Muscle Tissue Overview and Function

Muscle Tissue Overview

• Muscle tissue is responsible for movement in the human body
• Muscle cells are excitable, contractile, extensible, and elastic
• Types of movement include cilia and flagella, and body cell contraction

Muscle Tissue Function

• Movement: Muscle contraction causes body movement and material transport like blood, food
• Posture Maintenance: Continuous muscle contractions hold body in standing/sitting positions
• Joint Stabilization: Muscle tone strengthens and stabilizes joints
• Heat Generation: Muscle contraction generates heat as a byproduct

Muscle Classification

• Muscle tissue is categorized by appearance of contractile cells (striated or smooth)
• Striated muscle: Exhibit cross-striations under a microscope
  • Skeletal striated muscle: Attached to bone, controlling axial and appendicular skeleton movement along with posture and eye movement (extraocular muscles.)
  • Visceral striated muscle: Found in the tongue, pharynx, and parts of the diaphragm, contributing to speech, respiration and swallowing as well as esophagus
  • Cardiac striated muscle: Striated muscle found in the heart walls and large veins entering the heart, where it moves blood.
• Smooth muscle: Do not exhibit cross-striations

Skeletal Muscle Structure

• Composed of long tubular structures- muscle fibers
• Contains multiple nuclei beneath sarcolemma (plasma membrane)

Skeletal Muscle Nomenclature

• Mys/myos – muscle
• Myocyte – muscle cell
• Sarx/sarcós – meat
  • Sarcolemma- Cell membrane
  • Sarcoplasm – Cytoplasm
  • Sarcoplasmic reticulum – Smooth ER

Striated Muscle Organization

• Muscle → Fascicles → Muscle fibers → Myofibrils → Filaments (thick and thin)
• Connective tissues like tendons, fascia, epimysium, perimysium, and endomysium support skeletal muscle structure
• Arrangement of fibers, fascicles and tissues results in the ability to pull on bone during movement

Microscopic Anatomy of Skeletal Muscle

• Sarcomeres: Repeating units of myofilaments (actin and myosin) within myofibrils.
• Z lines: Define the boundaries of a sarcomere.
• A bands: Contain myosin filaments
• I bands: Contain actin filaments
• H zone: Region within A band where only myosin is present.
• M line: Middle of the sarcomere, supporting myosin filaments

Molecular Structure of Thick and Thin Filaments

• Thick filaments: Composed of myosin protein with multiple heads
• Thin filaments: Composed of actin, tropomyosin, and troponin
• Myosin head binds to actin, causing contraction.

Skeletal Muscle Contraction

• Calcium ions cause regulatory proteins troponin and tropomyosin to move
• Myosin heads bend, pulling actin filaments toward the center of the sarcomere
• ATP detaches myosin head from actin.
• ATP's energy reorients myosin and enables further contraction

Calcium Regulation in Muscle Contraction

• Calcium ions initiate the contraction process
• Calcium binding to troponin causes tropomyosin to move, exposing actin binding sites needed for contraction

Innervation of Skeletal Muscle

• Motor end plate: Synapse between motor neuron and muscle fiber
• Acetylcholine: Neurotransmitter released at the synapse, stimulating muscle contraction
• Junctional folds: Modifications of muscle fiber membrane for larger surface area for acetylcholine receptors.
• Motor neuron axon branches and forms synapse with muscle fibers.

Sensory Innervation of Skeletal Muscle

• Muscle spindles: Sense muscle length and tension changes, providing feedback to the nervous system

Muscle Fiber Regeneration

• Satellite cells: Muscle stem cells important for repair and regeneration of damaged muscle fibers, which are located within the basal lamina surrounding the muscle fiber.

Red and White Muscle Fibers

• Red muscle fibers: High myoglobin content, rich blood supply, slow contractions, crucial for sustained activity

• White muscle fibers: Low myoglobin content, poor blood supply, rapid contractions, are used for quick movements

Cardiac Muscle

• Location: Heart
• Function: Involuntary rhythmic contractions.
• Unit: Cardiomyocyte (cell)
  • Cylindrical shape
  • Branching and anastomosing (linking)
  • Single or multiple nuclei (usually one)
  • Intercalated discs: junctions between cardiomyocytes consisting of desmosomes and gap junctions which are important for the function of the heart.
• Modified cardiac cells- Generating rhythmic contractions
  • P cells (pacemaker)- Generate the heartbeat's inherent rhythm.
  • Modified cardiac cells make up Purkinje fibers
  • Autonomic Nervous system plays a role in modifying the heart rate

Smooth Muscle

• Location: Walls of hollow visceral organs, blood vessels, and eyes.
• Unit- Myocyte (cell)
  • Spindle shape
  • Arranged in sheets or layers (longitudinal, circular, oblique)
• Arrangement of layers to complete the function of contraction.
• Origin- Differentiate mesenchmal cells to form myoblasts to myocytes
  • Very good regeneration
• Functions: Changes in lumen of hollow organs, peristalsis of intestines and other organs, blood flow control by contraction/dilation, pupillary response/eye accommodation by contraction/relaxation, Childbirth(parturition)
• Mechanisms of contraction differ from striated muscles, requiring slow but sustained contractions
• Dense bodies act as the attachment points for actin filaments, replacing Z lines, which are not present in smooth muscles.
• Smooth muscles contain thin filaments of actin and thick filaments of myosin.
• Gap junctions electrically couple smooth muscle cells, enabling coordinated contractions.

Explore the fundamentals of muscle tissue, including its types, functions, and classifications. Understand how muscle contraction facilitates body movement, maintains posture, stabilizes joints, and generates heat. This quiz will test your knowledge of striated and smooth muscle types.