Neurons Structure And Environment

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 ch3 0

I II

ch3—n+—ch2—ch2—o— c — ch3 qh3 Acetylcholine

Amino acids nho

Monoamines

—ch2—ch2—nh2 Dopamines i— ch —ch2—nh2 oh Noradrenaline ho—i

—ch2—ch2—nh2 Dopamines i— ch —ch2—nh2 oh Noradrenaline ho.

nh nh

5-Hydroxytryptamine (5-HT)

Peptides

Tyr-Gly-Gly-Phe-Met Met-enkephalin

Arg-Pro-Lys-Pro-GIn-GIn-Phe-Phe-Gly-Leu-Met-N H2 Substance P

Arg = arginine, Gly = glycine, Gin = glutamine, Leu = leucine, Lys = lysine, Phe = phenylalanine, Pro = proline, Met = methionine

Figure 1.1 The chemical structures of the main neurotransmitters. The relatively simple structure of acetylcholine, the monoamines and the amino acids contrasts with that of the peptides, the simplest of which are the enkephalins which consists of five amino acids; substance P has eleven absence from sensory, i.e. initiating, neurons immediately suggests that their function is associated with the reception of signals (inputs) from other neurons. Neuron cell bodies vary in diameter from 5 p,m to 100 p,m and axons from 0.1 p,m to 10 p,m, although these are enlarged at their terminal endings. Axons are generally surrounded by an insulating myelin sheath which is important for the propagation of action potentials generated in the neurons and gives the axons and the pathways they form a white colour which contrasts with the grey appearance of those areas of the CNS dominated by the presence of neuron cell bodies and their dendrites.

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

Neural Synapse Black White

Figure 1.2 (a) Electron micrograph of three neuronal cell bodies in the anterior thalamic nucleus of the rat retrogradely labelled with horseradish peroxidase conjugated with cholera toxin B (dark bars) injected into the posterior cingulate cortex. N = nucleus of neurons, O = nucleus of oligodendrocyte, C = capillary, D = dendrite, G = Golgi apparatus, M = myelinated fibre, r = ribosome, l = lipofuscin pigment, g = granular endoplasmic reticulum. Picture kindly provided by Professor A. R. Lieberman (University College London). Reproduced by permission of Springer-Verlag GmbH & Co. KG, from Wang et al, Exp. Brain Res. 126: 369-382 (1999)

Figure 1.2 (a) Electron micrograph of three neuronal cell bodies in the anterior thalamic nucleus of the rat retrogradely labelled with horseradish peroxidase conjugated with cholera toxin B (dark bars) injected into the posterior cingulate cortex. N = nucleus of neurons, O = nucleus of oligodendrocyte, C = capillary, D = dendrite, G = Golgi apparatus, M = myelinated fibre, r = ribosome, l = lipofuscin pigment, g = granular endoplasmic reticulum. Picture kindly provided by Professor A. R. Lieberman (University College London). Reproduced by permission of Springer-Verlag GmbH & Co. KG, from Wang et al, Exp. Brain Res. 126: 369-382 (1999)

Axo Axonic

Figure 1.2 (b) Schematic representation of a neuron. The main features of a neuron are shown together with different synaptic arrangements (A) axo-dendritic, (B) axo-somatic, (C) axo-axonic and (D) dendro-dendritic. For more detail see section on 'Morphological correlates of synaptic function' and Fig. 1.7

Figure 1.2 (b) Schematic representation of a neuron. The main features of a neuron are shown together with different synaptic arrangements (A) axo-dendritic, (B) axo-somatic, (C) axo-axonic and (D) dendro-dendritic. For more detail see section on 'Morphological correlates of synaptic function' and Fig. 1.7

terminal are also found (axo-axonal) and occasionally even between dendrites (dendro-dendritic) (see Fig. 1.2(b)). The morphology of synapses is considered later.

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability.

Microtubules (about 20 nm in diameter) and solid neurofilaments (10 nm) extend from the cell body into the axon and are found along its length, although not continuous. They give structure to the axon but are not involved in the transport of material and vesicles to the terminal, which despite its high level of activity does not have the facility for molecular synthesis possessed by the cell body. Such transport is considered to be fast (200-400 mm per day), compared with a slower transport (1 mm per day) of structural and metabolic proteins. Although axonal flow is mainly towards the terminal (ortho or anterograde) there is some movement (fast) of waste material and possibly information on synaptic activity back to the cell body (retrograde). The neuron is obviously very active throughout the whole of its length.

In addition to neurons the CNS contains various neuroglia (often just called glia). These can outnumber neurons by up to 10:1 in some areas and include star-like astro-cytes with their long cellular processess which not only enable them to provide structural support for the nerve cells but also facilitate NT degradation and the removal of metabolites. Oligodendrites are glial cells which are involved in myelin formation and although they also have long processes, these are spirally bound rather than extending out as in the astocytes.

Neurons and glia are bathed in an ion-containing protein-free extracellular fluid which occupies less of the tissue volume (20%) in the brain than in other organs because of the tight packing of neurons and glia. In fact the whole brain is really suspended in fluid within its bony casing. The brain and spinal cord are covered by a thin close-fitting membrane, the pia mater and a thicker loose outer membrane, the dura mater. In the space between them, the subarachnord space, is the cerebrospinal fluid (CSF). This also flows into a series of ventricular spaces within the brain as well as a central canal in the cord and arises mainly as a secretion (ultra filtrate) of blood from tufts of specialised capillaries (the choroid plexus), which invaginate the walls of the ventricles. While the CSF is contiguous with the extracellular fluid within the brain and contributes to it, much of this fluid comes directly from the copious network of capillaries found throughout the brain. In fact neurons are never far from a capillary and their high metabolic rate means that despite contributing only 2% towards body weight, the nervous system receives 15% of cardiac output.

In most parts of the body, substances, other than large molecular ones like proteins, are filtered from the blood into the extracellular space through gaps between endothelial cells in the capillary wall. Such gaps are much narrower, almost nonexistent, in brain capillaries and it is likely that any filtering is further reduced by the manner in which astrocytes pack around the capillaries. This constraint is known as the blood-brain barrier (BBB). It protects the brain from inappropriate substances, including all NTs and many drugs. To enter the brain as a whole is therefore almost as difficult for a substance as entering a neuron and again it has to be either very lipid soluble, when it can dissolve in and so pass through the capillary wall, or be transported across it.

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