Muscle Structure and Function

• The muscular system facilitates movement, posture, circulation, breathing, digestion, and temperature regulation.

Muscle Structure Hierarchy

• Myofibrils contain actin and myosin filaments.
• Multiple muscle fibers form a fascicle.
• Fascicles are bundled together.
• Fascia surrounds the entire muscle.
• Tendons connect muscle to bone.

Connective Tissue Components

• Perimysium surrounds fascicles.
• Epimysium (dense irregular connective tissue) surrounds the entire muscle.
• Endomysium surrounds individual muscle cells.
• Dense irregular connective tissue, primarily collagen fibers, make up perimysium, epimysium, and endomysium.

Tendons and Ligaments

• Dense regular connective tissue forms tendons and ligaments.

Muscle Growth and Repair

• Satellite cells (muscle stem cells) activate for muscle growth and repair.
• Injury or exercise stimulates satellite cells to proliferate and differentiate into myoblasts.
• Myoblasts fuse with existing muscle fibers to repair tissue or enlarge muscles.
• The process involves activation, proliferation, and differentiation.
• Satellite cells maintain a stem cell pool for continuous regeneration.

Skeletal Muscle

• Skeletal muscle attaches to bones to facilitate voluntary movement.
• Striated cylindrical fibers with multiple nuclei characterize its structure.
• Characteristics: long, cylindrical, heavily striated, and multinucleated.

Cardiac Muscle

• Cardiac muscle is exclusively in the heart, pumping blood through involuntary contractions.
• Branched fibers with a single nucleus and striations mark its structure.
• Characteristics: striated appearance, branching structure, single nucleus, intercalated discs, many mitochondria, involuntary control

Smooth Muscle

• Smooth muscle lines internal organ walls, such as the stomach and blood vessels.
• Spindle-shaped cells lacking striations, each with a single nucleus, characterize its structure.
• It facilitates slow, sustained contractions for organ functions like digestion and blood pressure regulation.
• Characteristics: Spindle-shaped, Single nucleus, non-striated, Involuntary contractions, function: slow contractions in internal organs

Intracellular structures

• Sarcoplasmic reticulum's primary function is to regulate calcium levels within the muscle cell by storing and releasing calcium ions when needed for muscle contraction; considered a specialized form of endoplasmic reticulum in muscle cells.
• Myofibrils are long, cylindrical structures within the muscle fiber contain repeating units called sarcomeres, which are the functional units of muscle contraction.
• Sarcoplasm is the cytoplasm of a muscle cell where various metabolic processes occur, including glycogen storage and enzyme activity.
• Sarcolemma is the plasma membrane of a muscle cell, responsible for maintaining the cell's internal environment and regulating the passage of substances.
• Transverse tubules (T-tubules) are invaginations of the sarcolemma that allow for rapid transmission of electrical impulses throughout the muscle fiber, ensuring coordinated muscle contraction.
• Terminal cisternae are enlarged sacs at the ends of the sarcoplasmic reticulum where calcium ions are primarily stored and released upon receiving an electrical signal.
• Mitochondria the powerhouse of the cell, generating ATP (adenosine triphosphate) which is the primary energy source for muscle contraction.

Sarcomeres

• Sarcomeres are the basic units of contraction

Motor Units

• Variable motor unit sizes enable graded muscle contractions.
• Small motor units produce less force.
• Large motor units generate greater force.

Steps of Muscle Contraction

• A nerve impulse reaches the neuromuscular junction.
• An electrical signal (action potential) goes down the motor neuron.
• Acetylcholine (ACh) releases into the synaptic cleft when calcium ions enter the neuron.
• ACh binds to receptors on the muscle fiber membrane (motor end plate).
• Binding of ACh opens sodium channels, causing muscle fiber depolarization and generating an action potential that travels along the sarcolemma.
• Action potential spreads through T-tubules to the sarcoplasmic reticulum.
• The sarcoplasmic reticulum releases calcium ions (Ca2+).
• Calcium ions bind to troponin, exposing myosin binding sites on actin and forming crossbridges.
• Myosin heads pull actin filaments, shortening the sarcomere and contracting the muscle fiber (Sliding filament theory).

Muscle Relaxation

• Acetylcholinesterase breaks down acetylcholine.
• Calcium ions are actively pumped back into the sarcoplasmic reticulum.
• Myosin heads detach from actin, as calcium is no longer bound to troponin.
• Elastic components within the muscle fiber passively return it to its original length.

Cardiac and Smooth Muscle Contraction Differences

• Cardiac and smooth muscle contraction differs from skeletal muscle contraction in speed, involuntary control, and initiation mechanisms.
• Contraction in smooth muscle is not regulated by the binding of Ca to the troponin complex

Muscle Fiber Types

• Slow oxidative (SO) fibers are fatigue resistant, produce low force, and use aerobic metabolism.
• Fast oxidative (FO) fibers have moderate fatigue resistance, generate higher force than SO fibers, and use both aerobic and anaerobic metabolism.
• Fast glycolytic (FG) fibers fatigue quickly, produce the highest force, and rely heavily on anaerobic glycolysis.

Muscle Roles

• Agonist: The primary muscle in a specific movement (prime mover).
• Antagonist: The muscle opposing the agonist.
• Synergist: The muscle assisting the agonist by stabilizing joints and contributing to the movement.

Muscle Attachment Points

• Origin: One point where the muscle attaches to a bone.
• Insertion: The other point where the muscle attaches to a bone.