Muscular System: Structure, Classification, and Function

Muscle Structure and Attachments

Origin and Insertion

The origin and insertion are fundamental concepts in muscle anatomy. Muscles attach to bones at two points: the origin (the more fixed attachment) and the insertion (the more mobile attachment). During contraction, the insertion moves toward the origin, resulting in movement at the joint the muscle crosses.

• Origin: The stable, proximal attachment of the muscle.

• Insertion: The movable, distal attachment of the muscle.

• Muscle Action: Muscles can only pull, not push, and always cross at least one joint.

• Movement: Contraction brings the insertion closer to the origin.

Classification of Muscles

Criteria Used in Naming Muscles

Muscles are classified and named based on several criteria, which help identify their location, function, and structure. Understanding these criteria is essential for recognizing muscle names and their roles in the body.

• Location: Indicates where the muscle is found (e.g., temporalis in the temple).

• Shape: Describes the muscle's form (e.g., trapezius is trapezoid-shaped).

• Size: Relative size compared to other muscles (e.g., gluteus maximus is the largest gluteal muscle).

• Direction of Fibers: Orientation of muscle fibers (e.g., rectus abdominis runs straight).

• Number of Origins: Number of tendinous attachments (e.g., biceps brachii has two origins).

• Location of Attachments: Points where the muscle attaches (e.g., sternocleidomastoid attaches to the sternum, clavicle, and mastoid process).

• Action: The primary movement produced (e.g., flexor digitorum flexes the fingers).

Muscles of Facial Expression

Major Muscles and Their Functions

Facial muscles are responsible for expressions and movements such as smiling, frowning, and blinking. These muscles are unique because they insert into the skin rather than bone, allowing for a wide range of expressions.

• Epicranius (frontalis and occipitalis): Raises eyebrows and wrinkles forehead.

• Orbicularis oculi: Closes the eyelids.

• Orbicularis oris: Closes and protrudes lips (kissing).

• Zygomaticus major and minor: Elevate corners of the mouth (smiling).

• Buccinator: Compresses cheek (whistling).

• Risorius: Draws corner of mouth laterally.

• Platysma: Tenses skin of neck.

Muscles of Mastication

Chewing Muscles

The muscles of mastication are responsible for moving the jaw during chewing. These include the masseter, temporalis, and buccinator.

• Masseter: Elevates the mandible (jaw closure).

• Temporalis: Elevates and retracts the mandible.

• Buccinator: Compresses cheek, assists in chewing.

• Orbicularis oris: Closes lips, assists in chewing and swallowing.

Muscles of the Head and Neck

Swallowing and Head Movement

Muscles in the head and neck facilitate swallowing, speech, and movement of the head. Key muscles include the digastric, mylohyoid, and sternocleidomastoid.

• Digastric: Depresses mandible, elevates hyoid during swallowing.

• Mylohyoid: Elevates floor of mouth.

• Stylohyoid: Elevates and retracts hyoid.

• Sternocleidomastoid: Flexes and rotates the head.

Muscle Actions and Roles

Prime Movers, Synergists, Antagonists, and Fixators

Muscles work together to produce movement. Each muscle can play a specific role in an action:

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

• Synergist: Assists the prime mover.

• Antagonist: Opposes the action of the prime mover.

• Fixator: Stabilizes the origin of the prime mover.

Muscles of the Trunk

Abdominal Muscles

The trunk muscles support posture, protect internal organs, and facilitate movements such as bending and twisting. The abdominal muscles include the rectus abdominis, external oblique, internal oblique, and transversus abdominis.

• Rectus abdominis: Flexes the trunk, compresses abdominal cavity.

• External oblique: Flexes and laterally flexes the trunk.

• Internal oblique: Flexes and laterally flexes the trunk.

• Transversus abdominis: Compresses abdominal cavity.

Abdominal Muscle Table

The following table summarizes the actions, origins/insertions, and innervation of the main abdominal muscles:

Muscle	Action	Origin/Insertion/Nerve
Rectus abdominis	Flexes the trunk, compresses abdominal cavity	Origin: Pubic bones; Insertion: Costal cartilage of ribs 5-7, xiphoid process; Nerve: Branches of anterior rami of lower six thoracic nerves
External oblique	Flexes and laterally flexes the trunk	Origin: Inferior eight ribs; Insertion: Iliac crest, pubic tubercle, linea alba; Nerve: Branches of lower six thoracic nerves
Internal oblique	Flexes and laterally flexes the trunk	Origin: Iliac crest, inguinal ligament; Insertion: Inferior three or four ribs, linea alba, pubic bone; Nerve: Branches of lower six thoracic nerves
Transversus abdominis	Compresses abdominal cavity	Origin: Inferior six costal cartilages, thoracolumbar fascia, iliac crest, inguinal ligament; Insertion: Linea alba, pubic bone; Nerve: Branches of lower six thoracic nerves

Muscles of the Shoulder and Arm

Superficial and Deep Muscles

The shoulder and arm muscles are responsible for movements of the upper limb. Superficial muscles include the deltoid and pectoralis major, while deep muscles include the pectoralis minor and serratus anterior.

• Deltoid: Abducts the arm.

• Pectoralis major: Flexes, adducts, and medially rotates the arm.

• Pectoralis minor: Stabilizes the scapula.

• Serratus anterior: Protracts the scapula.

Teres Minor vs. Teres Major

The teres minor and teres major are two muscles located in the shoulder region. They have different functions and attachments.

• Teres minor: Rotates the arm laterally; part of the rotator cuff.

• Teres major: Adducts and medially rotates the arm; not part of the rotator cuff.

Muscles of the Arm, Forearm, and Hand

Anterior and Posterior Views

The muscles of the arm, forearm, and hand allow for complex movements such as gripping, flexing, and extending the wrist and fingers.

• Flexor digitorum superficialis: Flexes fingers.

• Flexor carpi radialis/ulnaris: Flexes and abducts/adducts the wrist.

• Extensor carpi radialis/ulnaris: Extends and abducts/adducts the wrist.

• Brachioradialis: Flexes forearm at elbow.

Superficial Muscles: Anterior and Posterior Views

Overview of Major Muscles

Superficial muscles are those closest to the skin and are responsible for the visible contours of the body. They include muscles of the chest, abdomen, arms, legs, and back.

• Pectoralis major: Chest muscle, flexes and adducts the arm.

• Rectus abdominis: Abdominal muscle, flexes the trunk.

• Biceps brachii: Arm muscle, flexes the forearm.

• Quadriceps femoris: Thigh muscle, extends the knee.

• Gastrocnemius: Calf muscle, plantar flexes the foot.

Muscles that Move the Thigh and Knee

Hip and Thigh Muscles

Muscles of the hip and thigh are essential for walking, running, and maintaining posture. They include the gluteal muscles, adductors, and hamstrings.

• Gluteus maximus: Extends and laterally rotates the thigh.

• Gluteus medius/minimus: Abducts and medially rotates the thigh.

• Adductor magnus: Adducts the thigh.

• Hamstrings (biceps femoris, semitendinosus, semimembranosus): Flex the knee and extend the thigh.

Muscles That Move the Ankle and Foot

Lower Limb Muscles

The muscles of the lower leg and foot control movements such as dorsiflexion, plantar flexion, inversion, and eversion. These muscles are crucial for walking and balance.

• Tibialis anterior: Dorsiflexes the foot.

• Fibularis longus/brevis: Everts the foot.

• Gastrocnemius: Plantar flexes the foot.

• Soleus: Plantar flexes the foot.

• Extensor digitorum longus: Extends toes.

Muscle Table: Foot and Toe Movement

The following table summarizes the actions and attachments of muscles that move the foot and toes:

Muscle	Action	Origin/Insertion
Tibialis anterior	Dorsiflexes the foot	Origin: Lateral condyle and proximal diaphysis of tibia; Insertion: Medial cuneiform and first metatarsal
Fibularis longus	Everts the foot	Origin: Head and proximal lateral fibula; Insertion: Medial cuneiform and first metatarsal
Gastrocnemius	Plantar flexes the foot	Origin: Medial and lateral condyles of femur; Insertion: Calcaneus via Achilles tendon
Soleus	Plantar flexes the foot	Origin: Head of fibula, medial border of tibia; Insertion: Calcaneus via Achilles tendon
Extensor digitorum longus	Extends toes	Origin: Lateral condyle of tibia, anterior surface of fibula; Insertion: Middle and distal phalanges of toes

Putting It All Together: The Big Picture of Muscle Movement

Integrated Muscle Actions

Muscle movement is a coordinated process involving multiple muscle groups. For example, walking up stairs requires the activation of muscles in the chest, arms, thighs, and calves, each contributing to the overall movement.

• Pectoralis major and coracobrachialis: Flex the arm.

• Brachioradialis: Flexes the forearm.

• Hamstrings: Flex the knee.

• Quadriceps femoris: Extends the knee.

• Gluteus maximus: Extends the thigh.

• Gastrocnemius: Plantar flexes the foot.

Summary Table: Muscle Classification Criteria

Overview of Naming Criteria

The following table summarizes the main criteria used in naming muscles:

Criterion	Example
Location	Temporalis, intercostal
Shape	Trapezius, rhomboid
Size	Gluteus maximus
Direction of fibers	Rectus abdominis, transversus abdominis
Number of origins	Biceps brachii
Location of attachments	Sternocleidomastoid
Action	Flexor digitorum