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Autonomic Nervous System 101: Anatomy and Physiology

Alyssa Luck · Jul 12, 2022 · Leave a Comment

The autonomic nervous system (ANS) is responsible for controlling all of the unconscious (autonomous) functions of the body. These autonomous functions maintain internal homeostasis, and also prepare the body to support appropriate voluntary (ie, non-autonomous) responses to external stimuli.

As such, dysfunction in the ANS can contribute to problems maintaining internal homeostasis in general (including problems with digestion, heart rate, and blood pressure, among others), but also problems involving maladaptive responses to external stimuli.

Many levels of control and regulation ultimately feed into control of the ANS, so by no means is this “autonomous” system detached from conscious and voluntary thoughts and behaviors, but understanding the inner workings of the ANS is very helpful as a foundation for understanding the physiological regulatory processes of the body, including emotional and cognitive processes.

To that end, this article will provide a general overview of the structure and function of the ANS, and a brief discussion of some problems with the common “fight or flight” versus “rest and digest” dichotomy of ANS function.

Table of Contents
Divisions of the autonomic nervous system
Anatomy of the sympathetic nervous system
Anatomy of the parasympathetic nervous system
Anatomy of the general visceral afferents
Anatomy of the vagus nerve
Sympathetic and parasympathetic signaling
Physiology of the autonomic nervous system
        Sympathetic/parasympathetic antagonism
        Autonomic innervation of tissues
        Autonomic tone
Problems with the “fight or flight” vs “rest and digest” dichotomy
References

Pick up any large textbook of neuroscience and in its 1000+ pages you will find a relatively short chapter (∼25 pages) devoted to the autonomic nervous system (ANS) and the involuntary control of visceromotor function. This is in contrast to the several hundred pages devoted to the somatomotor nervous system that governs the voluntary control of skeletal muscle contraction to maintain posture and permit locomotion. At the 2015 Society for Neuroscience meeting, none of the scientific sessions over a 5-day meeting had titles that included the word autonomic, sympathetic, or parasympathetic; and less than 50 oral or poster presentations included one of these terms in its title. In light of this limited coverage of the ANS in prominent neuroscience textbooks and scientific meetings, one would think that the ANS is not a very important component of the neurosciences. To the contrary, the ANS influences the function of nearly every organ in the body via its innervation of smooth muscle, cardiac muscle, and glands. In fact, the ANS is responsible for the control of all innervated organs and tissues except skeletal muscles.

Wehrwein et al. Overview of the Anatomy, Physiology, and Pharmacology of the Autonomic Nervous System. (2016)

Divisions of the Autonomic Nervous System

The autonomic nervous system (ANS) includes three divisions that are anatomically and functionally distinct from each other: sympathetic, parasympathetic, and enteric.

The enteric nervous system is the nervous system of the gut, and is a relatively self-contained web of neurons. Although some sympathetic and parasympathetic neurons synapse into the enteric nervous system enabling adaptive control of digestive functions by the CNS, the gut can still carry out its digestive activities through local feedback loops if these connections are broken.

To keep things a bit more manageable, I’m going to leave further discussion of the enteric nervous system for another post, and focus just on the sympathetic and parasympathetic divisions of the ANS here. (Of note, the heart also has an intrinsic nervous system, although apparently it’s not a big enough deal to warrant inclusion as a division of the ANS.)

One additional division bears mentioning: the general visceral afferents.

All neurons are classified as either efferent (motor neurons that travel from the CNS to target tissues and trigger a response) or afferent (sensory neurons that travel from the target tissue back to the CNS carrying sensory data).

The distinction between sympathetic and parasympathetic only exists for efferent neurons. The afferent neurons of the ANS are simply referred to as general visceral afferents, and the sensory information they provide is integrated at many levels – from ganglia to the spinal cord to the brainstem – ultimately promoting a (hopefully) adaptive response to a given situation, whether via sympathetic neurons or parasympathetic neurons or some combination of the two.

It’s debated whether these afferents should be classified as part of the ANS at all, but this is more an issue of semantics than anything. No matter how you classify them, the sensory data provided by general visceral afferents is critical to the proper efferent function of the ANS and its complex regulatory circuits.

Anatomy of the Sympathetic Nervous System

Both the sympathetic and parasympathetic nervous systems generally consist of neuron pairs, where the first neuron originates in the central nervous system (CNS; either the brain or spinal cord) and synapses into a ganglion (a cluster of nerve cell bodies outside of the CNS), and the second neuron originates in the ganglion and synapses at its final effector location (such as a blood vessel, gland, or organ).

The nerves of the sympathetic nervous system (SNS) originate in the spinal cord and synapse onto ganglia just outside the spinal cord, with the postganglionic neurons traveling from these ganglia to the final destination (all organ systems of the body, including the heart, lungs, blood vessels, digestive and excretory organs, immune organs [like the spleen and thymus], and reproductive organs, as well as various glands).

The columns of ganglia on either side of the spinal cord are referred to as the “sympathetic chain” or “sympathetic trunk,” and I’ve often seen people refer to the entire sympathetic division as the “sympathetic chain.”

Four notable sympathetic ganglia that are located outside the sympathetic chain are the celiac, aorticorenal (not depicted in the image below), superior mesenteric, and inferior mesenteric ganglia. Sympathetic postganglionic neurons that innervate the abdominal visceral organs originate in these ganglia.

One notable exception to the paired-neuron structure of the SNS is the neurons to the adrenal glands. Rather than synapsing into a ganglion and then proceeding to the final target, these neurons travel all the way from the spinal cord to the adrenal medulla, synapsing directly onto chromaffin cells, which adopt the role of a postganglionic neuron and secrete norepinephrine and epinephrine directly into circulation.

Phillip Low, MD, College of Medicine, Mayo Clinic. Overview of the Autonomic Nervous System, Merck Manual. September 2021.

Anatomy of the Parasympathetic Nervous System

The structure of the parasympathetic nervous system (PNS) is different from that of the SNS in many respects. Like the SNS, the PNS is generally characterized by neuron pairs, but unlike the SNS, most of the parasympathetic neurons originate in the brainstem rather than the spinal cord. The only parasympathetic neurons that originate in the spinal cord are the pelvic nerves, which innervate the excretory organs (including the distal part of the colon, rectum, and bladder) and reproductive organs.

The rest of the parasympathetic neurons are cranial nerves, including the 10th cranial nerve (and the most significant part of the PNS): the vagus nerve. Note that not all cranial nerves are part of the PNS – only the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. The others are sensory or control voluntary, not autonomic, functions, and thus are not part of the PNS.

Most of the parasympathetic neurons, including those of the vagus nerve, synapse onto ganglia that are very close to the target organ, which is what gives the parasympathetic nervous system its name (“para” means “adjacent to”).

Phillip Low, MD, College of Medicine, Mayo Clinic. Overview of the Autonomic Nervous System, Merck Manual. September 2021.

Anatomy of the General Visceral Afferents

Most of the sensory information from visceral organs and tissues regulated by the ANS is transmitted to the CNS via the vagus nerve, terminating in the nucleus of the solitary tract (NTS) in the brainstem. In fact, only about 20% of vagal fibers are efferent (and part of the PNS), while the other 80% are afferent. Visceral afferent neurons are also found in a few of the other cranial nerves, and these terminate in the NTS as well.

Other general visceral afferents carry sensory data from visceral organs and tissues to the spinal cord, enabling local reflexive regulation of organ function without involvement of the brain (although plenty of these afferent signals make their way from the spinal cord up to the brain as well).

The visceral afferents collect sensory data from their target tissues using various receptor types, including baroreceptors (pressure), mechanoreceptors (deformation), thermoreceptors (temperature), chemoreceptors (small molecules), and nociceptors (pain).

Anatomy of the Vagus Nerve

The efferent part of the vagus nerve (the 20%) has two different branches that are distinct in both anatomy and function.

One branch of the vagus nerve originates in the dorsal motor nucleus of the vagus nerve (DMNV) and is composed of unmyelinated nerve fibers that supply the subdiaphragmatic organs, as well as some fibers to the heart and lungs.

The other branch of the vagus nerve originates in the nucleus ambiguus and is composed of myelinated nerve fibers that mostly serve the heart and lungs, as well as some of the tissues of the face, head, and throat.

While there is broad agreement about these anatomical features, there does not appear to be consensus about their significance. According to researcher Stephen Porges, this anatomical distinction is crucial for properly understanding the function of the vagus nerve, because the two branches have very distinct modes of operation. His evolutionary and physiological framework for viewing the function of the vagus nerve is known as the polyvagal theory.

However, a great number of modern papers entirely gloss over this distinction when discussing the vagus nerve, and I’ve seen very little acceptance of the polyvagal theory in the literature. It appears to be occasionally refuted, but mostly ignored.

I’ll write a separate post about the polyvagal theory, because I think it’s a fascinating perspective. For now, suffice it to say that these anatomically distinct branches of the vagus nerve exist, and that this knowledge may be useful for further exploration of ANS research.

[Update Nov 6 2022: Full analysis of polyvagal theory published here!]

Sympathetic and Parasympathetic Signaling

For autonomic signaling, two molecules play the leading roles: norepinephrine and acetylcholine.

Preganglionic neurons in both the SNS and PNS generally signal with acetylcholine at the ganglia. Postganglionic neurons in the PNS also signal using acetylcholine, while postganglionic neurons in the SNS generally use norepinephrine (with some exceptions).

Both neurotransmitters are very rapidly removed from the synaptic cleft, either through reuptake (norepinephrine) or degradation (acetylcholine). However, recall that the adrenal medulla functions as a specialized postganglionic neuron in the SNS, secreting its norepinephrine (along with epinephrine) directly into circulation. These circulating neurotransmitters/hormones must be taken up and degraded by the liver, which takes a couple minutes rather than a couple milliseconds.

Several different receptor types are sensitive to norepinephrine and acetylcholine, with the main three receptor categories being adrenergic (for norepinephrine) and muscarinic and nicotinic (for acetylcholine). Preganglionic acetylcholine signaling in both the SNS and PNS typically acts through nicotinic receptors, while postganglionic signaling typically acts through adrenergic receptors in the SNS and muscarinic receptors in the PNS.

Both sympathetic and parasympathetic neurons often release signaling peptides and other compounds in addition to the main neurotransmitter, which provides one of many mechanisms introducing complexity into autonomic signaling. The coexistence of acetylcholine or norepinephrine with another molecule in functional groups of neurons is called neurochemical coding, although it’s sometimes referred to as non-adrenergic non-cholinergic (NANC) transmission.

Another modulating factor in autonomic nervous transmission is the existence of autoreceptors and heteroreceptors on ANS neurons. This allows for autoinhibitory feedback (whereby release of either acetylcholine or norepinephrine by a neuron prevents further release of that same neurotransmitter), as well as reciprocal inhibition of cholinergic vs adrenergic signaling in tissues that are innervated by both sympathetic and parasympathetic neurons. However, it also allows for positive feedback, whereby neurotransmitter release can facilitate, rather than inhibit, the release of other neurotransmitters in the region.

I won’t get too into the complexity of signaling here, so suffice it to say that not only does each type of receptor have several subtypes (eg, alpha and beta adrenergic receptors), but the effect a given type of receptor will have on a specific tissue is totally dependent on the internal signaling cascade within the tissue itself. So despite only using two neurotransmitters (plus epinephrine), the sympathetic and parasympathetic nervous systems exert extremely granular control over their target tissues.

Physiology of the Autonomic Nervous System

“The ultimate responsibility of the ANS is to ensure that the physiological integrity of cells, tissues, and organs throughout the entire body is maintained (homeostasis) despite perturbations exerted by both the external and internal environments.

Wehrwein et al. Overview of the Anatomy, Physiology, and Pharmacology of the Autonomic Nervous System. (2016)

As mentioned in the introduction, the ANS is responsible for the unconscious functions of the body to maintain a state of homeostasis. Functionally, this means it controls all innervated tissues and organs except for skeletal muscle. It regulates airway resistance, blood flow, blood pressure, body temperature, digestion, energy balance, waste excretion, fluid volume, glandular secretions, heart rate, immune function, inflammatory processes, salt and water balance, sexual function, and urination. Notably, some autonomic functions, such as breathing and excretion, can also be brought under conscious voluntary control.

The hypothalamus and brainstem are the main regulatory centers that receive afferent feedback and trigger efferent autonomic impulses, which are carried out by the peripheral nerves of the SNS and PNS. Some sensory/motor integration and regulation also happens at the level of ganglia (bundles of nerve cells) and plexuses (groupings of ganglia), either in the spinal sympathetic chain or closer to the target organs.

Sympathetic/Parasympathetic Antagonism

Although there’s a common conception of the sympathetic and parasympathetic divisions as antagonistic, that’s only true insofar as each of the two divisions predominates during different externally-oriented physiological states (as discussed further in subsequent sections). In organs that receive dual innervation, the two divisions do tend to control opposite functions, but on the level of tissues, usually only sympathetic or parasympathetic innervation is present.

One likely reason for the persistent focus on antagonism between the two systems is the fact that historically, the heart has been the focus of much of the research on autonomic function and regulation, largely because it offers so many metrics (heart rate, blood pressure, etc) that can be measured noninvasively. And the heart is probably the best example of true antagonism between the SNS and PNS, with several tissues (including the sinoatrial note and atrial muscle) receiving both sympathetic and parasympathetic innervation that have opposing functions.

Autonomic Innervation of Tissues

However, in most other organs, the SNS and PNS innervate different tissues, allowing for complementary and synergistic control of organ function in different states. And in many cases, an entire gland or organ only receives input from one division of the ANS.

For instance, the lacrimal glands and nasopharyngeal glands only have parasympathetic innervation, while the adrenal medulla, most blood vessels, the gallbladder, kidney, and the immune organs receive only sympathetic innervation. It’s often claimed that brown adipose tissue, liver, and spleen are also exclusively sympathetic, but there’s some evidence that the dorsal vagus also innervates these tissues.

The following table shows some examples of target tissues and their response to SNS or PNS activation. A better and more detailed table that also includes parasympathetic receptors is available in this paper (Table 4). Table 1.1 in this book (available online) is also very good.

Laurie Kelly McCorry, PhD. Physiology of the Autonomic Nervous System, 2007.

Autonomic Tone

While this table shows the results of stimulation of sympathetic or parasympathetic nerves, it’s also important to note that both systems are tonically active, meaning they have a baseline level of activity to a specific set of tissues and organs.

Autonomic tone is probably best characterized in the heart, where the PNS (specifically the ventral vagus) is tonically active. In the absence of external nervous impulses, the heart would beat at around 100 bpm, so the baseline vagal tone functions as a “vagal brake” keeping resting heart rate below that in healthy humans.

Functionally, this allows for rapid increases in heart rate (up to 100 bpm) by decreasing vagal influence (taking the foot off the brake) without the need for sympathetic activation (putting the foot on the accelerator).

The iris of the eye is another autonomic muscle that is under tonic parasympathetic control. On the other hand, most blood vessels are under tonic sympathetic control, as are the kidney and the bladder. Sympathetic neurons in the bladder keep the urethral sphincter contracted and the bladder wall relaxed, allowing it to fill with urine; only when you pee does sympathetic activity withdraw, allowing parasympathetic neurons to trigger relaxation of the urethral sphincter and contraction of the bladder.

These are just a few examples to illustrate the cooperative control of autonomic body systems by the SNS and PNS in the ambient state.

Problems with the “Fight or Flight” vs “Rest and Digest” Dichotomy

So far, we’ve been pretty “zoomed in,” looking at the ANS on the level of tissues and organs. On the level of the organism, the scientific conception of the physiological roles of the sympathetic and parasympathetic divisions of the ANS is largely the same as the popular conception: that the SNS is responsible for states of “fight or flight,” while the PNS is responsible for states of “rest and digest.”

In a sense, this is true. The sympathetic division does predominate in states that require mobilization of the organism, including defensive “fight or flight” states as well as non-defensive mobilization (such as exercise). These physiological states require liberation of fuel and transport of oxygen to optimally power the skeletal muscles of the body. This results in the sympathetically-mediated physiological changes we’re all familiar with, such as increased heart rate and respiration, routing of blood flow toward skeletal muscle at the expense of the visceral organs, and release of stored glucose and fatty acids into circulation.

On the other hand, the parasympathetic division predominates in resting states, commonly referred to as “rest and digest,” promoting physiological processes such as digestion and storage of food energy and tissue repair.

However, there seems to be a common misconception that the SNS and PNS are reciprocal and inhibitory on a systemic level, leaving an organism in either a systemically sympathetic-dominant state of “fight or flight” where parasympathetic functions are suppressed, or a systemically parasympathetic-dominant state of “rest and digest” where sympathetic functions are suppressed.

This is not the case. Rather than a seesaw, the SNS and PNS function more like an orchestra, where different internal and external landscapes require differential recruitment of the two systems on a granular level.

Sex is a great example of this. Parasympathetic activity to the genitals is required for sexual arousal (lubrication and erection), while concomitant sympathetic influence on the heart and blood vessels increases heart rate and blood pressure, and sympathetic influence on the eyes and skin causes pupil dilation and flushing or sweating. And although parasympathetic activity to the genitals is necessary for arousal, sympathetic activity is necessary to achieve orgasm and ejaculation.

Another example is postprandial (i.e. post-meal) autonomic regulation. True to its “rest and digest” designation, the PNS stimulates digestive functions in the gut and other digestive organs (like the pancreas), but that doesn’t mean the entire body is in maximum “parasympathetic mode.” On the contrary – sympathetic outflow to the heart and vasculature is increased postprandially to maintain blood pressure homeostasis. (Source 1, 2, 3) There’s even some evidence that sympathetic nervous system activation is partially responsible for the thermogenic effect of food. (Source 1, 2)

A final example is the diving reflex, which is the ANS-mediated reflex found in all mammals in response to the face being submerged. To conserve oxygen, parasympathetic nerves trigger significant bradycardia, while sympathetic nerves constrict most peripheral blood vessels to constrain blood to the heart and brain. (Source)

Even within the heart itself, sympathetic and parasympathetic activation can co-occur to accommodate specific physiological demands. For instance, during the above mentioned heart rate decrease as a result of vagal effects on the sinoatrial node, there’s also a concurrent sympathetic activation to the ventricles to increase stroke volume. (Source) (For a really interesting discussion of the diving reflex, you can check out this Scientific American article from 1963 !)

So clearly, despite the predominance of the “sympathetic = fight or flight and parasympathetic = rest and digest” dichotomy, the two divisions of the ANS operate synergistically, and with regional control rather than necessarily systemic activation.

The point is that just because whole-body states of maximal SNS or PNS activation are possible, does not mean that these two divisions operate antagonistically on a systemic level. You are not always in either “fight or flight” or “rest and digest.” Always remember that the body is complex and does not lend itself to binaries and dichotomies.

I really like the way Shaun Morrison puts it in his 2001 review Differential control of sympathetic outflow, so I’ll close with that:

Recognition of the important role of the autonomic nervous system (ANS) in coping with life-threatening challenges led to the earliest concepts of the sympathetic nervous system as a monolithic effector, activated to globally enhance organ function and substrate availability for fight or flight and to protect cerebral perfusion in the event of significant injury. In contrast, the parasympathetic component of the ANS is engaged in the aftermath of such challenges to coordinate recovery, a reduction in energy utilization, and replenishment of energy stores.

However, compared with our ancestors, modern lifestyles have all but eliminated the danger of predatory aggression, and we rarely engage the central autonomic networks developed by evolutionary pressure to cope with the most stressful challenges to homeostasis. In their stead, we are faced with the increasing awareness and prevalence of diseases such as hypertension, obesity, cardiac arrhythmia, heart failure, and diabetes, in which altered development or control of the ANS can play a significant role.

Within this framework and, importantly, with the availability of more powerful and precise electrophysiological and anatomic techniques, research on the regulation of autonomic outflow has eclipsed earlier views with an organizational model that emphasizes the differential central control of sympathetic outflows to functionally specific targets and the hierarchical interactions among defined populations of neurons that produce the patterns of autonomic efferent activity supporting behavior and reflex responses.

The focus has evolved toward an understanding of the central neural networks generating and controlling the levels of ANS activity to specific tissues, including those with noncardiovascular functions, with the recognition that the fight-or-flight response is just one of many options in an extensive repertoire of effector activation states in which the central autonomic network may exist.

Shaun Morrison. Differential control of sympathetic outflow. (2001)

References

Ernsberger, et al. The sympathies of the body: functional organization and neuronal differentiation in the peripheral sympathetic nervous system. (2021)

Goldstein, David. Principles of autonomic medicine. (Version 3) [700+ page open-source PDF written with the layperson/patient in mind]

Karemaker, John. An introduction into autonomic nervous function (2017) [this gives a wonderful sense of the complexity of the physiology of the ANS and an evolutionary perspective, and also makes a point to mark sections that reflect the author’s as-yet speculative thoughts and hypotheses vs well-accepted facts]

Low, Phillip. Overview of the Autonomic Nervous System. (2021) [this is a good basic overview if you want something simple]

Mathias and Bannister. Autonomic Failure: A Textbook of Clinical Disorders of the Autonomic Nervous System. (2013)

McCorry, Laurie Kelly. Physiology of the Autonomic Nervous System (2007)

Morrison, Shaun. Differential control of sympathetic outflow. (2001)

Mueller et al. Structural and functional connections between the autonomic nervous system, hypothalamic-pituitary-adrenal axis, and the immune system: a context and time dependent stress response network. (2022)

Sanvictores and Tadi. Neuroanatomy, Autonomic Nervous System Visceral Afferent Fibers and Pain (2021)

Waxenbaum et al. Anatomy, Autonomic Nervous System. (2021)

Wehrwein et al. Overview of the Anatomy, Physiology, and Pharmacology of the Autonomic Nervous System. (2016) [awesome awesome review, highly suggest giving it a read if you want a more complex overview than the merck manuals thing above]

Related

Anatomy, Biochemistry, and Physiology autonomic, nervous system, parasympathetic, sympathetic, vagus

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Hi! I’m Alyssa. I like thunderstorms and cats, hate wearing shoes, and enjoy devising extensive research projects for myself in my free time. This is me in Bali with a monkey on my shoulder. And this is my blog, where I muse about health-related topics and document my relentless self-guinea pigging. If you want to know more about me, click here!

alyssa.luck

alyssa.luck
Photo dump from the last year. Thanks to everyone Photo dump from the last year. Thanks to everyone who made 28 the best yet - excited for 29🥰

(PS. In case anyone wants to know what it’s like in my head, I was going to write something like “year 28” or “my 28th year” but then I realized that the year between your 28th and 29th birthdays is not your 28th year of life, it’s your 29th year. I am turning 29 because I have been alive for 29 years. So then I had a whole thing about how to word it without being inaccurate and ended up going with what you see above which is vague and weird but the point is it was a good year and I love all the people in my life dearly)
Biology of Belief (2005) was written by Bruce Lipt Biology of Belief (2005) was written by Bruce Lipton, who earned a PhD in developmental biology in 1971 and was an anatomy professor and academic researcher in the 70s and 80s. Despite the book's presentation and Lipton's background, this is not a science book. It is an exposition of an ideology, supported by haphazard and poorly contextualized nuggets of evidence, rhetorical leaps, and a mind-boggling overuse of analogies. 

The book largely failed to deliver on its promised content. What it does is argue for the primacy of the environment over DNA in controlling life; propose that the cell membrane rather than the nucleus is the "brain" of the cell; invoke quantum physics to explain why modern medicine fails; explain that our behavior is largely controlled by our subconscious mind; inform parents that they therefore have a great deal of control over the destiny of their children; and conclude that humans must become nonviolent protectors of the environment and of humanity because Everything Is Connected.

It’s not that these points aren’t relevant to the topic at hand - they are. But they were not connected in a coherent way that would explain how “belief” actually works (like…biologically), and the treatment of scientific concepts throughout was careless, or perhaps disingenuous.

I think he's correct about many things, some of them being common knowledge. For instance, the "new" science of epigenetics is now old news, as is the critical role of parenting and early environment in shaping a child’s future. But however important these and attendant concepts may be, the book did not do a good job explaining, supporting, or connecting them. 

As far as practical guidance, he refers the reader to a list of resources on his website, which is fine, but I expected some scientific insight into how/why those modalities work. None was given. 

On the plus side, the book was quite thought-provoking, and I came away with loads of references and topics to follow up on. My favorite line? "There cannot be exceptions to a theory; exceptions simply mean that a theory is not fully correct."
Friedrich Nietzsche, The Gay Science (section 382) Friedrich Nietzsche, The Gay Science (section 382), as quoted in the introduction to Thus Spoke Zarathustra because I like the translation better.
This paper totally changed the way I think about e This paper totally changed the way I think about early nervous system development and the relationship between physiology and sociality. 

The authors propose that newborn babies are not inherently social, and have just one goal in life: physiological homeostasis. I.e. staying alive. This means nutrients, warmth, and regulation of breath and heart rate, i.e. autonomic arousal (it’s well-accepted that newborns sync their breathing and heart rate with caregivers through skin to skin contact). 

All these things are traditionally provided by a loving caregiver. So what the baby experiences during the first weeks of life, over and over, is a shift from physiological perturbation to homeostasis (a highly rewarding event inherently) REPEATEDLY PAIRED with things like the sound of a caregiver’s voice and seeing their face. Thus, over time, the face/voice stimuli become rewarding as well. 

The authors argue that THIS is the beginning of humans’ wiring for sociality, and may explain why loving social interactions can have such a profound regulating effect on physiology throughout life: because the brain was trained for it at an early age. 

This framework holds all kinds of fascinating implications for what happens if that initial “training” isn’t so ideal. What if the return to nutritional homeostasis via feeding is paired with negative expressions and vocalizations rather than loving ones, perhaps as could occur with PPD? What happens if the caregiver has poor autonomic regulation, such that social stimuli become paired with cardiorespiratory overexcitement in the baby? Could that have potential for influencing later introversion vs extroversion? (Because if social interaction is paired with autonomic overexcitement, that could lead to social interaction literally being more energetically draining, which is what introverts experience. Thoughts?)

For my energy metabolism enthusiasts: Table 1 in the paper draws a link between metabolic rate and sociality across species. Swipe for a screenshot. 

Anyway, check out the paper! It’s free, just google “growing a social brain pdf.”
I’ll be under general anesthesia in a couple day I’ll be under general anesthesia in a couple days to have two tooth implants placed, and I think I’ll take the opportunity to have a little heart-to-heart with my subconscious mind. A bit of medically-assisted self-hypnosis, if you will. 

I randomly stumbled upon these papers a couple months ago - an RCT showing reduced post-op pain in patients who listened to recorded positive messages while under general anesthesia, plus a post-hoc analysis of the same data that found reduced post-op nausea and vomiting in a subset of high-risk patients. 

The full review paper from the first slide is unfortunately in German, but it has long been recognized that even when unconscious, the patient is listening (for better or for worse). 

It boggles my mind that it isn’t standard of care to have patients listen to recordings like this while under sedation, considering that almost nothing could be easier, safer, or cheaper, and we have at least some evidence of significant efficacy. I mean c’mon, what more could you want from an intervention? 

(Yeah, I know. Profit. If anyone still thinks that our medical system operates with patient well-being as the foremost goal, you’re deluding yourself.)

“There should be a fundamental change in the way patients are treated in the operating room and intensive care unit, and background noise and careless conversations should be eliminated.”

“Perhaps it is now time to finally heed this call and to use communication with unconscious patients that goes beyond the most necessary announcement of interventions and is therapeutically effective through positive suggestions. When in doubt, assume that the patient is listening.”
If you've seen "vagus nerve exercises" that have y If you've seen "vagus nerve exercises" that have you moving your eyes or tilting your head, you've probably encountered the work of Stanley Rosenberg. The exercises he created and introduced in his 2017 book now appear in instructional videos all over the internet. 
 
The book itself has much to recommend it: it's accessible, it's practical, it's inspiring. But it has one major flaw: the solid practical and informational content regarding the cranial nerves is framed in terms of the scientifically dubious polyvagal theory. 
 
I particularly enjoyed the book as an introduction to the therapeutic arena of bodywork, of which Rosenberg is a skilled practitioner. His book is full of case reports that demonstrate how immensely powerful extremely subtle movements and physical manipulations can be. These do need to be kept in perspective: it's a small sample size of the most remarkable cases, and the results were achieved within the supportive clinical environment of a skilled practitioner. You can tell from his descriptions how refined his technique is. But nevertheless, it was a paradigm-shifting read for me, and the exercises give you something concrete to play around with. 
 
The book also brought the cranial nerves and the concept of “social engagement” to the fore as arbiters of health. Rosenberg has a solid background in cranial nerve anatomy and shares many interesting tidbits and considerations that you don’t typically hear; for instance, the potential impact of dental and orthodontic work on cranial nerve function.
 
So, is it worth reading? If any of the above piques your interest, go for it! Just read my post on polyvagal theory first – you can use the book to practice separating the wheat (solid informational content) from the chaff (pseudoscientific framing). If nothing else, the book is a nice reminder that genuine healers who get lasting results for their patients do exist.

But if you just want to try the exercises, you can easily find them all on YouTube. 

“You learn techniques to understand principles. When you understand the principles, you will create your own techniques.” -Stanley Rosenberg
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