Exercises: Receptors and effectors

Classify sensory receptors according to the type of stimulus they detect (photoreceptors, mechanoreceptors, chemoreceptors, thermoreceptors, and nociceptors) and give a body example of each.

Differentiate between exteroceptors, interoceptors, and proprioceptors, indicating where they are located and what information each type provides.

Name the three types of effector organs involved in the interaction function and give an example of a coordinated response for each.

Name the five "classical" senses and add at least two lesser-known internal senses (balance, proprioception, hunger, thirst, visceral pain), indicating what information each one provides.

Explain the path of light through the eye (cornea, pupil, lens, vitreous humor, retina) and the function of each element, indicating where the image is formed and where it is transformed into a nerve signal.

Distinguish between the rods and cones of the retina: what each type detects (dim light versus color and detail), where each abounds (periphery versus fovea) and why at night we see in black and white.

Describe the path of sound through the ear (pinna, ear canal, eardrum, ossicles, cochlea) and explain in which structure vibrations are converted into nerve impulses.

Distinguish between taste and smell, and explain why with a heavy cold food "tastes like nothing", resorting to retronasal olfaction and to the notion of flavor as a combination of both senses.

The skin contains several types of receptors (touch, pressure, temperature, and pain). Explain the advantage of this sensory integration and describe what happens when you touch something that is cold, rough, and sharp all at the same time.

Compare the compound eye of an insect with the eye of a vertebrate: the basic structure of each, the type of image they form, and the advantage each design offers to the animal that has it.

In response to a sudden loud noise, the pupils dilate and the heart rate accelerates, as summarized in the diagram. Indicate which element of the pathway (receptor, nervous center, or effector) each numbered box (1 to 4) represents, and which part of the nervous system (voluntary or autonomic) controls each step.

Explain at which point the image is focused in a myopic eye and in a hyperopic one (in front of or behind the retina), which type of lens corrects each defect (diverging or converging), and which factors are related to the increase of myopia in young people (close-up work, few hours outdoors).

The inner ear is also the organ of balance. Explain what the semicircular canals and the utricle detect (turns and accelerations, the position of the head) and why getting seasick on a boat is a conflict between what the eyes say and what the inner ear says (motion sickness).

Nociceptors warn us of damage. Explain the protective function of pain and discuss why people with congenital insensitivity to pain suffer burns, fractures, and infections that they do not detect in time.

The fingertips can distinguish two pinpricks one millimeter apart; the back can do so only if they are several centimeters apart. Explain the two-point discrimination test, what determines it (receptor density and cortical territory), and what it says about the sensitivity of each area.

What is the sensory homunculus? Explain how the body is represented in the somatosensory cortex, why the lips and hands occupy much more territory than the back or the thighs, and what this means about the importance of each area for touch.

A research team records the electrical activity of two distinct sensory neurons in response to a continuous mechanical stimulus of constant intensity applied for 10 seconds:

• In Neuron A, the action potential frequency rapidly drops to zero at 0.8 seconds, even though the stimulus remains active.

• In Neuron B, the action potential frequency decreases slightly at the beginning, but remains elevated and constant throughout the entire 10 seconds.

1. a) Deduce with reasoning which of these recordings corresponds to a rapidly adapting (phasic) receptor and which to a slowly adapting (tonic) receptor. Associate each behavior with a functional example in the human body: the perception of clothing brushing against the skin versus the detection of pain from an ongoing burn (nociception).
2. b) Justify, from an evolutionary and survival perspective, why it would be maladaptive or lethal for an animal if nociceptors exhibited the same sensory adaptation pattern as olfactory or tactile receptors in the presence of a constant noxious stimulus.

In the Adelson checker shadow illusion, an opaque cylinder casts a shadow over checkered squares. Patch A (located in an illuminated area and perceived as a dark gray square) and patch B (located inside the shadow and perceived as a white square) reflect toward the eyes exactly the same physical irradiance (the same number of photons per square millimeter stimulates the retinal photoreceptors).

1. a) If the scene is covered with an opaque cardboard having two small pinholes that exclusively isolate squares A and B from their context and the shadow, predict what the observer will perceive. Based on this result, justify the formal difference between visual sensation and perception.
2. b) If both squares send an identical sensory signal, why does the visual cortex insist on "seeing" square B much lighter than A when the complete image is viewed? Explain the concept of color/lightness constancy and justify what adaptive advantage it offered to primates compared to the alternative of recording only the absolute physical photometry of their environment.

An ethology team studies the sensory mechanisms of two predators using experimental setups:

• Experiment 1: A shark swims in a sandy tank. It attacks with precision a live flatfish buried in the bottom. If the fish is replaced with pieces of fresh meat wrapped in insulating plastic impermeable to water and electricity, the shark swims past it. However, if an electric dipole emitting a continuous microcurrent of 5 nV/cm (similar to the branchial neuromuscular activity of the prey) is buried under the sand without any chemical trace of food, the shark lunges to bite the electrodes.

• Experiment 2: Insectivorous bats are recorded capturing moths in the dark. Upon introducing moths of the family Erebidae, it is observed that they emit trains of ultrasonic clicks when they detect that the bat accelerates its echolocation calls; faced with this, the bat's capture rate drops by 80%.

1. a) Based on Experiment 1, deduce which sensory receptor guides the shark's final attack (ampullae of Lorenzini versus olfactory chemoreceptors) and justify why the plastic barrier prevented detection despite the smell of meat.
2. b) Based on Experiment 2, formulate a neurophysiological hypothesis about how the ultrasound emitted by the moth disrupts the bat's echolocation system and interpret this interaction as a case of sensory predator-prey coevolution.
3. c) Use both examples to argue Jakob von Uexküll's concept of "Umwelt" and justify why designing a pest trap based exclusively on human sensory capabilities is usually ineffective.

Topic Challenge. How does your body respond to an alert or physical exertion? Design a home investigation on reaction time and neuroendocrine integration.

1. Data collection: With the help of someone you live with, perform the ruler drop test at rest and after $1\text{ min}$ of intense physical activity (such as jumping or doing squats). Measure the drop distance ($d$, in meters) five times per condition and calculate the average reaction time using the formula $t = \sqrt{2d/g}$ (with $g = 9.8\text{ m/s}^2$). Also record your pulse at rest and when active.

2. Processing: Design a data table with the calculated averages and create a bar graph comparing reaction time and heart rate in both situations.

3. Physiological report: Write a brief report reasoning through the results: identify the activated sensory receptors, describe the afferent neural pathways and muscular effectors involved, and explain the endocrine modulation exerted by adrenaline on the cardiac response and alertness.

Evaluation criteria:

• Rigor in obtaining empirical data and proper mathematical calculation with a decimal point.

• Clear, orderly, and properly labeled graphical representation.

• Accurate conceptual integration of receptors, effectors, and the coordination between the nervous and endocrine systems.