Regional anatomy 7 -- the neck 1

The neck –

• A tube providing continuity from the head to the trunk

• It extends – 

  • Anteriorly from lower border of the mandible to the upper surface of the manubrium of sternum

  • Posteriorly from the superior nuchal line on the occipital one of the skull to the intervertebral disk between C7 & T1 vertebrae –

Triangles of the neck —

Divided into anterior & posterior triangles –

• The boundaries of the anterior triangle are the anterior border of the sternocleidomastoid muscle, the inferior border of the mandible, and the midline of the neck

• The boundaries of the posterior triangle are the posterior border of the sternocleidomastoid muscle, the anterior border of the trapezius muscle, and the middle one-third of the clavicle 

Four compartments in the tube provide longitudinal organization –

• Visceral compartment –

  • Anterior and contain parts of the digestive & respiratory system, and several endocrine glands

• Vertebral compartment –

  • Posterior and contains the cervical vertebrae, spinal cord, cervical nerves, and muscles associated with the vertebral 

• 2 Vascular compartments —

  • One on each side – lateral & contain the major blood vessels (Carotid, jugular vein) & the vagus nerve (X) 

Neck fascia –

• The soft connective tissue of the neck gets denser & creates a generally vascular plane – used as surgical planes 

• 2 types —

  • Superficial fascia

  • Deep cervical fascia

Superficial fascia

• Contains a thin sheet of muscle (the platysma) –

  • Begins in the superficial fascia of the thorax & runs upwards to attach to the mandible & fuse with the mimic muscles of the face –

  • Innervated by the cervical branch of the facial nerve (VII), and is only found in this location

The deep cervical fascia –

• Organized into several distinct layers, including – 

• An investing layer

  • Surrounding all structures in the neck

• The prevertebral layer

  • Surrounds the vertebral column and the deep muscles associated with the back

• The pretracheal layer

  • Encloses the viscera of the neck

• The carotid sheaths –

  • Receive a contribution from the other three fascial layers and surround the 2 major neurovascular bundles on either side of the neck

The investing layer –

• Completely surrounds the neck

• Attaches posteriorly to the ligamentum nuchae and the spinous process of the CVII vertebrae, 

• Splits as it posses forward to enclose the trapezius muscle & reunites into a single layer as it forms the roof of the posterior triangle

• Splits again to surround the sternocleidomastoid muscle, and reunites again to join its twin from the other side

• Anteriorly, it surrounds the infrahyoid muscles

Prevertebral layer –

• A cylindrical layer of fascia surrounding the vertebral column and muscles associated to it

• These include – 

  • The prevertebral muscles, the anterior, middle, & posterior scalene muscles, and the deep muscles of the back 

Pretracheal layer –

• Consists of a collection of fascias surrounding the trachea, esophagus, & thyroid gland

• Anteriorly – consists of a pretracheal fascia crossing the neck just posteriorly to the infrahyoid muscles

  • Covers the trachea & thyroid gland

• Begins superiorly at the hyoid bone & ends inferiorly in the upper thoracic cavity

• Laterally, this fascia continues & covers the thyroid & the esophagus

• Posteriorly, is referred to as the buccopharyngeal fascia –

  • Separates the pharynx & esophagus from the prevertebral layer

  • Begins superiorly at the base of the skull & ends inferiorly in the thoracic cavity 

Fascial spaces

• Betweeen the investing layer of cervical fascia covering the posterior surface of the infrahyoid muscles and the pretracheal fascia, which passes between the neck

  • Pretracheal space — covering the anterior surface of the trachea and the thyroid gland

• Retropharyngeal space between the buccopharyngeal fascia and the prevertebral fascia, which extends from the base of the skull tothe upper part of the posterior mediastinum

  • Buccopharyngeal fascia — posterior surface of pharynx & esophagus

  • Prevertebral fascia — anterir surface of the transverse processes and bodies of the cervical vertebrae

• Prevertebral space — In the vertebral layer covering the anterior surface of the transverse processes

Danger space —

• Bounded at the top by the skull base, at front by alar fascia and behind by the prevertebral fascia

• Ends at the level of the diaphragm

• The retropharyngeal space —

  • Anterior to the danger space, between the alar fascia & buccopharyngeal fascia

• There exists a midline raphe in this space so some infections of this space appear unilateral

Carotid sheath –

• A column of fascia surrounding the common carotid artery the internal carotid artery, the internal jugular vein, and the vagus nerve as these structures pass through the neck

• It receives contributions from the investing, prevertebral, & pretracheal layers, though the extend of each component’s contribution varies

Anterior triangle of the neck subdivision –

• Further subdivided into several smaller triangles as follows –

  • The submandibular triangle

    • Outlined by the inferior border of the mandible superiorly * anterior and posterior bellies of the digastric muscle inferiorly

  • The submental triangle –

    • Outlined by the hyoid bone inferiorly, the anterior belly of the digastric muscle laterally, and the midline 

  • The muscular triangle

    • Outlined by the hyoid bone superiorly, the superior belly of the omohyoid muscle, & the anterior border of the sternocleidomastoid muscle laterally, and the midline

  • The carotid triangle –

    • Outlined by the superior belly of the omohyoid muscle anteroinferiorly, the stylohyoid muscle and posterior belly of the digastric superiorly, and the anterior border of the sternocleidomastoid muscle posteriorly 

Suprahyoid vs infrahyoid muscles –

• Muscles in the anterior triangle of the neck can be groups based on their location relative to the hyoid bone 

• Suprahyoid muscles –

  • Stylohyoid, digastric, mylohyoid, and geniohyoid

  • Give mobility to the hyoid bone & form the floor of the oral cavity 

• Infrahyoid muscles

  • Omyhyoid, sternohyoid, thyrohyoid, and sternothyroid 

Stylohyoid muscle

• Suprahyoid muscle

• Arises from the base of the styloid process and passes anteroinferiorly to attach to the lateral area of the body of the hyoid

• During swallowing it pulls the hyoid bone posterosuperiorly and it is innervated by the facial nerve (VIII)

Geniohyoid muscle

• Suprahyoid muscle

• Parallel to the anterior belly of the digastric muscle

• Superior to the floor of the oral cavity — not generally considered a muscle of the anterior trignalge of the neck, but regarded as a suprahyoid muscle

• Narrow & superior to the medial part of each mylohyoid muscle

• The muscles from each side are next to each other in the midline

• The geniohyoid arises from the inferior mental spine of the mandible and passes backward and downward to insert on the body of the hyoid bone

• 2 functions depending on which bone is fixed —

  • Fixation of the mandible elevates and pulls the hyoid bone forward

  • Fixation of the hyoid bone pulls the mandible downward and inward

• Innervated by CI

Digastric muscle —

• Suprahyoid muscle

• 2 muscular bellies connected by an intermediate tendon & innervated by 2 different nerves — thus digastric

  • Anterior belly —

    • Mylohyoid nerve (branch of mandibular (trigeminus) inserting in the digastric fossa

    • Inferior apex of the roots of the anterior belly are superior to the mylohyoid line

  • Posterior belly —

    • Facial nerve — starting from tip of mastoid on the mastoid notch, and continues at the anterior belly of the muscle at the level of the intermediate tendon

    • Posterior roots finish posterior to the mylohyoid lines

• Intermediate tendon —

  • Attaches to body of hyoid bone & point of insertion of both bellies

  • Due to this arrangement, multiple actions depending on whivch bone is fixed —

    • When mandible is fixed, functions to raise hyoid bone

    • When hyoid bone is fixed, opens mouth by lowering mandible

• Roots —

  • Inferior apex of anterior belly roots — superior to mylohyoid, while posterior finish posteriorly —

  • Important due to infectons —

    • Infections from the posterior belly directly go into the neck — infection of the morals can thus directly spread to neck

Mylohyoid muscle

• Suprahyoid muscle

• Origin —

  • Mylohyoid line of mandible

• Insertion —

  • Mylohyoid raphe, body of hyoid bone

• Action —

  • Forms floor of oral cavity

  • Elevates hyoid bone & floor of mouth

  • Depresses mandible

• Innervated —

  • Nerve to mylohyoid (of inferior alveolar nerve (CN V3))

Sternohyoid

• Infrahyoid muscle

• Origin —

  • Posterior aspect of sternoclavicular joint & adjacent manubrium of sternum

• Insertion —

  • Body of hyoid bone medial to attachment of omohyoid muscle

• Innervation —

  • Anterior rami of C1 to C3 through the ansa cervicalis

• Function —

  • Depresses hyoid bone after swallowing

Omohyoid muscle

• Infrahyoid muscle — also called the inferior digastrc muscle

• Lateral to the sternohyoid muscle — consists of 2 bellies with an intermediate tendon in both the posteiror & anterior triangles of the neck —

  • Inferior belly —

    • Begins on superior border of scapula (medial to the suprascapular notch) and passes forward and upward across the posterior triangle ending at the tendon

  • Superior belly

    • Begins at intermediate tendon & ascends to attach to the body of the hyoid bone just lateral to the sternohyoid

  • Intermediate tendon —

    • Attached to the clavicle, near its medial end, by a fascial sling

• Function —

  • Depresses & fixes the hyoid bone

• Innervation —

  • The anterior rami of C1 to C3 through the ansa cervicalis

Ansa cervicalis –

• Gives innervation to pre-laryngeal muscle

• Descending branch of hypoglossal nerve (superior root of ansa cervicalis), and posteriorly connects to CI, CII, & CIII, (cervical roots) (sometimes even CIV) – thus creating an innervation to all superficial prelaryngeal muscles

• Loop of nerves made from anterior branches of C1 connected to posterior branches of C3 (& everything in between) that supply infrahyoid muscles

Thyrohyoid muscle —

• Deep to the superior parts of the omohyoid and sternohyoid

• Origin — oblique line on the outer lamina of the thyroid cartilage

  • Passes upward to insert into greater horn & adjacent aspect of the body of the hyoid bone

• Functions — depend on which bone is fixed

  • Generally depresses hyoid

  • When hyoid is fixed it raises the larynx (ex. when high notes are sung)

• Innervation

  • Innervated by fibers from the anterior ramus of C1 that travel with the hypoglossal nerve (XII)

Sternothyroid muscle

• Beneath the sternohyoid & in continuity with the thyrohyoid

• Arises from posterior surface of the manubrium of the sternum & passes upward to attach to the oblique line on the lamina of the thyroid cartilage

• Draws the larynx (thyroid cartilage) downward & is innervated by the anterior rami of C1 to C3 through the ansa cervicalis

Muscles overview

Muscles of the posterior triangle — image

Sternocleidomastoid muscle —

• Origin —

  • Sternal head

    • Upper part of anterior surface of manubrium of sternum

  • Clavicular head

    • Superior surface of medial one-third of clavicle

• Insertion —

  • Sternal head —

    • Lateral one-half of superior nuchal line

  • Clavicular head —

    • Lateral surface of mastoid process

• Innervation —

  • Accessory nerve (XI) and branches from anterior rami

Trapezius muscle

• Origin —

  • Superir nuchal line, external occipital protuberance, ligamentum nuchae, spinous processes of vertebrae CVII to TXII

• Insertion —

  • Lateral one-third of clavicle, aronium, spine of scapula

• Innervation —

  • Motor — accessory nerve (XI) proprioception — C3 and C4

• Function —

  • Assists in rotating the scapula during abduction of humerus above horizontal —

    • Upper fibers — elevate

    • Middle fibers — adduct

    • Lower fibers — depress scapula

Splenius capitis muscle

• Origin —

  • Lower half of ligamentum nuchae — spinous processes C3-T3

• Insertion —

  • Mastoid process, skull below lateral one-third of superior nuchal line

• Innervation —

  • Posterior rami of middle cervical nerves

• Function —

  • Together, draw head backward

  • Individually, draw & rotate head to one side

    • (turn face to same side)