The Muscular System Quiz

The Muscular System

• Muscles are responsible for all types of body movement, and they contract or shorten, acting as the body's machinery.
• Three basic muscle types exist: skeletal, cardiac, and smooth muscles.

Function of Skeletal Muscles

• Produce movement
• Maintain posture
• Stabilize joints
• Generate heat

Skeletal Muscle Wrapping

• Muscles are composed of fascicles (bundles of muscle fibers)
• Fascicles are surrounded by perimysium
• Muscle fibers are surrounded by endomysium
• The epimysium surrounds the entire muscle.
• Tendons connect muscles to bones.

Connective Tissue Sheaths of Skeletal Muscle

• Epimysium: surrounds the entire muscle
• Perimysium: surrounds fascicles (bundles of muscle fibers)
• Endomysium: surrounds individual muscle fibers

Structure and Organizational Levels of Skeletal Muscle

• Muscle (organ): Hundreds to thousands of muscle cells, connective tissues, blood vessels, and nerve fibers
• Epimysium: Outer sheath
• Fascicle: Bundles of muscle cells
• Perimysium: Connective tissue surrounding fascicles
• Muscle Fiber (cell): Elongated multinucleate cells
• Endomysium: Connective tissue surrounding individual muscle fibers
• Sarcolemma: Muscle cell membrane
• Myofibril: Contractile unit within a muscle fiber
• Myofilaments: Actin and myosin filaments within a myofibril
• Sarcomere: Repeating functional unit within a myofibril
• Z disc: Boundary between sarcomeres
• H zone: Center of the sarcomere
• A band: Dark band
• I band: Light band
• M line: Middle of the sarcomere

Microscopic Anatomy of a Skeletal Muscle Fiber

• Myofibrils: Rod-like contractile elements within muscle fiber
• Sarcolemma: Plasma membrane of a muscle fiber
• Mitochondria: Provide energy for muscle contractions

Relationship of Sarcoplasmic Reticulum and T Tubules to Myofibrils

• Sarcoplasmic reticulum (SR) and T tubules have a critical role in muscle excitation-contraction coupling.
• The SR stores calcium ions.
• T tubules transmit action potentials deep into the muscle fiber.
• The SR releases calcium ions in response to an action potential, enabling muscle contraction.
• Triads are sites where T tubules and SR membranes interact.

Structure and Organizational Levels of Skeletal Muscle

• Myofibril or Fibril: Complex organelle made of many myofilaments
• Sarcomere: Contractile unit of myofibrils, composed of protein filaments
• Myofilaments: Thin (actin) and thick (myosin) filaments arranged within sarcomeres, resulting in banding pattern
• Thin Filaments: Composed of actin, troponin, and tropomyosin
• Thick Filaments: Composed of myosin molecules

Skeletal Muscle Activity

• Irritability: Ability to receive and respond to a stimulus.
• Contractility: Ability to shorten when an adequate stimulus is received.

Motor Unit

• A motor neuron and all the muscle fibers it innervates.
• Axons branch to innervate multiple muscle fibers.
• Axon terminals at neuromuscular junctions.

The Phases Leading to Muscle Fiber Contraction

• Motor neuron stimulates muscle fiber
• Ion permeability of sarcolemma changes
• Local change in membrane voltage
• Local depolarization triggers action potential
• Action potential travels across entire sarcolemma
• Action potential travels along T tubules
• SR releases Ca2+
• Ca2+ binds to troponin; myosin-binding sites exposed
• Myosin heads bind to actin; contraction begins

The Neuromuscular Junction

• Synaptic cleft: Space between axon terminal and muscle fiber
• Synaptic vesicles: Contain acetylcholine (ACh)
• Acetylcholine receptors (AChR): On the muscle fiber's sarcolemma
• Acetylcholinesterase (AChE): Degrades ACh

Transmission of Nerve Impulse to Muscle

• Action potential spreads rapidly along the sarcolemma.
• Na+ diffuses into the cell.
• Myoneural junction is a junction between nerve and muscle.

Channels Involved in Initiating Muscle Contraction

• Nerve impulse reaching the axon terminal opens voltage-gated calcium channels.
• Calcium entry triggers ACh release into the synaptic cleft.
• Released ACh binds to receptors on sarcolemma opening chemically gated Na+-K+ channels.
• Local depolarization opens voltage-gated sodium channels.
• Sodium influx causes further depolarization (end-plate potential).
• Action potential propagates along T tubules.

Steps in E-C Coupling

• Action potential travels down T tubules.
• Calcium ions released from sarcoplasmic reticulum.
• Calcium binds to troponin, exposing myosin-binding sites on actin.
• Myosin heads bind to actin, initiating contraction.

The Sliding Filament Theory of Muscle Contraction

• Activation (by nerve stimulation) causes myosin heads to attach to binding sites on the thin filaments.
• Myosin heads then bind to the next site on the thin filament.
• The action of the myosin heads sliding along the actin filaments shortens the muscle.

Contraction of a Skeletal Muscle

• Muscle fiber contraction is "all or none"
• Graded responses
• Muscle force depends on the number of fibers stimulated

Types of Muscle Contractions

• Isotonic contractions: Tension remains constant while the muscle shortens.
• Isometric contractions: Tension increases but there is no change in muscle length.

Muscles and Body Movements

• Muscles attach to at least two points: origin and insertion.
• Origin: Attachment to immovable bone
• Insertion: Attachment to moveable bone

Energy for Muscle Contraction

• Direct Phosphorylation
• Aerobic Respiration
• Anaerobic Glycolysis

Comparison of Skeletal, Cardiac, and Smooth Muscle

• Characteristics differ in body location, cell shape, appearance, presence of myofibrils, presence of T-tubules, and regulation of contraction

Naming of Skeletal Muscles

• Muscles named based on direction of fibers, relative size, location, number of origins, shape, location of origin/insertion, and action
• Examples: rectus, maximus, temporalis, triceps, sterno

Head and Neck Muscles

• Includes muscles that control facial expressions, chewing, and head movements.

Muscles of the Abdominal Wall

• Includes muscles like the rectus abdominis, external oblique, internal oblique, and transversus abdominis.

Superficial Muscles of the Thorax and Shoulder Acting on the Scapula and Arm

• Deltoid, pectoralis major, serratus anterior, etc.

Muscles Crossing the Shoulder and Elbow Joints

• Muscles that move the arm and forearm.

Muscles of the Anterior Fascial Compartment of the Forearm

• Muscles involved in wrist and finger movements.

Muscles of the Posterior Fascial Compartment of the Forearm

• Muscles responsible for extending wrist and fingers

Muscles of the Hip and Thigh

• Muscles supporting activities involved in hip and thigh movement

Muscles of the Leg

• Muscles responsible for movements of the lower leg

Types of Body Movements

• Flexion, Extension, Hyperextension, Abduction, Adduction, Circumduction, Rotation, Dorsiflexion, Plantar Flexion, Inversion, Eversion, Supination, Pronation, Opposition.

Types of Skeletal Muscles

• Prime Movers: Muscles primary responsible for a movement
• Antagonists: Muscles that oppose or reverse a prime mover
• Synergists: Muscles that help a prime mover in a movement