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Neural Pathways for Cognitive Command and Control of Hand Movements Esther P

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Neural Pathways for Cognitive Command and Control of Hand Movements Esther P COMMENTARY COMMENTARY Neural pathways for cognitive command and control of hand movements Esther P. Gardnera,1 A piece of fruit—a raisin—swings on a stick in front of a they placed microelectrodes into lateral area 5, where monkey (Fig. 1). He likes raisins; he wants this one. He Mountcastle et al. (1) had recorded “command” neu- stretches out his arm, opens his hand with the fingers rons, and evoked grasping type movements of the spread wide apart, and tries to capture it; he misses thumb and fingers following trains of electrical pulses (bursts B and D). He tries again, this time successfully delivered by the electrodes. Grasping movements (burst F). He grabs the raisin, pulls it off the stick, were evoked from both the anterior bank of the intra- brings it to his mouth, and eats it. Delicious! What parietal sulcus (area PEip), as well as the adjacent cor- happens in the brain when the animal performs these tical surface caudal to the S1 hand area (area PE). actions? The chart above the images in Fig. 1 tracks To determine the anatomical projection targets of responses of a neuron recorded in lateral area 5 bor- these physiologically identified zones, Rathelot et al. (3) dering the intraparietal sulcus, a subregion of the injected lateral area 5 with cholera-toxin subunit B (CTb), posterior parietal cortex (PPC) originally studied by a highly sensitive anterograde tracer molecule. CTb up- Mountcastle et al. (1, 2), and described as a hand- take into neurons is mediated by absorptive endocytosis “ ” manipulation command neuron engaged in pur- when it binds to monosialoglycoside GM1 in neuronal poseful actions of the hand: reaching and grasping membranes, and is actively transported bidirectionally an object of behavioral interest. within the neuron, retrograde (back toward the cell soma Mountcastle et al. (1) proposed that these regions and dendrites) and anterograde (forward along the axon of the PPC, to the synaptic terminals) (14, 15). Rathelot et al. (3) dem- ...receive afferent signals descriptive of the po- onstrate that anterograde transport of CTb produces sition and movement of the body in space, and dense labeling of axon terminals in the medial dorsal contain a command apparatus for operation of horn of the spinal cord from C2 to T2. This region is the limbs, hands, and eyes within immediate populated by spinal interneurons that receive somato- extrapersonal space. This general command sensory input from mechanoreceptors in the hand (16); function is exercised in a holistic fashion. It relates many of them also participate in reflex pathways to hand to acts aimed at certain behavioral goals and not motoneurons. The descending corticospinal projection to the details of muscular contraction during ex- from area 5 is contralateral and excludes the ventral horn ecution. These details are, on this hypothesis, where motoneurons reside. The pathway from area 5 is made precise by the motor system, for which it is disynaptic and does not excite motoneurons directly. well suited by virtue of its powerful mechanisms In a second series of experiments, Rathelot et al. (3) for specifying movement exactly. injected rabies virus into specific hand muscles to de- However, no anatomical substrate was defined in that lineate the circuitry of anatomical projections to their report beyond “the motor system.” spinal motoneurons. In previous studies, these authors In PNAS, Rathelot et al. (3) use a state-of-the-art neu- and others demonstrated retrograde transneuronal roanatomical pathway tracing to demonstrate a direct transport of rabies virus from hand muscles to moto- pathway from lateral area 5 to interneurons of the spinal neuron pools of the spinal cord, and subsequently to cord, providing an efficient, rapid route for modulating corticomotoneuronal cells of the primary motor cortex hand and arm movements during goal-directed behav- (13, 14, 17–19). Multiple levels of retrograde trans- iors. These authors harnessed bacterial toxins and viruses neuronal transport depend upon the survival time af- to label the biological circuits that implement the com- ter viral injection. Rathelot et al. (3) demonstrate that mand functions originating from the PPC, providing a short survival times labeled the cell somas of specific wiring diagram that complements the traditional volun- motoneurons as well as those of interneurons in lam- tary movement pathways from area 5 to corticomoto- inae IV–VIII. Moreover, there was much overlap be- neuronal fiber tracts to motoneurons (4–13). tween the retrogradely virus-labeled interneurons Rathelot et al. (3) take the reader, step-by-step, projecting to specific hand motoneurons and the ante- through the pathway from the PPC to action. First, rogradely labeled nerve terminals from the CTb injections aDepartment of Neuroscience and Physiology, New York University School of Medicine, New York, NY 10016 Author contributions: E.P.G. wrote the paper. The author declares no conflict of interest. See companion article on page 4255. 1Email: [email protected]. 4048–4050 | PNAS | April 18, 2017 | vol. 114 | no. 16 www.pnas.org/cgi/doi/10.1073/pnas.1702746114 Downloaded by guest on October 4, 2021 Clip 3 – 00:20:14:21 Hobbes Unit 17094-131-3.2 IIII IIIIIIIIIIIIIIIIIIIIIIIIIIIII I IIIII I II IIII I II IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII III II IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII I III III I II II II III I 250 Experimenter Raisin swings on stick feed Right Right reach 200 reach and miss Right acquire Right hand Left hand 150 Gaze 100 Task stage Knob Spikes per sec Burst 50 Burst threshold 0 51A B C D 01E F G 5 Time in clip (sec) Burst B Burst D Fig. 1. (Upper) Burst analysis of neural responses to spontaneous reaches and attempted grasps of a raisin on a stick moving through the workspace. The green burst trace marks intervals when firing rates are one SD greater than the mean during the 3-min analysis period. Firing rates are highest as the animal reaches toward the raisin and attempts to grasp it (bursts B, D, and F), and decay when the hand withdraws to a rest position. (Lower) Frame-by-frame tracings of the hand kinematics, gaze direction, and target location in digital video clips recorded simultaneously with neural responses. Same neuron as in figures 4–7 of Gardner et al. (30). from area 5. These data indicate that descending projections from into proximal muscles of the elbow and shoulder. Retrogradely area 5 modulate activity of motoneurons innervating hand muscles labeled corticospinal neurons from lateral area 5 were only about through a disynaptic pathway mediated by last-order interneurons in half as frequent as those in M1, but nearly 1.5 times more numerous themedialdorsalhorn. than those in the dorsal premotor cortex. Longer survival times labeled third-order transneuronal pro- Rathelot et al. (3) conclude that the “lateral region within area jections from the cerebral cortex to the virus-injected hand 5 has corticospinal neurons that are directly linked to the control muscles. These regions include the primary motor cortex (M1), of hand movements” and that “a localized region within the pos- several regions of the premotor cortex, and importantly, layer V of terior parietal cortex has” a more direct “route to access motor lateral area 5. The retrograde label in areas PE and PEip overlapped output” than pathways from premotor areas. Their data clearly fit the anterograde projection sites demonstrated in the CTb and Mountcastle’s view of a command function for the PPC (1, 2). microstimulation experiments. Moreover, the density of transsy- It is significant that the descending area 5 projections dem- naptically labeled neurons in the PPC was substantially greater onstrated by Rathelot et al. (3) terminate on interneurons, not when rabies virus was injected into distal muscles of the hand, than motoneurons, thus modulating the excitability of motoneuron Gardner PNAS | April 18, 2017 | vol. 114 | no. 16 | 4049 Downloaded by guest on October 4, 2021 pools. Terminating on motoneurons would simply confer a direct or eyes, it is seeking some information or trying to acquire a motor role on the PPC. Instead, by terminating on interneurons that desired object, such as food. These actions persist until the goal is activate specific motor pools, PPC cells may rev up the engine or slow achieved. Sensory feedback confirms subjective expectations or it down, enabling facilitation or inhibition of specific actions. Addi- alters actions to achieve that goal. tionally, by terminating on interneurons in a somatosensory projec- Later studies from other neurophysiologists using trained tasks tion zone, as demonstrated by Rathelot et al., PPC neurons may clearly established that PPC neurons in areas 5 and 7 are engaged enhance relevant sensory inputs, and suppress or gate irrelevant, in planning goal-directed movements of the hand, arm, and eyes distracting signals when subjects engage in purposeful acts. to obtain a reward, and provide relevant sensory feedback The 1975 Mountcastle et al. paper (1) was a game-changer, a concerning achievement of task goals (20). These functions are paradigm shift in neuroscience, because it demonstrated that it anatomically segregated into specific subregions of the PPC. Me- was possible to study higher functions of the brain in nonhuman
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