In order to circumvent problems with self-report measures of pain we conducted an experimental analysis of MPQ pain descriptors using a Stroop task. In this task subjects are asked to name the colours in which stimulus words are written. Previous research has demonstrated that words with emotional significance interfere (indexed by increased latencies to respond) with a person's ability to name the colour. We predicted that: (1) chronic pain patients, compared with normal controls, would show more interference to words drawn from the MPQ, and (2) affective/evaluative descriptors would produce greater interference than sensory descriptors. There was support for the first hypothesis but not the second. Possible reasons for these findings are discussed.

The single excitor motoneuron to the limb opener muscle in the crayfish Procambarus clarkii provides multiterminal innervation to individual muscle fibers. At low impulse frequencies, these neuromuscular synapses generate a threefold larger junctional potential in fibers of the proximal region of the muscle compared to those in the central region. Focal extracellular recording from synapse-bearing "boutons" showed more quantal release at low frequencies in the proximal region. Structural correlates for the physiological differences were sought. Fluorescence microscopy of surface innervation stained with a vital fluorescent dye, 4-Di-2-Asp, showed that density of innervation was not greater in the proximal region and thus could not account for the overall differences in synaptic strength. Freeze fracture studies showed that the intramembrane organization of excitatory synapses and their active zones was qualitatively similar in proximal and central sites. Serial section electron microscopy of several innervation sites in proximal and central regions showed homogeneity in number and size of synapses. However, presynaptic dense bars (at release sites, or active zones) were longer and occurred at a higher density in proximal than in central synapses. The differences in number and length of presynaptic dense bars correlate positively with the differences in synaptic strength represented by junctional potential amplitudes and quantal contents of individual surface recording sites. Since many individual proximal synapses have multiple dense bars, co-operativity among these may serve to enhance transmitter output. It is concluded that occurrence of dense bars is a significant presynaptic correlate of synaptic strength in this neuron.

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