Experiment: Activate your Sympathetic Nervous System

Background

Most of our human physiology experiments have dealt with the voluntary nervous system (Neuromuscular Neuroscience) or perception (Sensory Neuroscience); but here we will talk about the "involuntary" part of the nervous system, the autonomic nervous system. The autonomic nervous system controls things we are both aware and unaware of but generally do not have much control over - digestion, homeostasis, sweating, blood pressure, heart rate, and many others. It is traditionally divided into two systems, the sympathetic division (which activates the coined "fight or flight" response) and the parasympathetic division (which activates the also-coined "rest and digest" response).

We can study the effects of the sympathetic nervous activation by measuring heart rate. In our previous heart experiment we observed the electrical impulses of the heart and the increase in heart rate during exercise. Let's go a little deeper into heart rate physiology and try to specifically activate your sympathetic nervous system's "fight or flight" response. What happens in "fight or flight?" As you can imagine, if faced with a threat, say, a large shadow moving in the night, the heart rate increases, sweating begins, respiration increases, digestion is inhibited, the eyes dilate, and many other things.

Alternatively, the parasympathetic activates the "rest and digest" response, which performs the opposite of all the things listed above (heart rate decreases, digestion activates, salivation increases, etc). Many of these reactions in both the sympathetic and parasympathetic systems are controlled by hormones, which can be helpful to think of as "neurotransmitters" but that enter the bloodstream instead of the synaptic cleft to find their targets, and instead of response times of 1 ms in the brain, hormones have response times in the scale of seconds to minutes on multiple structures in the body.

For example, when the sympathetic nervous system is activated, the pituitary gland, which anatomically branches off of the hypothalamus in the brain, releases Adrenocorticotropic hormone (ACTH) into the bloodstream, increases cortisol levels, causing various physiological changes including heart rate increase. Simultaneously, the adrenal gland, a neural ganglion located on the kidneys, releases norepinephrine and has a similar effect on the heart.

To activate your sympathetic nervous system, we will use the famous "ice water" stimulus. This is often used in pain studies as humans can tolerate it, everyone has experienced cold hands before, it is not scary and does not result in psychological damage, and is a a good model stimulus easy to replicate in labs around the world. The longer you keep your hand in ice water, the more painful it becomes, activating your sympathetic nervous system, which will lead to an increase in heart rate.

Now let's see it for ourselves!

Downloads

Before you begin, make sure you have the Backyard Brains Spike Recorder installed on your computer. The Backyard Brains Spike Recorder program allows you to visualize and save the data on your computer when doing experiments. We have also built a simple lab handout to help you tabulate your data.

Spike Recorder Computer Software

Class Handout

Video

Print Materials

If you're looking for a PDF to print and scribble on, or a google doc to edit, check out this repository of print resources here!

Procedure

Note if the electrocardiogram appears weird (upside down), interchange the two red alligator clips to opposite forearms.

If you repeat enough observations, you can do statistical hypothesis testing.

Variant- The Diving Response

While we are set up for this experiment, we also examine another effect on the heart rate, called the diving reflex. When a seal lion or other marine mammal dives, its heart rate decreases, and the veins and arteries in peripheral tissues and limbs contract. This limits blood flow to organs not related to the dive, reduces oxygen consumption of the heart, and maintains blood flow to the brain.

But did you know that this response exists in all mammals, including you? When cold water contacts your face and you hold your breath, we can see the "Diving Reflex" as a decrease in heart rate. We can do variations to separate out the effects of water contact on the face versus simply holding your breath.

Heart Rate Diving Response Experiment

Note: Instructor Supervision is advised when doing this experiment in the classroom

Notes