MUSCLE TISSUE

Question 1

Smooth muscle tissue

Answer 1

GENERAL- Muscle Tissue:

• Specialized contractile elements
• Origin: mesoderm: skeletal and cardiac muscle mesenchyme: smooth muscle
• Ability to contract is based on contractile proteins - actin and myosin and their mutual interaction
• Types:
  
  • Striated muscle (displays cross-striations in the light microscope):
    
    • Skeletal muscle (attached to bones, provides for movement, visceral striated muscle)
    • Cardiac muscle (myocardium of the heart wall)
  • Smooth muscle (cells do not display cross-striations)

SMOOTH MUSCLE

• does not display cross-striations
• Leimocytes= smooth muscle cells

Shape and Size

• Spindle/fusiform shaped cell, with a thin nucleus rod-like – found in the center of each cell.
• 5-10mm in diameter, 20- 500 mm in length, nucleus with several nucleoli
• Dense bodies, thin myofilaments, thick myofilaments, intermediate filaments
  
  • Dense bodies:
    
    • Oval structures in sarcoplasm
    • Place where actin fibers are anchored
    • Provide uniform contraction
    • IMF (desmin, Dense body vimentin)​
  • Sarcoplasmic reticulum is less developed
• Cytoplasm staining: medium eosinophilic

Nucleus:

• Found in the center of the cell’s broadest part, single nucleus
• Dark, cigar shaped

Contraction:

• Slow and involuntary-peristalsis.
• ATP hydrolosis occurs much slower than in straited muscle
• Actin Myosin – Do not exhibit organization such as in skeletal muscles!
  
  • Don’t form myofibrils
  • Bundles of actin myosin crisscrossed obliquely through the cell.
  • This provides a weaker but more massive contraction
  • Can contract up to 70% of its resting length
  • Contraction is regulated by autonomic nerves, hormones and local conditions.
• Conductive connections in gap junctions cause a group of smooth muscle cells to contract together.

No troponin present! The protein biding calcium ions here -Calmodulin.

Junctions and environment:

• Around the cells we can see Lamina basila, Reticular fibrils and Elastic fibers.
• Gap junctions: connect smooth muscle cells to each other- rapid spread of contraction

Location:

• In the wall of vessels- tunica media is formed by smooth muscles with circular arrangement
• Muscle tissue of organs
  
  • digestive tube,
  • respiratory passages,
  • lower urinary passages
  • reproductive system
  • Bile ducts

Production:

• synthesize collagen, elastin and proteoglycans – ECM components normally created by fibroblasts.
• RER is found in small amounts – helps in the production of collagen type 3 and Elastin and Fibrilin.

Innervation:

• Multi-unit smooth muscle (Visceral)
  
  • Dominant in the body, walls of tubular organs
  • Conductive connections in gap junctions cause a group of smooth muscle cells to contract together.
  • in which each cell is innervated and can contract independently.
• Unitary smooth muscle
  
  • Isolated smooth mucle cells without gap junctions
  • Separate innervation and fine motor regulation
  • Located in iris in which only a few cells are innervated but all cells are interconnected by gap junctions which allow the stimulus for contraction to spread as a synchronized wave among adjacent cells.

Because Smooth muscle is usually spontaneously active without nervous stimuli, its nerve supply serves primarily to modify activity rather than initiate it

Question 2

Cross-striated skeletal muscle tissue, myofibril

Answer 2

General:

• Cellular Aspect:
  
  • Cross striations bundles of long, cyndrical and multinucleated cells.
  • Nuclei are found in the periphery of the cells, just under the Sarcolemma.
  • Multinucleation results from the fusion of embryonic mesenchymal cells called myoblasts.
  • Size of cell:
    
    • Diameter of 10 – 100um, Length: 4 – 40 cm. It is multinucleated
• Contraction:
  
  • Quick, forceful and usually voluntary-
  • Caused by the contraction of Actin-Myosin filaments.

Organization of the skeletal muscle:

• Epimysium:
  
  • Sheath of dense connective tissue that surrounds the fascicles of a muscle.
  • Large vessels and nerves penetrate the epimysium and course through it.
  • Myotendinous junctions: place where the epimysium is continuous with the dense regular connective tissue of a tendon
• Muscle
• Perimysium:
  
  • Loose connective tissue, surrounds a group of fibers, forming a bundle or fascicle.
  • Fascicle is functional unit of the muscle in which fibers work together.
• Endomysium:
  
  • Each muscle fiber that is found in a fascicle is surrounded by even thinner connective tissue called the Endomysium reticular fibers. (lamina basalis + network of ret. Fibers)
  • Ground substances, few fibroblasts around each muscle fiber.
  • Capillaries and nerve endings run through here

The muscle fiber:

• Sarcolemma: (plasma membrane, basal lamina)
  
  • T- Tubules:
    
    • interconnected system of channels in plasma membrane
    • invagination of sarcolemma at the border between A and I band
      • allow synchronized contraction
      • fun perpindicular to the lonf axsis of muscle fiber fast transfer of stimulus from surface of muscle fiber to all the myofibrils
• Sarcoplasm: (cytoplasm)
  
  • Nuclei: peripheri
  • Myofibrils: fill almost the entire sarcoplasm
  • Mitochondria: high in number
  • Sarcoplasm reticulum: (smooth ER)
    
    • network covers each and every myofibril
    • Terminal cisternae
      
      • storage of calcium cations
      • create triads together sith adjacent T tubules
      • Transverse
    • L cisterns: longitudinal
• Sarcomere:
  
  • Average length 2-3 um, contains muscle myofillament
  • Subunit of myofibrils
  • Lines and bands:
    
    • Z line
      
      • 2 Z lines define 1 sarcomere
      • actin attached with alfa- actinin
    • M line
      
      • connection point between two Myosin myofilaments in the middle of the H-zone, made by Myomesin protein, that holds the thick filaments in place and creatine kinase which catalyzes transfer of phosphate groups to ADP (helping to supply ATP for muscle contraction).
    • I band:
      
      • actin
      • lighter
      • Z line in midline
    • A band:
      
      • myosin partially overlapping with actin
      • darker, in middle of sarcomere
      • M line in midline of A band
    • H band
      
      • bit lighter zone – a region consisting only of Myosin with no Actin present
      • during contraction its narrows until dissapring completly

1. Thin myofilaments (actin myofilaments)

• Double-helix of F-actin- (subunits are globular protien G actin)
• tropomyosin and troponin (a complex of 3 subunits)

G-actin

(a binding site for myosin)

2. Thick myofilament (myosin myofilament)

• contains myosin II
• Myosin molecule is composed of two heavy chains and two pairs of light chains
  
  • tail (double helix from 2 polypeptide chains)
    • = 2 heavy chains
  • Globular heads: has binding sites
    • for actin
    • for ATP
    • Myosin light chains

Assosiated protiens:

Regulatory proteins:

• Tropomyosin
  
  • double helix of 2 polypeptide chains, winds around F actin helix.
  • Stabilizes thin myofilaments
  • Binds to Troponin (TnT)
• Troponin, complex, made of 3 subunits:
  
  • TnT which attaches to tropomyosin
  • TnC which binds to Ca2+ (the calcium resleased from sarcoplasmic reticulum at beggining of contraction)
  • TnI which regulates the actin-myosin interaction, if no calcium is bound to TnC, TnL covers the binding site for myosin in the actin fiber- Contraction cant occur

Additional accessory proteins:

• Titin
  
  • In band I: attaches thick myofilaments to Z-line
• Myomesin
  
  • Bind thick filaments to M line
• Myomesin (middle) and C protein (paired)
  
  • maintain regular arrangement of myosin in sarcomere
  • Bind myosin to M line
• Nebulin
  
  • Twists around thin myofilaments helps to attach them to Z-line
• Tropomodulin:
  
  • Regulates the length of thin filaments
• α- actinin
  
  • attaches thin myofilaments do Z line

Mechanism of contraction: sliding of fillaments

• Impulse transduction:
  
  • Release of acetylcholine from synapse
  • Ach binds to receptors at the sarcolemma (postsynaptic membrane).
  • Motor end-plate (efferent nerve ending in the skeletal muscle)
• Change in ion permeability leads to depolarization of sarcolemma, which spreads along the membrane of T-tubules.
• Membrane depolarization of T tubules opens voltage gated Ca2+ ion channels of sarcoplasmic reticulum membranes.
• Ca2+ ions released from sarcoplasmic reticulum enter cytosol and bind to TnC subunit of troponin.
• Spatial configuration of troponin-tropomyosin complex is changed upon Ca2+ binding to TnC, binding sites for myosin on actin (active sites) are uncovered and myosin heads bind to actin.
• Myosin binding to actin results in hydrolysis of ATP (head has the binding site for ATP): ATP→ADP + Pi
• The energy released leads to flexion of the myosin head, which causes shifting of actin myofilament (sliding of the myofilament) towards the center of the sarcomere. Next the myosin head is disengaged and binds again to the next actin molecule.
• This cyclic process: binding, flexion, detachment of the myosin head repeats many times and leads to a complete insertion of actin filaments and shortening of the sarcomere (as a consequence the muscle fiber or cardiomyocytes contract as well)
• End of contraction (repolarization of sarcolemma)–transportofCa2+ back to sarcoplasmic reticulum, tropomyosin covers again myosin binding sites on G actin and actin myofilaments return to their original position.

Types of muscle fibers:

Energy source: fast – ATP and phosphocreatine Fatty acids, glucose

anaerobic glycolysis → lactate

Type I. slow oxidative muscle fibers:

• red muscle fibers
• contains high amount of myoglobin, rich capillary network
• metabolize fatty acids
• slow and continuous contraction (long term, low intensity)

Type II.

A: fast oxidative-glycolyic muscle fiber

• red muscle fiber less myoglobin, rich capilary network
• high glykolysis,
• fast and short contractions, medium intesity

B: fast glycolyic muscle fiber

• white muscle fiber
• low myoglobin, mitochondria, capillary
• low oxidation ability
• highest glykolysis,
• fast and short contractions- maximal intesity, low endurance
• anerobic domiance

Question 3

Cross-striated cardiac muscle tissue, intercalated disc

Answer 3

Cardiomyocyte=Its basic morphological and functional unit

General:

• Cylindrical shape, 15-20μm*85-100μm
• Oval nucleus(1-2), located in the center of each cell
• high number of mitochondria (40% of the volume), glycogen particles, lipid droppletes
• Atrial cardiomyocytes – granules with atrial natriuretic peptide (ANP)
• pigment lipofuscin
• Surrounding cardiomyocytes is the - endomysium (richly vascularized loose connective tissue with fine reticular and colagen fibers, C= capillary)

Shape and structure

• Cylindrical shape, with specific short processes
• Cross striations, individual cells, parallel to one another.
• At the end: intercalated disks, only in cardiac muscle!
• 15-20μm*85-100μm

Sarcolemma: plasma membrane

• T tubules: form diads,
• next to Z line (form diads with a single terminal cisterna)

Sarcoplasm: cytoplasm

• Strongly eosinophilic
• Endoplasm: sarcoplasm around nucelous without myofibirls where cells organells are located:
  
  • Rich in Mitochondria, glycogen particles, lipid droplets and Myofibrils.
    
    • Mitochondria are 40% of the sarcoplasm
• Sarcoplasmic reticulum: less developed than skeletal muscle, cardiac muscle dosent stop working, so it is not nesissary to store calcium cations during relaxation
• • Atrial cardiomyocytes – granules with atrial natriuretic peptide
• Pigment lipofuscin

Intercalated disks:

• Connections between cardiomyocytes
• They appear as a thin cross line
• The intercalated discs have 3 basic cells junctions:

Transverse:

• Desmosomes: macula adherens
  
  • binding adjacent cardiomyocytes
• Fascia adherents:
  
  • direct connection of the actin of the last sarcomere to the sarcolemma (Cell membrane) of the cardiomyocyte

Lateral

• Nexus/Gap Junctions:
  
  • Provides ionic continuity between cells for uniform contractions.
• Desmosomes: macula adherens
  
  • ​strength of junction

Contraction: Involuntary, rhythmic.

• Contractile structures – myofibrils; T tubules next to Z line (form diads with a single terminal cisterna)
• Mechanism is the same as skeletal muscle
• Strong rhythmic contraction, forever!
• At rest: mainly fatty acids, lactic acid and glucose
• At load: lactic acid use increases
• action potential arrises in SA node, and is conducted by specilized cardiomyocytes- electric conduction system if the heart

Control of cardiac activity

• Action potential arrises in SA node
• Heart rate and strength is affected by autonomic nerve fibers
• Spread of contraction wave is enabled by:
• Interconnection of cardiomyocytes via gap junctions
• Network structure of cardiomyocytes
• Electric conduction system, of the heart

The Transverse Tubules: more common in cardiac muscle, larger than in skeletal muscle, the sarcoplasmic reticulum is less developed. It contains numerous mitochondria, occupy 40% of the cytoplasmic volume (only 2% in skeletal muscle), reflecting the need for continuous aerobic metabolism in heart muscle. Fatty acids are the main fuel for the cardiac muscles, and storage in lipid droplets which seen in many of the cells.