Myology Chapter 11 Study Notes

Myology 2 - Chapter 11: Fiber Arrangement, Muscle Actions, and Naming of Skeletal Muscles

Introduction

  • Skeletal Muscles Count and Weight

    • There are more than 600 skeletal muscles in the human body.

    • Skeletal muscles constitute between 40% to 50% of body weight.

  • Functions

    • Muscles, together with the skeleton, determine the form and contour of the body.

Superficial Muscles of the Body

  • Figure 10.5: Anterior View of Superficial Muscles

    • Head and Neck:

    • Epicranius (frontal belly)

    • Orbicularis oculi

    • Zygomaticus

    • Orbicularis oris

    • Temporalis

    • Masseter

    • Platysma

    • Sternohyoid

    • Sternocleidomastoid

    • Thorax:

    • Pectoralis minor

    • Pectoralis major

    • Serratus anterior

    • Intercostals

    • Abdomen:

    • Rectus abdominis

    • External oblique

    • Internal oblique

    • Transversus abdominis

    • Thigh:

    • Tensor fasciae latae

    • Sartorius

    • Adductor longus

    • Gracilis

    • Leg:

    • Gastrocnemius

    • Soleus

    • Fibularis longus

    • Extensor digitorum longus

    • Tibialis anterior

  • Figure 10.6: Posterior View of Superficial Muscles

    • Neck:

    • Epicranius (occipital belly)

    • Sternocleidomastoid

    • Trapezius

    • Shoulder:

    • Deltoid

    • Infraspinatus

    • Teres major

    • Rhomboid major

    • Latissimus dorsi

    • Hip:

    • Gluteus medius

    • Gluteus maximus

    • Adductor magnus

    • Biceps femoris

    • Leg:

    • Gastrocnemius

    • Soleus

    • Fibularis longus

    • Structures include: Calcaneal (Achilles) tendon and Iliotibial tract.

Skeletal Muscle Structure

  • Variability in Muscles

    • Size: Range from extremely small to large masses.

    • Shape: Can be broad, narrow, long, tapering, short, blunt, triangular, quadrilateral, irregular, flat sheets, or bulky masses.

    • Arrangement:

    • Muscles can be arranged in various patterns—parallel to the long axis, convergent, oblique, pennate (winged), bipennate, or curved.

    • The direction of fibers affects the function and efficiency of muscle contraction.

  • Figure 10.2: Fascicle Arrangement Patterns

    • Convergent (e.g., Pectoralis major)

    • Circular (e.g., Orbicularis oris)

    • Fusiform (e.g., Biceps brachii)

    • Parallel (e.g., Sartorius)

    • Multipennate (e.g., Deltoid)

    • Bipennate (e.g., Rectus femoris)

    • Unipennate (e.g., Extensor digitorum longus)

Attachment of Muscles

  • Definitions

    • Origin: The point of attachment that does not move when the muscle contracts, usually the proximal end.

    • Insertion: The point of attachment that moves when the muscle contracts, usually the distal end.

    • Belly: The central body portion of the muscle.

Muscle Actions

  • Muscle Coordination

    • Most movements result from the coordinated action of several muscles; some contract while others relax.

  • Muscle Types

    • Prime Movers (Agonists): Muscles or groups that directly perform a specific movement.

    • Antagonists: Muscles that oppose prime movers. They relax when prime movers contract to facilitate movement and provide precision and control.

    • Synergists: Muscles that assist prime movers in their actions, leading to more efficient movements.

    • Fixators: Muscles that stabilize joints, providing support during movement.

Lever Systems

  • Human Body as Lever Systems

    • In the body:

    • Bones serve as levers.

    • Joints serve as fulcrums.

    • Contracting muscles pull on bones at points of attachment, causing movement around the fulcrum (joint).

  • Components:

    • Rigid bar (bone)

    • Fulcrum (joint)

    • Load (weight being moved)

    • Effort (muscle contraction producing movement)

  • Types of Levers

    • First-Class Levers: Fulcrum lies between the effort and the load. Rare in the human body; functions mainly for stability.

    • Second-Class Levers: Load lies between the fulcrum and the effort. Not widely accepted as present in the human body.

    • Third-Class Levers: Effort is between the fulcrum and the load. Common in the body; allows for rapid and extensive movement.

  • Examples of Lever Systems

    • First-Class Lever: Raising the head off the chest, where:

    • Effort by posterior neck muscles

    • Fulcrum is the atlanto-occipital joint

    • Load is the facial skeleton

    • Second-Class Lever: Standing on tip-toe, where:

    • Effort is from calf muscles

    • Fulcrum is the joints of the ball of the foot

    • Load is body weight

    • Third-Class Lever: Flexing the forearm via the biceps brachii, where:

    • Effort is applied on the proximal radius

    • Fulcrum is the elbow joint

    • Load is the hand and distal end of the forearm.

Naming Skeletal Muscles

  • Criteria for Naming

    • Location: Associated bone or body region (e.g., Pectoralis = pectoral region).

    • Shape: Descriptive names based on shape (e.g., Deltoid = triangle).

    • Size: Comparative terms (e.g., maximus for largest, minimus for smallest, longus for long, brevis for short).

    • Fiber Orientation:

    • Rectus: Fibers run straight.

    • Transversus: Fibers run at right angles.

    • Oblique: Fibers run at angles.

  • Additional Naming Criteria

    • Number of Origins:

    • Biceps = two origins

    • Triceps = three origins

    • Locations of Attachments: Named according to origin and insertion points (origin listed first).

    • Muscle Action: Named for their action (e.g., flexor or extensor).

    • Combination: Multiple criteria can be used; e.g., extensor carpi radialis longus. The muscle name often gives clues about its function and actions.

Important Skeletal Muscles

  • Facial Muscles: Include muscles of expression, with at least one attachment to deeper skin layers (e.g., Buccinator for smiling).

  • Mastication Muscles: In charge of chewing movements (e.g., Masseter).

  • Head Movement Muscles: Paired neck muscles facilitating head movement (e.g., Sternocleidomastoid).

Trunk and Other Muscle Groups

  • Thorax Muscles:

    • Essential for respiration (e.g., External intercostals elevate ribs during inspiration; Internal intercostals lower ribs during forced expiration).

  • Abdominal Wall Muscles: Arranged in three layers, enhancing strength with fibers running in various directions.

  • Back Muscles: Provide bending and stabilization.

  • Pelvic Floor Muscles: Support pelvic cavity structures, with Levator ani being predominantly involved.

Upper Limb Muscles

  • Shoulder Girdle Muscles: Attach upper extremity to torso; allow extensive movement.

  • Upper Arm Muscles: Originate on clavicle/scapula; shoulder joint is a synovial joint allowing diverse movement.

    • Anterior-side muscles act as flexors, while posterior-side muscles act as extensors.

  • Rotator Cuff Muscles: Include Supraspinatus, Infraspinatus, Teres minor, and Subscapularis (memory aid: SITS).

  • Deltoid: Commonly used site for intramuscular injections.

Lower Limb Muscles

  • Pelvic Girdle and Lower Extremity: Key in locomotion and maintaining stability.

    • Intramuscular Injection Site: Gluteus medius.

  • Ankle and Foot Muscles:

    • Extrinsic muscles situated in the leg, controlling actions like dorsiflexion, plantar flexion, inversion, and eversion.

  • Toe Muscles: Intrinsic foot muscles located within the foot, responsible for toe movements like flexion, extension, abduction, and adduction.