Faculty Bibliography

The electrophysiological properties of guinea-pig dorsal vagal motoneurons were studied in an in vitro slice preparation. Antidromic, orthodromic and direct stimulation of the neurons demonstrated that the action potential is comprised of several distinct components: a fast initial spike followed by afterdepolarization and an early and a late afterhyperpolarizations. The fast initial spike and the early afterhyperpolarization were blocked by tetrodotoxin and tetraethylammonium ions, respectively. The afterdepolarization (present on the falling phase of the spike) and the late afterhyperpolarization were blocked by the addition of ions known to block calcium conductance (CdCl2, CoCl2 or MnCl2), indicating close association between these two potentials. Prolonged outward current injection through the recording electrode produced two different firing patterns, depending on the initial level of the membrane potential. From resting potential (usually -60 mV) the firing pattern was characterized by a short train of action potentials appearing shortly after the onset of the depolarization step. By contrast, when the depolarization was delivered from a hyperpolarized membrane potential level, a short train of repetitive firing appeared after an initial delay of 300-400 ms. The membrane current responsible for this initial reduction in excitability was studied by means of a single-electrode voltage-clamp technique. The magnitude, direction and kinetics of such current flow are consistent with the presence of early potassium current (IA), partly inactive at the resting potential. Synaptic activation of vagal motoneurons could be obtained by electrical stimulation of the tissue surrounding the vagal nucleus or by direct activation of the vagal nerve. Perivagal stimulation generated excitatory and inhibitory synaptic potentials which could be reversed by shifting the membrane potential. Vagal nerve stimulation, in addition to the antidromic activation of the cells, generated depolarizing responses which were unitary in nature and did not show much sensitivity to shifts in membrane potential. Perivagal and vagal nerve-evoked depolarizations could generate action potentials as well as partial dendritic spikes. We conclude that spike electroresponsiveness in vagal motoneurons is generated by voltage-dependent Na+ and Ca2+ conductances. In addition, the Ca2+-dependent current triggers a K+ conductance which is responsible for modulating the firing frequency obtained from the normal resting level.(ABSTRACT TRUNCATED AT 400 WORDS)

Here we present an elaboration and a quantitative example for a hypothetical neuronal process, implementing what we refer to as the metaorganization principle. This process allows the internalization of external (body) geometries into the central nervous system (CNS) and a reciprocal and equally important action of the CNS geometry on the external (body) geometry. The hypothesis is based on the distinction, within the CNS, between covariant sensory and contravariant motor vectorial expressions of the extrinsic geometry. These sensory and motor expressions, given in natural co-ordinate systems, are transformed from one to the other by a neuronal network which acts as a metric tensor. The metric tensor determines the relationship of these two expressions and thus comprises the functional geometry of the system. The emergence through metaorganization of networks that implement such metric function is viewed as the result of interactions between the covariant motor execution which generates a physical action on the external world (via the musculoskeletal system) and the covariant sensory proprioception which measures the effect of such motor output. In this transformation of contravariants to covariants by the physical geometry of the motor system, a covariant metric tensor is expressed implicitly. However, co-ordinated motor action requires its dual tensor (the contravariant metric) which is assembled in the CNS based on the metaorganization principle, i.e. the ability of CNS and external geometries to mold one another. The two metric transformations acting on each other detect error signals whenever the match of the physical and functional geometries is imperfect. Such error signals are utilized by the metaorganization process to improve the match between the two metrics, so that with use the internal representation becomes increasingly homeometric with the geometry of the external world. The proposed physical process by which the metaorganization principle is implemented is based on oscillatory reverberation. If covariant proprioception is used as a recurrent signal to the motor apparatus, as if it were a contravariant motor expression, then reverberations at their steady-state yield the eigenvectors and eigenvalues of the system. The stored eigenvectors and eigenvalues can serve, respectively, as a means for the genesis of a metric (in the form of its spectral representation) with the given eigenvectors and as a means of comparing the eigenvalues that are implicit in the external body geometry and those of the internal metric.(ABSTRACT TRUNCATED AT 400 WORDS)

In order to observe the fine details of intracellularly stained neurons in brain slices the slices must be cleared of opaque matter. This clearing process involves dehydration of the slice, which typically results in significant shrinkage of the cleared tissue. However, how this shrinkage affects neuronal morphology has not been demonstrated to date. In this paper we detail the artifacts induced in the morphology of stained neurons by this clearing process. During dehydration-induced shrinkage of the brain slices, neurons stained with the water-soluble dye, Lucifer yellow, demonstrated a dramatic decrease in size to less than two-thirds of their original dimensions. In contrast neurons stained with the horseradish peroxidase/diaminobenzidine reaction-product did not shrink with the slice; instead the dendrites bent and curled during dehydration with no loss in cell-soma size. We have managed to circumvent these artifacts by using as a clearing agent the solvent dimethyl sulfoxide, which is miscible in both aqueous and organic phases. This solvent will clear tissue slices without inducing the concomitant artifacts caused by tissue shrinkage occurring with the alcohol-dehydration process.

Quantitative modeling indicates that, in presynaptic terminals, the intracellular calcium concentration profile during inward calcium current is characterized by discrete peaks of calcium immediately adjacent to the calcium channels. This restriction of intracellular calcium concentration suggests a remarkably well specified intracellular architecture such that calcium, as a second messenger, may regulate particular intracellular domains with a great degree of specificity.

Synapsin I and calcium/calmodulin-dependent protein kinase II were pressure-injected into the preterminal digit of the squid giant synapse to test directly the possible regulation of neurotransmitter release by these substances. Neurotransmitter release was determined by measuring the amplitude, rate of rise, and latency of the postsynaptic potential generated in response to presynaptic depolarizing steps under voltage clamp conditions. Injection of dephosphosynapsin I decreased the amplitude and rate of rise of the postsynaptic potential, whereas injection of either phosphosynapsin I or heat-treated dephosphosynapsin I was without effect. Conversely, injection of calcium/calmodulin-dependent protein kinase II, which phosphorylates synapsin I on site II, increased the rate of rise and amplitude and decreased the latency of the postsynaptic potential. The effects of these proteins were observed without any detectable change in the initial phase of the presynaptic calcium current. A synapsin I-like protein and calcium/calmodulin-dependent protein kinase II were demonstrated by biochemical and immunochemical techniques to be present in squid nervous tissue. The data support the hypothesis that synapsin I regulates the availability of synaptic vesicles for release; we propose that calcium entry into the nerve terminal activates calcium/calmodulin-dependent protein kinase II, which phosphorylates synapsin I on site II, dissociating it from the vesicles and thereby removing a constraint in the release process.

The central nervous system expresses its function in natural frames of reference. A most conspicuous feature of such frames is their non-orthogonality. Gaze stabilization and, in particular, the sensorimotor transformations performed by the vestibulo-ocular reflex, are prime examples of such general coordinate transformations between and within multidimensional non-orthogonal frames. Since such operations can be described by tensor formalisms in an abstract manner, this methodology is applied here to develop a tensorial computer model of gaze stabilization. The representation of sensorimotor transformations by a reference-frame independent method obviates the necessity to simplify the intrinsic coordinate systems either by a reduction of the dimensionality or by a presumption of orthogonality. The frames of reference intrinsic to vestibulo-ocular reflex transformation (the vestibular semicircular canals and extraocular muscles) as well as the covariant character of the sensory input and the contravariant character of the motor output are physically obvious. A model built on these intrinsic systems of coordinates first serves to quantitate the degree of non-orthogonality in the extraocular muscle system, and thus to demonstrate both the necessity and the applicability of representing them by a formalism suitable for non-orthogonal systems, such as tensor network theory. The actual non-orthogonality of the gaze-stabilization system can be quantitated on the basis of the difference of covariant and contravariant expressions as follows. Tensor network theory describes sensorimotor transformations by employing a covariant embedding procedure. This, however, yields a covariant intention-type motor vector. If the central nervous system were to transmit these sensory-type components directly to the extraocular muscle motor mechanism, an error-angle would occur since covariants do not physically compose the intended movement. The error in every direction of gaze would be zero only if the extraocular muscle system would constitute an orthogonal set of rotation axes. Otherwise, the error, called refraction angle, is a measure of non-orthogonality. The complexity of the quantitation of non-orthogonality is compounded by the fact that these rotation axes change with the moving eye. Calculation of eye movements, executed both by covariant and contravariant vectors from primary and secondary eye positions, is based on the simplest assumption that the central nervous system establishes the covariant-contravariant transformation in the retinal tangent plane.(ABSTRACT TRUNCATED AT 400 WORDS)