Sensory receptors

Match each of the five senses with the type of stimulus it detects and with the organ where its main receptors are located.

Explain what a sensory receptor is and why we say that the senses are the "gateway" for information entering the nervous system.

Explain the difference between a stimulus and a sensation, and give an example of each using a cup of hot chocolate.

State whether these statements are true or false and justify your answer: a) each taste is perceived only in one fixed area of the tongue; b) olfactory cells are located in the nasal cavities; c) information from the senses reaches the brain through the nerves.

In the cross-sectional image of the eye, several structures have been marked with numbers. Write the name of the five marked structures and state in which of them light receptor cells are located.

Describe the path followed by light information from the moment light from an object enters the eye until the image is formed, naming the structures it passes through and the location where it is converted into a nerve impulse.

When you go from a dark room to a very sunny street, you are dazzled for a few seconds. Explain which structure of the eye regulates the amount of light entering, how it acts in that situation, and what this adaptation mechanism is called.

In dim light we see little detail and practically no colours. Knowing that the retina has two types of receptor cells, some very sensitive to faint light and others specialised in colours, explain why this happens.

Many older people hold the newspaper further away to read it, a problem known as "tired eyesight". Explain which structure of the eye loses flexibility with age and why this makes it difficult to focus on nearby objects.

A research team analyses the retina of three vertebrate species and obtains the following photoreceptor data:

• Species A: 98% rods and 2% cones; large pupillary dilation capacity.

• Species B: 65% cones (of three different pigment types) and 35% rods.

• Species C: 96% rods and 4% cones (sensitive only to a narrow spectral band).

1. a) Reasonably deduce which of these species most likely corresponds to a nocturnal bird of prey (such as an owl) and which to a diurnal bird that feeds by identifying ripe fruits and flowers among vegetation. Justify your choice based on the function of cones and rods.
2. b) If Species A were suddenly exposed to full sunlight at midday, predict what visual limitation it would experience compared to its night vision, justifying how rods respond to high light intensity.
3. c) Explain why natural selection has favoured nocturnal predators sacrificing chromatic discrimination in exchange for sensitivity to dim light.

Two patients visit the ophthalmology department with severe vision loss:

• Patient 1: Perceives images very blurry, faded, and as if through a fogged glass. Their retina and optic nerve are completely healthy, but the lens located behind the iris has become whitish and opaque due to age.

• Patient 2: Has a perfectly transparent cornea, crystalline lens, and vitreous and aqueous humours, but uncontrolled glaucoma has necrosed the nerve fibres that form the optic nerve.

1. a) Diagnose which pathology Patient 1 suffers from, indicating the name of the altered structure, and explain how this alteration affects the path of light rays to the back of the eye.
2. b) The ophthalmologist proposes a procedure for Patient 1 consisting of replacing the damaged lens with a transparent artificial lens (intraocular lens). Predict and justify whether this operation will restore clear vision.
3. c) Reasonably justify whether that same surgical procedure would help Patient 2 regain vision, distinguishing the function of the transparent media of the eyeball from the function of the nerve transmission pathways.

Name the three parts into which the ear is divided and state in which of them the receptor cells that transform sound into nerve impulses are found.

Explain what the eardrum is, where it is located, and what happens to it when sound waves reach it.

When diving or when a plane takes off, we feel pressure in our ears, which is relieved by swallowing or yawning. Explain which duct connects the throat with the middle ear and what function it performs in these situations.

Some people with an inner ear infection feel dizzy and lose their balance. Taking into account that the inner ear does not only detect sounds, explain what other information it sends to the brain and which structures are responsible for it.

A young man visits the doctor after noticing severe hearing loss due to continuous headphone use at very high volume. Diagnostic tests reveal that his tympanic membrane and ossicular chain are intact and transmit vibrations completely normally, but the ciliated mechanoreceptors of the cochlea (inner ear) have been destroyed.

1. a) Justify why the patient suffers from deafness if the elements responsible for capturing and transmitting sound in the outer and middle ear are working properly. In your answer, distinguish between the physical transmission of vibrations and their transduction into nerve impulses.
2. b) The patient asks if the injury will heal in a few weeks with rest, just as a skin wound or a perforated eardrum heals. Argue why doctors rule out this possibility based on the regenerative capacity of receptor cells compared to epithelial cells.
3. c) Predict with reasoning whether a conventional hearing aid (which amplifies sound vibration in the ear canal) would solve the hearing loss at the frequencies where the ciliated receptors have completely disappeared.

When we accidentally touch a hot frying pan, we feel pain instantly. Explain which skin receptors warn us of the danger and why this sensation, although unpleasant, is useful to the body.

Taste buds and olfactory cells work together when we eat: this is why, with your nose held, a raw apple and a raw onion "taste" almost the same. Explain what each sense contributes to the perception of food flavour.

With your eyes closed, you hold a very cold glass that is slipping through your fingers. As you squeeze it tightly to keep it from falling, a broken edge causes a superficial cut on your fingertip. Simultaneously and immediately, you distinguish three sensations: the coldness of the water, the pressure you exert on the glass, and the pain from the cut.

1. a) Identify which specific type of cutaneous receptor captured each of those three stimuli and classify them according to the nature of the stimulus they detect.
2. b) Knowing that all nerve fibres transmit information via identical electrochemical impulses, justify how the brain manages to distinguish and interpret all three sensations at once without confusing pain with cold or pressure.
3. c) If a local anaesthetic exclusively blocked the nociceptors in the area, predict what stimuli you would still perceive and explain why the congenital insensitivity to pain poses a serious threat to survival.

In a laboratory practical, the "two-point discrimination test" is applied using a blunt-ended caliper to measure the minimum distance at which a person detects two separate contacts instead of just one:

• On the fingertip, they clearly perceive two points separated by only 2.5 mm.

• On the skin of the back, two independent points are not perceived until the separation exceeds 35,0,mm; at smaller distances, a single touch is felt.

1. a) Interpret these results by justifying what differences exist in the density of mechanoreceptors and in the size of their receptive fields between the fingertips and the back.
2. b) Based on these data, justify why the Braille system (whose raised dots are spaced about 2.5 mm apart) is read with the fingertips, and predict what insurmountable difficulty would arise if a person tried to read it with their forearm or back.
3. c) In blind individuals proficient in Braille, neuroimaging studies show that the area of the cerebral cortex that processes touch from the fingertips is much larger than in sighted individuals. Justify this phenomenon by linking it to the plasticity of the nervous system.

Aoife burned her tongue while drinking boiling broth and for three days could barely perceive the flavours of food; on the fourth day, however, she regained her taste normally. That same weekend, a relative suffered trauma that severed the fibres of the olfactory nerve, causing a permanent loss of smell.

1. a) Explain what injury the receptor cells of Aoife's taste buds suffered and justify why the sense of taste recovers in a few days, comparing it with her relative's case: why is the renewal of olfactory neurons, which also occurs, not enough to restore smell when their fibres are severed?
2. b) During the days she was burned, Aoife claimed: "Since I burned my tongue, I can't smell coffee either." Refute her claim in a reasoned manner by distinguishing taste chemoreceptors from olfactory chemoreceptors, and explain why in everyday life we often mistakenly attribute sensations that depend on retronasal olfaction (flavour) to taste.

Many people suffer from dizziness and nausea if they read their mobile phone in the back seats of a moving car; however, if they stare at the road through the front windscreen or if they are the ones driving, they do not feel unwell. In contrast, a user sitting motionless on the sofa playing with a virtual reality headset experiences motion sickness when visually travelling on a virtual roller coaster.

1. a) Analyse the receptors and sensory organs involved (retina of the eye and vestibular system of the inner ear) and explain what contradiction of information (sensory conflict) the brain receives in each of the two described cases.
2. b) Justify why fixing your gaze on a distant point on the horizon eliminates sensory conflict while travelling by car.
3. c) Predict in a reasoned manner whether a person who suffered complete destruction of the mechanoreceptors in the semicircular canals and otoliths in both ears would experience motion sickness (kinetosis) when travelling on a boat subject to waves. Argue your answer.