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7: The Motor System Cognitive Neuroscience David Eagleman Jonathan Downar Chapter Outline Muscles The Spinal Cord The Cerebellum The Motor Cortex The Prefrontal Cortex Basal Ganglia Medial and Lateral Motor Systems Did I Really Do That? 2 Muscles Skeletal Muscle: Structure and Function The Neuromuscular Junction 3 Skeletal Muscle: Structure and Function Bringing about movement is the ultimate goal of the brain. Muscles attach to the skeleton at the origin and insertion. Muscles are collections of many muscle fibers. 4 Skeletal Muscle: Structure and Function 5 Skeletal Muscle: Structure and Function Muscle spindles and Golgi tendon organs provide proprioceptive information from the muscles. Muscles are organized into antagonistic pairs, with extensors extending the joint and flexors contract the joint. 6 The Neuromuscular Junction Motor neurons release neurotransmitters to cause muscle contraction at the neuromuscular junction. The neurotransmitter acetylcholine binds to ionotropic receptors, causing depolarization. If there is enough localized depolarization, voltage-gated ion channels will open. 7 The Neuromuscular Junction The rapid depolarization caused by the opening of voltage-gated ion channels causes the release of calcium. Calcium inside the muscle causes actin and myosin proteins to interact, which brings about a muscle contraction. Acetylcholinesterase removes the neurotransmitter and ends the contraction. 8 The Neuromuscular Junction 9 The Spinal Cord Lower Motor Neurons Spinal Motor Circuits: Reflexes Spinal Motor Circuits: Central Pattern Generators Descending Pathways of Motor Control 10 Lower Motor Neurons Lower motor neurons project from the ventral horn of the spinal cord. Alpha motor neurons cause contraction of the skeletal muscles. Gamma motor neurons adjust the tension in the muscle spindle fibers so they can accurately detect a stretch. The motor unit is the alpha motor neuron and all the muscle fibers it innervates. 11 Lower Motor Neurons 12 Spinal Motor Circuits: Reflexes Reflexes are simple movements coordinated by the spinal cord. Proprioceptors detect a stretch and trigger a motor response to counteract the stretch. The deep tendon reflex, or knee-jerk reflex, is an example of this. 13 Spinal Motor Circuits: Reflexes 14 Spinal Motor Circuits: Central Pattern Generators Neurons within the spinal cord influence rhythmic behaviors, such as walking. Excitatory interneurons stimulate alpha motor neurons to cause a muscle contraction. Inhibitory interneurons are also stimulated, eventually overwhelming the excitation. After a period of inactivity, excitation resumes. Inhibitory interneurons cross the midline, causing alternating contraction and relaxation. 15 Spinal Motor Circuits: Central Pattern Generators 16 Descending Pathways of Motor Control Upper motor neurons from the primary motor cortex project to the spinal cord. About 80% of the axons of the upper motor neurons decussate at the medulla, forming the lateral corticospinal tract. About 10% decussate at the point where they exit the spinal cord. The remainder remain ipsilateral. 17 Descending Pathways of Motor Control 18 Descending Pathways of Motor Control Other descending pathways also influence movement. The rubrospinal tract influences the limbs. The vestibulospinal tract influences balance of the trunk. The tectospinal tract coordinates movements to capture or avoid targets. The reticulospinal tract coordinates startle and escape reflexes. 19 Descending Pathways of Motor Control 20 The Cerebellum The Circuitry of the Cerebellum Motor Functions of the Cerebellum Nonmotor Functions of the Cerebellum 21 The Circuitry of the Cerebellum The cerebellum is important for motor coordination. Injury to the cerebellum results in impairments to the coordination, accuracy, and timing of movements. 22 The Circuitry of the Cerebellum There are three cellular layers of the cerebellum Granule cell layer Purkinje cell layer Molecular cell layer 23 The Circuitry of the Cerebellum 24 The Circuitry of the Cerebellum Purkinje cells generate the output of the cerebellum via inhibitory projections to deep cerebellar nuclei. These nuclei send excitatory connections to the brain and spinal cord. 25 The Circuitry of the Cerebellum Mossy fibers send excitatory input to the granule cells, which excite the molecular cell layer. Climbing fibers project from the olivary nuclei to provide excitatory input to the Purkinje cell bodies. Basket cells and stellate cells provide lateral inhibitory connections. 26 Motor Functions of the Cerebellum Cerebellum may provide forward modeling to fine-tune motor control. It combines sensory and motor information to predict where an object will be at some future point in time. 27 Motor Functions of the Cerebellum 28 Nonmotor Functions of the Cerebellum The cerebellum sends projections to the frontal lobe and influences cognition, emotion, motivation and judgement. Damage to the cerebellum impairs cognition, language perception, and grammar. 29 The Motor Cortex Motor Cortex: Neural Coding of Movements Motor Cortex: Recent Controversies 30 Motor Cortex: Neural Coding of Movements The primary motor cortex (M1) is in the frontal lobe, immediately anterior to the central sulcus. There is a motor homunculus in M1, similar to the somatosensory homunculus found in S1. Areas with more motor control or sensory input are larger in the homunculus. 31 Motor Cortex: Neural Coding of Movements 32 Motor Cortex: Neural Coding of Movements The lateral premotor area, supplementary motor area, and pre-supplementary motor area are anterior to M1. These are motor planning areas and each have their own somatotopic map. 33 Motor Cortex: Neural Coding of Movements 34 Motor Cortex: Neural Coding of Movements The upper motor neurons of M1 project to the lower motor neurons via the corticospinal tracts. They also connect with the interneurons of the spinal cord to influence reflexes and central pattern generators. M1 seems to use population coding to encode direction of movement. 35 Motor Cortex: Neural Coding of Movements 36 Motor Cortex: Recent Controversies Newer research with longer stimulation of M1 suggests the map may be more complex than the homunculus. Longer stimulation evokes complete movements, like moving the hand to the mouth and opening the mouth. There is no obvious population coding of direction with longer stimulation. 37 Motor Cortex: Recent Controversies 38 The Prefrontal Cortex: Goals to Strategies to Tactics to Actions The Functional Organization of the Prefrontal Cortex in Motor Control Sensory Feedback Mirror Neurons in Premotor Cortex Control Stages of the Motor Hierarchy 39 The Functional Organization of the Prefrontal Cortex Actions are the body’s way of transforming needs into goals and then into behaviors. Primary motor cortex and premotor cortex have direct connections to spinal cord to influence movement. Prefrontal cortical areas influence M1 and the premotor cortex, not the spinal cord directly. 40 The Functional Organization of the Prefrontal Cortex 41 The Functional Organization of the Prefrontal Cortex Most motor areas receive extensive input from somatosensory areas. The frontopolar cortex receives no sensory input and connects with other prefrontal areas. This helps set and maintain long-term goals. 42 Sensory Feedback Tactile, proprioceptive, and nociceptive somatosensory feedback helps guide movements. The intraparietal sulcus contains several areas that represent the location of objects in space in relation to different parts of the body. 43 Sensory Feedback 44 Mirror Neurons in Premotor Cortex Mirror neurons are active when performing an action or when observing another individual perform a similar action. Mirror neurons are found in the ventral premotor cortex. 45 Mirror Neurons in Premotor Cortex The action must be goal-directed to cause motor neurons to fire. These neurons may be important for our ability to understand the thoughts and feelings of others. 46 Mirror Neurons in Premotor Cortex 47 Control Stages of the Motor Hierarchy Posterior lateral premotor areas select actions based on sensory input. Intermediate lateral premotor areas choose which sensory rules to use in the current context. Anterior lateral premotor areas select the appropriate context of choosing an action. Most anterior areas keep track of overall goals. 48 Control Stages of the Motor Hierarchy 49 Basal Ganglia Components of the Basal Ganglia Circuitry of the Basal Ganglia Diseases of the Basal Ganglia 50 Components of the Basal Ganglia The basal ganglia project to areas involved in motor control, cognition, and judgement. The basal ganglia are gray matter structures deep within the white matter. The basal ganglia initiate and maintain activity in the cortex. 51 Components of the Basal Ganglia There are three components of the basal ganglia. Striatum Caudate Putamen Globus Pallidus The subthalamic nucleus and the substantia nigra are functionally connected to the basal ganglia. 52 Components of the Basal Ganglia 53 Circuitry of the Basal Ganglia Every area of the cortex interacts with the basal ganglia via recursive loop circuits. There are at least five distinct loops. Motor loop Oculomotor loop Dorsolateral prefrontal loop Lateral orbitofrontal loop Other loops and open circuits also exist. 54 Circuitry of the Basal Ganglia There are two main pathways within the basal ganglia. Indirect pathway is inhibitory. Direct pathway is excitatory. These pathways modulate cortical activity. 55 Circuitry of the Basal Ganglia 56 Diseases of the Basal Ganglia Huntington’s Disease A neurodegenerative disease caused by a dominant genetic mutation. The gene produces huntingtin, and the altered form is toxic to the caudate and putamen. Patients display nonvoluntary rhythmic movements, called chorea. The disease progresses to dementia with psychiatric symptoms. 57 Diseases of the Basal Ganglia 58 Diseases of the Basal Ganglia Parkinson’s Disease Caused by progressive destruction of the dopaminergic neurons of the substantia nigra. The indirect pathway (inhibitory) becomes more active, decreasing excitation to the thalamus and cortex. Symptoms include slow movements and difficulty initiating movements. Treatments involve stimulating dopamine receptors. 59 Medial and Lateral Motor Systems Organization of Medial Motor Areas Functions of Medial and Lateral Motor Systems 60 Organization of Medial Motor Areas The medial motor system controls movements guided by internal motivations. The supplementary motor area and presupplementary motor are part of the medial motor system. Activity in the pre-supplementary motor area begins several seconds before selfinitiated movements. 61 Functions of Medial and Lateral Motor Systems 62 Functions of Medial and Lateral Motor Systems The lateral motor system controls movements guided by external cues. The medial motor system becomes more active when internal signals are needed to select the appropriate action. Damage to the medial motor system results in a lack of spontaneous behavior and excessive externally-driven behavior. 63 Functions of Medial and Lateral Motor Systems 64 Did I Really Do That? The Neuroscience of Free Will Research has tried to identify the brain regions associated with planning a movement. The intent to move occurred about 200 msec before the movement. There was activity in the frontopolar cortex 8 – 10 seconds before the movement. What is the role of free will? 65 Did I Really Do That? The Neuroscience of Free Will 66