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3-1 Cells of the Nervous System

The nervous system is composed of two kinds of cells: neurons, which transmit information, and glia, which support neuronal function. Sensory neurons may act as receptors to convey information from the body to the brain; motor neurons command muscles to move; and interneurons link up sensory and motor neuron activities.

Like neurons, glial cells can be grouped by structure and function. Ependymal cells produce CSF. Astrocytes structurally support neurons, help to form the blood–brain barrier, and seal off damaged brain tissue. Microglia aid in brain cell repair and waste removal. Oligodendroglia and Schwann cells myelinate axons in the CNS and in the somatic division of the PNS, respectively.

A neuron is composed of three parts: a cell body, or soma; multiple branching extensions called dendrites, designed to receive information; and a single axon that passes information along to other neurons. Numerous dendritic spines greatly increase a dendrite’s surface area. An axon may have branches (axon collaterals), which further divide into telodendria, each ending at a terminal button (end foot). A synapse is the almost connection between a terminal button and another cell’s membrane.

3-2 Internal Structure of a Cell

A surrounding cell membrane protects the cell and regulates what enters and leaves it. Within the cell are a number of organelles, also enclosed in membranes. These compartments include the nucleus (containing the cell’s chromosomes and genes), the endoplasmic reticulum (where proteins are manufactured), the mitochondria (where energy is gathered and stored), the Golgi bodies (where protein molecules are packaged for transport), and lysosomes (which break down wastes). A cell also contains a system of tubules (microfilaments) that aid its movements, provide structural support, and act as highways for transporting substances.

To a large extent, the work of cells is carried out by proteins. The nucleus contains chromosomes—long chains of genes, each encoding a specific protein the cell needs. Proteins perform diverse tasks by virtue of their diverse shapes. Some act as enzymes to facilitate chemical reactions; others serve as membrane channels, gates, and pumps; still others are exported for use in other parts of the body.

A gene is a segment of a DNA molecule made up of a sequence of nucleotide bases. Through transcription, a copy of a gene is produced in a strand of messenger RNA. The mRNA travels to the endoplasmic reticulum, where a ribosome moves along the mRNA molecule, translating it into a sequence of amino acids. The resulting amino acid chain is a polypeptide. Polypeptides fold and combine to form protein molecules with distinctive shapes that serve specific purposes in the body.

3-3 Genes, Cells, and Behavior

From each parent, we inherit one of each chromosome in the 23 chromosome pairs that constitute the human genotype. Because all but the sex chromosomes are matched pairs, a cell contains two alleles of every gene. Sometimes the paired alleles are homozygous (the same), and sometimes they are heterozygous (different).

An allele may be dominant and expressed as a trait, recessive and not expressed, or codominant and expressed along with the other allele in the organism’s phenotype. One allele of each gene is designated the wild type—the most common in a population—whereas the other alleles are called mutations. A person might inherit any of these alleles from a parent, depending on the parent’s genotype.

Genes have the potential to undergo many mutations—of a single base pair, part of the chromosome, or the entire chromosome. Mutations can be beneficial, harmful, or neutral in their effects on nervous system structure and behavioral function. Genetic research seeks to prevent the expression of genetic and chromosomal abnormalities and to find cures for those that are expressed.

Selective breeding is the oldest form of genetic manipulation. In genetic engineering, an animal’s genome is artificially altered. The genetic composition of a cloned animal is identical to that of a parent or sibling. In transgenic animals, a new or altered gene may be added or a gene removed.

The genome encodes a range of phenotypes. The phenotype eventually produced is determined by epigenetics and further influenced by experience and the environment. Epigenetic mechanisms such as DNA methylation can influence whether genes are transcribed or transcription is blocked without changing the genetic code itself.