Porth's Essentials of Pathophysiology, 4e

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Organization and Control of Neural Function

C h a p t e r 3 4

of cell, the ependymal cell, forms the lining of the neural tube cavity, the ventricular system. In some areas, these cells combine with a rich vascular network to form the choroid plexus, where production of the cerebrospinal fluid (CSF) takes place. Metabolic Requirements of Nervous Tissue Nervous tissue has a high rate of metabolism. Although the brain makes up only 2% of the body’s weight, it receives approximately 15% of the resting cardiac out- put and consumes 20% of its oxygen. Despite its sub- stantial metabolic requirements, the brain can neither store oxygen nor engage in anaerobic metabolism. An interruption in the blood or oxygen supply to the brain rapidly leads to clinically observable signs and symp- toms. Without oxygen, brain cells continue to function for approximately 10 seconds. Unconsciousness occurs almost simultaneously with cardiac arrest, and the death of brain cells begins within 4 to 6 minutes. Interruption of blood flow also leads to the accumulation of meta- bolic by-products that are toxic to neural tissue. Glucose is the major fuel source for the nervous system. Unlike muscle cells, neurons have no glycogen stores and must rely on glucose from the blood or the glycogen stores of supporting glial cells to meet their energy needs. Persons receiving insulin for diabetes may experience signs of neural dysfunction and unconscious- ness (i.e., insulin reaction or shock) when blood glucose drops because of insulin excess (see Chapter 33). ■■ Anatomically, the nervous system can be divided into two basic components: the central nervous system (CNS), consisting of the brain and spinal cord; and the peripheral nervous system (PNS), which relays afferent or sensory input to the CNS for processing and transmitting efferent or motor output from the CNS to effector organs throughout the body. ■■ The nervous system contains two major types of cells: neurons, which are functioning cells of the nervous system, and neuroglial cells, which protect the nervous system and supply metabolic support. ■■ Neurons (nerve cells), which are the functional components of the nervous system, are composed of three parts: a cell body, which controls cell activity; dendrites, which conduct information toward the cell body; and an axon, which carries impulses from the cell body. SUMMARY CONCEPTS

Nerve Cell Communication Neurons are characterized by their ability to commu- nicate with other neurons and body cells through elec- trical impulses or action potentials, which are abrupt, pulsatile changes in the membrane potential that last a few ten thousandths to a few thousandths of a second. The frequency and pattern of action potentials consti- tute the code used by neurons to transfer information from one location to another. Action Potentials The cell membranes of excitable tissue, including those of nerve and muscle cells, contain ion channels that are responsible for generating action potentials (see Chapter 1, Understanding Membrane Potentials). These ion channels are guarded by voltage-dependent gates that open and close with changes in the membrane potential. Separate voltage-gated channels exist for the sodium, potassium, and calcium ions. Each type of ion channel has a characteristic membrane potential that opens and closes its channels. Also present are ligand- gated channels that respond to chemical messengers such as neurotransmitters, mechanically gated channels that respond to physical changes in the cell membrane, and light-gated channels that respond to fluctuations in light levels. Action potentials can be divided into three phases: the resting or polarized state, depolarization, and repo- larization (Fig. 34-4). The resting membrane potential (approximately −90 mV for large nerve fibers) is the undisturbed period of the action potential during which the nerve is not transmitting impulses. During this period, the membrane is said to be “ polarized” because of the −90 mV negative membrane potential (i.e., posi- tive on the outside and negative on the inside) that is present. The resting phase of the membrane potential continues until some event causes the membrane to increase its permeability to sodium. A threshold potential ■■ The neuroglial cells that provide support and protection for the neurons consist of the Schwann and satellite cells of the PNS and the oligodendrocytes, astrocytes, microglial cells, and ependymal cells of the CNS.The Schwann cells of the PNS and the oligodendrocytes of the CNS form the myelin sheath that allows for rapid conduction of impulses. ■■ The nervous system has a high level of metabolic activity, requiring a continuous supply of oxygen and glucose. Although the brain makes up only 2% of the body’s weight, it receives approximately 15% of the resting cardiac output and consumes 20% of its oxygen.