the memory “trace” back in those areas of the neocortex.
The neurobiology of memory depends both on modifiable synaptic weights, such as with NMDA receptors in hippocampus and cortex, and the creation of new neurons in memory areas such as the hippocampus. The discovery of the birth of neurons in the adult hippocampus overturns the old idea of zero neurogenesis in the adult brain. Some senile dementia and even depression appear to be associated with failure of this mechanism.
The frontal lobes and executive brain
The frontal lobes are responsible for planning and executing behavior. Generally speaking, the output of the frontal lobe is in its most posterior portion, the primary motor cortex. Neurons in primary motor cortex send their axons down the spinal cord (or out some cranial nerves) to drive motor neurons that cause muscles throughout the body to contract.
Anterior to the primary motor cortex are the supplementary motor area and premotor cortex that organize the firing of groups of muscles. Anterior to those areas are the frontal eye fields and other areas called prefrontal cortex (even though they are in the frontal lobe) that are involved in more abstract aspects of planning.
It is generally held that there is relative expansion of the frontal lobe compared to the rest of the brain in humans compared to other primates, and primates compared to other mammals. Some exceptional non-primate mammals such as the echidna have large frontal lobes, however. This has led to debate among neuroscientists about whether these frontal areas are really homologous across mammalian species. Whatever the result of that debate, we know that damage to prefrontal cortex in humans produces distinctive cognitive deficits such as impulsive behavior and profound changes in affect.
Language, emotions, lateralization, and thought
True grammatically ordered language distinguishes humans from all other species on earth. Recent evidence has suggested an important role for a gene called FOXP2 in generating language capability, although how this gene changes the brain to allow language isn’t clear.
The human brain does not contain any distinct anatomical structures or types of neurons associated with language. The human brain areas most crucial for language, Wernicke’s and Broca’s areas on the left side (in most humans), have homologous areas in other primates, but these areas do not support language. Yet all normal human infants learn, without any explicit instruction, whatever language to which they are exposed, but other animals do not learn grammatical, word-order based language despite extensive instruction.
The capacity for learning language is built in, but neuroscience does not now know how. One clue may be brain lateralization, however. Left- versus right-side specialization for some types of audio processing and production exists in other mammals, and even some birds, but is nowhere near as extensive as in humans.
Neuroscience’s view of emotions has changed markedly in the last decades. Earlier views regarded emotions as leftovers from our evolution from non-rational species. Star Trek’s Mr. Spock could be taken as a model of a superior, more evolved humanoid. However, we now know that emotions are a means of nonverbal communication within our brains. Hunches and anxiety in certain situations are signs of danger and the need to be cautious.
Developmental, Neurological, and Mental Disorders and Treatments
One of the most important reasons to understand neurobiology is to understand mental disorders and treatments. The good news is that great progress is being made in this field now. We know the genetic bases of many developmental disorders, such as Fragile X and William’s syndrome. The bad news is that many disorders remain that we do not know about, and, even among disorders with known genetics, how the gene alteration produces the disorder, and what to do about it, are not clear. Chapters 15 through 18 discuss the background and current treatment approaches (if any) of many common neurological disorders.
Developing the brain and nervous system
The set of genes that define an organism is not a blueprint that is executed by a builder, but a set of procedures that brings about the development of the organism.
Developing cells have genetically coded responses to substances they detect by their membranes or ingest, including cell identity and brain location marker molecules. Cell responses include movement, division, and secretion of other markers and agents. The interactions among cells that have these responses in the embryological environment builds the brain.
Much of the genome is only expressed extensively during development, a time when the organism is also particularly susceptible to toxins that mimic or interfere with these markers and agents. The result of this interference is the construction of an improperly set-up brain, which is typically much worse than inferring temporarily with a properly constructed brain later in life, which often can be reversed.
Movement disorders and symptoms
Movement disorders can originate with brain damage that compromises the control of movement, or neurons that drive muscles, or the muscles themselves. Chapter 16 discusses some of the most common movement disorders. Cerebral palsy and epilepsy typically involve brain damage. Multiple sclerosis is caused by demyelination of axons of motor and other neurons. Myasthenia gravis is an autoimmune disease involving the cholinergic receptors on muscle cells.
Some well-known movement disorders, such as Parkinson’s and Huntington’s diseases, occur only later in life. Neither of these diseases
