My Graduate School Years
I entered the graduate program of Boston University as a Presidential University Graduate Fellow in the Fall of 1989. I worked in the laboratory of Dr. Jelle Atema, Director of the Boston University Marine Program in the Marine Biological Laboratory in Woods Hole, MA.
Our laboratory's research focused on chemoreception (smell and taste) in the American lobster, Homarus americanus, an animal that relies heavily on its sense of smell and taste to find food and to interact with other members of its species. My research focused on the neurophysiology of chemoreceptor cells (or the "smell receptor cells") and how they might help the lobster find an odor source.
To find out more about my research, you can look through the abstracts or summaries of each research project that I did:
- Dissertation Abstract of my thesis research, a summary of what I did for 5 years
- Stimulus integration time of lobster olfactory receptor cells
- Flicker fusion rates of chemoreceptor cells
- Effects of stimulus concentration on flicker fusion rates
- Time course of recovery from adaptation of lobster chemoreceptor cells
Above: The lobster lateral antennule, shown with the high-resolution stimulus measurement probe
Some of my grad school friends. From left to right: Kathleen Berg, me, Heather Eisthen, Paul Bushmann, Lynda Farley, and Rainer Voigt
TEMPORAL FILTER PROPERTIES OF OLFACTORY RECEPTOR CELLS OF THE AMERICAN LOBSTER, HOMARUS AMERICANUS
Odors in the environment are distributed in a patchy or turbulent fashion. Patches of odor moving over a measuring point such as a chemoreceptor will appear as concentration pulses with a variety of temporal features. A spatial gradient in the distribute ion of features exists in these odor plumes. Animals such as lobsters could use such gradients for chemotactic orientation. To facilitate chemotaxis,olfactory receptor cells should selectively filter such temporal features that most reliably indicate spatial gradients. This study focused on the common hydroxyproline-sensitive cells of the lobster's olfactory organ, the lateral antennules, critical for efficient chemotaxis.
To investigate the temporal filter properties of olfactory receptor cells, accurate stimulus control was essential. Square pulses of odor (hydroxyproline) with different lengths (durations) and heights (concentrations) were reliably delivered using focal stimulation with concentric pipettes coupled with high (30 micromolar spatial and 5 millisecond temporal) resolution stimulus measurement in situ.
When given 1 to 100 micromolar odor steps with durations of 50, 100, 200, 500 and 1000 milliseconds, the cells responded primarily with phasic spike bursts. Steps of 200 milliseconds elicited maximal responses: longer pulses did not cause greater cell responses and shorter pulses gave smaller responses. These cells thus integrate stimuli over a 200 millisecond time period. The cells were rapidly adapting: response differences to different concentration steps were most evident 160 to 300 milliseconds following stimulus onset, indicating that this was the optimal time period for stimulus intensity discrimination. Following complete adaptation, most cells recovered exponentially and regained full sensitivity in 30 seconds.
When stimulated with ten 100 millisecond 100 micromolar odor steps at different repetition frequencies, cell responses accurately encoded repetition frequencies of 0.5, 1 and 2 Hz; their responses to 4 Hz stimulation fused and resembled responses to a lon g square pulse. Lower stimulus concentrations resulted in higher flicker fusion frequency.
Thus, optimal temporal resolution exists for low supra-threshold stimuli of 200 milliseconds duration; although full recovery takes 30 seconds, pulse frequencies of 2-3 Hz can be resolved. These temporal filter properties set clear limits on information that can be used during chemotactic orientation in turbulent odor plumes.
Recent investigations of the kinetics of olfaction have shown that olfactory systems are capable of resolving rapid events. Second messenger production in vitro and receptor potential generation in isolated olfactory receptor neurons occur within a hundred milliseconds. This study attempted to determine the time resolution oflobster olfactory receptor cells in situ by quantifying their stimulus integration time.
At a fixed odor concentration, odor steps of 200 ms duration elicited maximum, extracellularly recorded spiking responses from receptor cells; shorter odor steps did not drive the cells to their maximum response and longer odor steps did not result in stronger responses. Excitatory processes peaked within 220 ms. At 160 to 300 ms, stimulus intensity discrimination was most evident. Adaptation processes reduced response magnitude to near-baseline levels within 1000 ms.
Olfactory receptor cells are thus designed to resolve events within a few hundred milliseconds: this time window corresponds to of olfactory sampling dynamics as well as rapid fluctuations that may occur in natural odor plumes. The stimulus integration time of 200 ms may play a role in the filtering of information that lobsters use to orient to distant odor sources.
Adaptation and disadaptation rates of sensory receptor cells determine their temporal response properties. This important filter feature is still poorly understood in chemoreception. We studied the time course of disadaptation in lobster antennular chemoreceptor cells using in situ high resolution stimulus measurement and extracellularly recorded spike responses. Fifteen receptor cells were each tested with a series of three odor (hydroxyproline) pulses: a 200 ms test pulse, a 5 s adapting pulse, and a 200 ms probe odor pulse after time intervals ranging from 1 to 60 s. Following complete adaptation by the adapting pulse, individual cells recovered at different rates. After 1 s, a third of the cells responded with a mean response of 3 spikes per cell. The cells fully recovered between 10-30 s. Mean full recovery was within 25 s with a time constant of 12 s, independent of stimulus concentration.
Reference for this work: Gomez, G. and Atema, J. 1996. Temporal resolution in olfaction II: time course of recovery from adaptation in lobster chemoreceptor cells. J. Neurophysiol. 76:1340-1343.
1. We developed a high resolution, on-line stimulus measurement system for accurate control of chemical stimulus applications for Homarus americanus lateral antennule chemoreceptors. Focal stimulus presentations in an electrophysiological preparation with the receptor sensilla intact were measured at small spatial (30 micrometer) and time(5 milliseconds) scales.
2. Individual hydroxyproline-sensitive receptor cells showed differences in their capabilities to resolve pulses. Fifty percent of the cells surveyed were capable of resolving short (100 millisecond)pulses at 2 Hz while the others could not. At 4 Hz, only a small proportion (20%) could accurately encode individual stimulus pulses. Increased stimulation frequency resulted in poorer pulse resolution. As a population, pulse resolution capabilities of the hydroxyproline sensitive cell population on the lateral antennule was about 2 Hz.
3. Repetitive stimulation caused the number of spikes to decrease and first spike latency to increase. Increased stimulation frequency resulted in a greater decline in spike number and a greater increase in first spike latency.
4. Since individual differences in adaptation and disadaptation rates of the receptor cells could not be attributed to stimulus variability, we hypothesize that this diversity allows the system to perform frequency analysis tasks of pulsatile stimuli that occur in natural turbulent odor plumes.
Reference for this work: Journal of Comparative Physiology A, vol. 174, pp. 803-811, 1994.The data was presented in the Annual Meeting of the Association of Chemoreception Sciences, 1992.
1. To understand how chemoreceptor systems may extract temporal information from odor plumes, we investigated the frequency filter properties of lobster chemoreceptor cells. We used rapid stimulation and high-resolution stimulus measurement for accurate stimulus control and recorded extracellular responses from chemoreceptors on the lobster lateral antennule.
2. We tested sixteen hydroxyproline-sensitive cells with a series often 100 ms pulses at 10, 100 and 1000 uM at stimulation frequencies from 0.5 to 4 Hz. Receptor cell responses could accurately encode pulses delivered at 0.5 and 1 Hz at all three stimulus concentrations.Responses to 1000 uM at 2 Hz stimulation and to 100 uM at 3 Hz stimulation were fused while responses to 10 uM were still distinct. Responses to 4 Hz stimulation at all stimulus concentrations were fused. Thus lower stimulus concentration rations resulted in improved stimulus following capabilities.
3. When given pulsed stimulation, the effects of stimulus concentration on cell response magnitude were most pronounced after the first pulse: higher stimulus concentrations resulted in a greater number of spikes. However, the cumulative effects of adapt ation caused a large decline in response from the first to the second pulse. Succeeding pulses produced a less significant decline, and the cell responses attained a steady state; cumulative adaptation resulted in a loss of stimulus intensity discrimination. This effect was especially pronounced at 2 Hz stimulation.
4. Individual cells showed differences in their stimulus pulse following capabilities, as measured by the synchronization coefficient. These individual differences may form a basis for coding temporal features of an odor plume in an across-fiber pattern.
Reference for this work: Gomez, G., Voigt, R., Atema, J. 1999. Temporal resolution in olfaction III. Flicker fusion and concentration-dependent synchronization with stimulus pulse trains in lobster chemoreceptor cells. J. Comp. Physiol. A 185:427-436. Data from this paper was presented in the Annual Meeting of the Association of Chemoreception Sciences, 1993.