Lecture 8: Histology of Muscles, Heart, and Blood Vessels

• The lecture discusses muscle, heart, and blood vessel histology.

Muscle Tissue

• There are four basic tissue types, muscle being one
• Contractions occur via myofilaments (actin and myosin).
• Myofilaments come in thin(actin) and thick(myosin) versions.
• Sarcoplasm exists within muscle tissue.

Types of muscle

• Smooth Muscle

  • Tapered cells
  • Nuclei located in the center of the cell
  • Gap junctions facilitate intercellular communication.
  • Contains thin actin and thick myosin II filaments.
  • Usually found in organs
  • Can be stimulated by mechanical impulses, electrical depolarization, or chemical stimuli.
  • Myosin filaments are side polar
  • Actin connects to dense bodies.
  • Thick myosin is scattered throughout the sarcoplasm.
  • Thin filaments contain actin.

• Skeletal Muscle

  • Striated.
  • Syncytium - multinucleated cell.
  • Muscle cell = a muscle fiber
  • Nucleus is located at the side of the cell.
  • Sarcolemma: Plasma membrane.
  • Organization of Skeletal Muscle
    • Muscle -> fascicle - muscle fiber -> myofibril -> myofilament.
    • Epimysium: surrounds fascicles.
    • Perimysium: surrounds fascicle
    • Endomysium: surrounds muscle fibers.
  • Three types of skeletal muscle fibers:
    • Type I (slow oxidative) fibers are slow-twitch, resistant to fatigue, and contain many mitochondria and myoglobin.
    • Type IIa (fast oxidative glycolytic) fibers- many mitochondria, myoglobin, glycogen, fatigue resistant, and are fast-twitch, high tension
    • Type IIb (fast glycolytic) fibers- low mitochondria and myoglobin, fatigue-prone with very high tension, contain high glycogen and are fast-twitch
    • Deeply stained indicates Type I.
    • Lightly stained indicates Type IIb.
    • Stains for oxidative enzymes in mitochondria.

• Cardiac Muscle

  • Found in the heart and is striated.
  • Nuclei is found in the center of the cell.
  • Branched cells
  • Intercalated discs
  • Intercalated Discs
    • Fascia adherens (where sarcomeres anchor to the plasma membrane).
    • Maculae adherentes (desmosomes) bind cells together.
    • Gap junctions allow ionic continuity.
  • Lots of mitochondria.

Myofibrils and Myofilaments

• Myofibrils are subunits of muscle fiber.
• Composed of myofilaments
• The functional unit of myofibrils is the sarcomere.
• Thick filament: Myosin II.
• Thin filament: F-actin, tropomyosin, and troponin.

Sarcomere

• Contains:
  • A band (darker)
  • I band (lighter)
  • Z line
  • M line
  • H band
• Troponin covers a myosin binding site on actin, and is uncovered by calcium (by binding to troponin C).

Heart Structure

• A four-chambered organ that facilitates unidirectional blood flow.
• Contains two ventricles and two atria.
• Composed of cardiac muscle, coronary arteries, and cardiac veins.
• Fibrous skeleton of the heart consists of dense irregular connective tissue, four fibrous rings, and two fibrous trigones.
• The membranous portion of the interventricular system is located between the atria and ventricles.
• Heart Wall Layers:
  • The epicardium adheres to the outer layer of the heart.
  • Myocardium
    • Cardiac muscle (thicker in ventricle than in atrium). -Endocardium
    • Endothelial and subendothelial connective tissue.

Conducting System

• Sinoatrial (SA) node generates electrical impulses, is called the pacemaker of the heart
• SA node impulse spreads along cardiac muscle fibers of atria
• Impulse is then taken up by atrioventricular (AV) node and then carried along the AV bundle (of His).
• Purkinje fibers carry the generated electrical signals down to muscles
• Purkinje fibers- specialized cardiac muscle cells, are larger than ventricular muscle cells

Blood Vessel Histology

• General Features of Blood Vessels:

  • Tunica intima (inner layer): endothelial cells and connective tissue.
  • Tunica media (middle layer): smooth muscle.
  • Tunica adventitia (outermost layer): connective tissue.

• Large (elastic) Arteries:

  • E.g Aorta and pulmonary arteries: brachiocephalic, common carotid
  • Walls contain multiple sheets of elastic lamellae.
  • Tunica intima: endothelium, subendothelial connective tissue, and inconspicuous internal elastic membrane.
  • Tunica media: multiple layers of smooth muscle cells separated by elastic lamellae which is the thickest layer
  • Tunica adventitia: thin layer of collagen and elastic fibers.
  • Vaso vasorum: blood vessels and nervi vascularis.

• Medium (Muscular) Arteries:

  • Contains more smooth muscle and less elastin than elastic arteries.
  • Most "named" arteries
  • Internal elastic membrane distinguishes from elastic arteries
  • Tunica intima is thinner than in elastic arteries.
  • Tunica media is almost entirely smooth muscle.
  • Tunica adventia is thick and separated by an external elastic membrane.

• Small Arteries and Arterioles:

  • Differ based on amount of smooth muscle layers.
  • Tunica adventia is thin and ill-defined for both.
    • Small arteries: -Tunica intima contains internal elastic membrane. -Tunica media has up to 8 layers of smooth muscle. -Arterioles: -Tunica intima may or may not have internal elastic membrane. -Tunica media has up to 1-2 layers of smooth muscle.

• Types of Capillaries:

  • Continuous: found in muscle, lung, and CNS where only small molecules can pass
    • Pericytes present, surrounding the capillary
  • Fenestrated: Endocrine glands, gallbladder, kidney, intestinal tract for fluid, metabolite absorption
    • contain 80-100 nm fenestrations
  • Discontinuous: found in liver, bone marrow, and spleen, characterized by the presence of Kupffer cells and a discontinuous basement membrane

• Venules:

  • Collect blood from capillaries.
  • Two types: post-capillary and muscular.
  • Post-capillary: Tunica intima with endothelial lining and basal lamina and pericytes.
    • No tunica media
  • Muscular:
    • Tunica intima- endothelial lining with basal lamina.
    • Tunica media- 1-2 layers of smooth muscles cells.
  • Small veins- continuous with venules.

• Medium Veins:

  • Veins with diameters up to 10 mm (e.g., radial, popliteal vein).
  • Tunica intima- endothelium and basal lamina, occasional smooth muscle cells.
  • Tunica media- several layers of circularly arranged smooth muscle cells with interspersed collagen and elastic fibers.
  • Tunica adventitia- typically thicker than the tunica media and consists collagen fibers and elastic fibers.

• Large Veins:

  • Veins with diameters greater than 10 mm (e.g., SVC, IVC, subclavian).
  • Tunica intima- Endothelial lining, basal lamina, connective tissue
  • Tunica media- thin, circumferentially arranged smooth muscle cells, collagen fibers and some fibroblasts.
  • Tunica adventitia- thickest layer, contains collagen, elastic fibers, fibroblasts, and longitudinally arranged smooth muscle cells.