The major functions of the nervous system are to detect, analyze, and transmit information. Information is gathered by sensory systems, integrated by the brain, and used to generate signals to motor and autonomic pathways for control of movement and of visceral and endocrine functions. These actions are controlled by neurons, which are interconnected to form signaling networks that compose motor and sensory systems. In addition to neurons, the nervous system contains neuroglial cells that serve a variety of immunologic and support functions and modulate the activity of neurons. Understanding the pathophysiology of nervous system disease requires knowledge of neural and glial cell biology and the anatomy of neural networks. The first part of this chapter reviews several basic aspects of histology, cellular physiology, and anatomy of the nervous system.
Understanding the causes of neurologic diseases requires knowledge of molecular and biochemical mechanisms. Discoveries in the fields of molecular biology and genetics have made available important information about the mechanisms of several disease states. Several neurologic disorders for which some of the molecular mechanisms of pathogenesis are known are discussed later in this chapter, including cerebellar ataxia, motor neuron disease, Parkinson disease, myasthenia gravis, epilepsy, Alzheimer disease, and stroke. Exciting advances in our understanding of these diseases, including how they overlap, are leading to new therapeutic targets and the hope of better treating these devastating diseases.
NORMAL STRUCTURE & FUNCTION OF THE NERVOUS SYSTEM
The major function of neurons is to receive, integrate, and transmit information to other cells. Neurons consist of three parts: dendrites, which are elongated processes that receive information from the environment or from other neurons; the cell body, which contains the nucleus; and the axon, which may be up to 1 m long and conducts impulses to muscles, glands, or other neurons (Figure 7–1). Most neurons are multipolar, containing one axon and several dendrites. Bipolar neurons have one dendrite and one axon and are found in the cochlear and vestibular ganglia, retina, and olfactory mucosa. Spinal sensory ganglia contain pseudounipolar neurons that have a single process that emanates from the cell body and divides into two branches, one extending to the spinal cord and the other extending to the periphery. Axons and dendrites usually branch extensively at their ends. Dendritic branching can be very complex, with the result that a single neuron may receive thousands of inputs. Axon branching allows several target cells to simultaneously receive a message from one neuron. Each branch of the axon terminates on the next cell at a synapse, which is a structure specialized for information transfer from the axon to muscle, glands, or another neuron. Synapses between neurons most often occur between axons and dendrites but may occur between an axon and a cell body, between two axons, or between two dendrites.