • Contraction starts slowly & lasts longer – no transverse tubules & very little SR – Ca+2 must flow in from outside • In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin (in place of the role of troponin in striated muscle) – calmodulin activates the enzyme myosin light chain kinase, which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate.

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Smooth Muscle Tone • The prolonged presence of calcium ions in the cytosol of smooth muscle fibers provides for smooth muscle tone, a state of continued partial contraction. • Smooth muscle fibers can stretch considerably without developing tension; this phenomenon is termed the stress-relaxation response. • Useful for maintaining blood pressure or a steady pressure on the contents of GI tract

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The End

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