Lab 10/27

The Muscular System I

Skeletal Muscles: Functional Groups

  • Major functional groups of skeletal muscles include:
    • Agonists/Prime movers: These muscles are the primary force producers for specific movements.
    • Antagonists: They oppose or reverse a particular movement made by the agonists.
    • Synergists: These muscles assist agonists by adding force to a movement or reducing undesirable or unnecessary movement.
    • Change Force Application: They can stabilize a joint or position during actions.
    • Fixators: A subtype of synergists; these muscles immobilize a bone or a muscle’s origin to stabilize the entire body during movement.

Naming Skeletal Muscles

  • Skeletal muscles can be named based on several criteria:
    • Location: e.g., frontalis (muscle located at the front of the skull).
    • Shape: e.g., deltoid refers to a triangular shape.
    • Relative size: Terms like maximus (largest), minimus (smallest), and longus (long) denote the size of the muscles.
    • Direction of fibers or fascicles: The arrangement indicated by terms like rectus (straight), transversus (across), and oblique (angle).
    • Number of origins: Indicates how many attachment points there are; e.g., biceps (two heads) and triceps (three heads).
    • Location of attachments: Muscles named according to the point of origin or insertion.
    • Action: Common descriptors include flexor and extensor, which indicate whether a muscle decreases (flexor) or increases (extensor) the angle between body parts.

Fascicle Arrangements

  • All skeletal muscles consist of fascicles, which are bundles of fibers arranged in specific patterns:
    • Circular: Fascicles arranged in concentric rings; an example is the orbicularis oris (muscle around the mouth).
    • Convergent: Has a broad origin with fascicles converging towards a single tendon insertion; an example is pectoralis major.
    • Parallel: Fascicles run parallel to the long axis of a straplike muscle; e.g., sartorius.
    • Fusiform: These are spindle-shaped muscles that have parallel fibers; an example is biceps brachii.
    • Pennate: Fascicles attach at an angle to a central tendon running the length of the muscle:
    • Unipennate: Fasicles attach to one side of the tendon; e.g., extensor digitorum longus.
    • Bipennate: Fasicles insert from opposite sides of the tendon; e.g., rectus femoris.
    • Multipennate: Appearance resembles feathers inserting into one tendon; an example is deltoid.

Major Skeletal Muscles of the Body

  • Muscles grouped by function and location, each categorized with:
    • Name and description: Reflective of their function or position.
    • Origin and insertion: Generally involving a joint that exists between these two points.
    • Origin: Proximal attachment of muscle (the end that remains stationary).
    • Insertion: Distal attachment of muscle (the end that moves during contraction).
    • Action: Identified by which insertion moves toward the origin; understanding is aided by acting out muscle movements physically.

Muscle Anatomy

  • Muscles exert force on tendons, pulling attached bones.
    • Joint Mechanics: A joint is formed by two bones:
    • One bone remains stationary (the origin), while the other moves (the insertion).
    • Definitions:
    • Origin: Proximal attachment point of the muscle on bone.
    • Insertion: Distal attachment point that moves during muscle contraction.

Muscle Actions and Joint Mechanisms

  • The action of a muscle can be inferred from its position relative to the joint it crosses:
    • Shoulder Joint Examples:
    • Flexion: A muscle crossing anteriorly produces flexion, exemplified by pectoralis major (in anterior view).
    • Extension: A muscle that crosses posteriorly produces extension, such as latissimus dorsi (in posterior view).
    • Lateral Movement: Muscles that cross on the lateral side of a joint produce abduction; example includes the deltoid middle fibers.
    • Medial Movement: Muscles crossing on the medial side of a joint produce adduction; the teres major serves as an antagonist to the deltoid, producing adduction.

Flexion and Extension

  • Flexion: A decreasing angle between body parts.
  • Extension: An increasing angle between body parts.

Abduction and Adduction

  • Abduction: Movement away from the midline of the body.
  • Adduction: Movement toward the midline of the body.

Rotation vs Circumduction

  • Rotation: Movement around a single axis.
    • Internal Rotation: Rotation of the hip or shoulder toward the midline.
    • External Rotation: Rotation of the hip or shoulder away from the midline.
  • Circumduction: A tri-planar, circular motion at the hip or shoulder, combining flexion, extension, abduction, and adduction.

Supination and Pronation

  • Supination: Arm/hand orientation likened to a waiter carrying a soup bowl (palms facing up).
  • Pronation: Similar to a professional basketball player's hand positioning when dribbling (palms facing down).

Dorsiflexion and Plantar Flexion

  • Dorsiflexion: Movement that brings the top of the foot toward the anterior leg (lifting the foot).
  • Plantar Flexion: Movement that points the toes downward (standing on tiptoes).

Inversion and Eversion

  • Inversion: Turning the sole of the foot medially (toward the midline).
  • Eversion: Turning the sole of the foot laterally (away from the midline).

Elevation and Depression

  • Elevation: Movement that raises a body part.
  • Depression: Movement that lowers a body part.

Protraction and Retraction

  • Protraction: Moving a body part forward (e.g., mandible protrusion).
  • Retraction: Moving a body part backward (e.g., mandible retraction).

Head & Neck Muscles

  • Epicranial Aponeurosis: A broad, thin layer of connective tissue covering the scalp.
  • Muscles of the Neck:
    • Digastric Muscle: Has two muscle bellies; aids in swallowing.
    • Mylohyoid Muscle: Forms the floor of the mouth.
    • Epicranius Front: Involved in raising the eyebrows.
    • Temporalis: Strong muscle used for chewing.
    • Orbicularis Oculi: Closes the eyelids.
    • Zygomaticus Minor & Major: Muscles involved in smiling.
    • Orbicularis Oris: The muscle around the mouth helping in expressions and speech.
    • Sternocleidomastoid: Rotates and tilts the head.
    • Platysma: Facial expression muscle that tenses the skin of the neck.

Chewing Muscles

  • Masseter: Strong muscle used for closing the jaw.
  • Temporalis: Assists masseter in elevating the mandible.
  • Pterygoid Muscles (Lateral & Medial): Assist in chewing movements, enabling the side-to-side motion of the jaw.

Muscles of the Back

  • Trapezius: Elevates, retracts, and rotates the scapula.
  • Latissimus Dorsi: Extends and adducts the shoulder joint.
  • Levator Scapulae: Elevates the scapula.
  • Rhomboid Minor & Major: Retract and stabilize the scapula.
  • Spinalis, Longissimus, Iliocostalis: Groups of muscles that form the erector spinae, crucial for maintaining posture and spinal stability.

Abdominal Muscles

  • Pectoralis Major: Major muscle of the chest, crucial for shoulder movement.
  • External Oblique: Rotates and laterally flexes the trunk.
  • Rectus Abdominus: Muscle responsible for flexing the lumbar spine.
  • Transversus Abdominus: Compresses abdominal contents and aids in posture.