Upper And Lower Motor Neurones Flashcards
Where are the cell bodies of the upper + lower motor neurones located?
- upper: primary motor cortex
- lower: brain stem + ventral horn of spinal cord
Distribution of upper + lower motor neurones in CNS and PNS
- upper: only in CNS
- lower: cell bodies in CNS + axons in PNS
Where can lower motor neurones be damaged?
Severed axon
Destruction of cell body
List lower motor neurone lesion signs
- weakness
- hyporeflexia
- areflexia
- hypotonia
- muscle atrophy
- fasciculaitons
Define hyperreflexia, areflexia + hyporeflexia
- hyperreflexia: excessive reflex
- areflexia: no reflex
- hyporeflexia: reduced reflex
What are fasciciualtions?
Uncoordinated muscle contraction
Why does muscle atrophy occur in lower motor neurone lesions?
Reduce ACh to muscle at neuromuscular junction + reduction of growth factor
Why do fasciculations occur in lower motor neurone lesions?
- reduction of ACh to muscle
- upregulation of Ach receptors
- causes hypersensitivity
- uncoordinated muscle contraction
In the knee jerk reflex, for the quadriceps to contract, the hamstrings must relax. Explain the mechanism behind how this happens
- upper motor neurones at L3 cause contraction of quadriceps at
- the same UMN sends a projection to L5 + synapse at inhibitory interneurones > relaxation of hamstrings
Describe the inputs to lower motor neurones
- majority of inputs via inhibitory interneurones from upper motor neurones
- a strong excitatory input is needed to overcome inhibition to elicit a response
- net effect of UMNs on LMNs is inhibition
What pathway allows for communication between CN 3,4+6 for extra ocular eye muscle innervation?
Medial longitudinal fasciculus
What are signs of upper motor neurone lesion?
- weakness
- spasticity: hypertonia, hyper reflexia + clasp knife rigidity
- (atrophy)
- spinal shock
What is clasp knife rigidity?
- Flexed limb
- resistance to being pulled out then sudden relaxation
What is considered spasticity?
Hypertonia
Hyper reflexia
Clasp knife rigidity
What is spinal shock?
A phenomenon that occurs hours-days post upper motor neurone lesion
- initially flaccid paralysis with areflexia
- then hypertonia +hyperreflexia
What is meant by the saying ‘upper spares upper’?
Upper motor neurones lesion spare upper face (forehead is spared)
Describe the descent of upper motor neurones
- UMN cell body in primary motor cortex
- travel down through corona radiata
- compress together at internal capsule
- travels to medulla via cerebral peduncles
TWO PATHWAYS: - 85% decussate at decussation of pyramids in caudal medulla
- form lateral corticospinal tract
- synapse to LMN > limb musculature
OR - 15% travel down ipsilaterally
- form anterior corticospinal tract
- cross at level of LMN + synapse > trunk musculature
What pathway do 85% of upper motor neurones take at the medulla?
- 85% decussate at decussation of pyramids in caudal medulla
- form lateral corticospinal tract
- synapse to LMN > limb musculature
What pathway do 15% of upper motor neurones take at the medulla?
- 15% travel down ipsilaterally
- form anterior corticospinal tract
- cross at level of LMN + synapse > trunk musculature
What is referred to as the upper + lower face?
upper: above eyes (frontalis)
lower: below eyes
Where does the lower half of the facial motor neurone receive input from?
Contralateral upper motor neurones
Where does the upper half of the facial motor neurone receive input from?
Bilateral upper motor neurones
What is the clinical significance of the bilateral input to the upper facial motor nuclei?
Lesion prior to facial motor neurone is forehead sparing as there is another input from the opposite hemisphere
Distribution of upper motor neurones in the internal capsule
- only in posterior limb of internal capsules
- face at genu (bend)
- arms
- trunk
- legs
