L 29 Anterior & Lateral Neck Muscles – Comprehensive Study Notes

General Boundaries

• Skeletal landmarks
– Suprasternal (jugular) notch
– Clavicle (entire length; articulates with manubrium and acromion)
– Acromion process (lateral shoulder; palpable)
– C7 spinous process (vertebra prominens; surface landmark for cervicothoracic junction)
– External occipital protuberance (EOP; attachment for nuchal ligament)
– Mastoid process (posterolateral skull; SCM insertion)
– Mandibular angle
– Inferior border of mandible
– Mental protuberance (chin)

• Additional palpable/functional structures
– Nuchal ligament (mid-line fibrous septum; maintains head posture)
– Laryngeal prominence (thyroid cartilage; “Adam’s apple”)
– Hyoid bone (C3 level; only bone without bony articulation, serves as muscular sling anchor)

• Fascial layers (create potential spaces & guide infection spread)
– Superficial cervical fascia: contains platysma m. & cutaneous vessels/nerves
– Deep cervical fascia (three discrete laminae + sheath)
• Investing layer: surrounds entire neck; encloses SCM & trapezius
• Pretracheal layer: visceral portion (trachea, thyroid, esophagus) & muscular portion (infrahyoid mm.)
• Prevertebral layer: surrounds vertebral column & deep neck musculature
• Carotid sheath: formed by contributions of all layers; encloses common/internal carotid aa., internal jugular v., CN X, deep cervical LNs, sympathetic fibers
– Retropharyngeal space: between pretracheal & prevertebral layers; subdivided by alar fascia; “danger space” for infection to mediastinum

Triangles of the Neck (conceptual compartments)

Posterior Triangle

• Boundaries
– Posterior: trapezius m.
– Anterior: sternocleidomastoid (SCM) m.
– Inferior: clavicle
– Apex: junction of SCM & trapezius
– Floor: prevertebral fascia over splenius capitis, levator scapulae, posterior/middle/anterior scalenes
– Roof: investing fascia + platysma + superficial cervical fascia

• Major contents (superficial ➜ deep)
– Omohyoid (inferior belly)
– Spinal accessory n. (CN XI)
– Roots & trunks of brachial plexus (+ dorsal scapular n., suprascapular n.)
– Cervical plexus sensory branches (nerve point: midpoint of posterior SCM border)
– Phrenic n. on anterior scalene
– External jugular v.
– Subclavian v. (anterior to anterior scalene) & subclavian a. (posterior to anterior scalene) with branches

Anterior Triangle

• Boundaries
– Posterior: SCM
– Anterior: median cervical line
– Inferior: suprasternal notch
– Superior: inferior mandible border
– Floor: pharynx, larynx, thyroid (pretracheal fascia covered)
– Roof: investing fascia + platysma + superficial fascia

• Contents
– Suprahyoid mm.: mylohyoid, geniohyoid, stylohyoid, digastric (both bellies)
– Infrahyoid/strap mm.: sternohyoid, omohyoid (superior belly), sternothyroid, thyrohyoid
– Arteries: common, internal, external carotid and branches
– Veins: internal jugular + tributaries
– Nerves: ansa cervicalis, cervical plexus branches, nerve to mylohyoid (CN V3), CN VII, IX, X, XII, sympathetic trunk
– Visceral: submandibular gland, thyroid, parathyroids, larynx, proximal trachea/esophagus

Musculature (AOII = Action, Origin, Insertion, Innervation)

Shared to Both Triangles

• Platysma – superficial facial expression muscle; tenses neck, depresses mandible corners; O: fascia/skin of upper thorax; I: mandible/skin of lower face; N: CN VII
• Sternocleidomastoid – key landmark; unilateral: ipsilateral flexion & contralateral rotation; bilateral: neck flexion; O: manubrium + medial clavicle; I: mastoid process & superior nuchal line; N: CN XI (motor) + C2-C3 (proprioception)
• Omohyoid (both bellies) – depresses/retracts hyoid, steadies during speech/swallow; O: superior scapular border; I: hyoid; N: ansa cervicalis (C1-C3)

Posterior Triangle–specific

• Trapezius – scapular elevator/depressor/retractor & superior rotation; O: midline occiput to T12; I: lateral clavicle, acromion, spine of scapula; N: CN XI + C3-C4 proprioception
• Splenius capitis – unilateral lateral flexion & ipsilateral rotation; bilateral extension of head/neck; O: nuchal lig., C7-T3 spines; I: mastoid + sup nuchal line; N: dorsal rami cervical nerves
• Levator scapulae – elevates scapula, downward rotates glenoid; O: C1-C4 transverse processes; I: medial scapular border above spine; N: dorsal scapular n. (C5)
• Scalenes (anterior, middle, posterior) – lateral neck flexion & rib elevation during forced inspiration; O: cervical transverse processes; I: 1st rib (anterior & middle) / 2nd rib (posterior); N: anterior rami cervical nerves

Anterior Triangle – Suprahyoids

• Mylohyoid – elevates floor of mouth during swallowing/speech; O: mylohyoid line; I: hyoid & median raphe; N: nerve to mylohyoid (CN V3)
• Geniohyoid – pulls hyoid anterior/superior; O: mental spine; I: hyoid; N: C1 fibers via CN XII
• Digastric – two bellies share intermediate tendon on hyoid
– Anterior belly: O digastric fossa; N: nerve to mylohyoid
– Posterior belly: O mastoid notch; N: CN VII
Action: elevate/steady hyoid, depress mandible
• Stylohyoid – elevates & retracts hyoid; O: styloid process; I: hyoid; N: CN VII

Anterior Triangle – Infrahyoids (Strap)

• Sternohyoid – depresses hyoid after swallowing; O: manubrium & clavicle; I: hyoid; N: ansa cervicalis
• Sternothyroid – depresses hyoid & larynx; O: manubrium; I: thyroid cartilage; N: ansa cervicalis
• Thyrohyoid – depresses hyoid, elevates larynx (high-pitch phonation); O: thyroid cartilage; I: hyoid; N: C1 via CN XII

Prevertebral Group (deep, ant. to vertebral bodies)

• Longus colli/capitis, rectus capitis anterior/lateralis (mentioned implicitly; stabilize and flex cervical spine)

Nerves of the Neck

• Spinal Accessory (CN XI) – motor SCM & trapezius; exits jugular foramen ➜ dives deep to SCM ➜ crosses posterior triangle superficially to levator scapulae en route to trapezius (vulnerable to iatrogenic injury during LN biopsy)

• Vagus (CN X) – in carotid sheath posterior to vessels; branches: superior laryngeal n., recurrent laryngeal n., pharyngeal plexus, cardiac branches; parasympathetic to thoracoabdominal viscera

• Hypoglossal (CN XII) – crosses external carotid superficially; supplies intrinsic & extrinsic tongue mm.; C1 hitchhikes to geniohyoid & thyrohyoid + superior root of ansa cervicalis

• Glossopharyngeal (CN IX) – taste & sensory posterior 1\,\,\frac13 tongue; motor stylopharyngeus; gives carotid sinus n.

• Nerve to Mylohyoid (terminal CN V3) – motor mylohyoid + digastric ant. belly; passes inferior to mylohyoid line

• Cervical Plexus (C1-C4) – cutaneous branches emerge at posterior SCM midpoint (lesser occipital, great auricular, transverse cervical, supraclavicular nn.); motor loop = ansa cervicalis (C1-C3) innervates strap mm.

• Brachial Plexus (C5-T1) – roots in interscalene triangle between anterior & middle scalene; trunks in posterior triangle; dorsal scapular & suprascapular branch here

• Phrenic n. (C3-C5) – sole motor to diaphragm ("C3,4,5 keeps the diaphragm alive"); descends on anterior scalene within prevertebral fascia

• Sympathetic trunk – paravertebral; usually superior, middle, inferior (stellate) cervical ganglia; post-ganglionic fibers form periarterial plexuses (e.g., around carotids)

Vasculature

Arterial Supply

• Arch branches: right common carotid & subclavian from brachiocephalic trunk; left counterparts directly from aortic arch

Subclavian Artery (1st part ➜ vertebral groove)

– Vertebral a. (brain/posterior circulation)
– Thyrocervical trunk
• Transverse cervical a. (superficial branch to trapezius; deep branch = dorsal scapular a.)
• Suprascapular a. (supraspinatus/infraspinatus supply)
• Inferior thyroid a. (feeds thyroid/parathyroids, larynx, esophagus)
– Costocervical trunk (deep cervical & superior intercostal aa.)
– Internal thoracic a. (ant. chest wall, pericardium)

Common Carotid Artery (within carotid sheath)

– Bifurcates at C3-C4 vertebral level; carotid body (chemoreceptor) & sinus (baroreceptor, CN IX)
– Internal carotid a.: no cervical branches, ascends into skull ➜ cerebral circulation
– External carotid a. (not in sheath) – eight branches (memory aid: “Some Anatomists Like Freaking Out Poor Medical Students”)
1. Superior thyroid a. (gives superior laryngeal a.)
2. Ascending pharyngeal a.
3. Lingual a.
4. Facial a.
5. Occipital a.
6. Posterior auricular a.
7. Maxillary a.
8. Superficial temporal a.

Venous Drainage & Lymphatics

• Internal Jugular v. – begins at jugular foramen (sigmoid sinus continuation); joins subclavian v. to form brachiocephalic v. at the venous angle; thoracic duct (left) & right lymphatic duct empty here
• External Jugular v. – formed by retromandibular + posterior auricular vv.; superficial to SCM; distention suggests ↑ central venous pressure
• Anterior Jugular v. – drains anterior neck; communicates via jugular venous arch

Visceral Structures

• Submandibular gland – major salivary gland wrapping mylohyoid posterior free edge (see L36)

• Thyroid gland – bilobed endocrine organ joined by isthmus (sometimes pyramidal lobe); anterolateral to larynx/trachea; supplied by superior & inferior thyroid aa.; venous plexus drains to internal jugular & brachiocephalic vv.; hormone secretion regulated by pituitary (TSH); goiter may compress trachea/recurrent laryngeal nn.

• Parathyroid glands – usually two superior & two inferior on posterior thyroid; regulate serum Ca^{2+} via PTH; variable location critical in thyroidectomy

• Larynx – cartilaginous airway (C3-C6); phonation; continuation with trachea (detailed in L37)

• Thymus – immune organ in inferior neck/superior mediastinum; prominent in children, involutes after puberty ➜ adipose

Clinical / Real-World Connections

• Central venous catheterization targets internal jugular v. inferior to SCM & lateral to carotid; knowledge of carotid sheath vital to avoid arterial puncture.
• Spinal accessory n. superficial course in posterior triangle makes it vulnerable during lymph node dissection ➜ trapezius paralysis, shoulder droop.
• Retropharyngeal space infections (e.g., abscess) can descend to posterior mediastinum (“danger space”) causing mediastinitis.
• Inter-scalene brachial plexus blocks anesthetize upper limb but may also affect phrenic n. (contraindicated in severe lung disease).
• Atherosclerotic plaques at carotid bifurcation → transient ischemic attacks; carotid endarterectomy requires identification of CN X, XII, ansa cervicalis.

Self-Study Questions (from transcript)

• Which muscles form the anterior & posterior borders of the posterior triangle and which nerve innervates each?
• List the cranial nerves present in the neck and their primary functions.
• Between which two muscles do the roots/trunks of the brachial plexus emerge?
• Trace blood flow from the heart to the right superior laryngeal artery (hint: brachiocephalic ➜ common carotid ➜ external carotid ➜ superior thyroid ➜ superior laryngeal).
• Where precisely are the parathyroid glands located relative to the thyroid lobes?

Atlas Plates & Additional Reading

• Netter 8th ed. plates 36, 48-58, 75, 87, 98-100, 103-109, 155-157, 163, 165
• Gilroy Atlas of Anatomy 4th ed. pp 24-27, 33, 62-63, 512-540, 582-589, 642, 645, 652-653, 664-665, 669

Use these notes alongside clinical correlations and cadaveric dissection to consolidate spatial relationships among fascial planes, neurovascular bundles, and visceral organs within the neck.