S5: motor system Flashcards
Describe lower motor neurones
Cell bodies are found in the ventral horn and in cranial nerve nuclei
Typically activated by incoming impulses from sensory neurones that communicate with muscle spindles, but can also be inhibited
When activated, will cause muscle contraction
Why do primitive spinal reflexes in babies disappear?
The reflexes disappear as a baby grows due to maturation of descending upper motor neurone pathways
Eg. up going plantars, moro reflex and palmar grasp
Describe the signs seen in lower motor neurone damage
Weakness
Flaccid
Areflexia
Marked atrophy – due to loss of trophic support to the muscle from the LMN across the neuromuscular junction
Hypotonia
Fasciculation (uncoordinated muscle contractions) – up-regulation of muscle nAChRs to try and compensate for denervation
Describe upper motor neurones
Found in the primary motor cortex & synapse onto LMNs directly in the ventral horn or CN motor nuclei
Net effect of UMNs on LMNs is inhibition
The axons descend from the motor cortex through the following: corona radiata, internal capsule, cerebral peduncle in the midbrain, pons, medullary pyramids, decussation of pyramids, lateral CST, ventral horn & synapse
Describe when UMN lesions involving the face will spare the forehead
Facial motor nucleus is split into two halves – one supplies the superior face and one the inferior face
Part of the facial motor nucleus that supplies the upper half of the face receives UMNs from both hemispheres, whilst the part that supplies the lower face only receives a contralateral UMN input
Hence, UMN lesions involving the upper half of the face will spare the forehead
Describe the signs seen in upper motor neurone damage
Weakness
Initially hypotonia -> hypertonia
Initially areflexia -> hyperreflexia
Disuse atrophy
Describe spinal shock
Normally, LMN held in inhibition by the UMN
UMN lesion
Initially, flaccid paralysis (spinal shock), causing decreased tone
Later, LMN still stimulated by sensory spindles & has lost all its inhibitory influences
-fires more frequent action potentials, causing increased tone (get flexed posture)
What is the internal capsule?
A bidirectional white matter pathway connecting the cerebral hemisphere with the rest of the CNS
Superiorly continuous with the corona radiata, inferiorly continuous with the cerebral peduncle of the midbrain
Primarily contains descending axons of upper motor neurones but also has ascending axons of third order sensory neurones
Describe the three anatomical divisions of the internal capsule
Anterior limb – connects axons connecting the motor cortex with the cerebellum
Genu – the ‘bend’, contains axons of upper motor neurones supplying the face
Posterior limb – contains axons of UMNs supplying upper limb, trunk and lower limb from anterior to posterior (also contains third order sensory axons connecting thalamus to postcentral gyrus)
Describe the ventral corticospinal tract
At the decussation of the pyramids, around 85% of axons decussate to form the lateral CST
Remaining 15% remain ipsilateral and descend in the ventral funiculus as the ventral CST
At the level of the target LMN, the ventral CST axons decussate
Ventral CST supplies proximal and trunk muscles
Describe the cervical enlargement
Correlates with the presence of the brachial plexus
There are many LMN cell bodies and second order sensory neurone cell bodies at these levels to supply the upper limbs
Describe why the spinal cord is narrow at thoracic levels
Relatively few LMN cell bodies and second order sensory cell bodies due to relatively small dermatomes and myotomes at thoracic levels
Describe the lumbosacral enlargement
Wide due to presence of lumbosacral plexus
There are many LMN cell bodies and second order sensory neurone cell bodies at these levels to supply the lower limbs
Describe the distribution of white matter in the spinal cord
At top of the cord, maximum amount of white matter since it contains UMN axons yet to be distributed as well as the convergence of all sensory axons from levels below
Net effect = gradual increase in total white matter as we ascend the cord
