3. The specificity of the HDA stimulation was investigated by generalized

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In the normal guinea pig a map of auditory space appears, in the deeper layers of the superior colliculus, at 32 days after birth (DAB). The animal is unable to construct this collicular map of auditory space in the absence of developmental visual experience. Auditory receptive fields of animals dark-reared from birth are typically large, occupying most of the contralateral hemifield. There is no topographic relationship between the collicular location of the recording electrode and the spatial position from which auditory stimuli elicit a maximal response. The fields of dark-reared animals resemble, in their tuning parameters, the spatially undifferentiated fields typical of young postnatal normal guinea pigs. To investigate the time-course during which visual experience is required for map emergence, animals received normal visual experience until either 18 or 26 DAB and were then dark-reared until the terminal mapping experiment. Maps developed in neither group. Animals provided with a normal visual environment until 30 DAB, and then placed in the dark did, however, construct topographically organized spatial maps with discrete spatial receptive fields. Maps also failed to emerge in animals receiving normal visual experience both before and after a 4-day period of visual deprivation between 26 and 30 DAB. We conclude that this 4-day period, or part of it, constitutes a 'crucial' period during which visual experience is required for the normal elaboration of the collicular map of auditory space.

1. The postsynaptic action of carotid sinus nerve (SN), aortic nerve (AN), superior laryngeal nerve (SLN) and vagal nerve (VN) stimulation has been studied on neurones in the nucleus of the tractus solitarius (NTS) in vivo. 2. Three distinct patterns of postsynaptic responses were evoked by SN stimulation, an EPSP, an EPSP-IPSP sequence and an IPSP, observed separately in individual neurones. This diversity of response was represented in cells proven to receive baroreceptor input by inflation of a balloon-tipped catheter within the ipsilateral carotid sinus. 3. Virtually none of the neurones identified as baroreceptive exhibited pulse-related discharge. 4. A variety of influences to AN, SLN and VN stimulation were observed in neurones receiving baroreceptor afferent information. This wide convergence of input implies that this region of the brain stem is important in the integration of cardiovascular reflexes. 5. The hypothesis was tested that respiratory 'gating' of the baroreceptor reflex is produced by synaptic actions within the NTS. There was an absence of any modification of PSPs by lung inflation and by variations in the timing of the stimulation of the afferent nerves within the respiratory cycle. These observations indicate that respiratory modifications of the baroreceptor reflex must occur at later stages in the reflex pathway.

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Guinea pigs, reared from birth in an environment of omnidirectional white noise, fail to develop a map of auditory space in the deeper layers of the superior colliculus. Collicular responses from such noise-reared animals reveal large auditory spatial receptive fields. The representation of auditory space in the colliculus shows no topographic order. Exposing developing animals to the noise environment only for restricted time periods showed that animals reared normally up to 26 days after birth (DAB) and then placed in the noise chamber could not construct spatial maps, whereas animals reared normally to 30 DAB and then placed in the noise chamber until the terminal mapping experiment could construct topographically organized spatial maps with local receptive fields. Limiting the noise exposure to the period between 26 and 30 DAB was sufficient to prevent spatial map formation. The failure to form a map of auditory space did not reflect environmental damage to the cochlea or the functional organization of the primary auditory pathway. The response thresholds of cochlear microphonics and of auditory responses in both the inferior and superior colliculus were normal in noise-reared animals. Similarly normal were the tonotopic organization and frequency tuning characteristics of inferior collicular neurons. The rearing environment thus appears to exert a selective effect upon the maturation of the superior collicular map of auditory space. We attribute this effect to the masking, by the omnidirectional broad-band noise, of discrete localized auditory stimuli. Cues deriving from these latter stimuli would appear to be necessary for the elaboration of the map of auditory space. This auditory experience operates during a 4 day crucial developmental period from 26 to 30 DAB. This is the same developmental time window as that during which visual experience is required for the construction of the map.

The location within the brainstem of vagal preganglionic motoneurons has been determined in the dogfish Scyliorhinus canicula L. by means of the retrograde transport of horseradish peroxidase and cobalt applied to the vagus nerve and its component branches. Labelled vagal motoneurones were located in the ipsilateral caudal rhombencephalon from 2.1 mm caudal to 2.73 mm rostral to obex. The motoneurons of the vagal motor column are arranged as four distinct groups. Caudal to obex the column contains dorsomedial and ventromedial divisions, whilst rostrally it consists of a single rostromedial division and a short lateral division. The cells in the ventromedial division are approximately twice the size (mean area 1,094 microns 2) of the other vagal neurons. The dorsomedial division contains neurons that supply the heart and viscera; the ventromedial division supplies the viscera. The heart is also innervated by the neurons of the lateral division and the visceral nerve also receives axons from the rostromedial division. All neurons supplying axons to the gill arches are located in the rostromedial division. There is a sequential topographical representation of the vagus nerve in the vagal motor column. Neurons supplying the gastrointestinal tract are located caudally; those supplying the cardiac nerves lie in the midportion of the column, and the proximal supply to the gills is given by the most rostral neurons. There is some overlap between the pools of neurons supplying adjacent branches of the vagus.

SUMMARYStimulation of a medial region of the cerebellar uvula cortex produces sympathetically mediated cardiovascular effects in the decerebrate rabbit. In the present study the afferents to this cerebellar area have been determined using electrophysiological and neuroanatomical methods. Field potentials of prevalent mossy origin evoked by single-shock stimulation of different peripheral nerves have been recorded from sublobules IXa and IXb. No response to vagus and aortic nerves stimulation has been found. A modulation of Purkinje cell discharge following natural vestibular stimulation has been recorded from sublobules IXb and IXc. The retrograde transport of horseradish peroxidase from the uvula cortex has revealed a widespread afferent input to this cerebellar region arising from brain stem reticular and sensory nuclei. On the basis of the results obtained, together with the results of experiments performed in conscious rabbits, contained in the following paper (Bradley, Ghelarducci, La Noce & Spyer, 1990), it is suggested that the uvula participates in co-ordination of the visceral and somatic components of the alerting reaction in the rabbit.