5.1 Motor Systems Flashcards
Where are the cell bodies of Lower motor neurones found?
Cell bodies are found in the:
Ventral Horn and
Cranial nerve motor nuclei (e.g. oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus etc…)
Give 3 ‘primitive’ spinal reflexes that exist in babies
- Up-going plantar reflex
- Moro reflex (response when dropped)
- Palmar grasp reflex (finger in babies hand they grasp)
Why do ‘primitive’ spinal reflexes in babies disappear as we age?
These reflexes disappear due to maturation of descending upper motor neuron pathways
What signs can be seen in Lower Motor Neurone damage? And why are these signs seen?
- Weakness (due to denervation)
- Areflexia (due to denervation)
- Wasting (loss of trophic support to muscles from LMN across the neuromuscular junction)
- Hypotonia (due to loss of muscle activation)
- Fasciculations (up-regulation of muscle nAChRs to try to compensate for denervation)
Where are Upper motor neurones found?
They are found in the primary motor cortex (precentral gyrus)
Where do Upper motor neurones synapse onto lower motor neurones?
They synapse onto LMN’s directly (or indirectly) in the:
Ventral horn OR
Cranial nerve motor nuclei
This is also where the cell bodies for the LMN’s are found.
What is the pathway through the motor cortex as the axons descend?
- Corona Radiata
- Internal Capsule
- Cerebral peduncle in the MIDBRAIN
- PONS
- MEDULLARY pyramids
- Decussation of pyramids (in caudal MEDULLA)
- Lateral corticospinal tract (in the lateral funiculus of the cord)
- Ventral horn
- Synapse onto LMNs (usually indirectly via inhibitory interneurones)
Where does decussation of the pyramids occur?
In the caudal medulla
Where are the cerebral peduncles found?
In the midbrain
Where is the lateral corticospinal tract found in the cord?
In the lateral funiculus of the cord
What do the lateral corticospinal tracts specifically control?
They are mainly involved with fine motor control in the limbs (primarily the distal extremities - but all of the limb can be affected by a UMN lesion)
What pathway do facial structure UMN’s take?
Facial structure’s are supplied by cranial nerves NOT spinal nerves.
Therefore, they leave the pathway in the brainstem and form CORTICOBULBAR TRACT (aka CORTICONUCLEAR TRACT) which innervates LMNs in the cranial nerve motor nuclei
Explain why UMN lesions involving the face will spare the forehead…
Facial motor nucleus is split into two halves - one half supplying superior face (occipitofrontalis) and the other half the inferior face.
Upper half of the face receives UMNs from BOTH hemispheres.
Lower face only receives a contralateral UMN input
What signs are seen with UMN lesions and why?
- Weakness (due to loss of direct excitatory inputs onto LMNs from UMNs)
- Hypertonia (due to loss of descending inhibition)
- Hyperreflexia (same as hypertonia - overactive reflex arc)
- Extensor plantar reflexes (due to loss of descending modulation of spinal reflexes)
What is spinal shock?
Spinal shock is a phenomenon that occurs in the days immediately following a UMN lesion. Initially there is flaccid paralysis with areflexia (like LMN lesions) but then tone increases (becoming hypertonia) and reflexes become exaggerated (hyperreflexia). Mechanism is unclear but is related to neuroplasticity in the spinal cord.
