6) Motor System Flashcards
What 2 neurones make up the motor system ?
2 descending neurones
1) Upper Motor Neurones
2) Lower Motor Neurones
Upper motor neurone from the primary motor cortex and projects down onto the lower motor neurones which descends down te spinal cord and exit to the target spinal cord
Describe the anatomy of LMN
Location of Axon , Cell body
> Cell Body is found within the Ventral Horn or Spinal Cord or Motor Nuclei in the brain stem of the CNS
Axon is found in the in the PNS, which projects onto target peripheries
What is the function of LMN
They cause muscle contraction.
They are important components of spinal reflexes
Describe a Spinal Reflex involving LMN e.g. Patella Reflex
1) Spindle fibres detect stretch
2) Sensory neurone activates LMN cause contraction of the quadriceps at L3
3) Sensory neurone also branches with a inhibitory neurone at L5 to cause hamstring relaxation
What signs can be seen in damage to LMN
> Weakness (Denervation)
Areflexia
Muscle wastage (Loss of trophic factors such as GH which is supplied by LMN
Fasciculation (Uncoordinated Muscle Contraction, nAchRs try to compensate for Denervation)
Hypotonia (Decreased Muscle Tone)
Describe the anatomy of UMN
Cell body and Axon
Cell body and Axon are both found in the primary motor cortex of the CNS
> They synapse onto a LMN in the ventral horn or motor nuclei
What is the function of UMN
Excitation and inhibition of LMMs
Overall net effect is inhibitory
Due to the overall net effect of UMN on LMN what is the consequence of a lesion ?
A lesion would lead to hyperactivity as you would lose overall inhibition
Describe the pathway taken by UMN when travelling to Spinal Nerves
1) Cell bodies of UMN located in the motor cortex (Depends on homunculus and target muscle
2) Axons descend through the Corona Radiata
3) Passing through the internal capsule - Between Thalamus and Lentiform Nucleus
4) Descend through Cerebral Peduncle in the brain stem -> Pons and Medullary Brain stems
5) Decussate at Medullary Pyramids
6) Axon descends down the Lateral Corticospinal Tract found in the lateral funiculus to the ventral horn of the required spinal level
What is the Corona Radiata ?
White matter made up of . bundles of radiating fibres
Describe the pathway taken by UMN when travelling to CNs (Facial Nerve)
Corticobulbar / Corticonuclear
1) UMN cell body is found laterally in the motor cortex and projects onto the VII motor nucleus in the brain stem
2) Motor Nucleus is split into upper and lower face
3) UMNs Supplying upper face will project both Contra and Ipsilaterally
4) UMNs Supplying lower face will project only Contralaterally
5) Synapse onto LMN in the brain stem / Motor Nuclei `
How will a Stroke compare to a Bell’s Palsy in presentation ?
In a stroke affecting UMNs the forehead is spared
Whereas Facial Nerve Palsies will leave the patient unable to raise their eyebrows.
What signs can be seen in UMN neurone damage ?
1) Weakness (Loss of excitatory inputs onto LMNs by UMNs)
2) Hyperreflexia (Loss of Net inhibition)
3) Hypertonia
4) Extensor Plantar Reflex (Babinski’s Reflex, occurs in babies where toe extends instead of flexing when there is sensation to the sole)
5) Spinal Shock
What is Spinal Shock ?
A phenomenon which occurs immediately following a UMN lesion,
=> Initially there is flaccid paralysis (LMN presentation) but then tone increases and reflexes become exaggerated
