Guyton and Hall Physiology Chapter 56 PDF - Cortical and Brain Stem Control of Motor Function

CHAPTER 56 UNIT XI Cortical and Brain Stem Control of Motor Function Most “voluntary”...

CHAPTER 56 UNIT XI Cortical and Brain Stem Control of Motor Function Most “voluntary” movements initiated by the cerebral near the apex of the brain; and the leg and foot areas, in cortex are achieved when the cortex activates “patterns” the part of the primary motor cortex that dips into the of function stored in lower brain areas—the cord, brain longitudinal fissure. This topographical organization is stem, basal ganglia, and cerebellum. These lower centers, demonstrated even more graphically in Figure 56-2, in turn, send specific control signals to the muscles. which shows the degrees of representation of the different For a few types of movements, however, the cortex has muscle areas as mapped by Penfield and Rasmussen. This almost a direct pathway to the anterior motor neurons of mapping was done by electrically stimulating the different the cord, bypassing some motor centers on the way. This areas of the motor cortex in human beings were undergo- is especially true for control of the fine dexterous move- ing neurosurgery. Note that more than half of the entire ments of the fingers and hands. This chapter and Chapter primary motor cortex is concerned with controlling the 57 explain the interplay among the different motor areas muscles of the hands and the muscles of speech. Point of the brain and spinal cord to provide overall synthesis of stimulation in these hand and speech motor areas on voluntary motor function. rare occasion causes contraction of a single muscle, but most often, stimulation contracts a group of muscles. To MOTOR CORTEX AND express this in another way, excitation of a single motor CORTICOSPINAL TRACT cortex neuron usually excites a specific movement rather than one specific muscle. To do this, it excites a “pattern” Figure 56-1 shows the functional areas of the cerebral of separate muscles, each of which contributes its own cortex. Anterior to the central cortical sulcus, occupying direction and strength of muscle movement. approximately the posterior one third of the frontal lobes, is the motor cortex. Posterior to the central sulcus is the somatosensory cortex (an area discussed in detail in earlier Motor Sensory chapters), which feeds the motor cortex many of the sig- Primary nals that initiate motor activities. Supplementary motor The motor cortex is divided into three subareas, each area cortex Somatic of which has its own topographical representation of area 1 Legs Somatic muscle groups and specific motor functions: (1) the pri- association Feet area mary motor cortex; (2) the premotor area; and (3) the supplementary motor area. Trunk Arm 7 PRIMARY MOTOR CORTEX 4 6 Hand 5 The primary motor cortex, shown in Figure 56-1, lies in 1 the first convolution of the frontal lobes anterior to the Face 3, 2, central sulcus. It begins laterally in the sylvian fissure, Mouth spreads superiorly to the uppermost portion of the brain, and then dips deep into the longitudinal fissure. (This area is the same as area 4 in Brodmann’s classification of the brain cortical areas, shown in Figure 48-5.) Premotor Figure 56-1 lists the approximate topographical repre- area sentations of the different muscle areas of the body in the primary motor cortex, beginning with the face and mouth Figure 56-1. Motor and somatosensory functional areas of the cere- region near the sylvian fissure; the arm and hand area, in bral cortex. The numbers 4, 5, 6, and 7 are Brodmann’s cortical areas, the midportion of the primary motor cortex; the trunk, as explained in Chapter 48. 697 UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology Supplemental Primary and premotor motor areas cortex Th F Armnk ips Fe s g et um ing s Le ru H Hip Knee Trunk Shoulder Elbow Wrist Hand skills Hand b ers Mi Ring finger Neck T ex fing r Indddle finge Th fing er Head rotation um er An oes b le Contralateral ck Litt Lips kle Ne ow T eye movements Vocalization B r Jaw Choice all e yeb Tongu ing of words e w a n d Swalloing lid Face Chew Eye Eye fixation Lips Vocalization Word formation Jaw (Broca’s area) Tongu Swa e llowin g Mastication Figure 56-3. Representation of the different muscles of the body in Salivation the motor cortex and location of other cortical areas responsible for specific types of motor movements. Figure 56-2. Degree of representation of the different muscles of studies indicate that these neurons transform sensory the body in the motor cortex. (Modified from Penfield W, Rasmussen T: The Cerebral Cortex of Man: A Clinical Study of Localization of representations of acts that are heard or seen into motor Function. New York: Hafner, 1968.) representations of these acts. Many neurophysiologists believe that these mirror neurons may be important for understanding the actions of other people and for learn- PREMOTOR AREA ing new skills by imitation. Thus, the premotor cortex, The premotor area, also shown in Figure 56-1, lies 1 to basal ganglia, thalamus, and primary motor cortex consti- 3 centimeters anterior to the primary motor cortex. It tute a complex overall system for the control of complex extends inferiorly into the sylvian fissure and superiorly patterns of coordinated muscle activity. into the longitudinal fissure, where it abuts the supple- mentary motor area, which has functions similar to those SUPPLEMENTARY MOTOR AREA of the premotor area. The topographical organization of The supplementary motor area has yet another topographi- the premotor cortex is roughly the same as that of the cal organization for the control of motor function. It lies primary motor cortex, with the mouth and face areas mainly in the longitudinal fissure but extends a few centime- located most laterally; as one moves upward, the hand, ters onto the superior frontal cortex. Contractions elicited by arm, trunk, and leg areas are encountered. stimulating this area are often bilateral rather than unilateral. Nerve signals generated in the premotor area cause For example, stimulation frequently leads to bilateral grasp- much more complex “patterns” of movement than the ing movements of both hands simultaneously; these move- discrete patterns generated in the primary motor cortex. ments are perhaps rudiments of the hand functions required For example, the pattern may be to position the shoul- for climbing. In general, this area functions in concert with ders and arms so that the hands are properly oriented the premotor area to provide body-wide attitudinal move- to perform specific tasks. To achieve these results, the ments, fixation movements of the different segments of the most anterior part of the premotor area first develops a body, positional movements of the head and eyes, and so “motor image” of the total muscle movement that is to be forth, as background for the finer motor control of the arms performed. Then, in the posterior premotor cortex, this and hands by the premotor area and primary motor cortex. image excites each successive pattern of muscle activity required to achieve the image. This posterior part of the SOME SPECIALIZED AREAS OF MOTOR premotor cortex sends its signals either directly to the pri- CONTROL FOUND IN THE HUMAN mary motor cortex to excite specific muscles or, often, by MOTOR CORTEX way of the basal ganglia and thalamus back to the primary motor cortex. A few highly specialized motor regions of the human cere- A special class of neurons called mirror neurons bral cortex (shown in Figure 56-3) control specific motor becomes active when a person performs a specific motor functions. These regions have been localized either by task or when he or she observes the same task performed electrical stimulation or by noting the loss of motor func- by others. Thus, the activity of these neurons “mirrors” tion when destructive lesions occur in specific cortical the behavior of another person as though the observer areas. Some of the more important regions are described was performing the specific motor task. Brain imaging in the following sections. 698  CHAPTER 56 Cortical and Brain Stem Control of Motor Function Broca’s Area (Motor Speech Area). Figure 56-3 shows Motor cortex a premotor area labeled “word formation” lying immedi- ately anterior to the primary motor cortex and immedi- ately above the sylvian fissure. This region is called Broca’s area. Damage to it does not prevent a person from vo- UNIT XI calizing but makes it impossible for the person to speak Corpus callosum whole words rather than uncoordinated utterances or an occasional simple word such as “no” or “yes.” A closely as- Posterior limb of sociated cortical area also causes appropriate respiratory internal capsule function, so respiratory activation of the vocal cords can occur simultaneously with the movements of the mouth and tongue during speech. Thus, the premotor neuronal activities related to speech are highly complex. “Voluntary” Eye Movement Field. In the premotor area immediately above Broca’s area is a locus for controlling voluntary eye movements. Damage to this area prevents a person from voluntarily moving the eyes toward different Basis pedunculi of mesencephalon objects. Instead, the eyes tend to lock involuntarily onto specific objects, an effect controlled by signals from the occipital visual cortex, as explained in Chapter 52. This frontal area also controls eyelid movements such as blink- Longitudinal ing. fascicles of pons Head Rotation Area. Slightly higher in the motor associ- ation area, electrical stimulation elicits head rotation. This area is closely associated with the eye movement field; it directs the head toward different objects. Pyramid of medulla oblongata Area for Hand Skills. In the premotor area immediately anterior to the primary motor cortex for the hands and fingers is a region that is important for “hand skills.” That Lateral corticospinal tract is, when tumors or other lesions cause destruction in this Ventral corticospinal tract area, hand movements become uncoordinated and non- Figure 56-4. Corticospinal (pyramidal) tract. (Modified from Ran- purposeful, a condition called motor apraxia. son SW, Clark SL: Anatomy of the Nervous System. Philadelphia: WB Saunders, 1959.) TRANSMISSION OF SIGNALS FROM THE MOTOR CORTEX TO THE MUSCLES nucleus and the putamen of the basal ganglia) and then downward through the brain stem, forming the pyramids Motor signals are transmitted directly from the cortex to of the medulla. Most of the pyramidal fibers then cross in the spinal cord through the corticospinal tract and indi- the lower medulla to the opposite side and descend into rectly through multiple accessory pathways that involve the the lateral corticospinal tracts of the cord, finally termi- basal ganglia, cerebellum, and various nuclei of the brain nating principally on the interneurons in the intermediate stem. In general, the direct pathways are concerned with regions of the cord gray matter. A few terminate on sen- discrete and detailed movements, especially of the distal sory relay neurons in the dorsal horn, and a very few ter- segments of the limbs, particularly the hands and fingers. minate directly on the anterior motor neurons that cause muscle contraction. Corticospinal (Pyramidal) Tract A few of the fibers do not cross to the opposite side The most important output pathway from the motor in the medulla but pass ipsilaterally down the cord in the cortex is the corticospinal tract, also called the pyrami- ventral corticospinal tracts. Many, if not most, of these dal tract, shown in Figure 56-4. The corticospinal tract fibers eventually cross to the opposite side of the cord originates about 30% from the primary motor cortex, 30% either in the neck or in the upper thoracic region. These from the premotor and supplementary motor areas, and fibers may be concerned with control of bilateral postural 40% from the somatosensory areas posterior to the cen- movements by the supplementary motor cortex. tral sulcus. The most impressive fibers in the pyramidal tract are a After leaving the cortex, it passes through the poste- population of large myelinated fibers with a mean diam- rior limb of the internal capsule (between the caudate eter of 16 micrometers. These fibers originate from giant 699 UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology pyramidal cells, called Betz cells, that are found only in 2. Subcortical fibers that arrive through the corpus callo- the primary motor cortex. The Betz cells are about 60 sum from the opposite cerebral hemisphere. These fib- micrometers in diameter, and their fibers transmit nerve ers connect corresponding areas of the cortices in the impulses to the spinal cord at a velocity of about 70 m/sec, two sides of the brain. 3. Somatosensory fibers that arrive directly from the ven- the most rapid rate of transmission of any signals from the trobasal complex of the thalamus. These fibers relay brain to the cord. There are about 34,000 of these large mainly cutaneous tactile signals and joint and muscle Betz cell fibers in each corticospinal tract. The total num- signals from the peripheral body. ber of fibers in each corticospinal tract is more than 1 mil- 4. Tracts from the ventrolateral and ventroanterior nuclei lion, so these large fibers represent only 3% of the total. of the thalamus, which in turn receive signals from the The other 97% are mainly fibers smaller than 4 microm- cerebellum and basal ganglia. These tracts provide sig- eters in diameter that conduct background tonic signals nals that are necessary for coordination among the mo- to the motor areas of the cord. tor control functions of the motor cortex, basal ganglia, and cerebellum. 5. Fibers from the intralaminar nuclei of the thalamus. Other Fiber Pathways From the Motor Cortex These fibers control the general level of excitability of The motor cortex gives rise to large numbers of additional, the motor cortex in the same way they control the gen- mainly small fibers that go to deep regions in the cerebrum eral level of excitability of most other regions of the cer- and brain stem, including the following: ebral cortex. 1. The axons from the giant Betz cells send short collat- erals back to the cortex. These collaterals are believed to inhibit adjacent regions of the cortex when the Betz THE RED NUCLEUS SERVES AS cells discharge, thereby “sharpening” the boundaries of AN ALTERNATIVE PATHWAY FOR the excitatory signal. TRANSMITTING CORTICAL SIGNALS TO 2. A large number of fibers pass from the motor cortex THE SPINAL CORD into the caudate nucleus and putamen. From there, ad- ditional pathways extend into the brain stem and spinal The red nucleus, located in the mesencephalon, functions cord, as discussed in the next chapter, mainly to control in close association with the corticospinal tract. As shown body postural muscle contractions. in Figure 56-5, it receives a large number of direct fibers 3. A moderate number of motor fibers pass to red nuclei of from the primary motor cortex through the corticorubral the midbrain. From these nuclei, additional fibers pass tract, as well as branching fibers from the corticospinal down the cord through the rubrospinal tract. tract as it passes through the mesencephalon. These fibers 4. A moderate number of motor fibers deviate into the re- synapse in the lower portion of the red nucleus, the mag- ticular substance and vestibular nuclei of the brain stem; nocellular portion, which contains large neurons similar from there, signals go to the cord via reticulospinal and vestibulospinal tracts, and others go to the cerebellum in size to the Betz cells in the motor cortex. These large via reticulocerebellar and vestibulocerebellar tracts. neurons then give rise to the rubrospinal tract, which 5. A tremendous number of motor fibers synapse in the crosses to the opposite side in the lower brain stem and pontile nuclei, which give rise to the pontocerebellar fib- follows a course immediately adjacent and anterior to the ers, carrying signals into the cerebellar hemispheres. corticospinal tract into the lateral columns of the spinal 6. Collaterals also terminate in the inferior olivary nuclei, cord. and from there, secondary olivocerebellar fibers trans- mit signals to multiple areas of the cerebellum. Motor cortex Thus, the basal ganglia, brain stem, and cerebellum all receive strong motor signals from the corticospinal system every time a signal is transmitted down the spinal cord to cause a motor activity. Corticorubral tract Incoming Sensory Fiber Pathways to the Motor Cortex The functions of the motor cortex are controlled mainly by nerve signals from the somatosensory system but also, to some degree, from other sensory systems such as hearing and vision. Once the sensory information is received, the Interpositus motor cortex operates in association with the basal ganglia Red nucleus nucleus and cerebellum to excite appropriate motor actions. The Dentate more important incoming fiber pathways to the motor cor- nucleus Reticular formation tex are the following: Cerebellum 1. Subcortical fibers from adjacent regions of the cerebral Rubrospinal tract cortex, especially from (a) the somatosensory areas of the parietal cortex, (b) the adjacent areas of the frontal cortex anterior to the motor cortex, and (c) the visual Figure 56-5. The corticorubrospinal pathway for motor control, also and auditory cortices. showing the relation of this pathway to the cerebellum. 700  CHAPTER 56 Cortical and Brain Stem Control of Motor Function The rubrospinal fibers terminate mostly on the inter- from the cortical surface. The input signals all enter via neurons of the intermediate areas of the cord gray mat- layers 2 through 4, and the sixth layer gives rise mainly to ter, along with the corticospinal fibers, but some of the fibers that communicate with other regions of the cere- rubrospinal fibers terminate directly on anterior motor bral cortex. neurons, along with some corticospinal fibers. The red Each Column of Neurons Functions as an Integrative UNIT XI nucleus also has close connections with the cerebellum, Processing System. The neurons of each column oper- similar to the connections between the motor cortex and ate as an integrative processing system, using information the cerebellum. from multiple input sources to determine the output re- The Corticorubrospinal System Is an Accessory Path- sponse from the column. In addition, each column can way for Transmitting Relatively Discrete Signals function as an amplifying system to stimulate large num- From the Motor Cortex to the Spinal Cord. The mag- bers of pyramidal fibers to the same muscle or to synergis- nocellular portion of the red nucleus has a somatographic tic muscles simultaneously. This ability is important be- representation of all the muscles of the body, as does the cause stimulation of a single pyramidal cell seldom excites motor cortex. Therefore, stimulation of a single point in a muscle. Usually, 50 to 100 pyramidal cells need to be this portion of the red nucleus causes contraction of ei- excited simultaneously or in rapid succession to achieve ther a single muscle or a small group of muscles. However, definitive muscle contraction. the fineness of representation of the different muscles is Dynamic and Static Signals Are Transmitted by the far less developed than in the motor cortex, especially in Pyramidal Neurons. If a strong signal is sent to a mus- human beings, who have relatively small red nuclei. cle to cause initial rapid contraction, then a much weaker The corticorubrospinal pathway serves as an accessory continuing signal can maintain the contraction for long route for transmission of relatively discrete signals from periods thereafter. This process is the usual manner in the motor cortex to the spinal cord. When the corticospi- which excitation is provided to cause muscle contrac- nal fibers are destroyed but the corticorubrospinal path- tions. To provide this excitation, each column of cells way is intact, discrete movements can still occur, except excites two populations of pyramidal cell neurons, one that the movements for fine control of the fingers and called dynamic neurons and the other static neurons. The hands are considerably impaired. Wrist movements are dynamic neurons are excited at a high rate for a short pe- still functional, which is not the case when the corticoru- riod at the beginning of a contraction, causing the initial brospinal pathway is also blocked. rapid development of force. The static neurons then fire at Therefore, the pathway through the red nucleus to the a much slower rate, but they continue firing at this slow spinal cord is associated with the corticospinal system. rate to maintain the force of contraction as long as the Furthermore, the rubrospinal tract lies in the lateral col- contraction is required. umns of the spinal cord, along with the corticospinal tract, The neurons of the red nucleus have similar dynamic and terminates on the interneurons and motor neurons and static characteristics, except that a greater percentage that control the more distal muscles of the limbs. There- of dynamic neurons is in the red nucleus and a greater fore, the corticospinal and rubrospinal tracts together are percentage of static neurons is in the primary motor cor- called the lateral motor system of the cord, in contradis- tex. This may be related to the fact that the red nucleus tinction to a vestibuloreticulospinal system, which lies is closely allied with the cerebellum, and the cerebellum mainly medially in the cord and is called the medial motor plays an important role in rapid initiation of muscle con- system of the cord, as discussed later in this chapter. traction, as explained in the next chapter. EXCITATION OF THE SPINAL CORD Somatosensory Feedback to the Motor MOTOR CONTROL AREAS BY THE Cortex Helps Control Precision of Muscle PRIMARY MOTOR CORTEX AND RED Contraction NUCLEUS When nerve signals from the motor cortex cause a mus- Neurons in the Motor Cortex Are Arranged in Vertical cle to contract, somatosensory signals return all the way Columns. In Chapters 48 and 52, we pointed out that the from the activated region of the body to the neurons in cells in the somatosensory cortex and visual cortex are organ- the motor cortex that are initiating the action. Most of ized in vertical columns of cells. The cells of the motor cortex these somatosensory signals arise in the following: (1) the are also organized in vertical columns a fraction of a millim- muscle spindles; (2) the muscle tendon organs; or (3) the eter in diameter, with thousands of neurons in each column. tactile receptors of the skin overlying the muscles. Each column of cells functions as a unit, usually stim- These somatic signals often cause positive feedback ulating a group of synergistic muscles, but sometimes enhancement of the muscle contraction in the following stimulating just a single muscle. Also, each column has ways. In the case of the muscle spindles, if the fusimotor six distinct layers of cells, as is true throughout nearly muscle fibers in the spindles contract more than the large all the cerebral cortex. The pyramidal cells that give rise skeletal muscle fibers contract, the central portions of the to the corticospinal fibers all lie in the fifth layer of cells spindles become stretched and, therefore, excited. Signals 701 UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology from these spindles then return rapidly to the pyramidal helping to damp any oscillations of the motor movements cells in the motor cortex signal them that the large muscle initiated from the brain. This reflex probably also provides fibers have not contracted enough. The pyramidal cells at least part of the motive power required to cause mus- further excite the muscle, helping its contraction to catch cle contractions when the intrafusal fibers of the muscle up with the contraction of the muscle spindles. In the case spindles contract more than the large skeletal muscle fib- of the tactile receptors, if the muscle contraction causes ers, thus eliciting reflex “servo-assist” stimulation of the compression of the skin against an object, such as com- muscle, in addition to the direct stimulation by the corti- pression of the fingers around an object being grasped, cospinal fibers. the signals from the skin receptors can, if necessary, cause Also, when a brain signal excites a muscle, it is usu- further excitation of the muscles and, therefore, increase ally unnecessary to transmit an inverse signal to relax the tightness of the hand grasp. the antagonist muscle at the same time; this relaxation is achieved by the reciprocal innervation circuit that is Stimulation of the Spinal Motor Neurons always present in the cord for coordinating the function Figure 56-6 shows a cross section of a spinal cord segment of antagonistic pairs of muscles. demonstrating the following: (1) multiple motor and sen- Finally, other cord reflex mechanisms, such as with- sorimotor control tracts entering the cord segment; and drawal, stepping and walking, scratching, and postural (2) a representative anterior motor neuron in the middle of mechanisms, can each be activated by “command” signals the anterior horn gray matter. The corticospinal tract and from the brain. Thus, simple command signals from the the rubrospinal tract lie in the dorsal portions of the lateral brain can initiate many normal motor activities, particu- white columns. Their fibers terminate mainly on interneu- larly for such functions as walking and attaining different rons in the intermediate area of the cord gray matter. postural attitudes of the body. In the cervical enlargement of the cord where the hands and fingers are represented, large numbers of both corti- Effect of Lesions in the Motor Cortex or in the Corti- cospinal and rubrospinal fibers also terminate directly on cospinal Pathway the anterior motor neurons, allowing a direct route from Reduced Brain Blood Supply Caused by a Stroke. The the brain to activate muscle contraction. This mechanism motor control system can be damaged by the common ab- is in keeping with the fact that the primary motor cortex normality called a “stroke.” A stroke is caused by either a has an extremely high degree of representation for fine ruptured blood vessel that hemorrhages into the brain or control of hand, finger, and thumb actions. by thrombosis of one of the major arteries supplying blood to the brain. In either case, the result is loss of blood supply Patterns of Movement Elicited by Spinal Cord Cent- to the cortex or to the corticospinal tract where it passes ers. From Chapter 55, recall that the spinal cord can through the internal capsule between the caudate nucleus provide certain specific reflex patterns of movement in and the putamen. response to sensory nerve stimulation. Many of these Removal of the Primary Motor Cortex (Area Pyrami- same patterns are also important when the cord’s ante- dalis). Surgical removal of a portion of the primary mo- rior motor neurons are excited by signals from the brain. tor cortex—the area that contains the giant Betz pyramidal For example, the stretch reflex is functional at all times, cells—causes varying degrees of paralysis of the represent- ed muscles. If the sublying caudate nucleus and adjacent premotor and supplementary motor areas are not dam- Sensory neurons aged, gross postural and limb “fixation” movements can still occur, but there is loss of voluntary control of discrete movements of the distal segments of the limbs, especially of Propriospinal tract the hands and fingers. This does not mean that the hand Interneurons and finger muscles cannot contract; rather, the ability to control the fine movements is gone. From these observa- Corticospinal tract tions, one can conclude that the area pyramidalis is essen- from pyramidal cells of cortex tial for voluntary initiation of finely controlled movements, especially of the hands and fingers. Rubrospinal tract Muscle Spasticity Caused by Lesions That Damage Large Reticulospinal tract Areas Adjacent to the Motor Cortex. The primary motor Anterior motor neuron cortex normally exerts a continual tonic stimulatory effect on the motor neurons of the spinal cord; when this stimu- Motor nerve latory effect is removed, hypotonia results. Most lesions of Tectospinal and the motor cortex, especially those caused by a stroke, involve reticulospinal tracts not only the primary motor cortex but also adjacent parts of Vestibulospinal and the brain, such as the basal ganglia. In these cases, muscle reticulospinal tracts spasm almost invariably occurs in the afflicted muscle areas Figure 56-6. Convergence of different motor control pathways on on the opposite side of the body (because the motor path- the anterior motor neurons. ways cross to the opposite side). This spasm results mainly 702  CHAPTER 56 Cortical and Brain Stem Control of Motor Function from damage to accessory pathways from the nonpyramidal portions of the motor cortex. These pathways normally in- hibit the vestibular and reticular brain stem motor nuclei. When these nuclei cease their state of inhibition (i.e., are Pontine reticular nuclei “disinhibited”), they become spontaneously active and cause excessive spastic tone in the involved muscles, as we discuss UNIT XI more fully later in this chapter. This spasticity is that which normally accompanies a “stroke” in a human being. CONTROL OF MOTOR FUNCTIONS BY Vestibular nuclei THE BRAIN STEM The brain stem consists of the medulla, pons, and mes- encephalon (or midbrain). In one sense, it is an extension Medullary reticular nuclei of the spinal cord upward into the cranial cavity because it contains motor and sensory nuclei that perform motor and sensory functions for the face and head regions in the same way that the spinal cord performs these functions from the neck down. In another sense, however, the brain stem is its own master because it provides many special control functions, such as the following: Figure 56-7. Locations of the reticular and vestibular nuclei in the 1. Control of respiration brain stem. 2. Control of the cardiovascular system 3. Partial control of gastrointestinal function lospinal tract in the anterior column of the cord, as shown 4. Control of many stereotyped movements of the in Figure 56-8. The fibers of this pathway terminate on body the medial anterior motor neurons that excite the axial 5. Control of equilibrium muscles of the body, which support the body against grav- 6. Control of eye movements ity—that is, the muscles of the vertebral column and the Finally, the brain stem serves as a way station for “com- extensor muscles of the limbs. mand signals” from higher neural centers. Many of these The pontine reticular nuclei have a high degree of functions are discussed in other chapters in this text. In natural excitability. In addition, they receive strong excit- the following sections, we discuss the role of the brain atory signals from the vestibular nuclei, as well as from stem in controlling whole-body movement and equilib- deep nuclei of the cerebellum. Therefore, when the pon- rium. Especially important for these purposes are the tine reticular excitatory system is unopposed by the med- brain stem’s reticular nuclei and vestibular nuclei. ullary reticular system, it causes powerful excitation of antigravity muscles throughout the body, so much so that SUPPORT OF THE BODY AGAINST four-legged animals can be placed in a standing position, GRAVITY—ROLES OF THE RETICULAR supporting the body against gravity without any signals AND VESTIBULAR NUCLEI from higher levels of the brain. Figure 56-7 shows the locations of the reticular and ves- tibular nuclei in the brain stem. Medullary Reticular System Transmit Inhibitory Sig- nals. The medullary reticular nuclei transmit inhibitory Excitatory-Inhibitory Antagonism signals to the same antigravity anterior motor neurons via Between Pontine and Medullary Reticular a different tract, the medullary reticulospinal tract, locat- Nuclei ed in the lateral column of the cord, as also shown in Fig- The reticular nuclei are divided into two major groups: (1) ure 56-8. The medullary reticular nuclei receive strong pontine reticular nuclei, located slightly posteriorly and lat- input collaterals from the following: (1) the corticospinal erally in the pons and extending into the mesencephalon; tract; (2) the rubrospinal tract; and (3) other motor path- and (2) medullary reticular nuclei, which extend through ways. These tracts and pathways normally activate the the entire medulla, lying ventrally and medially near the medullary reticular inhibitory system to counterbalance midline. These two sets of nuclei function mainly antagonis- the excitatory signals from the pontine reticular system, tically to each other, with the pontine exciting the antigrav- so under normal conditions the body muscles are not ab- ity muscles and the medullary relaxing these same muscles. normally tense. Yet, some signals from higher areas of the brain can Pontine Reticular System Transmits Excitatory Sig- “disinhibit” the medullary system when the brain wishes nals. The pontine reticular nuclei transmit excitatory sig- to excite the pontine system to cause standing. At other nals downward into the cord through the pontine reticu- times, excitation of the medullary reticular system can 703 UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology inhibit antigravity muscles in certain portions of the body Lacking this input, the medullary reticular inhibitor to allow those portions to perform special motor activi- system becomes nonfunctional, full overactivity of the ties. The excitatory and inhibitory reticular nuclei consti- pontine excitatory system occurs, and rigidity develops. tute a controllable system that is manipulated by motor We shall see later that other causes of rigidity occur in signals from the cerebral cortex and elsewhere to provide other neuromotor diseases, especially lesions of the basal necessary background muscle contractions for standing ganglia. against gravity and to inhibit appropriate groups of mus- cles as needed so that other functions can be performed. VESTIBULAR SENSATIONS AND Role of the Vestibular Nuclei to Excite the MAINTENANCE OF EQUILIBRIUM Antigravity Muscles VESTIBULAR APPARATUS All the vestibular nuclei, shown in Figure 56-7, function in association with the pontine reticular nuclei to control The vestibular apparatus, shown in Figure 56-9, is the the antigravity muscles. The vestibular nuclei transmit sensory organ for detecting sensations of equilibrium. It is strong excitatory signals to the antigravity muscles via encased in a system of bony tubes and chambers located the lateral and medial vestibulospinal tracts in the ante- in the petrous portion of the temporal bone, called the rior columns of the spinal cord, as shown in Figure 56-8. bony labyrinth. Within this system are membranous Without this support of the vestibular nuclei, the pontine tubes and chambers called the membranous labyrinth. reticular system would lose much of its excitation of the The membranous labyrinth is the functional part of the axial antigravity muscles. vestibular apparatus. The specific role of the vestibular nuclei, however, is The top of Figure 56-9 shows the membranous to selectively control the excitatory signals to the different labyrinth. It is composed mainly of the cochlea (ductus antigravity muscles to maintain equilibrium in response to signals from the vestibular apparatus. We discuss this concept more fully later in this chapter. Anterior Ampullae The Decerebrate Animal Develops Spastic Rigidity. Utricle When the brain stem of an animal is sectioned below the Maculae and midlevel of the mesencephalon but the pontine and med- statoconia ullary reticular systems, as well as the vestibular system, Semi- are left intact, a condition called decerebrate rigidity de- circular canals velops. This rigidity does not occur in all the muscles of the body, but it does occur in the antigravity muscles— the muscles of the neck and trunk and the extensors of the legs. The cause of decerebrate rigidity is blockage of nor- mally strong input to the medullary reticular nuclei from the cerebral cortex, the red nuclei, and the basal ganglia. Saccule Ductus Posterior cochlearis Crista ampullaris Ductus endolymphaticus MEMBRANOUS LABYRINTH Gelatinous Statoconia mass of Medullary reticulospinal cupula Gelatinous tract layer Hair tufts Hair tufts Hair cells Hair cells Lateral vestibulospinal tract Nerve fibers Nerve Medial Pontine reticulospinal tract fibers vestibulospinal Sustentacular cells Sustentacular cells tract Figure 56-8. Vestibulospinal and reticulospinal tracts descending in CRISTA AMPULLARIS AND MACULA the spinal cord to excite (solid lines) or inhibit (dashed lines) the ante- Figure 56-9. Membranous labyrinth and organization of the crista rior motor neurons that control the body’s axial musculature. ampullaris and the macula. 704  CHAPTER 56 Cortical and Brain Stem Control of Motor Function cochlearis), three semicircular canals, and two large cham- Kinocilium bers, the utricle and saccule. The cochlea is the major sen- sory organ for hearing (see Chapter 53) and has little to do with equilibrium. However, the semicircular canals, the Stereocilia utricle, and the saccule are all integral parts of the equilib- UNIT XI rium mechanism. Filamentous attachments “Maculae”—Sensory Organs of the Utricle and Sac- cule for Detecting Orientation of the Head With Re- spect to Gravity. Located on the inside surface of each utricle and saccule, shown in the top diagram of Figure 56-9, is a small sensory area slightly greater than 2 mil- limeters in diameter called a macula. The macula of the utricle lies mainly in the horizontal plane on the inferior surface of the utricle and plays an important role in deter- mining orientation of the head when the head is upright. Conversely, the macula of the saccule is located mainly in a vertical plane and signals head orientation when the person is lying down. Each macula is covered by a gelatinous layer in which many small calcium carbonate crystals called statoconia are embedded. Also in the macula are thousands of hair cells, one of which is shown in Figure 56-10; these hair cells project cilia up into the gelatinous layer. The bases and sides of the hair cells synapse with sensory endings of the vestibular nerve. The calcified statoconia have a specific gravity two to three times the specific gravity of the surrounding fluid and tissues. The weight of the statoconia bends the cilia in Nerve fiber the direction of gravitational pull. Directional Sensitivity of the Hair Cells—Kinocilium. Each hair cell has about 100 small cilia called stereocilia, plus one large cilium, the kinocilium, as shown in Figure Figure 56-10. A hair cell of the equilibrium apparatus and its syn- 56-10. The kinocilium is always located to one side, and apses with the vestibular nerve. the stereocilia become progressively shorter toward the other side of the cell. Minute filamentous attachments, al- bent toward the kinocilium, the impulse traffic increases, most invisible even to the electron microscope, connect often to several hundred per second; conversely, bending the tip of each stereocilium to the next longer stereocili- the cilia away from the kinocilium decreases the impulse um and, finally, to the kinocilium. traffic, often turning it off completely. Therefore, as the Because of these attachments, when the stereocilia orientation of the head in space changes and the weight bend in the direction of the kinocilium, the filamentous of the statoconia bends the cilia, appropriate signals are attachments tug in sequence on the stereocilia, pulling transmitted to the brain to control equilibrium. them outward from the cell body. This movement opens In each macula, each of the hair cells is oriented in a dif- several hundred cation channels in the neuronal cell ferent direction so that some of the hair cells are stimulated membrane around the bases of the stereocilia, and these when the head bends forward, some are stimulated when channels are capable of conducting large numbers of posi- it bends backward, others are stimulated when it bends to tive ions. Therefore, positive ions pour into the cell from one side, and so forth. Therefore, a different pattern of exci- the surrounding endolymphatic fluid, causing receptor tation occurs in the macular nerve fibers for each orienta- membrane depolarization. Conversely, bending the pile tion of the head in the gravitational field. It is this “pattern” of stereocilia in the opposite direction (backward, away that apprises the brain of the head’s orientation in space. from the kinocilium) reduces the tension on the attach- ments; this movement closes the ion channels, thus caus- Semicircular Ducts. The three semicircular ducts in each ing receptor hyperpolarization. vestibular apparatus, known as the anterior, posterior, and Under normal resting conditions, the nerve fibers lead- lateral (horizontal) semicircular ducts, are arranged at ing from the hair cells transmit continuous nerve impulses right angles to one another so that they represent all three at a rate of about 100 per second. When the stereocilia are planes in space. When the head is bent forward about 30 705 UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology degrees, the lateral semicircular ducts are approximately FUNCTION OF THE UTRICLE AND SACCULE horizontal with respect to the surface of the Earth; the IN THE MAINTENANCE OF STATIC anterior ducts are in vertical planes that project forward EQUILIBRIUM and 45 degrees outward, whereas the posterior ducts are It is especially important that the hair cells are all oriented in in vertical planes that project backward and 45 degrees different directions in the maculae of the utricles and saccules outward. so that with different positions of the head, different hair cells Each semicircular duct has an enlargement at one of become stimulated. The “patterns” of stimulation of the dif- its ends called the ampulla, and the ducts and ampulla ferent hair cells apprise the brain of the position of the head are filled with a fluid called endolymph. Flow of this fluid with respect to the pull of gravity. In turn, the vestibular, cer- through one of the ducts and through its ampulla excites ebellar, and reticular motor nerve systems of the brain excite the sensory organ of the ampulla in the following man- appropriate postural muscles to maintain proper equilibrium. ner: Figure 56-11 shows a small crest in each ampulla This utricle and saccule system functions extremely called a crista ampullaris. On top of this crista is a loose effectively for maintaining equilibrium when the head is gelatinous tissue mass, the cupula. When a person’s head in the near-vertical position. Indeed, a person can deter- begins to rotate in any direction, the inertia of the fluid mine as little as half a degree of disequilibrium when the in one or more of the semicircular ducts causes the fluid body leans from the precise upright position. to remain stationary while the semicircular duct rotates with the head. This process causes fluid to flow from Detection of Linear Acceleration by the Utricle and Sac- the duct and through the ampulla, bending the cupula cule Maculae. When the body is suddenly thrust forward— to one side, as demonstrated by the position of the col- that is, when the body accelerates—the statoconia, which ored cupula in Figure 56-11. Rotation of the head in have greater mass inertia than the surrounding fluid, fall the opposite direction causes the cupula to bend to the backward on the hair cell cilia, and information of disequi- opposite side. librium is sent into the nervous centers, causing the person Hundreds of cilia from hair cells located on the ampul- to feel as though he or she were falling backward. This sensa- lary crest are projected into the cupula. The kinocilia of tion automatically causes the person to lean forward until these hair cells are all oriented in the same direction in the the resulting anterior shift of the statoconia exactly equals cupula, and bending the cupula in that direction causes the tendency for the statoconia to fall backward because of depolarization of the hair cells, whereas bending it in the the acceleration. At this point, the nervous system senses a opposite direction hyperpolarizes the cells. Then, from state of proper equilibrium and leans the body forward no the hair cells, appropriate signals are sent via the vestibu- farther. Thus, the maculae operate to maintain equilibrium lar nerve to apprise the central nervous system of a change during linear acceleration in exactly the same manner as they in rotation of the head and the rate of change in each of operate during static equilibrium. the three planes of space. The maculae do not operate for the detection of linear velocity. When runners first begin to run, they must lean far forward to keep from falling backward because of initial acceleration, but once they have achieved running speed, if they were running in a vacuum, they would not have to lean forward. When running in air, they lean forward to maintain equilibrium only because of air resistance against their bodies; in this case, it is not the maculae that make them lean but air pressure acting on pressure end-organs Cupula in the skin, which initiate appropriate equilibrium adjust- Cristae ments to prevent falling. Ampulla ampullaris DETECTION OF HEAD ROTATION BY THE SEMICIRCULAR DUCTS When the head suddenly begins to rotate in any direc- tion (called angular acceleration), the endolymph in the semicircular ducts, because of its inertia, tends to remain Hair cells stationary while the semicircular ducts turn. This mecha- Nerve nism causes relative fluid flow in the ducts in the direction opposite to head rotation. Figure 56-12 shows a typical discharge signal from a sin- gle hair cell in the crista ampullaris when an animal is rotated Figure 56-11. Movement of the cupula and its embedded hairs at for 40 seconds, demonstrating the following: (1) even when the onset of rotation. the cupula is in its resting position, the hair cell emits a tonic 706  CHAPTER 56 Cortical and Brain Stem Control of Motor Function discharge of about 100 impulses per second; (2) when the The function of the semicircular ducts can be animal begins to rotate, the hairs bend to one side, and the explained by the following illustration: if a person is rate of discharge increases greatly; and (3) with continued running forward rapidly and then suddenly begins to rotation, the excess discharge of the hair cell gradually sub- turn to one side, he or she will fall off balance a frac- sides back to the resting level during the next few seconds. tion of a second later unless appropriate corrections UNIT XI The reason for this adaptation of the receptor is that are made ahead of time. However, the maculae of the within the first few seconds of rotation, back resistance utricle and saccule cannot detect that the person is off to the flow of fluid in the semicircular duct and past the balance until after the loss of balance has occurred. The bent cupula causes the endolymph to begin rotating as semicircular ducts, however, will have already detected rapidly as the semicircular canal itself. Then, in another that the person is turning, and this information can eas- 5 to 20 seconds, the cupula slowly returns to its resting ily apprise the central nervous system of the fact that position in the middle of the ampulla because of its own the person will fall off balance within the next fraction elastic recoil. of a second or so unless some anticipatory correction When the rotation suddenly stops, exactly opposite is made. effects take place: The endolymph continues to rotate In other words, the semicircular duct mechanism pre- while the semicircular duct stops. This time, the cupula dicts that disequilibrium is going to occur and thereby bends in the opposite direction, causing the hair cell causes the equilibrium centers to make appropriate to stop discharging entirely. After another few seconds, anticipatory preventive adjustments, which helps the the endolymph stops moving and the cupula gradually person maintain balance before the situation can be returns to its resting position, thus allowing hair cell corrected. discharge to return to its normal tonic level, as shown Removal of the flocculonodular lobes of the cerebel- at the right in Figure 56-12. Thus, the semicircular lum prevents normal detection of semicircular duct duct transmits a signal of one polarity when the head signals but has less effect on detecting macular signals. begins to rotate and of opposite polarity when it stops It is especially interesting that the cerebellum serves as rotating. a “predictive” organ for most rapid movements of the “Predictive” Function of the Semicircular Duct body, as well as for those involving equilibrium. These System in the Maintenance of Equilibrium. Because other functions of the cerebellum are discussed in Chap- the semicircular ducts do not detect that the body is off ter 57. balance in the forward direction, in the side direction, or in the backward direction, one might ask, “What is the Vestibular Mechanisms for Stabilizing the Eyes function of the semicircular ducts in the maintenance When a person changes direction of movement rapidly of equilibrium?” All they detect is that the person’s or even leans the head sideways, forward, or backward, it head is beginning or stopping to rotate in one direction would be impossible to maintain a stable image on the reti- or another. Therefore, the function of the semicircular nas unless the person had some automatic control mecha- ducts is not to maintain static equilibrium or to main- nism to stabilize the direction of the eyes’ gaze. In addition, tain equilibrium during steady directional or rotational the eyes would be of little use in detecting an image unless movements. Yet, loss of function of the semicircular they remained fixed on each object long enough to gain a ducts does cause a person to have poor equilibrium clear image. Fortunately, each time the head is suddenly ro- when attempting to perform rapid, intricate changing tated, signals from the semicircular ducts cause the eyes to rotate in a direction equal and opposite to the rotation of body movements. the head. This movement results from reflexes transmitted through the vestibular nuclei and the medial longitudinal fasciculus to the oculomotor nuclei. These reflexes are de- 400 Rotation scribed in Chapter 52. Other Factors Concerned With Equilibrium Impulses per second 300 Neck Proprioceptors. The vestibular apparatus detects the orientation and movement only of the head. Therefore, Tonic level of it is essential that the nervous centers also receive appro- 200 discharge Stop rotation priate information about the orientation of the head with respect to the body. This information is transmitted from 100 the proprioceptors of the neck and body directly to the ves- tibular and reticular nuclei in the brain stem and indirectly Begin rotation 0 by way of the cerebellum. 0 10 20 30 40 50 60 70 80 90 Among the most important proprioceptive information Seconds needed for the maintenance of equilibrium is that transmit- ted by joint receptors of the neck. When the head is leaned Figure 56-12. Response of a hair cell when a semicircular canal is stimulated first by the onset of head rotation and then by stopping in one direction by bending the neck, impulses from the rotation. neck proprioceptors keep the signals originating in the ves- 707 UNIT XI The Nervous System: C. Motor and Integrative Neurophysiology tibular apparatus from giving the person a sense of disequi- the semicircular ducts. In fact, destruction of these lobes librium. They perform this function by transmitting signals results in almost exactly the same clinical symptoms as de- that exactly oppose the signals transmitted from the ves- struction of the semicircular ducts. That is, severe injury to tibular apparatus. However, when the entire body leans in either the lobes or the ducts causes loss of dynamic equilib- one direction, the impulses from the vestibular apparatus rium during rapid changes in direction of motion but does are not opposed by signals from the neck proprioceptors; not seriously disturb equilibrium under static conditions. It therefore, in this case, the person does perceive a change in is believed that the uvula of the cerebellum plays a similar equilibrium status of the entire body. important role in static equilibrium. Proprioceptive and Exteroceptive Information From Signals transmitted upward in the brain stem from both Other Parts of the Body. Proprioceptive information from the vestibular nuclei and the cerebellum via the medial parts of the body other than the neck is also important in longitudinal fasciculus cause corrective movements of the the maintenance of equilibrium. For example, pressure eyes every time the head rotates, so the eyes remain fixed sensations from the footpads tell one (1) whether weight on a specific visual object. Signals also pass upward (either is distributed equally between the two feet and (2) whether through this same tract or through reticular tracts) to the weight on the feet is more forward or backward. cerebral cortex, terminating in a primary cortical center for Exteroceptive information is especially necessary for equilibrium located in the parietal lobe deep in the sylvian the maintenance of equilibrium when a person is running. fissure on the opposite side of the fissure from the auditory The air pressure against the front of the body signals that a area of the superior temporal gyrus. These signals apprise force is opposing the body in a direction different from that the psyche of the equilibrium status of the body. caused by gravitational pull; as a result, the person leans Functions of Brain Stem Nuclei in Controlling Subcon- forward to oppose this force. scious, Stereotyped Movements Importance of Visual Information for Maintaining Equi- librium. After destruction of the vestibular apparatus, and Rarely, a baby is born without brain structures above even after loss of most proprioceptive information from the the mesencephalic region, a condition called anenceph- body, a person can still use the visual mechanisms reason- aly. Some of these babies have been kept alive for many ably effectively for maintaining equilibrium. Even a slight months. They are able to perform some stereotyped move- linear or rotational movement of the body instantaneously ments for feeding, such as suckling, extrusion of unpleas- shifts the visual images on the retina, and this information ant food from the mouth, and moving the hands to the is relayed to the equilibrium centers. Some people with bi- mouth to suck the fingers. In addition, they can yawn and lateral destruction of the vestibular apparatus have almost stretch. They can cry and can follow objects with move- normal equilibrium as long as their eyes are open, and all ments of the eyes and head. Also, placing pressure on the motions are performed slowly. However, when moving rap- upper anterior parts of their legs causes them to pull to idly or when the eyes are closed, equilibrium is immediately the sitting position. It is clear that many of the stereotyped lost. motor functions of the human being are integrated in the brain stem. Neuronal Connections of the Vestibular Apparatus With the Central Nervous System Figure 53-13 shows the connections in the hindbrain of the vestibular nerve. Most of the vestibular nerve fibers terminate in the brain stem in the vestibular nuclei, which Dentate nucleus Fastigial are located approximately at the junction of the medulla nucleus Medial longitudinal and the pons. Some fibers pass directly to the brain stem fasciculus reticular nuclei without synapsing and also to the cerebellar fastigial, uvular, and flocculonodular lobe nuclei. The fibers Red nucleus that end in the brain stem vestibular nuclei synapse with second-order neurons that also send fibers into the cerebel- lum, the vestibulospinal tracts, the medial longitudinal fas- Reticular ciculus, and other areas of the brain stem, particularly the substance reticular nuclei. Fastigioreticular The primary pathway for the equilibrium reflexes tract begins in the vestibular nerves, where the nerves are Vestibular nucleus excited by the vestibular apparatus. The pathway then passes to the vestibular nuclei and cerebellum. Next, sig- Flocculo- nodular lobe Vestibular nerve nals are sent into the reticular nuclei of the brain stem, as well as down the spinal cord via the vestibulospinal Vestibulospinal tract and reticulospinal tracts. The signals to the cord control Rubrospinal tract the interplay between facilitation and inhibition of the many antigravity muscles, thus automatically controlling Reticulospinal tract equilibrium. Figure 56-13. Connections of vestibular nerves through the vestibu- The flocculonodular lobes of the cerebellum are espe- lar nuclei (large pink oval area) with other areas of the central nervous cially concerned with dynamic equilibrium signals from system. 708  CHAPTER 56 Cortical and Brain Stem Control of Motor Function Bibliography Nachev P, Kennard C, Husain M: Functional role of the supplemen- tary and pre-supplementary motor areas. Nat Rev Neurosci 9:856, Cembrowski MS, Spruston N: Heterogeneity within classical cell types 2008. is the rule: lessons from hippocampal pyramidal neurons. Nat Rev Proske U, Allen T: The neural basis of the senses of effort, force and Neurosci 20:193, 2019. heaviness. Exp Brain Res 237:589, 2019. Cullen KE: Vestibular processing during natural self-motion: implica- Proske U, Gandevia SC: Kinesthetic senses. Compr Physiol 8:1157, UNIT XI tions for perception and action. Nat Rev Neurosci 20:346, 2019. 2018. 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