Spinal Reflexes, UMN and Control of Movement Flashcards
Spinal interneurons receive input from?
- Primary sensory axons (Ia and Ib fibres).
- Descending axons from the brain.
- Collaterals (branches) of LMNs.
- Other interneurons.
Input to spinal interneurons may be either?
Excitatory or inhibitory.
Interneurons integrate incoming information to generate what?
An output.
Inhibitory interneurons mediate?
- Inverse myotatic response.
- Reciprocal inhibition between extensor and flexor muscles.
Describe the myotatic reflex.
Causes the homonymous extensor muscle (quadriceps) to contract - but for the leg to extend, the antagonist flexor muscle (hamstring) must simultaneously relax.
With regards to myotatic reflex:
Describe the connection between the Ia afferent from the muscle spindle extensor and the homonymous muscle.
Ia afferent from muscle spindle extensor makes excitatory monosynaptic contact with the a-MN innervating the homonymous muscle.
With regards to myotatic reflex:
Which pathway involves an inhibitory interneurone?
A polysynaptic pathway.
With regards to myotatic reflex:
The Ia fibre inhibits what?
The a-MN supplying the flexor muscle.
Reciprocal inhibition is important in the initiation of movement by which cortex?
Motor cortex.
At a joint, voluntary contraction of an extensor will stretch an antagonist flexor, initiating what?
The myotatic reflex.
Descending pathways activating the a-MN controlling extensor muscles also inhibit a-MNs supplying antagonist muscles via what?
Inhibitory neurons.
Descending pathways activating the a-MN controlling extensor muscles also inhibit a-MNs supplying antagonist muscles to allow what?
Unopposed extension.
Excitatory interneurons mediate?
- Flexor reflex.
- Crossed extensor reflex.
With regards to the flexor reflex, noxious stimulus causes a limb to flex by?
- Contraction of flexor muscles via excitatory interneurones.
- Relaxation of extensor muscles via excitatory and inhibitory interneurones.
With regards to the crossed extensor reflex, noxious stimulus causes a limb to extend by?
- Contraction of extensor muscles via excitatory interneurons.
- Relaxation of flexor muscles via excitatory and inhibitory interneurons.
What reflex enhances postural support during withdrawal of a foot from a painful stimulus?
Crossed extensor reflex.
A simple spinal central pattern generator (CPG) can command what limb activity?
- Rhythmic, alternating activity that moves a limb.
What is the essential feature for an excitatory interneurone to display?
Oscillatory or pacemaker activity.
Controlled movements of the body (motor control) are initiated/influenced by?
- Multiple sensory inputs.
- A need to move using internal mechanisms.
What is the function of high level brain structures e.g. neocortical association areas and basal ganglia, in motor control?
- Strategy.
What is the function of middle level brain structures e.g. motor cortex and cerebellum, in motor control?
- Tactics.
What is the function of low level structures e.g. brain stem and spinal cord, in motor control?
- Execution.
Describe “strategy” in terms of motor control.
- What is the aim of the movement?
- How is the movement best achieved?
Describe “tactics” in terms of motor control.
- What sequence of muscle contractions and relaxations in time and space will fulfil the strategic aim?
Describe “execution” in terms of motor control.
- Activation of motor pools and interneuron pools that command desired movements and make essential postural adjustments.
Descending spinal tracts originate where?
- Cerebral cortex.
- Brain stem.
Descending spinal tracts are involved in what?
- Control of movement.
- Muscle tone.
- Spinal reflexes.
- Spinal autonomic functions.
- Modulation of sensory transmission to higher centres.
Lateral descending pathways are under control from what?
- Cerebral cortex.
Lateral descending pathways are important for voluntary control of what?
- Distal musculature, particularly in discrete, skilled movements.
Ventromedial descending pathways are under control from what?
The brainstem.
Ventromedial descending pathways are important for control of what?
Posture and locomotion.
The major lateral descending pathway is what?
Corticospinal (pyramidal) tract.
Where are the cell bodies of the corticospinal tract?
- Motor cortex (about 2/3).
- Somatosensory areas of parietal cortex (1/3).
Axons of the corticospinal tract course to the base of the medulla to form a tract known as?
The medullary pyramid.
Most fibres of the corticospinal tract decussate where?
Pyramidal decussation to form lateral corticospinal tract (75-90%).
The majority of corticospinal fibres decussate at the pyramidal decussation, what happens to the rest?
Remainder stay ipsilateral to form the ventral corticospinal tract and decussate more caudally.
Where do axons of the corticospinal tract terminate?
In the dorsolateral region of the ventral horn and intermediate grey.
The termination of axons of the corticospinal tract is the location of what?
LMNs and interneurons controlling distal muscles - particularly flexors.
Irrespective of the level of decussation, the left hemisphere of the brain controls which musculature?
Right musculature.
Name a minor lateral descending pathway that is phylogenetically older than the corticospinal tract.
Rubrospinal tract.
Where are the cell bodies of the rubrospinal tract located?
Red nucleus.
The cell bodies of the rubrospinal tract are located in the red nucleus, which receives input from what?
- Motor cortex and the cerebellum.
Where do axons of the rubrospinal tract decussate?
Ventral tegmental decussation.
Where do axons of the rubrospinal tract descend after decussating?
The spinal cord ventrolateral to the lateral corticospinal tract.
Where do axons of the rubrospinal tract terminate?
Ventral horn.
The rubrospinal tract exerts control over what?
Limb flexor muscles by exciting LMNs of those muscles.
Lesions of the lateral columns are associated with?
- Loss of fractionated movements.
- Slowing and impaired accuracy of voluntary movements.
- Little effect on normal posture.
A person with: - Loss of fractionated movements. - Slow and impaired accuracy of voluntary movements. - Little effect on normal posture. Most likely has a lesion where?
Lateral columns.
A lesion of the corticospinal tract alone initially causes what?
Deficits as profound as a lesion of the lateral column, but may recover over time.
- But weakness of distal flexors and inability to move fingers independently persist.
A lesion of the corticospinal tract will recover with time, but this recovery is reversed if there is also a lesion where?
The rubrospinal tract.
Where do cell bodies of the vestibulospinal tracts reside?
In the vestibular nuclei (lateral and medial).
Cell bodies of vestibulospinal tract reside in the lateral and medial vestibular nuclei that receive input via which CN?
CN VIII - vestibular nerve.
Cell bodies of vestibulospinal tract reside in the lateral and medial vestibular nuclei that receive input via CN VIII from where?
Vestibular labyrinths + cerebellum.
Axons of the vestibulospinal tracts from the lateral vestibular nucleus (Deiter’s nucleus) descend ipsilaterally as what?
Lateral vestibulospinal tract.
Axons of the vestibulospinal tracts from the lateral vestibular nucleus (Deiter’s nucleus) descend ipsilaterally as the lateral vestibulospinal tract as far as where?
The lumbar spinal cord.
The lateral vestibulospinal tract aids in what?
Holding body upright and balancing posture by facilitating extensor MNs of anti-gravity muscles.
Axons of the vestibulospinal tracts from the medial vestibular nucleus descend as what?
The medial vestibulospinal tract.
Axons of the vestibulospinal tracts from the medial vestibular nucleus descend until where?
The cervical spinal cord.
Axons of the vestibulospinal tracts from the medial vestibular nucleus activate what?
Cervical spinal circuits controlling the neck and back muscles that guide head movements.
Where do cell bodies of the tectospinal tract reside?
Superior colliculus/optic tectum.
The superior colliculus/optic tectum receives direct input from where?
The retina, visual cortex and afferents conveying somatosensory and auditory information.
Axons of the tectospinal tract decussate where?
Dorsal tegmental decussation.
Axons of the tectospinal tract decussate at the dorsal tegmental decussation and then descend where?
Close to midline as the tectospinal tract.
The tectospinal tract influences what?
Movement of the muscles of the neck, upper trunk and shoulders in reflex to visual stimuli.
The superior colliculus may guide the orientation of the head and eyes to what?
Important, new visual stimuli.
The pontine and medullary reticulospinal tracts arise from what?
The reticular formation.
What is the reticular formation?
Diffuse mesh of neurons located along the length and at the core of the brainstem.
The pontine (medial) reticulospinal tract descends?
Ipsilaterally.
What is the function of the pontine (medial) reticulospinal tract?
- To enhance antigravity reflexes of the spinal cord.
- Aids maintenance of standing posture by facilitating contraction of extensors of lower limbs.
The medullary (lateral) reticulospinal tract descends?
Bilaterally.
Function of the medullary (lateral) reticulospinal tract?
- Oppose action of the medial tract.
- Release antigravity muscles from reflex control.
Pontine and medullary reticulospinal tracts are what type of pathways?
Ventromedial.
Activity in both the pontine and medullary reticulospinal tracts is controlled by what?
Descending signals from the cortex.
