NEET MDS Lessons
Anatomy
Blood Supply to the Head and Neck
- Most arteries in the anterior cervical triangle arise from the common carotid artery or one of the branches of the external carotid artery.
- Most veins in the anterior cervical triangle are tributaries of the large internal jugular vein.
The Common Carotid Arteries
- The right common carotid artery begins at the bifurcation of the brachiocephalic trunk, posterior to the right sternoclavicular joint.
- The left common carotid artery begins arises from the arch of the aorta and ascends into the neck, posterior to the left sternoclavicular joint.
- Each common carotid artery ascends into the neck within the carotid sheath to the level of the superior border of the thyroid cartilage.
- Here it terminates by dividing into the internal and external carotid arteries.
The Internal Carotid Artery
- This is the direct continuation of the common carotid artery and it has no branches in the neck.
- It supplies structures inside the skull.
- The internal carotid arteries are two of the four main arteries that supply blood to the brain.
- Each artery arises from the common carotid at the level of the superior border of the thyroid cartilage.
- It then passes superiorly, almost in a vertical plane, to enter the carotid canal in the petrous part of the temporal bone.
- A plexus of sympathetic fibres accompany it.
- During its course through the neck, the internal carotid artery lies on the longus capitis muscle and the sympathetic trunk.
- The vagus nerve (CN X) lies posterolateral to it.
- The internal carotid artery enters the middle cranial fossa beside the dorsum sellae of the sphenoid bone.
- Within the cranial cavity, the internal carotid artery and its branches supply the hypophysis cerebri (pituitary gland), the orbit, and most of the supratentorial part of the brain.
The External Carotid Arteries
- This vessel begins at the bifurcation of the common carotid, at the level of the superior border of the thyroid cartilage.
- It supplies structures external to the skull.
- The external carotid artery runs posterosuperiorly to the region between the neck of the mandible and the lobule of the auricle.
- It terminates by dividing into two branches, the maxillary and superficial temporal arteries.
- The stems of most of the six branches of the external carotid artery are in the carotid triangle.
The Superior Thyroid Artery
- This is the most inferior of the 3 anterior branches of the external carotid.
- It arises close to the origin of the vessel, just inferior to the greater horn of the hyoid.
- The superior thyroid artery runs anteroinferiorly, deep to the infrahyoid muscles and gives off the superior laryngeal artery. This artery pierces the thyrohyoid membrane in company with the internal laryngeal nerve and supplies the larynx.
The Lingual Artery
- This arises from the external carotid artery as it lies on the middle constrictor muscle of the pharynx.
- It arches superoanteriorly, about 5 mm superior to the tip of the greater horn of the hyoid bone, and then passes deep to the hypoglossal nerve, the stylohyoid muscle, and the posterior belly of digastric muscle.
- It disappears deep to the hyoglossus muscle.
- At the anterior border of this muscle, it turns superiorly and ends by becoming the deep lingual artery.
The Facial Artery
- This arises from the carotid artery either, in common with the lingual artery, or immediately superior to it.
- In the neck the facial artery gives off its important tonsillar branch and branches to the palate and submandibular gland.
- The facial artery then passes superiorly under the cover of the digastric and stylohyoid muscles and the angle of the mandible.
- It loops anteriorly and enters a deep groove in the submandibular gland.
- The facial artery hooks around the inferior border of the mandible and enters the face. Here the pulsation of this artery can be felt (anterior to the masseter muscle).
The Ascending Pharyngeal Artery
- This is the 1st or 2nd branch of the external carotid artery.
- This small vessel ascends on the pharynx, deep to the internal carotid artery.
- It sends branches to the pharynx, prevertebral muscles, middle ear and meninges.
The Occipital Artery
- This arises from the posterior surface of the external carotid near the level of the facial artery.
- It passes posteriorly along the inferior border of the posterior belly of digastric.
- It ends in the posterior part of the scalp.
- During its course, it is superficial to the internal carotid artery and three cranial nerves (CN IX, CN X and CN XI).
The Posterior Auricular Artery
- This is a small posterior branch of the external carotid artery.
- It arises from it at the superior border of the posterior belly of the digastric muscle.
- It ascends posteriorly to the external acoustic meatus and supplies adjacent muscles, the parotid gland, the facial nerve, structures in the temporal bone, the auricle, and the scalp.
The Internal Jugular Vein
- This is usually the largest vein in the neck.
- The internal jugular vein drains blood from the brain and superficial parts of the face and neck.
- Its course corresponds to a line drawn from a point immediately inferior to the external acoustic meatus to the medial end of the clavicle.
- This large vein commences at the jugular foramen in the posterior cranial fossa, as the direct continuation of the sigmoid sinus.
- The dilation at its origin is called the superior bulb of the internal jugular vein.
- From here it runs inferiorly through the neck in the carotid sheath.
- The internal jugular vein leaves the anterior triangle of the neck by passing deep to the SCM muscle.
- Posterior to the sternal end of the clavicle, it unites with the subclavian vein to form the brachiocephalic vein.
- Near its termination is the inferior bulb of the jugular vein contains a bicuspid valve similar to that of the subclavian vein.
- The deep cervical lymph nodes lie along the course of the internal jugular vein, mostly lateral and posterior.
Tributaries of the Internal Jugular Vein
- This large vein is joined at its origin by the: inferior petrosal sinus, the facial, lingual, pharyngeal, superior and middle thyroid veins, and often the occipital vein.
- Provides a rigid support system
- Protects delicate structures (e. g., the protection provided by the bones of the vertebral column to the spinal cord)
- Bones supply calcium to the blood; are involved In the formation of blood cells (hemopoiesis)
- Bones serve as the basis of attachment of muscles; form levers in the joint areas, aIlowing movement
Innervation of the Pharynx
- The motor and most of the sensory supply of the pharynx is derived from the pharyngeal plexus of nerves on the surface of the pharynx.
- The plexus is formed by pharyngeal branches of the vagus (CN X) and glossopharyngeal (CN IX) nerves, and by sympathetic branches for the superior cervical ganglion.
- The motor fibres in the pharyngeal plexus are derived from the cranial root of accessory nerve (CN XI), and are carried by the vagus nerve to all muscles of the pharynx and soft palate.
- The exceptions are stylopharyngeus (supplied by CN IX) and the tensor veli palatini (supplied by CN V3).
- U-shaped bone
- Body
- Greater horn
- Lesser horn
- Suspended by ligaments from the styloid process
3 basic functions
o protection of respiratory tract during swallowing food/air pathways cross.
epiglottis provides protection
o control intra-thoracic pressure (in coughing) - close off airway to build pressure then rapidly open to release stuff
o production of sound (in speaking, singing, laughing)
Important structures
o hyoid bone
o thyroid cartilage
o arytenoids cartilage: vocal and muscle process
sits on slope on posterior side of cricoid - spin and slide
o cricoid cartilage: signet ring
o thyroepiglottic ligament
Membranes and ligaments
o membrane: general; ligament: thickening of membrane
o folds: free edges of membranes or ligaments
o names: tell you where located
Important membranes:
quandrangular/vestibular membrane—from epiglottis to arytenoids
• inferior edge: false vocal fold
thyrohyoid membrane
conus elasticus = cricothyroid = cricovocal
• superior/medial edge = vocal fold
• vocal ligaments: true folds, top of cricothyroid membrane
The Muscles of Facial Expression
- These lie in the subcutaneous tissue and are attached to the skin of the face.
- They enable us to move our skin and change our facial expression. They produce their effects by pulling on the skin but do not move the facial skeleton.
- These muscles surround the facial orifices and act as sphincters and dilators.
- All facial muscles receive their innervation from the branches of the facial nerve (CN VII)-temporal, zygomatic, buccal, marginal mandibular, cervical.
Tongue
Appears at 4th week.
Musculature derived from mesoderm of occipital somites. Precursor muscles cells migrate to region of tongue and are innervated by general sensory efferent fibers of CN XII.
Mucosa derived from anterior endoderm lining arches 1-4; accordingly, innervation depends on arch derivation:
Mucosa of anterior 2/3 of tongue comes from the first arch -> CN V
Mucosa of posterior 1/3 of tongue comes from third and forth arch -> CN IX, X
Special taste of anterior 2/3 of tongue comes from CN VII.
Special taste of posterior 1/3 of tongue comes from CN X.
Tongue freed from floor of mouth by extensive degeneration of underlying tissue. Midline frenulum continues to anchor tongue to floor of mouth.
Thyroid Gland
Develops as in growth of mucosal epithelium located in the midline of the tongue (at foramen cecum). It descends along front of pharyngeal gut, but remains connected to tongue by thyrooglossal duct, which is obliterated later in development. Thyroid gland descends to a point just caudal to laryngeal cartilages.
Facial structures (general)
a) medial nasal prominence forms midline of nose, philtrum and primary palate
b) lateral nasal prominence forms alae of nose
c) maxillary prominence forms cheek region and lateral lip
d) clefts can form at inter-prominence fusion lines
Nose
At the time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour. Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP. Mesenchyme swells around nasal placode producing a medial and lateral nasal prominence (nasomedial and nasolateral processes). These nasal prominences form the nose.
Mouth
Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch. The first pharyngeal arch forms the dorsal maxillary prominence and ventral mandibular prominence. The maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion. Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (first 4 teeth).
Nasolacrimal structures
Maxillary and lateral nasal prominences are separated by deep furrow, the nasolacrimal groove. Ectoderm in floor of groove forms epithelial cord, which detaches from overlying ectoderm. The epithelial cord canalizes to form the nasolacrimal duct. The upper end of the duct widens to form the lacrimal sac. After detachment of the cord, the maxillary and lateral nasal prominences merge with each other, resulting in the formation of a nasolacrimal duct that runs from the medial corner of the eye to the inferior meatus of the nasal cavity.
The maxillary prominences enlarge to form the cheeks and maxillae.
The lateral nasal prominences form the alae of the nose.
Secondary (hard) palate
Main part of definitive palate formed by two palatine shelves derived from intraoral bilateral extensions of the maxillary prominences. These appear at the 6th week. They are directed obliquely downward on each side of the tongue; they move down when mandible gets bigger.
At the seventh week, they ascend to attain a horizontal position, then fuse to form the secondary palate. At the time the palatine shelves fuse, the nasal septum (an outgrowth of median tissue of the frontonasal prominence) grows down and joins the cephalic aspect of the newly formed palate
Anteriorly, shelves fuse with triangular primary palate. The incisive foramen marks the midline between the primary and secondary palate.
External Ear
The auricle is derived from 6 auricular hillocks (mesenchymal proliferations) along the dorsal aspect of arches 1 (top of ear) and 2 (bottom of ear). These fuse to form the definitive auricle. At the mandible grows, the ear is pushed upward and backward from its initial horizontal position on the neck.
The EAM is derived from the 1st pharyngeal arch.
The eardrum (tympanic membrane) is composed of 3 layers of cells: 1) ectodermal epithelial lining of bottom of EAM; 2) endodermal epithelium lining of tympanic cavity; 3) intermediate layer of connective tissue.
The eardrum is composed of multiple cell layers because it represents the first pharyngeal membrane, and thus lies at the junction of the first pharyngeal pouch and cleft.
Middle Ear
The middle ear consists of an auditory tube (from the 1st pharyngeal pouch, along with tympanic cavity) and the ossicles (from pharyngeal arches 1 and 2 cartilage).
The first arch cartilage forms the malleus and incus. The tensor tympani (muscle of the malleus) is derived from the fourth somitomere (associated with the first arch) and is therefore innervated by CN V.
The second arch cartilage forms the stapes. The stapedius (muscles of the stapes) is derived from the sixth somitomere (associated with the second arch) and is therefore innervated by CN VII.
The ossicles are initially embedded in mesenchyme, but in the 8th month, the mesenchyme degenerates and an endodermal epithelial lining of the tympanic cavity envelops the ossicles and connects them to the wall of the cavity in a mesentery-like fashion.
Inner Ear
The inner ear is derived thickening of surface ectoderm on both sides of the hindbrain (otic placodes). The placodes invaginate to form otic vesicles (otocytes). The vesicles then divide into ventral and dorsal components.
The ventral component forms the saccule and cochlear duct.
The dorsal component forms the utricle and semicircular canals and endolymphatic duct.
Cochlear Duct
Derived from an outgrowth of the saccule during the 6th week. The outgrowth penetrates the surrounding mesenchyme in a spiral fashion. The surrounding mesenchyme forms the cartilage and undergoes vacuolization.
The scala vestibule and scale tympani form and surround the cochlear duct. They are filled with periplymp to receive mechanical vibrations of ossicles. The mechanical stimuli activates sensory (ciliary) cells in the cochlear duct.
Semicircular canals
The utricle is initially three flattened outpocketings, which lose the central core. From this three semicircular canals are forms, each at 90 degree angles from one another. Sensory cells arise in the ampulla at one end of each canal, in the utricle and saccule.