Muscular Tissues Overview

Muscular Tissues

• Muscles are the fourth type of basic tissue, alongside epithelium, connective tissue, and nervous tissue
• Muscles are composed of cells that optimize the property of contractility
• Actin microfilaments and associated proteins generate the forces needed for muscle contraction
• Muscle contraction drives movement within organ systems, blood flow, and the body as a whole
• All muscle cells originate from mesoderm
• Muscle cells lengthen through a process, and synthesize actin and myosin proteins

Function of Muscles

• Mobility
• Stability- maintaining body posture
• Circulation
• Respiration
• Digestion
• Urination
• Temperature regulation (shivering)
• Protection of inner organs
• Vision
• Childbirth

Types of Muscles

• Skeletal muscle:
  • Consists of bundles of very long, multinucleated cells with cross-striations
  • Contraction is quick, forceful, and usually under voluntary control
• Cardiac muscle:
  • Has cross-striations and is composed of elongated, often branched cells
  • Connected by intercalated discs
  • Contraction is involuntary, vigorous, and rhythmic
• Smooth muscle:
  • Consists of collections of fusiform cells
  • Lack striations and have slow, involuntary contractions

Structure of Myofibrils

• M line- center/medial line of thick muscle
• H band- actin-free zone of thick filaments
• Z line- lateral boundaries of sarcomere, anchors thin, titin, and nebulin filaments
• A band- contains both thick and thin filaments
• I band- from two adjacent sarcomeres, meets at Z line

Contraction of Muscle

• Tropomyosin- 40-nm-long coil of two polypeptide chains that lie in the groove between actin strands
• Troponin- attaches to tropomyosin, regulates actin-myosin interaction
• Sliding Filament Theory- tropomyosin blocks myosin-binding sites on actin, preventing cross-bridge formation; tropomyosin moves to expose binding sites when Ca2+ is present; cross-bridge cycling proceeds until Ca2+ and ATP are no longer available

Mechanism of Contraction

• Tropomyosin shifts position to uncover myosin-binding sites on actin
• Troponin, regulated by calcium, regulates tropomyosin
• Calcium binds to troponin which causes conformational change in tropomyosin and uncovers binding sites
• Cross-bridges form between actin and myosin
• Actin slides over myosin toward the Z line
• Contraction produced by repetitive cycling of myosin heads, requiring ATP
• Myosin heads detach from actin as ATP is hydrolyzed

Neuromuscular Junction

• Skeletal muscle fibers have sarcoplasmic reticulum containing protein pumps for Ca2+ sequestration.
• Calcium release is triggered by membrane depolarization from a motor nerve
• Sarcolemma has transverse (T) tubules
• T-tubules are adjacent to expanded terminal cisternae of sarcoplasmic reticulum

Innervation

• In the absence of ATP, actin-myosin cross-bridges become stable, resulting in rigor mortis
• Each axonal branch forms a dilated termination in a trough on muscle cell surface
• Motor end plates(MEP) contain mitochondria and synaptic vesicles filled with acetylcholine
• Acetylcholine is released into synaptic cleft and binds to its receptors initiating action potential
• Acetylcholine receptor is non-selective cation channel that opens upon neurotransmitter binding causing sodium influx into muscle initiating action potential

Medical Importance

• Myasthenia gravis- an autoimmune disorder where circulating antibodies target acetylcholine receptors
• Antibodies interfere with acetylcholine's action, causing skeletal muscle weakness, particularly in extraocular muscles

Smooth Muscles

• Specialized for slow, steady contraction under influence of autonomic nerves or various hormones
• Found in blood vessels and various tracts (digestive, respiratory, urinary, reproductive)
• Cells range from 20 µm to 500 µm in length, have a single, elongated nucleus, and appear uniformly stained, with close packing
• No T-tubules, caveolae present instead
• Contraction mechanism similar to skeletal muscle but involves calmodulin activating myosin kinase, which phosphorylates myosin for contraction.
• Contraction is regulated by action potentials from autonomic nervous system, stretching, hormones (e.g., epinephrine), or stress (like hypoxia, ischemia).

Cardiac Muscles

• Involuntary, striated muscle forming walls of heart
• Myocardium (middle layer of heart wall) between epicardium and endocardium
• Myocytes 100-150 µm in length and 30-40 µm in width
• Myocardium is sandwiched between Epicardium and Endocardium supplying blood via coronary circulation, connecting chambers and covering cardiac valves
• Coordinated contraction of sheets permit squeezing in various directions

Heart Anatomy

• Includes structures like superior/inferior vena cava, aorta, pulmonary artery, pulmonary veins, right/left atrium, right/left ventricle, septum, valves (tricuspid, mitral, aortic, pulmonary)
• Contraction uses considerable energy, requiring constant blood flow for oxygen and nutrients
• Blood is drained into right atrium by the coronary veins

Cardiac Conduction System

• Composed of cardiomyocytes (heart muscle cells)—joined by intercalated discs;
• Cells contract in coordinated way—functional syncytium
• Electrical impulses rapidly transmit through network
• Individual cells joined at their ends via intercalated disks

Intercalated Discs

• Network of cardiomyocytes connected by intercalated discs
• Allow rapid transmission of electrical impulses
• Composed of three types of cell-cell junctions:
  • Adherens junctions (actin filaments)
  • Desmosomes (intermediate filaments)
  • Gap junctions (allow ion passage)

Cardio Myopathies

• Conditions caused by restricted blood supply to heart muscle (e.g., angina, myocardial infarction) and other heart muscle diseases.

Explore the fundamental aspects of muscular tissues in this quiz. Learn about the muscle types, their functions, and the unique structures that enable muscle contraction and movements. Understand how these tissues contribute to essential bodily functions and overall mobility.