Good afternoon!
Case problem 5.1. Referring to part (b) of the figure, make a prediction about the relative size of the brain region devoted to processing lip sensations versus that for the brain region that processes sensations from the skin of your back.
Hint for answer 5.1. Although the skin area of your lips is much smaller than that of your back, the much larger number of sensory neurons originating in your lips requires a larger processing area within the somatosensory cortex of your brain.
Case problem 5.2. Applying a pressure stimulus to the fluid-filled capsule of an isolated Pacinian corpuscle causes a brief burst of action potentials in the afferent neuron, which ceases until the pressure is removed, at which time another brief burst of action potentials occurs. If an experimenter removes the capsule and applies pressure directly to the afferent neuron ending, action potentials are continuously fired during the stimulus. Explain these results in the context of adaptation.
Hint for answer 5.2. Pacinian corpuscles are rapidly adapting receptors, and that property is conferred by the fluid-filled connective-tissue capsule that surrounds them. When pressure is initially applied, the fluid in the capsule compresses the neuron ending, opening mechanically gated nonspecific cation channels and causing depolarization and action potentials. However, fluid then redistributes within the capsule, taking the pressure off the neuron ending; consequently, the channels close and the neuron repolarizes. When the pressure is removed, redistribution of the capsule back to its original shape briefly deforms the neuron ending once again and a brief depolarization result. Without the specialized capsule, the afferent neuron ending becomes a slowly adapting receptor; as long as pressure is applied, the mechanoreceptors remain open and the receptor potential and action potentials persist.
Case problem 5.3. A class of medications known as NSAIDs (nonsteroidal anti-inflammatory drugs) that includes aspirin and ibuprofen inhibits the activity of cyclooxygenase (COX) enzymes. Why would this make them effective as pain relievers?
Hint for answer 5.3. Cyclooxygenase enzymes mediate the production of prostaglandins from membrane phospholipids. Because prostaglandins are significant chemical stimulators of nociceptors, blocking their production can reduce the firing of afferent pain pathways.
Case problem 5.4. A woman has had a sore neck for a few days. Why might a clinician listen carefully to her chest and upper back with a stethoscope during the examination?
Hint for answer 5.4. Because the referred pain field for the lungs and diaphragm is the neck and shoulder, it is not unusual for individuals suffering from lower respiratory infections to complain of neck stiffness or pain. Lung infections are often accompanied by an accumulation of fluid in the lungs, which is detectable with a stethoscope as crackling or bubbling sounds during breathing.
Case problem 5.5. If an accident severed the left half of a person’s spinal cord at the mid-thoracic level but the right half remained intact, what pattern of sensory deficits would occur?
Hint for answer 5.5. Sensation of all body parts above the level of the injury would be normal. Below the level of the injury, however, there would be a mixed pattern of sensory loss. Fine touch, pressure, and body position sensation would be lost from the left side of the body below the level of the injury because that information ascends in the spinal cord on the side that it enters without crossing the midline until it reaches the brainstem. Pain and temperature sensation would be lost from the right side of the body below the injury because those pathways cross immediately upon entry and ascend in the opposite side of the spinal cord.
Case problem 5.6. Recall a general principle of physiology states that physiological processes are dictated by the laws of chemistry and physics. How is that principle evident here? What is the frequency of the electromagnetic wave shown in panel (b)? Would it be visible to the human eye?
Hint for answer 5.6. Sensory abilities in humans (and all animals) require structures that are capable of detecting a stimulus such as electromagnetic energy. Physical laws relate the wavelength and frequency of such radiation and determine its energy. Only certain wavelengths and energies are detected by the sensory apparatus of the human eye. Electromagnetic radiation that has more or less energy than a narrow band corresponding to a few hundred nanometers wavelength cannot be detected by the eye; this is what defines “visible” light. The frequency of the electromagnetic wave in this figure is [2 cycles/msec × 1000 msec/sec] or 2 × 103 Hz (2000 cycles per second). It would not be visible, because visible light frequencies are in the range of 1014 to 1015 Hz.
Case problem 5.7. Explain why one early symptom of vitamin A deficiency is impaired vision at night (often called night blindness).
Hint for answer 5.7. Vitamin A is the source of the chromophore retinal, which is the portion of the rhodopsin photopigment that triggers the response of rod cells to light. Because retinal is also used in cone photopigments, a severe vitamin A deficiency eventually results in impairment of vision under all lighting conditions, being generally most noticeable at night when less light is available.
Case problem 5.8. Three patients have suffered destruction of different portions of their visual pathway. Patient 1 has lost the right optic tract, patient 2 has lost the nerve fibers that cross at the optic chiasm, and patient 3 has lost the left occipital lobe. Draw a picture of what each person would perceive through each eye when looking at a white wall.
Hint for answer 5.8.
Case problem 5.9. What color was the image you saw while you stared at the square? Why did you perceive that particular colour?
Hint for answer 5.9. Most people who stare at the yellow background perceive an afterimage of a blue circle around the square. This is because prolonged staring at the color yellow activates most of the available retinal in the photopigments of both red and green cones, effectively fatiguing them into a state of reduced sensitivity. Because the red cones respond more to yellow light, their fatigue would be greater than that of the green cones. When you shift your gaze to the white background (white light contains all wavelengths of light), the blue cones respond strongly, the green respond weakly, and the red cones hardly respond, so you perceive a blue circle until the red and green cones recover.
Case problem 5.10. In what way does the process of hearing illustrate the general principle of physiology that physiological processes require the transfer and balance of matter and energy?
Hint for answer 5.10. Hearing begins with the arrival of sound energy reaching the eardrum. The energy is transferred to movement of the eardrum, which in turn transfers energy to the bones in the middle ear. That energy is transferred to the fluids of the inner ear, and then to the basilar membrane. In turn, energy from the movement of this membrane is transferred to the hair cells that, once activated, generate electrical signals that are sent to the brain. Therefore, energy from sound pressure in the environment undergoes a series of transformations until it ends up as electrical currents flowing across neuronal membranes.
Case problem 5.11. How might sounding an 80 dB warning tone just before the firing of an artillery gun (140 dB) reduce hearing damage?
Hint for answer 5.11. Though an 80 dB warning tone is not loud enough to cause hearing damage, it can activate the contraction of the stapedius and tensor tympani muscles. With those muscles contracted, the movement of the middle ear bones is dampened during the 140 dB gun blast, thus reducing the transmission of that harmfully loud sound to the inner ear.
Case problem 5.12. Furosemide is commonly used to treat high blood pressure because it increases the production of urine (it is a diuretic), which, in turn, reduces fluid volume in the body. It acts in the kidney by inhibiting a membrane protein responsible for pumping K+, Na+, and Cl− across an epithelial membrane. This protein is also present in epithelial cells surrounding the cochlear duct. Based on this information, propose a mechanism that might explain why one of the drug’s side effects is hearing loss.
Hint for answer 5.12. The transport protein responsible for reabsorbing K+ (along with Na+ and Cl−) in the kidney is also present in epithelial cells.
Good luck in your studies!