Functional Anatomy of the Thyroid & Parathyroid Glands Innervation of the Pharynx & Larynx Dr James Peerless June 2011

Objectives The Thyroid Gland The Parathyroid Gland Pharynx & Larynx Anatomy Endocrine function The Parathyroid Gland Ca2+ metabolism Pharynx & Larynx Innervation

The Thyroid Gland Largest endocrine gland in the body Origin Originates from the floor of pharynx Thyroglossal duct sometimes persists in adults Largest endocrine gland in the body Affects basal metabolic rate via the thyroid hormones: control CHO, protein metabolism, and influence growth, maturation and body temperature Originates from floor of the pharynx Thyroglossal duct marks path of gland from the base of the tongue to its final site

The Thyroid Gland Relations Deep to sternothyroid and sternohyoid muscles Level of C5 –T1 Bilobular; joined by the isthmus Anterolateral to the larynx and trachea (2nd & 3rd rings) Bound by a loose connective tissue capsule which is anchored to the cricoid cartilage and tracheal rings Upper and lower pole

Supplied by superior and inferior thyroid arteries Highly vascular Supplied by superior and inferior thyroid arteries Drainage via superior, middle and inferior thyroid veins 2+3 Superior thyroid artery is commonly the first branch of the external carotid Inferior thyroid artery originates from the thyrocervical trunk (from the subclavian artery). 10% of people have the unpaired thyroid ima artery – supplied directly from the aorta or the common carotid a. – important when planning tracheostomies as it runs arteriorly along the trachea. Sup. and middle drain into IJV, inf. into the brachiocephalic veins

The Thyroid Gland Nerve Supply Derived from superior, middle and inferior cervical sympathetic ganglia (from the sympathetic trunk) Fibres are vasomotor No secretomotor parasympathetic innervation: the gland is hormonally regulated

Ectopic glands Accessory tissue Pyramidal lobe Perseveration of the thyroglossal duct

Functional Anatomy of Thyroid Follicular structure lined by epithelial cells Alter shape depending on amount of activity Contain colloid Parafollicular ‘C’ cells Secrete calcitonin

Parafollicular Cells Thyroglobulin Colloid Follicular Cells

Thyroid Hormone Synthesis Function of the follicular cells: Thyroglobulin production (protein containing 123 tyrosine residues)  forms the colloid thyroid peroxidase synthesis storage of T3 and T4 1 iodide ions actively transported into the follicular cells from the plasma – 20-40x concentration 2 iodide ions reduced to iodine in the colloid by TP and combined with tyrosine to form MIT or DIT. 3 DIT/MIT combine to form T3/T4/thyroglobulin complex. 4 passes back into the follicular cell until released into the blood.

The Parathyroid Glands Four disc-like glands, embedded in the thyroid gland (dorsal surface) Contain two cell types: Chief cells producing PTH Oxyphil cells: function unknown Essential for maintenance of normocalcaemia – important as Ca2+ act as secondary messengers, blood coag., nerve impulse transmission, muscle contraction Four disc-like glands, embedded in the thyroid gland – two in each of the upper and lower poles Most people have 4 glands – 5% have more. Blood supply is most commonly provided by the inferior thyroid arteries (but can be any).

Sites of aberrant parathyroid glandular tissue

Parathyroid Hormone Polypeptide produced in the chief cells Preprohormone  Prehormone  PTH Increases Ca2+ via three main actions: Resorption Reabsorption Absorption Resorption: mobilization of Ca2+ from bone (clasts and blasts) Increased reabsorption from the distal tubule (and reduced reabsorption of PO42- in the proximal tubules) Increased production of 1,25 dihydroxycholecalciferol  increased absorption of Ca2+ from the small intestine Controlled by negative feedback loop.

Responds to low serum Ca2+ levels

The Pharynx External circular – constrictors (superior, middle and inferior constrictors) Internal longitudinal – elevate the larynx and shorten the pharynx during speaking and swallowing – stylopharyngeus, salpingopharyngeus, palatopharyngeus Anterior connections such as the mandible, hyoid and larynx – and join at the posterior midline raphe.

The Pharynx Nerve Supply Pharyngeal plexus provides motor and most of sensory Motor: All muscles – fibres from nucleus ambiguus of the accessory nerve carried via the vagus nerve Stylopharyngeus – CN IX Tensor veli palatini - CN V3 Lower pharynx also receives some motor fibres from recurrent laryngeal nerve Sensory: Glossopharyngeal (CN IX) – most of mucosa Maxillary (CN V2) – anterior nasopharynx Vagus (CN X) Branches from the cervical sympathetic chain

The Larynx Muscles (6+3) Intrinsic Extrinsic Cricothyroid Posterior cricoarytenoid Lateral cricoarytenoid Transverse arytenoid Aryepiglottic Thyroarytenoid All paired except the tranverse arytenoid Extrinsic Sternothyroid Thyrohyoid Inferior Constrictor

The Larynx Innervation Mucous membranes Muscles of the larynx Above cords: internal laryngeal nerve Below cords: recurrent laryngeal nerve Muscles of the larynx Cricothyroid: superior/external laryngeal nerve All others: recurrent laryngeal nerve

Objectives The Thyroid Gland The Parathyroid Gland Pharynx & Larynx Anatomy Endocrine function The Parathyroid Gland Ca2+ metabolism Pharynx & Larynx Innervation

Questions

Reference Smith T, Pinnock C, Lin T (2009). Fundamentals of Anaesthesia, 3rd Edition. Cambridge: Cambridge University Press Moore K, Dalley A (1999). Clinically Oriented Anatomy, 4th Edition. Philadelphia: Lippincott Williams & Wilkins