April 11th, 2023
Written by: Joe Stucynski
Picture this: It’s that time again and you’re just about to stand up in front of a large group of people to do some good old public speaking. Your hands are cold and sweaty, breathing is shallow, you’re shaking a little, and your heart feels like it’s about to pound out of your chest. In this moment you’re extremely nervous and anxious, and what’s happening in your body feels like it’s making it worse. We’ve all been there, whether it’s a presentation for school or work, a stage performance, or even a wedding speech. In fact, fear of public speaking is one of the most common phobias.1 When we’re nervous we often think of the emotion as what’s driving the bodily symptoms, but to what extent is the reverse true? Can a racing heart drive the emotional experience of anxiety and feed into a vicious cycle?
Emotional responses come from our brain and our body
Many parts of the brain are needed to regulate your emotions, but emotions also activate a part of the nervous system that extends beyond the brain, called the autonomic nervous system, which is responsible for the famous ‘fight or flight’ (sympathetic) and ‘rest and digest’ (parasympathetic) responses. Via the autonomic nervous system, the brain sends signals to internal organs like the heart, lungs, and gut but it also receives signals back. This sensation of your internal bodily state is called interoception. Much debate has taken place over just how much interoception influences emotion.2,3 Interestingly, it is known that manipulating your facial muscles into expressions like a smile can activate the autonomic nervous system to mimic how the body feels during happiness, which can then lead to the genuine emotion of happiness.4 But studying how much internal bodily sensations contribute to an emotion can be tricky. For instance, existing methods of influencing heart rate through electrical pacemakers or with heart medications can introduce unwanted side effects that may cause additional effects on emotions making it harder to study.
A new method to make a mouse’s heart race
In a recent paper, neuroscientists Brian Hsueh and Ritchie Chen from Stanford University introduced a solution to this problem with a new side-effect-free method of manipulating heart rate in mice. They used this novel method to ask whether increased heart rate alone can cause anxiety.5 The first part of this technique involves placing a special red light sensitive ion channel called ChRmine into the heart muscle cells. The second part involves attaching a red LED light to a vest that the mouse wears, and shining it through the skin onto the heart. — The red light can reach the heart because its lower wavelength allows it to pass through skin, in the same way that your hand glows red when you hold up a flash light to it. The flashlight isn’t showing red because of the blood in your hand, but because all of the wavelengths of light are blocked except for red which can pass through. — When the red light reaches the ion channels in the mouse’s heart, it causes the heart to contract. By controlling how fast the red light flashes, they can control the mouse’s heart rate.
Using ChRmine and red LED light, the researchers could artificially boost a mouse’s heart rate up to 900 beats per minute, about 30% higher than the normal heart rate of 500-800 beats per minute! However they only did this for half a second every 1.5 seconds in order to mimic a particular heart arrythmia called tachycardia, and to make sure it did not adversely affect the mouse’s health. When they turned the red light off, the mouse’s heart rate always returned to normal.
Elevating heart rate changes mouse brains and behavior
So what did they find? Mice don’t like to be in open or exposed areas, and when the researchers increased the mouse’s heart rate in a risky context like a spacious, open cage, the mice exhibited significantly more signs of apprehension and anxiety. Compared to when their hearts were not stimulated, they spent more time sticking to the edges of the enclosure and seeking safety. To visualize which areas of the brain were receiving interoceptive signals from the heart while it was artificially racing, the researchers then captured active neurons and performed whole brain clearing, in a technique similar to that discussed in a recent Neuroknow article. They found that highly active neurons in several brain areas including the brainstem which is known to receive sensations from the internal organs of the body, but also an area called the insula. This was interesting because the insula is known to connect areas of the brainstem with other parts of the brain that help produce emotions.
To confirm that the insula was a key player in producing anxiety during increased heart rate, they recorded from hundreds of insula neurons simultaneously using Neuropixels probes in live, awake mice and found that the insula neurons became active in real time when heart rate was elevated. To further convince themselves that the neurons in the insula were responsible for the increased anxiety that they saw in mice with artificially elevated heart rate, they used a similar technique to shut down the insula. They found that shutting down the insula blocked the signals coming from the heart and prevented the mice from acting more anxious during the various tests. Based on this, they concluded that the insula represents the interoceptive information from the heart and relays it to emotional centers in the brain.
In sum, this study presents some major advancements in both technology and our understanding of body-to-brain communication. In particular, this study reinforces the general idea that when people experience severe anxiety attacks in which their autonomic system goes into overdrive and their heart rate skyrockets, controlling bodily symptoms can help to alleviate the anxiety. In time, this may lead to improved treatments to help prevent such anxiety episodes. So, the next time you find yourself in a situation where you feel like you can’t handle the nerves, try to remember that’s it’s partly your body’s fault and take a deep breath.
References
- Ebrahimi O.V., Palleson, S., Kenter R.M. F., Tine, N. Psychological Interventions for the Fear of Public Speaking: A Meta-Analysis. Frontiers in Psychology, 2019
- Lange, C. G. & James, W. The Emotions (Williams & Wilkins, 1922)
- Schandry, R. Heart beat perception and emotional experience. Psychophysiology 18, 483–488 1981.
- Levenson, RW, Ekman, P, and Friesen, WV. Voluntary facial action generates emotion-specific autonomic nervous system activity. Psychophsyiology, 1990.
- Hsueh, B, Chen, R, et al. Cardiogenic control of affective behavioural state. Nature, 2023.
Image by DAMIAN NIOLET on Pixabay
PennNeuroKnow 
Leave a Reply