07-Control of Movement

11/13/2009 The Neurological Control of Movement Mary ET Boyle, Ph.D. Department of Cognitive Science UCSD Levels of Control of Movement • A change in the place or position of the Movement bodyyyp or a body part. • When neurological control of movement is working correctly we can … do anything! • If not, movement disorders such as myasthenia gravis, movement apraxia, ALS, Parkinson’s disease, and Huntington’s disease. 1 11/13/2009 Levels of Control of Movement: Simple to Complex The simplest movements are reflexive reactions withdrawing your hand after touching a hot stove or blinking when something gets in your eye more complex than reflexes, but less complex than other skills maintaining posture, sitting, standing, walking, and eye movement complex movements can be learned playing the violin, riding a bike, and operating exercise equipment 2 11/13/2009 Stimulation of Movement Most basic level of control is the spinal cord (e.g., spinal reflexes, such as the withdrawal reflex, are solely controlled by the spinal cord). Next level involves brain stem structures in the hindbrain and midbrain (e.g., visual pursuit of a light stimulus). 3 11/13/2009 Highest level of control involves the cerebral cortex and structures such as the dorsolateral prefrontal cortex, the primary and secondary motor cortex, and the somatosensory cortex. Basal Ganglia (main components: striatum, pallidum, substantia nigra, and subthalamic nucleus) Influences movement by smoothing out and refining it (gets rid of extraneous movement and acts to ensure that the selected movement occurs with sufficient, but not excessive, force); also responsible for muscle tone and postural adjustments. The corpus striatum and Huntington’s disease – leads to substantial enlargement of the lateral ventricles 4 11/13/2009 Cerebellum Plays a central role in translat ing uncoordinate d movements into a skilled action; receives feedback from sensory receptors that monitor movement and brain stem structures that initiate movement. 5 11/13/2009 Three types of muscle tissue in the body: 6 11/13/2009 7 11/13/2009 extension flexion Contraction of Contraction of the triceps muscle biceps muscle (extensor) (flexor) movement brings the extended away from the body limb back toward the body Skeletal muscle Muscle fibers Myofibrils Myofilaments Myosin Actin (thick filaments) (thin filaments) 8 11/13/2009 Neural Control of Muscle Contraction The motor neurons of the peripheral nervous system control the skeletal muscles. The cell bodies of motor neurons are located in the gray matter of the ventral horn of the spinal cord and in different parts of the brain stem. 9 11/13/2009 10 11/13/2009 Motor Neurons: Transmission of a Neural Impulse Transmission of a neural impulse from motor neuron to muscle fiber at the neuromuscular junction—similar to the transm iss ion of neural impu lses be tween neurons Motor neuron releases ACh into the synaptic cleft. ACh binds to receptor proteins on the postsynaptic membrane (the muscle fiber). EPSPs are then produced; upon sufficient excitation, an action potential is generated. Neuronal voltage gated calcium channel Skeletal muscle sodium c hanne l Potassium channel ACH ACH receptor 11 11/13/2009 As the action potential travels down the muscle fiber, it increases the permeability of the fiber membrane to Ca2+ ions, which causes myosin heads to form cross bridges with actin filaments. The myosin heads pivot, causing the myosin and actin filaments to slide past one another. The Motor Unit A motor neuron and the muscle fibers it controls form a motor unit Each branch of an axon synapses with a single muscle fiber. 12 11/13/2009 Patellar Reflex The reflex in which tapping the tendon of the knee stretches one of the muscles that extends the leg, and the resulting muscle contraction causes the leg to kick outward. Muscle spindle—A structure embedded within an extrafusal muscle fiber than enables the CNS to contract a muscle to counteract the stretching of the extrafusal muscle fiber. Intrafusal muscle fiber— A muscle fiber that extends the length of the muscle spindle that is surrounded by annulospiral endings (sensory receptors in the central part of the muscle fiber). 13 11/13/2009 Components of the Monosynaptic Stretch Reflex When the extrafusal muscle fibers are stretched, so are the intrafusal fibers stimulates the annulospiral endings, causing them to fire more rapidly This increased neural activity travels along the Ia fibers of the annulospiral endings entering the dorsa l root of the spinal cord and synapsing with alpha motor neurons. The Ia fibers have an excitatory influence on alpha motor neurons, causing the extrafusal muscle fibers to contract A Polysynaptic Reflex Withdrawal reflex— The automatic withdrawal of a limb from a painful stimulus the brain can influence the execution of polysynaptic reflexes the spinal cord can also inhibit reflexes to prevent damage to our muscles 14 11/13/2009 Golgi Tendon Organs Receptor located among the fibers of tendons that measures the total amount of force exerted by the muscle on the bone to which the tendon is attached Enables motor system to control extent of muscle contraction. Strength of muscle contraction reflects the force exerted by the muscle on the bone—the greater the contraction of the muscle fibers, the greater the force on the bone. With too much force, Golgi tendon organs reduce the contraction of extrafusal muscle fibers, resulting in the muscle exerting less pressure on the tendon and bone. Renshaw Cells An inhibitory interneuron excited by an alpha motor neuron that causes it to stoppg firing, preventing excessive muscle contraction. CbtCombats muscle damage that can result from fatigue, which results from muscles contracting often in a short period of time. 15 11/13/2009 Gamma Motor Neurons A neuron that synapses with intrafusal muscle fibers to produce continuous muscle tension. Continuous activity of gamma motor neurons produces a constant contraction of extrafusal muscle fibers (muscle tone) This muscle tone is maintained at all times, except during REM sleep. The gamma motor system also gives us the ability to anticipate certain movements and react quickly. Brain Control of Voluntary Movement • Hierarchically organized • Starts at dorsolateral prefrontal cortex. • Decision maker • Sensory input primarily integrated by the posterior parietal cortex. • Learning changes the locus of control over the movements. • Subcortical control. More efficient. 16 11/13/2009 Dorsolateral Prefrontal Cortex The top executive in the perception-action cycle in nonhuman primates and humans. cells in this area integrate sensory information across time with motor actions needed to deal with the information Secondary Motor Cortex Cortical area consisting of the supplementary motor and the premotor areas. Supplemental motor area --involved in the planning and sequencing of voluntary movements (internally generated Premotor cortex plans and stimuli) sequences externally guided movements. Receives input mostly from the visual cortex 17 11/13/2009 Supplemental motor area Part of the secondary motor cortex. receives input from the posterior parietal cortex and the somatosensory cortex Because most movements are guided by both intentions and external stimuli, the connections between the supplementary motor area and the premotor cortex coordinate movement planning. Cortical Control of Movement Primary motor cortex: initiates voluntary movements Directly involved in the control of motor neurons. Stimulation results in movements involving groups of muscles – not individual muscles. 18 11/13/2009 Mirror Neurons Neurons in the primate premotor cortex that are activated by performing an action or by watching another monkey or person performing an action. Also seems to exist in humans, in Broca’s area and the primary motor cortex. Humans learn many actions through the observation and imitation of the actions of others and the mirror neuron system provides a possible mechanism through which observation can be translated into action. Plasticity of the Primary Motor Cortex The primary motor cortex shows great plasticity in its response to sensory and motor changes. Any body part has multiple and widely distributed representations in the topography of the primary motor cortex. 19 11/13/2009 Plasticity of the Primary Motor Cortex Though the exact mechanism for this plasticity is not known, one possibility is long-term potentiation (LTP), a long-lasting increased excitability in a specific neural circuit caused by repetitive stimulation. May represent learning at the cellular level. Depen dent on NMDA receptor activat ion and GABAA receptor inhibition; weakened by the drug scopolamine. Primary Somatosensory Cortex The sensory receptors in the muscles and joints send information about the external environment to the somatosensory cortex and the posterior parietal cortex. It then goes to the dorsolateral prefrontal cortex, the secondary motor cortex, and then the primary motor cortex. The primary motor cortex then becomes aware of the status of the muscles that must be activate d and the location of the body parts that must be moved in order to exert the right amount of force. 20 11/13/2009 Posterior Parietal Cortex Posterior parietal cortex—Cortical area that integrates input from the visual, auditory, and skin senses and relays it to the primary motor cortex, which uses the information to guide our movements. Damage to this area results in difficulty responding to visual, auditory, or somatosensory

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