Distribution of oligosynaptic group I input to the cat medial gastrocnemius motoneuron pool

1985 ◽  
Vol 53 (2) ◽  
pp. 497-517 ◽  
Author(s):  
R. K. Powers ◽  
M. D. Binder
Keyword(s):  
Sural Nerve ◽  
Afferent Input ◽  
Medial Gastrocnemius ◽  
Resting Potential ◽  
Tibialis Posterior ◽  
Medial Branch ◽  
Motoneuron Pool ◽  
Group I ◽  
Flexor Digitorum ◽  
Stimulation Of

To characterize the oligosynaptic group I afferent input to the cat medial gastrocneumius (MG) motoneuron pool, the medial branch of the tibial nerve (MTIB: flexor digitorum and hallucis longus, popliteus, tibialis posterior and interosseous nerves), the nerves to flexor digitorum and hallucis longus (FDHL), or the nerves to the quadriceps muscles (QUAD) were stimulated at submaximal group I strength while recording intracellularly from MG motoneurons. Since previous work indicates that stimulation of these nerves at group I strength produces no significant monosynaptic Ia excitation or Renshaw inhibition of MG motoneurons, group I effects were assumed to be predominantly, though not exclusively, due to the action of Ib-fibers. Evidence supporting this assumption is presented in the following paper. MTIB, FDHL, and QUAD postsynaptic potentials (PSPs) were most commonly inhibitory. Since the MTIB, FDHL, and QUAD nerves are composed predominantly of fibers innervating muscles with extensor action, their inhibitory effect on MG motoneurons is consistent with previous findings that stimulation of Ib-afferents in nerves to extensor muscles produces di- and trisynaptic inhibition of extensor motoneurons. However, excitatory effects were observed in about one third of the motoneurons, indicating that oligosynaptic group I input is not homogeneously distributed within the MG motoneuron pool. Variations in QUAD, FDHL, and MTIB PSP pattern and amplitude were correlated with variations in the PSP pattern evoked by stimulation of the sural nerve: excitatory oligosynaptic group I PSPs generally appeared in motoneurons receiving excitatory cutaneous (sural nerve) input, whereas inhibitory PSPs generally appeared in motoneurons receiving some inhibitory cutaneous input and were largest in motoneurons receiving predominantly inhibition from the sural nerve. These variations in QUAD, FDHL, and MTIB PSP pattern and amplitude were not due to variations in resting potential and were only partly due to variations in intrinsic motoneuron properties or motoneuron "type." Our results indicate that activation of these cutaneous and group I muscle afferents can exert similar effects on the MG motoneuron pool. Moreover, the presence of a strong correlation between the distributions of cutaneous and oligosynaptic group I PSPs within a single motoneuron pool is consistent with the results of previous studies that have shown that some of the input to motoneurons from these peripheral afferents is mediated through common interneurons.

Determination of afferent fibers mediating oligosynaptic group I input to cat medial gastrocnemius motoneurons

1985 ◽  
Vol 53 (2) ◽  
pp. 518-529 ◽  
Author(s):  
R. K. Powers ◽  
M. D. Binder
Keyword(s):  
Electrical Stimulation ◽  
Medial Gastrocnemius ◽  
Flexor Hallucis Longus ◽  
Selective Activation ◽  
Group I ◽  
Afferent Fibers ◽  
Series Of Experiments ◽  
Group Ii ◽  
Flexor Digitorum ◽  
Stimulation Of

In the experiments described in the preceding paper electrical stimulation of the quadriceps (QUAD), medial tibial (MTIB), and flexor digitorum and hallucis longus (FDHL) muscle nerves was used to evoke oligosynaptic group I postsynaptic potentials (PSPs) in medial gastrocnemius (MG) motoneurons. In the present study, we attempted to specify the types of afferent fibers which mediate that oligosynaptic activity (FDHL to MG only). In one series of experiments, isolated single flexor digitorum longus (FDL) and flexor hallucis longus (FHL) afferents were identified as Ia, Ib, or group II fibers according to their conduction velocities, responses to muscle contraction, and mechanical thresholds to small amplitude triangular stretches applied to the parent muscles. We also determined the electrical thresholds of the identified afferent fibers by applying graded electrical stimulation to their muscle nerve. These results were used as criteria to define the types of afferents that mediated the electrically and stretch-evoked FDHL oligosynaptic PSPs recorded in MG motoneurons during a second series of experiments. The amplitudes of the oligosynaptic PSPs evoked in MG motoneurons increased as the strength of the electrical stimuli applied to the FDHL muscle nerves was raised to activate greater numbers of Ia- and Ib-fibers, but showed little or no additional increase when the stimulus intensity was raised further to include the majority of group II fibers. On this basis, a significant contribution by group II fibers to these oligosynaptic PSPs was considered unlikely. Simultaneous electrical activation of both Ia- and Ib-fibers produced distinct oligosynaptic PSPs in MG motoneurons, but these were likely due primarily to Ib-afferent activity, since selective activation of Ia-afferents (by stretch) rarely produced oligosynaptic PSPs in the same motoneurons. There was, however, evidence for some Ia contribution to these oligosynaptic PSPs. This is consistent with the demonstration that Ia- and Ib-afferent fibers converge onto common interneurons and that selective activation of Ia-fibers can produce PSPs similar to those evoked by concurrent stimulation of Ia- and Ib-fibers. On the basis of the present results and those of several related studies it is argued that the oligosynaptic PSPs evoked in MG motoneurons by submaximal group I stimulation of the FDHL, MTIB, or QUAD muscle nerves can be ascribed predominantly to the activation of Ib-afferent fibers, with only minimal Ia and probably no group II contribution.

A determination of excitability changes in dorsal spinocerebellar tract neurons from spike-train analysis

1977 ◽  
Vol 40 (3) ◽  
pp. 626-646 ◽  
Author(s):  
C. K. Knox ◽  
S. Kubota ◽  
R. E. Poppele
Keyword(s):  
Spike Train ◽  
Afferent Input ◽  
Complex Interactions ◽  
Group I ◽  
Long Latency ◽  
Cross Correlation Analysis ◽  
Output Spike ◽  
Type 3 ◽  
Stimulation Of

1. Responses of DSCT neurons to random electrical stimulation of peripheral nerves of the hindleg at group I intensity were studied using cross-correlation analysis of the output spike train with the stimulus. Three types of response were found: type 1 was due to monosynaptic activation of DSCT cells, type 2 resulted from inhibition of those cells, and type 3 was due to a long-latency excitation that was probably polysynaptic. 2. Most of the units studied responded to stimulation of both proximal and distal flexor and extensor nerves. The extensive convergence of afferent input on DSCT cells is much greater than has been observed previously, with type 2 and type 3 responses totaling 80% of the observed responses. We attribute this to the sensitivity of the analysis in detecting small changes in postsynaptic excitability. 3. The results of the study, particularly the derivation of postsynaptic excitability changes, generally confirm those of earlier work employing intracellular recording. 4. By varying stimulus rate and stimulus intensity in the group 1 range and simulating the resulting correlations, we conclude that excitability changes in DSCT cells are the net result of complex interactions involving excitation and inhibition. A summary of these findings is presented as a model for the minimum circuitry necessary to account for the observed behavior.

Recurrent inhibition to and from motoneurons innervating the flexor digitorum and flexor hallucis longus muscles of the cat

1990 ◽  
Vol 63 (3) ◽  
pp. 395-403 ◽  
Author(s):  
T. M. Hamm
Keyword(s):  
Tibialis Anterior ◽  
Recurrent Inhibition ◽  
Medial Gastrocnemius ◽  
Flexor Hallucis Longus ◽  
Flexor Digitorum Longus ◽  
Lateral Gastrocnemius ◽  
Hindlimb Muscles ◽  
Inhibitory Postsynaptic Potentials ◽  
Flexor Digitorum ◽  
Stimulation Of

1. Recurrent inhibitory postsynaptic potentials (IPSPs) to and from motoneurons innervating the flexor digitorum longus (FDL) and flexor hallucis longus (FHL) muscles of the cat were investigated to determine whether recurrent inhibitory projections involving these motoneurons are similar--as would be consistent with the Ia and anatomic synergism of FDL and FHL--or are dissimilar, as are the activities of these muscles during locomotion (O'Donovan et al. 1982). 2. Composite recurrent IPSPs were recorded in several species of motoneurons innervating hindlimb muscles in response to stimulation of a number of muscle nerves in cats allowed to become unanesthetized after ischemic decapitation. 3. No recurrent IPSPs from stimulation of the FDL nerve were observed in motoneurons innervating FDL, FHL, lateral gastrocnemius-soleus (LG-S), medial gastrocnemius (MG), plantaris (Pl), tibialis anterior (TA), or extensor digitorum longus (EDL). 4. The recurrent IPSPs produced by stimulation of FHL were larger and found more frequently in LG-S than in FDL motoneurons. Recurrent inhibition from FHL was also greater in Pl than in FDL motoneurons. 5. The recurrent IPSPs produced by stimulation of LG-S, PL, and MG were larger in FHL than in FDL motoneurons, and those from LG-S and MG were found more frequently in FHL than in FDL motoneurons. 6. Stimulation of the TA nerve produces recurrent IPSPs in FDL but not in FHL motoneurons. A few FDL and FHL cells (6 of 23 and 9 of 34, respectively) received small (less than 0.5 mV) recurrent IPSPs from stimulation of the EDL nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor-unit recruitment in the decerebrate cat: several unit properties are equally good predictors of order

1991 ◽  
Vol 66 (4) ◽  
pp. 1127-1138 ◽  
Author(s):  
T. C. Cope ◽  
B. D. Clark
Keyword(s):  
Motor Unit ◽  
Rank Order ◽  
Motor Units ◽  
Afferent Input ◽  
Medial Gastrocnemius ◽  
Size Principle ◽  
Recruitment Pattern ◽  
Decerebrate Cat ◽  
Recruitment Order ◽  
Stimulation Of

1. Recruitment order was studied in pairs of motor units of the medial gastrocnemius (MG) muscle of decerebrate cats with the use of dual microelectrode recording from intact ventral root filaments. Excitation was provided by stretch of MG, stretch of synergists [lateral gastrocnemius (LG), plantaris (PL), and soleus (SOL) muscles] or electrical stimulation of the caudal cutaneous sural (CCS) nerve. Motor units were characterized by axonal conduction velocity (CV), tetanic tension (Pmax), twitch contraction time (CT), and fatigue index (FI). 2. Consistent with the recruitment pattern described by others, most often in relation to either CV or Pmax, the first unit of a pair to be recruited by MG stretch was typically the one with the lower CV and Pmax, and the higher FI and CT. The proportion of pairs that agreed in rank order of each property and recruitment order was as follows: for CT, 94%; for CV, 87%; for Pmax, 84%; and for FI, 75%. With a single marginal exception (CT vs. FI), no motor-unit property proved to be significantly better than the others at predicting recruitment (G test; P greater than 0.05). 3. In all 11 tested pairs containing one slow (type S) and one fast (type F) unit, the S was more easily recruited by stretch. Type F units divided into groups with high (type FR), low (type FF), and intermediate (type FInt) values for FI were recruited in order from FR to FInt to FF in 8/11 pairs. Thus our findings were similar to earlier demonstrations that recruitment proceeds in order by type. 4. Stretch of MG synergists usually recruited units in the same order as MG stretch. In two S-S pairs, recruitment order was switched with synergist stretch. 5. Stimulation of the CCS nerve was generally excitatory to the MG units sampled. Most unit pairs were recruited by CCS stimulation in the same order as by MG stretch, but, for 6 of 39 pairs, CCS stimulation switched the order produced by stretch. Thus, whereas sural afferent input can preferentially excite some units over others as suggested by Kanda et al., that effect is not widespread or selective for unit type under these conditions. 6. Assuming that all MG motor units cooperate as a single functional pool in homonymous stretch reflexes, we support others in concluding that a motoneuron's recruitment threshold is not strictly determined by its size. However, our data do not distinguish other schemes that predict recruitment order more accurately than the size principle.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic organization of defined motor-unit types in cat tibialis anterior

1980 ◽  
Vol 43 (6) ◽  
pp. 1631-1644 ◽  
Author(s):  
R. P. Dum ◽  
T. T. Kennedy
Keyword(s):  
Motor Unit ◽  
Tibialis Anterior ◽  
Muscle Afferents ◽  
Medial Gastrocnemius ◽  
Synaptic Organization ◽  
Ia Afferents ◽  
Group I ◽  
Ia Epsp ◽  
Group I Afferents ◽  
Stimulation Of

1. Synaptic potentials were recorded intracellularly in tibialis anterior (TA) motoneurons following stimulation of a descending brain stem pathway, the medial longitudinal fasciculus (MLF), and three segmental inputs, the homonymous and heteronymous group Ia afferents, the group I afferents from the antagonist, and the cutaneous and muscle afferents. Intracellular stimulation of the motoneurons was used to classify them, based on the properties of the innervated muscle units, into types FF, F(int), FR, and S (6, 16). 2. The sum of the monosynaptic EPSP amplitudes resulting from stimulation of homonymous and heteronymous group Ia afferents (summed group Ia EPSP) was inversely related to motoneuron size, as assessed by motoneuron input resistance, and was inversely related to motor-unit tetanic tension. Type-FF, -FR, and -S motoneurons showed significant differences in the mean amplitude of their summed group Ia EPSPs. 3. The amplitudes of disynaptic IPSPs resulting from stimulation of group I afferents in the antagonist muscle also showed an inverse relationship to motoneuron size. The observed relationships between motoneuron size and the monosynaptic group Ia EPSP amplitude or the disynaptic group I IPSP amplitude are compatible with the “size principle” of motor-unit recruitment (26). 4. The amplitudes of the monosynaptic EPSPs evoked in TA motoneurons by stimulation of the MLF were distributed rather randomly among all types of TA motoneurons. A slight tendency of larger monosynaptic EPSPs to occur in motoneurons with larger tetanic tensions was observed. 5. The polysynaptic effects from cutaneous and muscle afferents in sural and gastrocnemius-soleus nerves were frequently excitatory on type-FF motoneurons, but were primarily inhibitory on type-FR and -S motoneurons. Clearly, the polysynaptic cutaneous and muscle inputs and the monosynaptic MLF input onto TA motoneurons show a different pattern of synaptic organization than the group I inputs. 6. In general, the synaptic organization of the TA motor nucleus is similar to that of its extensor antagonist, medial gastrocnemius (MG) (2--5, 7, 8), when analogous neural circuits are compared. This parallel organization suggests a commonality of motor-control systems for both flexor and extensor muscles.

Effects of ankle extensor muscle afferent inputs on hip abductor and adductor activity in the decerebrate walking cat

2012 ◽  
Vol 108 (11) ◽  
pp. 3034-3042 ◽  
Author(s):  
D. A. E. Bolton ◽  
J. E. Misiaszek
Keyword(s):  
Gluteus Medius ◽  
Extensor Muscle ◽  
The Body ◽  
Burst Duration ◽  
Medial Gastrocnemius ◽  
Decerebrate Cat ◽  
Group I ◽  
Adductor Muscles ◽  
Hip Abductor ◽  
Stimulation Of

Electrical stimulation of the lateral gastrocnemius-soleus (LGS) nerve at group I afferent strength leads to adaptations in the amplitude and timing of extensor muscle activity during walking in the decerebrate cat. Such afferent feedback in the stance leg might result from a delay in stance onset of the opposite leg. Concomitant adaptations in hip abductor and adductor activity would then be expected to maintain lateral stability and balance until the opposite leg is able to support the body. As many hip abductors and adductors are also hip extensors, we hypothesized that stimulation of the LGS nerve at group I afferent strength would produce increased activation and prolonged burst duration in hip abductor and adductor muscles in the premammillary decerebrate walking cat. LGS nerve stimulation during the extensor phase of the locomotor cycle consistently increased burst amplitude of the gluteus medius and adductor femoris muscles, but not pectineus or gracilis. In addition, LGS stimulation prolonged the burst duration of both gluteus medius and adductor femoris. Unexpectedly, long-duration LGS stimulus trains resulted in two distinct outcomes on the hip abductor and adductor bursting pattern: 1) a change of burst duration and timing similar to medial gastrocnemius; or 2) to continue rhythmically bursting uninterrupted. These results indicate that activation of muscle afferents from ankle extensors contributes to the regulation of activity of some hip abductor and adductor muscles, but not all. These results have implications for understanding the neural control of stability during locomotion, as well as the organization of spinal locomotor networks.

Contraction-induced excitation in cat peroneal motoneurons

1995 ◽  
Vol 73 (3) ◽  
pp. 974-982 ◽  
Author(s):  
N. Kouchtir ◽  
J. F. Perrier ◽  
D. Zytnicki ◽  
L. Jami
Keyword(s):  
Electrical Stimulation ◽  
Distal Portion ◽  
Afferent Input ◽  
Ventral Root ◽  
Triceps Surae ◽  
Group I ◽  
Peroneus Brevis ◽  
Repetitive Electrical Stimulation ◽  
Tendon Organ ◽  
Stimulation Of

1. Motoneurons innervating peroneal muscles were recorded intracellularly in anesthetized cats during sustained submaximal isometric contractions of peroneus brevis produced by repetitive electrical stimulation of motor axons in the distal portion of cut ventral root filaments. 2. In contrast with the inhibition previously observed during contractions of gastrocnemius medialis muscle in triceps surae motoneurons, the afferent input generated by peroneus brevis contraction elicited excitatory potentials in nearly all motoneurons supplying peroneus brevis, peroneus tertius, or peroneus longus muscles. 3. We ascribed the contraction-induced excitation of peroneal motoneurons to spindle afferents for two reasons. First, the amplitude of contraction-induced excitatory potentials increased when the ventral root stimulation strength was increased to recruit gamma-axons. Second, with stimulation strengths under gamma-threshold, peroneus brevis contraction still excited peroneal motoneurons, and we obtained evidence that activation of spindles by skeletofusimotor beta-axons could account at least partly for this excitation. 4. The lack of contraction-induced inhibition in peroneal motoneurons and the prevalence of contraction-induced excitation raised the possibility that, in contrast to the usual effects of tendon organ afferents, Ib afferents from peroneus brevis might exert an excitatory influence on homonymous motoneurons. The fact that electrical stimulation of group I afferents in the nerve to peroneus brevis only exceptionally evoked inhibition in peroneal motoneurons would appear compatible with this hypothesis. Furthermore, stimulation of cutaneous afferents, known to facilitate transmission in Ib pathways, only exceptionally revealed a weak contraction-induced inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasticity of the extensor group I pathway controlling the stance to swing transition in the cat

1995 ◽  
Vol 74 (6) ◽  
pp. 2782-2787 ◽  
Author(s):  
P. J. Whelan ◽  
G. W. Hiebert ◽  
K. G. Pearson
Keyword(s):  
Medial Gastrocnemius ◽  
Lateral Gastrocnemius ◽  
Group I ◽  
Excitatory Pathways ◽  
Extensor Muscles ◽  
Stimulus Train ◽  
Control Response ◽  
Control Limb ◽  
Adult Cats ◽  
Stimulation Of

1. This study examines whether the efficacy of polysynaptic group I excitatory pathways to extensor motoneurons are modified after axotomy of a synergistic nerve. Previously, it has been shown that stimulation of extensor nerves at group I strength can extend the stance phase and delay swing. Stimulation of the lateral gastrocnemius and soleus (LG/S) nerve prolongs stance for the duration of the stimulus train, whereas stimulation of the medial gastrocnemius (MG) nerve moderately increases stance. Our hypothesis was that after axotomy of the LG/S nerve the efficacy of the MG group I input would increase. 2. This idea was tested in 10 adult cats that had their left LG/S nerves axotomized for 3-28 days. On the experimental day the cats were decerebrated and the left (experimental) and right (control) LG/S and MG nerves were stimulated during late stance as the animals were walking on a motorized treadmill. A significant increase in the efficacy of the left MG nerve occurred 5 days after axotomy of the LG/S nerve when compared with the control response. By contrast, the previously cut LG/S nerve showed a reduction in efficacy after 3 days compared with the control limb. 3. Functionally, this plasticity may be an important mechanism by which the strength of the group I pathway is calibrated to different loads on the extensor muscles.

The long-latency component of cerebral evoked potentials in anesthetized cats

1986 ◽  
Vol 65 (3) ◽  
pp. 392-397 ◽  
Author(s):  
Kyu Ho Lee ◽  
Jun Kim ◽  
Jin Mo Chung
Keyword(s):  
Evoked Potentials ◽  
Sural Nerve ◽  
Evoked Potential ◽  
Afferent Input ◽  
Late Component ◽  
Content Type ◽  
C Fibers ◽  
Long Latency ◽  
Fine Print ◽  
Stimulation Of

✓ A late component of the cortical evoked potential elicited by somatosensory afferent input was studied in cats anesthetized with α-chloralose. Cortical evoked potentials were recorded from the somatosensory-motor cortex during stimulation of the sural nerve with graded intensities. The stimulus intensity was adjusted to activate Aαβ fibers only, then both Aαβ and Aδ fibers, and both A and C fibers, as judged by afferent volleys monitored from the sural nerve proximal to the stimulating site. In addition to early components reported previously, a very late component was identified at a latency of 400 to 600 msec following stimulation of the sural nerve with intensities above threshold for Aδ fibers. A further increase in stimulation intensity to include activation of C fibers did not reveal any more components. This late component was depressed by a systemic intravenous injection of morphine (2 mg/kg), and intravenous naloxone (0.1 mg/kg) reversed the effect of morphine. The late component of the evoked potential could also be recorded from subcortical tissue after decortication of the sensorimotor cortex. From these results, it appears that a very late component of the cortical evoked potential can be recorded from cats anesthetized with α-chloralose. The late component is evoked by activation of peripheral Aδ fibers. Furthermore, its morphine sensitivity suggests that this component may be elicited by nociceptive afferent fibers. If further investigations prove this, the late component, which is analogous to human long-latency potentials, could be used in an experimental model for pain research.

Two forms of inhibition of spinothalamic tract neurons produced by stimulation of the periaqueductal gray and the cerebral cortex

1991 ◽  
Vol 65 (6) ◽  
pp. 1567-1579 ◽  
Author(s):  
D. X. Zhang ◽  
C. M. Owens ◽  
W. D. Willis
Keyword(s):  
Cerebral Cortex ◽  
Sural Nerve ◽  
Background Activity ◽  
Periaqueductal Gray ◽  
Afferent Input ◽  
The Other ◽  
Inhibitory Effects ◽  
Spinothalamic Tract ◽  
Other Hand ◽  
Stimulation Of

1. Recordings were made from the lumbosacral spinal cord in anesthetized macaque monkeys. The inhibitory effects of electrical stimulation of the periaqueductal gray (PAG) and the cerebral cortex or cerebral peduncle (CP) were tested and compared by recording 1) cord dorsum potentials evoked by stimulation of the sural nerve, 2) discharges recorded extracellularly, and 3) membrane potentials recorded intracellularly from spinothalamic tract (STT) neurons at rest (background activity) or in response to stimulation of the sural nerve. 2. Stimulation of the cortex or in the CP preferentially reduced the amplitude of the N1 and N2 waves of the cord dorsum potential evoked by stimulation of the sural nerve, without affecting the N3 wave. Stimulation of the PAG, on the other hand, reduced the amplitude of the N3 wave with little effect on the N1 and N2 waves. 3. The activity of 62 STT neurons was recorded extracellularly. Stimulation of the PAG or the cortex/CP inhibited nonpreferentially the responses of the neurons in the superficial laminae to all afferent inputs. On the other hand, stimulation of the PAG or the cortex/CP inhibited preferentially the responses of most STT neurons in deep layers of the dorsal horn to the small or large afferent input, respectively. 4. Thirty-five neurons were recorded intracellularly. The membrane potential of the neurons averaged -45.5 +/- 10.1 (SD) mV. All neurons were recorded in laminae III-VI; the neurons were of the wide-dynamic-range (WDR) type and had background activity. 5. The inhibitory effects of stimulation of the PAG were tested on all 35 neurons. In 32 of the neurons, stimulation of the PAG evoked a hyperpolarization. The background activity of the neurons was reduced (generally it completely ceased) by the hyperpolarization. In three neurons stimulation of the PAG did not evoke a hyperpolarization and the background activity of the neurons did not change. Nevertheless, the responses of these three neurons to afferent input were inhibited by stimulation in the PAG. 6. The inhibitory effects of stimulating the cortex and/or the CP were tested in 26 of the 35 neurons. Stimulation of the cortex and/or the CP evoked a hyperpolarization in all the neurons, although, in 10 of the 26 neurons, stimulation of the CP also evoked a depolarization. The hyperpolarization generally blocked the background activity of the neurons. 7. The effective stimuli in the PAG and the cortex/CP to evoke a hyperpolarization in STT neurons were short, high-frequency trains of pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

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