Measurements of forces produced by the mitotic spindle using optical tweezers

We used a trapping laser to stop chromosome movements in Mesostoma and crane-fly spermatocytes and inward movements of spindle poles after laser cuts across Potorous tridactylus (rat kangaroo) kidney (PtK2) cell half-spindles. Mesostoma spermatocyte kinetochores execute oscillatory movements to and away from the spindle pole for 1–2 h, so we could trap kinetochores multiple times in the same spermatocyte. The trap was focused to a single point using a 63× oil immersion objective. Trap powers of 15–23 mW caused kinetochore oscillations to stop or decrease. Kinetochore oscillations resumed when the trap was released. In crane-fly spermatocytes trap powers of 56–85 mW stopped or slowed poleward chromosome movement. In PtK2 cells 8-mW trap power stopped the spindle pole from moving toward the equator. Forces in the traps were calculated using the equation F = Q′P/c, where P is the laser power and c is the speed of light. Use of appropriate Q′ coefficients gave the forces for stopping pole movements as 0.3–2.3 pN and for stopping chromosome movements in Mesostoma spermatocytes and crane-fly spermatocytes as 2–3 and 6–10 pN, respectively. These forces are close to theoretical calculations of forces causing chromosome movements but 100 times lower than the 700 pN measured previously in grasshopper spermatocytes

Synaptic Microcircuits in the Barrel Cortex

An elementary feature of sensory cortices is thought to be their organisation into functional signal-processing units called ‘cortical columns’. These elementary units process sensory information arriving from peripheral receptors; they are vertically oriented throughout all cortical layers and contain several thousands of excitatory and inhibitory synaptic connections. To understand how sensory signals are transformed into electrical activity in the neocortex it is necessary to elucidate the spatial-temporal dynamics of cortical signal processing and the underlying neurons and synaptic ‘microcircuits’.In the somatosensory barrel cortex there appears to be a structural correlate for the ‘functional’ cortical column. Therefore, it has become an attractive model system to study the synaptic microcircuitry in athe neocortex. Although many synaptic connections in whisker-related cortical ‘columns’ have been characterised over the past years our knowledge is far from complete, in particular with respect to inhibitory connections. In this chapter we will summarise recent data on different excitatory and inhibitory synaptic connections in a whisker-related ‘column’ of the somatosensory cortex and try to outline their function in the neuronal network. This requires an appreciation of the diverse types of excitatory and inhibitory neurons and their function within cortical columns and beyond. When necessary, we will also discuss the synaptic input from and to subcortical structures, in particular the thalamus. However, we will not provide a detailed description of the functional mechanisms of these connections; this is beyond the scope of this chapter