Spinal Reflexes, Upper Motor Neurones and Control of Movement Flashcards
What do spinal interneurones receive input from?
- Primary sensory axons.
- Descending axons from the brain.
- Collaterals (branches) of LMN’s.
- Other interneurons.
What can input to spinal neurones be either?
Excitatory
OR
Inhibitory
What generate incoming info to generate an output?
Interneurones
Interneurones are either excitatory or inhibitory
TRUE
What do inhibitory interneurones mediate?
- The inverse myotatic response.
* RECIPROCAL INHIBITION between extensor and flexor muscles
What is reciprocal inhibition important in the initiation of?
Movement by the motor cortex
At a joint, what will the voluntary contraction of an extensor stretch?
An antagonistic flexor
What does stretching of an antagonistic flexor do?
Initiates the myotatic reflex
Once this myotatic reflex is initiated, however, what happens to allow unopposed extension to still occur?
The descending pathways that activate the α-MN controlling the extensor muscles also, via inhibitory interneurons, inhibit the α-MNs supplying the antagonist muscles
What do excitatory interneurones mediate?
- The flexor reflex.
* The crossed extensor reflex
Describe the flexor reflex.
A noxious stimulus causes limb to flex by:
- contraction of flexor muscles by EXCITATORY INTERNEURONES.
- relaxation of extensor muscles via EXCITATORY + INHIBITORY INTERNEURONES.
Describe the crossed extensor reflex.
A noxious stimulus causes limb to extend by:
- contraction of extensor muscles via EXCITATORY interneurons.
- relaxation of flexor muscles via EXCITATORY + INHIBITORY interneurons
Why is the crossed extensor reflex important?
It enhances postural support during withdrawal of a foot from a painful stimulus so you don’t fall over
What are the controlled movements of the body influenced by?
- Multiple sensory inputs.
* A need to move using internal mechanisms
What is found high in the brain? What is its function?
Strategy area
Neocortical areas
Basal ganglia
What is found in the middle of the brain? What is its function?
Tactics area
Motor cortex
Cerebellum
What is found low in the brain? What is its function?
Execution
Brain stem
Spinal cord
Where do the descending spinal tracts originate from?
The cerebral cortex and brain stem
What functions are the descending spinal cord tracts concerned with?
1 – Control of movement. 2 – Muscle tone. 3 – Spinal reflexes. 4 – Spinal autonomic functions. 5 – Modulation of sensory transmission to higher centres
Name 2 important descending spinal tracts.
Lateral + Ventromedial
What are lateral pathways under the control of?
Cerebral cortex
What are lateral pathways important for?
Voluntary control of distal musculature, particularly discrete, skilled, movements (ie. hands and fingers in a ‘fractionated’ manner)
What are ventromedial pathways under the control of?
Brain stem
What are ventromedial pathways important for?
Posture and locomotion
Name a minor lateral pathway.
The rubrospinal tract
Where are the cell bodies of the rubrospinal tract located?
In the red nucleus, which receives input from the motor cortex and cerebellum
Where do axons of the rubospinal tract decussate?
At the ventral tegmental decussation
Once axons of the rubospinal tract have decussated, what do they do?
Descend the spinal cord ventrolateral to the lateral corticospinal tract
Where do axons of the rubospinal tract terminate?
Ventral horn
What are the effects of lesions of the lateral collumns?
- Loss of ‘fractionated’ movements ie. shoulders, elbow, wrist and fingers can’t be moved independently.
- Slowing and impairment of accuracy of voluntary movements.
- Little effect on normal posture (ie. standing and sitting).
Describe the deficits caused by lesions of the corticospinal tract alone.
Deficits are as profound as those caused by a lesion of the lateral columns, but over time, major recovery can occur
Although recovery can occur from deficits caused by lesions of the cortiospinal tract, what can occur?
Weakness of the distal flexors, and an inability to move fingers independently
When does recovery of lesions of the corticospinal tract not occur?
If the rubospinal tract is also lesioned
Where are the cell bodies of the vestibulospinal tract located?
In the vestibular nuclei (lateral and medial).
Where do vestibular nuclei receive input from? What is this via?
From the vestibular labyrinths – via CN VIII.
Cerebellar input is also important
Describe how axons from the lateral vestibular nucleus (Deiter’s nucleus) descend.
Descend ipsilaterally as the lateral vestibulospinal tract as far as the lumbar spinal cord
What do Deiter’s nucleus help to do?
Hold upright and balanced posture by facilitating extensor MNs of antigravity muscles (ie. of the leg).
What do the axons of the medial vestibular nucleus descend as? Where to?
Descend as the medial vestibulospinal tract, as far as the cervical spinal cord
What do medial vestibular nucleus axons activate?
Cervical spinal circuits that control neck and back muscles, guiding head movements
Where are the cell bodies of the tectospinal tract located?
In the superior colliculus (aka optic tectum)
Where does the superior colliculus receive input from?
- Direct input – from the retina.
- From visual cortex.
- Afferents conveying somatosensory and auditory information.
Where do axons of the tectospinal tract decussate?
In the dorsal tegmental decussation
After decussation, describe how axons of the tectospinal tract descend?
Descend close to the midline as the tectospinal tract to the cervical spinal cord, influencing the muscles of the neck, upper trunk and shoulders
What does the superior colliculus act as?
A map of the external world, guiding orientation of the head and eyes to an important, new, visual stimulus
Where do Pontine (medial) + Medullary (lateral) Reticulospinal Tracts arise from?
Reticular formation
What is the reticular formation?
A diffuse mesh of neurones that are located along the length, and at the core of the brainstem
How does the pontine tract descend in the spinal cord?
IPSILATERALLY
Where does the pontine tract enhance?
Anti-gravity reflexes of the spinal cord
What is the function of the pontine tract?
A standing posture.
By facilitating contraction of the extensors of the lower limbs
How does the medullary tract descend?
BILATERALLY
What does the medullary tract oppose?
The action of the medial tract
What does the medullary tract do to anti-gravity muscles?
Releases them from reflex control
What is activity in both the pontine and medullary reticulospinal tracts controlled by?
Descending signals from the cortex
