HBF-III LEC 26 Gross Anatomy Thyroid Root Neck Notes 2025 Xu PDF

Thyroid & Root of the Neck Page 1 of 14 Dr. Shunbin Xu Session Learning Objectives: By the end of this session, students should be able to accurately: 1. Use anatomical terminology to describe the thyroid and parathyroid glands A....

Thyroid & Root of the Neck Page 1 of 14 Dr. Shunbin Xu Session Learning Objectives: By the end of this session, students should be able to accurately: 1. Use anatomical terminology to describe the thyroid and parathyroid glands A. Describe the composition and anatomical locations of thyroid and parathyroid glands in relation to other neurovascular and muscular structures in the neck region B. Describe the vascular supply of the thyroid and parathyroid glands C. Describe the innervation of the thyroid and parathyroid glands D. Understand the anatomical relationship of recurrent laryngeal nerve to the thyroid and parathyroid glands and important clinical correlations 2. Describe the major muscular structures in the root of the neck A. Describe the origin and insertion of the three scalene muscles B. Describe the innervation and functions of the three scalene muscles in correlation to the skeletal framework C. Describe the anatomical relationship between the scalene muscles and other major neurovascular structures in the root of the neck region 3. Describe major vessels and branches in the root of the neck region A. Describe the anatomical relationships among the subclavian arteries, veins and major lymphatic ducts in the context of other neural and muscular structures B. Describe the major branches of the subclavian arteries in relation to neuromuscular structures and skeletal framework of the root of the neck region C. Describe the functions of the major branches of the subclavian arteries 4. Describe the nervous components in the root of the neck region A. List the major nerves and branches in the root of the neck region B. Describe the anatomical relationship in the context of other vascular and muscular structures in the root of the neck region C. Describe the functions of the major nerves and branches in the root of the neck region. Thyroid & Root of the Neck Page 2 of 14 Dr. Shunbin Xu Session Outline I. Thyroid and parathyroid glands A. The Thyroid Gland 1. Composition 2. Location 3. Vessels 4. Innervation B. Parathyroid gland C. Nerves in close relationship to the thyroid gland: the recurrent laryngeal nerves D. Clinical correlation of thyroid and parathyroid glands 1. Hyperthyroidism 2. Hypothyroidism 3. Ectopic thyroid gland and parathyroid glands 4. Complications of thyroidectomy II. The Root of the Neck A. Overview B. The scalene muscles and interscalene triangle C. Vessels in the root of the neck Subclavian vein Subclavian artery and major branches Lymphatics D. Nerves 1.Phrenic nerve 2. Vagus nerve 3. Sympathetic trunk 4. Brachial plexus 5. Cervical plexus Supplemental Reading Gray’s Anatomy for Students (4rd Ed) https://www.clinicalkey.com/#!/content/book/3-s2.0-B9780702051319000080?scrollTo=%23hl0008855 Thyroid & Root of the Neck Page 3 of 14 Dr. Shunbin Xu I. Thyroid & Parathyroid Glands A. Thyroid Gland 1. Composition An important endocrine gland, plays important roles in metabolism, growth and maturation of the human body by secreting thyroid hormones [Tri-iodothyronine (T3) and thyroxine (T4)] by follicular cells and regulating blood calcium level by secreting calcitonin by C cells. Two lobes: left and right, which are connected by a narrower isthmus (Fig. 1). Frequently, a pyramidal lobe exists extending superiorly from the isthmus. Developmentally, the thyroid gland originates from between the first and second pharyngeal pouches. The pyramidal lobe is a remnant of the thyroglossal duct, through which the thyroid gland migrates from the floor of the pharynx near the base of the tongue (foramen cecum of the tongue, details of which will be introduced in Fig.1 (Gray’s Fig.8.178A) later lectures), pass anterior to the body of the hyoid bone to its final adult location. The thyroglossal duct usually disappears early in Fig.2 (Gray’s Fig.8.178B) development. Remnants may persist along this path as “ectopic”, functional thyroid gland tissues. 2. Location (Fig.1-3): - In the visceral compartment, wrapped within the pretracheal layer of the deep fascia. Deep to the strap muscles; medial to the carotid sheath. - At vertebral levels C5-T1. - It wraps around the trachea, covering the anterior and lateral sides of the trachea. The isthmus crosses the anterior surface of tracheal rings 2 and 3 (Fig.3). 3. Vessels (Fig.4): Arteries: - Superior thyroid artery – the first brand of external carotid artery. - Inferior thyroid artery – a branch of the thyrocervical trunk from the subclavian artery. Fig.3 (Netter Plate 28) Thyroid & Root of the Neck Page 4 of 14 Dr. Shunbin Xu - Thyroid ima artery (Fig.5) (2%- 12% incidence) – a small branch of the brachiocephalic trunk or the arch of the aorta, enter the thyroid gland inferiorly, near the midline. Fig.5 (Grant’s Fig.7.11) Fig.4 (Netter Plate 76) Venous drainage (Fig.4-6): - Superior thyroid veins – drain to the internal jugular veins (Fig.4). - Middle thyroid veins – drain to the internal jugular veins (Fig.4). - Inferior thyroid veins - drain into brachiocephalic veins (Fig.5). Fig.6 (Netter Plate 77) Thyroid & Root of the Neck Page 5 of 14 Dr. Shunbin Xu Lymphatic drainage (Fig.7): Fig.7 (Gray’s Fig.207) - Paratracheal nodes - Deep cervical nodes along the internal jugular vein inferior to omohyoid muscle 4. Innervation of the thyroid gland (Fig.8): Principal innervation of the thyroid gland derives from the autonomic nervous system. It is believed that thyroid gland is rich in sympathetic and parasympathetic nerve fibers. Parasympathetic fibers come Deeper cervical nodes from the vagus nerves; sympathetic fibers are from the superior, middle, and inferior ganglia of the sympathetic trunk. These Jugulo-omohyoid node small nerves enter the gland along with the blood vessels. However, the influence of direct nerve impulses on the follicle activity is negligible in comparison with the humoral effect of thyrotropin or thyroid stimulating hormone (TSH). Fig.8 (Netter’s plate 141) The sympathetic nervous system is considered to be one of the most important systems that regulates the serum level of thyroid hormones and the level of their metabolic activity in the liver cells. However, autonomic nervous regulation of the glandular secretion is not clearly understood; most of the effect is postulated to be on blood vessels, hence the perfusion rates of the glands (Medscape; Abdreshov SN et al. 2019). B. Parathyroid glands (Fig.6) - Important endocrine glands, which play important roles in calcium metabolism by secreting parathyroid hormone. - they are two pairs (superior and inferior) of small (~5 mm in diameter), ovoid, yellowish structure on the posterior surface of the lateral lobes of the thyroid glands (Fig. 6). - darker in color, harder in texture compared to the thyroid gland - derived from the 3rd (the inferior parathyroid glands) and 4th (the superior parathyroid glands) pharyngeal pouches (refer to your Embryology and Histology courses), and translocate to their adult locations during development. - Positions of the glands can be highly variable –ectopic parathyroid gland. Thyroid & Root of the Neck Page 6 of 14 Dr. Shunbin Xu C. Nerves in close relationship to the thyroid gland: The recurrent laryngeal nerve (Fig. 6): - a branch of the vagus nerve (CN X) - looping around the subclavian artery on the right, and the arch of the aorta on the left - pass immediately posterior to the lobes of thyroid gland in the groove of between the trachea and the esophagus - it eventually reaches the larynx and is responsible for sensory innervation of the laryngel cavity below the vocal cord and motor to most intrinsic muscles of the larynx (This will be introduced in detail in later lectures of the Larynx). D. Clinical Correlations: The thyroid gland and parathyroid glands are important endocrine glands. Many disease conditions with complex etiologies are associated with these glands, which you will encounter in your future studies of medicine. 1. Hyperthyroidism – overactive thyroid. It can be accompanied by enlargement of the thyroid gland. Typical symptoms are related to increased actions of thyroid hormones, e.g. hot flashes, sweating, trembling, weight loss, racing heart, restlessness. One of common autoimmune diseases causing hyperthyroidism is Graves’ disease. 2. Hypothyroidism – with decreased level of thyroid hormones. One common cause is lack of iodine. To compensate the low function, the thyroid is often enlarged – goiter, also called struma. Typical symptoms of hypothyroidism include low energy, slow pulse, slowed metabolism, sensitive to cold, overweight, difficulties in concentration, mental slowness, dry skin, waxy skin, thickening and swelling (myxedema, severe hypothyroidism). 3. Ectopic thyroid gland and parathyroid glands. Ectopic thyroid tissue may be seen in any location from tongue (foramen cecum at base of tongue) to suprasternal notch (site of normal gland). Most common thyroid ectopia is at base of tongue (referred to as lingual thyroid).In general, ectopic thyroid tissue is benign. Malignant neoplasms may arise in thyroid ectopia, including papillary thyroid carcinoma and follicular carcinoma. (Diagnostic Pathology: Endocrine (2ed ed. 2018). Ectopic inferior parathyroid tissues are most frequently found in the anterior mediastinum, in association with the thymus or the thyroid gland, while the most common position for ectopic superior parathyroid is the tracheoesophageal groove and retroesophageal region (Noussios G et al. 2012. Exp Clin Endocrinol Diabetes) Fig. 9 (Gray’s Fig. 8.181) Thyroid & Root of the Neck Page 7 of 14 Dr. Shunbin Xu 4. Complications of thyroidectomy: - Accidentally removed all parathyroid glands, causing dysfunction of calcium metabolism. At least one of the four parathyroid glands must be kept to maintain proper serum calcium level without medication. - Accidentally damaged the recurrent laryngeal nerve because of their intimate relation (Fig. 9), causing paralysis of the vocal folds, leading to a hoarse voice and finally to an inability to speak (will be introduced in detail in later lectures). Assessment of the vocal folds is necessary before and after thyroidectomy surgery. II. The Root (Base) of the Neck (Fig.9) A. Overview - A junction between the thorax and the neck. Immediately superior to the superior thoracic aperture (a gateway to pleural cavities) and axillary inlets (a gateway to the upper limb) (Fig.10). - It is bounded by (1) the top of the manubrium of the sternum; (2) superior margin of the clavicle anteriorly; (3) the top of the thoracic vertebra T1; (4) the superior margin of the scapula to the coracoid process posteriorly. - All structures passing between the head and the thorax, and the thorax and neck and the upper limb must pass through the root of the neck. - At the base of the neck, the trachea is immediately anterior to the esophagus, which is directly anterior to the vertebral column; - Major veins, arteries, and nerves pass anterior and lateral to the trachea. - To the axillary inlet, structures such as blood Fig.10 (Gray’s Fig. 3.7 or 8.11) vessels pass over rib I when passing between the axillary inlet and thorax. - Cervical components of the brachial plexus pass directly from the neck through the axillary inlets to enter the upper limb Thyroid & Root of the Neck Page 8 of 14 Dr. Shunbin Xu B. The important landmarks of this Fig.11 (Netter plate 37) region: The scalene muscles and interscalene triangle (Fig.11). Anterior scalene muscle: Origin: Anterior tubercles of the transverse processes of vertebrae CIII to CVI. Insertion: Scalene tubercle and upper surface of rib I. Innervation: anterior rami of C4-C7. Function: Elevate rib I. Middle scalene muscle: Origin: Transverse processes of vertebrae CII to CVII. Insertion: Upper surface of rib I posterior to the groove for the subclavian artery. Innervation: anterior rami of C3-C7. Function: Elevate rib 1. Posterior scalene muscle: Origin: Posterior tubercles of transverse processes of Fig.12 (Gray’s Fig. 8. 180) vertebrae CIV to CVI. Insertion: Upper surface of rib II. Innervation: anterior rami of C5- C7. Function: Elevation of rib II Interscalene triangle: bordered by the anterior and middle scalene muscles and the superior border of the first rib form a triangle. Important structures passing through: the subclavian a. and brachial plexus (Figs.11). Finding the scalene muscles during dissection (Fig.12): - Inside the prevertebral layer of the deep cervical fascia in the prevertebral compartment (Fig.12), forming part of the floor of the posterior triangle - Posterior and/or deeper to the SCM and the omohyoid muscle (Fig.12); - Posterolateral to the carotid sheath (Fig.13). Thyroid & Root of the Neck Page 9 of 14 Dr. Shunbin Xu C. Vessels in the root of the neck - The subclavian vein runs Fig.13 (Netter’s Plate 32) anterior to the subclavian artery (Figs. 13, 14); - Both the subclavian vein and artery cross over the first rib to become the axillary artery and vein to the upper limb. However, the subclavian vein cross the first rib anterior to the anterior scalene muscle, while the subclavian artery cross the rib posterior to the anterior scalene muscle through the interscalene triangle (Fig.14). Major branches of the subclavian artery (Figs.14-19) 1st part of subclavian artery – medial to the anterior scalene muscle: 3 major branches: - Vertebral artery: enters the transverse foramen of vertebra C6 (Figs. 14-16) and ascends through the foramina in the transverse processes of cervical vertebrae and eventually pass through the foramen magnum to supply blood to the brain. - Thyrocervical trunk: three branches (Fig.14-19) 1) Inferior thyroid artery: Anterior to phrenic nerve and posterior to vagus nerve and the Fig.14 (Grant’s dissector Fig. 7.13) cervical sympathetic trunk. It gives off ascending cervical artery, which continues to ascend on the anterior surface of the prevertebral muscles, supplying these muscles and sending branches to the spinal cord. 2) Transverse cervical artery: divide into superficial and deep branches. The superficial branch continues on the deep surface of and supplies the trapezius muscle; the deep branch continues on the deep surface of the rhomboid muscles near the medial border of the scapula. Fig.15 (Grants’ dissector Fig. 7.13) 3) Suprascapular artery: passes superior to the superior transverse scapular ligament, supplies the supraspinatus and infraspinatus muscles (Fig.17, 18). Thyroid & Root of the Neck Page 10 of 14 Dr. Shunbin Xu - Internal thoracic artery (Figs.16,18): Arises from the anteroinferior surface of the subclavian artery and descends, passing posterior to the clavicle and the large veins in the region and anterior to the pleural cavity. It enters the thoracic cavity posterior to the ribs and anterior to the transversus thoracis muscle and continues to descend giving off numerous branches. Fig.16 (Gray’s Fig. 8.191) 2nd part of the subclavian artery – posterior to the anterior scalene muscle: 1 major branch: Costocervical trunk (Fig.16): ascends and passes posteriorly over the dome of the pleural cavity and continues in a posterior direction behind the anterior scalene muscle. Eventually it divides into two branches: 1) Deep cervical artery: Ascends in the back of the neck and anastomoses with the descending branch of the occipital artery. 2) Supreme intercostal artery: Fig.17 (Netter’s Plate 33) descends anterior to rib I and divides to form the posterior intercostal arteries for the first two intercostal spaces - posterior intercostal arteries 1 and 2. Fig.18 (Netter’s Plate 414) Thyroid & Root of the Neck Page 11 of 14 Dr. Shunbin Xu 3rd part of the subclavian artery – lateral to the anterior scalene muscle: 1 major branch Dorsal scapular artery: it passes between the superior and middle trunks of the brachial plexus (Figs.14,15,17); supplies the muscles of the scapular region (Figs.18,19). In ~30% of cases, it arises from the transverse cervical artery (Fig.14). Fig.19 (Netter’s Plate 414) Lymphatics in the root of the neck (Figs.14,20,21) On the left side: Thoracic duct Left jugular trunk left venous angle at the junction of left Left subclavian trunk subclavian and internal jugular veins Left bronchiomediastinal trunk On the right side: Right jugular trunk Right subclavian trunk right venous angle at the junction of right Right bronchiomediastinal trunk subclavian and internal jugular veins Fig.20 (Gray’s Fig.8.195) Fig.21 (Gray’s Fig.8.192) Thyroid & Root of the Neck Page 12 of 14 Dr. Shunbin Xu D. Nerves in the root of the neck 1. Phrenic nerve: C3-C5. Supplies the diaphragm with both sensory and motor innervation. Runs on the anterior surface of the anterior scalene within the prevertebral fascia; deep to the transverse cervical and Fig.22 (Netter’s Plate 137) suprascapular arteries; lateral to the vagus nerve and sympathetic trunk (Figs.14, 17). 2. The vagus nerve: - Descends through the neck within the carotid sheath, posterior to and between the common carotid artery and the internal jugular vein; - In the lower part of the neck, give off cardiac branches (Fig.22); - In the root of the neck, it passes anterior to the subclavian artery and posterior to the subclavian vein as it enters the thorax (Figs.17), while it gives off the recurrent laryngeal nerve; - the recurrent laryngeal nerves loop around the subclavian artery on the right and the arch of the aorta on the left, run upward in a groove between the trachea and the esophagus (Figs.14, 17). 3. The sympathetic trunk: two parallel cords that run from the base of the skull to the coccyx. - In the cervical region, it is anterior to the longus colli and longus capitis muscles, and posterior to the common carotid and internal carotid arteries in the carotid sheath; medial to phrenic nerve (Figs.14, 23, 24); - There are three cervical ganglia, which are collections of neuronal cell bodies outside the CNS: Fig.23 (Gray’s Fig. 8.194) Thyroid & Root of the Neck Page 13 of 14 Dr. Shunbin Xu Superior: large; at C1/C2 level; with gray rami communicantes (postganglionic nerve fibers) with spinal nerves C1-4; gives out superior cardiac nerves, and branches to the pharynx, and internal/external carotid arteries to form plexus around them (Figs.23,24); Middle: at the level of vertebrae C6; with gray rami communicantes to spinal nerves C5 and C6; gives out middle cardiac nerves (Figs.23,24); Inferior: at the level of vertebrae C7; often fuse with the first thoracic ganglion to form a very large “stellar ganglion” (Fig.24); posterior to subclavian artery and its branches, as well as the cervical pleura (cupula) (Fig.14); with gray rami communicantes with spinal nerves C7 to T1; gives off inferior cardiac nerves and branches to the vertebral artery, forming a plexus around this vessel (Figs.23,24). 4. Brachial plexus: Five roots: anterior rami of cervical nerves C5 to C8 Fig.24 (Netter’s plate 131) and thoracic nerve T1 (5 roots). Fig.25 (Netter’s plate 416) They pass between the anterior and middle scalene muscles through the interscalene triangle (Figs.14,15,17,20,25). Three trunks (Fig.25): - Upper (C5 and C6); - middle (C7); - lower trunk (C8 and T1). Six divisions: 3 anterior; 3 posterior (Fig.25) Thyroid & Root of the Neck Page 14 of 14 Dr. Shunbin Xu 5. Cervical plexus Fig.26 (Gray’s Fig.8.188) The cervical plexus is formed by the anterior rami of cervical nerves C1 to C4 (Fig.26). Cutaneous branches (Figs.26,27): Sensory nerve in the posterior triangle: Lesser occipital (C2) Greater auricular (C2, C3) Transverse cervical (C2, C3) Supraclavicular nerves (C3, C4) Muscular branches (do not necessarily mean motor only; some contain sensory fibers): - Nerve to the thyrohyoid muscle (C1): travel with the Hypoglossal nerve (CN XII). - Ansa cervicalis (C1-C3): (Fig.26) Superior root: C1 Inferior root: C2, C3. Innervates the infrahyoid muscles. - Phrenic nerve: C3-C5 (Figs.26,28). Supplies the diaphragm with both sensory and motor innervation. Fig.27 (Gray’s Fig.8.187) - Muscular branches to prevertebral and lateral vertebral muscles, including rectus capitis anterior, rectus capitis lateralis, longus colli, and longus capitis, anterior scalene muscle (C4-C7) and middle scalene muscle (C3-C7) (Fig.28). The posterior scalene muscle is innervated by C5- C7, which are not part of the cervical plexus Fig.28 (Gray’s Fig.8.189)