Colin F.L. Hinrichsen

How thick should a retina be? A comparative study of mammalian species with and without intraretinal vasculature.

In this study we describe a measurement method which closely approximates in vivo retinal thickness. Using this method we examined the laminar organisation of vascular and avascular retinae from placental and marsupial mammals. Thickness measurements on retinal wholemounts show that the avascular retinae of the placental guinea pig (140 microns) and the marsupial brushtail possum (170 microns) are thinner and show less centroperipheral taper than do the vascular retinae of placental cats (250 microns), rats (220 microns) and marsupial quolls (220 microns). In general, limitation in thickness of avascular retinae is borne by most retinal layers, but most particularly by the inner plexiform layer, the synaptic region farthest removed from the choroidal blood supply. Except for the absence of blood vessels, the histological organisation of the brushtail possums retina resembles closely that of its fellow marsupial, the quolls. In contrast, intraretinal organisation differs amongst the two avascular retinal species with the guinea pig displaying a much coarser photoreceptor grain.

Brain Stem Projections to the Facial Nucleus of the Rat

Horseradish peroxidase was injected into the medial and lateral columns of the facial nucleus of the rat. Following medial injections, cells were labelled by retrograde transport in the ipsilateral spinal trigeminal nucleus (caudalis) both medial vestibular nuclei, contralateral midbrain reticular formation and nucleus of the lateral lemniscus. The periaqueductal grey, interstitial nucleus and nucleus of Darkschewitch were also labelled ipsilaterally. Injections into the lateral column of the facial nucleus labelled the spinal trigeminal nucleus (oralis) and parabrachial nuclei ipsilaterally and the Darkschewitch and red nuclei contralaterally.

Localization of laryngeal motoneurons in the rat: Morphologic evidence for dual innervation?

Abstract The distribution in the brain stem of neurons supplying the laryngeal muscles cricothyroid (CT), posterior cricoarytenoid (PCA), and thyroarytenoid-lateral cricoarytenoid (TA-LCA) was investigated in the rat. Horseradish peroxidase (HRP) was injected into individual muscles and applied to the cut end of the cervical vagus, the superior laryngeal nerve, and the recurrent laryngeal nerves. The CT was represented rostrally and ventrolaterally to nucleus ambiguus. The PCA was represented in the middle of NA but not in a ventral or dorsal lamina. The TA-LCA overlapped the representation of PCA. Two cell types were labeled by both intramuscular injection and application of HRP to the cut end of nerves. Smaller cells were located in a dorsal lamina of nucleus ambiguus. Evidence is presented that these cell types are equivalent to the two types supplying laryngeal muscles from two separate nuclei in other species.

Respiratory roles of genioglossus, sternothyroid, and sternohyoid muscles during sleep.

We examined the respiratory activity of the genioglossus, sternothyroid, and sternohyoid muscles of the rat during nonrapid eye movement (non-REM) and REM sleep. Each animal carried implanted electrodes for recording the integrated EMG activity of respiratory muscles, the postural tone (EMG), and electrocortical activity (polygraphic identification of sleep-waking states). The three upper airway muscles exhibited inspiratory activity during non-REM sleep while rats breathed ambient air. Curled up postures promoted inspiratory activity of genioglossus and sternothyroid muscles, an effect enhanced by CO2 breathing but reduced by hypoxic breathing. During REM sleep, genioglossus and sternothyroid muscles lost their activity but the sternohyoid muscles retained their inspiratory activity. We conclude that the genioglossus and sternothyroid muscles contribute to upper airway patency during non-REM sleep, an effect CO2 augments but hypoxia reduces. The sternohyoid muscles have at least two functions during both sleep states: they contribute to maintenance of upper airway patency and to rib cage fixation, thereby optimizing the ventilatory action of the diaphragm.

The contractile properties, histochemistry, ultrastructure and electrophysiology of the cricothyroid and posterior cricoarytenoid muscles in the rat

SummaryThe contractile, histochemical, morphological and electrophysiological properties of two rat laryngeal muscles, the cricothyroid and posterior cricoarytenoid, have been measured. Both muscles act during respiration to maintain upper airway patency and an even distribution of air in the lungs. The cricothyroid and posterior cricoarytenoid are fast-twitch muscles, having contraction times of 3.4 and 7.2 ms respectively, high myosin ATPase activity, abundant sarcoplasmic reticulum (with average volumes of 9% and 15%, respectively, of the fibre volume) and T-system membrane (with average areas of 0.4 and 0.5 µm2 µ−3 of fibre). The large areas of T-tubule membrane are reflected in the average specific membrane capacities of 6.5 µF cm−2 to 10.5 µF cm−2, which are high considering the small diameter of the fibres (20–30 µm). Of the two muscles, the posterior cricoarytenoid has the faster contraction time and the more abundant sarcoplasmic reticulum content. In addition, the posterior cricoarytenoid is less resistant to fatigue and demonstrates lower succinic dehydrogenase activity. The fatigability of this muscle, coupled with its general lack of functional reserve, suggest that its failure may contribute to upper airway obstruction during respiratory distress.

Ventilatory and metabolic responses to cold and hypoxia in conscious rats with discrete hypothalamic lesions

We tested the hypothesis that hypothalamic nuclei involved in thermoregulatory control could represent a site of integration of the metabolic and ventilatory response to cold and hypoxia. Electrolytic lesions were performed bilaterally under stereotaxic guide, either within the anterior or posterior hypothalamic areas of adult rats. One week later, oxygen consumption (VO2) and ventilation (VE) were measured in the conscious animals during warm (27 degrees C) or cold (12 degrees C) conditions, in normoxia (21% O2) or hypoxia (10% O2), and compared to measurements obtained in control rats, which were either intact or sham-operated. VO2, VE, and body temperature did not differ between lesioned and control rats during warm normoxia. In cold and hypoxia, singly or combined, VE/VO2 was higher in the lesioned rats, because of higher VE. The differences in the cold were mostly confined to rats with anterior lesions, whereas differences in hypoxia were mostly in rats with posterior lesions. We conclude that the integrity of the anterior and posterior hypothalamic areas is important for the proper coupling of metabolism and ventilation during cold or hypoxic stimuli.

Hypoxia attenuates the respiratory response to injection of substance P into the nucleus of the solitary tract of the rat

Prolonged or repetitive bouts of hypoxia may desensitize the brain stem respiratory centres leading to reduced stimulation of ventilation. We investigated the possible involvement of changes in the sensitivity of the commissural nucleus of the solitary tract (cNTS) to the tachykinin peptide, substance P (SP). Urethane-anaesthetised rats were allowed to breath room air (normoxic) or subjected to four, 30 s bouts of hypoxia (10% O2/90% N2) prior to the injection of SP (750 pmol) into the cNTS. In normoxic rats (n = 5), SP produced a fall in frequency (f, 88+/-4% control) after 4 min and a maximum rise in tidal volume (VT) after 6 min (138+/-10% control) leading to an overall increase in minute ventilation (VE, maximum, 127+/-12% control after 2 min). In rats (n = 5) exposed to four bouts of hypoxia and allowed to recover for 10 min, injection of SP produced a similar fall in f but a delayed and significantly (P < 0.001) reduced VT (maximum after 10 min, 110+/-1% control) and hence, VE response (104+/-3% control). Sixty min after hypoxia, the f, VT and VE responses to SP were identical to those of normoxic rats. These data suggest that hypoxia desensitizes SP receptors in the cNTS and this may partly explain why the respiratory response to hypoxia declines over time.

Respiratory responses to combined hypoxia and hypothermia in rats after posterior hypothalamic lesions

Urethane-anaesthetised rats were exposed to hypoxia (7% O2 in N2) for 5 min periods while body core temperature (Tbc) was maintained within the normal range (37–38° C) using an abdominal heat exchanger. Animals were exposed to hypoxia and after placement of electrolytic lesions in either the anterior (n=6) or posterior hypothalamus (n=6). Neither lesion altered respiration while rats breathed air at either Tbc. At normal Tbc, rats responded to hypoxia with increased ventilation throughout the exposure period. This response was unchanged by lesions in either location. At reduced Tbc rats responded to hypoxia with an initial increase in ventilation followed by depression to below air-breathing levels. This depressive response was unchanged after anterior hypothalamic lesions but eliminated after posterior hypothalamic lesions. It is concluded that neurons either originating in the posterior hypothalamus, or passing through it, play a role in the interaction between cold and hypoxia which leads to inhibition of respiration.

Projections of the Midlateral Posterior Hypothalamic Area Influencing Cardiorespiratory Function in Rats

The influence of the midlateral posterior hypothalamic area (MLPHA) on arterial blood pressure and respiration was examined in the rats. Electrical stimulation produced a rapid transient increase in blood pressure and rapid shallow respiration. Glutamate, which stimulates neurons and not fibers of passage, however, caused a fall in blood pressure and slowing of respiration. Projections of MLPHA traced by horseradish peroxidase (HRP) and 3H leucine showed reciprocal connections with limbic forebrain centers but only indirect connections with brainstem cardiorespiratory centers. It is concluded that MLPHA may be a relay through which the limbic system can gain access to and depress cardiorespiratory function.

The size of motor units in laryngeal muscles of the rat.

The size of motor units in rat laryngeal muscles was determined by correlating the number of neurons labeled by i.m. injections of horseradish peroxidase with the number of motor end plates stained for acetylcholinesterase. The cricothyroid has a motor unit size of 8 muscle fibers per motor neuron and the posterior cricoarytenoid 4–5 muscle fibers per motor neuron.