What exactly are the characteristics of the nerve cells (neurons) that form circuits? Figure 6-5 shows a large pyramidal cell that controls memory, located in the hippocampus. Various kinds of neurons exist in our brains. For example, Purkinje cells in the cerebellum have almost an unlimited number of branched dendrites, and each one of these dendrites touches another neuron. Neurons, such as bidirectional retinal neurons, also have characteristic shapes.
Even individual nerve cells have a characteristic directional alignment. Round nerve cell bodies have long processes called axons. Substances created in the nuclei of neurons are transported through axons and secreted from the axon terminals (Fig. 6-6). Neurotransmitters secreted from the axon terminals are involved in the transport of stimuli (signals) among adjacent cells. In addition, the remaining substances are returned to the neurons by retrograde transport.
In addition, neurons possess dendrites, which receive signals from not only axons but also from other nerves. Therefore, each neuron has not just one but several axons and many dendrites. Neurons also have many points of contact (synapses) with each other, and one nerve usually synapses with thousands of other nerves.
Neurons cannot live independently; rather, they receive nutrition from the surrounding glia. Glia are cells that have various functions, such as astroglia for producing nutrients, oligodendroglia for producing myelin sheaths, and microglia, which are like macrophages. Glia are far more abundant than neurons.
Language and Genes
In 2001, a major genetic analysis was performed for reading disabilities characterized by symptoms such as mistaking pronunciations of words and being unable to understand them. The analysis revealed a genetic point mutation (out of 715 amino acids) in a gene called FOXP2. This gene is involved in transcription and is strongly expressed in the brain. It is now called the grammar gene.
This discovery has led to various debates such as whether FOXP2 is involved in the development of the speech area in the left brain, whether humans acquired language ability by a mutation in FOXP2 during evolution from apes to humans, and whether Neanderthal men lacked language because they lacked the FOXP2 mutation. From these detailed discussions, it was concluded that the FOXP2 gene is often conserved among species of mammals, that it is also present in the DNA of Neanderthal men, and that the sequence is almost same as the corresponding sequence in modern humans.