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 ID: 908436
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Slide1
7: The Motor System
Cognitive Neuroscience
David Eagleman
Jonathan
Downar
Slide2Chapter Outline
Muscles
The Spinal Cord
The CerebellumThe Motor CortexThe Prefrontal CortexBasal GangliaMedial and Lateral Motor SystemsDid I Really Do That?
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Slide3Muscles
Skeletal Muscle: Structure and Function
The Neuromuscular Junction
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Slide4Skeletal 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.
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Slide5Skeletal Muscle: Structure and Function
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Slide6Skeletal 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.
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Slide7The 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.
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Slide8The 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.
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Slide9The Neuromuscular Junction
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Slide10The Spinal Cord
Lower Motor Neurons
Spinal Motor Circuits: Reflexes
Spinal Motor Circuits: Central Pattern GeneratorsDescending Pathways of Motor Control10
Slide11Lower 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.
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Slide12Lower Motor Neurons
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Slide13Spinal 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.
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Slide14Spinal Motor Circuits: Reflexes
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Slide15Spinal 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.
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Slide16Spinal Motor Circuits: Central Pattern Generators
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Slide17Descending 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.
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Slide18Descending Pathways of Motor Control
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Slide19Descending 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.
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Slide20Descending Pathways of Motor Control
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Slide21The Cerebellum
The Circuitry of the Cerebellum
Motor Functions of the Cerebellum
Nonmotor Functions of the Cerebellum21
Slide22The 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.
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Slide23The Circuitry of the Cerebellum
There are three cellular layers of the cerebellum
Granule cell layer
Purkinje cell layerMolecular cell layer
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Slide24The Circuitry of the Cerebellum
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Slide25The 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.
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Slide26The 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.
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Slide27Motor 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.
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Slide28Motor Functions of the Cerebellum
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Slide29Nonmotor
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.
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Slide30The Motor Cortex
Motor Cortex: Neural Coding of Movements
Motor Cortex: Recent Controversies
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Slide31Motor 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.
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Slide32Motor Cortex: Neural Coding of Movements
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Slide33Motor 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.
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Slide34Motor Cortex: Neural Coding of Movements
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Slide35Motor 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.
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Slide36Motor Cortex: Neural Coding of Movements
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Slide37Motor 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.
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Slide38Motor Cortex: Recent Controversies
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Slide39The 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 CortexControl Stages of the Motor Hierarchy
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Slide40The 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.
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Slide41The Functional Organization of the Prefrontal
Cortex
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Slide42The 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.
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Slide43Sensory 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.
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Slide44Sensory Feedback
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Slide45Mirror 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.
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Slide46Mirror 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.
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Slide47Mirror Neurons in Premotor Cortex
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Slide48Control 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.
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Slide49Control Stages of the Motor Hierarchy
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Slide50Basal Ganglia
Components of the Basal Ganglia
Circuitry of the Basal Ganglia
Diseases of the Basal Ganglia50
Slide51Components 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.
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Slide52Components of the Basal Ganglia
There are three components of the basal ganglia.
Striatum
CaudatePutamenGlobus PallidusThe subthalamic
nucleus and the substantia nigra are functionally connected to the basal ganglia.
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Slide53Components of the Basal Ganglia
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Slide54Circuitry 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 loopOculomotor loopDorsolateral prefrontal loopLateral orbitofrontal loop
Other loops and open circuits also exist.
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Slide55Circuitry 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.
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Slide56Circuitry of the Basal Ganglia
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Slide57Diseases 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.
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Slide58Diseases of the Basal Ganglia
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Slide59Diseases 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.
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Slide60Medial and Lateral Motor Systems
Organization of Medial Motor Areas
Functions of Medial and Lateral Motor Systems
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Slide61Organization of Medial Motor Areas
The medial motor system controls movements guided by internal motivations.
The supplementary motor area and pre-supplementary motor are part of the medial motor system.
Activity in the pre-supplementary motor area begins several seconds before self-initiated movements.
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Slide62Functions of Medial and Lateral Motor Systems
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Slide63Functions 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.
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Slide64Functions of Medial and Lateral Motor Systems
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Slide65Did 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?
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Slide66Did I Really Do That? The Neuroscience of Free Will
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