Generalized fuzzy clustering for segmentation of multi-spectral magnetic resonance images.

An integrated approach for multi-spectral segmentation of MR images is presented. This method is based on the fuzzy c-means (FCM) and includes bias field correction and contextual constraints over spatial intensity distribution and accounts for the non-spherical cluster's shape in the feature space. The bias field is modeled as a linear combination of smooth polynomial basis functions for fast computation in the clustering iterations. Regularization terms for the neighborhood continuity of intensity are added into the FCM cost functions. To reduce the computational complexity, the contextual regularizations are separated from the clustering iterations. Since the feature space is not isotropic, distance measure adopted in Gustafson-Kessel (G-K) algorithm is used instead of the Euclidean distance, to account for the non-spherical shape of the clusters in the feature space. These algorithms are quantitatively evaluated on MR brain images using the similarity measures.

The role of sensory neuron outgrowth in long-term sensitization of the tail-elicited tail-siphon withdrawal reflex of Aplysia

Neuronal outgrowth has been proposed in many systems as a mechanism underlying memory storage. For example, sensory neuron outgrowth is widely accepted as an underlying mechanism of long-term sensitization of defensive withdrawal reflexes in Aplysia. The hypothesis is that learning leads to outgrowth and consequently to the formation of new synapses, which in turn strengthen the neural circuit underlying the behavior. However, key experiments to test this hypothesis have never been performed. ^ Four days of sensitization training leads to outgrowth of siphon sensory neurons mediating the siphon-gill withdrawal response in Aplysia . We found that a similar training protocol produced robust outgrowth in tail sensory neurons mediating the tail siphon withdrawal reflex. In contrast, 1 day of training, which effectively induces long-term behavioral sensitization and synaptic facilitation, was not associated with neuronal outgrowth. Further examination of the effect of behavioral training protocols on sensory neuron outgrowth indicated that this structural modification is associated only with the most persistent forms of sensitization, and that the induction of these changes is dependent on the spacing of the training trials over multiple days. Therefore, we suggest that neuronal outgrowth is not a universal mechanism underlying long-term sensitization, but is involved only in the most persistent forms of the memory. ^ Sensory neuron outgrowth presumably contributes to long-term sensitization through formation of new synapses with follower motor neurons, but this hypothesis has never been directly tested. The contribution of outgrowth to long-term sensitization was assessed using confocal microscopy to examine sites of contact between physiologically connected pairs of sensory and motor neurons. Following 4 days of training, the strength of both the behavior and sensorimotor synapse and the number of appositions with follower neurons was enhanced only on the trained side of the animal. In contrast, outgrowth was induced on both sides of the animal, indicating that although sensory neuron outgrowth does appear to contribute to sensitization through the formation of new synapses, outgrowth alone is not sufficient to account for the effects of sensitization. This indicates that key regulatory steps are downstream from outgrowth, possibly in the targeting of new processes and activation of new synapses. ^

Respiratory rhythm generation: A modeling study of the respiratory central pattern generator and the phrenic motor neuron

The respiratory central pattern generator is a collection of medullary neurons that generates the rhythm of respiration. The respiratory central pattern generator feeds phrenic motor neurons, which, in turn, drive the main muscle of respiration, the diaphragm. The purpose of this thesis is to understand the neural control of respiration through mathematical models of the respiratory central pattern generator and phrenic motor neurons. ^ We first designed and validated a Hodgkin-Huxley type model that mimics the behavior of phrenic motor neurons under a wide range of electrical and pharmacological perturbations. This model was constrained physiological data from the literature. Next, we designed and validated a model of the respiratory central pattern generator by connecting four Hodgkin-Huxley type models of medullary respiratory neurons in a mutually inhibitory network. This network was in turn driven by a simple model of an endogenously bursting neuron, which acted as the pacemaker for the respiratory central pattern generator. Finally, the respiratory central pattern generator and phrenic motor neuron models were connected and their interactions studied. ^ Our study of the models has provided a number of insights into the behavior of the respiratory central pattern generator and phrenic motor neurons. These include the suggestion of a role for the T-type and N-type calcium channels during single spikes and repetitive firing in phrenic motor neurons, as well as a better understanding of network properties underlying respiratory rhythm generation. We also utilized an existing model of lung mechanics to study the interactions between the respiratory central pattern generator and ventilation. ^

THE FUNCTIONAL ORGANIZATION OF SYNAPTIC VESICLE POOLS IN A RETINAL BIPOLAR NEURON

Ribbon synapses are found in sensory systems and are characterized by ‘ribbon-like’ organelles that tether synaptic vesicles. The synaptic ribbons co-localize with sites of calcium entry and vesicle fusion, forming ribbon-style active zones. The ability of ribbon synapses to maintain rapid and sustained neurotransmission is critical for vision, hearing and balance. At retinal ribbon synapses, three vesicle pools have been proposed. A rapid pool of vesicles that are docked at the plasma membrane, and whose fusion is limited only by calcium entry, a releasable pool of ATP-primed vesicles whose size also correlates with the number of ribbon-tethered vesicles, and a reserve pool of non-ribbon-tethered cytoplasmic vesicles. However evidence of vesicle fusion at sites away from ribbon-style active zones questions this organization. Another fundamental question underlying the mechanism of vesicle fusion at these synapses is the role of SNARE (Soluble N-ethylmaleimide sensitive factor Attachment Protein Receptor) proteins. Vesicles at conventional neurons undergo SNARE complex-mediated fusion. However a recent study has suggested that ribbon synapses involved in hearing can operate independently of neuronal SNAREs. We used the well-characterized goldfish bipolar neuron to investigate the organization of vesicle pools and the role of SNARE proteins at a retinal ribbon synapse. We blocked functional refilling of the releasable pool and then stimulated bipolar terminals with brief depolarizations that triggered the fusion of the rapid pool of vesicles. We found that the rapid pool draws vesicles from the releasable pool and that both pools undergo release at ribbon-style active zones. To assess the functional role of SNARE proteins at retinal ribbon synapses, we used peptides derived from SNARE proteins that compete with endogenous proteins for SNARE complex formation. The SNARE peptides blocked fusion of reserve vesicles but not vesicles in the rapid and releasable pools, possibly because both rapid and releasable vesicles were associated with preformed SNARE complexes. However, an activity-dependent block in refilling of the releasable pool was seen, suggesting that new SNARE complexes must be formed before vesicles can join a fusion-competent pool. Taken together, our results suggest that SNARE complex-mediated exocytosis of serially-organized vesicle pools at ribbon-style active zones is important in the neurotransmission of vision.