Upper Cervical Pathways

This was made to show that the connections from the upper neck have many projections into the nervous system and has the potential to help people with various conditions

Overview

Upper cervical (C1–C3) proprioceptive afferents split into seven parallel channels at their first synapses: segmental, cerebellar, vestibular, autonomic, conscious, orienting and noradrenergic. Unlike lower spinal levels, much of this input reaches brainstem gaze, vestibular and cardiorespiratory nuclei directly, with no spinal relay.

Past the dorsal root ganglion, nearly all of this map comes from tracing and recordings in rat, cat, guinea pig and monkey. None of it has been traced in humans, so human-level statements are inferences. Species differences are flagged where they change the picture.

Nociceptive routes such as the trigeminocervical complex and the ciliospinal reflex form a separate small-fibre system and appear here only for contrast.

Abbreviations: CCN, central cervical nucleus; COR, cervico-ocular reflex; CVLM/RVLM, caudal/rostral ventrolateral medulla; DRG, dorsal root ganglion; DVN, descending vestibular nucleus; ECN, external cuneate nucleus; EW, Edinger–Westphal nucleus; ICP/SCP, inferior/superior cerebellar peduncle; InM, intermedius nucleus of the medulla; LC, locus coeruleus; LVN/MVN, lateral/medial vestibular nucleus; MLF, medial longitudinal fasciculus; NTS, nucleus tractus solitarius; OCI/OCS, obliquus capitis inferior/superior; PBN, parabrachial nucleus; RCP, rectus capitis posterior; VPL/VPM, ventral posterolateral/posteromedial thalamic nucleus.

Pathway at a glance

Read each row left to right: the channel, the first relay its afferents reach, and where that relay projects. Dashed arrows mark links shown only by recordings or with no traced route.

Receptors, nerves and first-order neurons

First-order cell bodies sit in the C2 and C3 ganglia. C1 is inconsistent: in many people its sensory neurons lie along the spinal accessory nerve instead [1, 2].

Receptors

  • Muscle spindles (Ia primary and II secondary endings) and Golgi tendon organs (Ib) in the suboccipitals (RCP major and minor, OCS, OCI), semispinalis and splenius capitis, and the prevertebral longus capitis and rectus capitis anterior and lateralis.
  • Joint mechanoreceptors in the C0–C1, C1–C2 and C2–3 capsules.

Peripheral nerves

  • Suboccipital nerve (C1 dorsal ramus): all four suboccipital muscles; OCI often also receives a C2 branch.
  • C2 and C3 dorsal rami: the capitis extensors; the third occipital nerve (C3) supplies the C2–3 zygapophyseal joint.
  • C1 and C2 ventral rami: the atlanto-occipital and lateral atlantoaxial joints, respectively.
  • C1–C3 sinuvertebral nerves: the median atlantoaxial joint and its ligaments.

First-order cell bodies

  • C1, gross anatomy: in 40 cadavers (80 sides), 60% of sides had C1 dorsal rootlets. Of those, 30% had a distinct ganglion and 44% had the spinal accessory nerve joining the rootlets; every side had a C1 dorsal ramus [1].
  • C1, histology: a C1 dorsal root was present in 35.7% of specimens and always held sensory neurons. In about a quarter, sensory fibres reached C1 through an accessory nerve anastomosis; in 39.3%, no sensory component to C1 was found. Neuron clusters lay along CN XI at the C1 level in every specimen [2].
  • Where suboccipital neurons sit: in a small C1 ganglion, along the spinal accessory nerve, or in the C2 ganglion via the C1–C2 communicating branch (the last is an anatomical inference).
  • C2: a large ganglion outside a true foramen, between the posterior arch of the atlas and the C2 lamina, directly behind the lateral atlantoaxial joint.
  • C3: in the C2–3 intervertebral foramen.

Central entry

Large-diameter fibres enter through the medial division of the dorsal root and join the most lateral part of the fasciculus cuneatus. There they split into ascending and descending stems that give off collaterals.

Spinal terminations (C1–C4)

The central cervical nucleus is the key spinal relay. It receives neck spindle input and labyrinthine input, so it is the first site where the two meet.

  • Lamina IX: monosynaptic Ia contacts on neck motoneurons, the fast segmental limb of the cervicocollic stretch reflex.
  • Laminae V–VII: interneurons, including propriospinal neurons.
  • Central cervical nucleus (CCN): lies just lateral to the central canal in C1–C4. In the cat, almost all CCN neurons receive monosynaptic excitation from their own segment's dorsal root and from dorsal neck muscle nerves, including rectus capitis dorsalis and obliquus capitis caudalis (cat counterparts of the RCP group and OCI) [3]. Thresholds point to group I fibres, with group II input added at stronger stimulation; most neurons are driven by a single muscle [3]. CCN neurons also carry labyrinthine signals [4, 5].
  • Other spinocerebellar neurons: in the upper cervical segments, medial lamina VI neurons send uncrossed spinocerebellar axons; the CCN and laminae VII–VIII send crossed ones [6].
  • Descending stems: in the rat they reach upper thoracic levels, with collaterals to the intermediate zone and Clarke's column [7]. These segments contain the C8–T2 ciliospinal centre, but contacts on sympathetic preganglionic neurons have not been shown.

For contrast, nociceptive C1–C3 afferents converge with trigeminal afferents in the trigeminocervical complex; proprioceptive overlap with trigeminal circuits lies instead in subnucleus interpolaris and lateral laminae IV–V [7].

Direct brainstem projections

In rat and cat, upper cervical proprioceptors project straight into the caudal vestibular and perihypoglossal nuclei. In monkey tracing, the heavy direct target is the cuneate complex instead.

  • Rat, medial collaterals: ascending stems reach the pons. Medial collaterals, almost exclusively proprioceptive, end in the medial and descending vestibular nuclei, the perihypoglossal nuclei and the solitary nucleus [7].
  • Rat, lateral collaterals: mostly cutaneous, with a suboccipital proprioceptive share; they end in trigeminal subnucleus interpolaris and lateral laminae IV–V [7].
  • Rat, level specificity: injections into the C7 and L5 ganglia gave no significant vestibular or perihypoglossal labelling, so this is an upper-cervical feature [7].
  • Rat, retrograde: the main vestibular target is the caudal medial vestibular nucleus. Injections into rostral MVN, the superior, lateral and descending nuclei, the NTS and the reticular formation labelled no spinal ganglion cells [8], so direct NTS input is best treated as sparse.
  • Cat: cervical ganglion injections (C2–C8) label the caudal ipsilateral MVN and DVN, the zone holding vestibulospinal neurons that project back to the cervical cord [9].
  • Monkey: suboccipital afferents end most heavily in the ventral ipsilateral external (lateral) cuneate nucleus and in ventrolateral pars triangularis of the cuneate nucleus, partly overlapping extraocular muscle afferent terminals. Labelling is sparse in the CCN and light in group x of the vestibular complex [10].
  • Guinea pig: suboccipital injections label the CCN heavily and send ascending fibres to the ECN and group x, with only scanty vestibular and perihypoglossal projections [11].
  • InM, every species tested: primary afferent input to the intermedius nucleus of the medulla arises only from C1–C4, in rat, guinea pig, cat and monkey [12].

The often-cited direct neck-to-vestibular projection therefore rests mainly on rat and cat data.

End targets: cerebellum

The cerebellum receives the largest share. The CCN, the external cuneate and climbing fibres all deliver neck signals, and the rostral fastigial nucleus combines them with vestibular input into an estimate of body motion.

  • CCN route: axons cross at their segment of origin, then climb in the ventral and lateral funiculi and along the lateral medulla [13]. They enter through both cerebellar peduncles, mostly the superior [14].
  • CCN terminal field (cat): lobules I–VI, VIIb, VIII and IX, the paramedian lobule, crus I and II, and the simple lobule. Of the terminals, 67–80% lie in the anterior lobe, concentrated near the midline; in anterior-lobe sublobules the split is roughly 60:40 contralateral to ipsilateral [15].
  • CCN to cerebellar nuclei: the CCN also projects to the cerebellar nuclei [14], including direct input to the rostral fastigial nucleus [16].
  • ECN route: the cuneocerebellar tract runs through the ipsilateral inferior peduncle. ECN neurons respond to neck rotation, but their preferred 3D directions vary widely, so the useful neck–vestibular combination happens downstream [17].
  • Climbing fibres: neck afferent stimulation evokes climbing-fibre responses in the pars intermedia that converge on single Purkinje cells with extraocular, trigeminal and paw inputs [18].
  • Computation: rostral fastigial neurons combine neck proprioceptive and vestibular signals to compute the body's position and motion in space [19]. The interpositus shows similar convergence, and the anterior vermis responds to rotation about the C1–C2 axis [20].
  • Motor output: the rostral fastigial nucleus sends body-motion estimates to brainstem regions for postural control [21].
  • Autonomic output: fastigial stimulation in cats changes regional blood flow and cardiac dynamics in a pattern resembling orthostatic reflexes [22], relayed through the brainstem [23]. Doba and Reis tied the cerebellum to reflex cardiovascular adjustment to posture [24]. Localized ventral fastigial stimulation raises blood pressure, and one fastigial output module appears suited to adjusting cardiorespiratory reflexes during head tilt [25].

End targets: vestibular nuclei

The vestibular nuclei receive neck input twice, directly from primary afferents in rat and cat and through the crossed CCN relay. They pass it on to gaze, posture and sympathetic circuits.

  • CCN relay: CCN neurons reach the vestibular nuclei only on the opposite side, since their axons decussate in the cord. They target the lateral vestibular nucleus, the proposed upper-cervical link to the lateral vestibulospinal tract in tonic neck reflexes [5].
  • Vestibulospinal neurons: in cats, vestibulospinal neurons respond to sinusoidal rotation of the neck [26].
  • Primate recordings: in monkeys, neurons unrelated to eye movement and eye-head-velocity neurons were the most sensitive to passive neck rotation. Neck input shapes vestibular nucleus output for posture, gaze and perception [27].
  • Species caveat: vestibular nucleus neurons encode neck proprioception in cats and in squirrel and cynomolgus monkeys, but not in rhesus monkeys with intact labyrinths [16]. The fastigial route may carry more of the load in primates.

Outputs

  • Gaze: MVN and nucleus prepositus → abducens nucleus → (via the MLF) medial rectus motoneurons. This is the cervico-ocular reflex, normally weak in humans and upregulated after vestibular loss.
  • Posture: medial vestibulospinal tract → neck motoneurons; lateral vestibulospinal tract → limb extensor tone (tonic neck reflexes).
  • Perception: vestibular thalamus → vestibular cortex.
  • Sympathetic: vestibular neurons drive presympathetic cell groups in the RVLM and CVLM [28]. In cats, bilateral vestibular nerve section made blood pressure fall more steeply during nose-up tilt, the first demonstration of this vestibulosympathetic reflex [28, 29].

End targets: cardiorespiratory and oromotor (via InM)

The best-documented autonomic route runs through the InM. In rats, C2 stimulation alters breathing and perfusion pressure, and the InM projects to cardiorespiratory and oromotor nuclei.

  • The rat experiment: in a working heart–brainstem preparation, C2 nerve stimulation changed the central respiratory pattern and raised perfusion pressure. Direct InM stimulation reproduced the response [12].
  • Proprioceptive identity: afferent terminals in the InM co-express parvalbumin and VGLUT1, marking them as proprioceptive [12].
  • InM outputs: hypoglossal, facial and motor trigeminal nuclei; parabrachial nuclei; RVLM and CVLM; nucleus ambiguus [12]. The InM also sends excitatory and inhibitory projections to the NTS [30].
  • Cat: stimulating neck muscle afferents altered sympathetic and respiratory nerve activity [31], at intensities that recruit only group I fibres [12].
  • Human: dynamic sinusoidal neck stretch modulates muscle sympathetic activity to the legs in a frequency-dependent way [32]. Dynamic neck movement also raises skin sympathetic activity while static position does not; earlier human studies had found no neck effect [33].

End targets: conscious head-on-trunk sense

Neck muscle afferents reach the thalamus through relay cells in the caudal external cuneate and nucleus x. From there they follow the lemniscal route to parietal and vestibular cortex.

  • Medullary relay (rat): neck muscle afferents contact ventrobasal-thalamus-projecting neurons in the caudal third of the ECN and in nucleus x. Neck cutaneous afferents contact such neurons along the dorsolateral border of the rostral cuneate nucleus [34].
  • Lemniscal route: internal arcuate fibres → decussation → medial lemniscus → contralateral VPL, at its medial edge beside VPM.
  • Cortex: primary somatosensory areas 3a and 2, area 5, posterior parietal cortex and the parieto-insular vestibular cortex, where head-on-trunk and head-in-space signals combine.
  • Human: neck muscle input is also processed in motor cortex and at the parieto-temporal junction [35].

End targets: superior colliculus (orienting)

Whether neck spindle signals reach the superior colliculus is unsettled. Early cat recordings found strong neck muscle input; later ones did not.

  • For: in cats, neck muscle nerve stimulation excited collicular units, mostly bilaterally, with short- and long-latency populations that suggested two routes. Tectospinal neurons were identified in the same study [36].
  • Plausible relays: the deep layers receive input from the dorsal column nuclei (mostly cuneate), the perihypoglossal nuclei and the locus coeruleus [37].
  • Against: later work found no deep collicular input from the large superficial neck muscles, while C2 and C3 cutaneous nerves drove collicular neurons readily [38].
  • Output: the tectospinal projection back to C1–C4, which drives orienting head movements, is well established.

End targets: locus coeruleus and the pupil

Neck rotation modulates most locus coeruleus neurons in decerebrate cats, and LC activity dilates the pupil. No study has recorded the neck-to-pupil chain end to end.

  • Neck → LC: in decerebrate cats, 73 of 99 LC-complex neurons (74%) responded to 0.15 Hz, ±10° neck rotation, with the body turned under a fixed head. Of the 99, 14 were coeruleospinal or subcoeruleospinal [39]. The same population also responds to labyrinth stimulation [40].
  • Route in: not traced. The vestibular nuclei and reticular formation are the likely intermediaries (inference).
  • LC → pupil: LC activity dilates the pupil by inhibiting the Edinger–Westphal nucleus through α2-adrenoceptors and by driving C8–T2 sympathetic outflow to the superior cervical ganglion and dilator pupillae.
  • Coupling strength: in monkeys, LC activation reliably precedes pupil changes, but microstimulation shows the link is not LC-specific; intermediate superior colliculus and inferior colliculus also drive it [41]. In mice the LC–pupil relationship is graded but variable, and pupil predicts only a small fraction of moment-to-moment LC activity [42].
  • Not this route: the ciliospinal reflex dilates the ipsilateral pupil after noxious stimulation of the face, neck or upper trunk. It uses cervical pain fibres and C8–T2 sympathetic efferents, and is absent in Horner syndrome [43, 44].

Evidence strength by channel

The cerebellar, vestibular and InM channels are the best supported. The orienting and noradrenergic channels lack a traced route, and nothing here has been traced in humans.

Channel

First relay → target

Strongest evidence

Human evidence

Segmental reflex

Lamina IX → neck motoneurons

Cat physiology

Inferred

Cerebellar

CCN, ECN, climbing fibres → vermis, rostral fastigial

Cat and rat tracing; monkey fastigial recordings

Indirect

Vestibular

Caudal MVN/DVN (direct); CCN → contralateral LVN

Rat and cat tracing; monkey recordings, species-dependent

Behavioural only

Autonomic

InM → NTS, RVLM, CVLM, PBN; fastigial and vestibular → RVLM

Rat tracing and physiology; cat stimulation

Sympathetic modulation with dynamic neck stretch

Conscious

Caudal ECN, nucleus x → VPL → cortex

Rat tracing

Cortical processing of neck muscle input

Orienting

Possible cuneate or perihypoglossal relay → superior colliculus

Cat recordings, conflicting

None

Noradrenergic

Unmapped route → LC → EW and sympathetic outflow → pupil

Decerebrate cat (LC); monkey and mouse (LC–pupil)

None end to end

Open gaps: no human tract tracing, a thin direct vestibular projection in monkey tracing, an untraced route into the LC, and variable human C1 anatomy.

Efferents at a glance

The efferent side runs the afferent side in reverse. Six descending systems and segmental reflexes converge on one C1–C2 motoneuron pool and one nerve, with a separate sympathetic supply.

[embed: node/c34eabe8-3805]

Read left to right: each source converges on the C1–C2 pool, which leaves through the suboccipital nerve. Dashed arrows mark links shown only for other neck muscles.

Efferents: final common path

Every command to the four posterior suboccipitals leaves through one motoneuron pool in the upper cervical ventral horn and one nerve, the suboccipital nerve.

  • Pool location: in guinea pigs, OCS and RCP major motoneurons lie in C1 and OCI motoneurons in C2, all in or near the ventromedial nucleus [45]. In cats, OCI motoneurons were recorded at both C1 and C2 [46]. Vestibulocollic fibres target dorsal neck and suboccipital motoneurons in lamina VIII [47].
  • Human pools: not mapped. OCI's dual C1/C2 nerve supply fits a pool spanning both segments; RCP minor's pool is presumed to lie in C1.
  • Cell types: alpha motoneurons drive extrafusal fibres; gamma motoneurons in the same pool set spindle sensitivity. In muscles this spindle-dense, fusimotor output is probably a large share of the efferent traffic (inference).
  • Route out: C1 ventral rootlets form the C1 nerve, which runs beneath the vertebral artery in its groove on the posterior arch of the atlas. Its dorsal ramus, the suboccipital nerve, enters the suboccipital triangle between artery and arch.
  • Muscles supplied: RCP major, RCP minor, OCS, OCI and semispinalis capitis. OCI usually also receives a C2 dorsal ramus branch, and the nerve often communicates with the greater occipital nerve.
  • A constant motor path: even where the C1 dorsal root is missing, every side in the Tubbs series had C1 ventral roots and a C1 dorsal ramus [1].
  • Anterior group: rectus capitis anterior and lateralis belong to the C1 ventral ramus instead.

Efferents: vestibulospinal and reticulospinal drive

The fastest drive comes from the vestibular nuclei through three-neuron canal reflex arcs. The reticular formation adds strong monosynaptic excitation and inhibition.

  • Tracts: the medial vestibulospinal tract descends bilaterally in the MLF; cervical-projecting lateral vestibulospinal axons run mostly ipsilaterally in the lateral and ventrolateral funiculi [47].
  • Three-neuron arcs: canal afferent → vestibular neuron → neck motoneuron, first shown in the cat [48].
  • OCI (cat): excited by the ipsilateral anterior and posterior canals and the contralateral horizontal canal; inhibited by the contralateral anterior and posterior canals and the ipsilateral horizontal canal [46].
  • OCI routes: part of the inhibition from the contralateral vertical canals is trisynaptic. Excitation from the ipsilateral vertical canals survives cutting both MLFs, so it travels outside the MLF [46].
  • Why that pattern fits OCI: a turn toward the opposite side excites the contralateral horizontal canal, which drives OCI to turn the head back. OCI works as a yaw stabilizer.
  • OCS (cat): like splenius and longissimus, excited by all three contralateral canals and inhibited by all three ipsilateral canals. Both effects are disynaptic and run in the MLF on the motoneurons' side [49].
  • Synergies: single vestibulospinal, reticulospinal and tectospinal axons branch to several neck motor nuclei, wiring muscles into fixed groups [50].
  • Reticulospinal excitation (cat): n. reticularis pontis caudalis and the medullary gigantocellular and ventral nuclei monosynaptically excite all classes of neck motoneurons, more strongly on the same side [51].
  • Reticulospinal inhibition (cat): n. reticularis ventralis and the dorsal gigantocellular nucleus monosynaptically inhibit most ipsilateral neck motoneurons [51].
  • Hub role: these reticulospinal neurons also relay colliculus, cortex, vestibular and cerebellar input, so they are the common path for voluntary and orienting head movement.

Efferents: tectal, interstitiospinal and field H drive

Orienting head turns reach OCI and RCP major from the superior colliculus. The interstitial nucleus of Cajal and Forel's field H drive vertical head movement.

  • First recruited: in monkeys, OCI and RCP major fire during small turns and stay tonically active with the head only a few degrees off centre. They activate before movement onset [52].
  • Collicular drive: superior colliculus stimulation facilitates muscles that turn the head away from the stimulated side, including OCI and RCP major, and suppresses their antagonists [53, 54].
  • Proposed routes: two tectoreticulospinal pathways, one independent of gaze shifts and one gated by pontine omnipause neurons at gaze-shift onset [54]. Crossed tectospinal fibres also reach C1–C4 directly.
  • Frontal eye field: stimulation below saccade threshold evokes neck motor output in monkeys [55].
  • Interstitiospinal (cat): the interstitial nucleus of Cajal sends ipsilateral fibres in the MLF that monosynaptically excite dorsal neck motoneurons [56]. Biventer–complexus motoneurons responded consistently, splenius and trapezius less often; contralateral effects were weak [56].
  • Forel's field H (cat): monosynaptic excitation of dorsal neck motoneurons, plus a disynaptic route through pontomedullary reticulospinal neurons. It reached 91% of biventer–complexus motoneurons but only 10% of splenius motoneurons, consistent with vertical head movement [57].
  • Not tested: suboccipital motoneurons weren't sampled in the interstitial or field H studies.

Efferents: cerebellar, cortical and segmental inputs

The cerebellum and cortex act mostly through brainstem relays, while segmental reflexes close the loop within C1–C4.

  • Fastigiospinal (cat): crossed axons from the rostral fastigial nucleus reach the C2–C3 motor nuclei. They produce short-latency, apparently monosynaptic EPSPs in some trapezius and biventer motoneurons [58].
  • Fastigial relays: the larger fastigial output runs through the vestibular nuclei and reticular formation, carrying the body-motion estimate described in the afferent sections [19].
  • Human cortex: cortical stimulation evokes short-latency responses in contralateral splenius capitis and trapezius but not their ipsilateral counterparts. SCM responds bilaterally, with smaller, later ipsilateral responses [59].
  • Suboccipitals weren't recorded: surface EMG can't reach them, so their cortical input in humans is inferred.
  • Cat cortex: pyramidal effects on dorsal neck motoneurons are relayed, not direct. The shortest excitation is disynaptic through medullary reticulospinal neurons; later excitation and inhibition pass further relays, partly in the upper cervical cord [60].
  • Segmental loops: Ia afferents synapse on their own muscle's motoneurons, the fast limb of the cervicocollic reflex. Group II and Ib inputs act through interneurons, and commissural interneurons pair left and right muscles.

Efferents: sympathetic supply

Sympathetic fibres reach the suboccipitals from the superior cervical ganglion and act on muscle spindles as well as blood vessels.

  • Route: premotor neurons in the hypothalamus and RVLM drive preganglionic neurons in the upper thoracic lateral horn. Their axons climb the cervical sympathetic trunk to the superior cervical ganglion.
  • Postganglionic path: axons travel via gray rami to the C1–C4 spinal nerves, reaching the suboccipital nerve, and along the vertebral and occipital arteries.
  • Spindle effects (cat): 10 Hz cervical sympathetic stimulation depressed discharge in 73% of spindle afferents in trapezius and splenius and reduced Ia-like stretch sensitivity, independent of blood-flow changes [61].
  • Human spindles: in 232 spindles from lumbrical, biceps, levator scapulae and deep neck muscles, NPY and tyrosine hydroxylase marked sympathetic fibres inside the spindle, not only on its vessels [62].
  • Not tested: suboccipital spindles specifically.

Efferent evidence by pathway

Only the motoneuron location, the vestibular canal inputs and the collicular drive have been shown for suboccipital motoneurons themselves. The rest is inferred from other dorsal neck muscles.

Pathway

Shown for suboccipital motoneurons?

Species

Motoneuron location (C1–C2, ventromedial)

Yes: OCS, RCP major, OCI

Guinea pig, cat

Vestibulospinal canal inputs

Yes: OCI, OCS

Cat

Collicular drive

Yes: OCI, RCP major (EMG)

Monkey

Reticulospinal

Neck motoneurons generally

Cat

Interstitiospinal, field H

Other dorsal neck muscles

Cat

Fastigiospinal

Trapezius, biventer

Cat

Cortical

Other neck muscles

Human, cat

Sympathetic to spindles

Trapezius, splenius; human deep neck muscles

Cat, human

Open gaps: no human motoneuron mapping, and no suboccipital-specific data for reticulospinal, cortical or sympathetic inputs.

Human evidence

Human data confirm the receptors, the peripheral wiring and several end effects, but no central pathway in this doc has been traced in humans. Each row below shows what a link rests on in people.

Link

Human evidence

What it shows

Gap

Spindle supply

Fetal and adult histology [63, 64]

Fetal spindles per gram: OCI 242, OCS 190, RCP major and minor 98 each, against 2.2 in trapezius [63]. Adults: 199 spindles in all four suboccipitals from 5 donors; 22% linked in pairs or parallel; intrafusal make-up unlike limb spindles [64].

Small samples; fetal densities may differ from adult

C1 first-order neurons

Cadaver dissection and histology [1, 2]

C1 dorsal root often absent; sensory neurons lie along CN XI

Which afferents those neurons carry is unknown

Suboccipital nerve

Anatomy review [65]

C1 dorsal ramus, between the posterior arch and the vertebral artery; supplies the four suboccipitals; no cutaneous, meningeal or articular branches

Review, not new primary data

Central relays (CCN, vestibular nuclei, InM, ECN)

None

—

No human tracing; all animal

Neck-to-eye reflex

Eye-movement recording [66]

The cervico-ocular reflex is measurable in healthy adults and increases with age

Reflex only; no pathway

Cortical processing

fMRI [67]; review [35]

Posterior neck vibration in 16 adults activated areas 3a, 2 and S2, PIVC, intraparietal sulcus, premotor cortex and FEF [67]

Vibration drives many neck muscles, not suboccipitals alone

Sympathetic outflow

Microneurography [32, 33]

Dynamic neck stretch or movement modulates leg sympathetic activity; static position does not

Route unknown; not suboccipital-specific

Manipulation and autonomic or pupil outcomes

Pilot [68], RCT [69], systematic reviews [70, 71]

C1–2 thrust in 13 men shortened pupil cycle time, with no control group [68]. A sham-controlled RCT in 95 adults found no HRV change [69]. A meta-analysis of 14 RCTs found no significant autonomic change versus sham or control [70]; an overview of 12 reviews found inconsistent effects [71]. Certainty was low to very low.

Whole-intervention effects; cannot isolate a proprioceptive pathway

Motoneuron pools

None

—

Not mapped in humans

Suboccipital activity

Fine-wire EMG [72, 73]

RCP minor: 7.9–11.2% MVIC in neutral, higher with head retraction [72]. RCP major: 26–37% MVIC in retraction versus 10–15% in neutral [73].

Activation only; no pathway attribution

Vestibulocollic drive

cVEMP [74]

Saccular input inhibits ipsilateral SCM at about 12 ms through the medial vestibulospinal tract; responses also appear in trapezius and splenius capitis

Not recorded from suboccipitals

Cortical drive

Cortical stimulation [59]

Short-latency responses in contralateral splenius and trapezius; bilateral in SCM

Suboccipitals not recorded

Sympathetic supply to spindles

Histology [62]

NPY and tyrosine hydroxylase fibres inside spindles of deep neck muscles

Suboccipitals not singled out

For clinical context, Peng et al. review how neck pain disturbs cervical proprioception and how it is measured, mainly by joint position error [75].

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