The Nervous System: Introduction to Cells and Systems, Chapter 4, provides an overview of the neuronal structure and its major components. Figure 4.1 presents a schematic diagram of an idealized neuron, illustrating its key features and connections. Afferent input from axons of other cells terminates in synapses on the dendrites and cell body. The initial segment of the axon attaches at the axon hillock. This axon is myelinated, as indicated by the blue structures that encapsulate segments of the axon. The axon terminates on two postsynaptic neurons by forming synaptic terminals. The gaps between the myelin segments, known as Nodes of Ranvier, expose the axon membrane to the extracellular space. A higher-magnification view of the synapse is provided in Figure 4.1 (Redrawn from Blumenfeld H. Neuroanatomy Through Clinical Cases. 2nd ed. Sunderland, MA: Sinauer Associates; 2010.).

Neurons exhibit a wide range of morphological variations, with some lacking dendrites or an axon. However, these variations are not random and are instead related to the distinct functional properties of each neuronal class. In fact, specific regions of the CNS are often characterized by neurons with similar morphologies, reflecting the unique neuronal processing carried out in each CNS region.

The cell body, also known as the soma, serves as the main genetic and metabolic center of the neuron. It houses the nucleus and nucleolus and possesses a well-developed biosynthetic apparatus responsible for manufacturing membrane constituents, synthetic enzymes, and other chemical substances necessary for the specialized functions of nerve cells. This biosynthetic apparatus includes Nissl bodies, which are stacks of rough endoplasmic reticulum, as well as a prominent Golgi apparatus. The soma is also abundant in mitochondria and cytoskeletal elements, such as neurofilaments and microtubules.

In addition to its genetic and metabolic functions, the cell body acts as a region where the neuron receives synaptic input. This input comprises electrical and chemical signals from other neurons. Although the synaptic input to the soma is generally quantitatively less than that to dendrites, it often differs qualitatively from dendritic inputs. Furthermore, due to its proximity to the axon, inputs to the soma can override those to the dendrites (see
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