Muscle Origin, Insertion & Key Superficial Muscles of Face, Neck, and Upper Limb

Muscle anatomy vocabulary
• Origin = attachment point closer to the body’s midline / the more stable end.
• Insertion = attachment farther from the midline / the more mobile end.
• Rule of action: muscles always contract by pulling the insertion toward the origin – never pushing.
Functional implication
• During contraction the bone at the origin remains relatively stationary, while the bone at the insertion moves through the nearby joint.
• Naming tip: many muscles are literally named after their origins, insertions, or functions (e.g.
"biceps" = 2 heads/origins, "triceps" = 3 heads/origins).

Deltoid
• Origin: along clavicle & scapular spine ("closer to the body").
• Insertion: deltoid tuberosity on the humerus.
• Action: contracts upward → arm abduction (lifting away from midline).
Rhomboids (major & minor)
• Origin: spinous processes of thoracic vertebrae (central axis).
• Insertion: medial (vertebral) border of the scapula.
• Action: pull shoulder blades medially → scapular retraction.
Biceps brachii
• “Bi-” = two proximal origins on scapula.
• Insertion: radial tuberosity (bump on proximal radius).
• Action: flexes elbow by drawing radius toward scapula.
Triceps brachii
• “Tri-” = three heads/origins on scapula & posterior humerus.
• Insertion: olecranon of ulna.
• Action: extends the elbow.
Distribution note
• Fewer, larger muscles in the arm proper (move elbow) vs. many small muscles in the forearm (fine finger movements).

Laboratory focus = superficial muscles only (the ones visible on teaching models and cadavers).
Deep muscles of forearm/hand will not be quiz material.

Components
• Frontalis (frontal belly) – over the forehead.
• Occipitalis (occipital belly) – at posterior skull.
• Connected by the epicranial aponeurosis (a broad, tendon-like sheet).
Origins & insertions
• Occipitalis origin: occipital bone; insertion: epicranial aponeurosis.
• Frontalis origin: aponeurosis; insertion: skin & muscles around eyebrows.
Action: raises eyebrows, wrinkles forehead.
Clinical aside: some people can still wiggle the back of the scalp because the two bellies are linked.

Definition: muscle fibers arranged in a circle around an opening ("orbit").
Orbicularis oculi
• Encircles the eye.
• Action: forcibly or gently closes eyelids.
Orbicularis oris
• Encircles the mouth.
• Action: protrudes & purses lips ("kissing" / blowing).
Spelling caution: include the silent R – “Orbicularis,” not “Obicularis.”

Zygomaticus major & minor
• Origin: zygomatic bone.
• Insertion: skin & muscle at the corners of the mouth.
• Action: draw mouth corners superiorly & laterally → smiling.
• Major (lateral, larger) vs. minor (medial, smaller).
Buccinator
• Forms the muscular wall of the cheek (buccal region).
• Action: compresses cheek (blowing, whistling); when relaxed can store air (trumpet players – Louis Armstrong, Dizzy Gillespie).
• Also active when sucking in cheeks or biting inner cheek.
Risorius
• Origin (variable) on fascia over parotid gland; insertion at mouth corner.
• Action: draws mouth corners straight backward → “cynical” or horizontal smile.
Levator labii superioris
• Origin: infra-orbital margin of maxilla.
• Insertion: upper lip.
• Action: elevates & everts upper lip (snarl).

Temporalis
• Origin: temporal fossa on temporal bone.
• Insertion: coronoid process of mandible.
• Action: elevates mandible → closes jaw.
Masseter
• Origin: zygomatic arch (formed by zygomatic process of temporal bone + temporal process of zygomatic bone).
• Insertion: lateral surface & angle of mandible.
• Action: powerful jaw closure (primary chewing force).
• Palpation: clench teeth and feel the bulky muscle just anterior to ear.

Platysma
• Origin: fascia of upper chest & deltoid region.
• Insertion: lower border of mandible, skin of lower face.
• Action: tenses skin of neck (e.g., when men shave); assists in depressing mandible (opening mouth).
• Not present on many lab models → rarely tested.

Sternocleidomastoid (SCM)
• Dual origin: manubrium of sternum + medial clavicle.
• Insertion: mastoid process (temporal bone) & superior nuchal line.
• Action:
– Unilateral contraction → rotates head to opposite side (acts like a pulley behind the atlanto-occipital axis).
– Bilateral contraction → neck flexion.
• Innervation: Cranial Nerve $XI$ – Accessory nerve.
• Visible cords on either side of neck when turning head.

Correct nomenclature prevents medical errors; mis-spelling (e.g., leaving out the “R” in Orbicularis) can cause confusion.
Muscle names often encode location, attachments, fiber direction, or action – learn etymology to make recall easier (e.g., "levator" → lifts, "labii" → lip).

Shaving: contract platysma to pull skin taut, preventing cuts.
Instrument playing: trumpet players rely on buccinator relaxation & control.
Clinical palpation: feel masseter while clenching, SCM when rotating head, temporalis at temples when chewing.
Course logistics: tomorrow’s lab will revisit these muscles on physical models; bring lab worksheets.