Chapter 9: The Muscular System – Structure, Function, and Major Muscle Groups

Chapter 9: The Muscular System

Module 9.1 Overview of Skeletal Muscles

This module introduces the skeletal muscles, their primary functions, and the terminology used to describe their roles in movement. Understanding these concepts is essential for identifying how muscles work together to produce movement.

• Major Functions of Skeletal Muscles:

  • Produce movement by contracting and pulling on bones.

  • Maintain posture and body position.

  • Support soft tissues and protect internal organs.

  • Guard entrances and exits of the body (e.g., sphincters).

  • Generate heat to maintain body temperature (thermogenesis).

• Naming Skeletal Muscles:

  • Muscle names often indicate their action (e.g., flexor, extensor), appearance (e.g., deltoid = triangular), or location (e.g., brachii = arm).

  • Other naming conventions include direction of fibers (e.g., rectus = straight), number of origins (e.g., biceps = two heads), and size (e.g., maximus = largest).

• Muscle Roles in Movement:

  • Agonist (Prime Mover): Main muscle responsible for a specific movement.

  • Antagonist: Muscle that opposes the action of the agonist.

  • Synergist: Muscle that assists the agonist in performing its action.

  • Fixator: Muscle that stabilizes the origin of the agonist to allow efficient movement.

• Examples of Muscle Roles:

  • During elbow flexion: Biceps brachii is the agonist, triceps brachii is the antagonist, brachialis is a synergist, and muscles of the shoulder girdle act as fixators.

• Lever Systems in the Body:

  • Levers consist of a fulcrum (pivot point), force (effort applied), and load (resistance).

  • Three classes of levers:

    • First-class lever: Fulcrum between force and load (e.g., neck extension).

    • Second-class lever: Load between fulcrum and force (e.g., standing on tiptoe).

    • Third-class lever: Force between fulcrum and load (e.g., biceps brachii flexing the forearm).

  • Each lever type affects power and range of motion differently.

Module 9.2 Muscles of the Head, Neck, and Vertebral Column

This module covers the identification, description, and actions of major muscles in the head, neck, and vertebral column. Understanding their origin, insertion, and action is crucial for recognizing their roles in movement and support.

• Muscles of the Head: Include facial expression muscles (e.g., orbicularis oculi, zygomaticus), mastication muscles (e.g., masseter, temporalis).

• Muscles of the Neck: Include sternocleidomastoid (rotates and flexes the neck), scalenes, and splenius capitis.

• Muscles of the Vertebral Column: Include erector spinae group (extends the vertebral column), semispinalis, and multifidus.

• Origin, Insertion, and Action:

  • Origin: The fixed attachment point of the muscle.

  • Insertion: The movable attachment point.

  • Action: The movement produced when the muscle contracts.

• Example: The sternocleidomastoid originates on the sternum and clavicle, inserts on the mastoid process, and acts to rotate and flex the head.

Module 9.3 Muscles of the Trunk and Pelvic Floor

This module focuses on the muscles of the trunk and pelvic floor, their identification, and their functional roles in movement and support.

• Muscles of the Trunk: Include rectus abdominis (flexes vertebral column), external and internal obliques (rotate and laterally flex trunk), transversus abdominis (compresses abdomen), and diaphragm (main muscle of respiration).

• Muscles of the Pelvic Floor: Support pelvic organs and control openings of the urethra and anus (e.g., levator ani, coccygeus).

• Origin, Insertion, and Action: Each muscle's origin, insertion, and action should be identified for a comprehensive understanding.

• Example: The rectus abdominis originates on the pubic bone, inserts on the xiphoid process and costal cartilages, and flexes the vertebral column.

Module 9.4 Muscles of the Pectoral Girdle and Upper Limb

This module addresses the muscles that move the pectoral girdle and upper limb, including their identification, origin, insertion, and action.

• Muscles of the Pectoral Girdle: Include trapezius, levator scapulae, rhomboids, and serratus anterior.

• Muscles of the Upper Limb: Include deltoid (abducts arm), pectoralis major (flexes and adducts arm), biceps brachii (flexes elbow), triceps brachii (extends elbow), and forearm muscles.

• Origin, Insertion, and Action: Understanding these aspects is essential for each muscle's function.

• Example: The deltoid originates on the clavicle and scapula, inserts on the deltoid tuberosity of the humerus, and abducts the arm.

Module 9.5 Muscles of the Hip and Lower Limb

This module covers the muscles of the hip and lower limb, focusing on their identification, origin, insertion, and action.

• Muscles of the Hip: Include gluteus maximus (extends thigh), gluteus medius (abducts thigh), iliopsoas (flexes hip).

• Muscles of the Thigh: Include quadriceps femoris group (extends knee), hamstrings (flex knee, extend hip), adductors (adduct thigh).

• Muscles of the Leg and Foot: Include gastrocnemius (plantar flexes foot), tibialis anterior (dorsiflexes foot).

• Origin, Insertion, and Action: Each muscle's anatomical details should be identified.

• Example: The gastrocnemius originates on the femur, inserts on the calcaneus via the Achilles tendon, and plantar flexes the foot.

Module 9.6 The Big Picture of Muscle Movement

This module integrates knowledge of muscle locations and actions, emphasizing the coordination required for complex movements.

• Major Skeletal Muscles: Students should be able to locate and demonstrate the actions of the major skeletal muscles discussed in previous modules.

• Coordination of Muscle Groups: Movement often requires the coordinated action of agonists, antagonists, synergists, and fixators.

• Example: Walking involves coordinated actions of hip flexors, knee extensors, and ankle plantar flexors.

Additional info: For each muscle, students should be familiar with its anatomical location, origin, insertion, and primary action. Lever systems are fundamental to understanding how muscles produce movement efficiently in the human body.