The neck is the anatomical region of the human body that connects the head to the torso, situated between the mandible superiorly and the clavicle inferiorly, serving as a conduit for vital neurovascular structures, musculoskeletal support, and physiological functions such as swallowing and respiration.^[1] The skeletal framework of the neck primarily consists of the seven cervical vertebrae (C1 to C7), which form the cervical spine and provide structural support for the head's weight of approximately 10-13 pounds while allowing a wide range of motion including flexion, extension, rotation, and lateral bending.^[2] The uppermost vertebrae, C1 (atlas) and C2 (axis), are specialized: the atlas is ring-shaped to articulate with the skull, and the axis features a dens projection enabling rotational "no" movements of the head.^[2] Additional bony elements include the hyoid bone, a U-shaped structure suspended in the anterior neck that anchors muscles involved in swallowing and speech, and the clavicles at the base.^[1] Musculature in the neck comprises about 30 skeletal muscles, divided into anterior, lateral, and posterior groups, which stabilize the head and neck, facilitate movements, and aid in secondary functions like breathing and mastication.^[3] Key superficial muscles include the sternocleidomastoid, which rotates and flexes the head, and the trapezius, which elevates and retracts the scapula; deeper layers encompass suprahyoid muscles (e.g., digastric for jaw elevation) and infrahyoid muscles (e.g., sternohyoid for laryngeal depression during swallowing).^[1][3] These muscles are enveloped by layers of cervical fascia, including superficial, middle, and deep components, which compartmentalize structures and provide pathways for infection spread or surgical access.^[1] The neck also contains critical neurovascular elements within the carotid sheath, such as the common carotid arteries supplying oxygenated blood to the brain and face, and the internal and external jugular veins draining deoxygenated blood.^[1] Nerve supply arises from the cervical plexus (C1-C4) for sensory and motor innervation of the skin and muscles, the brachial plexus (C5-T1) extending to the upper limbs, and accessory nerves like the vagus (cranial nerve X) and spinal accessory (XI) for visceral and motor functions.^[1] The phrenic nerve (C3-C5) innervates the diaphragm, underscoring the neck's role in respiration.^[1] Functionally, the neck enables head mobility essential for vision, balance, and communication, while protecting the spinal cord, esophagus, trachea, and thyroid gland within its confines.^[2][1] Clinically, its complex anatomy makes it susceptible to trauma, such as whiplash or vascular dissections, and serves as a focal point for procedures like thyroidectomy or lymph node biopsies.^[1]

Anatomy

Bones and joints

The skeletal framework of the neck is primarily composed of the seven cervical vertebrae, designated C1 through C7, which form the cervical spine and provide structural support for the head while facilitating a wide range of motion. These vertebrae are the smallest and most mobile in the vertebral column, characterized by small, oval-shaped vertebral bodies, bifid spinous processes (except for C7), and transverse foramina that transmit the vertebral arteries and veins. Typical cervical vertebrae (C3–C6) feature uncinate processes on the lateral aspects of their bodies, which articulate with adjacent vertebrae to form uncovertebral joints that enhance lateral stability and limit excessive translation. The seventh cervical vertebra, known as the vertebra prominens, is distinguished by its longer, non-bifid spinous process, which serves as a palpable landmark at the base of the neck.^[4][5] The first two cervical vertebrae, C1 (atlas) and C2 (axis), are atypical in structure to accommodate the skull and enable pivotal head movements. The atlas lacks a vertebral body and spinous process, instead forming a ring-like structure with anterior and posterior arches and robust lateral masses that bear the weight of the head; its superior articular facets are concave to articulate with the occipital condyles of the skull. The axis features a prominent odontoid process (dens) projecting superiorly from its body, which acts as a pivot for rotation and is a remnant of the atlas's embryological vertebral body. These adaptations allow the upper cervical spine to support the cranium while permitting greater mobility than lower segments.^[4][5] The primary articulations involving these vertebrae include the atlanto-occipital and atlanto-axial joints, both synovial in nature and crucial for head movement. The atlanto-occipital joint is a paired ellipsoid (condyloid) synovial joint between the occipital condyles and the superior facets of the atlas, allowing primarily flexion and extension (nodding motions) with approximately 15–20 degrees of total range, along with limited lateral flexion of 5–8 degrees. This joint is stabilized by a loose articular capsule and lacks an intervertebral disc, with hyaline cartilage covering the articular surfaces. The atlanto-axial joint comprises three synovial articulations: two lateral plane synovial joints between the inferior facets of the atlas and superior facets of the axis, and a median pivot synovial joint between the dens and the anterior arch of the atlas, secured by the transverse ligament. This configuration permits primarily rotation, contributing about 40–50 degrees of axial rotation (half of the cervical spine's total rotational capacity), with minimal flexion or extension.^[6][7][5] Stability of the cervical vertebrae and joints is maintained by several key ligaments that resist excessive motion and protect the spinal cord. The anterior longitudinal ligament runs along the anterior surfaces of the vertebral bodies from the occipital bone to the thoracic spine, limiting hyperextension; the posterior longitudinal ligament courses along the posterior aspects of the bodies within the vertebral canal, preventing hyperflexion. The ligamentum flavum, an elastic band connecting adjacent laminae, facilitates return to neutral position after flexion and resists separation of the laminae. Interspinous ligaments connect the spinous processes, further limiting flexion between vertebrae. These ligaments collectively provide tensile strength and proprioceptive feedback, with the upper cervical ligaments (e.g., those at the atlanto-axial level) bearing significant load during rotation.^[5][8] A unique component of the neck's skeletal structure is the hyoid bone, a small, U-shaped or horseshoe-shaped solitary bone located in the anterior midline at the level of the third cervical vertebra, inferior to the mandible and superior to the thyroid cartilage. It consists of a central body and paired greater and lesser horns extending posteriorly, and unlike other bones, it does not articulate directly with any others but is suspended by ligaments and muscles to anchor the larynx and facilitate swallowing and speech. This floating nature allows flexibility in neck movements while maintaining airway patency.^[9][10]

Muscles and triangles

The muscles of the neck are broadly classified into anterior, lateral, and posterior groups, each contributing to distinct movements of the head and shoulders. The anterior group includes the suprahyoid and infrahyoid muscles, as well as the sternocleidomastoid and scalene muscles, which primarily facilitate flexion and lateral bending of the neck. The lateral group comprises the trapezius and levator scapulae, aiding in elevation and rotation of the scapula. Posterior muscles, such as the splenius capitis and cervicis, support extension and rotation of the head. The neck is anatomically divided into anterior and posterior triangles by the sternocleidomastoid muscle, which serves as the posterior boundary of the anterior triangle and the anterior boundary of the posterior triangle. The anterior triangle, located superficially on the front of the neck, is bounded superiorly by the inferior border of the mandible and laterally by the anterior border of the sternocleidomastoid; it contains subdivisions like the submandibular triangle, which houses the submandibular gland and lymph nodes. The posterior triangle, situated behind the sternocleidomastoid, is delimited anteriorly by the posterior edge of the sternocleidomastoid, posteriorly by the anterior border of the trapezius, and inferiorly by the middle third of the clavicle; its contents include the accessory nerve, cervical lymph nodes, and portions of the brachial plexus. Innervation of the neck muscles arises from the cervical plexus (C1-C4) and brachial plexus (C5-T1), with specific branches for each group; for instance, the sternocleidomastoid receives motor supply from the accessory nerve (cranial nerve XI) and proprioceptive fibers from C2-C3 via the cervical plexus. The scalene muscles are innervated by the anterior rami of C3-C8, while the trapezius is primarily supplied by the accessory nerve, with additional sensory input from C3-C4. Posterior muscles like the splenius capitis and cervicis are innervated by the dorsal rami of the cervical spinal nerves (C2-C6). Blood supply to these muscles is provided by branches of the subclavian and external carotid arteries; the sternocleidomastoid is nourished by the sternocleidomastoid branch of the superior thyroid artery and the transverse cervical artery, while the scalenes receive supply from the ascending cervical artery. The trapezius is vascularized by the transverse cervical and dorsal scapular arteries. Biomechanically, the sternocleidomastoid muscle plays a pivotal role in neck rotation, contributing to up to 80 degrees of contralateral rotation when acting with other muscles, and contributes to approximately 20-30 degrees of ipsilateral lateral flexion, aiding in the total range of about 45 degrees; its bilateral contraction supports flexion against gravity. The scalene muscles assist in elevating the first and second ribs during inspiration, serving as accessory muscles in forced inhalation, and facilitate lateral tilting of the neck by approximately 30 degrees. Posterior muscles like the splenius capitis generate extension torques essential for maintaining upright posture.

Fascia and compartments

The superficial fascia of the neck consists of a layer of subcutaneous connective tissue that lies immediately beneath the skin and invests the platysma muscle, a thin sheet-like muscle spanning from the mandible to the clavicle and facilitating subtle skin movements.^[11] Beneath the superficial fascia lies the deep cervical fascia, a robust connective tissue framework divided into three primary layers that encase and compartmentalize the neck's structures: the investing layer, the pretracheal layer, and the prevertebral layer. The investing layer, also known as the superficial layer of the deep cervical fascia, forms a tubular sheath surrounding the entire neck; it attaches superiorly to the mandible, zygomatic arches, mastoid processes, and superior nuchal line, and inferiorly to the manubrium, clavicles, and scapulae, while posteriorly it adheres to the ligamentum nuchae and vertebral spines, effectively enclosing muscles such as the sternocleidomastoid and trapezius.^[11] The pretracheal layer, or middle layer, subdivides into muscular and visceral divisions; the muscular division envelops the infrahyoid strap muscles and attaches superiorly to the hyoid bone and thyroid cartilage, extending inferiorly to blend with the clavipectoral fascia, whereas the visceral division surrounds the thyroid and parathyroid glands, trachea, and esophagus, attaching superiorly to the thyroid cartilage and continuing inferiorly as the fibrous pericardium, with a posterior buccopharyngeal fascia segment.^[11] The prevertebral layer, the deepest of the three, covers the vertebral column and prevertebral muscles like the longus colli and scalenes; it attaches superiorly to the base of the skull and transverse processes, and inferiorly to the anterior longitudinal ligament and suprapleural membrane, extending laterally into the axillary sheath.^[11] These fascial layers delineate four principal compartments within the neck, organizing vital structures and influencing pathological processes. The vertebral compartment, bounded by the prevertebral fascia, contains the cervical spine, spinal cord, and associated prevertebral muscles, providing structural support and protection.^[11] The visceral compartment, enclosed by the visceral division of the pretracheal fascia, houses the pharynx, larynx, thyroid gland, trachea, and esophagus, facilitating swallowing and respiration.^[11] The carotid compartments, formed by the carotid sheath—a condensation of fibers from all three deep fascial layers—encase the common and internal carotid arteries, internal jugular vein, and vagus nerve on each side, safeguarding these neurovascular elements.^[11] The retropharyngeal compartment, situated between the buccopharyngeal fascia (anteriorly) and the alar fascia (a thin division of the prevertebral layer posteriorly), contains loose areolar tissue and lymph nodes, extending from the skull base to approximately the T1-T2 level.^[11] The fascial planes and compartments play a critical role in the clinical context, particularly in containing or facilitating the spread of infections, as breaches in these barriers can lead to life-threatening complications. For instance, infections originating in the pharynx or tonsils may penetrate the buccopharyngeal fascia and enter the retropharyngeal space, allowing pus to track inferiorly through loose connective tissue into the superior mediastinum, potentially causing mediastinitis or airway obstruction.^[12] Similarly, deeper extensions into the "danger space"—the region between the alar and prevertebral fasciae—enable rapid dissemination to the posterior mediastinum and even the diaphragm due to its continuity and areolar composition, resulting in necrotizing infections, sepsis, or empyema with mortality rates ranging from 1% to 25% even with treatment, and higher if untreated.^[12][13] These pathways underscore the importance of fascial anatomy in guiding surgical drainage and antibiotic therapy for deep neck infections.^[12]

Nervous supply

The nervous supply of the neck encompasses several key neural structures that provide sensory, motor, and autonomic innervation to the region. The cervical plexus, formed by the anterior rami of spinal nerves C1 through C4, emerges from the intervertebral foramina and lies deep to the sternocleidomastoid muscle before branching into superficial and deep components.^[14] The superficial branches, known as the cutaneous branches, include the lesser occipital nerve (C2), which supplies sensation to the scalp posterior to the auricle; the great auricular nerve (C2-C3), innervating the skin over the parotid gland and angle of the mandible; the transverse cervical nerve (C2-C3), providing sensory input to the anterior and lateral neck; and the supraclavicular nerves (C3-C4), distributing to the skin of the upper chest and shoulder.^[15] Deep branches of the cervical plexus include the ansa cervicalis, a loop formed by the superior root (C1) and inferior root (C2-C3), which provides motor innervation to the infrahyoid muscles such as the omohyoid, sternohyoid, and sternothyroid.^[14] The brachial plexus, originating from the anterior rami of spinal nerves C5 through T1, traverses the posterior triangle of the neck where its roots emerge between the scalene anterior and medius muscles, unite to form three trunks (upper from C5-C6, middle from C7, lower from C8-T1), and then divide into anterior and posterior divisions before passing under the clavicle. This configuration positions the brachial plexus superficially in the posterior triangle, making it vulnerable to trauma; for instance, injury to the upper trunk (C5-C6) during birth or shoulder dystocia can result in Erb's palsy, characterized by paralysis of the deltoid, biceps, and supraspinatus muscles, leading to a "waiter's tip" posture of the arm.^[16] The roots also give rise to minor branches in the neck, such as the dorsal scapular nerve (C5) to the rhomboids and the long thoracic nerve (C5-C7) to the serratus anterior.^[17] Several cranial nerves course through the neck to innervate structures therein. The vagus nerve (cranial nerve X) descends within the carotid sheath, providing parasympathetic innervation to the pharynx, larynx, and thoracic and abdominal viscera, while its recurrent laryngeal branch supplies motor fibers to most intrinsic laryngeal muscles.^[14] The hypoglossal nerve (cranial nerve XII) exits the skull via the hypoglossal canal and travels in the neck to enter the tongue, delivering motor innervation to the intrinsic and extrinsic tongue muscles for movements essential to speech and swallowing.^[18] The accessory nerve (cranial nerve XI), comprising cranial and spinal roots, passes through the posterior triangle to innervate the sternocleidomastoid and trapezius muscles, facilitating head rotation and shoulder elevation.^[14] The sympathetic nervous system in the neck is mediated by the cervical portion of the sympathetic chain, a bilateral paravertebral chain of ganglia receiving preganglionic fibers from thoracic spinal levels T1-T5 via white rami communicantes.^[19] This chain includes three main ganglia: the superior cervical ganglion, located at the C1-C2 level anterior to the transverse processes, which sends postganglionic fibers to the dilator pupillae, superior tarsal muscle, and sweat glands of the head and neck; the middle cervical ganglion, at the C6 level near the inferior thyroid artery, contributing to cardiac and pharyngeal plexuses; and the inferior cervical ganglion (often fused with the first thoracic ganglion to form the stellate ganglion at C7-T1), which innervates the heart, lungs, and blood vessels of the upper limb.^[19] These ganglia provide sympathetic innervation to vascular smooth muscle, glands, and piloerector muscles throughout the head and neck.^[14] Within the vertebral compartment, the spinal cord's cervical segments (C1 through C8) occupy the upper neck, extending from the foramen magnum to approximately the C7 vertebral level, where the cord tapers into the conus medullaris lower in the spine.^[20] These segments give rise to the anterior and posterior roots that form the cervical spinal nerves, with the C1-C7 nerves exiting above their corresponding vertebrae and C8 below C7.^[20] The spinal cord in this region is enveloped by three meninges: the outermost dura mater, forming a tough dural sac; the middle arachnoid mater, creating a subarachnoid space filled with cerebrospinal fluid; and the innermost pia mater, adhering closely to the cord surface and extending into the anterior median fissure.^[20] This meningeal covering protects the cord and facilitates nutrient exchange via the cerebrospinal fluid.^[21]

Vascular and lymphatic supply

The arterial supply to the neck primarily arises from branches of the common carotid arteries and the subclavian arteries. The common carotid arteries, one on each side, ascend within the neck and bifurcate at the level of the upper border of the thyroid cartilage into the internal and external carotid arteries; the internal carotid supplies the brain and anterior neck structures, while the external carotid provides blood to the face, scalp, and superficial neck regions.^[22] The vertebral arteries, originating from the first part of the subclavian arteries, enter the neck by passing through the transverse foramina of the cervical vertebrae from C6 to C1, ultimately forming the basilar artery to supply the posterior brain and spinal cord.^[23] Additionally, the thyrocervical trunk, a short branch of the subclavian artery, gives rise to the inferior thyroid, suprascapular, transverse cervical, and ascending cervical arteries, which supply the thyroid gland, scapular region, and deep neck muscles.^[24] Venous drainage of the neck occurs mainly through the jugular and subclavian veins, forming a network that parallels the arterial supply. The internal jugular veins, paired structures lateral to the common carotid arteries, collect blood from the brain, face, and deep neck via tributaries such as the superior and inferior petrosal sinuses, and drain into the brachiocephalic veins; they are the primary conduits for cranial venous return.^[25] The external jugular veins, formed by the union of the posterior auricular and retromandibular veins, drain the superficial scalp and face, emptying into the subclavian veins.^[26] These veins converge with the subclavian veins to form the brachiocephalic veins, while venous plexuses around the cervical vertebrae, including the internal vertebral plexus, provide drainage for the spinal cord and deep neck structures, interconnecting with the external vertebral plexus.^[27] The lymphatic system of the neck consists of an extensive network of nodes and vessels that drain lymph from the head, neck, and upper thorax. Cervical lymph nodes are organized into chains, including the superficial and deep jugular chains along the internal jugular vein, which receive lymph from the scalp, face, oral cavity, and pharynx; the supraclavicular nodes, located above the clavicle, drain the lower neck, upper chest, and lungs.^[28] Lymph from the head and neck primarily flows through these nodes to the jugular lymphatic trunk on the left, which empties into the thoracic duct, and on the right, into the right lymphatic duct, both of which join the venous system at the junction of the internal jugular and subclavian veins.^[29] The carotid sheath, a condensation of deep cervical fascia, encases key neurovascular elements in the neck, integrating the common carotid artery (bifurcating superiorly into internal and external branches), the internal jugular vein, and the vagus nerve (cranial nerve X), which runs posteriorly between them; this arrangement facilitates efficient circulation and neural conduction.^[30] The common carotid pulse is palpable at the carotid tubercle (Chassaignac's tubercle) on the transverse process of C6, serving as a clinical landmark for assessing arterial flow.^[30] Anatomical variations in neck vasculature are common and can affect surgical planning. A notable example is the aberrant right subclavian artery (arteria lusoria), occurring in 0.5% to 1.8% of individuals, where the right subclavian artery arises directly from the descending aorta distal to the left subclavian, coursing retroesophageally behind the esophagus and potentially compressing adjacent structures.^[31]

Surface anatomy

The surface anatomy of the neck encompasses external features and palpable structures that serve as reliable guides for clinical assessment, procedural navigation, and correlation with deeper anatomy. These landmarks facilitate quick orientation during physical examinations, allowing practitioners to approximate underlying vertebral levels and soft tissue boundaries without invasive imaging. The neck's anterior and lateral surfaces are particularly accessible, with midline structures providing vertical references and lateral elements defining regional divisions. Prominent palpable landmarks include the thyroid cartilage, forming the laryngeal prominence or Adam's apple, situated at the C4-C5 vertebral level and serving as a key midline indicator. This structure is more pronounced in males due to a narrower 90-degree angle between its laminae compared to the 120-degree angle in females, a difference accentuated during puberty by testosterone-driven laryngeal growth. Inferior to it lies the cricoid cartilage at C6, the only complete ring of the larynx and a critical site for emergency airway access. Superiorly, the hyoid bone is palpable at C3, a U-shaped structure suspended by muscles and ligaments. At the neck's base, the clavicles form bilateral horizontal boundaries, while the sternocleidomastoid muscles create distinct edges running obliquely from the mastoid process to the sternum and clavicle, delineating the anterior and posterior cervical triangles. Pulse points are readily accessible for vascular assessment: the common carotid artery pulse is palpated medial to the sternocleidomastoid muscle at the level of the thyroid cartilage, reflecting central arterial pressure. The facial artery pulse emerges at the anterior border of the masseter muscle near the mandible's angle, supplying the face after ascending from the neck. Jugular venous distension is evaluated laterally along the sternocleidomastoid or in the supraclavicular fossa, with normal filling visible up to 3-4 cm above the sternal angle in semi-upright positions to gauge right atrial pressure. The skin overlying these features is thin (approximately 1-2 mm) and exhibits transverse creases that enhance mobility for head and neck movements, with subcutaneous tissue providing a loose, pliable layer that varies regionally—thinner anteriorly and thicker laterally. In radiographic imaging, surface landmarks align predictably with internal structures: on lateral cervical X-rays, the hyoid bone projects over C3, the thyroid cartilage over C4-C5, and the cricoid over C6, aiding in vertebral counting and anomaly detection. Computed tomography (CT) scans use these external references to standardize axial slices, where the thyroid cartilage's prominence orients midline views, and sternocleidomastoid edges help delineate compartmental boundaries in cross-sections. Sex-based variations, such as the more subtle laryngeal prominence in females, influence palpation ease and imaging interpretation, particularly in obese or pediatric patients where landmarks may be less distinct.

Function

Support and movement

The cervical spine serves as the primary load-bearing structure for the neck, supporting the weight of the head, which averages 4.5 to 5 kg in adults. This support is achieved through the seven cervical vertebrae and their intervertebral discs, with the natural lordotic curvature of the cervical region distributing compressive forces efficiently and maintaining upright balance. The lordosis acts as a shock absorber, aligning the head's center of gravity over the thoracic spine to minimize muscular effort during static postures.^[32][33] The neck enables extensive head movement through coordinated action of its synovial joints and musculature, providing a range of motion that includes approximately 50° of flexion, 80° of extension, 45° of lateral flexion to each side, and 80° of rotation to each side. These motions occur primarily at the atlanto-occipital and atlanto-axial joints superiorly, with contributions from the lower cervical facets, allowing for precise head orientation in space. Deep postural muscles, including the flexors such as the longus colli and capitis, and extensors like the semispinalis cervicis and multifidus, stabilize the cervical segments during dynamic activities such as standing and gait, preventing excessive sway and maintaining alignment. For instance, when controlling the head forward from overextension to neutral, posterior neck extensors (e.g., semispinalis capitis, splenius cervicis, upper trapezius, levator scapulae) perform eccentric contractions to resist gravity, while anterior flexors (e.g., sternocleidomastoid, deep neck flexors) perform concentric contractions to pull the head forward.^{[34][35][36][37]} Neck kinematics involve coupled movements to optimize efficiency and joint integrity, particularly at the atlanto-axial joint where rotation is inherently linked with contralateral lateral flexion. This coupling, driven by the joint's pivot morphology and ligamentous constraints, ensures that axial turning of the head (up to 50° at this level) is accompanied by side-bending away from the direction of rotation, enhancing overall mobility without isolated strain on any single plane.^[38]

Role in vital processes

The neck serves as a vital conduit for protecting the airway and esophagus, primarily through laryngeal structures such as the epiglottis and vocal folds, which seal the glottis during swallowing to prevent aspiration of boluses into the trachea.^[39] Pharyngeal muscles, including the superior, middle, and inferior constrictors innervated by the vagus nerve (cranial nerve X), contract sequentially to propel the bolus while elevating the hyoid bone and larynx, thereby maintaining airway closure via swallowing apnea—a brief cessation of respiration lasting 0.5–1.5 seconds.^[39] These mechanisms ensure that the aerodigestive tract's dual functions—respiration and digestion—do not interfere, with the upper esophageal sphincter (UES) relaxing under hyolaryngeal traction to direct contents safely to the esophagus.^[40] Swallowing, or deglutition, is coordinated across three phases, all reliant on neck structures for efficient bolus transport and airway safeguarding. The oral phase is voluntary, involving tongue propulsion of the prepared bolus from the mouth to the oropharynx.^[41] The pharyngeal phase, involuntary and lasting about 1 second, features hyoid elevation by suprahyoid muscles (e.g., geniohyoid, mylohyoid) that pulls the larynx upward and forward, inverting the epiglottis to cover the laryngeal inlet and close the vocal folds via adduction of the arytenoid cartilages.^[39] This elevation shortens the pharynx and opens the UES (typically 34–104 mm Hg resting pressure), allowing the bolus to pass at 20–40 cm/s while pharyngeal constrictors generate peristaltic waves.^[41] The esophageal phase follows with primary and secondary peristalsis propelling the bolus to the stomach at 3–4 cm/s through the lower esophageal sphincter.^[39] In speech production, the neck's larynx enables phonation through vibration of the vocal folds, which oscillate at 60–300 Hz under airflow from the lungs, generating fundamental frequencies that determine pitch.^[42] Resonance shapes this sound in the pharynx and larynx, where the vocal tract's configuration amplifies harmonics for timbre and clarity.^[43] Neck muscles, particularly the cricothyroid and thyroarytenoid, modulate pitch by adjusting vocal fold length, tension, and stiffness—elongation increases frequency for higher pitches, while contraction enhances medial compression for voice quality.^[43] The neck contributes to thermoregulation via its superficial skin vasculature and eccrine sweat glands, which promote heat loss when core temperature rises. Vasodilation of cutaneous vessels shunts warm blood to the surface for radiative and convective dissipation, while cholinergic sympathetic activation stimulates sweat secretion (up to 2–4 L/hour in heat stress), with evaporation accounting for about 22% of total heat loss (0.58 kcal per gram of water evaporated).^[44] In the neck specifically, countercurrent heat exchange between the carotid arteries and jugular veins cools arterial blood to the brain by up to 0.87°C during hyperthermia, augmented by radial conduction from superficial neck tissues.^[45]

Clinical significance

Neck pain and disorders

Neck pain is a prevalent musculoskeletal complaint, with a global age-standardized prevalence rate of 27.0 per 1000 population in 2019, affecting individuals across various age groups but increasing with age. As of 2020, the global age-standardized prevalence rate was 24.5 per 1000 population, with projections indicating a continuing upward trend. In 2021, there were approximately 206 million prevalent cases.^[46][47][48] Annual prevalence rates exceed 30% among adults, with point prevalence estimates ranging from 0.4% to 41.5% (mean: 14.4%) in the general population.^[49][50] Key risk factors include aging, which contributes to degenerative changes, and poor posture, often exacerbated by sedentary lifestyles or occupational demands such as prolonged computer use or repetitive motions.^[51][46] These factors can lead to chronic or recurrent episodes, significantly impacting quality of life and daily functioning. Musculoskeletal causes predominate in non-traumatic neck pain cases. Cervical spondylosis, a degenerative condition arising from age-related wear on intervertebral disks and facet joints, manifests as neck pain and stiffness, sometimes with narrowing of the spinal canal that compresses nerves.^[52][53] Whiplash-associated disorder, resulting from acceleration-deceleration injuries, commonly produces neck pain, reduced range of motion, and headaches, with about 50% of cases reporting persistent symptoms one year post-injury.^[54][55] Myofascial pain syndrome involves hypersensitive trigger points within neck muscles like the trapezius or sternocleidomastoid, causing localized deep aching pain that may worsen with activity or stress and restrict mobility.^[56][57] Systemic disorders can also underlie neck pain, often requiring differentiation from primary musculoskeletal issues. Subacute thyroiditis, an inflammatory condition of the thyroid gland typically following a viral infection, leads to anterior neck pain and tenderness that may radiate to the jaw or ears, accompanied by transient hyperthyroidism.^[58][59] Meningitis, inflammation of the brain and spinal cord's protective membranes, frequently causes severe neck stiffness and pain due to meningeal irritation, alongside systemic signs like fever and headache.^[60][61] Rheumatoid arthritis, an autoimmune disease, commonly affects the cervical spine through synovitis at the atlantoaxial joint, resulting in neck pain, instability, and potential neurological compromise in advanced cases.^[62][63] Symptoms of neck pain and associated disorders often include radiculopathy, where compressed cervical nerve roots produce sharp, radiating pain into the shoulders, arms, or hands, following a dermatomal pattern, along with possible numbness or weakness.^[64][65] Referred pain to adjacent areas like the occiput or scapula may occur without overt radicular features, complicating initial assessment. Diagnosis begins with a detailed history and physical examination, including evaluation of range of motion, sensory testing, and provocative maneuvers like the Spurling test for radiculopathy.^[66][67] Magnetic resonance imaging (MRI) serves as the gold standard for visualizing disc herniation, spinal stenosis, or soft tissue inflammation, guiding further management when symptoms persist beyond conservative measures.^[66][53]

Trauma and injuries

Trauma to the neck encompasses a range of acute injuries resulting from external forces, broadly classified into blunt, penetrating, and deceleration mechanisms. Blunt trauma often involves hyperextension or hyperflexion forces, such as those occurring in motor vehicle accidents, leading to soft tissue damage and skeletal disruption without skin penetration.^[68] Penetrating trauma, exemplified by stab wounds or gunshot injuries to the carotid artery, directly violates the skin and underlying structures, potentially causing immediate vascular or aerodigestive compromise.^[69] Deceleration injuries, common in high-speed collisions or falls, produce shearing forces across the cervical spine and vasculature due to rapid changes in momentum.^[70] Spinal trauma in the neck primarily affects the cervical vertebrae, with fractures like the hangman's fracture at C2 arising from hyperextension and axial loading, often seen in motor vehicle accidents or judicial hangings, resulting in bilateral pars interarticularis disruption and potential C2-C3 subluxation.^[71] These injuries can extend to the spinal cord, where damage at specific levels determines neurological deficits; for instance, a C5-level injury typically causes quadriplegia by interrupting motor pathways to the upper and lower extremities while sparing some diaphragmatic function.^[68] Cervical spinal cord injuries at C3-C5 levels further risk phrenic nerve involvement, leading to respiratory compromise due to impaired diaphragmatic innervation.^[72] Vascular trauma to the neck poses risks of hemorrhage, ischemia, or embolism, with carotid artery dissection frequently resulting from blunt mechanisms like rapid neck rotation in accidents, creating an intimal tear that propagates and may embolize to cause ischemic stroke.^[69] Laceration of the jugular vein, more common in penetrating injuries such as knife wounds, can lead to significant blood loss or air embolism if atmospheric air enters the venous system during injury.^[73] These vascular disruptions highlight the neck's vulnerability, as the carotid and jugular structures lie superficially within fascial planes, amplifying the potential for rapid decompensation.^[74] Soft tissue injuries from neck trauma include muscle strains and ligament sprains, often induced by whiplash in blunt deceleration events, where sudden hyperextension stretches paraspinal muscles like the trapezius or sternocleidomastoid and ligaments such as the anterior longitudinal ligament, causing microtears and inflammation.^[75] In severe cases, trauma-induced swelling within the confined fascial compartments of the neck can precipitate compartment syndrome, elevating intracompartmental pressure and compromising neurovascular structures, though this is less common than in extremities.^[76] Immediate effects of these soft tissue injuries manifest as pain, reduced range of motion, and potential airway obstruction from hematoma expansion.^[73]

Diagnostic and therapeutic approaches

Diagnosis of neck conditions typically begins with a thorough physical examination to identify signs of radiculopathy, myelopathy, or other disorders. The Spurling's test, involving neck extension, lateral bending, and axial compression, is a provocative maneuver used to assess for cervical radiculopathy by reproducing radicular symptoms, demonstrating high specificity (up to 93%) though moderate sensitivity (around 30-50%).^[77] Other physical tests, such as the shoulder abduction test or Valsalva maneuver, may complement this evaluation to suggest nerve root compression.^[78] Imaging modalities play a central role in confirming diagnoses and delineating pathology. Magnetic resonance imaging (MRI) is the preferred initial study for evaluating soft tissue structures, including discs, spinal cord, and neural elements, offering superior sensitivity for detecting herniations or stenosis without radiation exposure.^[79] Computed tomography (CT) angiography is utilized to assess vascular anomalies or compressions in the neck, particularly for vertebral artery evaluation in trauma or degenerative cases.^[80] Electromyography (EMG) and nerve conduction studies provide functional assessment of nerve integrity, aiding in the differentiation of radiculopathy from peripheral neuropathies by identifying denervation patterns.^[81] Therapeutic approaches for neck conditions are stratified by severity, starting with conservative measures. Physiotherapy, including strengthening exercises targeting key neck muscles such as deep flexors (e.g., longus colli), extensors (e.g., semispinalis cervicis), sternocleidomastoid, trapezius, and scalenes, and range-of-motion improvement, combined with nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen, forms the first-line management for acute or chronic neck pain, often providing relief in 70-80% of uncomplicated cases within weeks.^[82][83] For persistent radicular symptoms, interventional procedures such as cervical epidural steroid injections deliver corticosteroids to inflamed nerve roots, yielding short-term pain reduction in up to 60% of patients with disc herniation.^[84] Surgical interventions are reserved for refractory cases; anterior cervical discectomy and fusion (ACDF) removes herniated disc material and stabilizes the spine with grafting, achieving fusion rates of 90-95% and significant symptom improvement in degenerative disease.^[85] Thyroidectomy, involving partial or total gland removal via a low cervical incision, is indicated for nodules or cancer, with modern techniques minimizing tissue disruption.^[86] Recent advances have enhanced precision and reduced invasiveness in neck interventions. Robotic-assisted thyroidectomy, utilizing systems like da Vinci for remote-access approaches, offers superior cosmesis and comparable oncologic outcomes to open surgery, with complication rates under 5% in experienced centers post-2023 implementations.^[87] An AI algorithm reduced the median report turnaround time for CT cervical spine exams from 225.7 minutes to 99 minutes (a 56% reduction), as presented at RSNA 2024.^[88] Minimally invasive endoscopic techniques, such as transoral or gasless approaches for thyroidectomy, limit incisions to 2-3 cm, decreasing postoperative pain and recovery time compared to conventional methods.^[89] Complications from these approaches require vigilant monitoring. In thyroidectomy, recurrent laryngeal nerve palsy occurs in 1-3% of cases, potentially causing hoarseness or airway issues due to nerve traction or transection during dissection.^[90] ACDF may lead to dysphagia (up to 20% temporarily) or adjacent segment degeneration over years, while epidural injections carry risks of infection or dural puncture in less than 1% of procedures.^[91][92]

Anthropometry and variations

Neck circumference

Neck circumference is measured using a non-stretchable tape placed horizontally around the neck, just below the laryngeal prominence and midway between the midcervical spine and midanterior neck, perpendicular to the neck's long axis, with the subject standing upright, shoulders relaxed, and head in a neutral position.^[93][94] This anthropometric technique follows standardized protocols for reliability and reproducibility in clinical and research settings.^[95] Normal ranges for neck circumference in adults typically average 35–40 cm, with variations by sex, age, and ethnicity. Men generally have larger measurements than women, with medians around 38–41 cm for men and 33–35 cm for women at a BMI of 25 kg/m² across age groups from 35–74 years.^[96] More specifically, data from the ELSA-Brasil study provide median neck circumference values for adult men at various BMI levels: approximately 35.7 cm (14.1 in) at BMI 20 kg/m², 37.2 cm (14.6 in) at BMI 22.5 kg/m², 38.7 cm (15.2 in) at BMI 25 kg/m², 40.0 cm (15.7 in) at BMI 27.5 kg/m², 41.2 cm (16.2 in) at BMI 30 kg/m², and 43.2 cm (17.0 in) at BMI 35 kg/m². These are medians from percentile tables derived from population surveys; actual values vary by population, age group, and other factors.^[96] Fit or athletic individuals (e.g., in military or athletic populations) typically have smaller neck circumferences (around 37.9–38 cm or 14.9–15 in) than the general population at similar BMI, attributed to lower body fat percentages and differences in muscle distribution.^[97] Some men have narrower necks primarily due to genetic factors, including sex-specific variants near the NOG gene (such as rs227724) that influence neck circumference independently of BMI or generalized adiposity.^[98] Additional contributing factors include naturally lower muscle mass in the neck and trapezius muscles, ectomorphic body types with thinner frames, and low body fat percentage, which reduce neck girth. These represent normal anatomical variations rather than medical conditions, though targeted strength training can somewhat increase neck girth. For example, U.S. NHANES data from 2007–2010 indicate a mean neck circumference of approximately 34.6 cm for women aged 60–69 years (with average height in this group close to 160 cm), though standard anthropometric data do not provide values for precise combinations of exact age and height, as neck circumference correlates more strongly with body weight and BMI than with height. In multi-ethnic populations, such as those including European, African, and Asian ancestries, averages differ slightly; for instance, European adults show means of approximately 40.9 cm in men and 34.8 cm in women, while Asian cohorts often report lower values around 37–38 cm in men due to body size differences.^[99][98] Age has minimal impact on median values within adulthood, though interquartile ranges widen slightly with higher BMI.^[96] In overweight and obese adolescents, mean neck circumference in boys is generally lower than adult norms, typically ranging from 33 to 35 cm depending on age, population, and study. For example, a 2017 cross-sectional study of Indian adolescents aged 13-17 years reported a mean of 33.43 ± 2.3 cm for overweight/obese boys, while a 2018 study on school-going adolescents aged 11-18 years in South India reported 35.42 ± 2.26 cm for overweight/obese boys.^[100][101] Elevated neck circumference correlates strongly with central obesity and serves as a screening tool for metabolic syndrome, with cutoff values of ≥37 cm for men and ≥34 cm for women indicating overweight (BMI ≥25 kg/m²) and ≥39.5 cm for men and ≥36.5 cm for women indicating obesity (BMI ≥30 kg/m²).^[93] It is associated with increased cardiometabolic risks, including a pooled odds ratio of 2.17 (95% CI: 1.30–3.62) for type 2 diabetes mellitus in individuals with greater neck circumference compared to those with smaller measurements, independent of BMI.^[102] Specifically, each 1-cm increase in neck circumference raises the odds of type 2 diabetes by 16% (OR 1.16, 95% CI: 1.07–1.27).^[102] Recent studies highlight neck circumference as a predictor of disease severity in specific contexts. In hospitalized COVID-19 patients with respiratory failure, each 1-cm increase was linked to a 26% higher mortality risk (adjusted HR 1.26, 95% CI: 1.11–1.43) at 30 and 60 days, with "large neck" phenotypes (≥44 cm in men, ≥40 cm in women) showing over twofold mortality odds after adjusting for age, BMI, and comorbidities.^[94] Additionally, neck circumference exceeding 40.6 cm in women or 43.2 cm in men (equivalent to >16 inches in women or >17 inches in men) is a key risk factor for obstructive sleep apnea, as excess neck fat narrows the airway and promotes obstruction during sleep.^[103]

Anatomical and ethnic variations

The human neck exhibits considerable anatomical variation among individuals and populations, influenced by genetic, developmental, and environmental factors. Structural anomalies such as cervical ribs, which are supernumerary ribs arising from the seventh cervical vertebra, occur in approximately 0.5% to 1% of the general population, with prevalence rates ranging from 0.58% in certain African cohorts to up to 6.2% in some Turkish groups; these are more common in females (1.09% vs. 0.42% in males).^{[104][105][106]} Vertebral artery anomalies, including aberrant origins or courses through the cervical transverse foramina, affect up to 10% of individuals in some studies, with abnormal origins reported at 0.18% and higher incidences (up to 25.9%) in those with associated bony abnormalities; these variations can alter vascular pathways and increase procedural risks.^{[107][108][109]} Ethnic differences manifest in cervical spine dimensions, with African American individuals showing significantly wider, more elongated, and taller vertebral bodies and foramina compared to Caucasian Americans at levels C3-C7, potentially influencing load distribution and surgical planning.^[110] Asian populations, such as Chinese cohorts, exhibit smaller subaxial cervical endplates than White counterparts, which may contribute to mismatches in prosthetic sizing during disc arthroplasty and elevate complication risks in spine surgery.^[111] Additionally, congenital cervical stenosis prevalence is higher in Black and Asian patients, who demonstrate smaller spinal canal diameters (e.g., 1.5-2.5 mm narrower than in Whites) and larger lordotic angles, predisposing them to neurological vulnerabilities.^[112] Polynesian and Māori groups display narrower anteroposterior cervical canal diameters (approximately 1.5-2.5 mm less than New Zealand Europeans), affecting canal patency assessments in clinical evaluations.^[113] Age and sex contribute to further variations in neck anatomy. Males typically have larger neck circumferences and widths (20-24% greater than females), partly due to laryngeal prominence from androgen-driven growth during puberty, alongside broader vertebral geometries and greater muscle mass in posterior cervical structures.^[114][115] Females, conversely, present with smaller overall head and neck anthropometrics, thinner facet joint cartilage, and relatively narrower spinal canals, influencing biomechanical responses to loads.^[115][116] With advancing age, degenerative changes reduce cervical mobility, with elderly individuals showing 12% less flexion, 32% less extension, and 22% less lateral bending compared to younger adults, attributable to disc dehydration, facet arthrosis, and ligamentous stiffening.^[117] Genetic factors underlie certain congenital variations, such as Klippel-Feil syndrome, characterized by fused cervical vertebrae and a short neck, with an incidence of approximately 1 in 40,000 to 42,000 live births and a female predominance (60% of cases); this results from disruptions in somitogenesis genes like GDF6 or GDF3, leading to segmental fusion in 0.0058% to 0.71% of screened populations.^{[118][119][120]} These anomalies highlight the need for preoperative imaging to tailor interventions across diverse anatomical profiles.

Comparative anatomy

In non-human animals

In vertebrates, neck structures exhibit diverse adaptations tailored to ecological niches. Giraffes (Giraffa camelopardalis) possess seven elongated cervical vertebrae, enabling an extended neck reach of up to 2 meters to access high foliage for browsing, while providing flexibility for foraging and social behaviors such as male combat.^[121][122] In contrast, true seals (Phocidae), like harbor seals (Phoca vitulina), have shortened necks with seven shortened cervical vertebrae, contributing to a streamlined body form that minimizes drag during aquatic propulsion and diving, where they can reach depths exceeding 100 meters.^[123][124] Birds demonstrate remarkable cervical elongation, with most species featuring 13 to 25 vertebrae—far exceeding the seven in mammals—to support precise movements. Swans (Cygnus spp.), for instance, can have up to 23 cervical vertebrae, allowing extensive neck flexion for preening feathers across their bodies and probing aquatic environments for food during foraging.^[125][126] This heterogenous vertebral morphology facilitates a wide range of motion, essential for behaviors like grooming and capturing prey, while maintaining stability during flight.^[127] Among mammals, predatory felids such as lions (Panthera leo) and tigers (Panthera tigris) have robust, muscular necks reinforced by powerful sternocleidomastoid and trapezius muscles, which enable them to deliver lethal bites to the throat or neck of large prey, securing kills through suffocation or vascular damage.^[128][129] Primates adapted to arboreal lifestyles, including spider monkeys (Ateles spp.), exhibit enhanced cervical flexibility through shallow uncinate processes on vertebrae, permitting greater lateral and rotational motion to navigate complex forest canopies and maintain visual orientation during brachiation and suspension.^[130] Non-vertebrate animals feature analogous neck-like regions for mobility and sensory functions. In insects like blister beetles (Epicauta spp.), the prothorax serves as a constricted "neck" segment between the head and thorax, allowing independent head movement for detecting prey or mates while protecting vital structures during locomotion.^[131] In mollusks, cephalopods such as squids (Loligo spp.) possess a flexible neck region connecting the head to the mantle, incorporating the muscular siphon—a tubular structure that expels water for jet propulsion and manipulates the environment during hunting or escape.^[132]

Evolutionary aspects

The evolution of the neck in vertebrates marks a fundamental transition from the head-body fusion characteristic of early chordates and fish to a distinct, mobile cervical region that enhanced sensory perception, feeding efficiency, and locomotion. This innovation emerged in early tetrapods during the Late Devonian period, approximately 375 million years ago, as evidenced by fossils like Tiktaalik roseae, which exhibit the first clear separation of the skull from the shoulder girdle via a short neck comprising the atlas and axis vertebrae. This structural shift supported the weight of emerging limbs and allowed independent head movement, crucial for navigating shallow aquatic environments and eventual terrestrial colonization.^[133][134] Subsequent evolutionary adaptations diversified neck morphology across vertebrate lineages, driven by ecological pressures such as foraging height and predatory strategies. In sauropod dinosaurs of the Mesozoic era, extreme elongation enabled access to elevated vegetation; for instance, Brachiosaurus brancai possessed 13 cervical vertebrae, forming a neck up to 9 meters long, lightened by extensive pneumatization that invaded the vertebral bones to reduce mass while maintaining rigidity.^[135][136] Conversely, modern crocodylians evolved shortened necks with typically 9 cervical vertebrae, a derived condition from longer-necked archosaur ancestors, optimizing hydrodynamics for ambush hunting in aquatic habitats by minimizing drag and enhancing body streamlining.^[137] At the developmental level, neck evolution involves conserved genetic and cellular mechanisms that pattern its segmentation and support structures. Neural crest cells, a vertebrate innovation, migrate to contribute connective tissues, skeletal elements, and musculature in the neck and shoulder girdle, anchoring the head to the trunk and enabling coordinated movement—a pattern traced from fish to tetrapods through fate-mapping studies. Hox genes, a family of homeobox transcription factors, regulate this process by establishing anteroposterior identity along the vertebral column; their collinear expression domains specify cervical versus thoracic segments, with shifts in expression boundaries accounting for variations in neck length and vertebral count across species.^[138] Recent investigations, including genomic and embryological analyses post-2023, continue to elucidate these mechanisms, revealing how co-option of pre-existing muscle groups from the head-trunk interface facilitated neck functionality in early tetrapods, with implications for understanding adaptive radiations in diverse lineages.^[139]