L7 - Motor Pathways Flashcards
Where are the upper motor neurones found?
Cell body in the primary motor cortex
Found in the CNS
Axons descend via the corticospinal tract
Synapses in the ventral horn
UMN are not found in the cerebellum or basal ganglia
Where are lower motor neurones found?
The cell body exists in the brain stem or the ventral horn
The axons run via the peripheral nervous system to a muscle
Symptoms of a lower motor neurone lesion
Hypotonia Areflexia - loss of reflexes Weakness Paralysis - multiple cord segments Fasciculations Atrophy of denervation Fibrillation
Atrophy of denervation
Loss of trophic support via growth factors released by lower motor neurones
Fasciculations
Visible, uncoordinated, involuntary muscles contractions
Hypotonia
Less background contraction therefore floppy
Fibrillation
Uncoordinated muscle contractions that are not visible
How can a lower motor neurone lesion occur
Cell body destruction
Transected axon
Can occur due to damage of the CNS or PNS
How are the motor nuclei of cranial nerves created
The ventral horn extended from the spinal cord into the brain stem but were interrupted to form nuclei
Upper motor neurone lesion
Lesion in the CNS only causing loss of normal inhibition to LMN and lack of voluntary control of LMN
Therefore:
- hypertonic
- hyper-reflexia
- weakness
- clasped knife rigidity
- atrophy of disuse
- spasticity
Acute phase of UMN lesion
Flaccid paralysis - hypotonia causing spinal shock
Over a period of days to weeks hypertonia develops
UMN lesion spasticity
Antigravity muscles dominate therefore:
Upper limb - flexed
Lower limb - extended
Clasped knife rigidity
When pulling on limb, rigid up to a certain point where the limb gives way due to the Golgi tendon organ neurones
Golgi tendon organ neurones
High threshold neurones that are stimulated by high tension
Stimulate inhibitory interneurones that causes relaxation of muscles
UMN effects on lower motor neurones
- synapse onto inhibitory interneurones therefore background inhibition
- synapse directly onto LMN causing conscious excitation that overwhelms inhibition
UMN pathway from cerebral cortex to LMN
- Cell body in the primary motor cortex
- Axons descend via the corona radiata
- Axons converge at the internal capsule
- Axons enter the brain stem via the cerebral peduncle
- Axons descend down the anterior pons to the spinal cord via the corticospinal tract
- 85% of the axons decussate at the anterior, inferior medullary pyramids and run via the lateral corticospinal tract
- 15% continue ipsilaterally down the spinal cord and decussate at the level of the LMN of interest via the ventral corticospinal tract
Lateral corticospinal tract neurones
Decussate at the medullary pyramids
Supply the LMNs that supply the distal, peripheral muscles
Ventral corticospinal tract
- decussate at the level of the LMN of interest
- supply the LMNs that supply proximal postural muscles
Where is the internal capsule found
Between the lentiform nucleus and the thalamus
Corticonucleur tract
Some motor axons synapse and terminate in the brain stem
Facial nerve motor nucleus
Split into:
- superior aspect which supply LMNs that supply the upper face e.g. forehead and upper eyelid
- inferior aspect suppling the lower face from the lower eyelid
Facial nerve proper
Made up of LMNs only
UMNs of the upper face
- The cell body is within primary motor cortex laterally
- supply contralateral and ipsilateral superior aspects of facial nuclei
UMNs for lower face
- cell bodies in the primary motor cortex laterally
- only supply the contralateral side
Facial nerve proper lesion
Loss of LMNs supplying the upper and lower face therefore Bell’s Palsy on the ipsilateral side
Right UMN lesion of facial nerve
- The UMN of the upper face is affected contralaterally and ipsilaterally
- But the left contralateral and ipsilateral branch of the UMN supplying the upper face spares the forehead from weakness and paralysis on both sides
- the UMN supplying the contralateral lower face is affected therefore the left lower face becomes weak
- commonly caused by a stroke affecting the middle cerebral artery
Internal capsule segments
Anterior limb - between the caudate nucleus and lentiform nucleus
Genus - bend contains the facial upper motor neurones
Posterior limb - between the lentiform nucleus and the thalamus
Anterior limb
Allows the communication between the cerebral cortex and the cerebellum
Posterior limb
Contains the descending motor neurones of the:
- face
- arms
- trunk
- legs
Contains the 3rd order sensory neurones of the
- face
- arms
- trunk
- legs
