Chapter 10 Lecture Outline Control of Body Movement

Chapter 10 Lecture Outline* Control of Body Movement Eric P. Widmaier Boston University Hershel Raff Medical College of Wisconsin Kevin T. Strang University of Wisconsin - Madison *See Power. Point Image Slides for all figures and tables pre-inserted into Power. Point without notes. Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display. 1

Motor Control Hierarchy Fig. 10 -1 2

Cerebellum Fig. 10 -2 a 3

Subcortical and Brainstem Nuclei Fig. 10 -2 b 4

Voluntary and Involuntary Actions • Voluntary movements are accompanied by a conscious awareness of what we are doing, and our attention is directed toward the action or its purpose. • Involuntary movements are often characterized as unconscious, automatic or a reflex. • Most motor behavior is neither purely voluntary nor purely involuntary. 5

Local Control of Motor Neurons • Local control systems receive instructions from higher brain centers and make adjustments based on information received from sensory receptors in the muscles, tendons, and joints of the body part to be moved. 6

Interneurons Fig. 10 -3 7

Local Afferent Input Fig. 10 -4 8

Fig. 10 -5 9

Fig. 10 -6 10

The Withdrawal Reflex • Painful stimulation of the skin, as occurs from stepping on a tack, activates the flexor muscles and inhibits the extensor muscles of the ipsilateral (on the same side of the body) leg. • The resulting action moves the affected limb away from the harmful stimulus, and is thus known as a withdrawal reflex. • The same stimulus causes just the opposite response in the contralateral leg (on the opposite side of the body from the stimulus). • Motor neurons to the extensors are activated while the flexor muscle motor neurons are inhibited. This crossed-extensor reflex enables the contralateral leg to support the body’s weight as the injured foot is lifted by flexion. 11

The Withdrawal Reflex Fig. 10 -9 12

Cerebral Cortex • The cerebral cortex plays a critical role in both the planning and ongoing control of voluntary movements, functioning in both the highest and middle levels of the motor control hierarchy. • The term sensorimotor cortex is used to include all those parts of the cerebral cortex that act together to control muscle movement. • A large number of neurons that give rise to descending pathways for motor control come from two areas of sensorimotor cortex on the posterior part of the frontal lobe: the primary motor cortex (sometimes called simply the motor cortex) and the premotor area. • The neurons of the motor cortex that control muscle groups in various parts of the body are arranged anatomically into a somatotopic map. 13

Cerebral Cortex Fig. 10 -10 Fig. 10 -11 14

Cerebral Cortex • Other areas of sensorimotor cortex include the supplementary motor cortex, which lies mostly on the surface on the frontal lobe where the cortex folds down between the two hemispheres, the somatosensory cortex, and parts of the parietal-lobe association cortex. • Although these areas are anatomically and functionally distinct, they are heavily interconnected, and individual muscles or movements are represented at multiple sites. • The cortical neurons that control movement form a neural network, meaning that many neurons participate in each single movement. 15

Cerebral Cortex • The interactions of the neurons within the networks are flexible so that the neurons are capable of responding differently under different circumstances. • This adaptability enhances the possibility of integrating incoming neural signals from diverse sources and the final coordination of many parts into a smooth, purposeful movement. • It probably also accounts for the remarkable variety of ways in which we can approach a goal. For example, you can comb your hair with the right hand or the left, starting at the back of your head or the front. This same adaptability also accounts for some of the learning that occurs in all aspects of motor behavior. 16

Cerebral Cortex • Additional brain areas are also involved in the initiation of intentional movements, such as the association cortices and areas involved in emotion and motivation. • Association areas of the cerebral cortex also play other roles in motor control. For example, neurons of the parietal association cortex are important in the visual control of reaching and grasping. • These neurons play an important role in matching motor signals concerning the pattern of hand action with signals from the visual system concerning the three-dimensional features of the objects to be grasped. 17

Subcortical and Brainstem Nuclei • Numerous highly interconnected structures lie in the brainstem and within the cerebrum beneath the cortex, where they interact with the cortex to control movements. • Their influence is transmitted indirectly to the motor neurons both by pathways that ascend to the cerebral cortex and by pathways that descend from some of the brainstem nuclei. • It is not known to what extent, if any, these structures are involved in initiating movements. • Their role is to establish the programs that determine the specific sequence of movements needed to accomplish a desired action. 18

Subcortical and Brainstem Nuclei • Subcortical and brainstem nuclei are also important in learning skilled movements. • Prominent among the subcortical nuclei are the paired basal nuclei. • This explains why brain damage to subcortical nuclei following a stroke or trauma can result in either hypercontracted muscles, or flaccid paralysis—it depends on which specific circuits are damaged. 19

Parkinson Disease • The input to the basal nuclei is diminished, the interplay of the facilitory and inhibitory circuits is unbalanced, and activation of the motor cortex is reduced. • Clinically, Parkinson disease is characterized by a reduced amount of movement (akinesia), slow movements (bradykinesia), muscular rigidity, and a tremor at rest. • Other motor and nonmotor abnormalities may also be present. For example, a common set of symptoms includes a change in facial expression resulting in a masklike, unemotional appearance, a shuffling gait with loss of arm swing, and a stooped and unstable posture. 20

Parkinson Disease • Although the symptoms of Parkinson disease reflect inadequate functioning of the basal nuclei, a major part of the initial defect arises in neurons of the substantia nigra. These neurons normally project to the basal nuclei, where they release dopamine from their axon terminals. • The substantia nigra neurons degenerate in Parkinson disease, and the amount of dopamine they deliver to the basal nuclei is reduced. This decreases the subsequent activation of the sensorimotor cortex. 21

Parkinson Disease • It is not currently known what causes the degeneration of neurons of the substantia nigra and the development of Parkinson disease. It may be an inherited condition, exposure to environmental toxins such as manganese, carbon monoxide, and some pesticides may play a role. • The drugs used to treat Parkinson disease are all designed to restore dopamine activity in the basal nuclei, and fall into three main categories: 1. Agonists (stimulators) of dopamine receptors 2. Inhibitors of the enzymes that metabolize dopamine at synapses 3. Precursors of dopamine itself (ex. - Levodopa, also known as L-dopa) 22

Cerebellum Fig. 10 -2 a 23

Cerebellum • Is involved in posture and movement indirectly by means of input to brainstem nuclei and (by way of the thalamus) to regions of the sensorimotor cortex that give rise to pathways that descend to the motor neurons. • The cerebellum receives information both from the sensorimotor cortex (relayed via brainstem nuclei) and from the vestibular system, eyes, skin, muscles, joints, and tendons. • One role of the cerebellum in motor functioning is to provide timing signals for precise execution of the different phases of a motor program, in particular the timing of the agonist/antagonist components of a movement. It also helps coordinate movements and is involved in “muscle memory”. 24

Cerebellum • The cerebellum also participates in planning movements— integrating information about the nature of an intended movement with information about the surrounding space. • During movement, the cerebellum compares information about what the muscles should be doing with information about what they actually are doing and can send correction signals if needed. • People with cerebellar disease have uncoordinated movements, cannot start or stop movements quickly or easily, and they cannot combine the movements of several joints into a single smooth, coordinated motion (have trouble walking). 25

Descending Pathways • The influence exerted by the various brain regions on posture and movement occurs via descending pathways to the motor neurons and the interneurons that affect them. • The pathways are of two types: the corticospinal pathways, which, as their name implies, originate in the cerebral cortex; and a second group we will refer to as the brainstem pathways, which originate in the brainstem. 26

Descending Pathways Fig. 10 -12 27

Corticospinal Pathway • The nerve fibers of the corticospinal pathways have their cell bodies in the sensorimotor cortex and terminate in the spinal cord. • The corticospinal pathways are also called the pyramidal tracts or pyramidal system because of their triangular shape as they pass along the ventral surface of the medulla oblongata. • In the medulla oblongata near the junction of the spinal cord and brainstem, most of the corticospinal fibers cross to descend on the opposite side. So skeletal muscles on the left side of the body are controlled largely by neurons in the right half of the brain, and vice versa. 28

Brainstem Pathways • Axons from neurons in the brainstem also form pathways that descend into the spinal cord to influence motor neurons. These pathways are sometimes referred to as the extrapyramidal system. • Axons of most of the brainstem pathways remain uncrossed and affect muscles on the same side of the body (a minority do cross over to contralateral muscles). • The brainstem pathways are especially important in controlling muscles of the trunk for upright posture, balance, and walking. 29

Muscle Tone • Muscle tone is the resistance to stretch exhibited by a relaxed muscle. • It is due to passive elastic forces and to a state of partial contraction due to a slight degree of motor neuron activity. 30

Abnormal Muscle Tone • High muscle is called hypertonia, and It is due to a greater-than-normal level of motor neuron activity. – Hypertonia is accompanied by either spasticity, in which the excess tone diminishes as the muscles are stretched, or rigidity, in which the excess tone is continual. • Low muscle tone is called hypotonia, and it is due to disorders of motor neurons, neuromuscular junctions, or the muscles themselves. – Hypotonia is accompanied by weakness and atrophy. 31

Maintenance of Upright Posture and Balance • Your body needs the skeleton and muscles to work against gravity to keep a you upright. • Added to the problem of maintaining upright posture is that of maintaining balance. For stability, the center of gravity must be kept within the base of support the feet provide. • Once the center of gravity has moved beyond this base, the body will fall unless one foot is shifted to broaden the base of support. Yet, people can operate under conditions of unstable equilibrium because complex interacting postural reflexes maintain their balance. 32

Maintenance of Upright Posture and Balance Fig. 10 -14 33