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Biomedical subjects

S C Cannon

Publications and source records attributed to S C Cannon.

45 records · Page 3Linked to original sources
PubMed1988

Use of bacitracin for neurotologic surgery.

Wound infection, cerebrospinal fluid leak, and meningitis are serious potential complications of neurotologic procedures that transgress the posterior cranial fossa dura. A study of 236 patients was made to determine the effect of perioperative intravenous antibiotics and topical bacitracin irrigation on the incidence of these complications. Of the 236 patients, 170 (72%) underwent translabyrinthine resection of acoustic tumors, while 66 (28%) underwent retrolabyrinthine vestibular nerve section. Patients were divided into four groups: those who received no antibiotics, those who received perioperative intravenous antibiotics only, those who received topical bacitracin irrigation only, and those who received a combination of perioperative intravenous antibiotics and topical bacitracin irrigation. There were no untoward effects of either perioperative intravenous antibiotics or topical bacitracin. The results indicate that bacitracin irrigation reduced the incidence of wound infection from 9% to 2% (p less than 0.05); of cerebrospinal fluid leak from 12% to 5% (p less than 0.04); and of all targeted complications combined from 22% to 9% (p less than 0.006). Furthermore, the topical bacitracin irrigation only group showed a statistically significant reduction in wound infections compared to the perioperative intravenous antibiotic only group (p less than 0.02). The incidence of meningitis was statistically unaffected by any of our treatment protocols.

Bacitracin↗
PubMed1987

Loss of the neural integrator of the oculomotor system from brain stem lesions in monkey.

Eye movement were recorded from four juvenile rhesus monkeys (Macaca mulatta) before and after the injection of neurotoxins (kainate or ibotenate) in the region of the medial vestibular and prepositus hypoglossi nuclei, an area hypothesized to be the locus of the neural integrator for horizontal eye movement commands. Eye movements were measured in the head-restrained animal by the magnetic field/eye-coil method. The monkeys were trained to follow visual targets. A chamber implanted over a trephine hole in the skull permitted recordings to be made in the brain stem with metal microelectrodes. The abducens nuclei were located and used as a reference point for subsequent neurotoxin injections through cannulas. The effects of these lesions on fixation, vestibuloocular and optokinetic responses, and smooth pursuit were compared with predicted oculomotor anomalies caused by a loss of the neural integrator. Kainate and ibotenate did not create permanent lesions in this region of the brain stem. All the eye movements returned toward normal over the course of a few days to 2 wk. Histological examination revealed that the cannula tips were mainly located between the vestibular and prepositus hypoglossi nuclei, in their rostral 2 mm, bordered rostrally by the abducens nuclei. Dense gliosis clearly demarcated the cannula tracks, but for most injections there were no surrounding regions of neuronal loss. Thus the eye movement disorders were due to a reversible, not a permanent, lesion. The time constant for the neural integrator was determined from the velocity of the centripetal drift of the eyes just after an eccentric saccade in total darkness. For intact animals this time constant was greater than 20 s. Shortly after bilateral injections of neurotoxin, the time constant began to decrease and reached a minimum of 200 ms; every horizontal saccade was followed by a rapid centripetal drift with a time constant of approximately 200 ms. For vertical eye movements, in this acute phase, the time constant was approximately 2.5 s. The vestibuloocular reflex (VOR) was drastically changed by the lesions. A step of constant head velocity in total darkness evoked a step change in eye position rather than in velocity. In the absence of the neural integrator, the step velocity command from the canal afferents was not integrated to produce a ramp of eye position (normal slow phases); rather this signal was relayed directly to the motoneurons and caused a step in eye position. The per- and postrotatory decay of the head velocity signal was decreased to 5-6 s indicating that vestibular velocity storage was also impaired.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

An improved neural-network model for the neural integrator of the oculomotor system: more realistic neuron behavior.

The discharge rates of premotor, brain-stem neurons that create eye movements modulate in relation to eye velocity yet firing rates of extraocular motoneurons contain both eye-position and eye-velocity signals. The eye-position signal is derived from the eye-velocity command by means of a neural network which functions as a temporal integrator. We have previously proposed a network of lateral-inhibitory neurons that is capable of performing the required integration. That analysis centered on the temporal aspects of the signal processing for a limited class of idealized inputs. All of its cells were identical and carried only the integrated signal. Recordings in the brain stem, however, show that neurons in the region of the neural integrator have a variety of background firing rates, all carry some eye-velocity signal as well as the eye-position signal, and carry the former with different strengths depending on the type of eye movement being made. It was necessary to see if the proposed model could be modified to make its neurons more realistic. By modifying the spatial distribution of afferents to the network, we demonstrate that the same basic model functions properly in spite of afferents with nonuniform background firing rates. To introduce the eye-velocity signal a double-layer network, consisting of inhibitory and excitatory cells, was necessary. By presenting the velocity input to only local regions of this network it was shown that all cells in the network still carried the integrated signal and that its cells could carry different eye-velocity signals for different types of eye movements. Thus, this model stimulates quantitatively and qualitatively, the behavior of neurons seen in the region of the neural integrator.

Brain Stem↗
PubMed1983

A proposed neural network for the integrator of the oculomotor system.

Single-unit recordings, stimulation studies, and eye movement measurements all indicate that the firing patterns of many oculomotor neurons in the brain stem encode eye-velocity commands in premotor circuits while the firing patterns of extraocular motoneurons contain both eye-velocity and eye-position components. It is necessary to propose that the eye-position component is generated from the eye-velocity signal by a leaky hold element or temporal integrator. Prior models of this integrator suffer from two important problems. Since cells appear to have a steady, background signal when eye position and velocity are zero, how does the integrator avoid integrating this background rate? Most models employ some form of lumped, positive feedback the gain of which must be kept within totally unreasonable limits for proper operation. We propose a lateral inhibitory network of homogeneous neurons as a model for the neural integrator that solves both problems. Parameter sensitivity studies and lesion simulations are presented to demonstrate robustness of the model with respect to both the choice of parameter values and the consequences of pathological changes in a portion of the neural integrator pool.

Animals↗
PubMed1983

Predictions of the existence, frequency, and amplitude of physiological tremor in normal man based on measured frequency-response characteristics.

Mathematical models for 1) musculoskeletal dynamics, and 2) reflex feedback, based on the results of the authors' frequency-response measurements on normal adult male human subjects, are combined to produce a model for physiological tremor in such subjects. An analysis of this model shows that the system will be unstable to small disturbances (that is, tremor will occur) under certain conditions of external loading. Further, when the system is unstable, nonlinearities in the model produce responses in the form of limit cycles, and both the frequency and amplitude of the resulting tremor can be calculated. For constant loads applied through a constant compliance, the model predicts the onset of tremor at low loads, a maximum intensity of tremor at loads corresponding to 30-50 percent of maximum voluntary effort, and a decrease in the tremor amplitude at still higher loads.

Biomechanical Phenomena↗
PubMed1982

The mechanical behavior of active human skeletal muscle in small oscillations.

The results of an extensive series of small-amplitude frequency-response tests of the arm flexors and extensors of ten normal adult male subjects are reported. It is concluded that a quasilinear model for muscle which had been proposed previously by one of the authors provides an adequate quantitative representation of the observed system behavior. The identified model parameters, including the stiffness and the time constants of contraction dynamics are listed for all ten subjects, thus establishing the characteristic values and ranges of variation of these parameters in normal, adult males.

Adult↗
PubMedDate not supplied

Effect of electrical stimulation on middle latency response in the guinea pig.

A temporary threshold shift (TTS) has been demonstrated in the electrically evoked middle latency response (EMLR) following exposure to moderate levels of continuous electrical stimulation via a cochlear implant. The threshold at which the EMLR was elicited in chronically implanted guinea pigs was elevated by approximately 100% following 30 minutes of moderate intensity (200 microA or more) sinusoidal electrical stimulation of the cochlea. Results obtained under anesthesia varied unacceptably. In awake animals, EMLR thresholds were stable over time and consistent TTSs were observed. The threshold returned to prestimulation levels within 4 hours following termination of the stimulation. The possibility of histopathologic changes and the relevance of these findings in setting safe output levels for cochlear implant processors are discussed.

Animals↗
PubMedDate not supplied

The effect of the rotational magnification of corrective spectacles on the quantitative evaluation of the VOR.

The gain of the vestibulo-ocular reflex (slow-phase eye velocity/chair velocity, measured in the dark) was compared in 11 normal healthy subjects who habitually wore corrective spectacles of varying strength. The rotational magnification (or prismatic effect) induced by habitually wearing corrective spectacles caused the VOR gain measured in darkness to vary systematically with diopter of correction. Even when allowances were made for the inherent variability of measurement of the VOR gain, myopes tended to have lower gains and hyperopes higher gains. This study demonstrates that the clinician should account for spectacle adaptation to properly interpret the results of vestibular function tests.

Adaptation, Ocular↗