Descending Pathways

Question 1

Identify the main phases of motor learning.

Answer 1

• Voluntary movement (cognitive phase)
• Associative phase of motor learning
• Autonomous phase of motor learning (i.e. If you do movement repetitively, you can learn it and no longer to think about it)

Question 2

Identify inputs into the corticospinal tract.

Answer 2

Basal ganglia
Cerebellum
Inputs and reflexes from brainstem
Balance from inner ears

Question 3

Where are potential places that movement starts ?

Answer 3

• Bereitschaftspotential from Association MCx?
• Primary Motor Cx?
• Sensory input?
• Limbic system/basal ganglia?

Question 4

Describe the structure of a voluntary movement.

Answer 4

1. Sensory integration: Target identification; glass of wine needs to be identified as such and its location understood
2. Planning; movements required to bring glass of wine to mouth must be ascertained. Relative positions of wine and mouth need to be computed so movement can be planned
3. Execution; commands from cortical and brainstem centres need to be ordered and initiated in order to provide the correct strength grip and to smoothly move the glass from the table to the mouth
   Each phase involves the involvement of distinct areas of the cerebral cortex as well as feedback from basal ganglia and cerebellum

Question 5

Define Brodmann areas.

Answer 5

Areas in the brain distinguished by histological structure (presumed to also correspond to differences in function)

Question 6

Does electrical stimulation of the premotor/SMA produce movement ? Why ?

Answer 6

Electrical stimulation of the area in front of the primary motor area (AKA area 6, contains pre-motor cortex and SMA) also produces movement. However, stronger and more prolonged stimuli are necessary than for the primary motor area.

Question 7

What are the differences between the movements produced by the primary motor cortex and the pre-motor/SM area ?

Answer 7

Movements produced by premotor/SM areas are usually more complex:
• Can involve more than one joint
• Can be bilateral

Question 8

What is the effect of destruction of the primary motor cortex on the pre-motor/SM areas ? What does this mean ?

Answer 8

If primary motor area is destroyed, initially electrical stimulation of either premotor areas has no effect

HENCE, their principal actions are mediated through the primary motor cortex (everything goes through that primary motor cortex but the complicated movement patterns are produced in association cortex)

Question 9

What is the effect of destruction of pre-motor areas ?

Answer 9

• Damage to the premotor areas produces more complex deficits :
Apraxia -cannot perform tasks that involve a complex sequence of movements like brushing one’s hair or drawing a quick sketch (role has been investigated by measuring cerebral blood flow (fMRI))

Question 10

State which motor areas are stimulated in the following:

• Simple finger flexion
• Sequence of finger movements (actual)
• Sequence of finger movements (mental rehearsal)

Answer 10

• Simple finger flexion: Primary motor cortex and primary somatosensory cortex
• Sequence of finger movements (actual): Primary motor cortex and somatosensory cortex + SMA
• Sequence of finger movements (mental rehearsal): Only SMA

Question 11

Where does the premotor cortex connect to ?

Answer 11

It projects fibers to:
1. Brainstem motor nuclei
2. Spinal circuits controlling proximal and axial
muscles (mainly postural)

Question 12

Identify the main roles of the premotor cortex.

Answer 12

The roles of this area are currently being researched and its full function has yet to be described:

1) One function the premotor cortex is thought to be involved in is the plasticity of complex sequences of movements based on prior experience, and is influenced by memory and the limbic system
2) Neurons in this area begin to fire in primates prior to movement, but only when a stimulus requiring movement is detected. Shows an intent to move.
3) It is thought that this area is important in orienting the body in preparation for a voluntary movement
(contributing to establishment of an appropriate postural set)

Question 13

Distinguish between the location in the brain of the premotor cortex, and the SMA.

Answer 13

“The premotor cortex occupies the part of Brodmann area 6 that lies on the lateral surface of the cerebral hemisphere. The medial extension of area 6, onto the midline surface of the hemisphere, is the site of the supplementary motor area, or SMA.”

Question 14

Identify the functions of the parietal cortex.

Answer 14

1) Receives signals from a number of sources but is principally involved with spatial processing of visual stimuli. This information can be integrated in the planning of a sequence of motor outputs.

• somatic sensory area (about limb position)
• vestibular system (about head position)
• premotor areas (about motor plans)
• visual system
• limbic cortex (about motivational state)

The motor system as a whole then integrates information from these modalities to focus attention on relevant target and/or upon the spatial relationships of objects of interest

2) Some neurons in this area are context-specific and are only active when the hand is exploring an object of interest

Question 15

What are the primary spinal motor control routes ?

Answer 15

Corticospinal tracts

Question 16

How many nerve fibers do the corticospinal tracts contain ? How many of these come from the primary motor cortex ? Where does the remainder come from ?

Answer 16

These contain about 1,000,000 nerve fibres, about 1/3 of which come from the primary Motor cortex.
Most of the remainder come from pre-motor areas and association areas

Question 17

Describe the path of the corticospinal tracts.

Answer 17

• Corticospinal tract originates from primary motor cortex. Descending fibers form corona radiata and converge to pass through the posterior limb of the internal capsule.

1) Pyramidal or lateral corticospinal pathways: 90% of axons in the corticospinal tract follow the lateral corticospinal tract and decussate at the ventral pyramids in the brainstem, in the medulla, and descend contralaterally, until they terminate on the LMNs in the anterior horn of the spinal cord.
• The axons in the lateral tract are excitatory and control distal muscle (for example digits), for voluntary movements
- but have collateral branches which feed onto the upper motor nuclei in the brainstem (predictive postural set)
• Feed control to the distal muscles contralaterally
• Mainly controlled by cerebral cortex (via the corticospinal tracts)

2) Ventromedial (anterior/medial) pathways: 10% remain ipsilateral until they finally innervate bilaterally at the level of their ventral root. Cross the midline at the level where they terminate on the LMNs.
• Mainly control posture and locomotion
• Feed control to the axial muscles bilaterally (+ proximal muscles)
• Mainly controlled by the brainstem
• Mainly uncrossed

Question 18

To what extent is the primary motor cortex somatotopically organised ?

Answer 18

1. The primary motor cortex is somatotopically organised (there is a body map projected onto it)
2. Some areas of the body are represented on the somatotopic homunculus with a disproportionate size.
3. This ‘cortical magnification’ reflects the relative ability for precise movement in that part of the body
4. Ergo hands and lips and the tongue have a greater representation than the back
5. Many complex movements are controlled by just a few cortical neurons who exert their effects on interneurons in the spinal cord which then co- ordinate multiple LMNs

Question 19

Identify the main inputs to corticospinal output that fine tune movement.

Answer 19

3 Major sources of input:

1) Sensory receptors
-Either direct to primary motor area or indirectly
via the somatic sensory area, the premotor areas or the posterior parietal association cortex

2) From the Cerebellum
- Both planning (feed forward) movement and corrective feedback from proprioception etc.

3) From the Basal Ganglia
- Both initiating complex movement and correction from the reward system. Overlay of emotional component from limbic circuits.

All movement-related inputs to the cortex coming from other parts of the brain and spinal cord also pass though the Thalamus

Other sources (help to modulate motor system):

• Vestibulospinal pathway (postural stability)
• Reticulospinal pathway (postural adjustments, muscle tone)

Question 20

Distinguish between UMNs, LMNs, and interneurons.

Answer 20

UMNs orchestrate complex directed movements. UMN cell bodies are in brain or brainstem and do not project outside the CNS

LMNs are for single muscle innervation. LMN cell bodies in the ventral horn of spinal cord or motor nuclei of the brainstem, and project outside the CNS to muscle.

Interneurons coordinate groups of muscles

Question 21

Draw each of the main four spinal pathways.

Answer 21

Refer to slide 18 in lecture “Descending Pathways”

Question 22

What is the cause of the following:

• wasting of interossei
• wasting of thenar eminence
• hypertrophy calves
• bilateral wasting of anterior compartment of legs

Answer 22

• wasting of interossei: ulnar nerve compression (possibly at elbow)
• wasting of thenar eminence: median nerve compression
• hypertrophy calves: some muscle dystrophies (e.g. Duchenne’s)
• bilateral wasting of anterior compartment of legs: compression of fibular head (possibly due to tight boots), leading to compression of lateral popliteal nerve, causing wasting of tibialis anterior, and possible foot drop

Question 23

Define posture. Why is it important ?

Answer 23

• Posture is the position of a body and its parts relative to each other
• Vitally important in balance

Question 24

Which pathway is mainly responsible for posture ?

Answer 24

Ventromedial pathways

Question 25

Identify the main mechanisms that set posture. How can you interrupt these mechanisms ?

Answer 25

• adjusted predominantly by involuntary movement driven both

1) predictively (postural set) and
2) reflexively (compensation)

Stand with your heels to the wall and touch your toes to interrupt these processes

Question 26

Identify descending pathways other than corticospinal pathway.

Answer 26

Other descending pathways (which all feed into the corticospinal pathway) are the compensatory reflexes by the following systems:

• Vestibular system - body
• Collicular system – head/body (four colliculi in the midbrain, in area quadrigemina, 2 superior and 2 inferior. Enable us to respond with head movements in response to superior colliculi (visual input) and inferior (auditory input).
• Reticular system - body
• Tectal system– head and trunk

Question 27

What information do the compensatory reflexes depend on ? Where in the brain does this occur ?

Answer 27

Compensatory responses are based on information sent from:

1. Muscle proprioceptors (detect changes in muscle length and or tension)
2. Sense of balance derived from movements of the head relative to the Earths gravitational field (vestibular apparatus)
3. Visual inputs (detecting movements in visual field representing movement of the body)

These sensory inputs converge on nuclei in the brainstem and so the postural set is generated there.
These nuclei all receive information from the voluntary circuit collaterals which provides feed forward information

Question 28

Describe the path of the vestibular pathway.

Answer 28

First order neurons come from the cerebellum, and synapse either at the medial or lateral vestibular nucleus.

The medial vestibular nucleus projects bilaterally to cervical MNs to provide control over the position of the head

The lateral vestibular nucleus projects ipsilaterally to spinal MNs to provide extensor control over ipsilateral proximal limb as well as axial muscles

Question 29

Describe the function of the medial longitudinal fasciculus.

Answer 29

Medial longitudinal fasciculus- allow us to communicate between different nerve nuclei in the brainstem. It notably “links the three main nerves which control eye movements, i.e. the oculomotor, trochlear and the abducent nerves, as well as the vestibulocochlear nerve”, and is therefore essential for coordinating eye movement, but also for coordinating vestibular pathways.

Question 30

Identify examples of primitive reflexes. Why is this important clinically ?

Answer 30

Symmetrical Tonic Neck Reflex: “In an infant, flexion or extension of the arms in response to flexion and extension, respectively, of the neck”

Asymmetrical Tonic Neck Reflex (AKA cervicospinal reflex)

Grasp Reflex

Some of these reflexes can come back, in certain pathologies (e.g. Grasp reflex in dementia patients)

Question 31

Define asymmtrical tonic neck reflex.

Answer 31

Reflex that mediates limb movement as a function of head on trunk position and augments the vestibular reflexes (since the position of the neck differentially alters tone in the muscles of the arm). It sends messages via the vestibular nucleus along vestibulospinal tracts. Therefore, if turn baby’s head to one side, arm will go out towards that side and arm on opposite side will come up

Question 32

Describe a primitive reflex that may come back in a decerebrate patient (no cerebral or vestibular input).

Answer 32

Asymmetrical Tonic Neck Reflex

Question 33

What is the reticulospinal pathway ?

Answer 33

Part of the medial pathways

Question 34

Identify the main inputs, and outputs, of the reticulospinal pathway.

Answer 34

Information for integration in the reticulum comes from

1. Principally cortical areas (corticoreticular pathways)
2. Vestibular nuclei
3. Proprioception (ascending spinal), vision
4. Cerebellar output
   (5. Limbic system also influences on medullary reticulum)

THEN, information about how to counter any postural instability is processed in the reticulum, a course of action is decided upon and corrective output is routed via

1. Alpha motor neurons innervating trunk and proximal limb effectors (correct for shift in center of gravity etc.)
2. Excitation/inhibition of gamma motor neurons which increase muscle tone (correct for shift in weight bearing etc.)

Question 35

Describe the relationship between the reticulospinal pathway and the limbic system.

Answer 35

A further influence on the medullary reticulum (in addition to cortical areas, vestibular nuclei etc.) is that of the Limbic system (note the basal ganglia also are used to express emotion)

This influence manifests as the physical expression of emotion:

• A depressed person will adopt a depressed stance and generally appear sad
• A happy person will appear happier and more engaged

I.E. This forms the basis of body language

Question 36

Identify an example of tectal reflex.

Answer 36

Owl: maintains head in more or less exact same, upright position despite body movement, even if eyes are covered.

Question 37

Describe corticobulbar tracts.

Answer 37

“-Two-neuron white matter motor pathway connecting the motor cortex in the cerebral cortex to the medullary pyramids.

• Primarily involved in carrying the motor function of the non-oculomotor cranial nerves.”
• Cross at level of Cr N

Question 38

A lesion in the corticospinal pathway will cause a lesion where ?

Answer 38

A brainstem lesion in this pathway will produce contralateral UMN signs