Students' reluctance to engage in class activities is not surprising if classroom exercises are passive and not consistently well designed or executed as active learning exercises that students perceive as enhancing their learning through collaboration. Students' comments also suggest that their reluctance to participate regularly in class may be because they have not yet achieved the developmental level compatible with adult and active learning, on which the curriculum is based. Challenges include helping students better understand the nature of deep learning and their own developmental progress as learners, and providing robust faculty development to ensure the consistent deployment of higher-order learning activities linked with higher-order assessments.

Frequency facilitation is a process whereby neurosecretion increases as a function of stimulation frequency during repetitive synaptic activity. To examine the physiological basis underlying facilitation, we have estimated the frequency dependence of the synaptic parameters n (number of units capable of responding to a nerve impulse) and P (average probability of responding) at the lobster neuromuscular junction. Both n and P increase as a function of frequency, suggesting that the efficiency of quantal docking and quantal fusion is regulated by repetitive synaptic activity. In experiments in which facilitation is strong and quantal content does not saturate over the frequency range tested, the value of P saturates at low frequencies of stimulation, and increases in quantal content at higher frequencies of stimulation are due to an increase in n. Therefore the value of P does not limit facilitation. We propose that transmitter release is limited by the rates of quantal mobilization and demobilization, and that each excitatory stimulus causes additional mobilization of quanta to dock at the presynaptic release sites. In such a model the binomial parameter n will correspond to the number of quanta docked at the release sites and available for release. We have developed and solved kinetic equations that describe how the number of docked quanta changes as a function of time and of stimulation frequency. The stimulus-dependent mobilization model of facilitation predicts that the reciprocal value of the quantal content depends linearly on the reciprocal product of the stimulation frequency and the probability of release. Fits of the experimental data confirm the accuracy of this prediction, showing that the model proposed here quantitatively describes frequency facilitation. The model predicts that high rates of quantal demobilization will produce strong frequency facilitation.

SUMMARY The lobster Homarus americanus inhabits ocean waters that vary in temperature over a 25°C range, depending on the season and water depth. To investigate whether the lobster heart functions effectively over a wide range of temperatures we examine the temperature dependence of cardiac performance of isolated lobster hearts in vitro. In addition, we examined whether modulation of the heart by serotonin depends on temperature. The strength of the heartbeat strongly depends on temperature, as isolated hearts are warmed from 2 to 22°C the contraction amplitude decreases by greater than 60%. The rates of contraction and relaxation of the heart are most strongly temperature dependent in the range from 2 to 4°C but become temperature independent at warmer temperatures. Heart rates increase as a function of temperature both in isolated hearts and in intact animals, however hearts in intact animals beat faster in the temperature range of 12–20°C. Interestingly, acute Q10 values for heart rate are similar in vivo and in vitro over most of the temperature range, suggesting that temperature dependence of heart rate arises mainly from the temperature effects on the cardiac ganglion. In contrast to earlier reports suggesting that the strength and the frequency of the lobster heartbeat are positively correlated, we observe no consistent relationship between these parameters as they change as a function of temperature. Stroke volume decreases as a function of temperature. However, the opposing temperature-dependent increase in heart rate partially compensates to produce a relationship between cardiac output and temperature in which cardiac output is maximal at 10°C and significantly decreases above 20°C. Serotonin potentiates contraction amplitude and heart rate in a temperature-independent manner. Overall, our results show that although the parameters underlying cardiac performance show different patterns of temperature dependence, cardiac output remains relatively constant over most of the wide range of environmental temperatures the lobster inhabits in the wild.

The American lobster is a poikilotherm that inhabits a marine environment where temperature varies over a 25 degrees C range and depends on the winds, the tides and the seasons. To determine how cardiac performance depends on the water temperature to which the lobsters are acclimated we measured lobster heart rates in vivo. The upper limit for cardiac function in lobsters acclimated to 20 degrees C is approximately 29 degrees C, 5 degrees C warmer than that measured in lobsters acclimated to 4 degrees C. Warm acclimation also slows the lobster heart rate within the temperature range from 4 to 12 degrees C. Both effects are apparent after relatively short periods of warm acclimation (3-14 days). However, warm acclimation impairs cardiac function at cold temperatures: following several hours exposure to frigid (<5 degrees C) temperatures heart rates become slow and arrhythmic in warm acclimated, but not cold acclimated, lobsters. Thus, acclimation temperature determines the thermal limits for cardiac function at both extremes of the 25 degrees C temperature range lobsters inhabit in the wild. These observations suggest that regulation of cardiac thermal tolerance by the prevailing environmental temperature protects against the possibility of cardiac failure due to thermal stress.

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