Dendrites axons

Vesicular proteins and lipids that are destined for the plasma membrane leave the TGN sorting station continuously. Incorporation into the plasma membrane is typically targeted to a particular membrane domain (dendrite, axon, presynaptic, postsynaptic membrane, etc.) but may or may not be triggered by extracellular stimuli. Exocytosis is the eukaryotic cellular process defined as the fusion of the vesicular membrane with the plasma membrane, leading to continuity between the intravesicular space and the extracellular space. Exocytosis carries out two main functions it provides membrane proteins and lipids from the vesicle membrane to the plasma membrane and releases the soluble contents of the lumen (proteins, peptides, etc.) to the extracellular milieu. Historically, exocytosis has been subdivided into constitutive and regulated (Fig. 9-6), where release of classical neurotransmitters at the synaptic terminal is a special case of regulated secretion [54]. 

The function of the nervous system depends on communication between nerve cells (neurons). Structurally, all neurons are composed of a cell body, dendrites, axon, and terminal boutons (see figure 3-C). Dendrites are short, terminally branched structures projecting out from the cell body. They receive and conduct information to the cell body there may be one or several dendrites on each cell. Extremely fine projections on the dendrites are called dendritic spines. The axon is a single long fiber that ends in enlarged structures called terminal boutons. The axon serves to conduct impulses away from the cell body. 

A nerve consi.sts of myelinated or unmyelinated nerve fibers (Fig. 20-5a). These nerve fibers consist of chains" of neurons. The junction between adjacent neurons in the chain consists of the synaptic knob of the transmitting neuron separated by a gap of about 30 to 50 nm from either the dendrite, axon hilhx k. or cell body of the other neuron. This... 

Communication in the brain takes place between nerve cells or neurons. Psychoactive substances alter many aspects of communication between neurons, as will be discussed below. Neurons are highly speciahzed cells that exist in many shapes, sizes and varieties. However, they share the following basic structural regions cell body or soma, dendrites, axon, and terminal buttons. The cell body, or soma, is the metabolic centre of the neuron, and contains the nucleus and other structures that sustain the neuron. The nucleus plays a role in mature neurons, where it is used to synthesize proteins in response to a wide variety of stimuli (11, 17). 

The approach of biochemical purification combined with high-resolution mass spectrometiy used for the definition of the PSP could be easily applied to many other sub-cellular structures. Indeed, subcellular structures, such as axo-glial junctions and nodes of Ranvier would be prime candidates for such an approach. Also, refined or new biochemical approaches to isolate structures such as dendrites, axons and growth cones would facilitate definition of other neuronal proteomes. 

Figure 9-12. Biological neuron the dendrites receive the incoming information and send it to the cell body, The axon carries the information produced in the cell body to neighboring neurons.

The human brain is comprised of many millions of interconnected units, known individually as biological neurons. Each neuron consists of a cell to which is attached several dendrites (inputs) and a single axon (output). The axon connects to many other neurons via connection points called synapses. A synapse produces a chemical reaction in response to an input. The biological neuron fires if the sum of the synaptic reactions is sufficiently large. The brain is a complex network of sensory and motor neurons that provide a human being with the capacity to remember, think, learn and reason. 

Neurons have three parts the cell body and dendrites, the axon, and axon terminals. The cell body contains the nucleus and the organelles needed for metabolism, growth, and repair. The dendrites are branched extensions of the cell body membrane. The axon is a long, thin structure which transfers electrical impulses down to the terminals. The axon divides into numerous axon terminals and it is in this specialized region that neurotransmitters are released to transmit information from one neuron to its neighbors. The synapse has been defined as the space between two subsequent interrelated neurons. ... 

Tree like networks of nerve fiber called dendrites protrude outward from the neuron s cell body, or soma. Extending outward from the soma is also a long fiber called the axon that itself eventually branches out into a set of strands and sub strands. At the ends of these strands are the transmitting ends of communication junctions between nerve fibers called synapses. The receiving ends of these junctions exist both on dendrites and on the somas themselves. Each neuron is typically connected to several thousand other neurons. 

In the nervous system, DAT, NET, and SERT are distributed along axons, soma and dendrites. On the subcellular level they are localized at the... 

A synapse is a contact site between two neurones, where information is communicated from the axon of one neurone (the presynaptic) to the cell body, the dendrites or the axon of the second neurone (the postsynaptic). In most synapses, the information is communicated chemically ... 

The primary role of the sodium channels is to generate action potentials in excitable cells. In case of neurons, the sodium channel density is high at axon hillocks or axon initial segment where action potentials start to propagate. The sodium channels are also present in dendrites. The sodium channels contribute to amplifying synaptic inputs (particularly those distally located) and are actively involved in back propagation of action potentials into dendrites. Subtle differences in properties of sodium channels influence the dendritic processes of synaptic integration in and complex ways. 

The nervous impulse can be observed at the interface between the axon of a neuron and the dendrite of the next neuron. The ionic (Na K+, Ca2+) and chemical (neurotransmitters) nature of the nervous impulse has been stated and clarified during the past decades. Most of the neurotransmitters have an ionic nature. So the nervous impulse contains both ionic (electrical) and chemical information, and most of the carriers have been modulated in a different way. 

Microtubules (MT) are the largest of the cytoskeletal filaments with an outer diameter of about 25 nm, a wall thickness of about 5 nm, and a central lumen measuring about 15 nm. They consist of tubulin and associated proteins. Vertebrate brain tissue is a rich source of extractable tubulin because of the large numbers of microtubules that are present in axons and dendrites. Tubulin obtained from such a natural source is a heterodimer of 100 kD composed of a-tubulin and P-tubulin. Brain a-tubulin is a globular polypeptide that contains 451 amino acid residues, whereas P-tubulin, which is somewhat shorter, is made up of 445 amino acid residues. These two molecular species of tubulin share in common 40% of their amino acid residues. 

The MAPs vary in number and relative abundance in different cell types MAP2 is principally found in dendrites, whereas tau is restricted to axons. This selective distribution of MAP2 molecules is the result of subcellular sorting of its messenger RNA (Lewis et al., 1989). Currently, there is interest in the observation that tau is a component of the neurofibrillary tangles of Alzheimer s disease (Goedert et al., 1989). 

The microtubule-associated proteins MAP2 and tau both have two separate functional regions (Lewis et al., 1989). One is the microtubule-binding site, which nucleates microtubule assembly and controls the rate of elongation (by slowing the rate of assembly). The second functional domain shared by MAP2 and tau is a short C-terminal a-helical sequence that can cross-link microtubules into bundles by self-interaction. This domain has some of the properties of a leucine zipper. Likely it is responsible for the organization of microtubules into dense stable parallel arrays in axons and dendrites (Lewis et al., 1989). 

Lewis, S.A., Ivanov, I.E., Lee, G.-H., Cowan, N.J. (1989). Organization of microtubules in dendrites and axons is determined by a short hydrophobic zipper in microtubule-associated proteins MAP2 and tau. Nature 342,498-505. 

Microtubules, an integral component of the cellular cy-toskeleton, consist of cytoplasmic tubes 25 nm in diameter and often of extreme length. Microtubules are necessary for the formation and function of the mitotic spindle and thus are present in all eukaryotic cells. They are also involved in the intracellular movement of endocytic and exocytic vesicles and form the major structural components of cilia and flagella. Microtubules are a major component of axons and dendrites, in which they maintain structure and participate in the axoplasmic flow of material along these neuronal processes. 

The neurons from which NTs are released number more than 7 billion in the human brain. Each (Fig. 1.2) consists of a cell body, the soma or perikaryon, with one major cytoplasmic process termed the axon, which projects variable distances to other neurons, e.g. from a cortical pyramidal cell to adjacent cortical neurons, or to striatal neurons or to spinal cord motoneurons. Thus by giving off a number of branches from its axon one neuron can influence a number of others. All neurons, except primary sensory neurons with cell bodies in the spinal dorsal root ganglia, have a number of other, generally shorter, projections running much shorter distances among neighbouring neurons like the branches of a tree. These processes are the dendrites. Their... 

The axon terminals of one neuron synapse with other neurons either on the dendrites (axo-dendritic synapse) or soma (axo-somatic synapse). Synapses on another axon... 

Dendro-dendritic synapses have also been deseribed whieh show eharaeteristie synaptie eonneetions and we need to abandon the belief that one neuron ean only influenee another through its axon terminals. Dendro-dendritie synapses ean also be reeiproeal, i.e. one dendrite ean make synaptie eontaet with another and apparently be both pre- and postsynaptie to it. 

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