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Chapter 1 endnote 27 & 28, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

[note 27] Each behavior (fuming, crying, planning, withdrawing) is supported by a different physiological pattern in the body, a detail long known by physiologists who study the body for its own sake. [...] The sympathetic and parasympathetic nervous systems are together called the autonomic nervous system. They evolved to support your body’s movement (e.g., so you don’t faint when you stand up). It is well known that sympathetic activity is mobilized for the metabolic demands associated with actual movement behavior (cardio-somatic coupling; Obrist et al. 1970) or expected conditions (e.g., supra-metabolic activity; Obrist 1981).

[note 28] Each time your brain moves any part of your body, inside or out, it spends some of its energy resources: the stuff it uses to run your organs, your metabolism, and your immune system. You replenish your body’s resources by eating, drinking, and sleeping, and you reduce your body’s spending by relaxing with loved ones, even having sex. To manage all of this spending and replenishing, your brain must constantly predict your body’s energy needs, like a budget for your body.

Your brain must stay one step ahead of your body's energy needs for your body to move around, to grow, to reproduce, and so on. To do this in the most efficient manner, your brain anticipates what resources the body will need, such as glucose, oxygen, and salt, and then attempts to meet those needs before they arise. In the book, we name this process "body budgeting" for simplicity, but its scientific name is allostasis.[1][2][3]Allostasis is not a condition of the body, but a process for how the brain prepares the body according to costs (spending energy resources) and benefits (building energy stores).

Allostasis can mean various things. Sometimes allostasis means dynamically regulating resources (i.e., diverting glucose, electrolytes, water, etc. from one system to another or into and out of storage) to meet the body’s expected and current spending needs. For example, in advance of standing up, the heart beats faster and with greater force, blood vessels constrict, and blood pressure raises to ensure that the brain continues to receive blood and oxygen. Sometimes allostasis means signaling the need for resources before the body runs out (e.g., drinking before dehydration occurs) or preparing for the intake of resources in advance of their ingestion (e.g., preemptively salivating because saliva contains alpha-amylase, an enzyme that breaks down carbohydrates).  Sometimes it means spending energy to search for high value resources (e.g., what scientists call exploration), and sometimes it means rest or restricting movements (e.g., what scientists call exploitation).[4] 

Whatever else your brain is doing—thinking, feeling, or perceiving—it is also anticipating the specific actions that it might take, and readying your autonomic nervous system, your immune system, and your endocrine system for that challenge.

Allostasis is different from homeostasis, so named by William James’s old nemesis, the physiologist Walter Cannon.[5] Homeostasis refers to keeping the body within some ranging of optimal functioning, usually at some set-point.[6] It is more reactive than predictive. Allostasis, on the other hand, means anticipating the body's needs and meeting them before they arise so as to maintain energy-balance and metabolic efficiency; the actual values depend on the situation.

When changes in allostasis involve action (e.g., an anticipated need for glucose produces food seeking), this is called motivation, or motivated behavior.

Notes on the Notes

  1. Sterling, Peter. 2012. "Allostasis: a model of predictive regulation." Physiology & Behavior, 106 (1): 5-15.
  2. Sterling, Peter, and Simon Laughlin. 2015. Principles of Neural Design. Cambridge, MA: MIT Press.
  3. Ganzel, Barbara L., Pamela A. Morris, and Elaine Wethington. 2010. "Allostasis and the human brain: Integrating models of stress from the social and life sciences." Psychological Review 117 (1): 134-174.
  4. Cohen, Jonathan D., Samuel M. McClure, and J. Yu Angela. 2007. "Should I stay or should I go? How the human brain manages the trade-off between exploitation and exploration." Philosophical Transactions of the Royal Society of London B: Biological Sciences 362 (1481): 933–942.
  5. Cannon, Walter Bradford. 1932. The Wisdom of the Body. New York: Norton.
  6. For an up-to-date discussion of the differences between allostasis and homeostasis, see Ramsay, Douglas S., and Stephen C. Woods. 2014. "Clarifying the roles of homeostasis and allostasis in physiological regulation." Psychological Review 121 (2): 225-247.