All of us are aware of the many different states of brain activity, including sleep, wakefulness, extreme excitement, and even different levels of mood such as exhilaration, depression, and fear. All these states result from different activating or inhibiting forces generated usually within the brain. In Chapter 59 , we began a partial discussion of this subject when we described different systems that are capable of…

Control of behavior is a function of the entire nervous system. Even the wakefulness and sleep cycle discussed in Chapter 60 is one of our most important behavioral patterns. In this chapter, we deal first with the mechanisms that control activity levels in different parts of the brain. Then we discuss the causes of motivational drives, especially motivational control of the learning process and feelings of…

It is ironic that of all the parts of the brain, we are the least certain about the functions of the cerebral cortex, even though it is by far the largest and perhaps the most studied portion of the nervous system. However, we do know the effects of damage or stimulation of various portions of the cerebral cortex. In the first part of this chapter, the…

In addition to areas in the cerebral cortex that stimulate muscle contraction, two other brain structures are essential for normal motor function—the cerebellum and the basal ganglia . Neither of these structures can control muscle function by itself. Instead, these structures always function in association with other systems of motor control. The cerebellum plays major roles in timing of motor activities and in rapid, smooth progression…

Most “voluntary” movements initiated by the cerebral cortex are achieved when the cortex activates “patterns” of function stored in lower brain areas—the cord, brain stem, basal ganglia, and cerebellum. These lower centers, in turn, send specific control signals to the muscles. For a few types of movements, however, the cortex has almost a direct pathway to the anterior motor neurons of the cord, bypassing some motor…

Sensory information is integrated at all levels of the nervous system and causes appropriate motor responses that begin in the spinal cord with relatively simple muscle reflexes, extend into the brain stem with more complicated responses, and finally extend to the cerebrum, where the most complicated muscle skills are controlled. In this chapter, we discuss spinal cord control of muscle function. Without the special neuronal circuits…

The senses of taste and smell allow us to separate undesirable or even lethal foods from those that are pleasant to eat and nutritious. They also elicit physiological responses involved in the digestion and utilization of foods. The sense of smell allows animals to recognize the proximity of other animals or even individual animals. Finally, both senses are strongly tied to primitive emotional and behavioral functions…

This chapter describes the mechanisms whereby the ear receives sound waves, discriminates their frequencies, and transmits auditory information into the central nervous system, where its meaning is deciphered. Tympanic Membrane and the Ossicular System Conduction of Sound From the Tympanic Membrane to the Cochlea Figure 53-1 shows the tympanic membrane (commonly called the eardrum ) and the ossicles , which conduct sound from the tympanic membrane…

Visual Pathways Figure 52-1 shows the principal visual pathways from the two retinas to the visual cortex . The visual nerve signals leave the retinas through the optic nerves . At the optic chiasm , the optic nerve fibers from the nasal halves of the retinas cross to the opposite sides, where they join the fibers from the opposite temporal retinas to form the optic tracts…

The retina is the light-sensitive portion of the eye that contains the following: (1) the cones , which are responsible for color vision; and (2) the rods , which can detect dim light and are mainly responsible for black and white vision and vision in the dark. When either rods or cones are excited, signals are transmitted first through successive layers of neurons in the retina…

Physical Principles of Optics Understanding the optical system of the eye requires familiarity with the basic principles of optics, including such factors as the physics of light refraction, focusing, and depth of focus. A brief review of these physical principles is presented in this chapter, followed by discussion of the optics of the eye. Refraction of Light Refractive Index of a Transparent Substance Light rays travel…

Many ailments of the body cause pain. Furthermore, the ability to diagnose different diseases depends to a great extent on a physician’s knowledge of the different qualities of pain. For these reasons, the first part of this chapter is devoted mainly to pain and to the physiological bases of some associated clinical phenomena. Pain occurs whenever tissues are being damaged and causes the individual to react…

The somatic senses are the nervous mechanisms that collect sensory information from all over the body. These senses are in contradistinction to the special senses , which mean specifically vision, hearing, smell, taste, and equilibrium. Classification of Somatic Senses The somatic senses can be classified into three physiological types: (1) the mechanoreceptive somatic senses , which include both tactile and position sensations that are stimulated by…

Our perceptions of signals within our bodies and of the world around us are mediated by a complex system of sensory receptors that detect such stimuli as touch, sound, light, pain, cold, and warmth. In this chapter, we discuss the basic mechanisms whereby these receptors change sensory stimuli into nerve signals that are then conveyed to and processed in the central nervous system. Types of Sensory…

The nervous system is unique in the vast complexity of thought processes and control actions that it can perform. Each minute it receives literally millions of bits of information from the different sensory nerves and sensory organs and then integrates all these to determine responses to be made by the body. Before beginning this discussion of the nervous system, the reader should review Chapter 5, Chapter…

When people descend beneath the sea, the pressure around them increases progressively as they go to greater depths. To keep the lungs from collapsing, air must be supplied at very high pressure to keep them inflated. This maneuver exposes the blood in the lungs to extremely high alveolar gas pressures, a condition called hyperbarism . Beyond certain limits, these high pressures cause major alterations in body…

As humans have ascended to higher and higher altitudes in aviation, mountain climbing, and space exploration, it has become progressively more important to understand the effects of altitude and low gas pressures on the human body. This chapter deals with these problems and acceleratory forces, weightlessness, and other challenges to body homeostasis that occur at high altitudes and in space flight. Effects of Low Oxygen Pressure…

Diagnosis and treatment of most respiratory disorders depend heavily on understanding the basic physiological principles of respiration and gas exchange. Some respiratory diseases result from inadequate ventilation. Others are caused by abnormalities of diffusion through the pulmonary membrane or abnormal blood transport of gases between the lungs and tissues. Therapy is often entirely different for these diseases, so it is not satisfactory simply to make a…

The nervous system normally adjusts the rate of alveolar ventilation to meet the demands of the body almost exactly so that the oxygen partial pressure (P o 2 ) and carbon dioxide partial pressure (P co 2 ) in the arterial blood are hardly altered, even during heavy exercise and most other types of respiratory stress. This chapter describes the function of this neurogenic system for…

Once oxygen (O 2 ) has diffused from the alveoli into the pulmonary blood, it is transported to the tissue capillaries almost entirely in combination with hemoglobin. The presence of hemoglobin in the red blood cells allows the blood to transport 30 to 100 times as much O 2 as could be transported in the form of dissolved O 2 in the water of the blood.…