Muscular System Notes

Learning Outcomes

• Identify the different forms of muscle tissue: Recognize skeletal, smooth, and cardiac muscle types based on structure and function.

• Describe similarities and differences in the structure and function of the 3 types of muscle tissue and indicate their locations in the body, focusing on where each type is found and how their structures relate to their functions in the human body.

• Define and explain the role of:

  • Endomysium: A delicate connective tissue that surrounds individual muscle fibers, providing support and electrical insulation to prevent spread of action potentials between adjacent fibers.

  • Perimysium: A connective tissue sheath that encases a fascicle, providing a pathway for nerves and blood vessels, which supplies the muscle tissue and supports its functional integrity.

  • Epimysium: An outer layer of connective tissue that encases the entire muscle, allowing for a binding strength while keeping the muscle flexible, facilitating smooth movement against neighboring tissues.

  • Tendon: Connects muscle to bone; transmits the force generated by muscle contractions to create movement at joints and withstands high tensile forces.

Types of Muscle Tissue

Skeletal Muscle

• Enables movement via contraction and relaxation causing limb and body position changes.

• Allows for posture and positioning, playing a critical role in maintaining structural support via its origin and insertion points on bones.

• Supports and protects body cavities/openings such as the abdominal cavity and thoracic cavity, aiding in the protection of internal organs.

• Helps maintain body temperature through thermogenesis, which occurs when muscles contract, releasing heat as a byproduct.

• Characteristics:

  • Long, cylindrical fibers with multiple nuclei located peripherally, facilitating rapid contraction and high force production.

  • Voluntary contraction allowing conscious control derived from the somatic nervous system, enabling complex movements.

Smooth Muscle

• Involved in involuntary movements within organs and vessels, such as digestion and blood flow regulation, providing a lower resistance to flow through blood vessels.

• Characteristics:

  • Spindle-shaped fibers with a single central nucleus, enabling rhythmic contractions that are slower but more sustained than skeletal muscle.

  • Involuntary contraction regulated by the autonomic nervous system and hormones, coordinating functions across various organ systems.

Cardiac Muscle

• Responsible for pumping blood throughout the body, generating the necessary pressure to maintain circulation in the cardiovascular system.

• Characteristics:

  • Short branched fibers connected via intercalated discs that allow for synchronized contractions and effective communication between muscle cells.

  • Involuntary contraction regulated by intrinsic mechanisms and neural stimuli, ensuring constant and rhythmic heartbeats.

Skeletal Muscle Organization

• Components:

  • Fascia: A fibrous connective tissue that surrounds muscles, keeping them separated and providing a supportive structure facilitating movement.

  • Fascicle: Bundles of muscle fibers grouped together, enhancing overall strength and contraction efficiency.

  • Muscle fiber: Individual muscle cell; the basic contractile unit of muscle tissue.

• Connective tissues involved:

  • Epimysium: Surrounds the entire muscle, providing structural integrity to the muscle and facilitating interactions with tendons.

  • Perimysium: Surrounds fascicles, providing segmented compartments in the muscle that help prevent injuries by reducing the impact of forces.

  • Endomysium: Surrounds each muscle fiber, allowing for metabolic exchange and structural support between fibers for effective contractions.

Skeletal Muscle Fiber Structure

• Key Components:

  • Sarcoplasm: The cytoplasm of the muscle cell that contains organelles, myoglobin, and glycogen needed for muscle contractions.

  • Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum that stores calcium ions crucial for muscle contraction and relaxation cycles.

  • T-tubules: Extensions of the plasma membrane that penetrate into the muscle fiber, conducting action potentials deep into the fiber to ensure synchronous contractions.

  • Myofilaments: Comprise actin (thin) and myosin (thick) filaments responsible for the physical process of contraction, with specific interactions that facilitate the shortening of muscle fibers.

Sarcomere and Myofilaments

• The sarcomere is the functional unit of muscle, defined as the segment between two Z lines, crucial in muscle contraction mechanisms.

• Myofilaments:

  • Thick filaments: Composed of myosin, with heads that interact with actin to produce contraction through crossbridge cycling.

  • Thin filaments: Composed of actin, troponin, and tropomyosin, which regulate the binding sites available on actin for the myosin heads.

• Measurements:

  • Thick filament diameter: $15 ext{ nm}$

  • Thin filament diameter: $8 ext{ nm}$

Activation of Skeletal Muscle

1. Action Potential Initiation:

   • Arrival of an action potential at the axon terminal leads to the release of acetylcholine (ACh) from synaptic vesicles into the synaptic cleft, initiating muscle contraction.

2. ACh Binding:

   • ACh binds to ligand-gated ion channels on the motor end plate, allowing sodium ions (Na+) to enter the muscle fiber and depolarizing the membrane to trigger an action potential.

3. Propagation of Action Potential:

   • The endplate potential leads to an action potential propagating down the T-tubules, facilitating immediate muscle fiber activation.

4. Calcium Release:

   • The depolarization of the membrane opens calcium channels in the sarcoplasmic reticulum, increasing the intracellular calcium concentration necessary for the contraction cycle to begin.

Muscle Contraction Process

• Crossbridge Cycle Steps:

1. ATP Hydrolysis: ATP breaks down to ADP and inorganic phosphate, positioning the myosin head (cockerham) in readiness for binding.

2. Crossbridge Formation: Myosin head binds to the actin filament, forming a crossbridge that anchors both filaments together.

3. Power Stroke: The myosin head pivots and pulls the actin filament inward toward the center of the sarcomere, shortening the muscle fiber.

4. Release: ADP and phosphate detach from myosin, initiating a change in the myosin head that returns it to the original position ready for another ATP binding cycle, enabling sustained contraction responses.

Smooth Muscle Structure

• Characteristics:

  • Distributed throughout the circulatory, respiratory, digestive, and reproductive systems, providing essential functions such as peristalsis.

  • Composed of small, tapered cells with a single central nucleus, enhancing versatility and response time in various actions.

  • Actin and myosin filaments are not arranged in sarcomeres, which allows for a different contraction mechanism characterized by prolonged contractions without fatigue, ideal for visceral organ function.

Summary of Key Points

• Muscle Types: Skeletal, Cardiac, and Smooth; each with distinct properties that suit their functions.

• Muscle forms and functions depend on fascicle arrangement, influencing strength and direction of contraction.

• Skeletal muscle consists of myofibrils made of repeated units called sarcomeres, central to muscle contraction.

• Muscle contraction results from the intricate interaction between myosin and actin filaments, allowing for effective force generation and control of movement.