Muscle Actions: Origin, Insertion, Agonist

Muscle Definitions and Examples

• Origin refers to the fixed attachment point of a muscle.
• As an example, the scapula represents the origin for the biceps brachii.
• Insertion refers to the movable attachment point of a muscle.
• As an example, the radius represents the insertion for the biceps brachii.
• The agonist, or prime mover, signifies the main muscle responsible for a movement.
• As an example, the biceps brachii acts as the agonist in elbow flexion.
• The antagonist is the muscle that opposes the agonist.
• As an example, the triceps brachii serves as the antagonist during elbow flexion.
• The synergist assists the agonist.
• As an example, the brachialis acts as a synergist with the biceps brachii.
• The fixator stabilizes the origin of the agonist.
• As an example, rotator cuff muscles function as fixators during arm movements.

Fasciculus Orientation and Muscle Shapes

• Circular muscles feature fascicles arranged in rings.
• The orbicularis oculi serves as an example of a circular muscle.
• Convergent muscles have a broad origin converging to a single tendon.
• The pectoralis major exemplifies a convergent muscle.
• Parallel muscles exhibit fascicles that run parallel to the long axis.
• The sartorius is an example of a parallel muscle.
• Pennate muscles have fascicles attaching obliquely to a tendon.
• Unipennate muscles, such as the extensor digitorum longus, represent one subtype.
• Bipennate muscles, like the rectus femoris, represent another subtype.
• Multipennate muscles, such as the deltoid, represent another subtype.

Muscle Naming Rules

• Muscle names often reflect their location.
• As an example, the temporalis muscle is named for its proximity to the temporal bone.
• Muscle names often reflect their shape.
• As an example, the deltoid muscle has a triangular shape.
• Muscle names often reflect their action.
• As an example, the flexor digitorum muscle flexes the fingers.
• Muscle names often reflect their size.
• As an example, the gluteus maximus is the largest of the gluteal muscles.

Classes of Levers

• First-class levers have the fulcrum positioned between the effort and the load.
• Neck extension using the trapezius exemplifies a first-class lever.
• Second-class levers have the load positioned between the fulcrum and the effort.
• Standing on toes using the gastrocnemius exemplifies a second-class lever.
• Third-class levers have the effort positioned between the fulcrum and the load.
• The biceps brachii in elbow flexion exemplifies a third-class lever.

Neck Muscles

• The sternocleidomastoid originates at the sternum and clavicle and inserts at the mastoid process, flexing and rotating the head.
• The scalenes originate at the cervical vertebrae and insert at the first two ribs, elevating the ribs and assisting in neck flexion.

Head Movements

• The sternocleidomastoid facilitates flexion.
• The splenius capitis facilitates extension.
• The sternocleidomastoid and scalenes facilitate rotation.

Facial Expression Muscles

• The zygomaticus major facilitates smiling.
• The corrugator supercilii facilitates frowning.
• The buccinator facilitates cheek compression.

Mastication, Tongue Movement, and Swallowing

• Mastication involves the masseter and temporalis muscles.
• Tongue movement involves the genioglossus and styloglossus muscles.
• Swallowing involves the pharyngeal constrictors and suprahyoid muscles.

Hyoid Muscles

• The mylohyoid elevates the hyoid bone and tongue.
• The sternohyoid depresses the hyoid bone after swallowing.

Eye Movement Muscles

• The superior rectus elevates the eye.
• The lateral rectus abducts the eye.
• The inferior oblique elevates and laterally rotates the eye.

Vertebral Column Muscles

• The erector spinae group extends and flexes the spine.
• The multifidus stabilizes the vertebrae.

Thorax Muscles

• The diaphragm serves as the primary muscle of respiration.
• The external and internal intercostals assist in breathing.

Characteristics of Muscle Tissue

• Skeletal muscle is voluntary, striated, features multinucleated cells, and attaches to bones for movement and posture.
• Smooth muscle is involuntary, non-striated, has spindle-shaped cells, and is found in organ and vessel walls for digestion and circulation.
• Cardiac muscle is involuntary, striated, contains single-nucleus cells, is exclusively in the heart, and connects via intercalated discs for synchronized contractions.

Functions of the Muscular System

• Movement, such as walking and breathing
• Support and posture maintenance
• Protection of organs
• Heat generation via metabolism
• Blood circulation through cardiac muscle action

Functional Properties of Muscle Tissue

• Excitability involves the ability to respond to stimuli.
• Contractility involves the ability to shorten to generate force.
• Extensibility involves the ability to stretch without damage.
• Elasticity involves the ability to return to the original shape after stretching.

Connective Tissue Components of Skeletal Muscle

• The epimysium surrounds the entire muscle.
• The perimysium encloses fascicles.
• The endomysium surrounds individual muscle fibers.

Blood Supply and Innervation

• Skeletal muscles have a rich blood supply for nutrients and oxygen.
• Motor neurons innervate skeletal muscles for triggering contraction.

Origin of Muscle Fibers and Hypertrophy

• Muscle fibers originate from myoblast fusion during development.
• Hypertrophy occurs through increased protein synthesis in response to resistance training or stress.

Components of a Muscle Fiber

• Sarcolemma serves as the membrane.
• Sarcoplasmic reticulum provides calcium storage.
• Mitochondria produces ATP.
• Myofibrils are contractile units.
• Nuclei are present in muscle fibers.

Types of Myofilaments

• Actin (thin filament) contains binding sites for myosin.
• Myosin (thick filament) has heads for cross-bridge formation during contraction.

Sarcomere Diagram

• Sarcomeres contain alternating actin and myosin filaments arranged in repeating units.
• Z-discs mark sarcomere boundaries.

Sliding Filament Model & Band Changes

• During contraction, the A band remains constant.
• During contraction, the I band shortens.
• During contraction, the H zone disappears as filaments slide past each other.

Resting Membrane Potential

• Generated by ion gradients (Na+/K+) across the sarcolemma.
• Maintained by Na+/K+ pumps.