12-09-23 - Descending pathways - control of movement Flashcards
Learning outcomes
- Explain the neural pathways & processes involved in voluntary and involuntary movement
- Recall the signs of upper and lower motor neuron damage
- Recall examples of disease affecting upper / lower MN damage
Organisation of the motor system.
What are the 3 different types of neurons of the motor system?
Where do they each extend between?
What are they each responsible for?
- Organisation of the motor system
- 3 different types of neurons of the motor system:
1) Upper motor neurons (UMN)
* UMN cell bodies are in brain or brainstem and do not project outside the CNS
* Orchestrate complex directed movements
2) Interneurons
* Coordinate groups of muscles
3) Lower motor neurons (LMN)
* LMN cell bodies are in brainstem or spinal cord and project outside the CNS to muscles
* Single muscle innervation, with cell bodies in the ventral horn of spinal cord or motor nuclei of the brainstem
What are the 2 major systems of descending pathway from the cortex to motor centre?
What do they each control?
What are they controlled by?
- 2 major systems of descending pathway from the cortex to motor centre:
1) Lateral pathways
* Control voluntary movements
* Control distal muscles
* Mainly controlled by cerebral cortex (via the corticospinal tracts)
2) Ventromedial (medial) pathways
* Mainly control posture and locomotion
* Control the axial and proximal muscles
* Mainly controlled by the brainstem
* Mainly uncrossed
What are 4 characteristics of voluntary movements?
- 4 characteristics of voluntary movements:
1) Purposeful goal directed
2) Triggered either by imagination or by a wilful decision
3) Where the goal could be achieved by different strategies. e.g. writing on a small piece of paper or writing on a blackboard = Motor Equivalence – illustrates multiple levels of control of movement
4) Often learned may initially need much concentration (Driving or playing a guitar) but with practice movement can be achieved fluently. Efficiency and accuracy improves
What are 3 stages in the structure of a voluntary movement such as Drinking a glass of wine?
What does each phase involve?
- 3 stages in the structure of a voluntary movement such as Drinking a glass of wine:
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
Where does the upper motor neuron system run between?
What area on Broadmann’s cortical map are associated with the primary motor and primary somatosensory cortex?
- The upper motor neuron system stays in the CNS (Cortex to spinal cord/brain stem nuclei)
- In Broadmann’s cortical map, Area 4 is associated with the primary motor cortex
- Areas 1, 2, and 3 are associated with the primary somatosensory cortex
Cortical motor areas.
What can the primary motor cortex be used for during surgery?
How is the primary motor cortex organised?
How do sizes of areas on the somatotopic areas vary?
How has the primary motor cortex been mapped?
- Cortical motor areas
- During brain surgery, stimulation of the primary motor cortex elicits specific movements of defined parts of the body
- The primary motor cortex is somatotopically organised (there is a body map projected onto it)
- Some areas of the body are represented on the somatotopic homunculus with a disproportionate size.
- This ‘cortical magnification’ reflects the relative ability for precise movement in that part of the body
- Ergo hands and lips and the tongue have a greater representation than the back
- Lesions and electrical stimulation studies have mapped the primary motor cortex
What 3 cortical areas interact to generate movement?
- 3 cortical areas that interact to generate movement:
1) Premotor cortex
2) Supplementary motor cortex
3) Primary motor cortex
Premotor cortical areas (the primary motor cortex does not work alone).
What happens when the pre-motor cortex is electrically stimulated?
How does this differ from primary motor cortex stimulation?
How do movements differ?
- Premotor cortical areas (the primary motor cortex does not work alone)
- Electrical stimulation of the area in front of the primary motor area (pre-motor cortex) also produces movement
- However, stronger and more prolonged stimuli are necessary than for the primary motor area – implies a less direct path to Lower MNs
- Movements produced are usually more complex:
1) Can involve more than one joint
2) Can be bilateral
Where does the premotor cortex project fibres to?
What are 3 functions of the premotor complex?
- The premotor cortex projects fibres to:
1) Brainstem motor nuclei
2) Spinal circuits controlling proximal and axial muscles (mainly postural) - 3 functions of the premotor complex:
1) Plasticity of movements
* Plasticity of complex sequences of movements based on prior experience, and is influenced by memory and the limbic system
2) Intent to move
- 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) Orientation of body prior to movement
- It is thought that this area is important in orienting the body in preparation for a voluntary movement (contributing to establishment of an appropriate posture)
What happens if the primary motor areas is destroyed?
What does this prove?
What does damage to premotor areas cause?
What is apraxia?
- If primary motor area is destroyed, initially electrical stimulation of either premotor area has no effect
- Therefore, their principal actions are mediated through the primary motor cortex
- 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
What is the role of the supplementary cortex?
When is only the supplementary cortex stimulated?
- The role of the supplementary motor cortex is less clear.
- One function however has been demonstrated by measuring cerebral blood flow (fMRI)
- Mental rehearsal of complex movements stimulates only the supplementary cortex
What are 3 other major sources of inputs to corticospinal output that fine tune movement?
- 3 other major sources of inputs to corticospinal output that fine tune movement:
1) Sensory receptors
* 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 motor correction plus an overlay of an emotional component from limbic circuits.
General organisation of the descending neuronal tracts involved in voluntary movement.
Describe the simplified summary of descending pathways diagram (in picture).
What is the role of the anterior and lateral corticospinal tract?
Are they voluntary or involuntary?
- General organisation of the descending neuronal tracts involved in voluntary movement.
- Simplified summary of descending pathways diagram (in picture)
- Anterior corticospinal tract:
- Main role is postural function e.g picking up a weights from the floor with the left hand will cause the back to stiffen in anticipation
- Half voluntary, half involuntary
- Remains ipsilateral from the side its generated on
- Only really for the upper part of the spinal cord
- Lateral corticospinal tract:
- Controls distal muscles e.g feet, hands, legs – voluntary stuff that allows you to pick things up and move them around
- 90% of fibres belong to this tract
- Fibres descend down the spinal cord and branch at the appropriate level of the LMN, which go out to muscles
Organisation of the cortical tracts involved in voluntary movement. What is the primary spinal motor control route?
How many nerves fibres does this the corticospinal tract contain?
Where does the remainder come from?
Where do Most axons in the corticospinal tract decussate?
What happens to the remaining axons?
What is the role of axons in the lateral corticospinal tract?
- Organisation of the cortical tracts involved in voluntary movement
- The primary spinal motor control route is the corticospinal tract
- This contains about 1,000,000 nerve fibres, about 2/3 of which come from the primary Motor cortex.
- Most of the remainder come from pre-motor areas and association areas
- Most axons in the corticospinal tract decussate at the ventral pyramids in the brainstem (lateral corticospinal tract).
- 10-15% remain ipsilateral until they finally innervate bilaterally at the level of their ventral root (anterior/medial corticospinal)
- The axons in the lateral tract are voluntary motor and control distal muscle (for example digits)
What % of fibres follow the lateral and anterior corticospinal?
Where do these pathways lead to?
- 85-90% follow the Lateral corticospinal path (decussation)
- These pathways feed control to the distal muscles
- Contralaterally 10% follow the Anterior (medial) corticospinal path (no decussation)
- These pathways feed control to the axial muscles Bilaterally
Describe the diagram of the corticospinal tract (in picture)
How can we figure out where the lesion in the lateral corticospinal tract is?
- Because the lateral corticospinal tract axons decussate at the ventral pyramids in the brainstem, the loss of a particular motor output allows us to decide roughly where level of where the lesion may be
Neural elements of movement diagram
How common are Lesions of corticospinal neurons?
Why is this?
What are they most commonly caused by?
Which artery is particularly vulnerable?
What 3 other things can damage the corticospinal system?
- Lesions of corticospinal neurons are relatively common as these axons are very long (therefore vulnerable) These neurons are Upper motor neurons
- They are most commonly caused by infarcts related to cerebrovascular incidents
- The middle cerebral artery (supplying the lateral surface of the hemispheres and the internal capsule) is particularly vulnerable
3 other things can damage the corticospinal system:
1) Trauma
2) Tumors
3) Demyelinating diseases
What 2 ways do Lesions of corticospinal neurons present?
What can be a good diagnostic tool for locating the lesion?
What is an example of a positive sign seen following corticospinal lesions?
- Lesions of corticospinal neurons present as either positive or negative signs (based on descending inhibition or excitation)
- Negative is a loss of function, (weakness or paralysis)
- NB the site of weakness or paralysis provides a good diagnostic tool for locating the possible site of lesion (above the point of muscle weakness will be the location of a lesion)
- Positive is the appearance of an abnormal response.
- Extensor Planter reflex (Babinski sign) is an example of a positive sign seen following corticospinal lesions
What do Corticospinal lesions give rise to?
What do UMN synapse with?
How does UMN of the internal capsule differ?
Compare UMN syndrome and LMN lesions in the following areas (in picture):
* Muscle weakness
* Atrophy
* Fasciculation
* Muscle tone
* Stretch reflexes
* Abnormal reflexes
- Corticospinal lesions give rise to Upper motor neurone syndrome
- Upper motor neurons (UMN) synapse with lower motor neurons (LMN), and lower motor neurons synapse with muscle.
- Upper motor neurons do not leave the CNS
- For UMN disease from lesions of the internal capsule, hypotonia precedes hypertonia
- Compare UMN disease and LMN lesions (in picture):
What are the 4 different types of UMN pattern weakness?
- 4 different types of UMN pattern weakness:
1) Quadriplegia
* Paralysis/muscle weakness of arms and legs
1) Paraplegia
* Being paralyze/muscle weakness from the waist down
2) Monoplegia
* Paralysis/muscle weakness in one limb
3) Hemiplegia
* Being paralyzed/muscle weakness on one side of the body
* Arm extensors weaker than flexors (flexors are stronger)
* Leg flexors weaker than extensors (extensors are stronger)
Involuntary (ventromedial) motor pathways. Ventromedial pathways – Posture.
What is posture?
What is it important for?
How is posture adjusted?
How can we interrupt this process?
What is vital for seamless movement and adaptation to the environment?
Describe the postural adjustment flow chart (in picture)
- Involuntary (ventromedial) motor pathways
- Ventromedial pathways - Posture
- Posture is the position of a body and its parts relative to each other
- It is vitally important in balance
- adjusted predominantly by involuntary movement driven both predictively (postural set) and reflexively (compensation).
- Stand with your heels to the wall and touch your toes to interrupt these processes
- The interplay between voluntary and involuntary movement is vital for seamless movement and adaptation to the environment
- Postural adjustment flow chart (in picture)
Compensatory reflex pathways.
What are the 4 different Principal compensatory reflexes?
What part of the body are they each for?
- Compensatory reflex pathways
- 4 different Principal compensatory reflexes:
1) Vestibular - body
2) Collicula – head/body
3) Reticular - body
4) Tectal – head and trunk
What is the vestibular apparatus? What does it allow us to do?
What happens to information in the vestibular apparatus?
What is the role of the corpora quadrigemina?
- Thes vestibular apparatus is a series of structures in the inner ear
- They allow us to sense where our body is in relation to gravity, and how fast we are accelerating in a particular plane
- Information in the vestibular apparatus ascends to the corpora quadrigemina
- The corpora quadrigemina control the reflex movements for sound and sight:
- 2 Superior colliculi of corpora quadrigmina control reflexes that respond to physical events e.g moving out of the way of a falling stick
- 2 inferior colliculi of corpora quadrigmina control reflexes that respond to sound e.g ducking from a large bang
Where do the 2 principal vestibular tracts arise from?
Where do they project?
What structures do they provide control over?
What are both of these pathways influenced by?
- There are 2 principal vestibulospinal tracts:
1) One arises from the medial vestibular nucleus
* It projects bilaterally to cervical MNs to provide control over the position of the head (as we fall our head will remain horizontal
2) The other arises from the lateral vestibular nucleus
* It projects ipsilaterally to spinal MNs to provide extensor control over ipsilateral proximal limb as well as axial muscles – ie antigravity muscles and righting reflexes
- Both of these pathways are influenced by the cerebellum
What are the 2 primary effects of the Lateral vestibulospinal pathway?
What are some of the neurons facilitated in this pathway?
How does affect stretch reflexes?
What is the overall effect of the Lateral vestibulospinal pathway?
- 2 primary effects of the Lateral vestibulospinal pathway:
1) To facilitate ipsilateral extensor motor neurones
2) Inhibit flexor motor neurones innervating the proximal and axial muscles
- Some of the neurones facilitated are Gamma motor neurones ( a type of lower MN)
- This then enhances muscle spindle mediated stretch reflex’s
- The overall effect of the Lateral vestibulospinal pathway is to increase the tone to the antigravity muscles when the vestibular system signals a loss of postural stability
Tectospinal pathway (also called colliculospinal pathway).
Where does the tectospinal pathway arise?
How far does this pathway project?
What are 2 functions of the tectospinal pathway?
- Tectospinal pathway (also called colliculospinal pathway).
- The Tectospinal pathway (contralateral pathway) arises in the superior colliculus which receives a direct input from the retina.
- This pathway only projects as far as the cervical spinal segments
- 2 functions of the tectospinal pathway:
1) Coordinate eye movements to head and neck movements.
2) Move the neck/head (inputs through the vestibular nucleus – not shown on diagram) So points of visual interest can be focussed on the fovea
