CHAPTER 4 The Nervous System: Introduction to Cells and Systems •Fig. 4.1 Schematicdiagramofanidealizedneuronanditsmajorcomponentsandconnections.A,Afferentinputfromaxonsofothercellsterminatesinsynapsesonthedendritesandcellbody.Theinitialsegmentoftheaxonattachesattheaxonhillock.Thisaxonismyelinated,asindicatedbythebluestructuresthatencapsulatesegmentsoftheaxon.Theaxonterminatesontwopostsynapticneuronsbyformingsynapticterminals.B,NodesofRanvierarethegapsbetweenthemyelinsegmentswheretheaxonmembraneisexposedtotheextracellularspace.C,Higher-magnificationviewofsynapse.(RedrawnfromBlumenfeldH.Neuroanatomy Through Clinical Cases. 2nded.Sunderland,MA:SinauerAssociates;2010.) soma), a variable number of processes that extend from the soma called dendrites, and an axon (
Fig. 4.1 ). A tremendous number of morphological variants of this basic template exist, including cases where dendrites or an axon may be absent (
Fig. 4.2 ). These variations do not occur randomly but rather relate to the distinct functional properties of each neuronal class. Indeed, neurons with similar morphologies often characterize specific regions of the CNS and reflect the distinct neuronal processing performed in each CNS region.
The cell body is the main genetic and metabolic center of the neuron. Correspondingly it contains the nucleus and nucleolus of the cell and also possesses a well-developed biosynthetic apparatus for manufacturing membrane constituents, synthetic enzymes, and other chemical substances needed for the specialized functions of nerve cells. The neuronal biosynthetic apparatus includes Nissl bodies, which are stacks of rough endoplasmic reticulum, and a prominent Golgi apparatus. The soma also contains numerous mitochondria and cytoskeletal elements, including neurofilaments and microtubules.
The cell body is also a region in which the neuron receives synaptic input (i.e., electrical and chemical signals from other neurons). Although quantitatively the synaptic input to the soma is usually much less than that to dendrites, it often differs qualitatively from dendritic inputs, and by virtue of the closeness of the soma to the axon, inputs to the soma can override those to the dendrites (see