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