Chapter 1: Studying The Nervous System
Genetics, Genomics, and the Brain
1. Gene: hereditary unit located on the chromosomes; genetic information is
carried by linear sequences on nucleotides in DNA that code for corresponding
sequences of amino acids.
2. Genomics: Scientific field focusing on the analysis of DNA sequences including
both proteincoding DNA (genes) and noncoding DNA.
3. Based on current estimates, the human genomes contains approximately 20 000
genes, of which some 14 000 are expressed in the developing and/or the mature
1. Of this division, about 8 000 are expressed in all cells and tissues. Thus, a
great deal of “brain specific” genetic information resides in the intron and
regulatory sequences that control timing, quantity, variability, and cellular
specificity of gene expression.
2. Despite the number of genes shared among the brain and other tissues,
individual genes vary in the level and location of expression in specific brain
regions and cells (i.e. the amount of RNA expressed from region to region).
1. These differences are the foundation of the diversity and complexity of
4. The realization that one or a few genes, when altered (mutated), can explain at
least some of the pathology of important neurological and psychiatric diseases.
1. Using genetic tools and genomics, singlegene mutations have been identified
that result in rare but devastating changes in brain development and function.
1. For example, a mutation in a single gene that regulates mitosis can result
in microcephaly, a condition in which the brain and head fail to gran and
brain function is diminished dramatically.
The Cellular Components of the Nervous System
5. Early in the 19 century, the cell was recognized as the fundamental unit of all
living organisms. It was not until well into the 20 century, however, that
neuroscientists agreed that nervous tissue, like all other organs, is made up of
these fundamental units.
6. The majth reason was that the first generation of “modern” neurobiologists in
the 19 century had difficulty resolving the unitary nature of nerve cells with the
microscopes and cell staining techniques then available.
7. Some biologists of that era even concluded that each nerve cell was connected to
its neighbours by protoplasmic links, forming a continuous nerve cell network or
reticulum (Latin “net”).
8. The Italian pathologist Camillo Golgi articulated and championed this “reticular
theory” of nerve cell communication.
1. Made many important contributions to medical science, including identifying
the cellular organelle eventually called the Golgi apparatus. 9. Cajal argued that nerve cells are discrete entities, and that they communicate
with one another by means of specialized contacts that are not sites of continuity
1. Sherrington who had been working on the apparent transfer of electrical
signals via reflex pathways called these specialized contacts synapses,
10. It was not until the advent of electron microscopy in the 1950s that any lingering
doubts about the discreteness of neurons were resolved.
1. The highmagnification, highresolution obtained with the electron
microscope clearly established that nerve cells are functionally independent
units; such micrographs also identified the junction Sherrington had named
11. Gap Junction: a specialized intercellular contact formed by channels that
directly connect the cytoplasm of two cells.
12. Nerve cells (neurons): cells specialized for the conduction and transmission of
electrical signals in the nervous system.
13. Glial cells: The support cells associated with neurons (astrocytes,
oligodendrocytes, and microglial cells in the central nervous system; Schwann
cells peripheral nerves; and satellite cells in ganglia).
14. In contrast to nerve cells, glial cells support rather than generate electrical
signals. They also serve additional functions in the developing and adult brain.
15. Glial are essential contributors to repair of the damaged nervous system, acting
as stem cells in some brain regions, promoting regrowth of damaged neurons in
regions where regeneration can usefully occur,, and preventing regeneration in
other regions where uncontrolled regrowth might do more harm than good.
16. Neurons and glia share the complement of organelles found in all cells including
endoplasmic reticulum, Golgi apparatus, mitochondria, and a variety of vesicular
1. In neurons and glia, however, these organelles are often more prominent in
distinct regions of the cell.
1. For example, mitochondria tend to be concentrated at synapses in
neurons, while proteinsynthetic organelles such as the endoplasmic
reticulum are largely excluded from axons and dendrites.
17. Neurons are distinguished by their specialization for intercellular communication
and momenttomoment electrical signalling.
1. These attributes are apparent in their overall morphology, in the organization
of their membrane components for longdistance signalling, and in the
structural and functional intricacies of the synaptic contacts between neurons.
18. The most obvious morphological sign of neuronal specialization for
communication is the extensive branching of neurons.
1. The two most noticeable aspects of this branching for typical nerve cells are
the presence of an axon, and the elaborate arborisation of dendrites that arise from the neuronal cell body in the for of dendritic branches (or dendritic
19. Dendrite: a neuronal process arising from the nerve cell body that receives
20. Axon: the neuronal process that carries the action potential from the nerve cell
body to a target.
21. Some neurons lack dendrites altogether, while others have dendritic branches that
rival the complexity of its dendritic nook: nerve cells that lack dendrites are
innervated by just one or a few other nerve cells, whereas neurons with
increasingly elaborate dendritic branches are innervated by s commensurately
larger number of other neurons.
22. The number of inputs to a single neuron reflects the degree of convergence, while
the number of targets innervates by and one neuron represents its divergence.
23. Presynaptic: referring to the component of a synapse specialized for transmitter
release; upstream at a synapse.
24. Postsynaptic: referring to the component of a synapse specialized for transmitter
reception; downstream at a synapse.
25. Synaptic cleft: the space that separates pre and postsynaptic neurons at
26. The number of synaptic inputs received by each nerve cell in the human nervous
system varies from 1 to about 100 000. This range reflects a fundamental purpose
of the nerve cells: to relay and integrate information from other neurons in a
27. The information conveyed by synapses on the neuronal dendrites is integrates and
“ read out: at the origin of the axon, the portion of the nerve cell specialized for
relaying electrical signals.
28. The axon is a unique extensions from the neuronal cell body that may travel a
dew hundred micrometres or much farther, depending on the type of neuron and
the size of the animal (some axons in large animals can be meters in length).
1. The axon also has a distinct cytoskeleton whose elements are central for its
functional integrity. Many nerve cells in the human brain have axons no more
than a few millimetres long, and a few have no axons at all.
29. Local circuit neurons (interneurons): technically a neuron in the pathway
between primary sensory and primary effects neurons; more generally, a neuron
whose relatively short axons branch locally to innervate other neurons.
30. Projection neurons: a neuron with long axons that project to distant targets.
31. Action potential: the electrical signal conducted along axons (or muscle fibres)
by which information is conveyed from one pace to another in the nervous system.
32. Axon hillock: point at the cell body that is the site of an action potential’s
33. Synaptic transmission: the chemical and electrical process by which the
information encoded by action potentials is passed from a presynaptic (initiating)
cell to a postsynaptic (target) cell.
34. Chemical synapses: synapses that transmit information via the secretion of
chemical signals (neurotransmitters). 35. Electrical synapses: synapses that transmit information via the direct flow of
electrical current at gap junctions.
36. Synaptic vesicles: spherical, membranebound organelles in a presynaptic
terminal that stores neurotransmitter molecules.
37. Neurotransmitter molecule (neurotransmitters): substance released by
synaptic terminals for the purpose of transmitting information from one cell (the
presynaptic cell) to another (the postsynaptic cell).
38. The position of synaptic vesicles at the presynaptic membrane and their fusion to
initiate neurotransmitter release is regulated by a number of proteins (including
several cytoskeletal proteins) either within or associated with the vesicle.
39. They are more numerous than neurons in the brain by the ration of perhaps 3:1.
40. They do not participate directly in synaptic interactions or in electrical
signalling, although their supportive functions help define synaptic contacts and
maintain the signalling abilities of neurons.
41. Like nerve cells, glial cells have complex processes extending from their cell
bodies, but these are generally less prominent and do not serve the same purposes
as neuronal axons dendrites.
42. Cells with glial characteristics are the only apparent stem cells retained in the
mature brain, and are capable of giving rise both to new glia as well as new
43. The word glia is Greek for “glue” and reflects the 19 century presumption that
these cells “held the nervous system together”.
1. The term has survived despite the lack of any evidence that glial cells actually
binds nerve cells together.
44. Astrocytes: one of the three major classes of glial cells found in the central
nervous system: important in regulating the ionic milieu of nerve cells and, in
some cases, transmitter reuptake.
1. A major function of astrocytes is to maintain, in a variety of ways, an
appropriate chemical environment for neuronal signalling.
45. Oligodendrocytes: one of the three major classes of glial cells found in the
central nervous system; the