GAP Ch10 Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is one of the four primary tissue types in the body, specialized for contraction and movement. It plays essential roles in various bodily functions, from locomotion to internal organ function. There are three primary types of muscle tissue:

A. Skeletal Muscle

• Structure: Skeletal muscle comprises elongated fibers that are striated and multinucleated. The striations are due to the arrangement of myofilaments (actin and myosin) within the fibers.

• Attachment: Skeletal muscles are typically attached to bones via tendons and are responsible for voluntary movements of the skeleton.

• Functions:

  • Facilitates body movements and locomotion.

  • Helps in maintaining posture and stabilizing joints.

  • Protects internal organs and supports bodily functions such as respiration.

  • Produces heat through metabolic processes associated with muscle contraction.

B. Smooth Muscle

• Structure: Smooth muscle fibers are spindle-shaped, non-striated, and typically have a single nucleus. The smooth appearance is due to the irregular arrangement of actin and myosin filaments.

• Location: Found in the walls of hollow organs such as the intestines, blood vessels, bladder, and uterus.

• Functions:

  • Involuntary control over internal movements like digestion, blood flow, and respiratory functions.

  • Regulates the diameter of blood vessels and the gastrointestinal tract.

  • Contracts rhythmically in a corkscrew motion due to the sliding filament model.

C. Cardiac Muscle

• Structure: Cardiac muscle fibers are striated like skeletal muscle but are shorter and branched, typically possessing one or two nuclei. The intercalated discs between cells enable electrical coordination.

• Location: Exclusively found in the heart.

• Functions:

  • Involuntary contraction helps pump blood throughout the body.

  • Functions as a syncytium due to interconnected cells allowing coordinated contractions for effective pumping.

  • Regulates heart rhythm and responds to various stimuli including neural and hormonal signals.

Common Properties of Muscle Tissue

All three muscle tissues exhibit excitability, allowing their plasma membranes to undergo changes in electrical states, sending action potentials that trigger contractions. Additionally, they vary in their microscopic organization of contractile proteins and their control mechanisms (voluntary vs. involuntary).

Functional Units and Sarcomere Composition

• Sarcoplasmic Reticulum: Specialized smooth endoplasmic reticulum in muscle cells that stores calcium ions (Ca²+) crucial for muscle contraction.

• Sarcomeres: The functional unit of muscle fibers that consists of overlapping actin (thin filaments) and myosin (thick filaments).

  • Z lines: Anchor points for actin, moving closer as the sarcomere contracts.

  • A Band: Contains thick myosin filaments and does not change length during contraction.

  • I Band: Contains only actin filaments and narrows during contraction as filaments slide over each other.

  • H Zone: Region between actin filaments within the A Band, decreases in width during contraction as actin overlaps with myosin.

Mechanisms of ATP Regeneration

Muscles require ATP (adenosine triphosphate) for contraction, and several mechanisms regenerate it:

• Creatine Phosphate: An energy reservoir for immediate ATP regeneration during short bursts of high-intensity activities (around 15 seconds).

• Glycolysis: The process of breaking down glucose for ATP, primarily during high-intensity exercise lasting approximately a minute. If oxygen is present, it can lead to the more efficient aerobic respiration.

• Aerobic Respiration: Takes place in mitochondria requiring oxygen, yielding approximately 36 ATP per glucose molecule, essential for prolonged muscle activity during endurance exercises.

Types of Muscle Contractions

1. Isotonic Contractions: Muscle tension remains constant while moving a load.

   • Concentric Contractions: Muscle shortens while generating force (e.g., lifting weights).

   • Eccentric Contractions: Muscle lengthens while maintaining tension (e.g., lowering weights).

2. Isometric Contractions: Muscle generates tension without changing the length (e.g., wall sits, planks).

Types of Skeletal Muscle Fibers

• Slow Oxidative (SO) Fibers: Slow contraction rates, primarily utilize aerobic respiration, suited for endurance activities.

• Fast Oxidative (FO) Fibers: Faster contraction rates, utilize both aerobic and anaerobic pathways, leading to quicker fatigue in sustained efforts.

• Fast Glycolytic (FG) Fibers: Rapid contractions relying mainly on anaerobic processes, resulting in quick fatigue and suited for short bursts of activity.

Conclusion

Muscle tissue plays diverse and vital roles in movement, protection, and bodily functions, with each type exhibiting unique structures and functionalities tailored to their specific roles in the body's overall operation.