Michael D. Mann

Effective stimulation distance for current from macroelectrodes

Effective spread of stimulating current from macroelectrodes was measured using antidromic responses of axons of the pyramidal tract as an indicator of excitation. Both monopolar and concentric bipolar electrode configurations were tested with stimulating distances as large as 7mm. The effective stimulation distance was greater from monopolar electrodes especially at greater current strengths, but differences between the two configurations were frequently small and reversals of this trend occurred. There was no statistically significant difference between the estimates of effective stimulation distance made using large and small axons. The shape of current-distance curves was approximately parabolic using both bipolar and monopolar stimulation. A current strength of 0.5 to 1.0 mA will confine effective current from a monopolar electrode to a sphere of 2-mm radius, but will not stimulate all elements within that area. Even in a brain area as homogeneous as the pyramidal tract, there is still a great deal of variability from mean values in effective stimulation distance. Presumably, the variability would be even greater in more heterogeneous regions.

Should IACUCs review scientific merit of animal research projects

Whether IACUCs should review animal research protocols for scientific merit is not addressed in the federal regulations, resulting in ongoing confusion on the subject. The authors examine this issue, discuss the pros and cons, suggest how IACUCs can go about reviewing protocols for scientific merit, and question what effect recent changes in regulations will have on this issue.

Brain-Body Size Relations in Grasshopper Mice

Cranial volumes were measured on museum specimens of two species of grasshopper mice, Onychomys leucogaster brevicaudus and Onychomys torridus pulcher. These were compared with body weights and body lengths, as recorded on the museum tags. On average, females were heavier and slightly longer than males, but they had smaller cranial volumes by about 3%. Statistically, the differences were marginally significant (with no difference in body length for one species), but the pattern of differences was the same in both species, and therefore probably real. This casts suspicion on the common notion that a larger body size requires a larger brain. It also questions the real meaning of encephalization quotients.

Excitability changes along pyramidal tract axons after sensory stimulation

Abstract Recordings were made extracellularly of the activity of single pyramidal tract (PT) neurons in the forepaw focus of the cats postcruciate cerebral cortex. Activity was evoked antidromically by near-threshold stimulation of the contralateral dorsolateral funiculus of the C6 spinal cord (corticospinal tract), the ipsilateral medullary pyramid, the ipsilateral cerebral peduncle, and the ipsilateral internal capsule. Orthodromic activity was initiated by shocks to each of the four paws. The excitability of PT axons at the spinal cord, medullary pyramid, cerebral peduncle, and internal capsule was tested by measuring the threshold to antidromic activation in conditioning-testing interaction. The PT axons with small contralateral receptive fields (small-field cells) showed no change in excitability at any of the four test sites after stimulation of any of the four paws. In contrast, most PT axons with large bilateral receptive fields (wide-field cells) showed increases in excitability after stimulation of the contralateral forepaw at one or more of the test sites, in many cases, at all four sites. Although not tested in all neurons, increases in excitability of wide-field axons were also seen after stimulation of the ipsilateral forepaw and the contralateral and ipsilateral hind paws. The increased excitability was detectable 20 to 25 ms after the contralateral forepaw stimulus, was maximum at 25 to 30 ms, and then returned to baseline by 75 to 100 ms. Maximum changes in excitability varied from 5% to more than 60% in different axons. The time course of the change in excitability was nearly the same at each site, but the magnitude of the change was most often greater at the pyramid and cerebral peduncle than at the internal capsule. In some cells, a period of decreased excitability, in most cases apparent inexcitability, followed the period of increased excitability. This could also be observed at all four test sites. The period of increased excitability most likely resulted from depolarization of terminals of branches of the PT axon near the testing site, whereas the period of decreased excitability may have resulted from strong depolarization or hyperpolarization of such terminals or from hyperpolarization of one or more nodes of Ranvier, the site situated nearer to the soma than the test site.

Excitation of wide-field ventralis posterolateralis neurons by contralateral thalamic stimulation

Abstract Interconnections between the nuclei ventralis posterolateralis (VPL) of the thalamus were studied using single-unit recordings from chloralose-anesthetized cats. Responses to stimulation of the contralateral VPL (cVPL) were evoked in 28.9% of the 218 neurons sampled. Those neurons responsive to cVPL stimulation were almost exclusively those with an ipsilateral component to their receptive fields, i.e., sb (receptive fields on both fore- or hind limbs) and m neurons (receptive fields on all four limbs), but not all neurons of either of these sets responded. Only 2 of 127 neurons with receptive fields on either the contralateral fore- or hind limb, sa neurons, responded to cVPL stimulation. cVPL-Responsive and -unresponsive m neurons differed little in the properties of their responses to skin stimulation and no differences were found in their distribution within the nucleus. Corticopetal neurons were found among both cVPL-responsive and -unresponsive neurons. VPL neurons were shown to exhibit coadunate behavior, like cortical neurons, but no evidence was found of modulation of m neuron excitability by local sa neurons. Responsiveness of m neurons to cVPL stimulation was blocked by application of ice or KCl crystals to the contralateral, but not the ipsilateral cortex. The possibility is considered that some responses to cVPL stimulation are antidromically conducted.

Brain Size/Body Length Relations Among Myomorph Rodents

The relation between cranial volume and body length was examined in 45 species of myomorph rodents to determine how accurately the former can be estimated from the latter. On average within species, cranial capacity was directly proportional to body length (proportional to the cube root of body weight). On average across species, cranial capacity was directly proportional to the square of body length (proportional to the two-thirds power of body weight). Geometric similarity held within species, but the differences in habitus among species within the same genus generated genus slopes that differed from those defined by strict geometric similarity. It was found that cranial capacity could be estimated as accurately from body length as from body weight, and that body length was better if any differences in habitus were involved (because body length is relatively insensitive to habitus). Therefore, body length (or some strong correlate thereof) may serve well as the primary variable in brain paleoallometry.

On the action of intravenous picrotoxin in the sensorimotor cortex of the cat.

The action of topical convulsants, such as strychnine, is not exerted equally on all sets of neurons in the sensorimotor cortex of cats. Small-field neurons near the surface are influenced whereas deeper, wide-field neurons are affected only indirectly. The agent, picrotoxin, was used to test whether or not the deeper neurons are spared the effects of topical convulsants simply because of their location. Picrotoxin, injected intravenously, produced enhancement of the corticofugal reflex discharge evoked by stimulation of any of the four paws and by auditory stimulation. The surface-recorded primary evoked response was also enhanced but only slightly, far less than with topical application. The evoked response in the medial lemniscus was not altered by picrotoxin, but that in the thalamic radiations was enhanced. The drug diazepam, injected intravenously, diminished the enhancements of both the corticofugal reflex discharge and the evoked potential, in some cases reducing the responses to amplitudes smaller than control values. Single-cell recordings in other laboratories have shown that intravenous picrotoxin converts small-field cells to wide-field cells, and that diazepam reverses this effect. The increase in the surface-recorded primary response and the size of the increase in the corticofugal reflex suggest that there is also a direct effect on wide-field neurons or on the pathway leading to them.

Ontogenetic studies of the primary evoked response and strychnine spike in motor-sensory cortex of kittens

Abstract The development of the primary evoked potential in the motor-sensory cerebral cortex and the corticofugal reflex discharge in the medullary pyramids were studied in the first 2 weeks of postnatal life in kittens. In α-chloralose-anesthetized, paralyzed kittens at 1 day of age, the primary evoked potential produced by electrical stimulation of the contralateral forepaw was usually a purely negative response of 50-ms latency, poor frequency-following capability, and slow time course. During the first 2 weeks, the response became positive-negative as the latency shortened to 24 ms. During the same period, the frequency-following capability improved and the duration of the potential decreased. The cortical responses to stimulation of the other three paws were smaller, longer-latency negative potentials, but reliably evoked even in the youngest animals. The corticofugal reflex was not seen in any animal less than 14 days of age and was seldom seen in 14-day-old kittens. Topical application of strychnine enhanced the amplitude of the cortical response to stimulation of the contralateral forepaw, but had no effect on responses to stimulation of the other three paws. No effect on response latency was seen. The strychnine effect developed in about 15 s as in adults, but lasted two to four times longer. Both positive and negative components of the primary response were enhanced by strychnine, and, in animals that had no positivity in control responses, one was induced, remaining in the record for at least 11 h. As far as can be determined from these studies, the effect of topical strychnine is the same in neonatal and adult cat cerebral cortex.