Formerly known as the ascending frontal gyrus, the precentral gyrus is the home of the primary motor cortex, which works in association with other motor areas to plan and execute movement. The primary motor cortex contains a variety of pyramidal neurons whose axons extend down the spinal cord and synapse with motor neurons. Shown above are pericellular nests around pyramidal cell bodies (b,c,d) in the primary motor cortex, formed by the profusely branched axons of other neurons. Cajal noted the presence of pericellular nests in the cerebellum, hippocampus, and cortex and hypothesized that they amplify signals from the presynaptic cells to the enveloped cell body. We now know that pericellular nests allow tight control of the targeted cell, crowding out input from other, more distant axons. While the precise function of the pericellular nests above remains unknown, recent research in the hippocampus has shown that they can influence memory and reinforcement of learned behaviors.
The primary visual cortex is the first processing center for visual signals within the brain, which is divided into six functionally distinct layers. Shown here are feline stellate cells (A,B) from Layer 4 of the primary visual cortex. Stellate cells receive the majority of their input from the lateral geniculate nucleus (LGN) of the thalamus, then send signals to pyramidal cells in layer 6 (H,F) for further processing. While studies of visual system gross anatomy dating from the 1940s indicated that information travels from the eye through the LGN and then into the cortex, more recent work has revealed that only 5-10% of the excitatory synapses onto stellate cells actually come from the LGN; most input comes from deeper cortical layers. The mechanism by which so few LGN inputs control stellate cells remains unknown – synapses from the LGN not appreciably larger than the others, nor are they clustered on stellate cell dendrites. One possible explanation is that signals from the LGN are highly synchronized with each other, whereas other inputs are more temporally dispersed.
