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 humour, 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 colour 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 of nothing", resorting to retronasal olfaction and to the notion of flavour 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: basic structure of each, the type of image they form, and the advantage each design offers to the animal possessing it.

In response to a sudden loud noise, the pupils dilate and the heart rate accelerates, as summarised in the diagram. Indicate which element of the pathway (receptor, nervous centre, 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 hypermetropic 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 distinguish two pinpricks one millimetre apart; the back, only if they are several centimetres 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 a medical clinic, the balance of two patients is assessed using the Romberg test (standing upright with feet together and arms at the sides, first with eyes open and then with eyes closed):

• Patient 1 remains stable with eyes open, but immediately loses balance and sways with a risk of falling as soon as they close their eyes.

• Patient 2 exhibits instability and an unsteady gait both with eyes open and closed, with virtually no visual improvement.

1. a) Knowing that one of the patients suffers from degeneration of the proprioceptive pathways of the posterior columns of the spinal cord and the other has cerebellar atrophy, deduce with justification which diagnosis corresponds to each patient based on the multisensory integration of balance.
2. b) A healthy person can maintain balance on a moving bicycle with their eyes closed for a few seconds, but fails if they try to do so with the bicycle completely stationary without putting their feet down. Explain what dynamic sensory information the semicircular canals of the inner ear and the pedaling motion provide compared to purely static balance.

In a neurophysiology laboratory, the effects of two toxins are analyzed:

• Toxin X specifically blocks nicotinic acetylcholine receptors at the skeletal muscle motor endplate, without altering muscarinic receptors.

• Toxin Y acts as a potent competitive antagonist of muscarinic acetylcholine receptors in viscera and glands.

1. a) Predict with reasoning the immediate effects that Toxin X poisoning will cause on pulmonary ventilation (diaphragm), intestinal peristalsis, and pupil diameter.
2. b) A student claims: "Since the somatic system controls voluntary actions and the autonomic system controls involuntary ones, Toxin X will not affect any spinal reflex, whereas Toxin Y will paralyze all automatic movements of the body." Refute their claim by analyzing what happens to the patellar reflex under the effect of Toxin X and to heart rate under Toxin Y.
3. c) Which of the two poisonings will urgently require intubation with mechanical assisted ventilation to prevent death from asphyxiation? Justify your answer based on the type of motor innervation of the diaphragm.

A patient with an amputation of the left arm above the elbow presents a striking sensory experience: when their left cheek is touched with a cotton swab with their eyes closed, they report distinctly feeling caresses on the thumb and index finger of their amputated left hand. In addition, they suffer from intense pain because they feel their phantom hand clenched in a rigid, tight fist, with the fingernails digging into the palm.

1. a) Based on the layout of Penfield's somatosensory homunculus in the parietal cortex, explain why tactile stimulation of the face triggers sensations in the fingers of the absent hand. What phenomenon of neuroplasticity and cortical reorganization underlies this finding?
2. b) The patient's brain continues to send motor commands to the hand to open the fist, but receives no returning proprioceptive signals, generating an ongoing pain loop. Design a mirror therapy protocol (Ramachandran's mirror box) and predict what visual signal the patient must generate to trick their sensorimotor cortex and relieve the phantom spasm.

The normal trichromatic human retina has three types of cones whose photopigments (opsins) exhibit absorption peaks at different wavelengths: S (blue, ~420 nm), M (green, ~530 nm), and L (red, ~560 nm). The genes encoding the M and L opsins are located in tandem on the X chromosome.

1. a) An individual with deuteranopia completely lacks the functional M opsin. If two beams of light of equal radiance are projected onto them, one at 540 nm and the other at 565 nm, deduce and justify whether their visual cortex will be able to discriminate both stimuli as different colors by comparing signals between cones, or if they will perceive them as indistinguishable shades.
2. b) In a population in genetic equilibrium, the prevalence of X-linked color blindness among males is 6,0% (0,06). Reasonably calculate the expected allele frequency, the percentage of color-blind females, and the percentage of carrier females with phenotypically normal vision.
3. c) A woman with normal vision whose father is color-blind has offspring with a man with normal vision. Justify the probability that their sons and daughters will suffer from color blindness. Why, due to the process of stochastic X-chromosome inactivation (lyonization), can carrier females present a cellular mosaic in the retina without clinically manifesting color blindness?

In an otolaryngology department, two cases of profound bilateral sensorineural hearing loss are treated:

• Patient A: A 19-year-old who suffered drug-induced ototoxicity that caused the irreversible destruction of the hair cells of the organ of Corti, but retains intact neurons in the spiral ganglion and the vestibulocochlear nerve (cranial nerve VIII).

• Patient B: A 50-year-old adult with advanced bilateral compression of the vestibulocochlear nerve due to acoustic neuromas that have disrupted axonal conduction.

1. a) Justify why a cochlear implant is an effective therapeutic alternative for Patient A but is completely inoperative for Patient B. Describe the pathway followed by the electrical impulse from the implant electrode to the primary auditory cortex in the viable case.
2. b) The basilar membrane exhibits tonotopy: its base is stiff and narrow (resonating to high frequencies or high-pitched tones), while the apex or vertex is compliant and wide (resonating to low frequencies or low-pitched tones). Explain how the medical team should program the spatial activation of the electrodes along the cochlea so that Patient A can differentiate a high-pitched voice (4000 Hz) from the low-pitched sound of a bass drum (150 Hz).

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 table with the calculated averages and create a bar chart 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 comma.

• Clear, orderly, and properly labeled graphical representation.

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