Quiz 118 · Sensory Communication to Spinal Cord

Answers to Quiz 117 · Introduction to Sensory System and Physiology of Receptors

1. The emotional component of a sensation, such as the unpleasantness of pain, is called its:

   Answer: Affect

   Affect is the emotional side of a sensation: neutral, positive or negative, and negative for pain. Modality is the type of sensation, and acuity the precision of its localisation. Revise sensation: general principles

2. The acuity of a sensation depends on each of the following EXCEPT:

   Answer: The affect of the sensation

   Acuity, the precision of localisation, varies with innervation density, receptive field size and the size of the cortical area for the body part. Affect is the emotional component and has no bearing on localisation. Revise sensation: general principles

3. The largest mechanoreceptor in the body is the:

   Answer: Pacinian corpuscle

   The Pacinian corpuscle is 0.5–2 mm long and 0.7 mm across, with 20–70 lamellae. Meissner's corpuscle is only about 150 µm long. Revise sensory receptors and their classification

4. The most superficial mechanoreceptor of the skin, lying in the epidermis, is the:

   Answer: Merkel's disk

   Merkel's disks lie in the epidermis of both hairless and hairy skin. Krause's end-bulbs are in the dermis, and Pacinian corpuscles in the skin and deep tissues. Revise sensory receptors and their classification

5. Baroreceptors and osmoreceptors are classed as:

   Answer: Interoceptors

   Interoceptors detect changes in the internal environment, such as blood pressure and the osmolality of body fluids. Proprioceptors report body position, and teleceptors detect stimuli far from the body. Revise sensory receptors and their classification

6. A pinprick causes sharp, fast pain. The nociceptors that signal it do not respond to heat or chemicals. They are:

   Answer: Aδ-mechanical nociceptors

   Aδ-mechanical nociceptors signal fast, sharp pain such as pricking and ignore thermal and chemical stimuli. C-polymodal nociceptors respond to many noxious stimuli, heat and chemicals included. Revise sensory receptors and their classification

7. Which of these mechanoreceptors adapts slowly?

   Answer: Ruffini ending

   Ruffini endings, like Merkel's disks, adapt slowly; they usually sense crude touch. Pacinian, Meissner's and Krause's corpuscles all adapt rapidly. Revise sensory receptors and their classification

8. Meissner's corpuscles are found in the:

   Answer: Dermal ridges of hairless skin

   Meissner's corpuscles sit in the dermal ridges of glabrous skin, where they sense vibration and touch. The epidermis holds Merkel's disks, and Golgi-Mazzoni corpuscles were first found in tendons and muscles. Revise sensory receptors and their classification

9. An action potential fires in the sensory nerve when the receptor potential reaches about:

   Answer: 10–15 mV

   The receptor potential grows in proportion to the stimulus. When it reaches about 10–15 mV, an action potential is generated in the sensory nerve. Revise receptor potential and properties of receptors

10. In a Pacinian corpuscle, the action potential is generated at the:

    Answer: First node of Ranvier

    The unmyelinated terminal makes the receptor potential, and the action potential arises at the first node of Ranvier inside the corpuscle. Blocking that node stops the action potential; stripping off the lamellae does not. Revise receptor potential and properties of receptors

11. Pressure on a Pacinian corpuscle produces the receptor potential by:

    Answer: Opening stretch-sensitive Na⁺ channels in the terminal

    Distortion of the lamellae opens stretch-sensitive sodium channels in the nerve terminal, the actual receptor, and the Na⁺ influx is the receptor potential. The lamellae themselves are not needed. Revise receptor potential and properties of receptors

12. Which of these receptors is tonic, adapting slowly or not at all?

    Answer: Carotid sinus baroreceptor

    Tonic receptors include the baroreceptors of the carotid sinus and aortic arch, and pain receptors. Pacinian, Meissner's and Krause's corpuscles adapt rapidly, so they are phasic. Revise receptor potential and properties of receptors

13. A man whose leg was amputated a year ago complains of itching in the missing foot. This is explained by the:

    Answer: Law of projection

    Wherever a sensory pathway is stimulated, the sensation is felt where its receptors are. So activity in the pathway that served the foot is still felt in the foot: the phantom limb. Revise receptor potential and properties of receptors

14. Stimulating a touch fibre at any point on its way to the cortex produces only the sensation of touch. This is the basis of:

    Answer: Müller's doctrine of specific nerve energy

    The sensation depends on the brain area the pathway finally activates, because each pathway is specific from sense organ to cortex. The law of projection is about where a sensation is felt, not what it is. Revise receptor potential and properties of receptors

15. The magnitude of a sensation is proportional to the logarithm of the stimulus intensity. This is the:

    Answer: Weber–Fechner law

    The Weber–Fechner law links the sensation felt to the log of stimulus intensity. The law of projection and Müller's doctrine deal with where and what a sensation is, not how strong. Revise receptor potential and properties of receptors

14 questions on chapter 118. Pick one answer for each, then save. Your answers stay in this browser, and the right ones are shown at the top of the next quiz.

1. The Bell–Magendie law states that:

The ventral root is sensory and the dorsal root is motorSensory fibers regrow faster than motor fibers after axotomyLarge fibers fail before small fibers when a nerve is compressedThe dorsal root is sensory and the ventral root is motor

2. A few afferent fibers break the Bell–Magendie law by running in the ventral root. What are most of them?

Aβ touch fibers from the skin of the limbsAδ fibers from cold receptors in the skinLarge myelinated afferents from muscle spindlesSmall unmyelinated nociceptors from the viscera

3. In a skin nerve, fast pain and cold are carried mainly by:

Unmyelinated C fibersSmall myelinated Aδ fibersLarge myelinated Aβ fibersLarge myelinated Aα fibers

4. A blood-pressure cuff is kept inflated on a volunteer's arm for 20 minutes. Which sensation in the hand is most likely to survive?

Light touchColdSlow, burning painFast, pricking pain

5. A local anesthetic is injected around a nerve. In what order are sensations lost?

Touch and pain at the same momentTouch only; pain is never lostTouch first, then painPain first, then touch

6. In carpal tunnel syndrome, which fibers of the median nerve suffer earliest and most?

Large fibers over 5 µm acrossFibers of every size equallySmall unmyelinated C fibersThin myelinated Aδ fibers

7. After an axon is cut, which change is seen in the part of the neuron still joined to the cell body?

Swelling of the cell body with chromatolysisSprouting of new fibers from the distal stumpWallerian degeneration of the proximal stumpShrinkage of the cell body with a dense nucleus

8. Which statement about regeneration after axotomy matches your book?

Regrowth begins from the degenerating distal partSensory fibers regenerate faster than motor fibersCentral axons regenerate faster than peripheral axonsMotor fibers regenerate faster than sensory fibers

9. The area of skin supplied by a single dorsal root ganglion is called a:

DermatomeMyotomeReceptive fieldSclerotome

10. A man with a slipped lumbar disc has pain, tingling and heightened sensitivity along one dermatome, but no numbness. What does this pattern point to?

A lesion in the dorsal columnComplete damage to the dorsal rootIrritation of the dorsal rootCompression of the ventral root

11. The substantia gelatinosa is Rexed lamina:

VIIIIX

12. Dorsal horn cells that respond to fine touch, vibration and proprioception lie mainly in laminae:

IX–XI–IIIII–VIVII–VIII

13. Which Rexed lamina is the main motor lamina?

XIXVIIVIII

14. Of the descending tracts your book lists, which ends in dorsal horn laminae III–VI but not in laminae I and II?

Medullary reticulospinalPontine reticulospinalRaphespinalCorticospinal

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