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Chapter 3

Chapter 3: The Brain and the Nervous System

Chapter Review

THE ORGANISM AS A MACHINE

  • Many scientists have tried to explain human and animal behavior in mechanistic terms. For Descartes, this definition involved the reflex concept: A stimulus excites a sense organ that transmits excitation upward to the brain, which in turn relays the excitation downward to a muscle or gland and so produces action.

BUILDING BLOCKS OF THE NERVOUS SYSTEM

  • The basic unit of communication in the nervous system is the neuron. Each neuron typically has dendrites, a cell body, and an axon. The vast majority of neurons are interneurons that connect to yet other interneurons.
  • The nervous system also contains glia. These cells have many functions, both during development and in supporting the function of the mature nervous system. The glia may also constitute a separate, slow signal system.

COMMUNICATION AMONG NEURONS

  • When the neuron’s membrane is stable, an excess of positively charged ions are on the outside, resulting in the negative voltage difference of the resting potential. When the membrane is sufficiently stimulated, though, ion channels in the membrane spring open. This change allows ion movement that leads to an excess of positively charged particles inside the membrane and produces the positive voltage swing of the action potential. The excitation spreads to neighboring regions and leads to the propagation of the action potential along the axon. This propagation is much faster if the axon is myelinated, which causes the depolarization to proceed down the axon by means of a number of skips or jumps. In all cases, though, the action potential obeys the all-or-none law: Once the action potential is launched, further increases in stimulus intensity have no effect on its magnitude.
  • Communication between neurons is made possible by the release of neurotransmitters. The transmitters cross the synapse and latch onto receptors on the postsynaptic cell, potentially triggering a response in that cell.Some transmitters are inactivated shortly after being discharged by “cleanup” enzymes. More commonly, neurotransmitters are reused by a process of synaptic reuptake.
  • The lock-and-key model proposes that transmitter molecules will affect the postsynaptic membrane only if the molecule’s shape fits into certain synaptic receptor molecules. However, drugs called agonists can enhance a neurotransmitter’s effect; antagonists impede its effect. Some agonists enhance a transmitter’s effect by blocking its synaptic reuptake; others act by counteracting the cleanup enzyme. Yet other drugs affect the synaptic receptors by mimicking the transmitter’s action.

COMMUNICATION THROUGH THE BLOODSTREAM

  • Blood circulation not only brings energy to the nutrient-hungry brain but also aids communication by carrying hormones secreted by the endocrine glands to various target organs throughout the body. Despite some important points of contrast, the endocrine system has much in common with the chemical communication between neurons, suggesting a shared evolutionary origin for these two types of communication.

METHODS FOR STUDYING THE NERVOUS SYSTEM

  • One source of data about the nervous system comes from single-cell recording, which can be done for an individual cell, or, in multi-unit recording, with a number of cells. Another source of data focuses on cases of brain damage. In some animal studies, this damage is produced in the laboratory; but neuropsychologists often study naturally occurring cases of brain damage. In still other cases, scientists can study the effects of temporary brain damage by using the technique of transcranial magnetic stimulation (TMS).
  • A variety of techniques are used to study the whole brain. Electroencephalography is a procedure that uses sensitive electrodes, placed on the scalp, to measure voltages produced by ordinary brain activity. Neuroimaging techniques are used to study the living brain. Some of these techniques (MRI and CT scans) study the brain’s anatomy—the size and location of individual structures. Other techniques, such as PET and fMRI scans, reveal which brain locations are particularly active at any moment in time. All of these techniques make it clear that most mental activities rely on many brain sites, so that activities like reading or making decisions are supported by the coordinated functioning of many different parts of the brain.

THE ARCHITECTURE OF THE NERVOUS SYSTEM

  • The nervous system is divided into the central nervous system (the brain and spinal cord) and the peripheral nervous system, which includes both efferent and afferent nerves. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.
  • The very top of the spinal cord forms the brain stem, which includes the medulla and the pons, and just behind these structures is the cerebellum. The midbrain is on top of the pons, and on top of all is the forebrain. The outer surface of the forebrain is the cerebral cortex. The cortex is a large, thin sheet of tissue, crumpled inside the skull. Some of the folds— or convolutions—in the cortex are actually deep grooves that divide the brain into sections, such as the frontal lobes, the parietal lobes, the occipital lobes, and the temporal lobes.
  • The entire brain is roughly symmetrical around the midline, so that most structures come in pairs—one on the left side, one on the right. The left and right structures are generally similar, but they can be distinguished both anatomically and functionally. Crucially, the two halves of the brain work as an integrated whole.

THE CEREBRAL CORTEX

  • Localization of function refers to the task of determining the function of each brain area. In the cortex, some parts serve as projection areas—they are the first receiving stations for information coming from the sense organs and the departure points for signals going to the muscles. Most projection areas have a contralateral organization. Each area is organized so that (for example) adjacent areas in the motor projection area represent adjacent parts of the body, and adjacent areas in the visual projection area represent adjacent regions of space. However, the assignment of cortical space is disproportionate, so that (for example) parts of the body that are most sensitive to touch receive more cortical space.
  • We have learned much of what we know about other parts of the cortex by studying cases of brain damage. Damage at identifiable sites can produce apraxias (disorders in action), agnosias (disorders in perception), or aphasias (disorders of language). Still other forms of brain damage produce disorders of planning or social cognition.

PLASTICITY

  • The nervous system is plastic—subject to alteration in the way it functions. Some of this plasticity involves changes in how much neurotransmitter a presynaptic neuron releases. Neurons can also change how sensitive they are to neurotransmitters. Moreover, by growing new dendritic spines, neurons can create entirely new connections.
  • Plasticity can also involve larger-scale changes, including changes in the brain’s overall architecture. The central nervous system can grow new neurons, although it appears unable to do so in cases of cortical injury. This promotes stability in the brain’s connections, but obviously it can be an obstacle to recovery from brain damage.
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