Neural Control of Human Movement

97818_ch19.qxd 8/4/09 4:16 PM Page 376 CHAPTER 19 Neural Control of Human Movement CHAPTER OBJECTIVES ➤ Draw the major structural components of the ➤ Outline motor unit facilitation and inhibition and brain, including the four lobes of the cerebral the contribution of each to exercise performance cortex and responsiveness to resistance training ➤ Discuss specific pyramidal and extrapyramidal ➤ Discuss variations in twitch characteristics, resist- tract functions ance to fatigue, and tension development in the different motor unit categories ➤ Diagram the anterior motor neuron and discuss its role in human movement ➤ Describe mechanisms that adjust force of muscle action along the continuum from slight to ➤ Draw and label the basic components of a maximum reflex arc ➤ Define fatigue and discuss factors that act and ➤ Define the terms (1) motor unit, (2) neuromuscular interact to induce neuromuscular fatigue junction, and (3) autonomic nervous system ➤ List and describe functions of the proprioceptors ➤ Summarize the events in motor unit excitation within joints, muscles, and tendons prior to muscle action 376 97818_ch19.qxd 8/4/09 4:16 PM Page 377 CHAPTER 19 Neural Control of Human Movement 377 The effective application of force during complex learned Brainstem movements (e.g., tennis serve, shot put, golf swing) depends on a series of coordinated neuromuscular patterns, not just on The medulla, pons, and midbrain compose the brain- muscle strength. The neural circuitry in the brain, spinal cord, stem. The medulla, located immediately above the spinal and periphery functions somewhat similar to a sophisticated cord, extends into the pons and serves as a bridge between computer network. In response to changing internal and ex- the two hemispheres of the cerebellum. The midbrain, only ternal stimuli, hundreds of millions of bits of sensory input 1.5 cm long, attaches to the cerebellum and forms a connec- automatically synchronize for near-instantaneous processing tion between the pons and cerebral hemispheres. The mid- by central neural control mechanisms. The input becomes brain contains parts of the extrapyramidal motor system, properly organized, routed, and transmitted with extreme effi- specifically the red nucleus and substantia. The reticular ciency to the effector organs, the skeletal muscles.27 formation integrates various incoming and outgoing signals that flow through it. These signals originate from the stretching of sensors in joints and muscles, from pain recep- NEUROMOTOR SYSTEM tors in the skin, and as visual signals from the eye and audi- ORGANIZATION tory impulses from the ear. Once activated, the reticular system produces either inhibitory or facilitory effects on The human nervous system consists of two major parts: other neurons. Twelve pairs of cranial nerves innervate pre- 1. Central nervous system (CNS) consisting of the dominantly the head region. Each cranial nerve has a name brain and spinal cord and associated number (originally derived by Galen about 2. Peripheral nervous system (PNS) consisting of 1800 years ago). nerves that transmit information to and from the CNS Cerebellum FIGURE 19.1 presents an overview of these two subdivisions. The cerebellum consists of two peach-sized mounds of folded tissue with lateral hemispheres and a central vermis. It Central Nervous System—The Brain functions by means of intricate feedback circuits to monitor Over time the human brain has remained remarkably com- and coordinate other areas of the brain and spinal cord plex, but with selective growth of different anatomic areas. involved in motor control. The cerebellum receives motor From a comparative perspective, the size of the human brain output signals from the central command in the cortex. This exceeds that of most (but not all) mammals. Evolution of the specialized brain tissue also obtains sensory information cortex, particularly the frontal and temporal lobes, coincides from peripheral receptors in muscles, tendons, joints, and skin with unique human functions like spoken and written lan- and from visual, auditory, and vestibular end organs. The guage, reasoning, and abstract thinking. Such differentiation cerebellum functions as the major comparing, evaluating, frames the hypothesis that larger, more complex brains allow and integrating center for postural adjustments, locomotion, greater neural circuitry within the cortex and hence increased maintenance of equilibrium, perceptions of speed of body intellectual and higher center functioning. movement, and other diverse reflex-related movement func- For decades, conventional wisdom maintained that the tions. Movement tasks first learned by trial and error, like number of brain cells was fixed at birth, unlike the cells of other riding a bicycle or swinging a golf club, remain coded as organ systems that continually renew themselves throughout coordinated patterns in the cerebellar memory banks. In life. Neurobiologists now believe that brain cells, spinal neu- essence, this motor control center “fine-tunes” all forms of 29 rons, and neural circuits are created throughout life, with elimi- muscular activity. nation of unneeded or redundant synapses in developing neural tissues. From birth through late adolescence, the brain probably Diencephalon adds billions of new cells, literally constructing new circuits from these newly formed cells.14 After adolescence, the plastic- The diencephalon, located immediately above the ity of neuronal addition and formation of new circuits slows but midbrain, forms part of the cerebral hemispheres. The does not stop, even into old age. Regular physical activity thalamus, hypothalamus, epithalamus, and subthalamus appears to contribute to the development and maintenance of compose the major structures of the diencephalon. The optimal neural circuitry in middle and older age. hypothalamus, situated below the thalamus, regulates FIGURE 19.2 categorizes the brain into six main areas: metabolic rate and body temperature. The hypothalamus medulla oblongata, pons, midbrain, cerebellum, dien- also influences activity of the autonomic nervous system cephalon, and telencephalon. Figure 19.2C depicts four (see p. 382); it receives regulatory input from the thalamus lobes of the cerebral cortex and associated sensory areas. As a and limbic brain system and responds to the effects of di- frame of reference, the body has roughly 10 million sensory verse hormones (see Chapter 20). Changes in arterial blood (afferent) neurons, 50 billion central neurons, and 500,000 pressure and blood gas tensions influence hypothalamic ac- motor (efferent) neurons. This represents a ratio of about 20 tivity via peripheral receptors located in the aortic arch and to 1 between the sensory and motor circuits. carotid arteries. 97818_ch19.qxd 8/4/09 4:16 PM Page 378 Peripheral Central Nervous Nervous Brain System System So ma ti c se n Skin s o ry f Spinal cord ib er Visceral s ens ory fiber Cardio- fiber tor vascular mo ic thet r fiber Sympa oto m Vertebral tic he column pat Parasym ic at m so f o Muscle er m ib te Spinal nerve r f s to sy Mo us nervo Nervous System Central Nervous System (CNS) Peripheral Nervous System (PNS) • Brain (including retinas) • Cranial nerves III—XII • Spinal cord • Spinal nerves • Integrative/control centers Afferent Division (sensory) Efferent Division (motor) • Somatic and visceral neurons • Motor neurons • Conducts impulses from • Conducts impulses from the receptors to CNS CNS to effectors Autonomic Nervous System Somatic Nervous System • Involuntary • Voluntary • Conducts impulses from the CNS • Conducts impulses from the CNS to cardiac muscle, smooth to skeletal muscles muscles, and glands Sympathetic Parasympathetic Figure 19.1 • The two divisions of the human nervous system. The central nervous system (CNS) contains the brain (including retinas), spinal cord, and integrating and control centers; the cranial nerves and spinal nerves compose the peripheral nervous system (PNS). The PNS further subdivides into the afferent (sensory) and efferent (motor) divisions. The efferent division consists of the somatic nervous system and autonomic nervous system (sympathetic and parasympathetic divisions). 97818_ch19.qxd 8/4/09 4:16 PM Page 379 CHAPTER 19 Neural Control of Human Movement 379 Telencephalon Telencephalon Diencephalon The telencephalon contains the two hemispheres of the Thalamus cerebral cortex, including the corpus striatum and medulla. Epithalamus The cerebral cortex makes up approximately 40% of the total brain weight. It divides into four lobes: frontal, temporal, parietal, and occipital. Neurons in the cortex provide specialized sensory and motor functions. Beneath each cerebral hemisphere and in close association with the thalamus lie the basal ganglia, which play an important role in the control of motor movements. Midbrain Brain Limbic System Pons stem Cerebellum In 1878, French surgeon, neurologist, and anthropologist Medulla oblongata A Paul Pierre Broca (1824–1880) described a group of areas on Spinal cord the medial surface of the cerebrum that were distinctly different from the surrounding cortex. Using the Latin word for Longitudinal fissure “border” (limbus), Broca named the area the limbic lobe be- cause its structures formed a ring or border around the brainstem and corpus callosum on the medial surface of the temporal lobe.3 Broca also discovered the speech center now known as Broca’s area, or the third circumvolution of the Motor frontal lobe. Broca should be credited as the founder of mod-

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