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7: The Motor System 7: The Motor System

7: The Motor System - PowerPoint Presentation

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7: The Motor System - PPT Presentation

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 ID: 577520

cortex motor basal neurons motor cortex neurons basal ganglia movements cerebellum spinal lateral areas muscle medial premotor control cord

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Slide1

7: The Motor System

Cognitive Neuroscience

David Eagleman

Jonathan

DownarSlide2

Chapter Outline

Muscles

The Spinal Cord

The CerebellumThe Motor CortexThe Prefrontal CortexBasal GangliaMedial and Lateral Motor SystemsDid I Really Do That?

2Slide3

Muscles

Skeletal Muscle: Structure and Function

The Neuromuscular Junction

3Slide4

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.

4Slide5

Skeletal Muscle: Structure and Function

5Slide6

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.

6Slide7

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.

7Slide8

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.

8Slide9

The Neuromuscular Junction

9Slide10

The Spinal Cord

Lower Motor Neurons

Spinal Motor Circuits: Reflexes

Spinal Motor Circuits: Central Pattern GeneratorsDescending Pathways of Motor Control10Slide11

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.

11Slide12

Lower Motor Neurons

12Slide13

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.

13Slide14

Spinal Motor Circuits: Reflexes

14Slide15

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.

15Slide16

Spinal Motor Circuits: Central Pattern Generators

16Slide17

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.

17Slide18

Descending Pathways of Motor Control

18Slide19

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.

19Slide20

Descending Pathways of Motor Control

20Slide21

The Cerebellum

The Circuitry of the Cerebellum

Motor Functions of the Cerebellum

Nonmotor Functions of the Cerebellum21Slide22

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.

22Slide23

The Circuitry of the Cerebellum

There are three cellular layers of the cerebellum

Granule cell layer

Purkinje cell layerMolecular cell layer

23Slide24

The Circuitry of the Cerebellum

24Slide25

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.

25Slide26

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.

26Slide27

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.

27Slide28

Motor Functions of the Cerebellum

28Slide29

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.

29Slide30

The Motor Cortex

Motor Cortex: Neural Coding of Movements

Motor Cortex: Recent Controversies

30Slide31

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.

31Slide32

Motor Cortex: Neural Coding of Movements

32Slide33

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.

33Slide34

Motor Cortex: Neural Coding of Movements

34Slide35

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.

35Slide36

Motor Cortex: Neural Coding of Movements

36Slide37

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.

37Slide38

Motor Cortex: Recent Controversies

38Slide39

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 CortexControl Stages of the Motor Hierarchy

39Slide40

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.

40Slide41

The Functional Organization of the Prefrontal

Cortex

41Slide42

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.

42Slide43

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.

43Slide44

Sensory Feedback

44Slide45

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.

45Slide46

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.

46Slide47

Mirror Neurons in Premotor Cortex

47Slide48

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.

48Slide49

Control Stages of the Motor Hierarchy

49Slide50

Basal Ganglia

Components of the Basal Ganglia

Circuitry of the Basal Ganglia

Diseases of the Basal Ganglia50Slide51

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.

51Slide52

Components 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.

52Slide53

Components of the Basal Ganglia

53Slide54

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 loopOculomotor loopDorsolateral prefrontal loopLateral orbitofrontal loop

Other loops and open circuits also exist.

54Slide55

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.

55Slide56

Circuitry of the Basal Ganglia

56Slide57

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.

57Slide58

Diseases of the Basal Ganglia

58Slide59

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.

59Slide60

Medial and Lateral Motor Systems

Organization of Medial Motor Areas

Functions of Medial and Lateral Motor Systems

60Slide61

Organization 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.

61Slide62

Functions of Medial and Lateral Motor Systems

62Slide63

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.

63Slide64

Functions of Medial and Lateral Motor Systems

64Slide65

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?

65Slide66

Did I Really Do That? The Neuroscience of Free Will

66