Download Eagleman Ch 7. The Motor System

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.

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Skeletal Muscle: Structure and
Function
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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.

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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.

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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.

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The Neuromuscular Junction
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The Spinal Cord
Lower Motor Neurons
 Spinal Motor Circuits: Reflexes
 Spinal Motor Circuits: Central Pattern
Generators
 Descending Pathways of Motor Control

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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.
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Lower Motor Neurons
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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.

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Spinal Motor Circuits: Reflexes
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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.
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Spinal Motor Circuits: Central
Pattern Generators
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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.

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Descending Pathways of Motor
Control
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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.
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Descending Pathways of Motor
Control
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The Cerebellum
The Circuitry of the Cerebellum
 Motor Functions of the Cerebellum
 Nonmotor Functions of the Cerebellum

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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.

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The Circuitry of the Cerebellum

There are three cellular layers of the
cerebellum
 Granule
cell layer
 Purkinje cell layer
 Molecular cell layer
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The Circuitry of the Cerebellum
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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.

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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.

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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.
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Motor Functions of the
Cerebellum
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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.

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The Motor Cortex
Motor Cortex: Neural Coding of
Movements
 Motor Cortex: Recent Controversies

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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.

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Motor Cortex: Neural Coding of
Movements
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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.

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Motor Cortex: Neural Coding of
Movements
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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.

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Motor Cortex: Neural Coding of
Movements
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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.

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Motor Cortex: Recent
Controversies
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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

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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.

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The Functional Organization of
the Prefrontal Cortex
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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.

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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.

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Sensory Feedback
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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.

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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.

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Mirror Neurons in Premotor
Cortex
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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.

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Control Stages of the Motor
Hierarchy
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Basal Ganglia
Components of the Basal Ganglia
 Circuitry of the Basal Ganglia
 Diseases of the Basal Ganglia

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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.
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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.
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Components of the Basal
Ganglia
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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.
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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.
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Circuitry of the Basal Ganglia
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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.
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Diseases of the Basal Ganglia
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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.
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Medial and Lateral Motor
Systems
Organization of Medial Motor Areas
 Functions of Medial and Lateral Motor
Systems

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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.

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Functions of Medial and Lateral
Motor Systems
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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.

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Functions of Medial and Lateral
Motor Systems
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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?

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Did I Really Do That? The
Neuroscience of Free Will
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