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Neurons and Associated Cells CH02.p3 3/11/05 8:41 AM Page 11 2 Neurons and Associated Cells Neuroplasticity Organelles and Components Neurotrophic Factors and Tropic Factors Structure of Peripheral Nerves and Ganglia Neuroglia (GLIA) Paraneurons Nerve Regeneration Plasticity and Axonal Sprouting resulting excitation rapidly to other portions of NEUROPLASTICITY the nerve cell, and to influence other neurons, muscle cells, and glandular cells. Neurons are Some 100–200 billion ([1–2] × 1011) neu- so specialized that most are incapable of repro- rons (nerve cells), as well as many more glial ducing themselves and they lose viability if cells, are integrated into the structural and denied an oxygen supply for more than a few functional fabric that is the brain. They exhibit minutes. a wide diversity of form and sizes. The neuron is the basic unit of the nervous system and is composed of four structurally defined regions: a cell body (soma) that emits a single nerve ORGANELLES AND COMPONENTS process called an axon, which ends at presy- naptic terminals, and a variable number of Each neuron consists of a large nucleus, a branching processes called dendrites (Figs. 2.1 plasma (cell) membrane and cytoplasm con- and 2.2). Each axon, including its collateral sisting of cytosol (fluid phase of cytoplasm), branches, usually terminates as an arbor of fine and a number of organelles, including the fibers; each fiber ends as an enlargement called endoplasmic reticulum, Nissl substance, Golgi a bouton,which is part of a synaptic junction. apparatus, mitochondria, lysosomes and cyto- At the other end of the neuron, there is a three- skeleton (Figs. 2.3 to 2.5). dimensional dendritic field, formed by the branching of the dendrites (Fig. 2.2). Nucleus The cell body is the genomic and metabolic The nucleus is delineated from the cyto- center of the neuron. Dendrites are the main plasm by a double-layer unit membrane called recipients of neural signals for communication the nuclear envelope. This membrane is perfo- between neurons and contain critical process- rated by nuclear pores, through which macro- ing complexes (Chap. 3). The axon is the molecules synthesized in the nucleus pass into conduit for conducting messages (action poten- the cytoplasm. In humans, the nucleus contains tials) to the presynaptic terminals where each 46 chromosomes formed from DNA (deoxyri- neuron is in synaptic contact with other neu- bonucleic acid) and proteins. The DNA that rons and, thus, is part of the network that encodes some functions is present in the mito- constitutes the nervous system. A neuron is chondria. The outer layer of the nuclear enve- designed to react to stimuli, to transmit the lope is continuous with the membranes of the 11 CH02.p3 3/11/05 8:42 AM Page 12 12 The Human Nervous System Figure 2.1: Diagram of (A) a neuron located wholly within the central nervous system and (B) a lower motoneuron located in both the central and peripheral nervous systems. The latter synapses with a voluntary muscle cell to form a motor end plate. Note the similarities, as recon- structed from electron micrographs, between (C) a synapse between two neurons and (D) a motor end plate. The X represents the border between the central nervous system (above the X) and the peripheral nervous system (below the X). The myelin sheath of neuron (A) is entirely the product of a glial cell, and that of neuron (B) is produced by a glial cell inside the central nervous system and by a Schwann (neurolemma) cell in the peripheral nervous system. CH02.p3 3/11/05 8:42 AM Page 13 Chapter 2 Neurons and Associated Cells 13 Figure 2.2: (A) The degrees of densities among dendritic arborizations are expressed as a sequence of selective arbors (A), sampling arbors (A') and space-filling arbors (A").The selective arbor (A) of an olfactory receptor neuron (ORN) is comprised of a receptive dendritic arbor of cilia, a cell body located within the olfactory mucosa in the nasal cavity (OM), and an axon (Ax) that terminates in the olfactory bulb (OB) (Chap. 14). The sampling arbor (A') with its intermediate arborization pattern is in a pyramidal neuron of the neocortex (Chap. 25). The space-filling arbor (A") is oriented in a plane as in the dendritic branches of Purkinje neurons of the cerebellum (Chap. 18). Arrow indicates axon. (B) Examples of some geometric shaped radiation domains of dendritic arbors. The dendritic arbor (B) radiates from the cell body to form a cylindrical domain (note axon). The dendritic arbor (B') radiates from the cell body to form a cone domain.The dendritic arbor (B") radiates bipolarly from the cell body to form two-cone domains. (C) Outline of an electron micrograph segment of a spiny dendrite illustrating a variety of shapes and sizes of spines described as simple to branched and with spine heads ranging from stubby to mushroom shaped. In vivo imaging has demonstrated that dendritic spines (S) form, collapse and reform, and rapidly change size and shape in response to a diverse array of stimuli. Spine mor- phology is activity dependent and dynamically responsive (Chap. 3). The arrow indicates the direc- tion of the passage of the nerve impulse of an axon. (Adapted from Fiala and Harris, 1999). CH02.p3 3/11/05 8:42 AM Page 14 14 The Human Nervous System Figure 2.3: Some of the cytoplasmic organelles and associated structures of a postganglionic neuron of the autonomic nervous system. The junction between the varicosity and smooth muscle cell A is a typical synapse (Fig. 15.3). The junction between two smooth muscle cells B is an elec- trical synapse (gap junction, nexus). The small circles in the cell body represent lysosomes. endoplasmic reticulum. The inner layer of the the cytosol to the messenger RNA to be syn- envelope has filaments that attach to the thesized into the peptide. Importantly, DNA nuclear chromatin and to other structures specifies RNA through the process known as involved in pore-diameter control. transcription. The RNA then specifies proteins The chromosomes contain sequences of via translation. Thus, the information coded DNA called genes. Through a process known into the DNA sequence of the gene is tran- as transcription, the genes determine the amino scribed to the mRNA, which carries it to the acid sequences of polypeptides and, thus, the ribosome, where it is translated into a specific structure and properties of proteins. The trans- amino acid sequence to the corresponding pep- lation of the DNA code into a protein is accom- tide. The ribosomes are the sites of protein syn- plished by a special ribonucleic acid (RNA) thesis. called messenger RNA (mRNA). The mRNA Like other cells, each neuron synthesizes migrates from the nucleus through the nuclear three classes of proteins, each with a specific pores to the cytoplasm and becomes associated physiological role. Except for a few proteins with a ribosome, which contains another RNA encoded by mitochondria, essentially all of the called ribosomal RNA (rRNA). The rRNA acts macromolecules of a neuron are made in the as a template upon which the amino acids are cell body from mRNAs. These three classes are assembled. Another RNA, called transfer RNA as follows: (1) Proteins synthesized in the (tRNA), conducts the activated amino acids in cytosol by free ribosomes and polysomes; they CH02.p3 3/11/05 8:42 AM Page 15 Chapter 2 Neurons and Associated Cells 15 remain in the cytosol. These proteins, distrib- and secretory (transmitter containing) vesicles uted by slow axoplasmic transport, include and to the plasma membrane for the mainte- enzymes essential to catalyze metabolic nance of its protein composition. processes of the cytoskeleton. (2) Proteins syn- The prominent nucleolus within the nucleus thesized in the cytosol by free ribosomes and is a ribosome-producing machine consisting polysomes, which are incorporated into the largely of RNA and protein along with some nucleus, mitochondria, and peroxisomes. These DNA. It is the site of ribosomal (rRNA) pro- include enzymes that are involved in the syn- duction and initial assembly. The nucleolus is thesis of RNA, DNA, transcription factors well developed in cells such as neurons, which regulating gene expression, and other proteins are active in peptide and protein synthesis. required by these organelles. Mitochondria are The brain utilizes more genes than any other distributed by slow axoplasmic flow. (3) Pro- organ in the body, estimated to be 30,000– teins synthesized in association with the mem- 40,000 genes, with about 15,000 unique to the brane systems attached to or within the lumen neural tissue. of the endoplasmic reticulum and Golgi appa- In females, nuclei of cells throughout the ratus (GA). They are disbursed by vesicles that body, including neurons, contain a condensa- bud off the GA and are distributed via fast axo- tion of one of the two X chromosomes (sex plasmic flow to such organelles as lysosomes chromatin) called a Barr body (Fig. 2.3). It Figure 2.4: Plasma membrane of a neuron. Several types of integral proteins (I) are embedded in the bilipid layer of the 5-nm-thick plasma membrane. The carbohydrate chains of the glycopro- teins are located on the outer membrane surface. The differential distribution of specific proteins is a basis for regional differences in the functional activities expressed by the membrane. The carbo- hydrate chains of the glycolipids are not illustrated. At right, integral proteins are schematized as (1) a transmembrane channel (ionophore) with about a 0.5-nm-wide pore and (2) a coupled sodium–potassium pump (Chap. 3). An ionophore acts as a selective channel for the preferential passage of an ion such as sodium or potassium (Chap.
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