Neck

Ansa Cervicalis and Neck Muscles

Innervation of Neck Muscles

  • Ansa Cervicalis: A nerve loop composed of branches from the anterior rami of C1, C2, and C3.
  • Innervated Muscles:
    • Sternothyroid: innervated by the ansa cervicalis.
    • Omohyoid: innervated by the ansa cervicalis.
    • Sternohyoid: innervated by the anterior ramus of C1.
    • Geniohyoid: innervated by the anterior ramus of C1.
    • Thyrohyoid: innervated by the anterior ramus of C1.

Objectives of the Study

  1. Identify the superficial fascia and the four deep cervical fascia layers in a cross-section of the neck.
  2. List and identify the structures surrounded by the fascial layers of the neck.
  3. Discuss the clinical significance of potential spaces within the neck.
  4. Identify the boundaries of the anterior and posterior triangle of the neck.
  5. Identify the boundaries of subtriangles within the anterior and posterior triangles.
  6. Identify and list the structures contained within these subtriangles.
  7. Discuss the origin, insertion, and action of the muscles within the anterior and posterior triangles.
  8. Discuss the anatomical location of the ansa cervicalis and cervical plexus.
  9. Identify and list the main structures innervated by the ansa cervicalis and the cervical plexus.
  10. Identify and list the origin, insertion, and actions of prevertebral muscles.

Neck Boundaries

Superior Boundaries

  • Inferior border of the mandible
  • Tip of the mastoid process
  • External occipital protuberance

Inferior Boundaries

  • Jugular notch
  • Clavicle
  • Acromion
  • Spinous process of C7

Superficial Fascia

  • Extends from superficial fascia of the thorax to the mandible.
  • Blends with the muscles of the face.
  • Contents:
    • Platysma Muscle
    • Superficial branches of cervical plexus
    • Superficial veins: external and anterior jugular veins.
    • Lymph nodes.

Deep Cervical Fascia

Types of Deep Cervical Fascia

  1. Investing Fascia: Surrounds neck structures, including trapezius and sternocleidomastoid muscles.
  2. Prevertebral Fascia: Surrounded by the vertebral column and associated muscles.
  3. Pretracheal Fascia: Surrounds the trachea and thyroid gland.
  4. Carotid Sheath: Contains major vascular structures.

Fascial Compartments

  • Organized into four longitudinal compartments by deep cervical fascia:
    1. Visceral compartment (anterior)
    2. Vertebral compartment (posterior)
    3. Right vascular compartment (lateral)
    4. Left vascular compartment (lateral)

General Functions of Fascia

  • Support for the viscera of the neck.
  • Compartmentalization of neck structures.
  • Prevents the spread of infections between compartments.
  • Provides flexibility and slipperiness allowing structures to glide over one another during actions such as swallowing.

Investing Fascia

  • Completely surrounds the neck, enveloping:
    • Trapezius
    • Sternocleidomastoid muscles
    • Parotid gland
    • Submandibular gland
  • Forms the roof of the posterior triangle of the neck.

Attachments of Investing Fascia

  • Superior: External occipital protuberance and superior nuchal line.
  • Lateral: Mastoid process and zygomatic arch.
  • Inferior: Spine of scapula, acromion, clavicle, and manubrium of sternum.

Prevertebral Fascia

  • A tubular sheath extending from the base of the skull to T3.
  • Forms the floor of the posterior triangle of the neck.

Attachments of Prevertebral Fascia

  • Anterior: Attaches to the transverse processes (TVPs) and bodies of C1–C7 and T1–T3, and the anterior longitudinal ligament.
  • Posterior: Attaches to the nuchal ligament.
  • Inferior: Continues with the transversalis fascia of the thorax.

Specialized Compartments of Prevertebral Fascia

  1. Axillary Sheath: Surrounds the brachial plexus as it extends into the axilla.
  2. Alar Fascia: Anterior subdivision of prevertebral fascia which fuses with buccopharyngeal fascia around T1 to T4.

Pretracheal Fascia

  • Located at the anterior neck and extends from the hyoid bone, forming a capsule around the thyroid gland.
  • The posterior thickening is known as the ligament of Berry.
  • Fuses to the pericardium of the heart within the upper thoracic cavity.

Divisions of Pretracheal Fascia

  • Muscular Portion: Surrounds infrahyoid muscles.
  • Visceral Portion: Surrounds the thyroid gland, trachea, and esophagus.

Buccopharyngeal Fascia

  • A component of the posterior boundary of the visceral portion of the pretracheal fascia.
  • Separates the pharynx from the prevertebral layer, extending from the base of the skull to the mediastinum.

Carotid Sheath

  • Formed from contributions of various fascia in the neck, surrounding:
    • Common Carotid Artery
    • Internal Carotid Artery
    • Internal Jugular Vein
    • Vagus Nerve
    • Ansa Cervicalis

Fascial Spaces

  • Description: Spaces located between fascial layers of the neck, allowing for the spread of infections and tumors between the neck and mediastinum.
  • Types of Spaces:
    1. Pretracheal Space: Between investing layer and pretracheal fascia, extending from hyoid bone to the superior mediastinum.
    2. Retropharyngeal Space: Between buccopharyngeal fascia and alar fascia, extending from the base of the skull to the posterior mediastinum.
    3. Danger Space: Between alar fascia and prevertebral fascia, extending from the base of the skull to the diaphragm.

Neck Triangles

Anterior Triangle Boundaries

  • Anterior border: Sternocleidomastoid muscle.
  • Inferior border: Mandible.
  • Midline: Neck.

Posterior Triangle Boundaries

  • Anterior border: Sternocleidomastoid muscle.
  • Posterior border: Trapezius muscle.
  • Base: Middle third of the clavicle.

Subdivisions of Anterior Triangle

  1. Submandibular Triangle
  2. Submental Triangle
  3. Muscular Triangle
  4. Carotid Triangle

Submandibular Triangle Contents

Borders

  • Superior: Inferior border of mandible.
  • Inferio-medial/lateral: Anterior and posterior bellies of the digastric muscles.

Muscles, Viscera, Vessels, and Nerves

  • Muscles: Mylohyoid, Hyoglossus.
  • Viscera: Submandibular gland, submandibular lymph nodes.
  • Vessels: Facial artery and vein, submental artery and vein, lingual artery and vein.
  • Nerves: Nerve to mylohyoid, Hypoglossal Nerve (CN XII).

Submental Triangle Contents

Borders

  • Lateral: Anterior belly of digastric muscle.
  • Inferior: Hyoid Bone.

Contents

  • Viscera: Anterior jugular veins, submental lymph nodes.

Muscular Triangle Contents

Borders

  • Superior: Hyoid Bone.
  • Lateral: Superior belly of the omohyoid muscle and anterior border of sternocleidomastoid muscle.

Muscles, Vessels, and Viscera

  • Muscles: Thyrohyoid, sternothyroid, sternohyoid.
  • Vessels: Superior and inferior thyroid arteries and veins, middle thyroid vein, anterior jugular veins.
  • Viscera: Thyroid gland, parathyroid glands, larynx, trachea, esophagus.

Carotid Triangle Contents

Borders

  • Anteroinferior: Superior belly of the omohyoid muscle.
  • Superior: Stylohyoid muscle and posterior belly of digastric muscle.
  • Posterior: Anterior border of sternocleidomastoid muscle.

Vessels and Nerves

  • Vessels: Common carotid artery, internal and external carotid arteries, internal jugular vein.
  • Nerves: Vagus (CN X), Spinal Accessory (CN XI), Hypoglossal (CN XII), Ansa Cervicalis (superficial to IJV).

Ansa Cervicalis

  • A loop of nerve fibers from the anterior rami of C1 to C3, involved in innervating infrahyoid muscles within the anterior triangle.
  • Location: Deep to sternocleidomastoid, within carotid sheath.

Superior Root of Ansa Cervicalis

  • Formed by nerve fibers from the anterior rami of C1 and C2, descends between the internal jugular vein and carotid arteries.
  • Motor Innervation: To the superior belly of the omohyoid muscle, upper portions of sternohyoid and sternothyroid muscles.

Inferior Root of Ansa Cervicalis

  • Directly branches from cervical plexus (C2 – C3) to complete the loop.
  • Motor branches: To the inferior belly of the omohyoid, lower parts of sternohyoid and sternothyroid muscles.

Suprahyoid and Infrahyoid Muscles

Suprahyoid Muscles (Above Hyoid Bone)

  • Muscles:
    1. Stylohyoid
    2. Digastric (anterior and posterior bellies)
    3. Mylohyoid
    4. Geniohyoid

Infrahyoid Muscles (Below Hyoid Bone)

  • Muscles:
    1. Omohyoid
    2. Sternohyoid
    3. Thyrohyoid
    4. Sternothyroid

Stylohyoid Muscle

  • Origin: Styloid process.
  • Insertion: Body of hyoid bone.
  • Action: Pulls hyoid bone posterosuperior.
  • Innervation: Facial Nerve (CN VII).

Digastric Muscle

Anterior Belly

  • Origin: Digastric fossa from the inside of mandible.
  • Insertion: Intermediate tendon attached to the hyoid bone.
  • Innervation: CN V3.
  • Action: Depends on which bone is fixed in place (Mandible fixed: raises hyoid bone; Hyoid fixed: opens mouth by lowering mandible).

Posterior Belly

  • Origin: Mastoid process.
  • Insertion: Intermediate tendon attached to the hyoid bone.
  • Innervation: Facial Nerve (CN VII).

Mylohyoid Muscle

  • Origin: Mylohyoid line of mandible.
  • Insertion: Body of hyoid bone.
  • Action: Inside-out depends on fixation. Elevates hyoid bone and floor of mouth (mandible fixed) & depresses mandible (hyoid fixed).
  • Innervation: CN V3.

Geniohyoid Muscle

  • Origin: Inferior mental spine of mandible.
  • Insertion: Body of hyoid bone.
  • Innervation: Anterior Ramus of C1.

Action

  • Mandible fixed: Elevates and pulls hyoid forward.
  • Hyoid fixed: Pulls mandible downward and inward.

Sternohyoid Muscle

  • Origin: Posterior aspect of sternoclavicular joint and manubrium of sternum.
  • Insertion: Body of hyoid bone.
  • Action: Depresses hyoid bone.
  • Innervation: Anterior rami of C1–C3 (ansa cervicalis).

Omohyoid Muscle

Inferior Belly

  • Origin: Superior border of scapula.
  • Insertion: Intermediate tendon.
  • Innervation: Anterior rami of C1–C3 (ansa cervicalis).

Superior Belly

  • Origin: Intermediate tendon.
  • Insertion: Body of hyoid bone.
  • Innervation: Anterior rami of C1–C3 (ansa cervicalis).

Action

  • Depresses and fixes hyoid bone.

Thyrohyoid Muscle

  • Origin: Thyroid cartilage.
  • Insertion: Greater horn and body of hyoid bone.
  • Action: Depresses hyoid bone.
  • Innervation: Anterior Ramus of C1.

Sternothyroid Muscle

  • Origin: Posterior surface of manubrium of sternum.
  • Insertion: Thyroid cartilage.
  • Action: Depresses the larynx.
  • Innervation: Anterior Ramus of C1–C3 (ansa cervicalis).

Thyroid Gland

  • Located lateral to thyroid cartilage, corresponding to vertebral column levels C5–T1.
  • Composed of two lateral lobes connected by an isthmus; surrounded by pretracheal fascia with relation to the pharynx, trachea, esophagus, and larynx.

Blood Supply to Thyroid Gland

Major Arteries

  1. Superior Thyroid Artery: First branch of the external carotid artery; divides into anterior and posterior glandular branches.
  2. Inferior Thyroid Artery: A branch of the thyrocervical trunk from the subclavian artery; divides into inferior branch (anastomoses with posterior glandular branch) and ascending branch (supplies parathyroid glands).

Venous Drainage of Thyroid Gland

Major Veins

  1. Superior Thyroid Vein: Drains into Internal Jugular Vein (IJV).
  2. Middle Thyroid Vein: Drains into IJV.
  3. Inferior Thyroid Vein: Drains into Brachiocephalic Veins.

Thyroid Gland Nodules

  • Very common; estimated half of Americans will have nodules by age 60, with 95% being benign.
  • Types of Nodules:
    • Benign Nodules (95%): Hyperplastic nodules (85%), adenomas (15%), cysts (<1%).
    • Malignant Nodules (5%): Papillary (81%), follicular and Hürthle-cell (14-16%), medullary (3%), and anaplastic (2%).

Clinical Consideration

  • During a thyroidectomy, the recurrent laryngeal nerve (CN X) is the cranial nerve most at risk of damage.

Parathyroid Glands

  • Two pairs of small, ovoid, yellowish structures on the deep surface of the lateral lobes of the thyroid gland (superior and inferior parathyroid glands).
  • Regulate the body’s calcium and phosphorus levels via parathyroid hormones.

Posterior Triangle of the Neck

Borders

  • Anterior: Posterior aspect of sternocleidomastoid (SCM).
  • Posterior: Anterior aspect of trapezius.
  • Base: Middle third of clavicle.
  • Apex: Occipital bone.
  • Roof: Investing layer of cervical fascia.
  • Floor: Prevertebral fascia.

Occipital Triangle Contents

  • Apex: Occipital bone.
  • Contents:
    • Spinal Accessory Nerve (CN XI).
    • Branches of cervical plexus.
    • Upper trunk of brachial plexus.
    • Occipital Artery.
    • Posterior cervical lymph nodes.

Cervical Plexus

  • Formed by the anterior rami of cervical nerves C1–C4 and C5, divided into motor and sensory branches.

Cutaneous Branches

  • Originates from Erb's point at the posterior border of sternocleidomastoid muscle and includes:
    1. Lesser Occipital Nerve (C2): Sensation to skin of neck and scalp posterior to ear.
    2. Great Auricular Nerve (C2, C3): Sensation to skin of parotid gland, ear, mastoid area.
    3. Transverse Cervical Nerve (C2, C3): Sensation to lateral and anterior portion of neck.
    4. Supraclavicular Nerves (C3, C4): Sensation over clavicle and shoulder.

Muscular Branches

  1. Phrenic Nerve (C3–C5): Motor and sensory to diaphragm, sensory to pericardium and mediastinal pleura.
  2. Superior and Inferior Roots of Ansa Cervicalis: Motor innervation to infrahyoid muscles.
  3. Muscular branches: To prevertebral muscles, scalenes, and levator scapulae.

Omoclavicular or Subclavian Triangle

Borders

  • Anterior: Posterior aspect of sternocleidomastoid.
  • Inferior: Middle third of clavicle.
  • Superior: Inferior border of the inferior belly of omohyoid muscle.

Contents

  • Subclavian artery and vein.
  • External Jugular Vein and tributaries (suprascapular and transverse cervical veins).
  • Trunks of Brachial Plexus.

Brachial Plexus

  • Formed by anterior rami of C5–T1, it is divided into roots, trunks, division, cords, and terminal branches.
  • Formation: Between anterior and middle scalene muscles, alongside the axillary artery:
    • C5 – C6 forms Superior Trunk.
    • C7 forms Middle Trunk.
    • C8 – T1 forms Inferior Trunk.

External Jugular Vein Drainage

  • Eventually drains into the subclavian vein.

Tributaries

  • Posterior External Jugular Vein
  • Anterior Jugular Vein
  • Transverse Cervical Vein
  • Suprascapular Vein

Muscles of Posterior Triangle

  • Muscles:
    • Sternocleidomastoid Muscle
    • Inferior Belly of Omohyoid Muscle
    • Trapezius
    • Splenius Capitis
    • Levator Scapulae
    • Scalene Muscles (Anterior, Middle, Posterior)

Splenius Capitis

  • Origin: Ligamentum nuchae and spinous processes of C7–T3.
  • Insertion: Mastoid process and lateral portion of superior nuchal line.
  • Innervation: Posterior rami of C2 and C3.
  • Action:
    • Unilateral: Lateral rotation.
    • Bilateral: Extension of head.

Levator Scapulae

  • Origin: Transverse processes of C1–C4.
  • Insertion: Upper medial border of scapula.
  • Innervation: Anterior rami of C3, C4, and dorsal scapular nerve.
  • Action: Elevates scapula.

Scalene Muscles

  • Anterior Scalene: Elevates rib 1, innervation by anterior rami C4–C7.
  • Middle Scalene: Elevates rib 1, innervation by anterior rami C3–C7.
  • Posterior Scalene: Elevates rib 2, innervation by anterior rami C5–C7.

Prevertebral Muscles

  • Muscles: Longus Capitis, Longus Colli (with superior oblique, vertical, and inferior oblique portions), Rectus Capitis Anterior, Rectus Capitis Lateralis.
  • Functions: Stabilize vertebral column, antagonistic to intrinsic back muscles.

Longus Capitis

  • Origin: Anterior tubercles of transverse processes C3–C6.
  • Insertion: Basilar portion of occipital bone.
  • Innervation: Anterior rami of C1–C3.
  • Action: Flexes head/neck and contralaterally rotates head.

Longus Colli

  • Origin: Transverse processes of C3–C5 and vertebral bodies of C5–T3.
  • Insertion: Anterior tubercle of atlas.
  • Innervation: Anterior rami of C2–C6.
  • Action: Flexes and laterally flexes the head/neck (bilateral/unilateral).

Rectus Capitis Anterior

  • Origin: Anterior surface and TVP of atlas.
  • Insertion: Basilar portion of occipital bone.
  • Action: Flexes head at atlanto-occipital joint.
  • Innervation: Anterior rami of C1 and C2.

Rectus Capitis Lateralis

  • Origin: Superior surface of transverse process of atlas.
  • Insertion: Jugular process of occipital bone.
  • Action: Lateral flexion of head.
  • Innervation: Anterior rami of C1 and C2.

Suboccipital Region

  • Deepest region of the posterior neck containing pyramidal shape muscle compartments, located inferior to the external occipital protuberance and posterior to C1 and C2.

Suboccipital Triangle

Boundaries

  • Medial: Rectus capitis posterior major.
  • Superolateral: Obliquus capitis superior.
  • Inferiolateral: Obliquus capitis inferior.
  • Floor: Posterior atlanto-occipital membrane and posterior arch of C1.
  • Roof: Semispinalis capitis; Apex: bifid spinous process of C2.

Muscles of Suboccipital Triangle

  • All muscles rotate and extend the head; innervated by the suboccipital nerve (dorsal rami of C1).

Contents of Suboccipital Triangle

  • Suboccipital Nerve
  • 3rd Part of Vertebral Artery (Atlantic Segment)
  • Suboccipital Venous Plexus
  • Sympathetic Plexus surrounding blood vessels.

Vertebral Artery

  • Supplies blood to the posterior portions of the brain, ascending through the transverse foramina of C1–C6.
  • Consists of four segments:
    1. V1: Pre-Foraminal Segment
    2. V2: Foraminal Segment
    3. V3: Atlantic, Extradural, or Extraspinal Segment
    4. V4: Intradural or Intracranial Segment.

Dorsal Rami of Cervical Spinal Nerves

  • C1: Suboccipital Nerve.
  • C2: Greater Occipital Nerve.
  • C3: 3rd Occipital Nerve.
  • Each dorsal ramus provides specific motor and sensory function in the neck region.

Greater Occipital Nerve

  • Arises from the dorsal rami of C2, providing sensory innervation to the skin over the posterior scalp and upper neck, ascends alongside occipital artery to penetrate trapezius muscle.

Third Occipital Nerve

  • Arises from dorsal rami of C3, providing sensory innervation to laterally and deep structures of the neck.

Dorsal Rami of Cervical Spinal Nerves Continuation

  • Continuing from C4 – C5 and C6 – C8, describes muscular and cutaneous branches that supply intrinsic back muscles.