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.
