Chapter 1

Neurons

Constance Hammond

Abstract

Neurons are independent cells making specific contacts called synapses, with hundreds or thousands of other neurons sometimes greatly distant from their cell bodies. The neurons connected together form circuits, and so the nervous system is composed of neuronal networks which transmit and process information. Neurons are excitable cells. Depending on the information they receive, neurons generate electrical signals and propagate them along their processes. This capacity is due to the presence of particular proteins in their plasma membrane which allow the selective passage of ions: the ion channels. Neurons are also secretory cells. Their secretory product is called a neurotransmitter. The release of a neurotransmitter occurs only in restricted regions, the synapses. The neurotransmitter is released in the extracellular space. The synaptic secretion is highly focalized and directed specifically on cell regions to which the neuron is connected. The synaptic secretion is then different (with only a few exceptions) from other secretory cells, such as from hormonal and exocrine cells which respectively release their secretory products into the general circulation (endocrine secretion) or the external environment (exocrine secretion).

Keywords

axon axon terminal axonal transport dendrite, dendritic spine dendritic transport dynein excitable cell Golgi type I neuron Golgi type II neuron kinesin network secretory cell synapse

Outline

1.1 Neurons have a cell body from which emerge two types of processes: the dendrites and the axon 4

1.2 Neurons are highly polarized cells with a differential distribution of organelles and proteins 6

1.3 Axonal transport allows bidirectional communication between the cell body and the axon terminals 11

1.4 Neurons connected by synapses form networks or circuits 18

1.5 Summary: the neuron is an excitable and secretory cell presenting an extreme functional regionalization 22

Further reading 23

By using the silver impregnation method developed by Golgi (1873), Ramon y Cajal studied neurons, and their connections, in the nervous system of numerous species. Based on his own work (1888) and that of others (e.g. Forel, His, Kölliker and Lenhossék), he proposed the concept that neurons are isolated units connected to each other by contacts formed by their processes: 'The terminal arborizations of neurons are free and are not joined to other terminal arborizations. They make contacts with the cell bodies and protoplasmic processes of other cellular elements.' As proposed by Cajal, neurons are independent cells making specific contacts called synapses, with hundreds or thousands of other neurons sometimes greatly distant from their cell bodies. The neurons connected together form circuits, and so the nervous system is composed of neuronal networks which transmit and process information. In the nervous system, there is another class of cells, the glial cells, which surround the various parts of neurons and cooperate with them. Glial cells are discussed in Chapter 2. Neurons are excitable cells. Depending on the information they receive, neurons generate electrical signals and propagate them along their processes. This capacity is due to the presence of particular proteins in their plasma membrane which allow the selective passage of ions: the ion channels. Neurons are also secretory cells. Their secretory product is called a neurotransmitter. The release of a neurotransmitter occurs only in restricted regions, the synapses. The neurotransmitter is released in the extracellular space. The synaptic secretion is highly focalized and directed specifically on cell regions to which the neuron is connected. The synaptic secretion is then different (with only a few exceptions) from other secretory cells, such as from hormonal and exocrine cells which respectively release their secretory products into the general circulation (endocrine secretion) or the external environment (exocrine secretion). Synapses are discussed in Chapter 6. Neurons are quiescent cells. When lesioned, most neurons cannot be replaced, since they are postmitotic cells. Thus, they renew their constituents during their entire life, involving the precise targeting of mRNAs and proteins to particular cytoplasmic domains or membrane areas.

1.1. Neurons have a cell body from which emerge two types of processes: the dendrites and the axon

Although neurons present varied morphologies, they all share features that identify them as neurons. The cell body or soma gives rise to processes which give the neuron the regionalization of its functions, its polarity and its capacity to connect to other neurons, to sensory cells or to effector cells.

1.1.1. The somatodendritic tree is the neuron's receptive pole

The soma of the neuron contains the nucleus and its surrounding cytoplasm (or perikaryon). Its shape is variable: pyramidal soma for pyramidal cells in the cerebral cortex and hippocampus; ovoid soma for Purkinje cells in the cerebellar cortex; granular soma for small multipolar cells in the cerebral cortex, cerebellar cortex and hippocampus; fusiform soma for neurons in the pallidal complex; and stellar or multipolar soma for motoneurons in the spinal cord (Figure 1.1). Figure 1.1 The neurons of the central nervous system present different dendritic arborizations.
(a) Photomicrographs of neurons in the central nervous system as observed under the light microscope. A - Purkinje cell of the cerebellar cortex; B - pyramidal cell of the hippocampus; C - soma of a motoneuron of the spinal cord. Golgi (A and B) and Nissl (C) staining. The Golgi technique is a silver staining which allows observation of dendrites, somas and axon emergence. The Nissl staining is a basophile staining which displays neuronal regions (soma and primary dendrites) containing Nissl bodies (parts of the rough endoplasmic reticulum). (b) Camera lucida drawings of neurons in the central nervous system of primates, revealed by the Golgi silver impregnation technique and reconstructed from serial sections: St, medium spiny neuron of the striatum; GP, neuron of the globus pallidus; Th, thalamocortical neuron; STN, neuron of the subthalamic nucleus; IO, neurons of the inferior olivary complex; Pu, Purkinje cell of the cerebellar cortex; SNc, dopaminergic neuron of the substantia nigra pars compacta. All these neurons are illustrated at the same magnification. Photomicrographs by Francoise Condé (aA), Olivier Robain (aB) and Paul Derer (aC). Drawings by Jérôme Yelnik, except OL and PU by Ramon Y Cajal (1911). One function of the soma is to ensure the synthesis of many of the components required for the structure and function of a neuron. Indeed, the soma contains all the organelles responsible for the synthesis of macromolecules. Most neurons in the central nervous system cannot further divide or regenerate after birth, and the cell body must maintain the structural integrity of the neuron throughout the individual's entire life. Moreover, the soma receives numerous synaptic contacts from other neurons and constitutes, with the dendrites, the main receptive area of neurons (see Figure 1.5 and Section 6.2). The neurons have one or several processes emerging from the cell body and arborizing more or less profusely. The two types of neuronal processes are the dendrites and the axon (Figures 1.1 and 1.3). This division is based on morphological, ultrastructural, biochemical and functional criteria. The dendrites, when they emerge from the soma, are simple perikaryal extensions, the primary dendrites. On average, between one and nine primary dendrites emerge from the soma and then divide successively to give a dendritic tree with specific characteristics (number of branches, volume, etc.) for each neuronal population (Figures 1.1 and 1.2). The dendrites are morphologically distinguishable from axons by their irregular outline, by their diameter, which decreases along their branchings, by the acute angles between the branches, and by their ultrastructural characteristics (Figures 1.1, 1.3 and 1.7). The irregular outline of dendrites is related to the presence of numerous appendices of various shapes and dimensions at their surface. The most frequently observed are the dendritic spines which are lateral expansions with ovoid heads binding to the dendritic branches by a peduncle that is variable in length (Figure 1.3). Some neurons are termed 'spiny' because there are between 40 000 and 100 000 spines on the surface of their dendrites (e.g. pyramidal...