Muscle Tissue Types and Functions

The text describes muscle tissue, focusing on its types, characteristics, and functions.

General Muscle Tissue Information

Muscle tissue's primary function is contraction, facilitated by specialized cells.

Muscle tissue is responsible for movement and shape alteration.

Types of Muscle Tissue

The classification of muscle tissue depends on how the cells appear.

Two main classifications are: striped and unstriped

There are also three types of striated muscle tissue, skeletal, visceral and cardiac

Striated Muscle Tissue

Shows transverse bands under a light microscope.

Includes skeletal, visceral, and cardiac muscle.

Skeletal muscle attaches to bone, allows movement, and maintains posture.

Visceral striated muscle is in soft tissues like the tongue, pharynx, and upper esophagus, and is also involved in speech, breathing, and swallowing.

Cardiac muscle is present around the heart and basal aspects of large veins which enter the heart

Smooth Muscle Tissue

Lacks transverse striations under a light microscope.

Responsible for involuntary movements

Muscle Fiber Composition

Muscle fibers are created by the fusion of individual myoblast cells.

The fibers contain multiple nuclei

Muscle fiber length varies.

Connective Tissue Sheaths

Provide structural support and organization for muscles.

Different functional layers, including the endomysium, perimysium, and epimysium

Connective tissue sheaths bind muscles together for support and also allow independent movement

Support transmission of mechanical forces

Endomysium

Reticular, loose connective tissue layer that surrounds individual muscle fibers

Perimysium

Consists of dense irregular connective tissue

Layer surrounds bundles of muscle fibers (fascicles)

Epimysium

Outermost layer of dense irregular connective tissue encompassing the entire muscle

Light under the Microscope

Reveals the nature of muscle cross sections

Myofibril sections appear evenly distributed or grouped.

Transverse sections show round muscle fibers polygonally shaped if compressed.

Cross Banding

Seen using polarized microscopy to reveal dark and light bands

Dark bands (A bands) exhibit anisotropy, meaning they refract light in two directions.

Light bands (I bands) are isotropic, meaning they do not alter the plane of polarized light.

Muscle Fiber Composition

Myofibrils consist of thick and thin myofilaments.

Thin filaments primarily consist of actin.

Thick filaments primarily consist of myosin II.

Myosin and actin constitute about 55% of total muscle protein.

Myofilaments are surrounded by the sarcoplasmic reticulum

Sarkomer is the smallest functional unit of all myofibrils

Each is between two Z lines

A Bands

Dark bands

H band is located inside the A band, in the centre

The H band has an M line running through

I Bands

Light bands

Have Z disks inside

Thin Filaments

Primarily consist of actin, troponin, and tropomyosin

G-actin’s polymerization forms two stranded F actin

Contains a negative and positive end

Tropomyosin

Two-polypeptide chain that binds F-actin

Works with troponin complexes to cover myosin binding sites.

Troponin

Contains three subunits

TnC binds calcium ions and starts muscle contraction

TnT binds tropomyosin, which stabilizes the bond

TnI binds Actin which inhibits actin-myosin interactions

Thick Filaments

Consist of myosin II, with two heavy and four light chains.

Formed by tail-to-tail aggregation into a bipolar arrangement.

Accessory Proteins

Help maintain spacing between filaments

Titin

Connects thick filaments to the Z disc.

Alpha-actinin

Attaches the thin filaments to Z disk

Nebulin

Runs alongside thin filaments and Z discs for structural support

Tropomodulin

Binds the free end of thin filaments and contributes to stability.

Myomesin

Found on the M line and functions to bind myosin filaments

C Protein

Myosin binding protein

Present on the M line and functions to bind myosin filaments

Desmin and Dystrophin

Support integrity of the sarcolemma.

Sarcolemma

Contains tubular invaginations, known as transverse or T tubules.

Contains openings for depolarization signals

Systems encircle the A-I bands

Releases calcium

Interacts with the T-tubules

Sarcoplasmic Reticulum

A network of smooth ER around the myofibrils, storing calcium.

Has terminal systems

Motor Innervation

Begins with motor units which connects spinalis or the brain

A motor neuron stimulates a group of muscle fibers.

Transmits acetylcholine across the synaptic cleft

Action potential is initiated by the influx of sodium ions

Action potentials

Spread along the muscle cell membrane and T tubules.

Triggers a conformational change in voltage-sensitive receptors.

Calcium from the sarcoplasmic reticulum is released

Leads to contractions

Sensory Innervation

Capsule receptors within the muscle itself

Includes muscle spindles and Golgi tendon organs

Muscle Spindles

Intrafusal

Sensitive to stretch

Golgi Tendon Organs

Located near the tendons

Help to determine contraction

Muscle Regeneration

Satellite cells regenerate damaged skeletal muscle by activating and fusing to form new muscle fibers

Cardiac Muscle

Characteristics of Cardiac Muscle

Typically have 1-2 centrally located nuclei

Form linked networks of cells

Show striations

Contain lots of Mitrochondria

T Tubules in relation to Z disks

Associated to Z disks

Less developed sarcoplasmic reticulum

T tubules associate with only one terminal cistern leading to diad formulation

More plentiful and larger in the ventricles

Intercalated Disks

Special junctions containing gap junctions and desmosomes which facilitate ion passage for synchronized contractions.

Cardiac Muscle Contraction

Typically involuntary rhythmic contractions

Initiated by the sinus node

Purkinje Fibers

Specialized cardiac cells responsible for impulse generation, located beneath the endocardium

Cardiac Muscle Metabolism

Rich In glycogen and mitochondria

Cardiac Muscle Contraction

Calcium ions from the sarcoplasmic reticulum and extracellular fluid mediate contraction.

Very limited capacity for regeneration

Smooth Muscle

Found in the walls of hollow organs

Smooth Muscle Characteristics

Have narrow, tapered and rod nuclei

Have central nuclei

Lack striations

Smooth muscle cells are linked by gap junctions for coordinated contractions.

Smooth Muscle Structure

Smooth muscle cells do not have T tubules

Contains scant sarcoplasmic reticulum

Plasma membranes have caveolae.

Smooth Muscle Composition

Includes actin and myosin filaments

Contain no troponin molecules

Contains calmodulin

Dense Bodies

α-actinin is enriched in these

Dense bodies attach to the internal parts of the sarcolemma as well as actin bundles

Regulation of Contraction

Triggered by calcium binding to calmodulin, activating myosin light-chain kinase (MLCK), initiating cross-bridge cycling.

Characteristically have slower rhythmic contractions

Smooth Muscle Functions

Maintains prolonged contractions for functions like vessel tone or peristalsis.

Smooth Muscle Contraction

Induced by mechanical, electrical, or chemical stimuli

Can regenerate from surrounding cells

Activated by oxytocin/vasopressin in the posterior pituitary and by epinephrine / norepinephrine in the adrenals

Inhibited by oestrogen and increased by progesterone