Chapter 15: Somatosensory System
A 49-year-old woman complains about loss of accuracy in identifying and localizing sensation (in particular, conscious proprioception, tactile sensation, and pressure) from the appropriate sites along her right hand and leg. A magnetic resonance imaging (MRI) scan indicates the presence of a tumor in the region of the right dorsal columns at the level of the spinal cord–medulla border. The presence of the tumor could account for both loss of sensation and loss of accuracy in identifying the sites on the limbs associated with the respective sensation because of a general property of inhibition associated with dorsal column functions. Which one or more of the following types of inhibition have been identified within the dorsal column nuclei that are disrupted by the tumor?
A. Feedforward inhibition using local interneurons only
B. Feedback inhibition using local interneurons only
C. Descending inhibition from fibers arising in the cerebral cortex only
D. Feedforward, feedback, and descending inhibition
E. Feedforward and descending inhibition only
D. To generate an excitatory focus with an inhibitory surround, 3 types of inhibition are present in the dorsal column nuclei. First-order neurons ascending in the dorsal columns make synaptic contact with different cells in the dorsal column nuclei and excite those cells. One such cell may be an inhibitory interneuron that makes synaptic contact with a neighboring dorsal column nuclear cell, thus inhibiting that cell (ie, feedforward inhibition). In addition, the dorsal column cell that is excited by the first-order neuron may make synaptic contact with another inhibitory interneuron (in addition to its classical ascending projection to the ventral posterolateral nucleus of the thalamus). This inhibitory interneuron makes synaptic contact with an adjacent dorsal column cell and inhibits that cell (ie, feedback inhibition). Finally, a descending fiber from the postcentral gyrus can make synaptic contact with inhibitory interneurons that inhibit dorsal column cells (descending inhibition).
A 57-year-old man is referred to a neurologist after he complains about difficulties in determining the directionality and orientation of movement of stimuli along his right arm. An MRI is taken of the patient, and a central nervous system (CNS) tumor is noted. Where in the regions listed below is the tumor most likely to be present?
B. Medial half of thalamus
E. As a general rule, neurons that are situated in the cortex in association with any of the sensory systems take on a much higher level of complexity than neurons that are situated at lower levels of the relay network. In the case of the somatosensory system, direction-sensitive cells in the somatosensory cortex will respond to 1 direction of movement of a stimulus along the receptive field and not to another direction. Orientation-sensitive neurons respond best to movement along 1 axis of the receptive field. This is not true of neurons that are situated in lower levels of the somatosensory pathway. Thus, a tumor or lesion of this region of cortex is likely to disrupt the neuronal process essential for such discriminations to be made.
A patient has been seeing a physician for almost a year because she complains of pain in her shoulder. After extensive analysis, the physician determines that the pain in her shoulder reflects referred pain that is arising from another source. In this case, which of the following best explains the basis for the referred pain?
A. Inhibitory fibers that block transmission of pain impulses along a given pathway and then transfer the impulses to a different pathway associated with a different part of the body
B. A massive discharge along a given pathway that results in the activation of a separate pathway because of the principle of divergence
C. A convergence of primary afferent fibers from a given region onto second-order neurons that normally receive primary afferents from a different body part
D. The disruption of lateral spinothalamic fibers
E. The blockade of substance P from primary afferent terminals
C. Referred pain is a phenomenon in which pain impulses, arising from primary afferent fibers from 1 part of the body (eg, from deep visceral structures), terminate on dorsal horn projection neurons that normally receive cutaneous afferents from a different part of the body (eg, the arm). In this situation, a person who is suffering a heart attack experiences pain that appears to be coming from the arm. It is the convergence of these distinctly different inputs onto the same projection neurons that provides the basis for this phenomenon. None of the other possible mechanisms listed in this question have an anatomic or physiologic basis.
A car door is accidentally closed on the hand of a teenage boy. As a result, he experiences significant pain that persists for a while. In terms of the neurochemical events that take place at the afferent terminals of the first-order pathway that conveys the pain sensation to the spinal cord, which of the following transmitters will be released onto dorsal horn neurons of the spinal cord from these primary afferent fibers?
D. Glutamate and substance P
E. Enkephalins, substance P, and glutamate
D. Primary nociceptive afferent fibers would have to release an excitatory transmitter in order for normal transmission to take place. Two excitatory transmitters have been identified in association with different classes of primary nociceptive afferents: (1) substance P and (2) excitatory amino acids. The best candidate as an excitatory amino acid is glutamate. Because enkephalins have been shown to be inhibitory transmitters in the pain system, they are not likely to be released from the primary afferents. Instead, other CNS neurons impinge upon the primary afferents, and enkephalins are released from those neurons.
A patient is experiencing severe pain. If it were possible to place an electrode into the gray matter around the cerebral aqueduct of the midbrain and stimulate the cells in this region, it would induce an analgesic response. Which of the following best explains such an effect?
A. Activation of a pathway that ascends directly to the cortex and mediates analgesia
B. A descending pathway that blocks nociceptive inputs at the level of the dorsal horn
C. Activation of local interneurons that block ascending nociceptive signals at the level of the midbrain
D. Activation of an ascending inhibitory pathway that projects to the ventral posterolateral nucleus of the thalamus
E. Activation of cholinergic neurons in the basal forebrain
B. Perhaps one of the most important discoveries in pain research made over the past 25 years is that of a descending pathway that originates in the midbrain periaqueductal gray and makes synaptic contacts in the medulla. From the medulla, this pathway descends to the dorsal horn, where these fibers provide the anatomic substrate for suppression of pain inputs that enter the spinal cord from the periphery. There are no known inputs to the cortex that directly produce analgesia. The mechanism governing analgesia appears to operate at lower brainstem and spinal cord levels. The ascending fibers for transmission of pain impulses reach thalamic nuclei directly, and thus, local interneurons within the midbrain would not be able to interfere with such transmission. The pathway to the ventral posterolateral nucleus of the thalamus is an excitatory one and is not known to have any inhibitory properties. Cholinergic neurons in the basal forebrain have been implicated in memory functions and are not known to have any role in the regulation of pain sensation.