# Lecture 14: Neck Vessels and Nerves ## Unit 1.5: Human Gross Anatomy III - Pelvis and Head/Neck --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the course and branches of the common, internal, and external carotid arteries 2. Identify the internal and external jugular veins and their tributaries 3. Describe the cervical plexus and its branches 4. Identify the brachial plexus roots, trunks, and branches in the neck 5. Describe the course of the vagus and phrenic nerves 6. Correlate anatomical features with clinical conditions --- ## Common Carotid Artery The common carotid arteries are the principal arteries supplying the head and neck. Their origins differ on the two sides: the right common carotid arises from the bifurcation of the brachiocephalic artery behind the sternoclavicular joint, while the left common carotid arises directly from the arch of the aorta and ascends through the superior mediastinum before entering the neck. Both common carotid arteries ascend within the carotid sheath, positioned posterior to the sternocleidomastoid muscle, medial to the internal jugular vein, and with the vagus nerve lying posteriorly between the two vessels. The arteries give off no branches in the neck, reserving their entire blood flow for the brain and face. At the upper border of the thyroid cartilage (approximately at the level of C4), each common carotid artery divides into the internal and external carotid arteries. This bifurcation is an important surgical landmark and is the most common site for atherosclerotic disease affecting the carotid system. Two specialized structures are located at the carotid bifurcation. The carotid sinus is a dilation at the proximal portion of the internal carotid artery containing baroreceptors in its wall that monitor arterial blood pressure. The carotid body is a small collection of chemoreceptor cells at the bifurcation that senses blood oxygen, carbon dioxide, and pH levels. Both structures are innervated primarily by the glossopharyngeal nerve (CN IX), with contributions from the vagus. Pressure on the carotid sinus, either pathological or as part of therapeutic carotid sinus massage, can trigger reflex bradycardia and hypotension. Panel A: Right common carotid arising from brachiocephalic artery behind the sternoclavicular joint and left common carotid ascending from aortic arch through superior mediastinum. Panel B: Both arteries ascending in the carotid sheath (translucent tube) with internal jugular vein lateral and vagus nerve between. Panel C: Bifurcation at C4 level showing carotid sinus (dilated proximal ICA) and carotid body at the angle. Panel D: CN IX innervation to the carotid sinus and carotid body indicated. --- ## External Carotid Artery The external carotid artery supplies the extracranial structures of the head and neck, including the face, scalp, tongue, pharynx, and meninges. It begins at the bifurcation of the common carotid at the level of the upper thyroid cartilage and terminates within the substance of the parotid gland by dividing into its two terminal branches. Initially, the external carotid lies anteromedial to the internal carotid, but as it ascends it becomes more lateral. It passes through the carotid triangle, then deep to the posterior belly of the digastric and the stylohyoid muscles, and finally enters the parotid gland. The external carotid gives off eight branches, which can be remembered using the mnemonic "Some Anatomists Like Freaking Out Poor Medical Students" (Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Maxillary, Superficial temporal). The superior thyroid artery is typically the first branch, arising near the bifurcation and descending to the upper pole of the thyroid gland. It gives off the superior laryngeal artery, which accompanies the internal laryngeal nerve through the thyrohyoid membrane. The ascending pharyngeal artery is the smallest branch, arising from the medial surface and ascending along the pharynx to supply the pharyngeal wall, tonsils, and auditory tube. The lingual artery arises at the level of the greater horn of the hyoid bone, loops upward and then passes deep to the hyoglossus muscle to supply the tongue. Its relationship to the hypoglossal nerve (which passes superficial to hyoglossus) is a key surgical landmark. The facial artery arises just above the lingual artery, loops upward deep to the digastric and stylohyoid muscles, and then passes deep to the submandibular gland, grooving its surface. It emerges to cross the lower border of the mandible just anterior to the masseter muscle, where it is palpable. The artery then ascends tortuously across the face to terminate as the angular artery at the medial angle of the eye. The occipital artery arises from the posterior surface of the external carotid, passes posteriorly along the inferior border of the posterior belly of digastric, grooves the mastoid bone, and pierces the fascia between the trapezius and sternocleidomastoid to supply the posterior scalp. The posterior auricular artery arises above the digastric and ascends between the mastoid process and the ear to supply the scalp behind the ear. The maxillary and superficial temporal arteries are the terminal branches, dividing within or just above the parotid gland. The maxillary artery is the larger terminal branch, passing deep to the neck of the mandible into the infratemporal fossa to supply the deep structures of the face, including the muscles of mastication, the teeth, the palate, the nasal cavity, and the meninges (via the middle meningeal artery). The superficial temporal artery emerges from the parotid gland anterior to the ear, where it is readily palpable, and ascends to supply the temporal region and scalp. Panel A: External carotid artery with anterior branches -- superior thyroid descending to thyroid gland (with superior laryngeal branch), lingual looping deep to hyoglossus, and facial crossing mandible to face. Panel B: Medial branch (ascending pharyngeal ascending medially) and posterior branches (occipital passing posteriorly to scalp and posterior auricular ascending behind ear). Panel C: Terminal division in parotid into maxillary artery (entering infratemporal fossa) and superficial temporal artery (anterior to ear). Panel D: Mnemonic "Some Anatomists Like Freaking Out Poor Medical Students" listed alongside all eight branches. --- ## Internal Carotid Artery The internal carotid artery supplies the brain, the orbit, and the forehead. Unlike the external carotid with its numerous branches, the internal carotid gives off no branches in the neck, reserving its entire flow for intracranial distribution. The internal carotid begins at the bifurcation of the common carotid (at C4) as the larger of the two terminal branches. At first it lies posterolateral to the external carotid, but as it ascends it becomes more medial. The artery ascends vertically to the skull base and enters the cranial cavity through the carotid canal in the petrous portion of the temporal bone. Within the carotid sheath, the internal jugular vein lies lateral to the artery and the vagus nerve lies in the posterior groove between the two. The sympathetic trunk lies posterior to the sheath on the prevertebral fascia, and sympathetic fibers accompany the artery as the internal carotid plexus. The cervical portion of the internal carotid is susceptible to atherosclerotic disease, particularly just beyond the bifurcation. Carotid stenosis at this location is a major cause of ischemic stroke due to thromboembolism or, less commonly, hemodynamic compromise. The artery may also be involved in dissection, which can occur spontaneously or following trauma. Panel A: Internal carotid artery from bifurcation at C4 ascending without branches to enter the skull through the carotid canal. Panel B: Cross-section showing position within the carotid sheath with internal jugular vein lateral and vagus nerve posterior between vessels. Panel C: Sympathetic trunk on prevertebral fascia posteriorly with the internal carotid plexus. Panel D: Comparison with external carotid showing the external carotid initially anteromedial then becoming lateral and giving multiple branches while the internal carotid ascends without branching. --- ## Subclavian Artery The subclavian arteries supply the upper limb, part of the neck, and contribute to the blood supply of the brain via the vertebral arteries. Their origins differ: the right subclavian artery arises from the bifurcation of the brachiocephalic artery behind the sternoclavicular joint, while the left subclavian artery arises directly from the aortic arch, passing through the superior mediastinum before entering the neck. Each subclavian artery arches over the apex of the lung and crosses the first rib between the anterior and middle scalene muscles. At the lateral border of the first rib, it becomes the axillary artery. The anterior scalene muscle divides the artery into three parts for descriptive purposes. The first part of the subclavian artery, medial to the anterior scalene, gives off three branches. The vertebral artery ascends to enter the transverse foramen of C6 and continues through the transverse foramina to the atlas, from which it enters the skull through the foramen magnum and joins its partner to form the basilar artery. The thyrocervical trunk is a short trunk that divides into the inferior thyroid artery (to the thyroid and parathyroid glands), the suprascapular artery (to the scapular anastomosis), and the transverse cervical artery (to the trapezius and deep back muscles). The internal thoracic artery descends behind the ribs to supply the anterior chest wall and gives rise to the superior epigastric artery. The second part of the subclavian artery, behind the anterior scalene, gives off the costocervical trunk, which divides into the supreme intercostal artery (supplying the first two intercostal spaces) and the deep cervical artery (to the deep muscles of the neck). The third part of the subclavian artery, lateral to the anterior scalene, gives off the dorsal scapular artery in some individuals (though this may arise from the transverse cervical). This part lies in the supraclavicular triangle and is closely related to the brachial plexus trunks, which lie superior and posterior to it. The subclavian vein lies anterior and inferior to the subclavian artery, separated from it by the anterior scalene muscle. The phrenic nerve descends on the anterior surface of the anterior scalene and passes between the artery and vein as it enters the thorax. Panel A: First part of subclavian artery (medial to anterior scalene) giving vertebral artery ascending to transverse foramina, thyrocervical trunk branching into inferior thyroid, suprascapular, and transverse cervical arteries, and internal thoracic descending behind ribs. Panel B: Second part (behind anterior scalene) giving costocervical trunk. Panel C: Third part (lateral to anterior scalene) in the supraclavicular triangle with brachial plexus trunks posterior and superior. Panel D: Phrenic nerve shown on anterior scalene surface, passing between the subclavian artery and vein to enter the thorax. --- ## Vertebral Artery The vertebral arteries provide approximately twenty percent of cerebral blood flow and are the main supply to the posterior fossa (brainstem and cerebellum). The vertebral artery is the first and usually largest branch of the subclavian artery. The artery is divided into four segments. V1 (preforaminal) extends from the origin to the transverse foramen of C6. V2 (foraminal) courses through the transverse foramina from C6 to C2. V3 (atlantic or extradural) exits the transverse foramen of C1, curves posteriorly and medially around the lateral mass of the atlas, and then passes through the posterior atlanto-occipital membrane to enter the foramen magnum. V4 (intracranial) extends from the foramen magnum to the junction with the opposite vertebral artery at the lower border of the pons, forming the basilar artery. The V2 segment is protected by the bony transverse foramina, while the V3 segment, with its loop around the atlas, is potentially vulnerable during cervical spine trauma or manipulation. Extreme rotation or extension of the neck can compress or stretch the artery. The vertebral artery is also at risk during posterior cervical spine surgery. Vertebrobasilar insufficiency results from reduced flow through the vertebral-basilar system, often due to atherosclerosis or compression. Symptoms include vertigo, diplopia, dysarthria, drop attacks, and visual disturbances. Subclavian steal syndrome occurs when proximal subclavian stenosis causes blood to flow retrograde through the vertebral artery to supply the arm, "stealing" blood from the posterior circulation; symptoms worsen with arm exercise. Panel A: V1 segment (preforaminal) from subclavian artery to C6 transverse foramen. Panel B: V2 segment (foraminal) ascending through transverse foramina from C6 to C2. Panel C: V3 segment (atlantic) looping posteriorly around lateral mass of atlas then medially to the foramen magnum. Panel D: V4 segment (intracranial) joining with opposite vertebral to form basilar artery at pontomedullary junction, with inset showing vulnerability of V3 during neck rotation. --- ## Internal Jugular Vein The internal jugular vein is the principal venous drainage pathway for the brain and most of the face and neck. It is the continuation of the sigmoid sinus, beginning at the jugular foramen in the posterior cranial fossa. The vein descends through the neck within the carotid sheath, lying lateral to the internal carotid artery superiorly and to the common carotid artery inferiorly. The vagus nerve lies in the posterior groove between the vein and the artery. At its termination behind the sternoclavicular joint, the internal jugular vein joins the subclavian vein to form the brachiocephalic vein. Major tributaries include the inferior petrosal sinus (draining directly from the skull base), the facial vein, the lingual vein, the pharyngeal veins, and the superior and middle thyroid veins. The vein also communicates with the external jugular vein. The internal jugular vein is a common site for central venous catheter placement. Access is typically gained in the neck using landmarks (the triangle between the two heads of the sternocleidomastoid, with the carotid pulse as a medial guide) or, preferably, with ultrasound guidance. The vein is compressible by the ultrasound probe, distinguishing it from the artery. Complications of internal jugular catheterization include carotid artery puncture, pneumothorax, hemothorax, and thrombosis. Panel A: Internal jugular vein from jugular foramen descending in the carotid sheath, positioned lateral to carotid arteries throughout its course. Panel B: Junction with subclavian vein behind the sternoclavicular joint forming the brachiocephalic vein. Panel C: Major tributaries draining into the vein -- inferior petrosal sinus at jugular foramen, facial vein, lingual vein, pharyngeal veins, and thyroid veins. Panel D: Inset showing ultrasound appearance with compressible vein lateral to non-compressible artery. --- ## External Jugular Vein The external jugular vein is a superficial vein that drains the scalp and face. It is formed behind the angle of the mandible by the union of the posterior auricular vein and the posterior division of the retromandibular vein. The vein descends obliquely across the sternocleidomastoid muscle, beneath the platysma, and pierces the investing layer of deep cervical fascia just above the clavicle to drain into the subclavian vein. A valve near its termination prevents reflux. Tributaries include the posterior external jugular vein, the transverse cervical vein, the suprascapular vein, and the anterior jugular vein. The anterior jugular veins descend near the midline and are connected by a jugular venous arch in the suprasternal space before joining the external jugular veins. The external jugular vein is visible when distended, making it a useful clinical indicator of central venous pressure. Distension while the patient is sitting at 45 degrees (jugular venous distension, JVD) suggests elevated right heart pressure. The vein can be used for emergency intravenous access, though the internal jugular or subclavian veins are preferred for central access. Panel A: External jugular vein forming behind the mandible angle from posterior auricular and retromandibular vein tributaries. Panel B: Vein descending superficially across the sternocleidomastoid (beneath platysma, shown as transparent layer) and piercing deep fascia above the clavicle to join the subclavian vein. Panel C: Anterior jugular veins near the midline connected by the jugular venous arch. Panel D: Patient diagram showing normal versus distended external jugular vein for assessment of venous pressure. --- ## Cervical Plexus The cervical plexus is formed by the ventral rami of the first four cervical spinal nerves (C1-C4). It lies deep to the sternocleidomastoid muscle, on the anterior surface of the middle scalene and levator scapulae muscles. The plexus provides sensory innervation to the skin of the neck, ear, and shoulder region, as well as motor innervation to several neck muscles and the diaphragm (via the phrenic nerve). The cutaneous (sensory) branches emerge at Erb's point, located at the midpoint of the posterior border of the sternocleidomastoid, and radiate to supply their respective territories. The lesser occipital nerve (C2) ascends along the posterior border of the sternocleidomastoid to supply the skin of the scalp behind the ear. The great auricular nerve (C2, C3) ascends across the sternocleidomastoid to supply the skin over the parotid gland and the ear. The transverse cervical nerve (C2, C3) passes horizontally across the sternocleidomastoid to supply the skin of the anterior neck. The supraclavicular nerves (C3, C4) descend across the clavicle to supply the skin of the lower neck and shoulder. The motor branches include the ansa cervicalis and contributions to the phrenic nerve. The ansa cervicalis is a nerve loop on the carotid sheath. Its superior root carries C1 fibers that travel briefly with the hypoglossal nerve before descending. Its inferior root carries C2 and C3 fibers. The loop supplies the infrahyoid muscles: the sternohyoid, sternothyroid, and the inferior belly of the omohyoid. (The superior belly of the omohyoid and the thyrohyoid are supplied by C1 fibers traveling with the hypoglossal nerve.) Additional motor branches supply the prevertebral muscles (longus colli, longus capitis, rectus capitis), the levator scapulae (with C5), and contribute to the trapezius supply along with the spinal accessory nerve. Panel A: Cervical plexus with C1-C4 ventral rami forming loops deep to the sternocleidomastoid. Panel B: Cutaneous branches emerging at Erb's point -- lesser occipital ascending behind ear, great auricular ascending to ear and parotid, transverse cervical crossing horizontally, and supraclavicular descending to shoulder. Panel C: Ansa cervicalis shown as a loop on the carotid sheath with superior root (from C1 via hypoglossal) and inferior root (C2-C3). Panel D: Ansa cervicalis innervation to the strap muscles (sternohyoid, sternothyroid, inferior belly of omohyoid) labeled. --- ## Phrenic Nerve The phrenic nerve is the sole motor supply to the diaphragm and is essential for breathing. It arises primarily from the fourth cervical nerve (C4) but receives contributions from C3 and C5, recalled by the mnemonic "C3, 4, 5 keeps the diaphragm alive." The nerve forms on the anterior surface of the anterior scalene muscle at the lateral border of the muscle's upper part. It descends vertically on the anterior surface of the anterior scalene, running from lateral to medial, and passes between the subclavian artery (posteriorly) and the subclavian vein (anteriorly) to enter the thorax. In the thorax, the right and left phrenic nerves take different courses. The right phrenic nerve descends along the right brachiocephalic vein, the superior vena cava, and the right side of the pericardium covering the right atrium, passing anterior to the right lung root to reach the diaphragm. The left phrenic nerve crosses the aortic arch, passes over the left ventricle on the pericardium, and descends anterior to the left lung root to the diaphragm. The phrenic nerve provides motor innervation to the diaphragm, enabling contraction for inspiration. It also carries sensory fibers from the central portion of the diaphragm, the mediastinal pleura, and the pericardium. Irritation of the diaphragm may cause referred pain to the shoulder (C3-C5 dermatome), as seen in conditions such as subphrenic abscess or hepatic pathology affecting the right hemidiaphragm. Hiccups result from involuntary spasmodic contraction of the diaphragm mediated by the phrenic nerve. Phrenic nerve injury causes paralysis of the ipsilateral hemidiaphragm, which rises paradoxically during inspiration (demonstrated by fluoroscopy or ultrasound, the "sniff test"). Bilateral phrenic nerve injury causes severe respiratory compromise. Causes of phrenic nerve injury include surgical trauma (especially cardiac surgery), central line placement, brachial plexus block, and tumors. Panel A: Phrenic nerve formation from C3-C5 on the anterior scalene, descending on its surface and passing between subclavian artery and vein to enter the thorax. Panel B: Right phrenic nerve descending along the SVC and right pericardium to the right hemidiaphragm. Panel C: Left phrenic nerve crossing the aortic arch and descending on the left pericardium to the left hemidiaphragm. Panel D: Inset showing fluoroscopy of hemidiaphragm paralysis (elevated, paradoxical movement) and diagram of referred pain to the shoulder (C3-C5 dermatome). --- ## Vagus Nerve in the Neck The vagus nerve (CN X) is the longest cranial nerve, extending from the brainstem through the neck and thorax to the abdomen. Its cervical course is essential for understanding its relationship to surgical landmarks. The vagus nerve exits the skull through the jugular foramen and descends through the neck within the carotid sheath. It lies in the posterior groove between the internal jugular vein (laterally) and the carotid artery (medially), positioned between the internal carotid artery superiorly and the common carotid artery inferiorly. Several important branches arise in the neck. The pharyngeal branches leave the upper vagus and, along with branches from the glossopharyngeal nerve and sympathetic trunk, form the pharyngeal plexus on the middle constrictor muscle. This plexus supplies motor innervation to all pharyngeal muscles except the stylopharyngeus (CN IX) and all palatal muscles except the tensor veli palatini (CN V3). The superior laryngeal nerve arises from the inferior vagal ganglion and descends alongside the pharynx, dividing into internal and external branches. The internal laryngeal nerve pierces the thyrohyoid membrane with the superior laryngeal artery to provide sensory innervation to the larynx above the vocal folds. The external laryngeal nerve descends on the inferior constrictor muscle to innervate the cricothyroid muscle, the only intrinsic laryngeal muscle not supplied by the recurrent laryngeal nerve. The external laryngeal nerve lies close to the superior thyroid artery and is vulnerable during ligation of this vessel in thyroid surgery. The cardiac branches arise in the neck and thorax and descend to contribute to the cardiac plexus. The recurrent laryngeal nerves are critical branches with different courses on the two sides due to embryological asymmetry. The right recurrent laryngeal nerve loops posteriorly around the subclavian artery and ascends in the tracheoesophageal groove to enter the larynx. The left recurrent laryngeal nerve has a longer course: it loops under the aortic arch (at the level of the ligamentum arteriosum), passes posterior to the arch, and then ascends in the tracheoesophageal groove. Both nerves supply motor innervation to all intrinsic laryngeal muscles except the cricothyroid and sensory innervation to the larynx below the vocal folds. Panel A: Vagus nerve in the neck within the carotid sheath, posterior to internal jugular vein and carotid arteries, with pharyngeal branches to pharyngeal plexus on the constrictor. Panel B: Superior laryngeal nerve dividing into internal branch (piercing thyrohyoid membrane) and external branch (to cricothyroid) with cardiac branches descending. Panel C: Right recurrent laryngeal nerve looping around the subclavian artery and ascending in the tracheoesophageal groove. Panel D: Left recurrent laryngeal nerve looping under the aortic arch with its longer intrathoracic course before ascending in the tracheoesophageal groove. --- ## Brachial Plexus in the Neck The brachial plexus provides motor and sensory innervation to the upper limb. It is formed by the ventral rami of C5, C6, C7, C8, and T1, with frequent contributions from C4 (prefixed plexus) or T2 (postfixed plexus). The roots of the brachial plexus emerge between the anterior and middle scalene muscles and unite to form three trunks in the posterior triangle of the neck, posterior and superior to the third part of the subclavian artery. The superior trunk is formed by C5 and C6. The middle trunk is the continuation of C7 alone. The inferior trunk is formed by C8 and T1. Within the neck (the supraclavicular part of the plexus), several branches arise directly from the roots and trunks. From the roots: the dorsal scapular nerve (C5) pierces the middle scalene and supplies the rhomboid muscles; the long thoracic nerve (C5, C6, C7) descends on the middle scalene and serratus anterior to supply that muscle. From the superior trunk: the nerve to subclavius supplies the subclavius muscle; the suprascapular nerve passes through the scapular notch to supply the supraspinatus and infraspinatus muscles. The trunks pass over the first rib, behind and above the subclavian artery, and enter the axilla, where each trunk divides into anterior and posterior divisions that recombine to form the lateral, posterior, and medial cords around the axillary artery. The brachial plexus is vulnerable to injury in the neck from several mechanisms. Erb-Duchenne palsy results from injury to the upper trunk (C5, C6), typically from excessive lateral neck flexion away from the shoulder (birth trauma with shoulder dystocia, or motorcycle accidents). The affected arm assumes the "waiter's tip" position: the shoulder is adducted and internally rotated, the elbow extended, the forearm pronated, and the wrist flexed. Klumpke palsy results from injury to the lower trunk (C8, T1), typically from excessive arm abduction (grasping during a fall, or birth trauma with breech delivery). The hand shows a "claw" deformity with weakness of the intrinsic muscles. Because the T1 root carries sympathetic fibers, Klumpke palsy may be associated with an ipsilateral Horner syndrome (ptosis, miosis, anhidrosis). Panel A: Brachial plexus roots (C5-T1) emerging between anterior and middle scalene muscles. Panel B: Three trunks (superior from C5-C6, middle from C7, inferior from C8-T1) posterior to third part of subclavian artery. Panel C: Branches from roots (dorsal scapular, long thoracic) and trunks (suprascapular, nerve to subclavius) labeled. Panel D: Inset showing Erb-Duchenne palsy arm position (waiter's tip) and Klumpke palsy claw hand deformity. --- ## Cervical Sympathetic Trunk The cervical sympathetic trunk lies posterior to the carotid sheath on the prevertebral fascia, anterior to the transverse processes of the cervical vertebrae. It is the continuation of the thoracic sympathetic chain and contains three cervical sympathetic ganglia. The superior cervical ganglion is the largest ganglion and lies at the level of C1-C4, behind the internal carotid artery. Its postganglionic fibers distribute along the internal and external carotid arteries (as the internal and external carotid plexuses) to reach targets in the head: the dilator pupillae (pupil dilation), Mueller's muscle (which assists in elevating the upper eyelid), smooth muscle of the orbit, blood vessels, and sweat glands of the face and scalp. Preganglionic fibers to this ganglion originate in the upper thoracic spinal cord and ascend through the sympathetic trunk. The middle cervical ganglion is inconstant and, when present, lies at the level of C6 near the inferior thyroid artery. It gives cardiac branches and gray rami communicantes to cervical spinal nerves. The inferior cervical ganglion lies at the level of C7 and is frequently fused with the first thoracic ganglion to form the stellate (cervicothoracic) ganglion. This ganglion gives cardiac branches and postganglionic fibers that accompany the subclavian and vertebral arteries to the upper limb and posterior fossa. The ansa subclavia, a loop of sympathetic fibers, connects the middle and inferior ganglia around the subclavian artery. Interruption of the cervical sympathetic pathway produces Horner syndrome: miosis (pupillary constriction due to unopposed parasympathetic tone), partial ptosis (drooping of the upper eyelid due to loss of Mueller's muscle tone), and anhidrosis (decreased sweating on the ipsilateral face due to loss of sudomotor fibers). Apparent enophthalmos (sunken eye) results from narrowing of the palpebral fissure. Causes include Pancoast tumor (apical lung tumor invading the sympathetic chain), carotid artery dissection, brainstem stroke, neck trauma, and iatrogenic injury during central line placement or neck surgery. Panel A: Cervical sympathetic trunk on prevertebral fascia posterior to carotid sheath with the superior cervical ganglion (largest, at C1-C4 level) sending fibers along internal and external carotid arteries to head structures. Panel B: Middle cervical ganglion (at C6, variable) and inferior cervical ganglion often fused with T1 as stellate ganglion. Panel C: Cardiac branches descending from the ganglia and ansa subclavia connecting middle and inferior ganglia. Panel D: Inset showing Horner syndrome features -- ptosis, miosis, and anhidrosis on the affected side compared to the normal side. --- ## Clinical Correlations Carotid artery disease, particularly atherosclerotic stenosis at the carotid bifurcation, is a major cause of ischemic stroke. Plaque rupture can cause thromboembolism to the brain, while severe stenosis can reduce cerebral perfusion. A carotid bruit on auscultation suggests turbulent flow from stenosis but correlates imperfectly with the degree of narrowing. Carotid endarterectomy, the surgical removal of atherosclerotic plaque, reduces stroke risk in appropriately selected patients. During this procedure, the hypoglossal nerve (crossing the carotid vessels), the vagus nerve (in the carotid sheath), and the recurrent laryngeal nerve are at risk. Subclavian steal syndrome results from stenosis or occlusion of the proximal subclavian artery, proximal to the origin of the vertebral artery. Blood flows antegrade through the contralateral vertebral artery, across the basilar artery, and then retrograde down the ipsilateral vertebral artery to supply the arm. During exercise of the affected arm, increased demand causes further "stealing" of blood from the posterior cerebral circulation, producing vertebrobasilar symptoms such as vertigo, diplopia, and syncope. Recurrent laryngeal nerve injury is a significant complication of thyroid surgery, parathyroid surgery, carotid endarterectomy, and thoracic procedures. The left nerve is more vulnerable due to its longer course. Unilateral injury causes hoarseness due to vocal cord paralysis in the paramedian position. Bilateral injury causes both cords to remain in the paramedian position, producing stridor and potentially life-threatening airway obstruction that may require tracheostomy. Horner syndrome may be caused by lesions anywhere along the sympathetic pathway from the hypothalamus to the eye. First-order neuron lesions (central) occur with brainstem strokes or syringomyelia. Second-order neuron lesions (preganglionic) occur with apical lung tumors (Pancoast tumor), lower brachial plexus injuries, or trauma to the neck. Third-order neuron lesions (postganglionic) occur with carotid artery dissection, cluster headaches, or lesions in the cavernous sinus or orbit. Brachial plexus injuries produce characteristic patterns depending on which elements are affected. Upper plexus injuries (Erb-Duchenne palsy, C5-C6) result from shoulder depression with lateral neck flexion, as in birth trauma or falls on the shoulder. Lower plexus injuries (Klumpke palsy, C8-T1) result from arm abduction, as in grasping during a fall or breech delivery. Complete brachial plexus injury, as may occur with severe traction injuries, results in a flail arm. Panel A: Carotid endarterectomy showing plaque removal at the bifurcation with hypoglossal and vagus nerves at risk. Panel B: Subclavian steal syndrome diagram showing proximal stenosis and retrograde vertebral flow pattern. Panel C: Comparison of Erb-Duchenne palsy (waiter's tip, upper trunk injury) and Klumpke palsy (claw hand, lower trunk injury) with causative mechanisms illustrated. Panel D: Horner syndrome showing asymmetric pupils (miosis) and ptosis on the affected side. --- ## Summary The common carotid artery bifurcates at C4 into the external carotid (eight branches supplying extracranial head and neck) and the internal carotid (no branches in the neck; supplies brain and orbit). The subclavian artery is divided by the anterior scalene into three parts, with the vertebral artery, thyrocervical trunk, and internal thoracic arising from the first part. The vertebral arteries ascend through the transverse foramina to form the basilar artery. The internal jugular vein lies within the carotid sheath lateral to the carotid arteries and is a common site for central venous access. The external jugular vein crosses the sternocleidomastoid superficially and is a clinical indicator of central venous pressure. The cervical plexus (C1-C4) provides cutaneous branches to the neck and shoulder and motor branches including the ansa cervicalis and contribution to the phrenic nerve. The phrenic nerve (C3-C5) is the sole motor supply to the diaphragm and descends on the anterior scalene before entering the thorax. The vagus nerve descends in the carotid sheath, giving pharyngeal, superior laryngeal, and recurrent laryngeal branches. The brachial plexus roots emerge between the scalene muscles and form trunks in the posterior triangle. The cervical sympathetic trunk lies posterior to the carotid sheath, and its interruption produces Horner syndrome. --- ## Key Terms | Term | Definition | |------|------------| | Carotid sinus | Baroreceptor-containing dilation at the proximal internal carotid artery | | Thyrocervical trunk | Branch of the first part of the subclavian artery giving the inferior thyroid, suprascapular, and transverse cervical arteries | | Ansa cervicalis | Nerve loop from C1-C3 on the carotid sheath supplying the infrahyoid (strap) muscles | | Phrenic nerve | Nerve from C3-C5 providing sole motor innervation to the diaphragm | | Stellate ganglion | Fusion of the inferior cervical and first thoracic sympathetic ganglia | | Erb's point | Point on the posterior border of the sternocleidomastoid where the spinal accessory nerve and cervical plexus cutaneous branches emerge | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*