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 autonomic nervous system controls involuntary body functions such as heart rate, digestion, and respiration, and is divided into two parts: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system’s primary function is to activate the fight-or-flight response to danger, but it is also active at a basal level to preserve homeostasis. The superior cervical ganglion, a section of which is shown here, is a part of the sympathetic nervous system in which neurons originating in the spinal cord form synapses with neurons that innervate the heart, head, and neck, and control responses such as heart rate and pupil dilation. The cell bodies shown above are surrounded by “receptive nets” (A, B) formed by their own dendrites and the axons of neurons originating in the thoracic spinal cord. While Cajal himself performed preliminary work on the sympathetic nervous system, his former student, Fernando de Castro, gained international recognition for his careful studies of the fine structure of the autonomic ganglia. Cajal recognized de Castro as an equal and entrusted him with supervising the technical training and research of fellows at the Cajal Institute between 1924-1932.