Key Concepts
     1. The nervous system reaches into every part of the body and coordinates every aspect of the functioning of the body, from seemingly automatic processes like heart rate, to the most sophisticated of behavioral responses.
     2. The immense amount of information processing that the nervous system performs requires an immensely complicated “computer”—more than 100 billion neurons of widely varying shapes and sizes, depending on the functions they perform. Nevertheless, neurons can be categorized according to general layout, size, and function. “Typical” neurons all share three main structural elements: (1) dendrites, which are the main receptive surfaces of the cell, (2) a cell body, which contains the nucleus and therefore the cell’s DNA, and (3) an axon, which carries nerve impulses away from the cell body. From a functional perspective, neurons share four basic elements: (1) an input zone (dendrites and cell body), (2) an integration zone (the axon hillock), (3) a conduction zone (the axon), and (4) an output zone (the axon terminals).
     3. Glial cells also serve important functions in the nervous system: they myelinate axons, exchange nutrients and other substances with neurons, and remove cellular debris.
     4. Information is transmitted from a neuron to other neurons, or muscles or glands, across specialized contacts called synapses. In most neurons, the axon terminals contain specialized “boutons” that release molecules of neurotransmitter in response to the arrival of an impulse along the axons; the neurotransmitter crosses the synaptic cleft and alters the electrical properties of postsynaptic cell via receptor proteins that recognize and react to the neurotransmitter.
     5. The nervous system consists of the central nervous system (the brain and spinal cord), and the autonomic nervous system (the sympathetic and parasympathetic nervous systems and the cranial nerves). A simplified view is that the sympathetic nervous system prepares the body for action, while the parasympathetic division has opposite effects (tending toward relaxation) for most bodily functions.
     6. The most prominent features of the brain are the two cerebral hemispheres, the surfaces of which are heavily convoluted with gyri (humps) and sulci (cracks). Each hemisphere contains the four major lobes—frontal, parietal, temporal, and occipital—made up of gray matter overlying white matter. Within the hemispheres are found the components of the basal ganglia and limbic system.
     7. Consideration of the embryonic development of the brain reveals the major divisions: (1) the forebrain, consisting of the telencephalon (cortex and basal ganglia are prominent) and diencephalon (thalamus and hypothalamus are prominent), (2) the midbrain, or mesencephalon (superior and inferior colliculi are prominent), and (3) the hindbrain, consisting of the metencephalon (the cerebellum and pons are prominent) and the myelencephalon (the medulla is prominent).
     8. The brain and spinal cord are protected by the three layers of the meninges (the dura mater, the arachnoid, and the pia mater), and are suspended in cerebrospinal fluid, which is produced in and circulates throughout the ventricular system of the brain.
     9. The vascular system of the brain is elaborate, consisting of the anterior, middle, and posterior cerebral arteries, plus numerous arterioles and a backup system called the circle of Willis. Alterations or disruptions in the flow of blood in these arteries, commonly called a stroke, cause lesions in the brain regions supplied by the affected artery, with concomitant deficits in brain function.
     10. The cells of the neocortex are arranged in six distinct layers, with different layers performing different functions. Cytoarchitectonic variations in the structure of the cortex seem to correspond reasonably well with functional variations—that is, the structural map of the cortex is at least roughly in agreement with the functional map of the cortex. Furthermore, clusters of cortical neurons are organized as functional units within “cortical columns”; there are about one million such cortical columns across the surface of the human neocortex.
     11. Numerous techniques have been developed to study the fine structure of neural tissue, such as the use of stains for labeling particular types of cells or structures, radioactive methods for identifying cells that are metabolically active during particular behaviors, immunohistochemical techniques for labeling particular constituent proteins, and tract-tracing techniques. There have also been tremendous advances in techniques that allow the study of brain structure in the brains of living people and animals. These include CT scans and MRI scans, which provide detailed cross-sectional views of the brain, and techniques that track the activation of brain structures during behavioral tests, such as PET, EEG, and fMRI.