Pre Lab Exercise 19-2 Autonomic Nervous System

Mastering Pre-Lab Exercise 19-2: A Comprehensive Guide to the Autonomic Nervous System

Success in any physiology or anatomy laboratory hinges on thorough preparation. Pre-lab exercise 19-2 is a critical and hands-on exploration designed to demystify the autonomic nervous system (ANS), the body’s unconscious command center for vital functions. This guide serves as your essential companion, transforming theoretical knowledge from textbooks into a clear, actionable plan for the lab. By the end, you will not only know what to do but why each step matters, ensuring a safe, insightful, and truly educational experimental experience. This pre-lab framework focuses on observing the ANS in action through simple, non-invasive tests on yourself and peers, directly linking neural pathways to tangible physiological responses.

Understanding the Objective: What Are We Really Testing?

The primary goal of this pre-lab exercise is to directly observe and document the antagonistic functions of the two main divisions of the ANS: the sympathetic nervous system (SNS), responsible for the "fight-or-flight" response, and the parasympathetic nervous system (PNS), governing "rest-and-digest" activities. You will act as both subject and investigator, using your own body as a living laboratory. The exercise typically involves measuring and comparing physiological parameters—such as pupil diameter, heart rate, and skin temperature/conductance—under contrasting conditions designed to stimulate one division while inhibiting the other. This isn't just about collecting numbers; it's about witnessing homeostasis in real-time and understanding the delicate balance that maintains internal stability.

Essential Materials and Safety Precautions

Before stepping into the lab, ensure you have the following materials, often listed in your official lab manual:

• Ruler with millimeter (mm) markings or a caliper for precise pupil measurement.
• Stopwatch or timer (a smartphone works perfectly).
• Heart rate monitor (optional but ideal) or the ability to manually take a radial or carotid pulse accurately for 30 seconds.
• Thermometer (digital is best) for measuring skin temperature, typically on the fingertip.
• Galvanic Skin Response (GSR) sensor (if available) to measure skin conductance, a sensitive indicator of sympathetic arousal.
• A well-lit area and a dimly lit or dark area for pupil tests.
• A quiet, comfortable space for baseline measurements.

Safety is paramount. This exercise is low-risk but requires ethical and practical care. Always obtain informed consent from any partner. Do not perform tests on individuals with known cardiovascular conditions, epilepsy, or severe anxiety without instructor approval. The stress-inducing tasks (like the cold pressor test) should be brief and immediately discontinued if the subject feels significant discomfort, dizziness, or pain. Never use excessive force or extreme temperatures.

Step-by-Step Procedure: From Baseline to Stimulus

Follow this structured protocol to gather coherent data.

Part 1: Establishing Baselines (Parasympathetic Dominance)

1. Resting State: Have your subject sit comfortably and quietly for 5-10 minutes. This allows their ANS to reach a stable, resting state, where parasympathetic tone is dominant.
2. Measure & Record:
   • Pupil Size: In consistent, moderate room lighting, have the subject look at a distant, neutral point. Using the ruler held gently against the outer canthus (corner) of the eye, measure the horizontal diameter of the pupil in millimeters. Record.
   • Heart Rate (HR): Take a 30-second pulse count and multiply by two to get beats per minute (BPM). Repeat for accuracy.
   • Skin Temperature/Conductance: Measure the temperature of a fingertip (e.g., index finger) and record. If using a GSR sensor, note the baseline microsiemens (µS) reading.

Part 2: Activating the Sympathetic Nervous System

Now, introduce a controlled physiological stressor. The cold pressor test is standard and effective.

1. Preparation: Fill a bowl with cold water (ideally 1-5°C / 34-41°F) and ice. Have a towel ready.
2. Immersion: Instruct the subject to submerge one hand (typically the non-dominant hand) completely in the ice water up to the wrist. They should keep it there for up to 2 minutes maximum, or until they can no longer tolerate the discomfort.
3. Continuous Monitoring: During immersion, the subject should avoid talking or moving excessively. You will measure the responses at the 1-minute mark and/or at the moment they withdraw.
4. Measure & Record (Sympathetic Activation):
   • Pupil Size: Quickly but carefully measure the pupil diameter in the same lighting as the baseline. Sympathetic activation causes mydriasis (pupil dilation).
   • Heart Rate: Take a 30-second pulse count immediately upon immersion or at the 1-minute mark. Expect an increase.
   • Skin Temperature/Conductance: Measure fingertip temperature. Sympathetic activation often causes vasoconstriction in the skin, lowering peripheral temperature. Conversely, GSR will increase significantly due to increased sweat gland activity.

Part 3: Rebound to Parasympathetic Dominance

1. Recovery: After removal, have the subject dry their hand and rest quietly for 3-5 minutes.
2. Measure & Record: Repeat all three measurements (pupil, HR, skin temp/GSR). You should observe a return toward baseline values, demonstrating the system's dynamic equilibrium.

Part 4: The Pupillary Light Reflex (A Pure Reflex Arc)

This isolates a specific, brainstem-mediated reflex.

1. In a dim room, have the subject look at a distant object to establish a baseline pupil size.
2. Shine a penlight briefly (1-2 seconds) into one eye from the side, not directly into the pupil to avoid a glare response.
3. Observe & Record: Note the direct response (pupil constriction in the illuminated eye) and the consensual response (simultaneous constriction in the other eye). This tests the integrity of the oculomotor nerve (CN III) and its parasympathetic fibers. Repeat for the other eye.

The Scientific Explanation: Connecting Observation to Neurobiology

Your observed changes are the visible outcomes of a complex neural cascade.

The Scientific Explanation: Connecting Observation to Neurobiology

Your observed changes are the visible outcomes of a complex neural cascade. The cold pressor test initiates a nociceptive signal from thermoreceptors and pain fibers in the hand. This afferent input ascends to the thalamus and hypothalamus, which act as the central integrators for the autonomic response. The hypothalamus then dispatches signals via two parallel pathways:

1. Sympathetic Activation: Preganglionic fibers descend from the hypothalamus to the thoracolumbar spinal cord (T1-L2). Postganglionic fibers release norepinephrine at target organs. This causes:

   • Mydriasis: Contraction of the radial iris muscle (α1-adrenergic receptors).
   • Tachycardia: Increased SA node firing rate and contractility (β1-adrenergic receptors).
   • Vasoconstriction & Sweating: Cutaneous vasoconstriction reduces heat loss (α1 receptors), while sympathetic cholinergic fibers stimulate eccrine sweat glands, dramatically increasing Galvanic Skin Response (GSR).

2. Parasympathetic Withdrawal: Concurrently, the hypothalamus inhibits the dorsal motor nucleus of the vagus in the brainstem. This reduces tonic parasympathetic (vagal) outflow to the heart, removing the "brake" and further contributing to the observed tachycardia.

During recovery, the cessation of the nociceptive input allows the hypothalamus to re-engage the parasympathetic system. Vagal tone increases (via acetylcholine acting on muscarinic receptors in the heart), slowing the heart rate. Sympathetic outflow diminishes, leading to vasodilation (warming the fingertips) and a decline in sweat gland activity (lowering GSR). The return to near-baseline values demonstrates the system's homeostatic plasticity.

The pupillary light reflex is a distinct, monosynaptic polysynaptic circuit. Light activates retinal ganglion cells, whose axons travel via the optic nerve (CN II) to the pretectal nucleus in the midbrain. From there, bilateral projections reach the Edinger-Westphal nuclei (parasympathetic nuclei of CN III). Preganglionic parasympathetic fibers then travel with CN III to the ciliary ganglion, where postganglionic fibers innervate the sphincter pupillae muscle, causing constriction. This reflex is autonomic but not emotional; it tests the integrity of this specific brainstem pathway and is modulated by a balance of sympathetic (dilator) and parasympathetic (constrictor) tone.

Conclusion

By sequentially challenging the autonomic nervous system with a physiological stressor and a pure reflex test, you have effectively mapped its dual-component architecture. The cold pressor test reveals the sympathetic "fight-or-flight" mobilization and its subsequent resolution by parasympathetic dominance. The pupillary light reflex isolates a fundamental brainstem-mediated parasympathetic circuit. Together, these simple, non-invasive measures provide a powerful window into autonomic flexibility and integrity. Deviations from the expected pattern—such as an absent heart rate recovery, blunted GSR, or a non-consensual pupillary reflex—can signal dysautonomia, ranging from peripheral neuropathies to central brainstem lesions. Thus, this protocol serves as both an educational demonstration of neurophysiology and a rudimentary clinical screening tool for autonomic nervous system function.