When neuroscientists employ the term "nucleus", they are usually referring to:_____.
​a. a cluster of glial cells in the dorsal-medial CNS.
​b. a neural unit composed of anatomically and functionally related neurons.
​c. a group of homologous ganglia located either in the cortex or in the cerebellum.
​d. cognitively synchronized clusters of apical dendrites anywhere in the PNS.