Dendrites, which are tapering and branching extensions of the soma, serve as the primary recipients of signals from other neurons. They play a crucial role in expanding and specializing the surface area of a neuron. In fact, dendrites may account for more than 90% of the available surface area for synaptic contact, including both the soma and dendrites themselves. Primary dendrites extend directly from the soma, while higher-order dendrites are daughter branches that extend from a more proximal branch. The main cytoplasmic organelles found in dendrites are microtubules and neurofilaments. However, primary dendrites can also contain Nissl bodies and parts of the Golgi apparatus.

The collection of dendrites belonging to a neuron is known as its dendritic tree. Dendritic trees can vary greatly between different types of neurons in terms of their size, number, and spatial organization. Some dendritic trees consist of only a few unbranched dendrites, while others are highly ramified with numerous branches. Individual dendrites can range in length from over 1 mm to as short as 10 to 20 µm. Another important morphological variation is the presence or absence of spines on dendrites. Spines are small protrusions shaped like mushrooms or lollipops that are specialized for synaptic contact, usually from excitatory inputs. The shape, size, and arrangement of the dendritic tree, as well as the distribution of channels in the dendritic membrane, all play a significant role in determining how synaptic input will impact the neuron.

On the other hand, the axon serves as an extension of the neuron that transmits the cell's output to other neurons or, in the case of a motor neuron, to muscle cells. Typically, each neuron possesses only one axon, which tends to have a uniform diameter. The length and diameter of axons vary depending on the type of neuron. While some axons are relatively short and do not extend much beyond the length of the dendrites, others can be a meter or more in length. Axons may have branches en passant, but they typically terminate in a spray of branches known as a terminal arborization. The size, shape, and organization of the terminal arborization determine which cells it will make contact with. The initial segment of the axon, called the axon hillock, arises from the soma or occasionally from a proximal dendrite. Unlike the soma and proximal dendrites, the axon lacks rough endoplasmic reticulum, free ribosomes, and a Golgi apparatus. The initial segment is usually where action potentials, or spikes, that propagate down the axon are initiated. An axon may terminate in a synapse or make synapses along its length. Synapses will be discussed in greater detail later on.
Dendrites are tapering and branching extensions of the soma and are the main direct recipients of signals from other neurons. They can be thought of as a way to expand and specialize the surface area of a neuron, and indeed, they may account for more than 90% of the surface area available for synaptic contact (soma plus dendrites). Dendrites can be divided into primary dendrites (those that extend directly from the soma) and higher-order dendrites (daughter branches extending from a more proximal branch, in •Fig. 4.2 A,Purkinjecell.B,Pyramidalcell.C,Golgicell.D,Granulecell.E,Inferiorolivecells.F,Bipolarcells.(A,CourtesyofBorisBarbour.B,CourtesyofT.F.Fletcher,from http://vanat.cvm.umn.edu/ neurHistAtls/pages/neuron3.html .C,FigurewasprovidedbyCourtHullandWadeRegehr,DepartmentofNeurobiology,HarvardMedicalSchool.D,FromDelvendahlIetal.Front Cell Neurosci 2015;9:93,Fig.1A.E,FromMathyA,ClarkBA.In:MantoMetal[eds].Handbook of the Cerebellum and Cerebellar Disorders. Dordrecht,Netherlands:SpringerScience+BusinessMediaDordrecht;2013.F,FromLiW,DeVriesSH.Nat Neurosci 2006;9:669-675,Fig.2.) which proximal refers to closeness to the soma). The main cytoplasmic organelles in dendrites are microtubules and neurofilaments; however, the primary dendrites can also contain Nissl bodies and parts of the Golgi apparatus.
A neuron’s set of dendrites is termed its dendritic tree. Dendritic trees differ tremendously between different types of neurons in terms of the size, number, and spatial organization of the dendrites. A dendritic tree can consist of just a few unbranched dendrites or of many highly ramified dendrites. Individual dendrites can be longer than 1 mm or only 10 to 20 µm in length. Another major morphological variation is whether or not a dendrite has spines, which are small mushroomor lollipop-shaped protrusions from the main dendrite. Spines are sites specialized for synaptic contact (usually, but not always) from excitatory inputs. The shape and size of the dendritic tree, as well as the population and distribution of channels in the dendritic membrane, are all important determinants of how the synaptic input will affect the neuron (see
Chapter 6).
The axon is an extension of the cell that conveys the output of the cell to other neurons or, in the case of a motor neuron, to muscle cells as well. In general, each neuron has only one axon, and it is usually of uniform diameter. The length and diameter of axons vary with the neuronal type. Some axons do not extend much beyond the length of the dendrites, whereas others may be a meter or more long. Axons may have orthogonal branches en passant, but they often end in a spray of branches called a terminal arborization (represented by the four terminal branches and their synaptic terminals in
Fig. 4.1A ). The size, shape, and organization of the terminal arborization determine which other cells it will contact. The first part of the axon is known as the initial segment and arises from the soma (or sometimes from a proximal dendrite) in a specialized region called the axon hillock. The axon differs from the soma and proximal dendrites in that it lacks rough endoplasmic reticulum, free ribosomes, and a Golgi apparatus. The initial segment is usually the site where action potentials (spikes) that are propagated down the axon are initiated (see ). An axon may terminate in a synapse and/or it may make synapses along its length. Synapses will be described in detail in