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Biomedical subjects

L M Martinez

Publications and source records attributed to L M Martinez.

At least 19 recordsLinked to original sources

The generation of receptive-field structure in cat primary visual cortex.

Cells in primary visual cortex show a remarkable variety of receptive-field structures. In spite of the extensive experimental and theoretical effort over the past 50 years, it has been difficult to establish how this diversity of functional-response properties emerges in the cortex. One of the reasons is that while functional studies in the early visual pathway have been usually carried out in vivo with extracellular recording techniques, investigations about the precise structure of the cortical network have mainly been conducted in vitro. Thus, the link between structure and function has rarely been explicitly established, remaining a well-known controversial issue. In this chapter, I review recent data that simultaneously combines anatomy with physiology at the intracellular level; trying to understand how the primary visual cortex transforms the information it receives from the thalamus to generate receptive-field structure, contrast-invariant orientation tuning and other functional-response properties.

Animals↗

Construction of complex receptive fields in cat primary visual cortex.

In primary visual cortex, neurons are classified into simple cells and complex cells based on their response properties. Although the role of these two cell types in vision is still unknown, an attractive hypothesis is that simple cells are necessary to construct complex receptive fields. This hierarchical model puts forward two main predictions. First, simple cells should connect monosynaptically to complex cells. Second, complex cells should become silent when simple cells are inactivated. We have recently provided evidence for the first prediction, and here we do the same for the second. In summary, our results suggest that the receptive fields of most layer 2+3 complex cells are generated by a mechanism that requires simple cell inputs.

Animals↗

A thermodynamic connection to the fragility of glass-forming liquids.

Although liquids normally crystallize on cooling, there are members of all liquid types (including molecular, ionic and metallic) that supercool and then solidify at their glass transition temperature, Tg. This continuous solidification process exhibits great diversity within each class of liquid-both in the steepness of the viscosity-temperature profile, and in the rate at which the excess entropy of the liquid over the crystalline phase changes as Tg is approached. However, the source of the diversity is unknown. The viscosity and associated relaxation time behaviour have been classified between 'strong' and 'fragile' extremes, using Tg as a scaling parameter, but attempts to correlate such kinetic properties with the thermodynamic behaviour have been controversial. Here we show that the kinetic fragility can be correlated with a scaled quantity representing excess entropy, using data over the entire fragility range and embracing liquids of all classes. The excess entropy used in our correlation contains both configurational and vibration-related contributions. In order to reconcile our correlation with existing theory and simulations, we propose that variations in the fragility of liquids originate in differences between their vibrational heat capacities, harmonic and anharmonic, which we interpret in terms of an energy landscape. The differences evidently relate to behaviour of low-energy modes near and below the boson peak.

Journal Article↗

Synaptic integration in striate cortical simple cells.

Simple cells in the visual cortex respond to the precise position of oriented contours (Hubel and Wiesel, 1962). This sensitivity reflects the structure of the simple receptive field, which exhibits two sorts of antagonism between on and off inputs. First, simple receptive fields are divided into adjacent on and off subregions; second, within each subregion, stimuli of the reverse contrast evoke responses of the opposite sign: push-pull (Hubel and Wiesel, 1962; Palmer and Davis, 1981; Jones and Palmer, 1987; Ferster, 1988). We have made whole-cell patch recordings from cat area 17 during visual stimulation to examine the generation and integration of excitation (push) and suppression (pull) in the simple receptive field. The temporal structure of the push reflected the pattern of thalamic inputs, as judged by comparing the intracellular cortical responses to extracellular recordings made in the lateral geniculate nucleus. Two mechanisms have been advanced to account for the pull-withdrawal of thalamic drive and active, intracortical inhibition (Hubel and Wiesel, 1962; Heggelund, 1968; Ferster, 1988). Our results suggest that intracortical inhibition is the dominant, and perhaps sole, mechanism of suppression. The inhibitory influences operated within a wide dynamic range. When inhibition was strong, the membrane conductance could be doubled or tripled. Furthermore, if a stimulus confined to one subregion was enlarged so that it extended into the next, the sign of response often changed from depolarizing to hyperpolarizing. In other instances, the inhibition modulated neuronal output subtly, by elevating spike threshold or altering firing rate at a given membrane voltage.

Action Potentials↗

Ascending projections of simple and complex cells in layer 6 of the cat striate cortex.

Receptive field properties vary systematically across the different layers of the cat striate cortex. Understanding how these functional differences emerge requires a precise description of the interlaminar connections and the quality of information that they transmit. This study examines the contribution of the two physiological types of neuron in layer 6, simple and complex, to the cortical microcircuit. The approach was to make whole-cell recordings with dye-filled electrodes in vivo to correlate visual response property with intracortical projection pattern. The two simple cells we stained projected to layer 4, as previously reported (Gilbert and Wiesel, 1979; Martin and Whitteridge, 1984). Six of the eight complex cells that we labeled projected to the superficial layers, a pathway not previously described in the cat. The remaining two cells targeted the infragranular layers. Layer 4 is dominated by simple cells, whereas layers 5 and 2+3 are mainly composed of complex cells (Hubel and Wiesel, 1962; Gilbert, 1977). Hence, our results indicate that the ascending projections of simple cells in layer 6 target other simple cells. In parallel, the ascending projections of a population of complex cells in layer 6 favor other complex cells. Anatomical experiments in several species (Lund and Boothe,1975; Burkhalter,1989; Usrey and Fitzpatrick, 1996; Wiser and Callaway, 1996) had also demonstrated that layer 6 gives rise to two separate intracortical pathways. Pooling the results of these anatomical studies with our own suggests a common feature of the laminar organization: cells that project to different intracortical targets have distinct functional characteristics.

Action Potentials↗

Conjunctival dermoid cyst seen on examination as a chronically red eye.

We describe a patient who was seen with a unilateral red eye and purulent discharge refractory to antibiotic treatment with multiple antibiotic regimens over the previous 4 months. Initially it was believed to be a bacterial conjunctivitis, but when conventional treatments failed it was thought to be a viral conjunctivitis with bacterial superinfection. Cultures yielded only Enterococcus. There was a small mass with a draining fistulous tract at the lateral canthus. Symptoms persisted despite courses of topical and systemic antibiotics followed by a tapering dose of steroids. The patient was taken to the operating room for an orbitotomy through the conjunctiva at the lateral canthus. Findings from pathologic examination of the excised mass demonstrated a dermoid cyst of the conjunctiva. This clinical appearance obscured the diagnosis.

Adult↗

Functional connectivity between simple cells and complex cells in cat striate cortex.

In the cat primary visual cortex, neurons are classified into the two main categories of simple cells and complex cells based on their response properties. According to the hierarchical model, complex receptive fields derive from convergent inputs of simple cells with similar orientation preferences. This model received strong support from anatomical studies showing that many complex cells lie within the range of layer IV simple-cell axons but outside the range of most thalamic axons. Physiological evidence for the model, however, has remained elusive. Here we demonstrate that layer IV simple cells and layer II and III complex cells show correlated firing consistent with monosynaptic connections. As expected from the hierarchical model, all connections were in the direction from the simple cell to the complex cell, most frequently between cells with similar orientation preferences.

Animals↗

Silicone oil adhesion to intraocular lenses: an experimental study comparing various biomaterials.

PURPOSE: To perform an in vitro experimental study comparing the degree of adherence of silicone oil to various rigid and foldable intraocular lens (IOL) designs and to the human lens capsule. SETTING: Center for Research on Ocular Therapeutics and Biodevices, Department of Ophthalmology, Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA. METHODS: Seven IOL styles comprising various biomaterials were studied: fluorine-treated (Fluorlens), heparin-surface-modified (HSM), hydrogel, Memory-Lens, Poly(methyl methacrylate) (PMMA), soft acrylic, and silicone lenses; the human crystalline lens was also studied. Each lens was immersed in silicone oil for 12 hours, than photographed, studied by scanning electron microscopy (except the crystalline lens), and subjected to computer-generated image analysis to determine the silicone oil coverage. RESULTS: Silicone oil coverage of dry silicone lenses was 100% and of lenses immersed in normal saline, 82.5%. The least coverage was on the heparin-surface-modified lens (mean score 9.4%). Coverage of the other four lenses ranged from approximately 15.1% to 33.7%. Mean coverage of the human lens capsule was 10.9%. CONCLUSION: Although a silicone IOL shows maximal adherence to silicone oil, other lens biomaterials are not immune to this complication. Silicone oil coverage was related to the dispersive energy component of the surface charge of the IOL biomaterial. Low dispersive energy materials had less silicone oil coverage, while those with higher dispersive energy had more oil coverage.

Adhesiveness↗

Indwelling silicone femoral catheters: experience of three haemodialysis centres.

The aim of this study is to describe the experience of three haemodialysis centres using indwelling femoral silicone catheter (model SSL 1220M, Medcomp, USA) in 55 patients, three with acute renal failure, one requiring plasmapheresis, and 51 with chronic renal failure but no other available vascular access. Sixty-four catheters were in place for a mean duration of 41.5 +/- 30 days. The rate of catheter-related complications, including mechanical problems, thromboses, and infections was low and they were never life-threatening. The results of the study suggest that femoral cannulation with modern flexible devices can be considered as a reliable temporary access, even for extended periods, with advantages exceeding those for subclavian and jugular routes.

Acute Kidney Injury↗

[Femoral indwelling silicone catheter: experience of three hemodialysis centers].

The use of femoral vein for temporary access in hemodialysis patients is still considered as a slightly desirable route. However recent technical improvements have made this approach more reliable because the new femoral catheters can be left in place for a long time and used for ambulatory treatment. We describe the experience of three hemodialysis centres with temporary indwelling femoral catheter made of silicone (SSL 1220 M, Medcomp) in 55 patients: 3 patients with acute renal failure, 1 requiring plasmapheresis and 51 with chronic renal failure but no other available vascular access. Sixty four catheters were implanted and left in place for a mean of 41.5 +/- 30 days. Complications (mechanical, thrombotic and infectious) were infrequent and never life-threatening. These results suggest that the femoral route can be used reliably for temporary access, and provides advantages over subclavian and jugular routes in certain circumstances.

Acute Kidney Injury↗

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Americas↗

Stretching in the rehabilitation of low-back pain patients.

The major objective of this investigation was to study the effectiveness of the physical conditioning program used at the University of Miami Comprehensive Pain and Rehabilitation Center in restoring certain elements of the functional abilities of individuals suffering from chronic low-back pain and to evaluate the effectiveness of using a systematically applied, aggressive stretching maneuver as an add-on therapy in the treatment of chronic low-back pain patients. The effect(s) of the proposed stretching maneuver was evaluated. Twenty-eight chronic low-back pain patients were randomly assigned to one of two groups. The control group underwent a multimodal rehabilitation program, and the experimental group underwent the same rehabilitation program in addition to the systematic stretching maneuver under investigation. The results obtained showed that patients undergoing the multimodal rehabilitation program with and without the proposed systematic stretching maneuver showed a significant improvement in their functional abilities as seen from the significant increase in the static strength of the back extensors, with corresponding significant increase in back muscle myoelectric signals. Also, a significant decrease in their pain level was reported after 2 weeks of continuous treatment. The use of the systematic stretching maneuver enhanced the functional gains of chronic low-back pain patients compared to the control group.

Back Pain↗

Coloplasty and free intestinal transplants in the treatment of extensive cancers of the pharyngo-oesophageal junction.

The authors report their surgical experiences with 55 extensive tumours of the pharyngoesophageal junction. Thirty-three of these cases were managed by circular pharyngolaryngectomy, total oesophagectomy with stripping and left coloplasty, and 22 were treated with circular pharyngolaryngectomy with free intestinal transplants. A comparative study of the post-operative courses of the patients and their oncological and functional outcomes shows the advantages, disadvantages and indications of these two techniques.

Adult↗