11.6 The Resting Hum

Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.

I · A loop that stopped, and a hum that would not drop

The bladder came back from the Spinal Cord module with a name and no explanation. After the October 2015 discectomy she could not feel it fill; a scan found 350 mL still in it after she thought she had emptied, the detrusor did not contract when it should have, and the urologist taught her to pass a catheter four times a day. <em>Neurogenic</em>, he wrote, correctly, and filed it under the spine. What he did not have — what no folder had — was the rest of her autonomic chart: a resting pulse that sat in the 90s between flares and climbed to the 130s during them, with a low variability figure buried in the report that nobody mentioned; a gut that was never settled; hands that were cold in a warm room. In January 2026 a urinalysis came back “awful” and two cultures grew nothing. The urologist wrote “colonized.” The word for a hum that will not drop is <strong>tone</strong>, and nobody had read hers as a baseline.

See it move: Scene 1 · The Resting Hum · Scene 2 · Three Ways They Interact

II · Tone, and three ways to interact

<strong>Autonomic tone</strong> is the baseline level of activity in both divisions, maintained by continuous low-level firing, and it exists so the body can adjust in either direction without switching anything on from zero. The number that makes it concrete is the heart: strip the nerves away and the <strong>SA node paces itself at roughly 100 beats per minute</strong>; your actual resting rate of about 70 is that intrinsic rate with a parasympathetic brake held gently down. So you have two ways to speed a heart — press the accelerator, or ease off the brake — and the body uses whichever is cheaper. The vascular side works differently, and the glossary gives it its own name: <strong>vasomotor tone</strong>, the standing level of sympathetic constriction in your arteriolesSmall arteries that regulate blood flow into capillaries through vasoconstriction and vasodilation. Most vessels get no parasympathetic supply, so there is no second pedal down there; blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. is regulated by one division turning a dial around a set point. The lecture notes list <strong>three ways the two divisions interact</strong>, and only the first one is famous. <strong>Antagonistic</strong> control is straightforward competition — two divisions, one organ, opposite effects, whichever is louder wins: the heart, the pupil, the gut. <strong>Tone</strong> is the second, and it is homeostatic control rather than competition: sympathetic tone sets vessel caliber and therefore blood pressure; parasympathetic tone sets the baseline for the heart and the GI tract — the myenteric plexus, as the enteric page says, is never inactive. <strong>Cooperative</strong> control is the third, where the divisions take one job in turns: the lecture notes’ example is the genitalia (parasympathetic for erection, sympathetic for emission), the glossary’s is salivation; both are right, and if a test asks, give the one your source gave you. The reason to hold all three is diagnostic. If you only know antagonism, every autonomic complaint reads as one division winning a fight. Much of the <strong>dysautonomia</strong> seen in inflammatory disease is a tone problem instead: a set-point that has drifted, in a system still perfectly capable of pushing both pedals. That distinction is the difference between calling a patient anxious and finding out what moved her baseline.

See it move: Scene 2 · Three Ways They Interact · Scene 3 · Loops Without a Brain

III · Loops without a brain, and the bladder with its explanation

A <strong>visceral reflex arc</strong> has the same five parts you learned in the Spinal Cord module — receptorA structure that detects stimuli., sensory neuron, integration center, motor neuron, effector — and every one of them has been modified. The receptor is visceral: a baroreceptor, a chemoreceptorSensory receptors that detect chemical stimuli, such as odors or blood pH., a stretch receptor, a visceral nociceptor. The afferent fiber travels with autonomic nerves but its cell bodyThe central part of a neuron containing the nucleus and organelles. still sits in a dorsalRelating to the back side of the body. root ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS). or a cranial sensory ganglion. Integration happens in the lateral hornA part of the spinal cord’s gray matter, containing autonomic motor neurons. or the brainstemThe lower part of the brain that connects to the spinal cord and controls vital functions., not the cortex. The motor limb is two neuronsThe functional cells of the nervous system that transmit signals. with a ganglion between them. The effector is smooth muscle, cardiac muscle or a gland. And it is noticeably slower — smooth muscle is not in a hurry. The <strong>baroreflex</strong> is the model: stretch receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter in the carotid sinus and the aortic arch are pressure gauges; the carotid sinus reports on cranial nerve IX, the aortic arch on X (you will see the aortic baroreceptor routed on IX in some sources; X is the one to carry); the cardiovascular centers of the medulla weigh the traffic; the answer returns on the vagus to the SA node, and pressure is corrected within a beat or two without consulting anything above the brainstem. It is why you can stand up out of a chair and not faint: gravity drags blood into the legs, carotid pressure drops, the baroreflex answers in a second or two with constriction and a faster heart. When the loops lag — as hers did on the beta blocker — you find out what they were doing. Not every loop even reaches the CNSComposed of the brain and spinal cord; integrates and processes information. : the enteric page’s <strong>short reflex</strong> stays entirely in the gut wall, the <strong>long reflex</strong> goes up and back. And most of what does come up stays out of consciousness; the sliver that reaches the visceral sensorySensory input from internal organs to the CNS. cortex arrives as a vague feeling with no clear address, which is precisely why visceral disease is so often mistaken for something else. The <strong>micturition reflex</strong> is the arc made visible. The detrusor is smooth muscle in the wall; the internal urethral sphincter below it is smooth muscle and not yours to command; the external sphincter is skeletal muscle on the pudendal nerve and very much is. <strong>Storage is sympathetic</strong>: NE at β3 receptors relaxes the detrusor so the bag can fill, and at α1 receptors holds the internal sphincter shut. <strong>Voiding is parasympathetic</strong>, out of the sacral cord at S2–S4: AChalso know as ACh A neurotransmitter that stimulates muscle contraction. at muscarinic receptorsCholinergic receptors found on the effector organs of the parasympathetic nervous system; they media contracts the detrusor and lets the internal sphincter open, and you release the external one when the moment is acceptable. The site’s own case study — Xing-mae Chang, sixty-eight, with an <em>overactive</em> bladder — walks three drugs around this exact loop: an antimuscarinic (Detrol) blocks the contract signal and dries the mouthThe opening of the digestive tract where food enters and mastication begins., stalls the bowel and fogs the mind, because muscarinic receptors are not a bladder specialty; a β3 agonist (Myrbetriq) enhances the relax signal and nudges blood pressure up; an α1 blocker (Hytrin) relaxes the internal sphincter and every arteriole with it, so the pressure drops and standing becomes a risk. (Careful with a line you will meet on that page: the parasympathetic relaxes the <em>internal</em> sphincter; the external one is skeletal muscle and stays yours.) Stina’s bladder is the mirror image — not overactive but <em>areflexic</em>: the sacral limb of the loop stopped answering, so the detrusor never gets the contract signal and the bag fills unnoticed. A catheter does what the reflex cannot. Seen here, it is not a plumbing fault; it is one autonomic loop out of several in a patient whose pulse and gut are also off their set-points — and a reservoir that never empties is exactly where “colonized” comes from.

See it move: Scene 3 · Loops Without a Brain · Scene 4 · The Bladder Returns · Scene 5 · Three Drugs on One Loop

5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: A set-point that drifted, in a system still pushing both pedals, Neither on its own, or A spinal bladder plus deconditioning, nothing more. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.