Chapter 10: The Muscular System – Structure, Function, and Major Muscles

Chapter 13: The Muscular System

Overview of the Muscular System

The muscular system is responsible for movement, posture, and heat production in the human body. It consists of skeletal muscles, which attach to bones and enable voluntary movement, as well as smooth and cardiac muscles, which control involuntary actions.

• Muscle Actions and Naming Conventions: Muscles are named based on their location, shape, size, direction of fibers, number of origins, and action.

• Major Muscles: Includes muscles of the head, neck, trunk, upper and lower limbs.

Prime Movers, Antagonists, Synergists, and Fixators

Muscles work together to produce movement. Their roles are classified as follows:

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

• Antagonist: The muscle that opposes the action of the prime mover.

• Synergist: A muscle that assists the prime mover by adding extra force or reducing unnecessary movement.

• Fixator: A type of synergist that stabilizes the origin of the prime mover.

Example: In elbow flexion, the biceps brachii is the prime mover, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Criteria Used in Naming Muscles

Muscles are named according to several criteria:

• Location: e.g., frontalis (on the frontal bone)

• Shape: e.g., deltoid (triangle-shaped)

• Size: e.g., maximus (largest), minimus (smallest)

• Direction of fibers: e.g., rectus (straight), oblique (angled)

• Number of origins: e.g., biceps (two origins), triceps (three origins)

• Location of attachments: e.g., sternocleidomastoid (attached to sternum, clavicle, and mastoid process)

• Action: e.g., flexor, extensor, adductor

Major Muscles of the Human Body

The following are key muscle groups and their compartments:

• Muscles of Mastication: Responsible for chewing (e.g., masseter, temporalis).

• Muscles that Move the Scapula/Shoulder: e.g., trapezius, serratus anterior.

• Anterior and Posterior Compartments of the Arm: Anterior: biceps brachii, brachialis (flexion) Posterior: triceps brachii (extension)

• Anterior and Posterior Compartments of the Forearm: Anterior: Flexors of wrist and fingers Posterior: Extensors of wrist and fingers

• Muscles of Respiration: diaphragm, intercostals

• Anterior, Posterior, and Medial Compartments of Thigh: Anterior: quadriceps femoris (extension of knee) Posterior: hamstrings (flexion of knee) Medial: adductors (adduction of thigh)

• Anterior, Posterior, and Lateral Compartments of Leg: Anterior: tibialis anterior (dorsiflexion) Posterior: gastrocnemius, soleus (plantarflexion) Lateral: fibularis (peroneus) muscles (eversion)

Table: Major Muscle Compartments and Their Actions

Region	Compartment	Main Muscles	Primary Action
Arm	Anterior	Biceps brachii, Brachialis	Flexion of elbow
Arm	Posterior	Triceps brachii	Extension of elbow
Thigh	Anterior	Quadriceps femoris	Extension of knee
Thigh	Posterior	Hamstrings	Flexion of knee
Thigh	Medial	Adductors	Adduction of thigh
Leg	Anterior	Tibialis anterior	Dorsiflexion of foot
Leg	Posterior	Gastrocnemius, Soleus	Plantarflexion of foot
Leg	Lateral	Fibularis (Peroneus) muscles	Eversion of foot

Origins, Insertions, and Actions

Each muscle has a specific origin (fixed attachment), insertion (movable attachment), and action (movement produced).

• Origin: The stationary bone where the muscle attaches.

• Insertion: The bone that moves when the muscle contracts.

• Action: The specific movement produced by muscle contraction.

Example: The biceps brachii originates from the scapula and inserts on the radius, producing flexion of the forearm.

Additional info:

• Muscle compartments are groups of muscles with similar functions and innervation, separated by connective tissue.

• Understanding muscle actions and compartments is essential for clinical assessment and treatment of injuries.