Descending motor pathways Flashcards
Where is the motor cortex?
- Anterior to the central cortical sulcus
* Occupies ~1/3 of the frontal lobe
What are the 3 subareas of the motor cortex?
- Primary motor cortex
- Premotor area
- Supplementary motor area
What proportion of the motor cortex control hands of the muscles and muscles of the speech?
More than half
What does the excitation of a single motor neurone do?
- Excites a specific movement (not one specific muscle)
* A pattern of separate muscles are excited
Describe the action of the premotor area
- More complex patterns of movement
- most anterior part develops an image for total muscle movement
- Posterior excitation of the specific muscle acitvity that is required
- Sends signals to the primary motor cortex so the specific muscles are excited
Describe the supplemental motor area
- Contractions are ofter bilateral e.g. grasping movement of both hands
- Usually functions together with the premotor area
how have the specialised areas of motor control been identified?
• Electrical stimulation or observation of loss of motor function with destructive lesions
What is Broca’s area
Motor speech area
What is the role of the corticospinal tract?
• Carries signals direct from the cortex to the spinal cord
Where does the corticospinal tract originate from ?
- 30% from the primary motor cortex
- 30% premotor
- 40% somatosensory
Describe the pathway of the corticospinal tract
- Cortex
- Medulla
- Majority cross in the lower medulla
- Lateral corticospinal tracts
- Mainly terminate of interneurones
What happens to the fibres in the corticospinal tract that do not cross in the lower medulla?
They pass ipsilaterally in the ventral corticospinal tracts
Which pathway is conscious movement associated with
Lateral pathways
Which pathway is unconscious movement associated with?
Ventromedial pathways
Describe the lateral pathway
- Principally controlled bu the cerebral cortex via 2 corticospinal tracts
- General control of voluntary moments
- Mainly associated with control of distal muscles
Describe the Ventromedial pathways
- Principally controlled in the brainstem
- Control of posture and rhythmic movements associated with locomotion
- control axial and proximal muscles
What does the brain stem contain?
• Motor and sensory nuclei
What special control functions does the brain stem control?
- Respiration
- Partial control of GI function
- Control of many stereotyped body movements, equilibrium, eye movements
What is the brainstem made up of?
- Medulla
- Pons
- midbrain
What is posture?
Portion of body and its parts relative to each other. Compromise between balance and movement
How to we maintain balance?
Posture is adjusted predominantly by involuntary movement driven both predictably and reactively
What is posture controlled by?
The brainstem
What is the mechanism for unexpected movement?
• Feedback
What is the mechanism for expected movement?
• Feedforward mechanism
How is sensory information integrated in regard to posture control?
- Muscle proprioceptors detect changes in muscle length and/or tension
- Sense of balance derived from movements of the head relative to the Earth’s gravitational field (vestibular apparatus)
- Visual inputs detect movements in visual field representing body movement
Where is sensory information regarding balance and posture predominantly integrated?
In the brainstem
What are the nuclei in the brianstem?
• Reticular nuclei
- pontine and medullary
• Vestibular nuclei
What do the pontine nuclei do?
Excite antigravity muscles
What do the medullary nuclei do
Relax antigravity muscles (works antagonistically to the pontine)
Describe the Pontine Reticular system
- Nuclei transmit signals into the cord via pontine reticulospinal tract
- Fibres terminate on the medial anterior motor neurones that excite axial muscles which support the body against gravity
- When the pontine reticular excretory system is unopposed by the medullary reticular system, there is a powerful excitation of antigravity muscles throughout the body
Where is the pontine reticulospinal tract?
In the anterior of the cord
Describe the excitability of the pontine reticular system
- High degree of natural excitability
* Receive strong signals from the vestibular nuclei and from the deep nuclei of the cerebellum
Describe the medullary reticular system
• Nuclei transmit inhibitory signals to antigravity anterior motor neurones via the medullary reticulospinal tract
Where is the medullary reticulospinal tract?
In the lateral column of the cord
Where do the medullary reticular nuclei receive strong input collaterals from?
- Corticospinal tract
- Rubrospinal tract
- Other motor pathways
When is the medullary reticular system activated?
• Activated to counterbalance pontine reticular system
How is standing caused?
Some signals from higher areas of the brain can disinhibit the medullary system to excite the pontine system
What is the specific role of the Vestibular nuclei?
Selectively control the excitatory signals to the different antigravity muscles depending on the particular posture
• Activated to counterbalance pontine reticular system Equilibrium in repose to signals from the vestibular apparatus
How do the vestibular nuclei transmit signals?
• Transmits strong excitatory signals to antigravity muscles via lateral and medial vestibulospinal tracts in the anterior columns of the spinal cord
What are upper motor neurones?
- in cortex and brainstem nuclei
- Restricted to the CNS and don’t contact muscle
- Executive function for lower motor neurones and circuits controlled LMNs
- Long
What are the lower motor neurones?
- Found in the brainstem and spinal cord
- Not restricted to the CNS and stimulate the muscle to contract
- motor function to muscles
- True alpha motor neuornes
Describe lesions in the upper motor neurones
• Relatively common as long axons
• Signs of lesion war positive or negative signs
- negative is a loss of function
- positive is the appearance of an abnormal response
• Babinski sign (extensor planter relflex) is a positive sign
What do corticospinal lesions give rise to?
Upper motor neurone syndrome
babinski sign
In response to movement of the foot, the toes curl up rather than down (pressure on heel e.g. using a pen)
Describe lower motor neurone lesions
- No atrophy or fasciculations
- Muscle paralysis
- Reduced motor tone
- Reduced stretch reflex
- Fasiculation
- Atrophy
Describe the location of the lower motor neurone
Axon leaves the CNS and innervate muscle
What is hypertonia caused by?
Hyperexcitable spindle neurones which induce over reactive stretch responses
