Karen A. Manning

Not looking while leaping: the linkage of blinking and saccadic gaze shifts

Many vertebrates generate blinks as a component of saccadic gaze shifts. We investigated the nature of this linkage between saccades and blinking in normal humans. Activation of the orbicularis oculi, the lid closing muscle, EMG occurred with 97% of saccadic gaze shifts larger than 33°. The blinks typically began simultaneously with the initiation of head and/or eye movement. To minimize the possibility that the blinks accompanying saccadic gaze shifts were reflex blinks evoked by the wind rushing across the cornea and eyelashes as the head and eyes turned, the subjects made saccadic head turns with their eyes closed. In this condition, orbicularis oculi EMG activity occurred with all head turns greater than 17° in amplitude and the EMG activity began an average of 39.3 ms before the start of the head movement. Thus, one component of the command for large saccadic gaze shifts appears to be a blink. We call these blinks gaze-evoked blinks. The linkage between saccadic gaze shifts and blinking is reciprocal. Evoking a reflex blink prior to initiating a voluntary saccadic gaze shift dramatically reduces the latency of the initiation of the head movement.

A role for the basal ganglia in nicotinic modulation of the blink reflex

SummaryIn humans and rats we found that nicotine transiently modifies the blink reflex. For blinks elicited by stimulation of the supraorbital branch of the trigeminal nerve, nicotine decreased the magnitude of the orbicularis oculi electromyogram (OOemg) and increased the latency of only the long-latency (R2) component. For blinks elicited by electrical stimulation of the cornea, nicotine decreased the magnitude and increased the latency of the single component of OOemg response. Since nicotine modified only one component of the supraorbitally elicited blink reflex, nicotine must act primarily on the central nervous system rather than at the muscle. The effects of nicotine could be caused by direct action on lower brainstem interneurons or indirectly by modulating descending systems impinging on blink interneurons. Since precollicular decerebration eliminated nicotines effects on the blink reflex, nicotine must act through descending systems. Three lines of evidence suggest that nicotine affects the blink reflex through the basal ganglia by causing dopamine release in the striatum. First, stimulation of the substantia nigra mimicked the effects of nicotine on the blink reflex. Second, haloperidol, a dopamine (D2) receptor antagonist, blocked the effect of nicotine on the blink reflex. Third, apomorphine, a D2 receptor agonist, mimicked the effects of nicotine on the blink reflex.

Thalamocortical projections to rat auditory cortex from the ventral and dorsal divisions of the medial geniculate nucleus

The ventral and dorsal medial geniculate (MGV and MGD) constitute the major auditory thalamic subdivisions providing thalamocortical inputs to layer IV and lower layer III of auditory cortex. No quantitative evaluation of this projection is available. Using biotinylated dextran amine (BDA)/biocytin injections, we describe the cortical projection patterns of MGV and MGD cells. In primary auditory cortex the bulk of MGV axon terminals are in layer IV/lower layer III with minor projections to supragranular layers and intermediate levels in infragranular layers. MGD axons project to cortical regions designated posterodorsal (PD) and ventral (VA) showing laminar terminal distributions that are quantitatively similar to the MGV‐to‐primary cortex terminal distribution. At the electron microscopic level MGV and MGD terminals are non‐γ‐aminobutyric acid (GABA)ergic with MGD terminals in PD and VA slightly but significantly larger than MGV terminals in primary cortex. MGV/MGD terminals synapse primarily onto non‐GABAergic spines/dendrites. A small number synapse on GABAergic structures, contacting large dendrites or cell bodies primarily in the major thalamocortical recipient layers. For MGV projections to primary cortex or MGD projections to PD or VA, the non‐GABAergic postsynaptic structures at each site were the same size regardless of whether they were in supragranular, granular, or infragranular layers. However, the population of MGD terminal‐recipient structures in VA were significantly larger than the MGD terminal‐recipient structures in PD or the MGV terminal‐recipient structures in primary cortex. Thus, if terminal and postsynaptic structure size indicate strength of excitation then MGD to VA inputs are strongest, MGD to PD intermediate, and MGV to primary cortex the weakest. J. Comp. Neurol., 2012.

Pattern of extraocular muscle activation during reflex blinking

SummaryStudies in humans and rabbits suggest that cocontraction of extraocular muscles occurs with reflex and voluntary blinks. We determined the pattern of extraocular muscle activity elicited by blink-evoking visual and trigeminal stimuli by electromyographically recording antagonistic pairs of extraocular muscles in alert rabbits. In addition, we recorded the activity of antidromically identified oculomotor motoneurons in response to the same stimuli in alert rabbits. The data demonstrate that all extraocular muscles except the superior oblique transiently increase their activity in response to blink-evoking stimuli. The pattern of extraocular muscle activity with reflex blinks mirrors that occurring in the lid-closing muscle, orbicularis oculi, but the latency of extraocular muscle activation is longer.

Histamine-immunoreactive neurons and their innervation of visual regions in the cortex, tectum, and thalamus in the primate Macaca mulatta

The histaminergic system is involved in the control of arousal in the brain and may impact significantly on visual processing. However, little is known about the histaminergic innervation of visual areas, or the histamine system in the primate brain, in general. We examined in Macaca mulatta the location of histamine‐immunoreactive neurons and the innervation of important cortical and subcortical visual areas by histamine‐immunoreactive axons. Brain sections were treated with an antibody to histamine and processed with standard immunohistological procedures.

MUSCARINIC RECEPTOR SUBTYPES IN THE LATERAL GENICULATE NUCLEUS : A LIGHT AND ELECTRON MICROSCOPIC ANALYSIS

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, m1–m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3‐positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2‐positive synaptic contacts were classified as F2‐type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype‐specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic‐mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons. J. Comp. Neurol. 404:408–425, 1999.

The histaminergic innervation of the lateral geniculate complex in the cat

The histaminergic innervation of the thalamic dorsal and ventral lateral geniculate nuclei and the perigeniculate nucleus of the cat was examined immunohistochemically by means of an antibody to histamine. We find histamine-immunoreactive neurons in the cat brain are concentrated in the ventrolateral portion of the posterior hypothalamus, confirming a previous report. However, this cell group also spreads into medial, dorsal, and extreme lateral regions of the posterior hypothalamus and extends as far rostral as the optic chiasm. Histamine-labeled fibers cover all regions of the lateral geniculate complex, but the density of labeling varies. The ventral lateral geniculate nucleus (vLGN) is most densely labeled, the A laminae of the dorsal lateral geniculate are sparsely labeled, and the geniculate C laminae and the perigeniculate nucleus show intermediate amounts of label. Thus, histaminergic fibers demonstrate a predilection for zones innervated by the W-cell system. Labeled fibers exhibit few branchings and numerous en passant swellings, lending a beaded appearance. The vLGN showed more instances of fibers with larger-sized swellings (up to 2 microns). Following injections of biotinylated tracers into the hypothalamus, we find labeled fibers throughout the lateral geniculate complex. The anterogradely labeled fibers resemble the histaminergic fibers in morphology, distribution, and relative bouton size. Thus, the hypothalamus appears to be the source of the histaminergic fibers in the lateral geniculate complex. Histamine-labeled fibers in the dorsal lateral geniculate nucleus (dLGN) exhibit uncommon ultrastructural morphology. Many extremely large, round, or elliptical vesicles fill the fiber swellings. Swellings are directly apposed to a variety of other dendritic and axonal profiles, but thus far no convincing synaptic contacts have been seen. The distribution and appearance of these histaminergic fibers resembles those reported for serotonergic fibers. Our results support the idea that histamine works nonsynaptically as a neuromodulator in the lateral geniculate complex, affecting the level of visual arousal.

Histaminergic and non-histamine-immunoreactive mast cells within the cat lateral geniculate complex examined with light and electron microscopy

Mast cells and their location in the cat lateral geniculate complex of the thalamus were examined by means of histamine immunohistochemistry and the mast cell stain pinacyanol erythrosinate. Brain sections from seven normal adult pigmented cats were processed for light or electron microscopy. Histamine-containing and pinacyanol erythrosinate-stained mast cells were widespread throughout the dorsal and ventral lateral geniculate nuclei and the surrounding regions. Mast cells were especially numerous rostrally in the complex and in the geniculate C laminae. The cells were found consistently in association with blood vessels, ranging from capillary size to vessels c. 150 microns diameter, and twice as often with arterioles as with venules. Large clusters of many mast cells associated with single blood vessels were seen. Individual mast cells were typically 8 microns in diameter and somewhat oval, although multipolar and crescent-shaped cells were also seen, up to twice as long. The amount of histamine labeling varied across cells. When histamine-labeled material was secondarily stained with pinacyanol erythrosinate, many mast cells were double labeled. In addition, there was a small population of mast cells that stained only with pinacyanol erythrosinate, but was otherwise identical to the histamine-immunoreactive mast cells. Electron microscopic examination showed that the mast cells lie on the brain side of the blood-brain barrier. Mast cells were found in close proximity to the thalamic neuropil, primarily apposed to the processes of astrocytes, but also apposed to neural elements. The distinctive electron-dense cytoplasmic granules in the fully granulated, mature state were largely amorphous in appearance and as large as 700 nm in diameter. Histamine was dispersed throughout some granules and contained within restricted areas of other granules. In degranulated mast cells, large, irregularly shaped, electron-lucent granules were seen fused with the cell membrane on the neuropil side, as well as the lumen side of the mast cell. More mast cells were observed at the electron microscopic level than were expected from the light level observations, which suggests that, despite the numbers of mast cells labeled, these results may still underestimate the total mast cell population present in this region of the thalamus. Mast cells, by their numbers, their distribution and the potent chemical substances they contain, may significantly influence vascular and neural function, directly and indirectly, in the cat lateral geniculate complex.

Preferential effect of isoflurane on top-down vs. bottom-up pathways in sensory cortex

The mechanism of loss of consciousness (LOC) under anesthesia is unknown. Because consciousness depends on activity in the cortico-thalamic network, anesthetic actions on this network are likely critical for LOC. Competing theories stress the importance of anesthetic actions on bottom-up “core” thalamo-cortical (TC) vs. top-down cortico-cortical (CC) and matrix TC connections. We tested these models using laminar recordings in rat auditory cortex in vivo and murine brain slices. We selectively activated bottom-up vs. top-down afferent pathways using sensory stimuli in vivo and electrical stimulation in brain slices, and compared effects of isoflurane on responses evoked via the two pathways. Auditory stimuli in vivo and core TC afferent stimulation in brain slices evoked short latency current sinks in middle layers, consistent with activation of core TC afferents. By contrast, visual stimuli in vivo and stimulation of CC and matrix TC afferents in brain slices evoked responses mainly in superficial and deep layers, consistent with projection patterns of top-down afferents that carry visual information to auditory cortex. Responses to auditory stimuli in vivo and core TC afferents in brain slices were significantly less affected by isoflurane compared to responses triggered by visual stimuli in vivo and CC/matrix TC afferents in slices. At a just-hypnotic dose in vivo, auditory responses were enhanced by isoflurane, whereas visual responses were dramatically reduced. At a comparable concentration in slices, isoflurane suppressed both core TC and CC/matrix TC responses, but the effect on the latter responses was far greater than on core TC responses, indicating that at least part of the differential effects observed in vivo were due to local actions of isoflurane in auditory cortex. These data support a model in which disruption of top-down connectivity contributes to anesthesia-induced LOC, and have implications for understanding the neural basis of consciousness.

Evaluation of inputs to rat primary auditory cortex from the suprageniculate nucleus and extrastriate visual cortex.

Evidence indicates that visual stimuli influence cells in the primary auditory cortex. To evaluate potential sources of this visual input and how they enter into the circuitry of the auditory cortex, we examined axonal terminations in the primary auditory cortex from nonprimary extrastriate visual cortex (V2M, V2L) and from the multimodal thalamic suprageniculate nucleus (SG). Gross biocytin/biotinylated dextran amine (BDA) injections into the SG or extrastriate cortex labeled inputs terminating primarily in superficial and deep layers. SG projects primarily to layers I, V, and VI while V2M and V2L project primarily to layers I and VI, with V2L also targeting layers II/III. Layer I inputs differ in that SG terminals are concentrated superficially, V2L are deeper, and V2M are equally distributed throughout. Individual axonal reconstructions document that single axons can 1) innervate multiple layers; 2) run considerable distances in layer I; and 3) run preferentially in the dorsoventral direction similar to isofrequency axes. At the electron microscopic level, SG and V2M terminals 1) are the same size regardless of layer; 2) are non‐γ‐aminobutyric acid (GABA)ergic; 3) are smaller than ventral medial geniculate terminals synapsing in layer IV; 4) make asymmetric synapses onto dendrites/spines that 5) are non‐GABAergic and 6) are slightly larger in layer I. Thus, both areas provide a substantial feedback‐like input with differences that may indicate potentially different roles. J. Comp. Neurol. 518:3679–3700, 2010.