Topic 6: Major Skeletal Muscles (Muscular System II) / บทที่ 6: กล้ามเนื้อโครงร่างมัดสำคัญและกายวิภาคอาจารย์ใหญ่ (ระบบกล้ามเนื้อ 2)

Muscles of Facial Expression (Innervated by Facial Nerve - CN VII)

• Frontalis (Frontal belly of Occipitofrontalis): Raises eyebrows, wrinkles forehead in surprise/curiosity.
• Orbicularis oculi: Circular sphincter around eye; closes eyelids tightly (blinking, squinting, sleeping).
• Levator palpebrae superioris: Elevates upper eyelid (innervated by CN III Oculomotor; damage causes ptosis).
• Corrugator supercilii: Draws eyebrows medially and down (frowning, anger, concentration).
• Zygomaticus major & minor: Elevates angle of mouth upward and laterally (smiling and laughing).
• Risorius: Draws corner of mouth straight laterally.
• Buccinator: Compresses cheek against teeth (whistling, blowing trumpet, sucking straw, keeping food on teeth during chewing).
• Orbicularis oris: Oral sphincter; closes, purses, and protrudes lips (kissing, speaking, whistling).
• Mentalis: Wrinkles chin, protrudes lower lip (pouting, doubt).
• Platysma: Superficial thin sheet; tenses skin of neck, depresses mandible and lower lip (horror, fright).

Muscles of Mastication (Innervated by Mandibular Nerve - CN V3)

Muscle	Origin & Insertion	Action
Masseter	Zygomatic arch ➔ Ramus and angle of mandible	Elevates mandible (closes jaw); strongest masticatory muscle.
Temporalis	Temporal fossa ➔ Coronoid process of mandible	Elevates and retracts mandible (closes jaw and pulls backward).
Medial Pterygoid	Pterygoid process & maxilla ➔ Medial surface of mandibular ramus	Elevates mandible, assists in side-to-side grinding chewing.
Lateral Pterygoid	Greater wing & lateral pterygoid plate ➔ Mandibular condyle & TMJ capsule	DEPRESSES MANDIBLE (OPENS MOUTH), protracts jaw, side-to-side grinding. (Only masticatory muscle that OPENS jaw).

Muscles of Respiration

• Diaphragm: Dome-shaped partition separating thorax and abdomen. Origin: xiphoid process, ribs 7–12, lumbar vertebrae. Insertion: Central tendon of diaphragm. Chief muscle of inspiration (~75% air volume); flattens during contraction. Innervation: Phrenic nerve (C3, C4, C5: "C3, 4, 5 keep the diaphragm alive").
• External Intercostals: Fibers run obliquely down and forward ("hands in pockets"). Elevate ribs during quiet/forced inspiration. Intercostal nerves.
• Internal Intercostals: Fibers run down and backward. Depress ribs during active, forced expiration.

Abdominal Wall Muscles (4 Layers)

1. External Abdominal Oblique: Outermost flat muscle; fibers run inferomedially; lower aponeurosis folds to form the inguinal ligament.
2. Internal Abdominal Oblique: Middle flat muscle; fibers run superomedially perpendicular to external oblique.
3. Transversus Abdominis: Deepest flat muscle; horizontal fibers; compresses abdominal contents.
4. Rectus Abdominis: Vertical muscle strap from pubic crest to xiphoid process and costal cartilages 5–7. Segmented by 3 tendinous intersections ("six-pack"); enclosed by rectus sheath; midline fusion forms the fibrous linea alba.

Rotator Cuff Muscles (SITS) — Crucial Exam Target!

Four scapulohumeral muscles whose tendons encircle and dynamically stabilize the shallow shoulder joint:

Muscle	Origin & Insertion	Action	Innervation
S - Supraspinatus	Supraspinous fossa ➔ Greater tubercle of humerus (superior facet)	Initiates arm abduction (first 0° to 15°); stabilizes humeral head	Suprascapular nerve (C5, C6)
I - Infraspinatus	Infraspinous fossa ➔ Greater tubercle (middle facet)	Lateral (external) rotation of arm	Suprascapular nerve (C5, C6)
T - Teres minor	Lateral border of scapula ➔ Greater tubercle (inferior facet)	Lateral (external) rotation and weak adduction	Axillary nerve (C5, C6)
S - Subscapularis	Subscapular fossa ➔ Lesser tubercle of humerus	Medial (internal) rotation of arm	Upper & Lower subscapular nerves

Shoulder & Arm Movers

• Deltoid: Multipennate shoulder cap; middle fibers abduct arm from 15° to 90°; anterior fibers flex and medially rotate; posterior fibers extend and laterally rotate (Axillary nerve).
• Biceps Brachii: Two heads (long from supraglenoid tubercle, short from coracoid process); inserts onto radial tuberosity; powerful supinator of forearm and flexor of elbow when supinated (Musculocutaneous nerve).
• Brachialis: Lies deep to biceps; origin on anterior humerus, insertion on coronoid process of ulna; chief powerful flexor of elbow in all forearm positions (Musculocutaneous nerve).
• Triceps Brachii: 3 heads (long, lateral, medial); inserts on olecranon process of ulna; prime extensor of elbow (Radial nerve).
• Serratus Anterior: Origin on ribs 1-8, inserts on medial scapular border; protracts scapula ("boxer's muscle"). Damage to Long Thoracic Nerve causes Winged Scapula.

Gluteal & Thigh Muscle Groups

• Gluteus Maximus: Largest and heaviest; powerful extensor and lateral rotator of thigh at hip (climbing stairs, rising from seated position, running). Inferior gluteal nerve.
• Gluteus Medius & Minimus: Abducts and medially rotates thigh; stabilizes pelvis preventing pelvic tilt during walking. Superior gluteal nerve. Weakness produces positive Trendelenburg sign. Major safe site for intramuscular (IM) ventrogluteal injections (avoids the sciatic nerve).
• Iliopsoas (Psoas major + Iliacus): Inserts onto lesser trochanter of femur; prime, most powerful flexor of thigh at hip.
• Sartorius: Longest strap-like muscle crossing anterior thigh; flexes, abducts, and laterally rotates hip and flexes knee ("cross-legged tailor sitting position").
• Quadriceps Femoris (4 heads): Rectus femoris (crosses hip and knee; flexes hip, extends knee), Vastus lateralis, Vastus medialis, Vastus intermedius. Insert via patellar tendon/ligament onto tibial tuberosity. Prime extensor of knee. Femoral nerve.
• Hamstrings (Posterior Thigh): Biceps femoris, Semitendinosus, Semimembranosus. Origin on ischial tuberosity; flex knee and extend hip. Sciatic nerve (Tibial division).

Leg Muscles (Crus)

• Tibialis Anterior: Anterior compartment; deep fibular nerve; dorsiflexes and inverts foot. Damage to common fibular nerve causes Foot Drop.
• Gastrocnemius & Soleus (Triceps Surae): Posterior compartment; insert via Achilles (calcaneal) tendon onto calcaneus; powerful plantarflexion of foot at ankle (walking, jumping, standing on tiptoes). Tibial nerve.

Memorize these official numbered pin locations directly from the course dissection review guide:

Pin #	Muscle / Structure	Region & Key Identifying Feature
1	Platysma	Superficial neck thin muscle sheet
2	Pectoralis major	Anterior chest large fan-shaped muscle
3	Deltoid	Rounded shoulder cap muscle
4	Rectus abdominis	Vertical anterior abdominal muscle (six-pack)
5	External abdominal oblique	Lateral abdominal wall with inferomedial fibers
6	Biceps brachii	Anterior arm muscle with two heads
7	Triceps brachii	Posterior arm three-headed extensor
8	Trapezius	Large diamond-shaped superficial back muscle
9	Brachioradialis	Lateral superficial forearm muscle
10	Latissimus dorsi	Broad lower back swimmer's muscle
11	Gluteus maximus	Large superficial buttock muscle
12	Biceps femoris	Lateral hamstring muscle behind thigh
13	Sartorius	Long diagonal ribbon crossing anterior thigh
14	Vastus medialis	Teardrop medial muscle of quadriceps
15	Rectus femoris	Central superficial muscle of quadriceps
16	Adductor magnus / group	Medial compartment of thigh
17	Tibialis anterior	Anterior shin muscle next to tibia crest
18	Gastrocnemius	Two-bellied superficial calf muscle
19	Fibularis (peroneus) longus	Lateral compartment of leg
21	Iliotibial (IT) tract	Thick lateral fascial band of thigh
23	Quadriceps femoris tendon	Common tendon attaching into patella