May 31, 2022
Written by: Kara McGaughey
Have you ever felt your heart begin to race and a constrictive, queasy feeling creep into your stomach as you commute to work on the morning of an important presentation? Or maybe you’ve noticed that as you lie on your back at the end of a yoga class the tension you’ve been holding begins to dissipate, your heart rate begins to slow, and your breath begins to move a bit more easily.
Both of these examples reflect the body’s response to a specific situation or context, but their physical — and cognitive — experiences couldn’t be more different. At one extreme, the feelings of anticipation associated with giving a presentation correspond to a high arousal state. On the other side of the spectrum, the yoga-induced relaxation represents a low arousal state. You may have heard this dichotomy referred to as “fight or flight” vs. “rest and digest,” but it’s not as simple as the body just being in one state or the other. Instead, each catchy one-liner represents one end of an arousal spectrum or continuum that we’re continually sliding along (Figure 1).
It’s the nature of our current environment that slides us along this arousal continuum and settles us in new places. This constant repositioning means that we’re able to adapt our physical and cognitive state to the current context and generate behaviors that best meet its demands. This adaptation is one of the most important things we do, but how exactly does it work?
How does the body adapt our physical state to current environmental demands?
The arousal continuum and the movement of our physical state along its axis are made possible by two branches of our body’s nervous system: the sympathetic and parasympathetic. These nervous system branches affect the same tissue (Think: organs), but release different chemicals that allow them to have opposite functions in regulating our physical state.
The sympathetic nervous system is responsible for ramping us up towards high arousal “fight or flight” states. Through the release of the chemical messenger norepinephrine, the sympathetic nervous system produces changes throughout the body that support our functioning in these higher stress environments.1 Returning to our example of giving an important presentation, your increased heart rate is due to the release of norepinephrine on cardiac muscles that help pump well-oxygenated blood throughout the body. The queasy feeling in your stomach results from the release of norepinephrine on the digestive system, which shuts down food processing and stops you from feeling hungry. In other words, by increasing the function of some organs and decreasing the function of others performing less-essential jobs, the sympathetic nervous system prepares you for success in the current environment. (You can read more about this connection between arousal, stress, and the digestive system in this recent PennNeuroKnow article).
In contrast, the parasympathetic nervous system predominates and supports our functioning during low arousal “rest and digest” states. The parasympathetic system’s release of the neurotransmitter acetylcholine toggles the body’s resources toward longer-term goals, like decreasing heart rate and increasing digestive function and nutrient absorption.1 While digesting what you ate for breakfast isn’t a priority mid-presentation, it does need to happen for your continued survival. Your body takes advantage of the lulls in arousal that occur in safe, predictable environments (like your yoga mat) to redelegate its resources towards that goal.
So, how do these opposing systems that seem to map onto the extremes of arousal come together to form a continuum that our physical state can slide smoothly along? The answer lies in the fact that both our sympathetic and parasympathetic nervous systems are always active.1 So, rather than alternating between high arousal states with purely sympathetic activity and low arousal states with purely parasympathetic activity, our organs receive constant input from both systems. The relative strength of these inputs determines which state you are in — where you lie on the continuum — at a given time. In other words, the competition between sympathetic and parasympathetic nervous systems (and corresponding norepinephrine and acetylcholine release) allows for finer tuning of how far you’re pushed towards one end of the continuum or the other. This ability to toggle and tune our nervous system activity makes sense when we consider the gradient of experiences (like trying a new restaurant, riding a roller coaster, or sleepily sipping coffee) that we can have along the continuum between attending a yoga class and negotiating a stressful presentation.
How does the brain adapt our cognitive state to current environmental demands?
Navigating the diversity of situations in our day-to-day lives involves more than just adjustment of our physical state. For example, as your body prepares you for your important meeting by increasing your heart rate and decreasing your digestion, your brain is making adjustments, too. Fascinatingly, work in neuroscience suggests that these changes in brain state, like changes in physical state, rely on many of the same neurotransmitters, like norepinephrine and acetylcholine.2
In the periphery (a term neuroscientists use to refer to all parts of the body outside of the brain and the spinal cord) norepinephrine and acetylcholine are released by the sympathetic and parasympathetic nervous systems onto target organs to change how they function. In the brain, norepinephrine and acetylcholine are released from tiny clusters of cells in the brainstem — the locus coeruleus (LC)4 and the basal nucleus of Meynert (BNM),2 respectively — onto neurons to change how they communicate. These changes in neuronal communication are thought to facilitate changes in ongoing brain activity (which we can think of as our brain state or cognitive state).
Adjusting our brain activity seems like a big responsibility for some small pools of neurons. However, despite their small size, the LC and the BNM make a massive number of connections throughout the brain and brainstem. This means they’re in prime position to use their release of norepinephrine and acetylcholine to evoke large-scale changes in brain activity.3 In other words, because of the wide-reaching nature of their connections, subtle changes in the amount of norepinephrine and acetylcholine released by the LC and the BNM can have big effects not only on local activity of specific brain regions, but also on ongoing neural activity at the whole-brain level.
Critically, like with adjustment of physical state in the periphery, many of these brain state adjustments are arousal-related. In fact, the LC and the BNM are two key players in our arousal system and activity in these regions is thought to reflect various aspects of arousal, like attention, surprise, and expectation.2.3 So, similar to how the activity of our sympathetic and parasympathetic nervous systems can toggle our physical state along an arousal continuum, changing the way our body functions, activity of our brainstem arousal system can toggle our brain state, changing the way our brain functions. (In our example of presentation preparation, this might look like increasing activity in brain regions related to current goals and actions, like remembering your talking points.) However, given that the brain and its 100 billion neurons with 100 trillion connections is incredibly complex, imagining the range of cognitive states is not quite as simple as a one-dimensional axis with high and low arousal state extremes.6 Nonetheless, changes to the activity of our LC and BNM (and all the wide-reaching effects that ensue) are still intimately linked to the nature of our current environment and experience.
What does all this mean?
As we navigate the world, our moment-to-moment experience is constantly changing. We experience moments of intense stress and arousal, moments of stillness, and everything in between. One of the most important functions of our body (and our brain) is to keep us balanced as we slide along this continuum of arousal states.
So, the next time you notice your heart rate speeding up, impending waves of nausea, and a loss of appetite when you feel stressed, take a deep breath and remember that your body is working with you, not against you, in the perpetual process of adjusting your internal experience to meet external environmental demands.
- McCorry L. K. (2007). Physiology of the autonomic nervous system. American journal of pharmaceutical education, 71(4), 78. https://doi.org/10.5688/aj710478
- Munn B. R., Muller E. J., Wainstein G, and Shine J. M. (2021). The ascending arousal system shapes neural dynamics to mediate awareness of cognitive states. Nature communications, 12(6016). https://doi.org/10.1038/s41467-021-26268-x
- Samuels, E. R., & Szabadi, E. (2008). Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organization. Current neuropharmacology, 6(3), 235–253. https://doi.org/10.2174/157015908785777229
- Vazey E., M, Moorman D. E., Aston-Jones G. (2018). Phasic locus coeruleus activity regulates cortical encoding of salience information. PNAS, 115(40), E9439-E9448. https://doi.org/10.1073/pnas.1803716115
- Hasselmo M. E., & Sarter M. (2011). Modes and models of forebrain cholinergic neuromodulation of cognition. Neuropsychopharmacology, 36, 52-73. https://doi.org/10.1038/npp.2010.104
6. Zimmer, C. (2011). 100 Trillion connections: New efforts probe and map the brain’s detailed architecture. Scientific American, https://www.scientificamerican.com/article/100-trillion-connections/
Cover Image adapted from Pixabay https://pixabay.com/illustrations/stress-relax-arrows-signs-pole-5576017/