L09 - Motor Control: Role of the Motor Cortex & Spinal Reflexes Flashcards
What are central pattern generators?
Central pattern generators are networks of neurones in the spinal cord which can generate specific learned movements such as chewing, walking etc without initiation from higher centres
*NB input from the brain can still superimpose on this activity
Which fibres connect the ventral horn of the spinal cord to muscle fibres in the periphery?
⍺ motoneurones
How are motoneurones of the descending pathways arranged in the spinal cord?
Motoneurones of the descending pathway are located in the ventral horn
- Ventromedial motoneurones in the ventral horn are the involuntary component responsible for the innervation of axial muscles and proximal muscles of the limbs
- Lateral motoneurones are the voluntary component responsible for the innervation of distal muscles (hands, feet and digits)
List the lateral descending pathways.
Lateral descending pathways:
1 - Corticospinal tract
2 - Rubrospinal tract
List the ventromedial descending pathways.
Ventromedial descending pathways:
1 - Tectospinal tract
2 - Vestibulospinal tract
3 - Pontine reticulospinal tract
4 - Medullary reticulospinal tract
Give an example of a structure involved in the modulation of the lateral descending pathways.
Where is this structure located?
- The red nucleus is involved in the modulation of the lateral descending pathways
- The red nucleus is located in the medial tegmentum, dorsal to the substantia nigra
List 3 structures involved in the modulation of ventromedial descending pathways.
Where are these structures located?
Structures involved in the modulation of ventromedial descending pathways:
1 - The superior colliculus (paired structures on the posterior midbrain)
2 - Vestibular nuclei (in the brainstem - nucleus of CN VIII)
3 - Reticular formation (in tegmentum surrounding aqueduct)
Which tract is the main tract used for voluntary movement?
The corticospinal tract
What would be expected from a lesion of the lateral pathways?
Slowed voluntary movements but unaffected posture (as posture is controlled by ventromedial pathways)
How is loss of control of muscle contraction due to corticospinal tract damage compensated for?
The rubrospinal tract is able to compensate almost entirely for the loss of corticospinal tract function, with the exception of fine digit control
Where do lateral descending pathways originate?
Where do ventromedial pathways originate?
- Lateral descending pathways originate in the motor cortex
- Ventromedial descending pathways originate in the brainstem nuclei
Are the lateral descending pathways ipsilateral or contralateral?
Are the ventromedial descending pathways ipsilateral or contralateral?
- Lateral descending pathways are all contralateral
- Some ventromedial descending pathways are contralateral, whereas some are ipsilateral
With which structures do the ventromedial pathways synapse?
1 - The pontine reticulospinal and medullary reticulospinal tracts synapse with the nuclei of the reticular formation
2 - The vestibulospinal and tectospinal tracts synapse with the superior colliculus and vestibular nuclei
What are the functions of the pontine reticulospinal tract?
1 - It enhances anti-gravity reflexes of the spinal cord
2 - It facilitates the leg extensors to maintain standing posture
What are the functions of the medullary reticulospinal tract?
1 - It frees anti-gravity muscles from reflex control
2 - It facilitates voluntary override
What are the functions of the vestibulospinal tract?
1 - Relays gravitational sensory info from vestibular labyrinth in inner ear & stretch receptors in axial muscles, maintaining head & neck position with respect to gravity
2 - Helps maintain upright, balanced position by facilitating extensors in leg
What is the function of the tectospinal tract?
Superior colliculus receives visual input from retina & visual cortex and the tract orientates head & eye movement to visual & auditory stimuli
What are the sensorimotor areas of the cortex that constitute the motor cortex?
- Primary motor cortex
- Premotor cortex
- Supplementary motor cortex
What is the function of area 4 of the motor cortex (primary motor cortex)?
Does area 4 have a high or low threshold for excitation?
- Control of distal musculature (fine motor control)
- Lowest stimulus threshold
What is the function of area 6 of the motor cortex (supplementary and premotor areas)?
Does area 6 have a high or low threshold for excitation?
- Premotor cortex – control of proximal musculature (posture, balance), movement sequencing, & prep for movement & initiation
- Supplementary motor area – planning & initiation, & bimanual coordination (crosses to other side)
What are Betz cells?
Primary motor cortical output neurones (upper motor neurones) with cell bodies in cortical layer V that send their axons down spinal cord via the corticospinal tract
List the effects of damage to upper motoneurones.
Is this repairable?
- Initial muscle weakness
- Eventual spasticity (↑ resistance to passive movement)
- ↑ muscle tone (hypertonia)
- ↑ reflex responses (hyperreflexia)
- Affects contralat side to damage
- Recovery possible -> primary motor cortex circuitry shows adaptive alterations