Nerves and Hormones: The Body's Communication Networks

Drew Bennett

Now, here's the deal. Your body runs two communication networks at the same time, and nearly every vital sign you will ever take is a readout of both of them working together. Blood pressure, blood glucose, temperature, heart rate. Today we're taking apart the nervous system and the endocrine system, cell by cell, and then we'll spend some time on how you actually sense the world around you.

Lizzie Stevens

And I'll say up front that I get a little nervous when the names start piling up. Oligodendrocyte is a lot of syllables. So if I start drifting into jargon, Drew, stop me. Let's begin with the big picture. Both of these systems keep the whole body in homeostasis, but they deliver their messages very differently, right?

Drew Bennett

Very differently. The nervous system uses electrical impulses along a neuron, and then chemical messengers called neurotransmitters, released into a microscopic gap called the synaptic cleft. It's fast, on the order of milliseconds. The effect is short lived. And it adapts quickly if the stimulus just keeps on going. The endocrine system uses hormones, which are chemical messengers released by ductless glands straight into the bloodstream. It responds in seconds to days. The effects last longer. And it adapts slowly to continuous stimulation.

Lizzie Stevens

The slide deck gives a nice way to picture the hormone side. The hormone is the message, the blood is the delivery system, and the receptor is the correct mailbox. A hormone travels past every cell in the body, but only cells with the matching receptor respond. Those are called target cells. If a cell has no receptor, it's return to sender.

Drew Bennett

That's a solid one, and I'll let it slide without teasing you. Ductless is the word to hang on to. A gland with ducts, like a sweat gland or the digestive part of the pancreas, sends its product through a tube to a surface or a cavity. An endocrine gland has no tube. It dumps hormone into the blood, and the blood takes it everywhere.

Lizzie Stevens

Why does that matter at the bedside? Because when a hormone system goes sideways, the patient doesn't come in saying, my pituitary is off. They show up with altered blood glucose, abnormal blood pressure, fluid and electrolyte problems, temperature intolerance, weight changes, fatigue, or a changed stress response. Those are the clues you'll be reading.

Drew Bennett

And the two systems overlap more than people expect. You'll see in a few minutes that the center of the adrenal gland is really a modified piece of the sympathetic nervous system, and the hypothalamus, which is brain tissue, is the boss of the whole endocrine hierarchy. So let's start with the faster network, the nervous system, and work out who lives in there.

Lizzie Stevens

Quick map first. The central nervous system is the brain and spinal cord. It's the decider. The peripheral nervous system is everything outside those two, the delivery system. It has 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Drew Bennett

The peripheral side splits by direction of traffic. Afferent, sensory fibers carry impulses from peripheral receptors toward the CNS. A trick from the lecture: afferent arrives, efferent exits. Efferent, motor fibers carry commands away from the CNS to effectors, meaning muscles and glands. The efferent side divides again into the somatic system, which is voluntary control of skeletal muscle, and the autonomic system, which is involuntary control of smooth muscle, cardiac muscle, and glands.

Lizzie Stevens

And the autonomic system has two divisions that work at the same time, giving a background level called autonomic tone. Sympathetic is fight or flight and comes out of the thoracolumbar region. Parasympathetic is rest and digest and comes out of the craniosacral region. The balance between them shifts constantly depending on what the body needs.

Drew Bennett

Now the cells. Nervous tissue is made of two families: neurons, which conduct, and neuroglia, also called glial cells, which support. Neurons get the glory, but neuroglia outnumber them, and unlike neurons, glia keep the ability to divide. Neurons lose their mitotic apparatus around six months of age. So a neuron you lose is generally a neuron you don't get back, and that detail is going to matter later when we talk about injury.

Lizzie Stevens

Let's build a neuron from the inside out. There's a cell body, the soma, which is the control center and holds the nucleus. In there you also find neurofibrils, which help hold the cell's shape, and Nissl bodies. Those are also called chromatophilic substance, and they're basically rough endoplasmic reticulum, the protein factory. A neuron that is constantly making neurotransmitter needs a lot of protein machinery.

Drew Bennett

Sticking out of the soma are dendrites. Dendrites receive signals from other neurons and conduct them toward the cell body. Some neurons have one, some have thousands. The memory hook is dendrites receive, axons away. Where the axon leaves the cell body there's a region called the axon hillock, and that's the trigger zone. That's the spot that generates the action potential if the incoming signals add up enough.

Lizzie Stevens

Then the axon itself, one per neuron, carrying the signal away. It can be a few millimeters or up to about a meter long. Many axons are wrapped in myelin, which is mostly lipid and acts like insulation. There are gaps in that insulation at regular intervals, the nodes of Ranvier. And at the far end, the axon branches into terminals, each ending in a synaptic end bulb, which holds vesicles full of neurotransmitter.

Drew Bennett

Neurons also get classified by shape, and this is one nursing students tend to skip. Multipolar neurons have many dendrites and one axon. They're the most common type in the CNS and include the somatic motor neurons. Bipolar neurons have one axon and one dendrite, and you find them in the special senses, the retina, the inner ear, and the olfactory system. Unipolar neurons, sometimes called pseudounipolar, are the sensory neurons whose cell bodies sit in the dorsal root ganglia just outside the spinal cord.

Lizzie Stevens

And by function, the three classes line up with the pathway that runs through every reflex. Sensory, afferent neurons detect a stimulus and carry it to the CNS. Interneurons live only in the CNS and connect the incoming sensory pathway with the outgoing motor pathway. Motor, efferent neurons carry the command to a muscle or gland. Stimulus, afferent, CNS, efferent, response.

Drew Bennett

Now the glial cells. Four live in the CNS and two in the PNS. Start with astrocytes. Star shaped, the most numerous of all the glial cells. Each projection ends in a little foot. Some feet latch onto a capillary and some onto a neuron, so the astrocyte can funnel glucose from the blood to the neuron. Where those feet join with the endothelial cells lining the capillary, you get the blood brain barrier.

Lizzie Stevens

The slide calls it a selective security system for the brain. Small molecules like oxygen, carbon dioxide, and water diffuse across, but larger molecules are blocked. That protects the brain from foreign substances, and it's also why most medications can't reach brain tissue. Astrocytes also regulate the potassium and neurotransmitter concentrations around neurons, give the tissue a structural framework, and after an injury they form a glial scar.

Drew Bennett

Second, oligodendrocytes. These make myelin in the CNS. One oligodendrocyte sends out several projections and wraps multiple axons nearby. Third, microglia. Small, resident cleanup crew. They phagocytize bacteria, debris, and dead tissue. Fourth, ependymal cells. They line the ventricles of the brain and the central canal of the spinal cord. Some secrete cerebrospinal fluid, and some have cilia that help propel it along.

Lizzie Stevens

Over in the peripheral system, Schwann cells make myelin around axons, and satellite cells surround the neuron cell bodies in peripheral ganglia and help regulate their chemical surroundings. The key contrast is myelin: oligodendrocytes in the CNS, Schwann cells in the PNS. And it's not just a trivia question, because that difference decides who can heal.

Drew Bennett

Here's why. A Schwann cell wraps its membrane around a single segment of one axon, layer after layer. The inner layers become the myelin sheath. The nucleus and most of the cytoplasm get pushed to the outermost layer, and that living outer layer is the neurilemma. An oligodendrocyte, though, keeps its nucleus off to the side, away from the sheath, so there's no neurilemma.

Lizzie Stevens

And the neurilemma forms a regeneration tube. If a peripheral nerve is cut but the cell body survives, the Schwann cells guide the axon as it regrows, which can take months. In the CNS there's no tube. On top of that, oligodendrocytes produce molecules that inhibit regrowth and astrocytes build a dense glial scar. So brain and spinal cord injuries are generally permanent. A caution from the source material on that, though: even in the PNS, function is never completely restored after a nerve is severed.

Drew Bennett

Myelin also connects to disease. Conditions like multiple sclerosis and Guillain Barré syndrome involve demyelination of the axon, and the expected effect is slowed signal conduction. That sets up the next piece, which is how a signal actually moves down an axon, and why myelin is such a big deal for speed.

Lizzie Stevens

This is the part I most want students to feel confident about, because it ties back to the sodium potassium pump we covered in the episode on atoms to anatomy. The pump moves three sodium ions out of the cell for every two potassium ions it pulls in. That uneven trade is a big part of why a resting neuron is electrically lopsided.

Drew Bennett

Right, and let's put numbers on it. A resting neuron sits at about minus 70 millivolts. That means the inside is negative relative to the outside. There's more sodium outside, more potassium inside, and inside there are also large negatively charged proteins and nucleic acids that can't leave. The membrane leaks a little potassium out all the time, and a little sodium in, and the pump constantly works to put everybody back on the right side. A resting neuron is polarized, and it has potential to react if a stimulus shows up.

Lizzie Stevens

Then a stimulus arrives. Chemicals, heat, pressure, or incoming signals from other neurons arrive at the dendrites and soma as small graded potentials. If enough of them add up so that the membrane at the axon hillock reaches threshold, about minus 55 millivolts, we get an action potential.

Drew Bennett

Here's the sequence. Voltage gated sodium channels open fast. Sodium rushes in, and the inside swings from negative to positive, up to about plus 30 millivolts. That's depolarization. The slide's shortcut is sodium in equals depolarize. Then the sodium channels inactivate and voltage gated potassium channels open. Potassium flows out and the membrane heads back toward minus 70. That's repolarization, potassium out. It can briefly overshoot into hyperpolarization before settling at rest.

Lizzie Stevens

And while that's happening, the cell is in a refractory period. Ions are on the wrong sides, so the neuron can't respond normally to another stimulus. The pump, sodium potassium ATPase, is the reset. It restores the original distribution. The refractory period has a second job, too. It keeps the impulse moving in one direction down the axon, because the segment behind it can't fire again.

Drew Bennett

Two rules to remember. First, all or none. If a stimulus reaches threshold, the action potential happens at full size every time. A stronger stimulus doesn't make a bigger action potential. Second, then how does the brain know a stimulus is strong? Frequency. A stronger stimulus makes the neuron fire more often, and more nerve fibers get recruited.

Lizzie Stevens

And conduction speed. In a myelinated axon, myelin blocks ion movement across the membrane, so ion exchange can only happen at the nodes of Ranvier. The current flows under the myelin to the next node and triggers another action potential there. The impulse seems to leap from node to node. That's saltatory conduction, and it can reach speeds up to 120 meters per second, compared with about 0.5 to 2 meters per second in unmyelinated fibers.

Drew Bennett

The lecture makes a sensible point about why not everything is myelinated. Fibers that stimulate skeletal muscle, where speed matters, are myelinated. Fibers doing things like stimulating stomach acid secretion, where speed isn't essential, are often unmyelinated. Also, myelination begins around the 14th week of fetal development and isn't complete until late adolescence.

Lizzie Stevens

Now, the signal reaches the end of the axon, and it has to cross a gap. Neurons don't yell across empty space. The sequence is electrical, then chemical, then electrical. The action potential arrives at the presynaptic terminal and voltage gated calcium channels open. Calcium floods in and triggers vesicles to fuse with the membrane and release neurotransmitter into the synaptic cleft.

Drew Bennett

The neurotransmitter binds to receptors on the postsynaptic membrane, and each neurotransmitter fits specific receptors. What happens next depends on the neurotransmitter and the receptor. An excitatory one opens sodium channels and produces a depolarizing potential, an EPSP. An inhibitory one hyperpolarizes the membrane, an IPSP, making firing less likely. Examples include acetylcholine, norepinephrine, and GABA.

Lizzie Stevens

And then the message has to end. The neurotransmitter is terminated three ways: broken down by enzymes, such as acetylcholinesterase breaking down acetylcholine, taken back up into the terminal or nearby cells, or it simply diffuses away. If the message never ended, the postsynaptic cell would keep firing.

Drew Bennett

Here's a clinical pearl that falls straight out of this. Local anesthetics like lidocaine reversibly block voltage gated sodium channels. No sodium influx, no depolarization, no action potential along the sensory fiber, and the patient doesn't feel the stimulus. That's the whole mechanism of numbing a laceration before suturing.

Lizzie Stevens

One more connection to the autonomic system, because it sets up the endocrine half. In the autonomic pathway, a myelinated preganglionic neuron runs to a ganglion and releases acetylcholine onto a postganglionic neuron. The postganglionic neuron then goes to the target. Parasympathetic fibers release acetylcholine there, while most sympathetic fibers release norepinephrine. The takeaway from the slides is that the effect depends on the receptor the neurotransmitter binds to.

Drew Bennett

And sympathetic preganglionic fibers branch to excite many postganglionic neurons at once, which is why a sympathetic response is widespread and immediate. Parasympathetic ganglia sit in or near the target organ, so the response is more localized. Keep that fast, widespread picture in mind, because the adrenal gland is about to take it into the bloodstream.

Drew Bennett

Time to shift from wires to the mail system. The endocrine hierarchy starts in the brain. The hypothalamus is the boss, it controls the pituitary, and the pituitary controls many target glands. The slide deck says, do not begin by memorizing hormone names. Understand the hierarchy first. Hypothalamus, pituitary, target gland.

Lizzie Stevens

The pituitary is pea sized and sits in the sella turcica, a cavity in the sphenoid bone. It's connected to the hypothalamus by a stalk called the infundibulum. And it's really two glands in one. The anterior pituitary, the adenohypophysis, is glandular tissue. The posterior pituitary, the neurohypophysis, is neural tissue.

Drew Bennett

Anterior first. Neurons in the hypothalamus make releasing hormones and inhibiting hormones. They drop them into a special set of blood vessels, the hypophyseal portal system, which carries them straight to the anterior pituitary. There they tell specific cells to release or hold back their hormones. Somatotrophs make growth hormone. Corticotrophs make ACTH. Thyrotrophs make TSH. Gonadotrophs make FSH and LH. Lactotrophs make prolactin.

Lizzie Stevens

And the names tell you the job. Thyrotropin releasing hormone from the hypothalamus tells the anterior pituitary to release thyroid stimulating hormone, which stimulates the thyroid to release thyroid hormone. Corticotropin releasing hormone leads to ACTH, which stimulates the adrenal cortex. Gonadotropin releasing hormone leads to FSH and LH. The slide says, thyroid stimulating hormone stimulates the thyroid. Revolutionary.

Drew Bennett

Now the exam trap. The posterior pituitary does not make hormones. The hypothalamus makes antidiuretic hormone and oxytocin, sends them down axons into the posterior pituitary, and the posterior pituitary stores them until the nervous system signals their release. So anterior makes and releases, posterior stores and releases.

Lizzie Stevens

Two jobs to know. ADH, also called vasopressin, acts on the kidneys to reduce urine volume and prevent dehydration. The hook is ADH equals add H2O. Oxytocin stimulates uterine contractions during childbirth and triggers milk release during nursing. And prolactin produces milk, oxytocin pushes it out.

Drew Bennett

Feedback is what keeps all this from running away. Take cold. The cold stimulates the hypothalamus to release TRH, which prompts TSH, which prompts thyroid hormones, which raise metabolism and warmth. And those thyroid hormones also inhibit further TSH release. Enough product, stop production. That's negative feedback, and it's the pattern for most of this system.

Lizzie Stevens

Let's go gland by gland. The thyroid is the largest endocrine gland. It wraps around the front and sides of the trachea, with two lobes connected by a narrow bridge called the isthmus. Its tissue is made of tiny sacs called follicles, filled with a thick fluid called colloid. The follicular cells make T3 and T4, which regulate metabolism, and unlike other glands, the thyroid can store its hormone for later.

Drew Bennett

Between the follicles sit the parafollicular cells, also called C cells. They secrete calcitonin when blood calcium rises, and calcitonin promotes calcium deposition in bone. Calcitonin lowers blood calcium. Which brings us to the parathyroids, usually four tiny glands on the back of the thyroid. Their chief cells release parathyroid hormone when blood calcium is low.

Lizzie Stevens

PTH raises blood calcium three ways. It stimulates breakdown of old bone, moving calcium and phosphate into the blood. It encourages the kidneys to reabsorb calcium. And it prompts the kidneys to activate vitamin D, which helps the intestines absorb calcium. The hook on the slide is PTH, put the calcium here, meaning into the blood. Calcitonin and PTH are antagonists.

Drew Bennett

Why do nurses care? Calcium is needed for normal nerve and muscle function, clotting, and more. Low calcium makes nerves excitable. You can see carpopedal spasm, which is Trousseau's sign, facial twitching with Chvostek's sign, laryngeal stridor, even dysrhythmias. High calcium goes the other direction: muscle weakness, lethargy, and kidney stones.

Lizzie Stevens

Now the adrenals, which sit on top of the kidneys. Each is really two glands. The inside, the medulla, is made of chromaffin cells, which are modified sympathetic neurons. When stimulated, they release catecholamines into the blood, mostly epinephrine and some norepinephrine. Heart rate and blood pressure go up, bronchioles dilate, and the liver breaks down glycogen and makes new glucose. Fight or flight, delivered by the bloodstream, so it prolongs the sympathetic response.

Drew Bennett

The outer part, the cortex, is glandular tissue with three layers making steroid hormones. The memory hook is salt, sugar, sex, from the outside in. Zona glomerulosa, the outermost, makes mineralocorticoids, mainly aldosterone. Zona fasciculata, the middle, makes glucocorticoids, mainly cortisol. Zona reticularis, the innermost, makes sex steroids, weak androgens that get converted into testosterone and some estrogen.

Lizzie Stevens

Aldosterone acts on the kidneys to retain sodium and excrete potassium, and water follows sodium. Cortisol helps the body handle prolonged stress. It breaks down fat and protein, makes glucose, has an anti inflammatory effect, and suppresses the immune system if secreted over the long term. It also helps keep blood pressure normal.

Drew Bennett

A distinction that matters for fluid balance. ADH retains water and can dilute serum sodium. Aldosterone saves sodium, dumps potassium, and water follows. Both show up when you assess dehydration, heart failure, and renal function. Different hormones, different mechanisms, similar-looking problems at the bedside.

Lizzie Stevens

And that stress pathway is the HPA axis. Stress is processed by the amygdala, which signals the hypothalamus to activate the sympathetic system. After about 10 seconds, as that first epinephrine surge starts to fade, the HPA axis kicks in. Hypothalamus releases CRH, pituitary releases ACTH, adrenal cortex releases cortisol. Short term, that's useful. Chronic low level stress keeps it switched on, and the slides list infection susceptibility, type 2 diabetes, obesity, cardiovascular disease, and depression.

Drew Bennett

Which brings us to the hormones that classically divide into two chemical families, and this explains how they get into cells. Steroid hormones, like cortisol, aldosterone, estrogen, and testosterone, are made from cholesterol. They're lipid soluble, slip through the membrane, bind receptors inside the cell, and modulate DNA transcription. Slips inside. Nonsteroid hormones, made from amino acids, can't cross the membrane. They bind receptors on the surface and trigger a second messenger cascade. Knocks on the door. The slide itself says, please don't let this become molecular biology.

Lizzie Stevens

Next, the pancreas, which is both exocrine and endocrine. Most of it is exocrine, the acini that send digestive enzymes through ducts into the small intestine. Scattered among them are clusters called the islets of Langerhans. Alpha cells make glucagon, beta cells make insulin, and delta cells make somatostatin.

Drew Bennett

Insulin puts glucose in cells. After a meal, blood glucose rises, beta cells release insulin, cells take up glucose, and the liver stores it as glycogen. Glucagon gets glucose back into the blood. Between meals, when glucose falls, alpha cells release glucagon and the liver breaks down glycogen and makes new glucose. Delta cells release somatostatin, which works inside the pancreas to inhibit insulin, glucagon, and growth hormone.

Lizzie Stevens

Clinically, when beta cell insulin fails, glucose stays in the blood. The patient gets hyperglycemia, high urine output from osmotic diuresis, thirst, and cells starving even while glucose is high. Going the other way, acute hypoglycemia triggers a sympathetic surge, so tremors, sweating, and a fast heart rate. Those are the nervous system and the adrenal medulla showing up in a glucose problem, and they need to be recognized and treated right away.

Drew Bennett

Three more quick ones. The pineal gland sits on the roof of the third ventricle and makes melatonin. Darkness raises it, sleepiness follows. The thymus sits in the mediastinum behind the sternum, large in children, shrinking after puberty into mostly fat and fibrous tissue. It makes thymosin and thymopoietin, which help the immune system develop, so it belongs to both systems. And the gonads: ovaries secrete estrogen, and after ovulation the corpus luteum secretes progesterone. Testes secrete testosterone.

Lizzie Stevens

And the surprise, the gut microbiome as an endocrine organ. Gut microbes release signaling chemicals into the bloodstream, hundreds of products compared with one or a few from most glands. Serotonin is a main neurotransmitter in the brain, but over 90 percent of it is found in the intestine. The slides say the precise pathways haven't been completely deciphered, so treat the gut and brain link as an active area, not a finished story.

Drew Bennett

Four relationships to carry out of this section. Hypothalamus to pituitary to target gland. Posterior pituitary stores what the hypothalamus makes. Negative feedback means enough hormone, less secretion. And opposites: insulin lowers glucose while glucagon raises it, calcitonin lowers calcium while PTH raises it.

Lizzie Stevens

Last stretch, the sensory systems, and the principle first. A sensation starts when a specialized receptor detects a stimulus and, if it's strong enough, a sensory neuron sends impulses to the CNS. The sense organ detects. The brain interprets. Most receptors are specialized nerve cells or nerve endings, and the brain filters out most of what they detect, so you're not aware of it.

Drew Bennett

Receptors tell the brain three things: type, location, and intensity of a stimulus. Location depends on receptive fields. The fingertips have many receptors with tiny territories, so the brain can pinpoint a touch. The back has fewer, so localization is coarse. Intensity is the firing frequency again. And with a continuous stimulus, the firing slows and the sensation fades. That's adaptation, and it's why you stop noticing your watch.

Lizzie Stevens

Receptors are classified by what they detect. Chemoreceptors for chemicals, including odors, tastes, and blood gases. Mechanoreceptors for pressure, stretch, and vibration. Thermoreceptors for temperature. Nociceptors for pain and tissue damage. Photoreceptors for light, found only in the eyes. Proprioceptors for body position and movement, found in muscles, joints, and tendons.

Drew Bennett

Pain is protective information. Nociceptors respond to trauma, extreme temperatures, noxious chemicals, and a lack of oxygen. And the brain itself has no nociceptors. Fast pain is sharp and localized, carried on myelinated A delta fibers, abundant in skin and mucous membranes. Slow pain is dull and aching, carried on unmyelinated C fibers, and it's more common in deep structures.

Lizzie Stevens

The pain pathway has two branches. The signal enters the dorsal horn. The spinothalamic tract goes to the thalamus, which relays it to the postcentral gyrus, and that's where you become aware of the pain and where it is. At the same time, the spinoreticular tract sends signals to the brainstem reticular formation, and on to the hypothalamus and limbic system. That's the branch that produces fear, nausea, a racing heart, and sweating.

Drew Bennett

And that connects to referred pain. Sensory fibers from a deep organ and from an area of skin converge on a shared pathway, so the brain misidentifies the source. The classic: cardiac pain felt in the chest, shoulder, or arm, or the jaw. The big idea on the slide is that where the patient feels the pain isn't always where the problem is.

Lizzie Stevens

Special senses quickly. Taste is chemoreception. Chemicals dissolve in saliva, enter taste pores, and stimulate gustatory cells in taste buds, mostly around vallate, foliate, and fungiform papillae. Filiform papillae have no taste buds, they help with texture. Cranial nerves VII, IX, and X carry taste, depending on the location. Smell begins with odor molecules binding cilia on olfactory receptor cells. Axons pass through the ethmoid bone to the olfactory bulb and travel on cranial nerve I, the olfactory nerve. Its direct path to the limbic system is a reason smells tie so closely to emotion and memory.

Drew Bennett

Hearing follows a chain. Sound goes down the auditory canal and vibrates the tympanic membrane. The vibration passes through the malleus, incus, and stapes, the three smallest bones in the body, into the oval window of the inner ear. Fluid ripples in the cochlea stimulate hair cells in the organ of Corti, and the cochlear nerve, part of cranial nerve VIII, carries the signal to the auditory cortex in the temporal lobe. The eustachian tube connects the middle ear to the nasopharynx, equalizes pressure, and can also let infection spread from the throat.

Lizzie Stevens

Balance is also in the inner ear. The semicircular canals detect head rotation, dynamic equilibrium. The utricle and saccule in the vestibule detect head position and straight line movement, static equilibrium. Both use hair cells, and both report through cranial nerve VIII. And vision runs through the retina, where rods handle dim light and cones handle color and sharp detail, then the optic nerve, cranial nerve II, then the occipital lobe.

Drew Bennett

So let's put it together. A sensory receptor depolarizes and fires an afferent neuron, a neuron's sodium and potassium channels carry it along, myelin from Schwann cells or oligodendrocytes speeds it up, and calcium triggers neurotransmitter release at each synapse. Meanwhile, the hypothalamus is steering hormones through the blood to finish the job on a slower clock. Fast network, slow network, one body.

Lizzie Stevens

And if there's one habit to take away, it's this. When a patient has an odd symptom, ask which network is talking, how fast the problem arrived, and which receptor or cell type is involved. That question will get you further than memorizing any list.