Electrophysiological properties of identified trigeminal ganglion neurons innervating the cornea of the mouse.

The cornea is innervated by three functional types of neurons: mechanosensory, polymodal and cold-sensitive neurons, all of which are presumed to be nociceptive. To explore if corneal neurons constitute a heterogeneous population according to their electrophysiological properties, intracellular recordings were made in vitro from trigeminal ganglion neurons innervating the cornea of the mouse. Corneal neurons were labelled with FluoroGold applied after a corneal epithelial wound. Five days later, the trigeminal ganglion attached to the eye by its nerves was removed and placed in a superfusion chamber. FluoroGold-positive cells that also responded to electrical stimulation of the cornea were considered corneal neurons. Non-corneal neurons were also studied. Based on their conduction velocity at room temperature, corneal neurons were classified as myelinated A (>1.5m/s) or non-myelinated C (< or =1.5m/s) neurons. A and C neurons differed significantly in their passive and active electrical properties. Virtually all corneal C neurons and about two-thirds of A neurons exhibited a hump in the falling phase of the action potential (S neurons), while the remaining A neurons (F neurons) showed faster and narrower action potentials without a hump. Among non-corneal neurons, A neurons of the F type were found in a proportion of about 50%. Based on their ability to produce somatic action potentials in tetrodotoxin (0.1 microM), non-corneal neurons were classified as fully or partially tetrodotoxin sensitive, which were mainly of the Adelta type, and tetrodotoxin resistant, which were C neurons. Among the corneal neurons, those with a faster action potential, possibly associated to the expression of tetrodotoxin-sensitive Na(+) channels, may be pure corneal mechanosensory neurons, all of which are known to belong to the Adelta type. Neurons with a slower action potential showing a hump in the repolarization phase are both corneal Adelta and C polymodal nociceptive neurons, a type of cell in which tetrodotoxin-resistant Na(+) channels have been identified. The possibility is raised that the small population of neurons with a very high input resistance are cold-sensitive neurons. From the present results, we suggest that the electrophysiological properties of primary sensory neurons innervating the cornea are attributable not only to their conduction velocities, but also to the functional characteristics of their peripheral nerve terminals.

[The effects of axotomy on the nociceptive neurons of the leech]. / Efectos de la axotomía en neuronas nociceptivas de la sanguijuela.

INTRODUCTION: It has been described several similarities between mammalian and invertebrate nociceptors. Lateral N neuron, in the segmental ganglion of the leech Hirudo medicinalis, behave like polimodal nociceptors in mammals, meanwhile medial N cells is similar to mechano nociceptors (Pastor, Soria, Belmonte 1996). The effects of axotomy in sensory neurons in leeches are poorly understood. OBJECTIVE: We have focused on the electrophysiological properties of leech nociceptive neurons after axotomy. RESULTS: In N lateral neurons, axotomy induced an increase in the total membrane capacitance, membrane time constant, decrease in the steady state/maximum hyperpolarization rate, increment in the time constant of voltage relaxation of inward rectifier, increase in action potential duration, dV/dtmax, dV/dtmin, and maximum amplitude and duration of after hyperpolarization; firing threshold (always in not axotomized) and an increment in adaptation spike frequency. In N medial cells, axotomy produced a slight depolarization and a decrease in Rinput, a rise in the time constant of voltage relaxation of inward rectifier, a decrement of action potential maximum amplitude, dV/dtmax, dV/dtmin, and after hyperpolarization maximum amplitude, with an increase in action potential and after hyperpolarization duration. Finally, firing threshold increased both in axotomized and not axotomized after axotomy. Axotomy not only induced changes in injured cells, but modified neurons whose axons did not run into the cutting nerves. CONCLUSION: axotomy induces different changes in N lateral and N medial nociceptive cells, and it is suggested some kind of communication between axotomized and not axotomized cells

[Maps of visually evoked brain electrical activity in rabbits]. / Mapas de la actividad eléctrica cerebral evocada visualmente en conejos.

A basic model of topographic distribution of the electric response visually evoked in rabbits by means of flashes (0.69 joules/flash) has been obtained. The model is composed of four main parts--N0, P1, N1 y P2--linked to the VI visual area and displayed on a dipole shaped. The dipole turnaround time oscillates between 20 and 25 ms. The use of electrical activity brain maps on the study of the PE makes it possible to notice the phenomenon simultaneousness, thus facilitating its interpretation. A multinomial interpolation method of continuous function has been used to perform the maps.

Visual evoked potentials in response to pattern reversal in the cat cortex.

A model of the visually evoked potential (VEP) in the cerebral cortex of the cat after binocular stimulation by means of pattern reversal is presented. The VEP is defined by four components: P1, N1, P2 and N2, which appear during the 100 ms following the stimulus. This model is repeated for the majority of recording points although N1 and P2 do not appear to be homogeneous over the entire cortex. The variability of the VEPs recorded at the same point in different cats is lower than the one observed by means of stimulation with flashes. The possible origin of the four components in the primary visual area is presented as a hypothesis and a discussion is made of the differences which exist between the models proposed for flash and pattern reversal.

Visual evoked potentials in response to flashes in the cat cortex.

A model is presented of visually evoked potentials (VEPs) in the cerebral cortex of cats after binocular stimulation by means of flashes. The VEPs consist of four components: P1, N1, P2 and N2 which appear during the first 100 ms after the stimulation is produced. This model has been found in all the animals used in the experiments and is repeated with small variations at almost all the recording points. After studying the data obtained, a hypothesis is put forward for the possible origin of the four components in the primary visual area.

Effect of stimulus intensity on visually-evoked electrical brain activity maps in rabbit.

Stimulation by means of flashes is a commonly-used method in basic research into evoked potentials. Nevertheless, the different responses obtained at different luminous intensities, to which the inter-individual and intra-individual differences are added, determine the need to control this stimulus parameter for each experimental model. Maps of visually-evoked activity in the rabbit brain, obtained after monocular stimulation with flashes at different intensities of luminosity, are presented. Variation in the intensity of the luminous stimulus does not substantially affect the distribution of the electrical potential on the surface of rabbit brain described in previous articles.

The amygdaloid complex: anatomy and physiology.

A converging body of literature over the last 50 years has implicated the amygdala in assigning emotional significance or value to sensory information. In particular, the amygdala has been shown to be an essential component of the circuitry underlying fear-related responses. Disorders in the processing of fear-related information are likely to be the underlying cause of some anxiety disorders in humans such as posttraumatic stress. The amygdaloid complex is a group of more than 10 nuclei that are located in the midtemporal lobe. These nuclei can be distinguished both on cytoarchitectonic and connectional grounds. Anatomical tract tracing studies have shown that these nuclei have extensive intranuclear and internuclear connections. The afferent and efferent connections of the amygdala have also been mapped in detail, showing that the amygdaloid complex has extensive connections with cortical and subcortical regions. Analysis of fear conditioning in rats has suggested that long-term synaptic plasticity of inputs to the amygdala underlies the acquisition and perhaps storage of the fear memory. In agreement with this proposal, synaptic plasticity has been demonstrated at synapses in the amygdala in both in vitro and in vivo studies. In this review, we examine the anatomical and physiological substrates proposed to underlie amygdala function.

Corneal innervation and sensitivity to noxious stimuli in trkA knockout mice.

Most primary sensory neurones depend on neurotrophins for survival. Mutant mice in which TrkA, the high-affinity receptor for nerve growth factor (NGF), has been inactivated lack nociceptive neurones in sensory ganglia and do not respond to noxious stimuli. The cornea of the eye is innervated by trigeminal neurones that are activated by noxious mechanical, thermal and chemical stimuli. In the human cornea, these stimuli evoke only sensations of pain. We have analysed the innervation pattern and the response to noxious stimulation of the cornea of trkA (-/-) mutant mice. Corneal nerves were stained with the gold chloride impregnation method. Corneal sensitivity to noxious stimuli was assessed by counting blinking movements evoked by von Frey hairs, topical application of saline at different temperatures and application of acetic acid and capsaicin at different concentrations. In the cornea of trkA (-/-) mutant animals, we observed a drastic reduction in the number of nerve trunks and branches in the corneal stroma. Furthermore, quantitative analysis of the number of thin nerve terminals revealed a marked decrease in the corneal epithelium of trkA (-/-) mice when compared to those present in wild type and trkA (+/-) animals. The blinking response of trkA (-/-) mice to mechanical, thermal and chemical noxious stimuli was also significantly reduced. These results indicate that the population of corneal sensory neurones is markedly depleted in trkA (-/-) mutant mice. However, a small portion of corneal sensory neurones survive in these mice suggesting that they may be NGF independent. On the basis of our results, we propose that these surviving cells are polymodal nociceptive neurones, sensitive to mechanical stimulation, noxious heat and acid.