Receptors and Effectors
You walk into a taquería and the smell of al pastor meat fills your nose; five minutes later you barely notice it. Meanwhile, an agave bat "sees" cacti with its ears and a bee sees ultraviolet colors that you can't tell apart. Each animal perceives only the part of the world that its receptors can detect. Why does your nose "switch off" for a smell that is still there? We'll solve it at the end.
Classification of receptors
A drop of lemon on your tongue, the prick of a needle, or a blinding light cause very different sensations. Yet all of them travel to the brain as the same bioelectrical signal. How does the body detect such different stimuli and translate them into a common code?
Examples in the body: photoreceptors, the cones and rods of the retina; mechanoreceptors, those for pressure in the skin and the hair cells of the ear; chemoreceptors, those in the nose and the taste buds; thermoreceptors, the cold and heat receptors in the skin; nociceptors, the free nerve endings that warn you of a burn or a cut.
Classify sensory receptors according to the type of stimulus they detect (photoreceptors, mechanoreceptors, chemoreceptors, thermoreceptors and nociceptors) and give a…
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.
Differentiate between exteroceptors, interoceptors and proprioceptors, indicating where they are located and what information each type provides.
Effectors and the stimulus-response chain
When you hear a loud crash in the street, your pupils dilate and your heart speeds up without your deciding it, and if necessary, your legs start running. Which structures carry out these orders, and why do you control some consciously and others not?
Every response follows the same chain: the receptor picks up the stimulus, a sensory pathway carries the signal to the nerve center (brain or spinal cord), which works out an order, a motor pathway conducts it, and the effector responds. In the figure, the boxes are numbered 1 to 5 in the order the signal travels.
Now apply it to a specific case. In response to a sudden noise, the diagram simplifies to four boxes, because the signal is distributed from the nerve center to two effectors:
Name the three types of effector organs involved in the interaction function and give an example of a coordinated response for each.
Name the three types of effector organs involved in the interaction function and give an example of a coordinated response for each.
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,…
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.
Skin, vision, and hearing
You walk out of a movie theater into broad daylight and are dazzled. When you enter another dark theater, it takes you minutes to make out the steps, and you can't tell the color of the seats. How are the photoreceptors organized to work both in daylight and in near darkness?
The skin is a sensory organ with several receptors: for touch, pressure, heat, cold, and pain. This integration has an advantage: with a single touch you know what an object is, and pain warns you of danger. If you touch something cold, rough, and sharp, each type of receptor is activated separately and the brain combines the signals into a single sensation.
The skin contains several types of receptors (touch, pressure, temperature and pain). Explain the advantage of this sensory integration and describe what happens when…
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…
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.
Balance and proprioception
Police officers sometimes ask a person to close their eyes and stand still with their feet together. In medicine this test is called the Romberg test. Which senses keep you standing?
| Source | What it reports | If it fails… |
|---|---|---|
| Vision | Position relative to the surroundings | The other two compensate |
| Proprioceptors | Position of each part of the body | There is instability even with the eyes open |
| Inner ear | Rotations and position of the head | There is vertigo and loss of balance |
In the Romberg test, closing the eyes removes vision and forces the body to depend only on the proprioceptors and the inner ear. A person who sways when closing the eyes but is steady with them open was using vision to compensate for a proprioceptive failure. A person who is unsteady both ways, with no improvement from vision, has a failure that vision cannot compensate for, such as a vestibular or cerebellar problem.
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…
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):
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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.
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Patient 2 exhibits instability and an unsteady gait both with eyes open and closed, with virtually no visual improvement.
- 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.
- 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.
Sensory adaptation and the senses of other animals
Back to the taquería: at first the smell is very strong and then you barely notice it. The smell is still there, so what changes is the response of your receptors.
Neuron A (which goes silent at 0.8 s even though the stimulus continues) is a rapidly adapting receptor: it detects the start and end of contact and stops spending energy. Neuron B, which stays almost constant, is a slowly adapting receptor: it monitors a situation that lasts, such as the pressure of clothing on the skin or the position of a joint. Both strategies are useful: that way you can ignore the watch on your wrist but notice an insect crawling up your arm.
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…
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:
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In Neuron A, the action potential frequency rapidly drops to zero at 0.8 seconds, even though the stimulus remains active.
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In Neuron B, the action potential frequency decreases slightly at the beginning, but remains elevated and constant throughout the entire 10 seconds.
- 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).
- 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.
Reproductive strategies: energy cost and success
A golfina sea turtle leaves more than a hundred eggs on a beach in Oaxaca and goes away; a woman or a cow has a single offspring and cares for it for months. Both strategies work. Why are there so many ways of reproducing?
Reproduction ensures the continuity of the species, and each strategy is a balance between how much energy the parent spends and how many offspring reach adulthood. There are three main decisions:
| Decision | Options | Effect |
|---|---|---|
| How many parents? | Asexual (just one, identical offspring) or sexual (two gametes, offspring with new genetic combinations) | Asexual is fast and cheap; sexual generates variability |
| Where do the egg and sperm unite? | External fertilization (in the water) or internal (inside the body) | External needs many gametes because many are lost; internal protects the gametes |
| Where does the embryo develop? | Oviparous (egg outside the body), ovoviviparous (egg that hatches inside the mother), or viviparous (the embryo grows in the mother and is nourished by her) | The egg is cheap for the mother but leaves the embryo exposed; viviparity protects more but costs more energy |
Examples: the golfina turtle is oviparous with internal fertilization and lays many eggs with little care; many fish have external fertilization in the water, while the axolotl has internal fertilization without copulation (the male deposits a spermatophore and the female picks it up with her cloaca); mammals have internal fertilization, viviparity, and few, well-cared-for offspring. An agave plant can also reproduce asexually through the offsets that sprout from its base.
In a neurophysiology laboratory, the effects of two toxins are analyzed: Toxin X specifically blocks nicotinic acetylcholine receptors at the skeletal muscle motor…
In a neurophysiology laboratory, the effects of two toxins are analyzed:
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Toxin X specifically blocks nicotinic acetylcholine receptors at the skeletal muscle motor endplate, without altering muscarinic receptors.
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Toxin Y acts as a potent competitive antagonist of muscarinic acetylcholine receptors in viscera and glands.
- a) Predict with reasoning the immediate effects that Toxin X poisoning will cause on pulmonary ventilation (diaphragm), intestinal peristalsis, and pupil diameter.
- 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.
- 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.