Cohen and pfaff types of inputs(e.g, sensory, hormonal) can be inte- Nevertheless, as eukaryotic cells, they adhere to grated and relayed to other parts of the central ner- basic laws that govern cellular function. In many vous system or to motor or endocrine targets. ways, neurons are not so different from other cells Interneurons contribute to the formation of neural and qualities once ascribed only to neurons may he circuits and are to a great extent responsible for the found elsewhere. Egg membranes, for example, can relatively large size and extraordinary complexity of depolarize during fertilization and release granules in the mammalian nervous system response to Ca entry through specific channels, although the process takes much longer than compar 1.4. Neuronal Polarity Allows the Directed able signaling mechanisms in neurons. Sites of ribo- Flow of Electrical and Chemical Signals somal RNA synthesis, called nucleoli, are found in all A single neuron may receive a multitude of differ- eukaryotic cells but are especially pro rons. because there is a constant need for ribosomes nt inputs from a variety of sources. A motor neuron, fo for example, receives thousands of presynaptic term- for new protein synthesis. It appears that basic cellu inals from many different neurons on its dendritic lar mechanisms are present in nerve cells but that surface and on its somal and nd, to some extent axonal. some of these are amplified to meet the rigorous membranes, which may also bear postsynaptic recep- demands of neuronal form and function tors. This input has to be organized into a cohesive message that can be transmitted to its postsynaptic 2.1. The Nucleus Is the command Center neighbor of the neuron The neuronal plasma membrane plays a key role in The molecular and cellular diversity within and integrating and relaying the information in a directed among neurons reflects a highly controlled differen- manner and with exceptional speed. Information is tial expression of genes. Gene regulation also deter- conducted within neurons in the form of electrical mines nerve cell connectivity, which dictates patterns or shals, which are actually changes in the distribution of stereotyped behaviors. It is also becoming evident Charge distribution is highly regulated in neurons by learning, may require the synthesis of new proteins specific and selective proteins called ion channels which ultimately depends on the expression of parti- embedded in the plasma membrane. These transmem- cular genes. As eukaryotic cells, neurons sequester brane proteins control the flow of ions and, conse- their genome in the nucleus, the largest and most quently, the distribution of positive and negative conspicuous feature of the perikaryon(Fig. 2). The charges across the membrane In resting neurons, the nucleus contains chromosomal dna and the membrane potential is about -60 to -70 mV(i.e, an machinery for synthesizing and processing RNA excess of positive charges outside and negative charges which is subsequently transported to the cytoplasm, inside). When this potential becomes less negative, or where information encoded in the DNA is expressed depolarized, electrical excitation occurs in the mem- as specific proteins. The nucleus is separated from the brane. In axons, this excitation is known as the action rest of the cytoplasm by a porous double membrane potential.The action potential is generated when the the nuclear envelope, consisting of an outer nuclear membrane potential of the axonal membrane is membrane and an inmer muclear membrane: the inner decreased beyond a threshold value. An important and outer membranes are in contact with each other area of the cell body is the axon hillock, the site on at regions called pore membranes, which are described summation of excitatory and inhibitory input Struc- below. Beneath and in intimate contact with the inner turally, it is devoid of organelles, such as rough nuclear membrane is the nuclear lamina, consisting endoplasmic reticulum(see later) and, at the light of intermediate filaments, and which controls such microscopic level, appears as a lightly stained area. functions as the maintenance of nuclear shape, dis- After summation, the membrane potential reaches its assembly and assembly of the nucleus prior to and threshold, thereby generating the action potential following mitosis, organization of the chromatin spacing of nuclear pores, and transcriptional regula 2. MECHANISMS OF NEURONAL FUNCTION tion. At least 13 genetic disorders involving gene encoding some of the laminins have been described ompared with other cells, neurons are unsurpassed and include premature aging syndromes, myopathies in their complexity of form and ability to comm- and neuropathies, and lipodystrophies. The nuclear unicate with lightning speed over long distances. envelope protects the DNa molecules from mechanicaltypes of inputs (e.g., sensory, hormonal) can be inte￾grated and relayed to other parts of the central ner￾vous system or to motor or endocrine targets. Interneurons contribute to the formation of neural circuits and are to a great extent responsible for the relatively large size and extraordinary complexity of the mammalian nervous system. 1.4. Neuronal Polarity Allows the Directed Flow of Electrical and Chemical Signals A single neuron may receive a multitude of differ￾ent inputs from a variety of sources. A motor neuron, for example, receives thousands of presynaptic term￾inals from many different neurons on its dendritic surface and on its somal and, to some extent axonal, membranes, which may also bear postsynaptic recep￾tors. This input has to be organized into a cohesive message that can be transmitted to its postsynaptic neighbor. The neuronal plasma membrane plays a key role in integrating and relaying the information in a directed manner and with exceptional speed. Information is conducted within neurons in the form of electrical signals, which are actually changes in the distribution of electrical charges across the neuronal membrane. Charge distribution is highly regulated in neurons by specific and selective proteins called ion channels embedded in the plasma membrane. These transmem￾brane proteins control the flow of ions and, conse￾quently, the distribution of positive and negative charges across the membrane. In resting neurons, the membrane potential is about –60 to –70 mV (i.e., an excess of positive charges outside and negative charges inside). When this potential becomes less negative, or depolarized, electrical excitation occurs in the mem￾brane. In axons, this excitation is known as the action potential. The action potential is generated when the membrane potential of the axonal membrane is decreased beyond a threshold value. An important area of the cell body is the axon hillock, the site of summation of excitatory and inhibitory input. Struc￾turally, it is devoid of organelles, such as rough endoplasmic reticulum (see later) and, at the light microscopic level, appears as a lightly stained area. After summation, the membrane potential reaches its threshold, thereby generating the action potential. 2. MECHANISMS OF NEURONAL FUNCTION Compared with other cells, neurons are unsurpassed in their complexity of form and ability to comm￾unicate with lightning speed over long distances. Nevertheless, as eukaryotic cells, they adhere to basic laws that govern cellular function. In many ways, neurons are not so different from other cells, and qualities once ascribed only to neurons may he found elsewhere. Egg membranes, for example, can depolarize during fertilization and release granules in response to Ca+ entry through specific channels, although the process takes much longer than compar￾able signaling mechanisms in neurons. Sites of ribo￾somal RNA synthesis, called nucleoli, are found in all eukaryotic cells but are especially prominent in neu￾rons, because there is a constant need for ribosomes for new protein synthesis. It appears that basic cellu￾lar mechanisms are present in nerve cells but that some of these are amplified to meet the rigorous demands of neuronal form and function. 2.1. The Nucleus Is the Command Center of the Neuron The molecular and cellular diversity within and among neurons reflects a highly controlled differen￾tial expression of genes. Gene regulation also deter￾mines nerve cell connectivity, which dictates patterns of stereotyped behaviors. It is also becoming evident that more complex behaviors, such as memory and learning, may require the synthesis of new proteins, which ultimately depends on the expression of parti￾cular genes. As eukaryotic cells, neurons sequester their genome in the nucleus, the largest and most conspicuous feature of the perikaryon (Fig. 2). The nucleus contains chromosomal DNA and the machinery for synthesizing and processing RNA, which is subsequently transported to the cytoplasm, where information encoded in the DNA is expressed as specific proteins. The nucleus is separated from the rest of the cytoplasm by a porous double membrane, the nuclear envelope, consisting of an outer nuclear membrane and an inner nuclear membrane; the inner and outer membranes are in contact with each other at regions called pore membranes, which are described below. Beneath and in intimate contact with the inner nuclear membrane is the nuclear lamina, consisting of intermediate filaments, and which controls such functions as the maintenance of nuclear shape, dis￾assembly and assembly of the nucleus prior to and following mitosis, organization of the chromatin, spacing of nuclear pores, and transcriptional regula￾tion. At least 13 genetic disorders involving genes encoding some of the laminins have been described and include premature aging syndromes, myopathies and neuropathies, and lipodystrophies. The nuclear envelope protects the DNA molecules from mechanical 4 Cohen and Pfaff