Chapter 10 - The Muscular System: Gross Anatomy

Chapter 10 - The Muscular System: Gross Anatomy

Tendons

  • Function: Connect muscles to bones.
  • Types:
    • Cordlike tendons.
    • Broad/flat tendons referred to as aponeuroses.

Muscle Structure

  • Origin: The fixed attachment of a muscle, typically connected to a stationary bone.
  • Insertion: The attachment point of a muscle that is typically connected to a moving bone.
  • Belly: The central part of the muscle, situated between the origin and insertion.

Types of Muscle Interactions

  • Synergists: Muscles that work in conjunction to produce a specific movement.
  • Prime mover (Agonist): The muscle that predominantly facilitates the observed movement.
  • Antagonist: Muscles that oppose the action of the prime mover and synergists.
  • Fixator: A muscle that stabilizes the joint crossed by the prime mover to allow for efficient movement.
    • Example: In forearm flexion:
    • Prime Mover = Biceps brachii
    • Synergist = Brachialis
    • Antagonist = Triceps brachii

Classification of Muscles

  • Based on Location: E.g., Pectoralis (chest muscles).
  • Based on Size: Terms such as maximus (largest) and minimus (smallest).
  • Based on Shape: For instance, deltoid (triangular shape).
  • Based on Fiber Orientation: Descriptors like oblique (diagonal) and rectus (straight).
  • Based on Origin/Insertion: E.g., Sternocleidomastoid, which relates to its attachment points.
  • Based on Number of Heads: Muscles can have prefixes indicating number, such as biceps (two heads) or triceps (three heads).
  • Based on Function: For example, flexor carpi radialis is classified due to its function in flexing the wrist.

Mechanics of Muscle Contraction and Movement

  • Force: The pull applied to a lever results from muscle contraction.
  • Movement Types:
    • Flexion: Decrease in the angle between body parts.
    • Extension: Increase in the angle between body parts.
  • Lever Systems: Contextualizing Movement:
    • Definitions:
    • Lever: A bone functioning as a rigid shaft capable of rotating about a pivot point, which is defined as a joint.
    • Weight/Resistance: The object upon which the force of movement is exerted.
    • Fulcrum: The pivot point around which movement occurs.
Class I Lever
  • Mechanics: The arrangement involves the following components:
    • Weight.
    • Pivot point (fulcrum).
    • Force applied.
  • Example:
    • Location of Forces: Atlanto-occipital joint acts as fulcrum; posterior neck muscles apply force; head is the weight.
    • Action Description: This setup facilitates the depression of the back of the head, offering limited distance or weight lifted due to its position.
Class II Lever
  • Mechanics: Different arrangement than Class I.
  • Components:
    • Fulcrum,
    • Weight,
    • Applied Force.
  • Example:
    • Location of Forces: Calf muscles provide the force; the calcaneus acts as the lever; the entire body’s weight serves as the resistance, with the metatarsal-phalangeal joint functioning as the fulcrum.
    • Action Description: Example of standing on toes, allowing for lifting considerable weight, albeit over a short distance.
Class III Lever
  • Mechanics: Example of how most levers operate in the human body.
  • Components:
    • Fulcrum,
    • Force,
    • Weight.
  • Example:
    • Location of Forces: Biceps brachii applies the force; the ulna serves as the lever; the weight consists of the forearm/hand.
    • Action Description: This mechanism enables elbow flexion, which facilitates greater distances of movement but does not carry much weight, making it the most common lever system in the body.