Cartesian representation of stimulus direction: Parallel processing by two sets of giant interneurons in the cockroach

The cockroachPeriplaneta americana responds to wind puffs by turning away, both on the ground and when flying. While on the ground, the ventral giant interneurons (ventrals) encode the wind direction and specify turn direction, whereas while flying the dorsal giant interneurons (dorsals) appear to do so. We report here on responses of these cells to controlled wind stimuli of different directions. Using improved methods of wind stimulation and of positioning the animal revealed important principles of organization not previously observed.All six cells of largest axonal diameter on each side respond preferentially to ipsilateral winds. One of these cells, previously thought to respond non-directionally (giant interneuron 2), was found to have a restricted directional response (Fig. 3). The organization of directional coding among the ventral giant interneurons is nearly identical to that among the dorsals (Fig. 2). Each group contains, on each side, one cell that responds primarily to wind from the ipsilateral front, another primarily in the ipsilateral rear, and a third responding more broadly to ipsilateral front and rear.These results are discussed in terms of the mechanisms of directional localization by the assembly of giant interneurons.Abbreviations GI giant interneuron - vGI ventral giant interneuron - dGI dorsal giant interneuron - CF 5-carboxyfluorescein - A6 6th abdominal ganglion - TI thoracic interneuron - BED best excitatory direction     

Electrophysiological investigation of conduction of afferent impulses through the medial geniculate body

Extra- and intracellular reactions of 280 neurons of the pars principalis of the medial geniculate body (MGB) and of 408 auditory cortical neurons in area AI to stimulation of the inferior brachium of the midbrain and geniculocortical fibers were studied in cats immobilized with D-tubocurarine. Single electrical stimulation of the inferior brachium was shown to evoke a long and complex neuronal response in MGB in the form of excitation of some and inhibition of other neurons. The initial component of this response lasted 13 msec. Excitation of 72% of neurons participating in the response took place during the first 3 msec after the beginning of stimulation. In the same period 84% of IPSP arose. The inferior brachium was shown to contain a certain number of descending fibers. Some of them are axons of MGB neurons. Many fibers of the inferior brachium reach the auditory cortex without synaptic relay in MGB. Of all cells of MGB excited by stimulation of the inferior brachium monosynaptically, 76% are thalamocortical relay neurons; the rest are interneurons. Of the relay neurons of MGB 90% are excited monosynaptically, the rest by impulses passing through two or three synaptic relays in MGB. During stimulation of the inferior brachium, responses consisting of EPSP-IPSP and primary IPSP are recorded in many neurons of MGB. About 20% of primary IPSP arise monosynaptically, evidently in response to stimulation of inhibitory fibers of the inferior brachium. Most IPSP arise disynaptically, with the participation of an inhibitory interneuron located at the entrance to MGB. Inhibition observed in this case is direct afferent in nature.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 6, pp. 515–523, November–December, 1979.     

Centrally-mediated synaptic input: Effects on an identified crayfish mechanosensory interneuron

Summary High-level mechanosensory interneurons integrate a substantial amount of polysynaptic input. We have used an identified interneuron, the crayfish (Procambarus clarki) caudal photoreceptor (CPR), to examine the extent and specificity of interneuronal input as received by a physiologically complex, smalldiameter sensory unit. Presynaptic central neurons were identified by antidromic stimulation of connective fibers and characterized physiologically relative to the bilateral responses observed in the paired CPR's. Eleven inhibitory interneurons have been identified, including cells with ipsilateral (Fig. 4), contralateral (Fig. 5) and bilateral effects (Fig. 6). Seven excitatory interneurons have been identified, including examples from each of the respective categories above (Figs. 7–9). The results of this survey are summarized in Table 1; axon locations are presented in Fig. 10.It has also been demonstrated that several of these fibers are themselves ascending mechanoreceptive interneurons (e.g., fiber 122, Fig. 1). Thus, for the encoding of environmental stimuli, these results indicate that central integration involves a lateral exchange of tactile information among a set of interrelated sensory interneurons. However, the possibility still exists that some of these fibers represent descending pathways for central influence of local (segmental) integrative processes.Abbreviations CPR caudal photoreceptor - EPSP excitatory postsynaptic potential - IPSP inhibitory postsynaptic potential This work has been supported in part by a Research Fellowship from Bryn Mawr College (to G.A.M.) and by Research Grants from NIH (NS-12971-03) and the Whitehall Foundation. This paper is a contribution of the Tallahassee, Sopchoppy and Gulf Coast Marine Biological Association (No. 123).     

Influence of afferent impulses from the nerves of the anterior limbs on the insertion neurons of the lumbar section of the spinal cord

In anesthetized cats in conditions of muscular relaxation we have studied the participation of the interneurons of the lumbar section of the spinal cord in the interaction of the FRA systems of the fore and hind limbs. Using microelectrodes we have made extra- and intracellular recordings of the potentials. It has been shown that from the flexor afferents of the fore limbs both facilitating and inhibitory influences are transmitted. The former are expressed in increased frequency of the background impulse activity of the neurons, in the appearance of evoked responses of the "silent" cells and intensification of the test responses for short time intervals with paired heteronymous stimulation. The inhibitory influences prevail over the facilitating and are manifest in depression of the background activity and evoked segmental responses of the neurons. The maximum inhibition of the segmental responses was noted for intervals of 40–140 msec. The duration of inhibition varied from 100–500 msec and more. Depending on the intensity and duration of the inhibitory influences two groups of interneurons have been isolated. The role of the pre- and postsynaptic mechanisms in the transmission of inhibitory influences from the afferents of the fore limbs on the afferents of the hind limbs is discussed.Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 1, No. 3, pp. 235–242, November–December, 1969.     

Network activity of "interneurons" and large cells of amygdala in active and passive rabbits

By plotting cross-correlation histograms differences were found in interaction of conjectural small "interneurons" and large principal cells of the central and basal amygdalar nuclei in negative emotional situations. The network activity of "nterneurons" was higher than in principal cells. "Interneurons" more frequently had excitatory and inhibitory input or output connections with neighbouring cells, latency of their connections with other cells was smaller than in principal neurons. Interaction of "interneurons" and principal cells differed in animals with active and passive behavioural strategy in negative emotional situations. As compared to active animals, in passive rabbits inhibitory connections to "interneurons" from other cells occurred more frequently, excitatory or inhibitory connections from "interneurons" to principal cells appeared more rare.     

Cerebellar Cortex Granular Layer Interneurons in the Macaque Monkey Are Functionally Driven by Mossy Fiber Pathways through Net Excitation or Inhibition

The granular layer is the input layer of the cerebellar cortex. It receives information through mossy fibers, which contact local granular layer interneurons (GLIs) and granular layer output neurons (granule cells). GLIs provide one of the first signal processing stages in the cerebellar cortex by exciting or inhibiting granule cells. Despite the importance of this early processing stage for later cerebellar computations, the responses of GLIs and the functional connections of mossy fibers with GLIs in awake animals are poorly understood. Here, we recorded GLIs and mossy fibers in the macaque ventral-paraflocculus (VPFL) during oculomotor tasks, providing the first full inventory of GLI responses in the VPFL of awake primates. We found that while mossy fiber responses are characterized by a linear monotonic relationship between firing rate and eye position, GLIs show complex response profiles characterized by “eye position fields” and single or double directional tunings. For the majority of GLIs, prominent features of their responses can be explained by assuming that a single GLI receives inputs from mossy fibers with similar or opposite directional preferences, and that these mossy fiber inputs influence GLI discharge through net excitatory or inhibitory pathways. Importantly, GLIs receiving mossy fiber inputs through these putative excitatory and inhibitory pathways show different firing properties, suggesting that they indeed correspond to two distinct classes of interneurons. We propose a new interpretation of the information flow through the cerebellar cortex granular layer, in which mossy fiber input patterns drive the responses of GLIs not only through excitatory but also through net inhibitory pathways, and that excited and inhibited GLIs can be identified based on their responses and their intrinsic properties.     

Wind-activated thoracic interneurons of the cockroach: I. Responses to controlled wind stimulation

The cockroach escape response begins with a turn away from a wind puff such as that generated by an approaching predator. The presence and direction of that wind is detected by hairs on the animal's cerci, and this information is conducted to the thoracic ganglia via two populations of giant interneurons. In the thoracic ganglia, the giant interneurons excite a number of interneurons, at least some of which in turn excite motor neurons that control leg movement. In this paper we examine response properties of various thoracic neurons to wind stimuli originating from different directions. Three sets of thoracic neurons were distinguished on the basis of latency. Type A interneurons had short latencies to wind stimuli (1.3-2.25 ms). Type B interneurons had longer latencies (4-6 ms), and motor neurons had the longest latencies (5.6-17.0 ms). Individual type A interneurons either responded equally to wind from all directions or were biased in their response. Directionality was related to the presence of ventral branches near one or both sides of the midline of the ganglion. Cells with ventral median (VM) branches on either side tended to be omnidirectional or front-rear biased, whereas cells with VM branches on only one side were biased to that side. Although several type B interneurons had strong wind responses and were directionally sensitive, they did not have VM branches. We hypothesize that the presence of VM branches in type A interneurons permits connection with ventral giant interneurons, and this connection accounts for their short latency and directional properties. This hypothesis will be tested in the companion paper.     

A Guide to In vivo Single-unit Recording from Optogenetically Identified Cortical Inhibitory Interneurons

A major challenge in neurophysiology has been to characterize the response properties and function of the numerous inhibitory cell types in the cerebral cortex.We here share our strategy for obtaining stable, well-isolated single-unit recordings from identified inhibitory interneurons in the anesthetized mouse cortex using a method developed by Lima and colleagues¹. Recordings are performed in mice expressing Channelrhodopsin-2 (ChR2) in specific neuronal subpopulations. Members of the population are identified by their response to a brief flash of blue light. This technique – termed “PINP”, or Photostimulation-assisted Identification of Neuronal Populations – can be implemented with standard extracellular recording equipment. It can serve as an inexpensive and accessible alternative to calcium imaging or visually-guided patching, for the purpose of targeting extracellular recordings to genetically-identified cells. Here we provide a set of guidelines for optimizing the method in everyday practice. We refined our strategy specifically for targeting parvalbumin-positive (PV+) cells, but have found that it works for other interneuron types as well, such as somatostatin-expressing (SOM+) and calretinin-expressing (CR+) interneurons.     

A Quantitative Study of Inhibitory Interneurons in Laminae I-III of the Mouse Spinal Dorsal Horn

Laminae I-III of the spinal dorsal horn contain many inhibitory interneurons that use GABA and/or glycine as a neurotransmitter. Distinct neurochemical populations can be recognised among these cells, and these populations are likely to have differing roles in inhibiting pain or itch. Quantitative studies in rat have shown that inhibitory interneurons account for 25-40% of all neurons in this region. The sst2A receptor is expressed by around half the inhibitory interneurons in laminae I-II, and is associated with particular neurochemically-defined populations.Although much of the work on spinal pain mechanisms has been performed on rat, the mouse is now increasingly used as a model, due to the availability of genetically altered lines. However, quantitative information on the arrangement of interneurons is lacking in the mouse, and it is possible that there are significant species differences in neuronal organisation.In this study, we show that as in the rat, nearly all neurons in laminae I-III that are enriched with glycine also contain GABA, which suggests that GABA-immunoreactivity can be used to identify inhibitory interneurons in this region. These cells account for 26% of the neurons in laminae I-II and 38% of those in lamina III. As in the rat, the sst_2A receptor is only expressed by inhibitory interneurons in laminae I-II, and is present on just over half (54%) of these cells. Antibody against the neurokinin 1 receptor was used to define lamina I, and we found that although the receptor was concentrated in this lamina, it was expressed by many fewer cells than in the rat. By estimating the total numbers of neurons in each of these laminae in the L4 segment of the mouse, we show that there are around half as many neurons in each lamina as are present in the corresponding segment of the rat.     

Interneurons of the motor region of the neocortex]

Interneurons of motor area in the brain cortex have been studied in cats and monkeys. The greatest attention has been paid to pyramidal interneurons, among which six cell types have been described according to their axonal composition. Unlike stellate interneurons, all types of pyramidal interneurons possess less developed axonal collaterals. Interneuronal contacts are situated on dendrites or cell bodies of middle and large long-axonal pyramids. Functional role of cortical interneurons seems to be different. Some of them are of inhibitory nature (basket cells and, perhaps, other types of long-axonal stellate neurons), others are exciting elements. The latter include short-axonal stellate neurons and, perhaps, pyramidal interneurons. While comparing the cortex in cats and monkeys, it is evident that the neocortex in monkeys, especially its lower layers, is rich in pyramidal interneurons.     

Synchronous firing by specific pairs of cercal giant interneurons in crickets encodes wind direction

One prominent stimulus to evoke an escape response in crickets is the detection of air movement, such as would result from an attacking predator. Wind is detected by the cercal sensory system that consists of hundreds of sensory cells at the base of filiform hairs. These sensory cells relay information to about a dozen cercal giant and non-giant interneurons. The response of cercal sensory cells depends both, on the intensity and the direction of the wind. Spike trains of cercal giant interneurons then convey the information about wind direction and intensity to the central nervous system. Extracellular recording of multiple cercal giant interneurons shows that certain interneuron pairs fire synchronously if a wind comes from a particular direction. We demonstrate here that directional tuning curves of synchronously firing pairs of interneurons are sharper than those of single interneurons. Moreover, the sum total of all synchronously firing pairs eventually covers all wind directions. The sharpness of the tuning curves in synchronously firing pairs results from excitatory and inhibitory input from the cercal sensory neurons. Our results suggest, that synchronous firing of specific pairs of cercal giant interneurons encodes the wind direction. This was further supported by behavioral analyses.     

Evoked Electrical Activity and Immunocytochemical Peculiarities of Cultured Excitatory and Inhibitory Neurons of the Rat Hippocampus

Experiments were carried out on cultured hippocampal neurons using a patch-clamp technique in the whole-cell configuration. We studied the characteristics of regular series of action potentials (APs), which were generated with a low frequency by inhibitory and excitatory interneurons after their direct stimulation with long-lasting (500 msec) current pulses. Nearly all parameters of the evoked impulse activity (except the frequency of generation and duration of APs) in excitatory and inhibitory neurons were significantly different. According to immunocytochemical analysis, Kv1.2- and Kv4.2-type potassium channels were expressed in the membrane of excitatory neurons (granular cells), and somatostatin was present in all these cells. As to inhibitory interneurons, only a part of such cells (large units) demonstrated immunopositivity with respect to somatostatin. In inhibitory neurons, only Kv1.2-type potassium channels were expressed. Therefore, mechanisms responsible for the ability of hippocampal interneurons to generate impulse activity under conditions of direct stimulation (in our experiments, regular low-frequency series of APs) in inhibitory and excitatory neurons are rather dissimilar. Neirofiziologiya/Neurophysiology, Vol. 37, No. 3, pp. 207–216, May–June, 2005.     

Distributing coordinated motor outputs to several body segments: escape movements in the cockroach

In the escape behavior of the cockroach, all six legs begin to make directed movements nearly simultaneously. The sensory stimulus that evokes these leg movements is a wind puff. Posterior wind receptors excite giant interneurons that carry a multi-cellular code for stimulus direction — and thus for turn direction-to the three thoracic ganglia, which innervate the three pairs of legs. We have attemptd to discriminate among various possible ways that the directional information in the giant interneurons could be distributed to each leg's motor circuit. Do the giant interneurons, for instance, inform separately each thoracic ganglion of wind direction? Or is there one readout system that conveys this information to all three ganglia, and if so, might the identified thoracic interneurons, which are postsynaptic to the giant interneurons, subserve this function? We made mid-sagittal lesions in one or two thoracic ganglia, thus severing the initial segments of all the known thoracic interneurons in these ganglia, and thus causing their projection axons to the other thoracic ganglia to degenerate. This lesion did not sever the giant interneurons, however (Fig. 5). Following such lesions, the legs innervated by the intact thoracic ganglia made normally directed leg movements (Figs. 4, 6, 7). Thus, the projection axons of the thoracic interneurons are not necessary for normal leg movements. Rather, the giant interneurons appear to specify to each thoracic ganglion in which direction to move the pair of legs it innervates.     

Central organization of common inhibitory motoneurons in the locust: role of afferent signals from leg mechanoreceptors

Intracellular recordings of mesothoracic common inhibitory neurons (CI₁, CI₂ and CI₃) were made while tactile hairs of the middle legs of locusts (Locusta migratoria) were mechanically stimulated. Generally the three common inhibitory neurons were excited by stimulation of tactile hairs on the ventral and dorsal surface of femur and tibia. The response pattern of all three CI neurons was similar suggesting that they work as a functional unit. Touching hairs on the dorsal surface of tibia and tarsus in some cases led to inhibition of CIs. The connection between sensory cells of tactile hairs and common inhibitory neurons is polysynaptic.To identify interneurons which mediate afferent signals, simultaneous intracellular recordings from CIs and interneurons were made. Different spiking interneurons were identified which made excitatory or inhibitory monosynaptic connections with CIs. Interneurons with inhibitory input to CIs belonged to the ventral midline group of spiking local interneurons. Behavioral and electrophysiological results indicate that reflex movements of the leg are accompanied by activity of CI neurons. Further it appears that CI activity is inhibited when reflex movements of the leg are actively suppressed by the animal.Abbreviations CI common inhibitor - IN interneuron - LY Lucifer Yellow     

Physiological properties of afferents and synaptic reorganization in the rat cerebellum degranulated by postnatal X-irradiation.

Elimination of most granule, basket, and stellate interneurons in the rat cerebellum was achieved by repeated doses of low level x-irradiation applied during the first two weeks of postnatal life. Electrical stimulation of the brain stem and peripheral limbs was employed to investigate the properties of afferent cerebellar pathways and the nature of the reorganized neuronal synaptic circuitry in the degranulated cerebellum of the adult. Direct contacts of mossy fibers on Purkinje cells were indicated by short latency, single spike responses: 1.9 msec from the lateral reticular nucleus of brain stem and 5.4 msec from ipsilpateral forelimb. These were shorter than in normal rats by 0.9 and 2.1 msec, respectively. The topography of projections from peripheral stimulation was approximately normal. Mossy fiber responses followed stimulation at up to 20/sec, whereas climbing fiber pathways fatigued at 10/sec. The latency of climbing fiber input to peripheral limb stimulation in x-irradiated cerebellum was 23 +/- 8 (SD) msec. In x-irradiated rats, the climbing fiber pathways evoked highly variable extracellular burst responses and intracellular EPSPs of different, discrete sizes. These variable responses suggest that multiple climbing fibers contact single Purkinje cells. We conclude that each type of afferent retains identifying characteristics of transmission. However, rules for synaptic specification appear to break down so that: (1) abnormal classes of neurons develop synaptic connections, i.e., mossy fibers to Purkinje cells; (2) incorrect numbers of neurons share postsynaptic targets, i.e., more than one climbing fiber to a Purkinje cell; and (3) inhibitory synaptic actions may be carried out in the absence of the major inhibitory interneurons, i.e., Purkinje cell collaterals may be effective in lieu of basket and stellate cells.     

Indirect synaptic inputs from filiform hair sensory neurons contribute to the receptive fields of giant interneurons in the first-instar cockroach

The first-instar cockroach, Periplaneta americana, detects air movements using four filiform hair sensilla, which make synaptic connections to seven pairs of giant interneurons (GIs) in the terminal abdominal ganglion. The directional sensitivities of some of the GIs, predicted from their patterns of monosynaptic inputs, may not be the same as in the second instar or adult. Intracellular recordings were made to determine the contribution of polysynaptic inputs to the receptive fields of first-instar GIs. The ventral GI1, and the dorsal GI5, GI6, and GI7 were all found to have indirect synaptic inputs from filiform afferents. The indirect inputs were excitatory to GI1, GI5, and GI7, and inhibitory to GI6 and GI7. The indirect excitatory input to GI1 was predicted to alter qualitatively its receptive field, allowing it to respond to wind from the side of the animal, as in the adult. Inhibition was predicted to sharpen the receptive fields of GI6 and GI7. The inhibitory postsynaptic potentials reversed 6–8 mV below resting potential and were blocked by picrotoxin, indicating that they are GABAergic. Indirect excitation also altered the predicted receptive field of GI7, one of the inputs being an unusual “off-response” to movement of a filiform hair in its inhibitory direction. Accepted: 19 June 1998     

Anatomy and physiology of identified wind-sensitive local interneurons in the cricket cercal sensory system

1. A group of wind sensitive local interneurons (9DL Interneurons) in the terminal abdominal ganglion of the cricket Acheta domesticus were identified and studied using intracellular staining and recording techniques. 2. The 9DL interneurons had apparent resting potentials ranging from -38 mV to -45 mV. At this membrane potential, these cells produced graded responses to wind stimuli; action potentials were never observed at these resting potentials. However, when the 9DL interneurons were hyperpolarized to a membrane potential of approximately -60 mV, a single action potential at the leading edge of the wind stimulus response was sometimes observed. 3. The wind stimulus threshold of the 9DL interneurons to the types of stimuli used in these studies was approximately 0.01 cm/s. Above this threshold, the excitatory responses increased logarithmically with increasing peak wind velocity up to approximately 0.5 cm/s. 4. The 9DL interneurons were directionally sensitive; their response amplitudes varied with wind stimulus orientation. 9DL1 cells responded maximally when stimulated with wind directed at the front of the animal. The apparent peak in directional sensitivity of the 9DL2 interneurons varied between the side and the rear of the animal, depending upon the site of electrode penetration within the cell's dendritic arbor. 5. The locations of dendritic branches of the 9DL interneurons within the afferent map of wind direction were used to predict the excitatory receptive field of these interneurons.     

Inhibitory response in neurosecretory cells of the hypothalamic supraoptic nucleus to subiculum stimulation in lactating rats

The effects of subiculum stimulation were investigated in 80 antidromically identified hypothalamic supraoptic neurons in lactating rats. Inhibition manifesting as suppression of antidromic action potentials (or of their somatodendritic component) was revealed in 26% of cells, induced by applying conditioned and test stimuli to the subiculum and neurohypophysial stalk. In some instances inhibition arose following a latency of 5–25 msec after each subicular stimulus and lasted only briefly; it set in gradually in other cases, leading to stable long-term changes in the excitability of neurosecretory cells. No activation was produced by this stimulation. It is deduced that subicular inhibitory inputs follow different patterns, thus reflecting morphological organizational aspects of synaptic inhibitory inputs to neurosecretory cells.A. A. Ukhtomskii, Institute of Physiology, A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 20, No. 4, pp. 431–437, July–August, 1988.     

猫视皮层细胞对运动光棒刺激的反应特性的定量分析

数据分析方法的不完善影响了关于细胞视觉反应特性研究结果的可靠性.本工作在分离视觉反应中方向和取向选择性因素的基础上.采用“积分平均”的手段.且将取向选择性理解为“强度随取向的变化率”,得到一种较为准确全面地表现细胞有关反应特性的定量分析法,可以体现出一些以往各方法无法反映出的差别,我们用此方法分析了猫视皮层细胞对运动光棒刺激的反应特性及其相关性.发现取向选择性因素对总反应的贡献较方向选择性大;反应强度,方向选择性和取向选择性三者间存在着弱相关.最优方向、最优取向及方向选择性最强的方向三者间有一定的偏离.     

Neuronal and synaptic mechanisms of cortical inhibition

The latent periods, amplitude, and duration of IPSPs arising in neurons in different parts of the cat cortex in response to afferent stimuli, stimulation of thalamocortical fibers, and intracortical microstimulation are described. The duration of IPSPs evoked in cortical neurons in response to single afferent stimuli varied from 20 to 250 msec (most common frequency 30–60 msec). During intracortical microstimulation of the auditory cortex, IPSPs with a duration of 5–10 msec also appeared. Barbiturates and chloralose increased the duration of the IPSPs to 300–500 msec. The latent period of 73% of IPSPs arising in auditory cortical neurons in response to stimulation of thalamocortical fibers was 1.2 msec longer than the latent period of monosynaptic EPSPs evoked in the same way. It is concluded from these data that inhibition arising in most neurons of cortical projection areas as a result of the arrival of corresponding afferent impulsation is direct afferent inhibition involving the participation of cortical inhibitory interneurons. A mechanism of recurrent inhibition takes part in the development of inhibition in a certain proportion of neurons. IPSPs arise monosynaptically in 2% of cells. A study of responses of cortical neurons to intracortical microstimulation showed that synaptic delay of IPSPs in these cells is 0.3–0.4 msec. The length of axons of inhibitory neurons in layer IV of the auditory cortex reaches 1.5 mm. The velocity of spread of excitation along these axons is 1.6–2.8 msec (mean 2.2 msec).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 394–403, May–June, 1984.