The Nervous System · Part Two · How It Senses and Moves
Lesson 26 / 61
The Autonomic Nervous System: Circuitry and the Segmental Map
Where the wiring sits, what it releases, and which receptor reads it.
The autonomic nervous system is the involuntary circuitry that runs every organ, blood vessel, and gland. Its commands cross two neurons before they arrive. One leaves the brainstem or spinal cord. The second waits in a ganglion, where a single incoming fiber can reach hundreds of outgoing ones. Which receptor catches the transmitter decides what the organ does. The Unified Model of Tone reads this architecture as the route by which a spinal level reaches an organ.
Sympathetic outflow
T1 to L2, intermediolateral cell column
Parasympathetic outflow
Cranial nerves III, VII, IX, X and S2 to S4
Ganglionic transmitter
Acetylcholine, in both divisions
Receptor families
Nicotinic, muscarinic, alpha, beta
Autonomic nervous system
Its motor arm carries output to smooth muscle, cardiac muscle, and glands through the sympathetic and parasympathetic divisions. Its sensory arm carries visceral traffic back to the brainstem inside the same peripheral nerves. A third division, the enteric network in the gut wall, runs digestion through its own complete circuits.
The ganglion
Each cell of a paravertebral ganglion receives about 3.9 preganglionic inputs, and only about 1.2 of them fire it on their own. The rest sit below threshold, unused and available. In the Unified Model of Tone, that spare capacity is what lets one fixed anatomy produce different output in different states.
01The two-neuron chain
Every autonomic command crosses two neurons before it reaches an organ
The somatic motor system runs one neuron from the spinal cord to the muscle. The autonomic nervous system runs two. A preganglionic neuron leaves the brainstem or the cord and ends in a ganglion outside the neuraxis. A postganglionic neuron carries the signal the rest of the way. Acetylcholine crosses the synapse between them in both divisions.
In the guinea pig superior cervical ganglion, a single preganglionic axon contacts on the order of 50 to 200 ganglion cells Purves 1983. Cells sharing an axon sit scattered across the ganglion instead of clustering together. One command leaving one cord level therefore arrives spread across a field of targets.
Convergence runs the other way at the same time. McLachlan read the paravertebral ganglion as a distribution point and not an integrator, with one or two strong inputs carrying the traffic and the weak ones held in reserve McLachlan 2003.
The Unified Model of Tone takes divergence and convergence together as the reason autonomic output is regional. A cord level does not address an organ. It addresses a population of second neurons whose targets overlap, which is why one input changes several things at once.
Speed sorts the fibers into two classes
Of the postganglionic fibers leaving the lumbar chain, 86 percent are myelinated B fibers with maximal conduction velocities of 2.0 to 16.0 m/s. The remaining 14 percent are unmyelinated C fibers at 0.2 to 2.0 m/s Hartman 1984. Preganglionic axons run mostly myelinated, though between 8 and 24 percent are unmyelinated depending on the functional class the neuron belongs to Boczek-Funcke 1993. Axons arising at T1 and T2 conduct faster than those from lower segments.
Seventy-three percent of chain ganglion neurons in the same preparation received convergent input, from levels up to three segments above and two below Hartman 1984. The overlap is structural. One segment does not own one organ.
The adrenal medulla skips the second neuron
Preganglionic splanchnic fibers release acetylcholine directly onto chromaffin cells, and those cells answer with catecholamine output graded to stimulation frequency Akiyama 2004. Roughly 80 to 85 percent of them store adrenaline and 15 to 20 percent store noradrenaline Kent 1993. The medulla is a sympathetic ganglion whose second neurons became secretory cells.
Nerve endings deliver noradrenaline to tissue. The medulla delivers adrenaline to blood. One anatomy produces a local signal and the other a systemic one, from the same preganglionic command.
Prevertebral ganglia run their own loops
Intestinofugal neurons in the gut wall synapse onto prevertebral ganglia, a population making up 0.25 percent of myenteric neurons and falling to 0.05 percent in inflamed distal colon Linden 2012. These ganglia close reflex loops that never enter the spinal cord, so part of autonomic regulation happens outside the neuraxis entirely.
02Autonomic anatomy in numbers
What the research shows about autonomic circuitry
03The thoracolumbar column
Sympathetic outflow leaves the cord from one column of cells
Every sympathetic command in the body departs through the intermediolateral cell column, a strip of preganglionic neurons in the lateral gray matter running from T1 to L2. The column measures about 200 micrometers across and runs as clusters of 20 to 150 neurons spaced at roughly 300 micrometer intervals Oldfield 1981. About 75 percent of preganglionic neurons sit in the principal column and the rest occupy three smaller sub-nuclei Pyner 1995.
Morphometry in man counted 5,002 neurons on the right at T6, 5,004 at T7, and 4,654 at T8 Low 1977. About 370 cells are lost per decade of adult life, close to 8 percent of the sympathetic outflow. The Unified Model of Tone reads that as the range doing its work. Health is a defended window and not one setting, so a system can lose substrate and still hold the window.
Which level supplies which organ
The column is mapped along its length. Renal preganglionic neurons occupy T10 to T12 with the mode at T11, and the populations serving the cranial and caudal poles of the kidney separate within T11 Huang 2002. Neurons supplying the small intestine run from the fifth thoracic segment to the second lumbar segment and concentrate between T5 and T10 Mabon 1997. Cells projecting to the superior cervical ganglion span C8 to T6 Pyner 1995.
Stimulating individual rami shows the same order functionally. Forelimb resistance vessels answer to rami T2 to T8, peaking at T7 with a rise in perfusion pressure of 55 mmHg. Heart rate answers to more rostral rami T1 to T5 and peaks at T3 Backman 1999. Each organ has its own peak level along the same column.
The Unified Model of Tone takes the segmental map as a route and not a diagram. Because organs are reached through neurons housed at specific cord levels, a state held at a level has consequences in the organs that level serves.
When the column is lost
Loss of the column is measurable and it is specific. Counts fall to 17 percent of control in Shy-Drager syndrome and 52 percent in idiopathic orthostatic hypotension, with ventral-root B fiber counts falling to 21 and 41 percent alongside them Low 1978. In Parkinson disease the column holds 69 percent of control numbers at T2 and 57 percent at T9 Wakabayashi 1997. Surviving neurons carried Lewy bodies in 24 of 25 cases. Clarke's column, lying immediately beside the intermediolateral column, was untouched.
04Chemical coding
Postganglionic neurons are chemically coded by their target
Postganglionic neurons sort into function-specific pathways that are chemically distinct and separately controlled Jänig 1992. In guinea pig sympathetic ganglia, vasoconstrictor neurons make up as much as 60 percent of the population, pilomotor neurons about 30 percent, and non-noradrenergic vasodilator neurons about 15 percent Gibbins 1992.
The classes differ in size as well as chemistry. Secretomotor cell bodies average 32 micrometers, pilomotor 23, and cutaneous vasomotor 19, and only about half of ganglion cells carry neuropeptide Y at all Gibbins 1991. A single ganglion holds several populations that a stain can tell apart.
Sudomotor pathways are labeled from the cord down
Every one of more than 6,000 corticotropin-releasing-factor terminal baskets in the cat surrounded a sudomotor cell body, and between 96 and 99 percent of those target cells lacked tyrosine hydroxylase Shafton 1992. The chemical label runs the length of the pathway instead of sitting at its end. Rat thoracic chain carries sudomotor and periosteal cholinergic classes but no cholinergic vasodilator class Anderson 2006.
The transmitter is not fixed at birth. Target tissue determines the transmitter phenotype a sympathetic neuron adopts Schotzinger 1994. The chemistry of the pathway is set by where it ends, which makes the periphery a participant in its own regulation.
Vasoconstrictor neurons release noradrenaline and neuropeptide Y
Vasoconstrictor neurons release more than noradrenaline. Constricting cutaneous ear arteries of 80 to 140 micrometers required trains of 50 to 300 impulses. The response stayed frequency-dependent from 2 to 20 Hz, and a neuropeptide Y antagonist cut what survived alpha-1 blockade Morris 1999. In human mammary and radial vessels, stimulation released neuropeptide Y equal to 4 to 6 percent of total vessel content Donoso 2004.
The Unified Model of Tone reads the chemical coding as addressability. Each pathway is separately coded and separately driven, so the organization deciding which pathway runs is doing work that no single transmitter can explain.
05Craniosacral outflow
Parasympathetic fibers leave the neuraxis at two ends
Cranial nerves III, VII, IX, and X carry parasympathetic outflow above, and the pelvic splanchnic nerves carry it below. The vagus reaches the thorax and most of the abdomen. Of roughly 28,000 axons in the ferret cervical vagus, about 27,000 run unmyelinated Asala 1986.
Vagal efferents arise from two brainstem nuclei, the nucleus ambiguus and the dorsal motor nucleus, and cardiopulmonary neurons are mapped across both by target Hopkins 1998. Cardiac vagal motoneurons of the nucleus ambiguus conduct at 1.6 to 13.8 m/s, a median of 4.2 and inside the B fiber range. Of 33 recorded cells, 22 were silent at rest Rentero 2002. Vagal restraint is held by cells that are not continuously firing.
The vagus hands the gut over
Vagal control of the gut fades from front to back. Nearly every myenteric ganglion in the stomach and proximal duodenum receives vagal innervation, against up to 65 percent in the caecum and proximal colon Berthoud 1990. Past that point the enteric nervous system runs motility and secretion on site, with the prevertebral ganglia closing their own loops Linden 2012.
What leaves at the sacrum
Pelvic splanchnic nerves arise from S2 to S4, with an additional S1 root in 18 percent of 31 dissected female pelvises Aurore 2020. Brainstem stimulation reaches pelvic nerve efferents at latencies of 60 to 110 ms, so the bladder is governed by a long loop and not a local one McMahon 1982.
Whether the sacral component belongs to the parasympathetic family by lineage is disputed in the developmental literature, where tracing its ontogeny placed it with sympathetic precursors Espinosa-Medina 2018. The route is not in dispute. Sacral fibers leave S2 to S4 and reach the bladder, lower bowel, and reproductive organs, which is what the segmental map needs. Both sides of the lineage question are set out on the research page for autonomic state.
06The receptor decides
The receptor, not the transmitter, decides what the organ does
Two molecules run this entire system, and neither carries the instruction. Acetylcholine crosses every autonomic ganglion in both divisions. Noradrenaline carries most sympathetic output to tissue. What happens next depends on the receptor waiting for them.
Ganglionic and muscle nicotinic receptors use different subunits
In the superior cervical ganglion, alpha3beta4 accounts for 55 percent of nicotinic receptors, alpha3beta4alpha5 for 24 percent, and alpha3beta4beta2 for 21 percent David 2010. Muscle uses an entirely different combination: two alpha1 subunits with beta1 and delta, plus gamma in the fetal receptor and epsilon in the adult one Takahashi 2002.
Subunit composition sets sensitivity. Hexamethonium blocks wild-type ganglia at an IC50 of 389.2 micromolar and blocks alpha5beta4 knockout ganglia at 22.1 micromolar, a shift of more than tenfold from subunit content alone Simeone 2019. Deleting alpha3 removes five physiologically distinguishable receptor subtypes from the ganglion and produces megacystis and mydriasis Xu 1999.
Cardiac parasympathetic ganglia carry their own version. Alpha-conotoxin MII attenuates the sinus cycle length response by about 70 percent Bibevski 2000. The toxin works at all because the ganglionic subtype is molecularly distinct enough for a peptide to tell it apart Loughnan 1998.
Five muscarinic subtypes split between Gi and Gq coupling
Past the ganglion, acetylcholine acts through five muscarinic subtypes encoded by five separate genes. M2 and M4 couple to Gi and Go, while M1, M3, and M5 couple to Gq and G11 Andersson 2011. The same transmitter therefore inhibits at one target and excites at another. Removing M2 or M3 in mice abolishes vagal bradycardia and vagal bronchoconstriction respectively Fisher 2004.
Alpha and beta receptor families produce opposite effects from the same catecholamines
Alpha and beta families read the same catecholamines and produce opposing effects across tissues Motiejunaite 2021. Nonfailing human ventricle carries 77 percent beta-1 and 23 percent beta-2 receptors across 48 hearts Bristow 1986. At the beta-2 receptor, adrenaline acts with an EC50 of 175 nM against 18 micromolar for noradrenaline, a gap of roughly a hundredfold Suh 1999.
Deleting alpha-1b cuts the pressure response to phenylephrine by 45 percent Cavalli 1997. Alpha-2 agonists suppress evoked noradrenaline release from sympathetic neurons by 70 to 85 percent, which makes the transmitter its own brake Trendelenburg 2001. The reach of these subtypes runs past the vessel wall. Deleting alpha-2A doubles embolus formation after arterial injury Pozgajová 2006, and a beta-3 agonist raises energy expenditure by 35.6 kJ per hour in healthy people Loh 2019. Vascular tone, clotting, and metabolic rate all answer to one transmitter family through different receptors.
This is where the Unified Model of Tone locates specificity. Specificity is correspondence between an input and the organization already present, and it is not magnitude. Noradrenaline is a constant across all of these tissues. The reply changes because the receiving state changes, which is the same reason a well-matched input can reorganize a system that a larger one cannot reach.
The brain does not speak to the heart. It speaks to a ganglion, and the ganglion speaks to the heart.
07Spine to organ
Somatic input reaches autonomic output through the same segment
Lateral loads of 0.5 to 3.0 kg applied to individual spinal segments in anesthetized rats lowered blood pressure by 29.8 mmHg and heart rate by 6.1 beats per minute Sato 1984. Loading was applied at T10 to T13 and L4 to L7. The autonomic nervous system answered a load applied to bone and ligament.
Somato-cardiac sympathetic reflexes run at 41 ms through A fibers and 210 ms through C fibers Li 1996. The gastric vagal equivalent runs at about 120 ms and 360 ms Kimura 1996. Stimulating thoracic and lumbar interspinous tissues changes adrenal output through the same class of pathway Budgell 1997.
Why one segment reports for several
The medial branch of the primary dorsal ramus supplies facet joints and interspinous tissue one and two levels below its own origin Budgell 1997. Lamina I neurons from cervical levels C6 to C8 project onto thoracolumbar sympathetic nuclei, terminating most densely at T2 to T4 Craig 1993. Input entering at one level reaches sympathetic machinery several levels away.
Why visceral pain is felt in the body wall
Convergence in the dorsal horn explains referred pain, and most of the neurons recorded show it. Of 133 neurons recorded at T8 and T9 in the cat, 75 percent responded both to splanchnic nerve stimulation and to a costal cutaneous field Cervero 1983. Among T2 to T4 tract cells carrying cardiac input, 71 percent had somatic fields on the left forelimb and upper thorax, and 86 percent were high threshold Foreman 1984. Heart and chest wall report to the same cells.
Convergence runs in both directions on those neurons. A visceral signal reaching a somatic field is referred pain. A somatic signal reaching visceral machinery is a change in organ regulation. The anatomy does not distinguish them.
This is the mechanism the Unified Model of Tone builds on. Input to the spine can reach organs that no direct nerve connects to it, because the route runs through the segment and not through a wire from spine to organ. Chiropractic care delivers graded mechanical and sensory information into that machinery. The model makes its own claim about what follows. An input matched to the pattern a segment is holding changes the regulation that segment carries, and the organ effect follows the segmental map rather than the location of the symptom.
The model stakes itself on the unification claim beneath that prediction. It holds that cortical excitability, autonomic regulation, map organization, and tissue compliance are readings of one underlying variable. When those measures are recorded in the same people and load on one common factor, the model is confirmed.
08Autonomic tone
How the autonomic nervous system expresses tone
Every part of the nervous system expresses all of tone. In the autonomic nervous system three foundations carry the signature.
Coupling
Viscera and body wall report to the same cells. Of 133 neurons recorded at T8 and T9, 75 percent answered both splanchnic and cutaneous input.
Set point
Autonomic circuits defend a range and not a value. The column holds about 5,002 neurons at T6 and loses 370 a decade without leaving that range.
Input quality
Every autonomic reply begins with a report from the viscera and the body wall. A load of 0.5 to 3.0 kg on one segment moved pressure by 29.8 mmHg.
The remaining foundations run through the same circuitry. Gain: one preganglionic axon reaching 50 to 200 ganglion cells is amplification built into the anatomy. Oscillation: sympathetic and vagal traffic arrives in bursts timed to the cardiac and respiratory cycles instead of as steady drive. Prediction: autonomic output shifts before a movement or a meal, not after it. Load: sustained sympathetic outflow costs energy, which is why it cannot be held indefinitely without expense elsewhere. Constraint: a ganglion can only produce the replies its target tissue has receptors to read. Time course: reflex changes measured in milliseconds sit on top of receptor changes measured in days and column changes measured in decades.
09Neighboring autonomic pages
Where this circuitry is read across the library
The same system read as a state rather than as circuitry. Which instrument reads which channel, what a high or low value means, and what actually moves autonomic state. Start there for a measurement. Stay here for the wiring.
The loops that hold pressure and rate steady from moment to moment, including the baroreflex arithmetic running between the medulla and the cord.
The column the sympathetic outflow leaves from, and how its gray matter is arranged around the intermediolateral cells counted here.
The cranial parasympathetic outflow in detail, including the vagal nuclei supplying the thorax and the upper abdomen.
The single nerve carrying most of the parasympathetic reach below the neck, and what the evidence shows about stimulating it.
What happens when this regulation fails rather than merely shifts, and how that failure is separated from the ordinary variation of a healthy system.
10Frequently asked
Questions about autonomic circuitry
Where in the spinal cord does sympathetic output begin?
Sympathetic output begins in the intermediolateral cell column, a strip of preganglionic neurons in the lateral gray matter running from T1 to L2. The column measures about 200 micrometers across and runs as clusters of 20 to 150 neurons spaced roughly 300 micrometers apart. Morphometry in man counted 5,002 neurons on the right at T6. About 75 percent of preganglionic neurons sit in the principal column, and the rest occupy three smaller sub-nuclei alongside it. Every sympathetic command in the body departs through that column.
Which spinal levels supply which organs?
The intermediolateral column is mapped along its length. Renal preganglionic neurons sit at T10 to T12 with the mode at T11. Neurons supplying the small intestine run from the fifth thoracic segment to the second lumbar segment and concentrate between T5 and T10. Cells projecting to the superior cervical ganglion span C8 to T6. Stimulating rami T2 to T8 raises forelimb vascular resistance and peaks at T7, while heart rate answers to T1 through T5 and peaks at T3. Each organ has its own peak level along the same column.
Why does one transmitter speed the heart and relax the airway?
The receptor decides the reply, not the transmitter. Noradrenaline reaching a beta-1 receptor on cardiac tissue raises rate and force. The same molecule reaching a beta-2 receptor on airway smooth muscle relaxes it. Nonfailing human ventricle carries 77 percent beta-1 and 23 percent beta-2 receptors. Acetylcholine behaves the same way, exciting through nicotinic receptors at the ganglion and then acting through five muscarinic subtypes, two coupling to inhibitory G proteins and three to excitatory ones. Transmitter identity carries only half the message.
Which spinal roots carry the sacral parasympathetic outflow?
Pelvic splanchnic nerves arise from S2 to S4 and join the inferior hypogastric plexus, supplying the bladder, lower bowel, and reproductive organs. An additional root from S1 appeared in 18 percent of 31 dissected female pelvises. Brainstem stimulation reaches pelvic nerve efferents at latencies of 60 to 110 ms, so the bladder is governed by a long loop. Whether this outflow belongs to the parasympathetic family by lineage is disputed in the developmental literature. The route itself is not disputed, and it is what the segmental map needs.
How can input to the spine reach an organ?
Input to the spine reaches organs through segmental autonomic reflexes that have been measured directly. Lateral loads of 0.5 to 3.0 kg applied to individual spinal segments in anesthetized rats lowered blood pressure by 29.8 mmHg and heart rate by 6.1 beats per minute. Somato-cardiac sympathetic reflexes run at 41 ms through A fibers and 210 ms through C fibers. Dorsal horn neurons carry both visceral and somatic input on the same cells, so the route runs through the segment rather than through a wire from spine to organ.
Is the sympathetic nervous system one alarm signal?
The sympathetic nervous system is not one signal. Postganglionic neurons sort into function-specific pathways that are chemically distinct and separately driven. In guinea pig ganglia, vasoconstrictor neurons make up as much as 60 percent of the population, pilomotor about 30 percent, and non-noradrenergic vasodilator about 15 percent. Sudomotor neurons are cholinergic rather than noradrenergic, and every one of more than 6,000 corticotropin-releasing-factor terminal baskets in the cat surrounded a sudomotor cell body. Between 96 and 99 percent of those cells lacked tyrosine hydroxylase.
Why is visceral pain felt in the body wall?
Visceral and somatic input converge on the same dorsal horn neurons. Of 133 neurons recorded at T8 and T9 in the cat, 75 percent responded both to splanchnic nerve stimulation and to a costal cutaneous field. Among T2 to T4 tract cells carrying cardiac input, 71 percent had somatic fields on the left forelimb and upper thorax. The cell cannot report which source a signal came from, so a visceral signal is felt where the somatic field lies. Heart and chest wall report to the same cells.
How does a state held at one spinal level reach an organ?
Through the two-neuron chain and the segmental map. A preganglionic neuron at a given level reaches a ganglion, and a single preganglionic axon contacts 50 to 200 ganglion cells whose targets overlap. Because organs are supplied from defined stretches of the intermediolateral column, a state held at a level has consequences in the organs that level serves. The Unified Model of Tone treats that arrangement as a route into organ regulation rather than a diagram of separate wires. The organ effect follows the segmental map.
11The sources
References
Sources: primary literature, linked inline.
Related evidence