10 - Motor I

Question 1

Six areas that control movement

Answer 1

• Cerebral cortex
• Thalamus
• Basal ganglia
• Cerebellum
• Brainstem
• Spinal cord

Question 2

Role of cerebral cortex in movement and how it influences movement directly

Answer 2

Plans and executes complex, voluntary motor tasks

The primary and premotor cortical areas influence movement directly via projections to the brainstem and spinal cord.

Question 3

What four areas of the cerebral cortex are involved in motor activity?

Answer 3

1 - Supplementary motor area (SMA) and premotor cortex (PMC)
2 - Frontal eye field (FEF)
3 - Posterior parietal cortex
4 - Primary motor cortex

Question 4

Supplementary motor area (SMA) and premotor cortex (PMC) location

Answer 4

Frontal lobe (Brodmann’s area 6)

Question 5

Supplementary motor area (SMA) and premotor cortex (PMC) function

Answer 5

design and sequence complex movements involving groups of muscles

The “plan of movement” or “blueprint of motion” is created here

Question 6

Where is the “blueprint” sent for execution

Answer 6

Primary motor cortex

Question 7

What muscle regions does the supplementary motor area (SMA) and premotor cortex (PMC) control?

Answer 7

Control axial (trunk) and proximal limb (girdle) musculature of the upper and lower limbs.

This role helps to orient the trunk
and/or limbs toward the intended direction of movement

Question 8

Frontal Eye Field (FEF)

Answer 8

1. Corresponds to Brodmann’s area 8
2. Projects to brain stem centers that control ocular movements
3. Coordinates eye movements
4. Plays role in visual tracking

Question 9

Posterior Parietal Cortex

Answer 9

Brodmann’s area 7 (in superior parietal lobule)

• Is associated with visual guidance of movement
• Evaluates location or position of body / body parts, and forms a movement plan that would accomplish a task/reach a target

Question 10

The last four regions we went through are involved in motor activity, however there is also one area of the cerebral cortex that INFLUENCES movement

Answer 10

Primary somatosensory cortex (S-I)

Does NOT produce movement, but instead influences movement

Question 11

Location and fibers of primary somatosensory cortex

Answer 11

• It corresponds to Brodmann’s areas 3,1,2
• Is located in the postcentral gyrus (of parietal lobe)
• It gives rise to fibers that descend to terminate in the brain stem and spinal cord

Question 12

Function of primary somatosensory cortex

Answer 12

• Does NOT produce movement, instead, it influences movement by modulating the relay of sensory input from visceral and
  somatic structures to the spinal cord
• Our nervous system receives an immense amount of sensory information
• Some of that sensory information is “filtered” or attenuated by descending fibers
  arising from the somatosensory cortex, so that it does not reach higher
  brain centers and overwhelm the brain

Question 13

What is layer V of the cerebral cortex?

Answer 13

Layer V, the internal pyramidal layer, contains large pyramidal neurons which give rise to axons leaving the cortex and running down to subcortical structures (such as the basal ganglia)

It is very prominent in the motor cortex

Question 14

What does layer V contain?

Answer 14

It contains pyramidal cell bodies…

1. Pyramidal cells are output neurons of the motor cortex
2. Pyramidal cells are upper motor neurons (UMNs)
3. They give rise to axons that form the descending motor pathways
   a. axon terminals synapse mostly with interneurons
   b. interneurons in turn, synapse with motoneurons
   c. axon terminals synapse less often directly with motoneurons

Question 15

Upper motor neurons (UMN)

Answer 15

Their cell bodies reside in the motor cortex or the brainstem

They influence lower motor neurons (LMNs) located in the brainstem or spinal cord

Question 16

Examples of upper motor neurons

Answer 16

Examples of upper motor neurons are the neurons in the following tracts:

1. Corticonuclear tract
2. Anterior and lateral corticospinal tracts
3. Corticoreticular and reticulospinal tracts
4. Corticorubral and rubrospinal tracts
5. Vestibulospinal tracts

Question 17

Lower Motor Neurons (LMNs)

Answer 17

LMNs that control movement of the body, reside in the ventral horn of the spinal cord

Their axons run in peripheral nerves that terminate in skeletal muscle

Innervate skeletal muscle with motor innervation

Question 18

Descending motor tract of upper motor neurons (UMNs)

Answer 18

Corticospinal Tract

Question 19

Pathway of corticospinal tract

Answer 19

• The corticospinal tract arises from the cortex of the frontal and parietal lobes.
• It is a long descending motor tract.
• When it gets to the caudal medulla, it splits into 2 tracts - the lateral and anterior
corticospinal tracts that descend to terminate in the spinal cord where they are
involved in the motor control of the body (not the head).

Question 20

Function of corticospinal tract

Answer 20

• It is involved in skilled, voluntary, movement of the opposite side of the body.
• It is somatotopically organized through its entire path. The motor homunculus
display’s the disproportionate innervation of different parts of the body

Question 21

Origin of corticospinal tract

Answer 21

Origin:
1/3 of its fibers arise from Brodmann’s area 4
1/3 of its fibers arise from Brodmann’s area 6
1/3 of its fibers arise from Brodmann’s areas 5, 7 and 3,1,2

Question 22

Course of corticospinal tract

Answer 22

• Descends through the corona radiata, posterior limb of the internal capsule, basis pedunculi, pons and medulla.
• In the medulla, the corticospinal tract fibers assemble to descend in the pyramid.

Question 23

Decussation (crossing) of the corticospinal tract

Answer 23

When the corticospinal tract reaches the caudal medulla…

• about 85 – 90 % of its fibers decussate in the pyramidal decussation to the opposite side
• the remaining 10 – 15% of fibers do not decussate, but instead, descend on the same side of origin

Question 24

Describe the course of the CROSSED fibers of the corticospinal tract in the spinal cord

Answer 24

• Crossed fibers descend in the lateral funiculus of all spinal cord levels as the lateral corticospinal tract
• Fibers of this tract terminate and synapse at all spinal cord levels, but fiber termination is concentrated in the cervical and LS levels (for the control of the upper and lower limbs).

Question 25

Breakdown of where CROSSED fibers of the corticospinal tract terminate in the spinal cord

Answer 25

55% of fibers terminate in the cervical cord
20% of fibers terminate in the thoracic cord
25% of fibers terminate in the lumbosacral cord

Question 26

Function of the CROSSED fibers of the corticospinal tract

Answer 26

• This tract controls mainly the muscles of the upper and lower limbs, especially the distal muscles of the upper limb.
• It is involved in the execution of distinct, skilled, well-defined manipulative, and independent voluntary movement of the fingers

Question 27

Describe the course of the UNCROSSED fibers of the corticospinal tract in the spinal cord

Answer 27

• Uncrossed fibers descend in the anterior funiculus of the cervical and upper thoracic spinal cord levels as the anterior corticospinal tract.
• Although this tract does not cross in the caudal medulla, fibers DO eventually cross near their level of termination where they synapse in the gray matter.

Question 28

What is unique about the UNCROSSED fibers of the corticospinal tract?

Answer 28

** The descending axons of this tract are the only UMN axons that decussate in the spinal cord **

Recall that even though these fibers are called “uncrossed” they do eventually cross in the spinal cord near the place where they synapse. This is different than the crossed fibers which cross in the caudal medulla (in the pyramidal decussation)

Question 29

Function of the UNCROSSED fibers of the corticospinal tract

Answer 29

This tract controls the axial muscles (neck, shoulder and trunk)

Question 30

UMN lesion

Answer 30

An UMN lesion is a lesion involving UMNs anywhere from the motor cortex to their termination in the brainstem or spinal cord

Question 31

Spastic paralysis

Answer 31

A sign of an UMN lesion…

• Initially, a temporary muscle paresis (weakness) or flaccid paralysis, hypotonia and hyporeflexia develop
• After a variable amount of time, regain function of proximal limb musculature
• Eventually, there is an increase in resting muscle tone (muscles become hypertonic)

Question 32

Cause of spastic paralysis

Answer 32

Descending inhibitory influences are diminished or eliminated, resulting in overactive gamma motor neurons

Severe hypertonicity (spasticity) develops in the distal limb muscles

Spasticity is caused by the exaggeration of the stretch reflex

Spasticity causes resistance to passive motion

Question 33

What are other signs of UMN lesions?

Answer 33

• Hyperreflexia
• Mild muscle atrophy
• Babinski’s sign

Question 34

Describe hyperreflexia

Answer 34

Increase in stretch reflexes (hyperreflexia) due to excess stimulation of gamma motor neurons

Question 35

Describe Babinski’s sign

Answer 35

Extensor plantar response (Babinski’s sign) is the dorsiflexion of the big toe and fanning of the remaining 4 toes, following firm stroking of the sole of the foot

Question 36

LMN lesion

Answer 36

A LMN lesion is a lesion involving

• the cell bodies of the LMN’s in the ventral horn of the spinal cord or
• the axons of these neurons running in peripheral nerves

Question 37

Signs of a LMN lesion

Answer 37

• Decrease in muscle strength
• Decrease in muscle tone (hypotonia) due to lesion of α and γ motor neurons
• Weakening or absence of tendon (stretch) reflexes (hyporeflexia or areflexia)
• Flaccid paralysis
• Severe muscle atrophy (wasting)
• Fibrillations
• Fasciculations

Question 38

Describe the hyporeflexia or areflexia seen in LMN lesions

Answer 38

Normally, when a muscle is passively stretched, it sets off a reflex, causing the muscle to contract. For a normal reflex to occur, an intact sensory neuron and an intact motor neuron are necessary. With a LMN lesion the motor neurons (α and γ) that go directly to the muscle are damaged, interrupting the reflex arc

Question 39

Fibrillations

Answer 39

involuntary contraction of 1 muscle fiber - in electrical recordings

Question 40

Fasiculations

Answer 40

involuntary contraction of a group of muscle fibers - seen grossly

Question 41

Which areas of the corticospinal tract does the anterior cerebral artery supply?

Answer 41

Medial surface (hip, leg and foot area) of the precentral gyrus

Question 42

Which areas of the corticospinal tract does the middle cerebral artery supply?

Answer 42

Lateral surface (trunk, upper limb, head area) of the precentral gyrus

Question 43

The middle cerebral artery gives off lenticulostriate branches. Which areas of the corticospinal tract do they supply?

Answer 43

Posterior limb of the internal capsule

Question 44

The corticospinal tract in the midbrain is supplied by…

Answer 44

posterior cerebral artery, basilar artery

Question 45

The corticospinal tract in the pons is supplied by…

Answer 45

pontine arteries of the basilar artery

Question 46

The corticospinal tract in the rostral medulla is supplied by…

Answer 46

anterior spinal artery, vertebral artery

Question 47

The corticospinal tract in the caudal medulla is supplied by…

Answer 47

anterior spinal artery

Question 48

The corticospinal tract in the cervical spinal cord is supplied by…

Answer 48

anterior spinal artery

same as caudal medulla

Question 49

SUMMARY SLIDE

Answer 49

The corticospinal tract arises from the cortex of the frontal and parietal lobes
It splits into 2 tracts in the caudal medulla

• anterior corticospinal tract (terminates at C and upper T spinal cord levels) - It controls movement of the axial muscles (neck, shoulder, upper trunk)
• lateral corticospinal tract (terminates at all spinal cord levels) - It controls movement of the opposite side of the body

The lateral corticospinal tract is involved in the skilled, voluntary, movement of the body