Fig. 1.

Examining neural circuits by two-photon imaging of transgenic mice expressing GFP. (A) Two-photon image of cortical pyramidal neurons in coronal sections of GFP M-line mice; a subset of layer 5 cells express GFP. The main (apical) dendrite of these cells is branched and projects to layer 1; dendritic spines are located along the dendrite. (B) High magnification image of dendrite, dendritic spines, and axon imaged by intravital two-photon microscopy. Dendritic spines protrude from dendrites, allowing neurons to make synaptic connections. Note that the axon is thinner than dendrites and does not have spines. Image demonstrates dendritic spines synapsing with an axon. (C) Examples of dendritic spine plasticity, imaged in vivo: novel spines can form (formation), whereas existing spines can change shape and size (retract, grow, or become longer); spines can also be eliminated. Dendritic spines change morphology in response to numerous extracellular stimuli; this can be a consequence of synaptic activity or neuromodulatory stimuli. Intravital two-photon images were acquired with the aid and expertise of Dr. Jack Waters, Northwestern University. (D) Schematic of cortical circuitry rewiring. The strengthening or weakening of existing synaptic connections and the addition or elimination of synaptic connections allows for the bidirectional rewiring of cortical circuits.