Session 5: Motor System Flashcards
How are lower motor neurones controlled?
-They are controlled by upper motor neurones, which descend through the cord or brainstem and synapse on LMNs
Where are lower motor neurone cell bodies found?
Found in the ventral horn and in cranial nerve motor nuclei (oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus etc)
How are lower motor neurone tested?
- Participate in spinal reflexes, particularly deep tendon reflex
- Jendrassik Manoevre makes it more pronounced
How are lower motor neurone activated?
- Typically activated by incoming impulses from sensory neurones that communicate with muscle spindles (muscle stretch reflex)
- Can also be inhibited
- Best example is inhibition of antagonist muscles such as hamstrings following patellar reflex activation
What are primitive spinal reflexes?
- Exist in babies
- Moro reflex and Palmar grasp
- Reflexes disappear as a baby grows due to maturation of descending upper motor neurone pathways
What are the signs seen when lower motor neurone lesions?
Weakness (due to denervation): Interruption of information from neurone to muscle
Areflexia (due to denervation): Absence of reflexes due to damage to path
Wasting - due to loss of trophic support to the muscle from the LMN across the neuromuscular junction
Hypotonia: Decrease muscle tone due to loss of muscle activation. Appears like a baby
Fasciculation: due to up-regulation of muscle nAChRs to try to compensate for denervation
Where are upper motor neurone lesions found?
Found in primary motor cortex in precentral gyrus
Where do the UMN synapse?
- Synapse onto LMNs directly (or indirectly) in the ventral horn or cranial nerve motor nuclei
- Neurones in the basal ganglia and cerebellum are not upper motor neurones
What is the net effect of UMNs on LMNs?
- Inhibition
- Explains most of the features of UMN lesions
How do the axons descend from the motor cortex?
- Corona radiata
- Internal capsule
- Cerebral peduncle in the midbrain
- Pons
- Medullary pyramids
- Decussation of the pyramids (in the caudal medulla)
- Lateral corticospinal tract (in the lateral funiculus of the cord)
- Ventral horn
- Synapse (directly but usual indirectly via inhibitory interneurones) on LMNs
What is the lateral corticospinal tract?
Involved with fine motor control in the limbs, primarily the distal extremities (but all of limb can be affected by a UMN lesion)
What is the corticobulbar tract?
- Supplied by UMNs for facial structures (cranial nerve innervated).
- These UMN leave pathway in the brainstem and form the corticobulbar tract (aka corticonuclear) which innervated LMNs in the cranial nerve motor nuclei
How do UMN neurone lesions appear and why?
- One supplies the superior face (mostly occipitofrontalis) and one the inferior face.
- Part of the facial motor nucleus that supplies the upper half of the face receives UMNs from both hemispheres, whereas the part that supplies the lower face only receives a contralateral UMN input
- Hence UMN lesions involving the face will spare the forehead
How do true facial nerve lesions appear?
- True facial nerve palsies affect all of the muscles of facial expression
- Affect the entire ipsilateral half of the face
How do UMN lesions appear?
- Weakness (due to loss of direct excitatory inputs onto LMNs from UMNs)
- Hypertonia (due to loss of descending inhibition – remember that the net effect of UMNs on LMNs is inhibition)
- Hyperreflexia (same as hypertonia. It is an overactive reflex arc)
- Extensor plantar reflexes (this is a reversion to the situation in a baby, due to loss of descending modulation of spinal reflexes)
