Autonomic Regulation: The Hierarchy From Spinal Segment to Insular Cortex

01Four regulatory tiers

Autonomic regulation runs as four tiers, and each tier biases the one below it

The lowest tier of autonomic regulation is the intermediolateral cell column, the strip of sympathetic preganglionic neurons running from T1 to L2. Morphometry in man counted roughly 5,000 of those cells on one side of the cord at each of T6, T7, and T8 Low 1977. Above the column sit the rostral ventrolateral medulla and the nucleus of the solitary tract. Above them, the paraventricular nucleus of the hypothalamus. At the top, insular and infralimbic cortex. What passes between the tiers is bias rather than command. The two-neuron chain that carries the output past the cord, and the receptor families it meets at the organ, are set out in the autonomic nervous system.

The efferent arm leaves the neuraxis at two ends. Parasympathetic fibers depart through the Edinger-Westphal nucleus to the ciliary ganglion, through the superior and inferior salivatory nuclei, and through the vagus. Visceral afferents ride back alongside those same nerves, carrying reports from cardiovascular, gastrointestinal, and respiratory receptors toward the brainstem.

The classical scheme calls that outflow craniosacral, and the name is under live challenge. Fifteen phenotypic and ontogenetic features separate cranial parasympathetic neurons from thoracolumbar sympathetic neurons in mice, and the sacral outflow matched the thoracolumbar pattern on every one Espinosa-Medina 2016. John Horn answered that the sacral outflow behaves parasympathetically in function, and that a developmental marker is not the arbiter of a functional class Horn 2018. The question is open and textbooks still teach craniosacral, and the dispute is set out in full on the research page on the autonomic nervous system. The route to the bladder does not change. Which descending system is understood to bias that route does.

Cortex enters the loop through a mapped relay chain

The insular cortex sits at the top of the autonomic hierarchy, and the path up to it is viscerotopically organized. Visceral afferents terminate first in the nucleus of the solitary tract, relay through the parabrachial nucleus and the ventroposterior parvocellular thalamus, and reach the insula Cechetto 2014. The descending half of the same system runs from insular and infralimbic cortex onto cardiovascular output.

Stimulating that cortex moves the circulation. In five patients undergoing epilepsy surgery, electrical stimulation of the left insula produced bradycardia and depressor responses more often than tachycardia and pressor effects, at p less than 0.005 Oppenheimer 1992. Five patients is a small series, and later work has complicated the difference between the sides. The demonstration itself is what holds: a patch of thinking cortex has a line to the heart.

Cortical autonomic pathways also differ between people who mount a cardiovascular response to mental stress and people who do not Cechetto 2014. In the Unified Model of Tone that difference is the standing bias appearing at the top of the stack. A thought reaches the heart along the route a baroreceptor uses, and it arrives at a medulla that is already set one way.

02Findings

What the research shows

03Coding at the effector

The size of an autonomic command is settled at the target, and the target's settings drift

Autonomic neurons are chemically coded by function. Neuropeptide Y is colocalized in adrenergic vasoconstrictor neurons, and vasoactive intestinal polypeptide in cholinergic sudomotor neurons Benarroch 1994. Vasoconstrictor endings release adenosine triphosphate alongside norepinephrine, and sudomotor endings release nitric oxide alongside acetylcholine. One thoracolumbar outflow narrows a vessel at one target and floods a gland at another.

That coding is plastic. Preganglionic input raises neuropeptide Y and inhibits substance P, developing sympathetic neurons switch from a noradrenergic to a cholinergic phenotype, and sympathetic denervation triggers sensory sprouting Benarroch 1994. Part of autonomic regulation is therefore written into the periphery, and it can be rewritten there.

An autonomic command is divided differently at every target

Acetylcholine leaves the nerve as one signal and meets M2 and M3 receptors in different proportions at every organ. In isolated tissue from mice, the M2 and M3 contributions to contraction ran about 5 and 95 percent in the detrusor and about 25 and 75 percent in ileal longitudinal muscle Matsui 2002. Cholinergic transmission crosses the ganglion through ionotropic nicotinic receptors that open sodium channels, and reaches the organ through metabotropic muscarinic receptors working through G proteins on cAMP and IP3.

The same mice returned a result the acetylcholine account alone does not predict. Animals lacking both M2 and M3 had smaller pupils than animals lacking M3 alone, which puts M2 inside pupillary dilation Matsui 2002. Deleting the receptors that carry cholinergic smooth muscle contraction removed one route to regulation. It did not remove the regulation, and the animals stayed viable and fertile.

Delete two alpha-1 subtypes and norepinephrine stops raising pressure

The sympathetic command that raises blood pressure is read at the vessel wall, and deleting the receptors that read it abolishes the response. Norepinephrine infusion produced no pressor response at all in mice lacking both alpha1B and alpha1D adrenergic receptors Hosoda 2005. Alpha-1 receptors constrict skin, visceral, renal, and salivary vessels. Beta-1 receptors on heart and kidney raise rate and contractility and release renin, which drives angiotensin II.

Salt loading then split the two subtypes apart. In the same mice, deleting alpha1D blunted the resulting hypertension and lowered circulating catecholamines, while deleting alpha1B did not Hosoda 2005. In the Unified Model of Tone the split is regulation held at the vessel wall, where an identical salt load meets a wall that either carries the subtype to read it or does not.

04The medullary pressor engine

The rostral ventrolateral medulla generates sympathetic tone instead of relaying it

Tonically active reticulospinal cells in the rostral ventrolateral medulla supply a substantial part of the excitatory drive reaching vasomotor sympathetic preganglionic neurons Guyenet 1989. Some of those cells in the rat carry intrinsic pacemaker activity, release a glutamate-like substance, and are not phenotypically adrenergic. Resting vasomotor drive is manufactured in the medulla rather than passed through it.

The catecholaminergic C1 group projects onto the same preganglionic neurons and adds its own excitation. Restraint arrives from the baroreceptor-sensitive neurons of the nucleus of the solitary tract and from the caudal ventrolateral medulla. Vestibular neurons add excitation from the other side. The solitary nucleus is the first relay for visceral afferents and is organized viscerotopically, and it projects onward to hypothalamus, amygdala, and thalamus. This standing balance of excitation and restraint is the central integrative state of tone read at the brainstem.

Driving the input raises pressure, and blocking glutamate in the medulla stops it

Optogenetic stimulation of glutamatergic paraventricular neurons projecting to the RVLM raised renal sympathetic nerve activity and arterial pressure in rats, at least in part through RVLM C1 neurons Koba 2018. Blocking ionotropic glutamate receptors inside the RVLM suppressed the effect. The descending hierarchy is demonstrated here rather than inferred from a lesion, and its currency at this synapse is glutamate.

Silencing the engine changes little until the system is loaded

Optogenetic silencing of C1 neurons in conscious rats breathing room air produced only a very small fall in blood pressure Wenker 2017. The same silencing produced a much larger fall during hypoxia at 10 and 12 percent inspired oxygen, after sino-aortic denervation, and under isoflurane anesthesia. About 65 percent of transduced neurons were C1, and the degree of hypotension tracked the percentage transduced.

The Unified Model of Tone reads that pattern as the RVLM's contribution being set by the state it acts into. An unloaded animal has other routes to pressure. Load it, cut its baroreceptors, or anesthetize it, and this one population becomes the thing holding pressure up. How much a structure matters here changes with the state of everything around it, which is why the size of a medullary lesion predicts the size of the deficit so poorly.

05Baroreflex wiring and gain

The baroreflex reaches the RVLM through one glutamate synapse and one GABA synapse

The baroreceptor reflex defends arterial pressure beat to beat, and its central path is short. Stretch receptors in the aortic arch and carotid sinus report to the nucleus of the solitary tract. The solitary nucleus excites the caudal ventrolateral medulla through a glutamatergic synapse, and the CVLM inhibits the RVLM through GABA. In pentobarbital-anesthetized rats, bicuculline in the RVLM greatly reduced baroreceptor-evoked sympathoinhibition, and kynurenate in the CVLM did the same Potas 2003.

Several separate brakes compete at one nucleus

A second restraining route bypasses the CVLM entirely. Sympathoinhibition evoked from the caudal midline medulla survived kynurenate in the CVLM, while baroreceptor-evoked inhibition did not Potas 2003. The midline brake reaches the RVLM without the glutamatergic relay the baroreflex uses. Several separate braking inputs converge on one nucleus, so the structure holding sympathetic tone is also the structure where every source of restraint has to compete.

The gain of that loop is reported in milliseconds of cardiac cycle per millimeter of mercury, which grades the loop rather than the pump behind it. What the gain predicts about survival is read on the research page on gain.

Standing and straining are how the loop is read at the bedside

Standing unloads the baroreceptors, and the answer arrives in order. Heart rate climbs first through vagal withdrawal. Rate and peripheral resistance then climb more slowly through sympathetic activation. Valsalva's maneuver runs the same loop through four phases, with subjects exhaling into a mouthpiece at 40 mmHg expiratory pressure for 15 seconds Oka 2003. Phase 1 shows a transient pressure rise and a heart-rate fall as the thoracic aorta is compressed.

The diagnostic information sits later in the maneuver. Pressure recovery time correlates most strongly with late phase II reflex vasoconstriction and with the phase IV overshoot Vogel 2005. It keeps grading adrenergic failure after late phase II can no longer be recorded. Oka studied 62 patients with Parkinson disease and 53 controls, of whom 37 patients and 20 controls underwent the Valsalva maneuver. Cardiac sympathetic imaging tracked QTc and the phase IV pressure rise, and tracked neither baroreflex sensitivity nor vasomotor function Oka 2003. The four phases interrogate separable limbs of one loop, and no single autonomic number stands in for all of them.

The same barosensitive neurons put the brain to sleep

The barosensitive neurons of the solitary nucleus also put the brain into non-REM sleep. Mice tagged them by activity-dependent genetic labeling and confirmed them barosensitive by optrode recording and calcium imaging Yao 2022. Activating them promoted non-REM sleep while lowering blood pressure and heart rate, and the GABAergic CVLM neurons downstream did the same, partly by inhibiting the wake-promoting adrenergic neurons of the RVLM. The pressure loop and the sleep switch share cells, so the baroreflex is part of how the brain sets its own state.

06Descending bias and sign

Descending bias decides which way a segmental autonomic reflex runs

Somatic input reaches autonomic output through the spinal segment, and the sign of the response is set from above. Acupuncture-like stimulation changed adrenal catecholamine secretion and adrenal sympathetic nerve activity three ways in urethane-anesthetized rats. It raised them, lowered them, or left them unchanged, and mean arterial pressure followed the same three patterns Sato 1996. The needles were 340 microns across, inserted 10 mm into skin and underlying muscle, and twisted at about 1 Hz for 90 seconds. Severing the somatic afferent nerves innervating the abdomen and hindlimb eliminated the adrenal sympathetic nerve responses.

Spinalize the animal and the variety disappears. In spinalized rats the same stimuli produced only increases in catecholamine secretion and nerve activity, and abdominal stimulation produced a larger response than hindlimb stimulation Sato 1996. Both the cord and the structures above it act as reflex centers. The segmental machinery is excitatory and fixed, and the brain converts it into a generalized response that can run either way.

Human injury shows the same arithmetic in millimeters of mercury. Autonomic dysreflexia occurred in 43.3 percent of 282 urodynamic tests across 70 people with spinal cord injury at or above T6 Kirshblum 2021. Systolic pressure rose by a mean of 35.5 mmHg and diastolic by 19.0 mmHg. Bladder filling is an ordinary visceral report. Stripped of descending restraint, it becomes a hypertensive event.

No single nucleus holds the pressure setpoint

Destroying the commissural nucleus of the solitary tract in spontaneously hypertensive rats dropped mean arterial pressure to normotensive levels for one day. Nine days later the pressure was back at hypertensive levels, while the pressor component of the chemoreflex stayed blocked for 30 days Sato 2003. Other solitary subnuclei or other brainstem nuclei reorganized, restored the sympathetic activity, and brought the high pressure with it. The setpoint outlived the structure that had been carrying it.

Input at one tier reaches the tiers above and below it, so a segmental contact is read against the descending bias already standing there. Chiropractic care delivers graded mechanical and sensory information at one segment of the column. In the Unified Model of Tone the sign of what follows belongs to the bias, not to the contact. The anesthetized rats make that arithmetic visible. One needling protocol raised adrenal output, lowered it, or changed nothing at all, and which of the three occurred was decided above the segment. Two people can take the same contact at the same level and answer in opposite directions in arterial pressure, and neither answer means the contact missed. Remove the bias and one direction is left. That is what spinalization did to the rats, and what a filling bladder does to pressure after a high cord injury.

Tone shows up here in units a clinic can record. Baroreflex gain comes in milliseconds of cardiac cycle per millimeter of mercury, a dysreflexic bladder in millimeters of mercury, and a thinning intermediolateral column in cells per segment. The Unified Model of Tone expects those numbers to share one organization with cortical excitability, with map organization, and with tissue compliance, because each is sampled from a different depth of one hierarchy, and compensation decides which depth shows the change first. A number that moves alone was sampled too shallow.

07Where regulation fails

Autonomic failure appears at whichever tier of the hierarchy has thinned

Orthostatic hypotension is a sustained fall of 20 mmHg systolic or 10 mmHg diastolic within 3 minutes of standing, the criterion set by international consensus Freeman 2011. Pooled prevalence reaches 22.2 percent among 24,967 community-dwelling people over 60, and 23.9 percent among 2,694 people in long-term care Saedon 2020. What that threshold means for a person, and how the syndromes on either side of it divide, is set out on the research page on dysautonomia.

The threshold is also a probe. Supine rest duration, the frequency of measurement, and the timing of the standing reading varied substantially across those studies Saedon 2020. How hard a test loads the system decides what the test finds, which is a property of the regulation and not a flaw in the method. Several drug classes provoke the same drop with no cells lost at all Mosnaim 2010.

Falls follow the drop. Individual patient data from 1,022 people across three prospective cohorts gave a hazard ratio of 1.52 for time to first fall Hartog 2017. The same data showed no significant relationship in the logistic models. The hazard ratio caught the timing that the logistic models could not catch at all. A failure of regulation shows itself in when a fall arrives, not in whether one arrives.

The bottom tier can be counted

Intermediolateral neuron numbers fall in graded fashion across neurodegenerative disease. Against controls, human cords held 53 percent in Parkinson disease and 47 percent in dementia with Lewy bodies. Multiple system atrophy held 27 percent, with small ventral root fibers reduced alongside Hatsuta 2016. Within the Parkinson and Lewy body cases, those with orthostatic hypotension retained 77 percent of the intermediolateral neurons of those without.

The bladder fails at three levels at once

Fourteen patients with progressive autonomic failure lost the ability to initiate a voluntary micturition reflex, and the damage sat in pontine nuclei, medullary nuclei, and the sacral intermediolateral columns at once Kirby 1986. They were studied by urodynamic, electromyographic, and neurohistochemical means against age-matched controls. Reinnervated motor units in the striated urethral sphincter pointed to Onuf's nucleus, and acetylcholinesterase-containing nerve density in bladder muscle was significantly reduced.

Morphometry at the S3 level in five Shy-Drager cords, five cords from amyotrophic lateral sclerosis, and four age-matched controls sorted the damage by cell group Konno 1986. Onuf's nucleus runs as a slender column between the middle of S2 and the rostral third of S3, and the sacral autonomic column sits chiefly at S3 and S4. Shy-Drager syndrome, now classified as multiple system atrophy, took the sacral autonomic column, Onuf's nucleus, and the somatic motor neurons together. Amyotrophic lateral sclerosis severely depleted the somatic motor neurons while the sacral autonomic column was only modestly reduced. Two diseases enter the same segment and select different tiers of it.

A resting exam can pass while the reply fails

Pupillometry gives each limb of the outflow its own quantity. Constriction velocity and amplitude grade the parasympathetic side, dilation velocity and re-dilation time the sympathetic side. The iris circuit that produces them is set out in cranial nerves I to VI. What the autonomic hierarchy adds is that the resting value and the reply to a challenge come apart. Twenty migraine patients and 20 controls matched on every pupillary and cardiovascular measure at baseline and at two minutes of a cold pressor test. At five minutes, constriction velocity had risen to 5.59 mm/s in the migraine group against 5.16 mm/s in controls Eren 2018. The migraine pupil answered the challenge too hard.

Tone inside its range is health because the hierarchy keeps the freedom to answer in either direction, up or down, at whichever tier the demand arrives. Tone drifted outside that range is a hierarchy that can answer one way only. A bladder that raises systolic pressure 35.5 mmHg with no way to say no, a sphincter left by a lost sacral column, and a pupil that overshoots a cold challenge all share that.

08Tone

How this system expresses tone

Tone shows up here as the direction of the answer. That answer arrives in clinical units, in milliseconds of cardiac cycle per millimeter of mercury and in the pressure a filling bladder can raise. One needling protocol raised adrenal output in some anesthetized rats, lowered it in others, and left it unchanged in the rest. After spinalization the same protocol could only raise it.

Oscillation belongs to the medullary tier, where reticulospinal cells in the rat RVLM carry intrinsic pacemaker activity, so the resting drive is generated there rather than passed along. Prediction belongs to the cortical tier, where insular and infralimbic pathways differ between people who answer a mental stressor and people who do not. Load lands on the segmental tier, where a filling bladder raises systolic pressure 35.5 mmHg in injured people once descending restraint is gone. Input quality rides the afferent line into the medulla. After sino-aortic denervation in conscious rats, the same C1 silencing that barely moved pressure in room air collapsed it. Time course shows up at the effector, where pupillary constriction velocity slows before diabetic retinopathy is clinically visible in type 2 diabetes Bista Karki 2020. Coupling runs back through the medullary tier, where the barosensitive solitary and CVLM neurons that lower pressure also promote non-REM sleep in mice.

09Across the library

How this page relates to the rest of the library

10Frequently asked

Questions about this topic

11The sources

References

Sources: primary literature, linked inline.