Muscular Tissue: Skeletal & Cardiac

Muscular Tissue Overview

• Muscle tissue is specialized for contraction
• Muscle tissue is responsible for movement and force generation
• Three major types exist, each differing in structure and function

Skeletal Muscle

• Facilitates voluntary body movements
• Composed of long, cylindrical fibers
• Fibers contain multiple, peripherally located nuclei
• Exhibits striations due to sarcomere alignment
• Contains myofibrils, bundles of actin and myosin
• Epimysium: The outer sheath that covers the entire muscle
• Perimysium: Encloses fascicles
• Endomysium: Surrounds individual muscle fibers
• Regeneration is possible via satellite cells that activate upon injury

Cardiac Muscle

• Facilitates involuntary heart contractions
• Composed of branched cells, each with a centrally located nucleus
• Presence of intercalated discs, unique to cardiac tissue
• Has a striated appearance
• Intercalated discs contain desmosomes, fascia adherens, and gap junctions
• Intercalated discs enable synchronous contraction
• Limited regeneration; injuries typically replaced by scar tissue

Smooth Muscle

• Facilitates involuntary contractions in hollow organs
• Composed of spindle-shaped cells
• Each cell contains a single, central nucleus
• Lacks striations due to irregular arrangement of contractile filaments
• Does not contain sarcomeres and T-tubules
• Contains dense bodies, equivalent to Z discs
• Caveolae: Small sarcolemma invaginations, calcium entry points
• Regulation is controlled by calmodulin, instead of troponin
• Calmodulin activates myosin light chain kinase (MLCK) to initiate contraction

Sarcomere Components

• Sarcomere: Functional unit of striated muscle, extending from Z disc to Z disc
• Z disc anchors thin filaments and stabilizes sarcomere alignment
• Z disc is composed of α-actinin
• Thin filaments composed of actin, tropomyosin, and troponin
• Thick filaments made of myosin
• I band contains only thin filaments
• A band contains full length of thick filaments and overlaps thin filaments
• H zone contains thick filaments only
• M line stabilizes the sarcomere

Regulatory and Structural Proteins

• Tropomyosin covers myosin-binding sites on actin (at rest)
• Troponin complex contains TnC, TnT, and TnI
• TnC binds calcium
• TnT binds to tropomyosin
• TnI inhibits actin-myosin interaction
• Titin connects thick filaments to Z discs
• Titin provides elastic recoil and maintains sarcomere integrity
• Desmin intermediate filament protein links adjacent myofibrils and supports structure

Mechanism of Muscle Contraction

• Sliding filament theory explains muscle fiber shortening during contraction
• Myosin heads bind to Actin
• Thin filaments are pulled toward the M line
• Sarcomeres shorten, bringing Z discs closer, but filaments do not change length
• Calcium ions (Ca2+) are stored in the sarcoplasmic reticulum (SR)
• Calcium is released upon excitation and binds to troponin C or calmodulin
• ATP provides energy for myosin head pivoting/detachment
• Rigor mortis occurs when ATP is absent, locking muscle contraction

Muscle Fiber Types

• Type I (Slow-Twitch / Oxidative) fibers are rich in myoglobin and mitochondria
• Type I fibers use aerobic metabolism, fatigue resistant and adapted for endurance
• Type IIa (Fast-Twitch / Oxidative-Glycolytic) fibers have intermediate fatigue resistance
• Type IIa fibers use both aerobic and anaerobic pathways
• Type IIb (Fast-Twitch / Glycolytic) fibers are low in myoglobin
• Type IIb fibers rely on anaerobic glycolysis, fatigue quickly, and suited for short bursts

Muscle Plasticity

• Hypertrophy increases muscle fiber size through repeated stimulation
• Atrophy decreases muscle mass from disuse, aging, or denervation
• Hyperplasia increases muscle fiber number, more common in smooth muscle

Muscle Cell Ultrastructure

• T-tubules transmit action potentials into the cell
• T-tubules are absent in smooth muscle
• The sarcoplasmic reticulum (SR) stores and releases calcium,
• The sarcoplasmic reticulum works with T-tubules regulating calcium dynamics
• Caveolae function in calcium entry and signaling
• Caveolae substitute for the lack of T-tubules
• Caveolae initiate contraction via calcium-calmodulin pathway

Blood Vessel Structure

• Blood vessels (except capillaries) have three tunics
• Tunica intima is the innermost layer that consists of endothelium, subendothelial layer
• The endothelium is a single layer of squamous epithelial cells
• The endothelium minimizes friction and regulates blood-tissue interaction
• The subendothelial layer is thin connective tissue with fibroblasts
• The internal elastic lamina (IEL), in muscular arteries, allows vessel flexibility
• Tunica media is the muscular layer in vessel walls
• The tunic media is composed of concentric smooth muscle cells responsible for vasoconstriction/vasodilation
• Elastic fibers are abundant in elastic arteries
• Elastic arteries contain elastic lamellae and smooth muscle
• Elastic arteries absorb high pressure during systole, maintain flow during diastole
• Tunica adventitia is the outermost connective tissue layer that contains collagen fibers, longitudinal smooth muscle
• The tunica adventitia contains vasa vasorum and nerve fibers
• Tunica adventitia is most prominent in veins

Classification of Blood Vessels

• Arteries have elastic and muscular types
• Elastic arteries contain tunica media with abundant elastic lamellae
• Elastic arteries conduct blood away from the heart and dampen pressure pulsations
• Muscular arteries contain tunica media with smooth muscle
• Tunica media regulates blood flow to organs
• Arterioles control blood flow into capillary beds
• Arterioles are essential in blood pressure regulation
• Capillaries are composed of endothelium and basal lamina only
• Capillaries facilitate nutrient, gas, and waste exchange
• Continuous capillaries have tight junctions and are found in muscle, brain, and lungs
• Fenestrated capillaries have small pores and facilitate water and small molecule exchange
• Sinusoidal capillaries (discontinuous) allow passage of large proteins/cells and are found in liver, spleen, bone marrow
• Postcapillary venules collect blood from capillaries
• Postcapillary venules are a primary site of white blood cell migration
• Medium and large veins have thin tunica media and a thick tunica adventitia.
• Veins may contain valves that prevent backflow of blood against gravity

Vessel Specializations

• The internal elastic lamina (IEL) is a wavy band present in muscular arteries
• Vasa vasorum supplies oxygen and nutrients to vessel walls
• Arteriovenous shunts bypass capillaries and function in thermoregulation

Heart Wall Layers

• The endocardium is the innermost layer lined with endothelium, and supported by connective tissue
• Subendocardial layer contains Purkinje fibers
• The myocardium is the thickest layer with cardiac muscle fibers connected by intercalated discs
• Myocytes are joined by desmosomes and gap junctions
• The epicardium (visceral pericardium) is lined by mesothelium and contains blood vessels, nerves, and adipose tissue
• The fibrous cardiac skeleton is dense connective tissue that supports valves and insulates atria from ventricles
• Heart valves are covered with endocardium and have a fibrous core
• Chordae tendineae and papillary muscles prevent valve prolapse during contraction

Blood Components

• Erythrocytes (red blood cells) are anucleated biconcave disks with salmon-pink cytoplasm
• Erythrocytes function in oxygen and carbon dioxide transport
• Platelets (thrombocytes) are cell fragments with no nucleus and deep purple staining
• Platelets function in blood clot formation
• Neutrophils are the most abundant white blood cells
• Neutrophils have a multi-lobed nucleus and light pink cytoplasm with fine granules
• Neutrophils are first responders to infection and phagocytose bacteria
• Eosinophils have a bilobed nucleus and large, red-orange cytoplasmic granules
• Eosinophils are active in parasitic infections and allergic reactions
• Basophils have granules that obscure the nucleus and contain histamine and heparin
• Basophils are involved in hypersensitivity and inflammatory responses
• Lymphocytes have a round, dark nucleus and little cytoplasm
• Lymphocytes consist of T cells, B cells and NK cells
• Lymphocytes facilitate immune regulation and memory
• Monocytes are the largest white blood cells that differentiate into macrophages

Hematopoiesis

• Hematopoiesis is the formation and development of blood cells
• Prenatal hematopoiesis occurs in the yolk sac, liver, and spleen
• Postnatal hematopoiesis occurs in red bone marrow
• All blood elements originate from a pluripotent hematopoietic stem cell

Clinical Correlations in Blood

• Increased eosinophils may indicate parasitic infections
• Increased neutrophils may indicate bacterial infections
• Monocyte elevation often signals chronic infection
• Basophils are elevated in hypersensitivity
• Lymphocytosis is associated with viral infections

Lymphoid Organs and Immunoglobulins

• Lymphoid organs generate, house, and activate lymphocytes (T, B, NK cells) and antigen-presenting cells (APCs)
• Primary organs facilitate lymphocyte development and maturation
• In bone marrow B cells undergo maturation and in thymus T cells mature
• Secondary organs facilitate antigen recognition and lymphocyte activation
• Secondary lymph organs include lymph nodes, spleen, tonsils, and MALT

Lymphocyte Production

• B lymphocytes mature in bone marrow and can produce antibodies after antigen stimulation
• T lymphocytes mature in the thymus, including CD4+ helper T cells and CD8+ cytotoxic T cells
• Antigen-presenting cells (APCs) uptakes pathogens
• APCs process antigens into peptide fragments
• APC loads fragment onto MHC
• APC present it to naïve t-cells in lymphoid organs
• MHC Class I presents endogenous antigens to CD8+ T cells, presents in all nucleated cells
• MHC Class II presents exogenous antigens to CD4+ T cells, presents on APCs
• Activated helper T cells stimulate B cells, produce specific antibodies

Lymphoid Organs

• The thymus is the primary lymphoid organ and site of T cell selection & maturation
• Thymus is divided into the cortex and medulla
• Does not contain B cells
• Lymph nodes are secondary lymphoid organs that filter lymph
• Lymph nodes contain a cortex, paracortex, and medulla.
• Paracortex is T-cell rich
• The spleen filters blood has white and red pulp
• Red pulp destroys/recycles red blood cells
• White pulp creates an adaptive immune response
• Tonsils are located at the entrance of the respiratory and digestive tracts

Immunoglobulins (Antibodies)

• Antibodies are glycoproteins from plasma or B cells
• Two heavy and two light chains
• Variable region binds specific antigens
• Constant region determines class and function
• IgA protects against pathogens
• IgD has a possible role in B cell activation
• IgE binds to Fc receptor
• Key role in parasitic responses
• IgG provides long term immunity
• IgM allows strong agglutination

Antibody Functions

• Antibodies may neutralize
• Antibodies may facilitate opsonization by macrophages/neutrophils
• Antibodies activate the classical complement
• Antibodies carry out antibody dependent cytotoxicity
• Antibodies promote agglutination and precipitation

Clinical Correlation with Lymphoid Organs

• IgE mediate hypersensitivity is related to asthma
• Elevated IgA indicates mucosal infections
• Babies receive passive immunity from mom
• Vaccines stimulate IgG production
• Hypergammaglobulinemia indicate chronic immune

Integument System

• The integumentary system includes skin, hair, glands, and nails
• The skin is the largest organ
• The integumentary system performs barrier protection
• The integumentary system helps with thermoregulation and excretion
• The integumentary system aids sensation
• The integumentary system has an immunity component
• The integumentary system is involved in metabolism
• The integumentary system has three main layers: epidermis, dermis, and hypodermis.

Skin Structure

• Epidermis is derived from the ectoderm is avascular and has keratinized stratified squamous epithelium
• The dermis is derived from mesoderm, is vascularized and innervated, contains follicles
• Hypodermis or subcutaneous tissue contains tissue for insulation, cushioning, and energy storage
• The epidermis is made of keratinocytes
• The epidermis also contains stem cells, Merkel cells, and melanocytes
• Epidermis layers include stratum basal, spinosum, granulosum, lucidium, and corneum
• Cells have assembly of tonofibrils, and excretion
• Hemidesmosomes bind keratinocytes
• Melanocytes protect nuclei from UV radiation
• Merkel cells are touch receptors
• The papillary layer is connective tissue
• The reticular layer is dense connective tissue

Skin Structures

• Thick skin is on palms and soles
• Thin skin is on the rest of the body
• Eccrine sweat glands are found on most of the body
• Sweat glands function is thermoregulation
• Apocrine sweat glands are in the axilla
• Apocrine glands are activated at puberty
• Sebaceous glands secrete to lubricate
• Hair are made of keratin

Integumentary System Summary

• Basement membrane defects leads to bullous disorders
• Sebaceous gland hyperactivity are common in acne
• Excema and psoriasis are related to the barrier of the dermis
• Sweat issues can lead to heat issues

Endocrine Overview

• The endocrine system consists of hormones release into the body
• Glands have a rich supply, lack ducts, have clusters, or follicles
• Hormones can be peptides, steroids, or amino acids

Pancreatic Histology

• Pancreas contains islets of langerhans which contain beta, delta, and alpha cells
• Beta secretes insulin
• Alpha cells secrete glucagon
• Delta cells secrete somatostatin
• Insulin lowers blood glucose by uptakes to the liver, enhances, and inhibit
• Related clinic note with type 1 diabetes where there is destruction and type 2 resistance is when the body has insulin resistance instead of an issue of secretion

Thyroid Gland

• The thyroid gland composed of cuboidal epithelial cells
• The cells have colloid with thyroglobulin

Pituitary Overview

• Posterior is an extension of the hypothalamus
• The posterior lobe stores hormones: oxytocin
• Oxytocin causes uterine contractions and contraction
• The adrenal glands has three zona: glomerulosa, fasciculata, and reticularis
• Fasciculata creates cortisol, and it helps with energy, suppresses immunity, stimulates via ACTH
• Testes make testerone
• Pineal glands makes melatonin

Medulla

• The medulla creates NE, to mediate flight/flight
• The ovaries made of theca make estrogen
• The parathyroid secrete PTH which increases calcium
• The anterior petutiary creates growth hormone