Muscle Tissue Overview

Cardiac Muscle Characteristics

  • Location: Found in the heart.

  • Control: Involuntary; cardiac muscles generate contraction without nerve stimulation.

  • Cell Characteristics:

    • 1 to 4 nuclei per cell.

    • Cells are short, branched, and striated.

    • Neighbouring cells are joined by intercalated disks.

    • High concentration of mitochondria, indicating significant energy requirements for function.

Cellular Structure of Cardiac Muscle

  • Cellular Characteristics:

    1. Cylindrical and branching cells.

    2. Generally contain a single central nucleus with 2 nucleoli.

    3. Cells are interconnected by intercalated disks which enhance intercellular communication.

    4. Almost 50% of the cell's volume is composed of mitochondria, providing ATP for contractions.

    5. Abundant supply of myoglobin, necessary for oxygen storage and metabolism.

Intercalated Disks

  • Function of Intercalated Disks:

    • Form “stairstep” junctions between adjacent cardiac muscle cells.

    • Composed of two main components:

    • Desmosomes: Anchoring junctions that provide mechanical strength and resist pulling forces during contraction (located in the transverse portion of the disk).

    • Gap Junctions: Membrane protein channels that electrically couple all cardiac myocytes, functioning collectively as a syncytium (located in the lateral portion).

Cardiac Muscle Contraction Process

  1. Triggering Contraction:

    • Contractile cells receive a depolarizing current through gap junctions, initiating action potentials.

    • Action potentials propagate along the plasma membrane and in T tubules.

  2. Calcium Release:

    • Voltage-gated calcium channels open in the plasma membrane and sarcoplasmic reticulum (SR).

    • Increased intracellular calcium induces further calcium release from the SR.

  3. Myosin Activation:

    • Calcium binds to troponin, leading to the exposure of myosin-binding sites on actin.

    • The crossbridge cycle commences, facilitating muscle fiber contraction.

  4. Relaxation:

    • Calcium is actively transported back into the SR and extracellular fluid (ECF).

    • Tropomyosin then blocks myosin-binding sites, leading to muscle fiber relaxation.

Smooth Muscle Characteristics

  • Location: In the walls of internal organs.

  • Control: Exclusively involuntary.

  • Cell Characteristics:

    • Smooth muscle cells are small and spindle-shaped (fusiform).

    • They possess the ability to stretch.

Smooth Muscle Cellular Structure

  • Unique Cytoskeletal Arrangement:

    • Composed of an extensive network of intermediate filaments forming a cytoskeleton.

    • Dense Bodies: Connect intermediate filaments within the sarcoplasm.

    • Dense Plaques: Anchor intermediate filaments to the inner sarcolemma.

  • Contractile Proteins:

    • Arranged between dense bodies; lack formal sarcomeres, thus no striations observed.

    • Thin filaments contain actin and tropomyosin, but no troponin present.

    • The sarcoplasmic reticulum is absent; cavernous structures called caveolae serve for calcium storage.

  • Key Proteins for Contraction:

    • Calmodulin: Protein that binds calcium.

    • Myosin Light-Chain Kinase (MLCK): Enzyme that adds a phosphate group to activate myosin heads.

    • Myosin Light-Chain Phosphatase: Enzyme responsible for inactivating myosin by removing the phosphate group.

Mechanism of Smooth Muscle Contraction

  1. Stimulus Activation:

    • External stimuli such as nerve signals, hormonal release, or muscle stretching trigger the opening of calcium channels, allowing calcium to flow from caveolae into the cytoplasm.

  2. Calcium-Mediated Activation:

    • Calcium binds to calmodulin, forming a calcium-calmodulin complex.

    • This complex activates MLCK.

  3. Phosphorylation and Contraction:

    • MLCK phosphorylates myosin heads, activating them for crossbridge cycling.

    • Crossbridge cycling results in muscle contraction, compressing the cell in three dimensions.

  4. Relaxation Process:

    • Requires cessation of stimulation, removal of calcium from the sarcoplasm, and deactivation of myosin via myosin light-chain phosphatase.

Characteristics of Smooth Muscle Contraction

  • Contraction Properties:

    • Long latent period before contraction.

    • Long duration of contraction.

    • Fatigue-resistant due to energy-efficient contraction mechanisms.

Innervation of Smooth Muscle

  • Functional Categories:

    • Multiunit Smooth Muscle:

    • Composed of cells arranged in separate motor units; each unit contracts individually based on stimulation.

    • Examples include:

      • Iris muscles of the eye.

      • Arrector pili muscles of the skin.

      • Larger air passageways in the respiratory system.

      • Walls of larger arteries.

    • Single-unit Smooth Muscle:

    • Most common type, with cells organized into sheets that contract together.

    • Stimulation by neurons passing closely to the cells.

    • Examples include walls of the digestive tract, urinary tract, reproductive tract, portions of the respiratory tract, and most blood vessels.