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K M Spyer

Publications and source records attributed to K M Spyer.

At least 127 records · Page 7Linked to original sources
PubMed1980

Vestibular inputs to the fastigial nucleus; evidence of convergence of macular and ampullar inputs.

1. Experiments have been undertaken on 11 decerebrate cats to investigate the effects of natural vestibular stimulation on the activity of cerebellar fastigial neurons. 2. From recordings in the rostral portion of the nucleus during sinusoidal lateral (roll) and horizontal (yaw) rotation, distinctive patterns of response were observed. 3. The majority of neurons sensitive to vestibular stimulation showed responses to a single modality of vestibular activation. During lateral tilt some neurones showed positional sensititivy, others gave responses related tothe velocity of movement. Other neurones responded in phase with the velocity of movement in the horizontal plane. 4. Aside from these neuronal responses, others provided indications of a convergence of inputs from different sets of vestibular receptors. In particular, several neurons showed a pattern of response that indicated tht they received inputs from otolith receptors and ampullar receptors of the vertical canal. At low velocities of movement their response was positional but with inreasing velocity the magnitude of the response increased and there was a marked phase shift of the discharge towards head velocity. 5. Neurons responding to horizontal rotation often showed positional responses during lateral tilt. There were also indications of a convergence of ampullar inputs from both vertical and horizontal canals. 6. The neural pathways mediating these resonses are discussed in consideration of previous neuroanatomical and neurophysiological data. We consider it likely that several pathways may act to evoke the patterns of response observed, and a role of the cerebellar cortex is indicated.

Acoustic Maculae↗
PubMed1979

The origin of the hind limb vasodilatation evoked by stimulation of the motor cortex in the cat.

In cats under Althesin anaesthesia, the hind limb area of the motor cortex has been stimulated by means of monopolar, semi-micro-electrodes with careful experimental control so as to avoid reflex effects evoked through stimulation of meningeal afferent fibres or stimulus spread to non-cortical structures. 2. Localized cortical stimulation which elicited muscle contractions in the contralateral hind limb also elicited vasodilation in the same limb: the stimulus threshold was the same for both effects, and the magnitude of the dilatation was related to the strength of contraction. 3. Reduction of the somatic motor response, caused by lesions in the medullary pyramidal tract, was accompanied by a parallel reduction of the vascular response. 4. Prevention of the motor response by gallamine or by spinal cord section at L4--L5 (which leaves the sympathetic outflow to the hind limbs intact) led to abolition of the vascular response. During recovery from gallamine, contraction and vasodilatation returned in parallel. 5. The muscle vasodilatation was insensitive to atropine or guanethidine. 6. It is concluded that the hind limb vasodilatation observed on stimulation of the motor cortex is simply a post-contraction hyperaemia, and that it is independent of the sympathic nervous system. Previous conclusions of a sympathetically mediated vasodilatation probably resulted from inadequate control of the stimulus or a failure to recognize weak muscle contractions.

Animals↗
PubMed1979

Studies on the excitability of sinus nerve afferent terminals.

1. The excitability of sinus nerve afferent terminals within the nucleus of the tractus solitarius has been studied in cats and rabbits using the technique of antidromic activation. 2. Conditioning stimuli to the hypothalamic defence area increased the excitability of some glossopharyngeal nerve afferents, though no such effects were observed on sinus nerve terminals. 3. Although the excitability of superior laryngeal nerve afferent terminals was observed to fluctuate in phase with the central respiratory cycle, no equivalent variation in sinus nerve terminal excitability were observed. 4. It is concluded that sinus nerve afferent terminals are not influenced by presynaptic mechanisms. Possible sites for the observed modulations of baroreceptor and chemoreceptor reflexes are discussed in the light of these results.

Animals↗
PubMed1978

Two types of vagal preganglionic motoneurones projecting to the heart and lungs.

1. A study has been made of eighty-four cells in the cat's nucleus ambiguus whose axons projected to the cardiac (seventy-four) and pulmonary (ten) branches of the right vagus. Their axonal conduction velocities were all in the range of B fibres (2.8-15.5 m/sec).2. Pulmonary branch projecting neurones were usually spontaneously active (nine out of ten) and fired in phase with inspiration. Their activity showed no pulse modulation.3. Ten cardiac branch projecting neurones had properties indistinguishable from those of pulmonary branch projecting neurones. Inspiratory-firing cells projecting to either branch are believed to be bronchoconstrictor in function.4. The remaining sixty-four cells that projected to the cardiac branch had properties expected of cardioinhibitory neurones. Most (fifty-four) were silent until activated by ionophoresis of excitant amino acids. All showed an expiratory discharge when active, and of twenty-seven tested twenty-three showed a cardiac modulation of their discharge. When the aortic baroreceptors were denervated, the cardiac rhythm was always abolished reversibly by carotid occlusion.5. Ionophoretic activation of expiratory firing (presumed cardioinhibitory) cells slowed the heart (fifteen out of eighteen neurones tested). Excited inspiratory-firing cells never had this effect (eleven tested).6. Both types of neurones were found in the nucleus ambiguus, but presumed cardioinhibitory cells tended to be found more caudally and ventrally than presumed bronchoconstrictor neurones.

Action Potentials↗
PubMed1978

The baroreceptor input to cardiac vagal motoneurones.

1. A study has been made of twenty-three cardiac vagal motoneurones (c.v.m.s) in the nucleus ambiguus of chloralose-anaesthetized cats.2. C.v.m.s were identified as described previously (McAllen & Spyer, 1978). They were either spontaneously active (five units) or induced to fire by the ionophoresis of DL-homocysteic acid (DLH). Evidence is presented that above threshold, the DLH dose is not critical for the ability to show a baroreceptor influence.3. The influence of the baroreceptors was determined by plotting pulse-triggered histograms of c.v.m. activity. If the aortic baroreceptors were denervated, the pulse synchronous activity disappeared reversibly on carotid occlusion (eight out of eight cases). In five out of fifteen cells studied, when aortic baroreceptor pathways were intact, a small degree of cardiac modulation survived carotid occlusion.4. Estimates of the central delay of the baroreceptor-vagal reflex were made by timing the c.v.m. response to both electrical stimulation of the sinus nerve and natural baroreceptor stimulation. When both methods were used on individual units, electrical stimulation invariably gave shorter values. Natural stimulation gave values that fell into two ranges (20-60 msec and 70-110 msec), suggesting the involvement of two pathways.5. Using higher currents, four c.v.m.s were induced to fire tonically during inspiration. Their activity in this phase showed a cardiac rhythm just as during expiration. Similarly, they responded to sinus nerve stimulation throughout the respiratory cycle.6. This result is discussed in relation to the observation of others that the baroreceptor-cardiac reflex is ineffective during inspiration. It is proposed that there is no need to invoke a ;gate' early in the pathway but that this phenomenon and the present observations are best explained by summation of influences at the c.v.m. cell membrane.

Action Potentials↗
PubMed1977

Studies on the termination of sinus nerve afferents.

Compound action potentials evoked by electrical stimulation in the brainstem of anaesthetized, paralyzed cats and rabbits were recorded distally in the carotid sinus nerve. The latencies of the components of the responses were indicatives of fibres with conduction velocities between 0.5 and 32 m/s. These include both myelinated and unmyelinated fibres. Using histological reconstruction of the 'responsive sites', the termination of the primary afferent fibres of the sinus nerve within the medulla was mapped. In both cat and rabbit these responsive sites were restricted to the doromedial medulla in the vicinity of the NTS 0--3 mm rostral to the obex. Although in the cat the fast conducting myelinated fibres (conduction velocities greater than 12.5 m/s) were discretely localized to the ventrolateral area of the NTS, the slower conducting myelinated and unmyelinated fibres were found more diffusely, spreading into areas dorsal and medial to the NTS. In the the rabbit the terminals of all types of fibre were mainly confined to the NTS although some C fibres appeared to be grouped in the juxta-alar region. Responses were never evoked from the medial reticular formation or nucleus ambigus in either cats or rabbits. The organization of the sinus nerve input to the medulla is discussed in the light of these results.

Animals↗
PubMed1977

The carotid chemoreceptor input to the respiratory neurones of the nucleus of tractus solitarus.

1. An investigation has been made into the connexions between the carotid body chemoreceptors and the dorsal respiratory neurones of the cat's medulla.2. In confirmation of previous work these neurones were found to be all inspiratory in firing pattern and to fall into two categories, Ralpha (forty-four units) which fire only with the central inspiratory rhythm and Rbeta (thirty-two neurones) that are also excited by lung inflation. Both categories were shown to be excited by stimuli delivered to the carotid bodies during inspiration but, with a single exception, not during expiration.3. When Rbeta neurones were made to fire tonically in expiration by maintained lung inflation, chemoreceptor activation inhibited this discharge in 7/11 cases, the remainder being unaffected.4. Iontophoretically applied DL-homocysteic acid or glutamate made both Ralpha and Rbeta neurones fire tonically in expiration. Chemoreceptor stimulation during expiration inhibited this activity in all neurones tested (nine Ralpha and three Rbeta cells).5. Using the measurement of the antidromic latency to spinal stimulation as an index of membrane potential, evidence was obtained that any subthreshold influence of the chemoreceptors on dorsal respiratory neurones during expiration was inhibitory (9/18 cases).6. It is concluded that chemoreceptors do not even subliminally excite dorsal inspiratory neurones during expiration; such influence as they have then is inhibitory. Possible reasons for this difference in chemoreceptor influence during inspiration and expiration are discussed. It is suggested that chemoreceptor excitation reaches them only as part of an enhanced central inspiratory drive from an as yet unknown source.

Action Potentials↗
PubMed1976

The location of cardiac vagal preganglionic motoneurones in the medulla of the cat.

1. Electrophysiological techniques have been used to locate the origin of preganglionic vagal motoneurones supplying the heart of the cat. 2. The right cardiac vagal branches were identified anatomically and their ability to slow the heart was assessed by electrical stimulation. Control experiments revealed that contamination of cardiac branches by bronchomotor and oesophageal efferent fibres was likely to be small. 3. Fifty-seven neurones in the medulla were activated antidromically on stimulating the cardiac branches at up to 5 times the threshold for cardiac slowing. They had axons with conduction velocities between 3 and 15 m/sec, corresponding to B fibres. 4. None of these were located in the region of the dorsal motor nucleus of the vagus, in spite of repeated sampling there, but all were located in the region of the nucleus ambigus. Histological examination of marked neurones (forty-six of the fifty-seven neurones) revealed that they were associated with its principal column, rostral to the obex. 5. Sampling motoneurones of the dorsal motor nucleus revealed that most sent axons down the thoracic vagus below the cardiac branches. Only three of thirty-three could be activated antidromically by high intensity stimulation of the cardiac branches, but on the basis of their thresholds and conduction velocities, it is argued that they were unlikely to be cardio-inhibitory neurones. 6. It is concluded that preganglionic cardio-inhibitory neurones arise not in the dorsal motor nucleus, but in the principal column of the nucleus ambiguus.

Animals↗
PubMed1975

The sinus nerve and baroreceptor input to the medulla of the cat.

Electrical stimulation within the medulla of cats revealed that myelinated primary afferent fibres of the sinus nerve terminated within the immediate vicinity of the tractus solitarius and its nucleus. 2. The activity of neurones within this area was also evoked on sinus nerve stimulation, although few (17%) were activated within a latency compatible with monosynaptic excitation. Additional projections over polysynaptic pathways have been shown to the parahypoglossal area and to the area of the nucleus ambiguus. 3. These three areas were shown to contain neurones whose activity was enhanced by stimulation of the baroreceptor endings of the ipsilateral carotid sinus. 4. No evidence for a projection of sinus nerve afferents to the medial reticular formation (an area extending medially from the hypoglossal nucleus and nerve tract and including the paramedian reticular nucleus) was obtained in either antidromic or orthodromic studies. 5. The organization of the central pathway of the carotid sinus baroreceptor reflex is discussed in the light of these results.

Animals↗