Dr. Sami Zaqout IUG

Dr. Sami Zaqout IUG

Types of muscles

• There are three types of muscles:

• 1. Skeletal

• 2. Smooth

• 3. Cardiac

Dr. Sami Zaqout IUG

General characteristics of muscles

• The structural and functional units of muscles are formed of special elongated cells known as muscle fibers.

• The cell membrane of these muscle fibers is known as sarcolemma.

• The cytoplasm of these muscle fibers is known as sarcoplasm.

• Smooth endoplasmic reticulum is called sarcoplasmic reticulum.

• The muscle fibers may have transverse striations as the skeletal and cardiac muscle fibers, or they may show no striations as the plain or smooth muscles.

Dr. Sami Zaqout IUG

Dr. Sami Zaqout IUG

1. SKELETAL MUSCLES

Dr. Sami Zaqout IUG

Sites

• The skeletal muscles are attached to the skeleton.

• They are present also in the diaphragm, tongue, muscles of the face, eye, pharynx and upper third of esophagus.

• Skeletal muscles are voluntary muscles except in: – The upper third of esophagus

– Some muscles of the pharynx

– Cremasteric muscles of the spermatic cord.

Dr. Sami Zaqout IUG

Connective tissue around muscle fibers and bundles

The epimysium is the C. T. around the whole muscle.

The perimysium is the C. T. septa between the muscle bundles.

Each muscle fiber is surrounded by C. T. endomysium.

• In the connective tissue of the muscles, B. V., nerves and lymph vessels are present.

• C.T. Mechanically transmit the forces generated by contracting muscle cells

Dr. Sami Zaqout IUG

Characteristics of skeletal muscles

• They are formed of striated muscle fibers.

• Each muscle fiber is a single cell which varies in length from 1 mm up to 30 cm.

• The muscle fibers do not branch except in the tongue and face muscles.

Dr. Sami Zaqout IUG

Characteristics of skeletal muscles

• Each muscle fiber is a multinucleated cell, it has many oval nuclei.

• The nuclei are peripheral in position, present under the sarcolemma.

• The sarcoplasm is acidophilic rich in glycogen and myoglobin.

• The myoglobin is formed of pigmented protein.

Dr. Sami Zaqout IUG

Characteristics of skeletal muscles

• The sarcoplasm contains many mitochondria and many smooth endoplasmic reticulum which is called sarcoplasmic reticulum.

• The sarcoplasm contains also longitudinal fibrils known as myofibrils.

• Mature muscle cells have negligible amounts of rough endoplasmic reticulum and ribosomes

Dr. Sami Zaqout IUG

The myofibrils of skeletal muscles

• The myofibrils are the contractile threads which are arranged longitudinally in the sarcoplasm of each muscle fiber.

• The arrangement of myofibrils near each other shows transverse striations.

• The transverse striations in the muscle fibers are due to presence of alternating dark and light bands on each myofibril.

Dr. Sami Zaqout IUG

The myofibrils of skeletal muscles

• Each dark band of one myofibril is present beside the dark band of the adjacent myofibril.

• These arrangements of dark and light bands give the muscle fiber the appearance of transverse striations.

Dr. Sami Zaqout IUG

The myofibrils of skeletal muscles • The dark bands have double refraction to

polarized light. They are called Anisotropic bands or A-Bands.

• Each dark band is further subdivided by a pale area in its centre called Hensen's zone or H-Band.

• Bisecting the H band is the M-line, a region at which lateral connections are made between adjacent thick filaments.

• The major protein of the M-line is creatine kinase.

• The light bands are not refractile to polarized light.

• They are called Isotropic bands or I-Bands.

• Each light band is also further subdivided by a darkly-stained zone present at its center and is called Z-line.

Dr. Sami Zaqout IUG

The sarcomere

• The area of the muscle fiber enclosed between two Z-Discs is called Sarcomere.

• The sarcomeres are the functional contractile units of the muscle fiber.

• Each Sarcomere includes the whole dark band and the two halves of the two light bands on both sides.

• The sarcomeres of each muscle fiber contract and relax as one unit.

• Their contractions are due to the presence of longitudinally arranged very fine electron microscopic threads known as myofilaments.

Dr. Sami Zaqout IUG

Myofilaments

• The myofilaments are 2 types: – Thin myofilaments – Thick myofilaments

• 1. Thin filaments or actin

filaments which are formed mainly by actin protein.

• They extend from the Z line till the middle of the dark band

• They terminate just before the middle of the dark band, therefore, the middle of the dark band appears light and is called H-band.

Dr. Sami Zaqout IUG

Thin myofilaments

Dr. Sami Zaqout IUG

Thick myofilaments

• Thick filaments or myosin filaments which are formed of protein known as Myosin.

• They extend in the dark bands only.

• Both ends of the thick filament are free, while the thin filament has only one free end and the other end is attached to the Z-line.

Dr. Sami Zaqout IUG

Thick myofilaments

• Myosin, a much larger complex, can be dissociated into two identical heavy chains and two pairs of light chains.

• Myosin heavy chains are thin, rodlike molecules made up of two heavy chains twisted together.

• Small globular projections at one end of each heavy chain form the heads, which have ATP-binding sites as well as the enzymatic capacity to hydrolyze ATP (ATPase activity) and the ability to bind to actin.

• The four light chains are associated with the head

Dr. Sami Zaqout IUG

Cross-bridges between thin and thick filaments

• These bridges, which are known to be formed by the head of the myosin molecule plus a short part of its rodlike portion, are involved in the conversion of chemical energy into mechanical energy.

Dr. Sami Zaqout IUG

Sarcoplasmic reticulum & transverse tubule system

• To provide for a uniform contraction, skeletal muscle possesses a system of transverse T tubules.

• These fingerlike invaginations of the sarcolemma form a complex anastomosing network of tubules that encircles the boundaries of the A-I bands of each sarcomere in every myofibril.

• Adjacent to opposite sides of each T tubule are expanded terminal cisternae of the sarcoplasmic reticulum.

Dr. Sami Zaqout IUG

The triad of tubular system

• The triad of tubular system includes: – One Transverse Tubule

Surrounded By – Two Sarcoplasmic Tubules

Dr. Sami Zaqout IUG

The role of tubular system in muscular contractions

During muscular contraction the nerve impulse reaches the sarcolemma, then it passes through the transverse T-tubules.

The impulse then goes into the two tubules of the endoplasmic reticulum.

These tubules pump calcium ions between the myosin and actin molecules.

This will result in muscular contraction.

Dr. Sami Zaqout IUG

Contraction of muscles

• The energy needed for muscle contraction comes from the transformation of ATP into ADP.

• This energy causes the gliding of the thin filaments over the thick filaments.

Dr. Sami Zaqout IUG

Contraction of muscles

• The thin filaments thus slide towards the middle of the sarcomere.

• This will result in pulling the two Z-lines behind them.

• The H-Zones disappear during muscular contractions, because they will contain both thick and thin filaments.

Dr. Sami Zaqout IUG

Contraction of muscles

Dr. Sami Zaqout IUG

Innervation of muscles • Myelinated motor nerves

branch out within the perimysial connective tissue, where each nerve gives rise to several terminal twigs.

• At the site of innervation, the nerve loses its myelin sheath and forms a dilated termination that sits within a trough on the muscle cell surface.

• This structure is called the motor end plate , or myoneural junction

Dr. Sami Zaqout IUG

Myasthenia gravis

• An autoimmune disorder characterized by progressive muscular weakness caused by a reduction in the number of functionally active acetylcholine receptors in the sarcolemma of the myoneural junction.

• This reduction is caused by circulating antibodies that bind to the acetylcholine receptors in the junctional folds and inhibit normal nerve muscle communication.

Dr. Sami Zaqout IUG

Dr. Sami Zaqout IUG

Types of skeletal muscle fibers

• According to the amount of myoglobin, the type of innervation and the mode of contraction, the muscle fibers are classified into:

• 1. Type I: Red Muscle Fibers – They have large amounts of myoglobin, mitochondria

and cytochrome.

– They have small diameters.

– They can sustain contraction for a long time without fatigue.

– Their energy is derived from oxidation of fatty acids.

Dr. Sami Zaqout IUG

Types of skeletal muscle fibers

• 2. Type II: White Muscle Fibers – They have small amounts of myoglobin and few

mitochondria. – They have wide diameters. – Their contractions are quick, but they become fatigued easily. – Their energy is derived from glycolysis.

• 3. Intermediate Muscle Fibers

– They have intermediate characters between red and white fibers.

Dr. Sami Zaqout IUG

Comparison of types of skeletal muscle fibers

Dr. Sami Zaqout IUG

Hypertrophy vs. Hyperplasia

• Tissue growth by an increase of cell volume, is called hypertrophy.

• Tissue growth by an increase in the number of cells is called hyperplasia

• Hyperplasia does not occur in either skeletal or cardiac muscle but does take place in smooth muscle, whose cells have not lost the capacity to divide by mitosis.

• Hyperplasia is rather frequent in organs such as the uterus, where both hyperplasia and hypertrophy occur during pregnancy.

Dr. Sami Zaqout IUG

Development of skeletal muscles

• In the embryo muscle fibers develop from myoblast cells.

• In adults, they develop from satellite cells present under the sarcolemma.

• Growth of muscles and repair of torn muscles occur by the proliferation of the satellite cells which are present under the sarcolemma and can differentiate into new muscle fibers.

Dr. Sami Zaqout IUG

Muscle spindles

• All human striated muscles contain encapsulated proprioceptors known as muscle spindles.

• These structures consist of a connective tissue capsule surrounding a fluid-filled space that contains a few long thick muscle fibers and some short thinner fibers collectively called intrafusal fibers.

Dr. Sami Zaqout IUG

Golgi tendon organs

• In tendons, near the insertion sites of muscle fibers, a connective tissue sheath encapsulates several large bundles of collagen fibers that are continuous with the collagen fibers that make up the myotendinous junction.

• These structures, known as Golgi tendon organs, contribute to proprioception by detecting tensional differences in tendons.

Dr. Sami Zaqout IUG

2. CARDIAC MUSCLE

Dr. Sami Zaqout IUG

Characteristics of cardiac muscle fibers

• Cardiac cells form complex junctions between their extended processes.

• Cells within a chain often branch and bind to cells in adjacent chains.

• 15µm in diameter and from 85 to 100µm in length

• They exhibit a cross-striated banding pattern identical to that of skeletal muscle.

Dr. Sami Zaqout IUG

Characteristics of cardiac muscle fibers

• Unlike multinucleated skeletal muscle each cardiac muscle cell possesses only one or two centrally located pale-staining nuclei.

• Surrounding the muscle cells is a delicate sheath of endomysial connective tissue containing a rich capillary network.

Dr. Sami Zaqout IUG

Characteristics of cardiac muscle fibers

• Cardiac muscle cells contain numerous mitochondria, which occupy 40% or more of the cytoplasmic volume.

• Fatty acids are stored as triglycerides in the numerous lipid droplets seen in cardiac muscle cells.

• A small amount of glycogen is present.

• Lipofuscin pigment granules often seen in long-lived cells, are found near the nuclear poles of cardiac muscle cells.

Dr. Sami Zaqout IUG

Characteristics of cardiac muscle fibers

• Membrane-limited granules are found at both poles of cardiac muscle nuclei and in association with Golgi complexes in this region.

• These granules are most abundant in muscle cells of the right atrium.

• These atrial granules contain the high-molecular-weight precursor of a polypeptide hormone known as atrial natriuretic factor.

Dr. Sami Zaqout IUG

• Intercalated disks represent junctional complexes found at the interface between adjacent cardiac muscle cells.

• The junctions may appear as straight lines or may exhibit a steplike pattern.

• Two regions can be distinguished in the steplike junctions: – Transverse portion

– Lateral portion

Characteristics of cardiac muscle fibers

Dr. Sami Zaqout IUG

Intercalated disks

• There are three main junctional specializations within the disk.

Fasciae adherentes

the most prominent membrane specialization in transverse portions of the disk.

Maculae adherentes (desmosomes) are also present in the transverse portion and bind the cardiac cells together

Gap junctions

On the lateral portions of the disk, provide ionic continuity between adjacent cells

Dr. Sami Zaqout IUG

Sarcoplasmic reticulum & transverse tubule system

• The T tubule system and sarcoplasmic reticulum are not as regularly arranged in the cardiac myocytes.

• Cardiac T tubules are found at the level of the Z band.

• The sarcoplasmic reticulum is not as well developed and wanders irregularly through the myofilaments.

Dr. Sami Zaqout IUG

The diads of tubular system

• Triads are not common in cardiac cells, because the T tubules are generally associated with only one lateral expansion of sarcoplasmic reticulum cisternae.

• Thus, heart muscle characteristically possesses diads composed of: – One T tubule

– One sarcoplasmic reticulum cisterna.

Dr. Sami Zaqout IUG

3. SMOOTH MUSCLES

Dr. Sami Zaqout IUG

Characteristics of smooth muscles

• Smooth muscle is composed of elongated, nonstriated cells, each of which is enclosed by a basal lamina and a network of reticular fibers.

Dr. Sami Zaqout IUG

Characteristics of smooth muscles

• Smooth muscle cells are fusiform.

• They may range in size from 20 µm in small blood vessels to 500 µm in the pregnant uterus.

• Each cell has a single nucleus located in the center of the broadest part of the cell.

Dr. Sami Zaqout IUG

Characteristics of smooth muscles

• The borders of the cell become scalloped when smooth muscle contracts, and the nucleus becomes folded or has the appearance of a corkscrew.

• Two types of dense bodies appear in smooth muscle cells, both contain α-actinin: – One is membrane associated

– The other is cytoplasmic.

Dr. Sami Zaqout IUG

Characteristics of smooth muscles

• Concentrated at the poles of the nucleus are mitochondria, polyribosomes, cisternae of rough endoplasmic reticulum, and the Golgi complex.

• Pinocytotic vesicles are frequent near the cell surface.

Dr. Sami Zaqout IUG

Characteristics of smooth muscles

• A rudimentary sarcoplasmic reticulum is present; it consists of a closed system of membranes, similar to the sarcoplasmic reticulum of striated muscle.

• T tubules are not present in smooth muscle cells.

Dr. Sami Zaqout IUG

Contraction in smooth muscle cells

• In smooth muscle cells, bundles of myofilaments crisscross obliquely through the cell, forming a latticelike network.

• These bundles consist of thin filaments containing actin and tropomyosin and thick filaments consisting of myosin.

• Ca2+ in a smooth muscle complexes with calmodulin.

• The Ca2+ - calmodulin complex activates myosin light-chain kinase, the enzyme responsible for the phosphorylation of myosin.

Dr. Sami Zaqout IUG

Dr. Sami Zaqout IUG

Intermediate filaments

• Desmin has been identified as the major protein of intermediate filaments in all smooth muscles

• Vimentin is an additional component in vascular smooth muscle.

Dr. Sami Zaqout IUG

Sites Of Smooth Muscles

• 1. Digestive system: Muscles in the wall of the lower third of esophagus, the wall of stomach, intestine, gall bladder and wall of salivary and pancreatic ducts.

• 2. Respiratory system: Wall of trachea, bronchi and bronchioles.

• 3. Urinary system: Wall of ureter, urinary bladder and urethra.

Dr. Sami Zaqout IUG

Sites Of Smooth Muscles

• 4 Male genital system: Epididymis, vas deferens, prostate and penis.

• 5. Female genital system: Fallopian tube, uterus and vagina.

• 6. All the media (middle part) of blood and lymph vessels.

• 7. Capsule of glands and spleen, ciliary muscles and iris of the eye and also in the arrector pili muscles of the hairy skin.

Dr. Sami Zaqout IUG

Regeneration of Muscle Tissue

• Cardiac muscle has almost no regenerative capacity beyond early childhood. Defects or damage in heart muscle are generally replaced by scars.

• Skeletal muscle the tissue can undergo limited regeneration. The source of regenerating cells is believed to be the satellite cells.

• Smooth muscle is capable of an active regenerative response.

Dr. Sami Zaqout IUG

Dr. Sami Zaqout IUG