Motor System Flashcards
Describe the two types of motor neurones found in the CNS
Upper motor neurones - have cell body in cerebral cortex of primary motor cortex. Axons project on to dendrites of LMN
Lower motor neurones - cell body in brainstem or ventral cord. Axons projects on to skeletal muscle
What are the signs of a lower motor neurones lesion
Weakness
Wasting - in muscle distribution. Due to loss of trophic factors
Hypotonia - localised to myotomes. Have loss of normal tone
Areflexia - hyporeflexia or areflexia
Fibrillations - due to upregulation of nAChR -> muscles more sensitive to ACh
What are the signs of an upper motor neurone lesion
Weakness
Hypertonia - can manifest as spasticity or clasp knife reflex. Due to overactivity of LMN as there is no inhibition by UMN
Hyper-reflexia
Clonus
Extensor-plantar reflex/positive Babinski’s sign - interruption of descending inhibitory neurones causes reflexes to go back to how they were as a baby
What is spasticity and what is clasp knife reflex
Spasticity - arises from hypertonia of all muscles in a limb. There is equal amount of tone in all compartments but one set of muscles is stronger than the other, giving distinct limb positions
Clasp knife reflex - if given enough force, rigid limb will give way
What is spinal shock
Spinal shock is where immediately after UMN lesion, LMNs decrease their level of activity so in the early phase, a pateint will have hypotonia and hyporeflexia
After a few days to weeks, this will convert back to classic UMN syndrome
Describe the lateral corticospinal tract
UMN cell bodies in the cerebral cortex project their axons into the corona radiata
The axons coalesce to form the internal capsule which passes between the thalamus and lentiform nucleus
Axons enter the brainstem through the cerebral peduncles and descend to the caudal medulla
Axons decussate and then descend to level of innervation where they synpase with LMN which innervates the muscle
What is the corona radiata
Corona radiata - white matter found in each hemisphere
Contains UMN axons which are funnelled between the thamalus and lentiform nucleus
What is the internal capsule
Internal capsule is a bundle of axons running between the thalamus and lentiform nucleus
Contains both sensory and motor axons
Also encapsulates the thalamus
Has an anterior limb, posterior limb and genu - axons near genu supply face while those that are distal to genu supply lower down
Name some different somatic motor tracts and the differences between them
Lateral corticospinal tracts - innervates distal muscle. Decussate in the medulla
Ventral/anterior corticospinal tracts - innervates proximal muscles. Decussate at level of innervation
Corticobulbar/Corticonuclear tracts - supply LMNs in brainstem. May or may not decussate
Describe the UMN innervation to the LMN in the facial nerve
Facial nerve recieves UMNs from the lateral part of the motor cortex
Upper half of the facial nerve nuclei receives innervation from both hemispheres
Lower half of the facial nerve nuclei receives innervation from the contralateral side so the left UMN supplies right lower facial motor nucleus
What clinical implications are there in regards to the innervation of the facial nerve nuclei
Lesion to motor cortex supplying face will result in no innervation to contralateral lower half of the facial motor nucleus but there is still innervation to contralateral upper half of nucleus -> lesion is forehead sparing
Lesion to CN VII affects both upper and lower halves of face as both LMN axons are damaged
Name some other motor tracts in the CNS
Rubrospinal tract - no function in adults
Medullary reticulospinal tract - keeps person awake and keeps muscle tone
Lateral vestibulospinal tract - connects input from vestibular system to corticospinal tract and helps maintain posture
Tectospinal tract - helps respond to auditory and visual stimuli
