Motor System Flashcards
What are the types of motor neurones?
Upper motor neurones
Lower motor neurones
Where is the cell body of upper motor neurones located?
Cell body and axon are found in the primary motor cortex in the pre-central gyrus of the frontal lobe
Where does the axon of all upper motor neurones travel?
Descends in the corona radiata and internal capsule
What is the corona radiata?
White matter structure formed by many upper motor neurone axons running together
What happens to the corona radiata?
Becomes condensed into the internal capsule
Where is the internal capsule located?
The thalamus is medial to it
The lentiform nucleus is lateral to it
Where does the axon of upper motor neurones supplying the trunk and limbs travel?
After the internal capsule, it descends down the medullary pyramids
At the end of the medullary pyramids, it decussates
Descends down the spinal cord in the lateral corticospinal tract
What forms the lateral corticospinal tract?
Upper motor neurone axons in the spinal cord
Where is the lateral corticospinal tract located?
Lateral funiculus of spinal cord
What does the upper motor neurone axon do in the lateral corticospinal tract?
Synapses onto lower motor neurone or inhibitory interneurone
Where does the upper motor neurone synapse onto the inhibitory neurone or the lower motor neurone in the corticospinal tract?
In the ventral horn of the grey matter of the spinal cord
What is the function of upper motor neurones in the corticospinal tract?
Stimulate lower motor neurones
Or stimulate inhibitory interneurones
What do inhibitory interneurones do?
Synapse onto lower motor neurones
Inhibit them
Are lower motor neurones excited more or inhibited more by upper motor neurones and inhibitory interneurones?
Inhibited more
Where does the axon of upper motor neurones supplying the head and neck travel?
After the internal capsule, it travels to motor nuclei in the brainstem
What are brainstem motor nuclei?
Continuation of ventral horn of grey matter of spinal cord
How is the facial nerve motor nucleus organised?
Split into two
One half corresponding to upper face
Other half corresponding to lower face
How do upper motor neurones innervate the facial nerve motor nucleus?
Upper motor neurones supplying the upper half of the face innervate both facial nerve motor nuclei i.e. both right and left sides
Upper motor neurones supplying the lower half of the face innervate the contralateral facial nerve motor nucleus i.e. opposite side
What are the motor abnormalities with damage to upper motor neurones from the right side of the brain supplying the face?
Upper half of face is normal due to innervation by upper motor neurones from left side of brain
Lower half of face on right side is normal due to innervation by upper motor neurones from left side of brain
Lower half of face on left side is weak or paralysed due to loss of innervation by upper motor neurones from right side of brain
What causes upper motor neurone damage?
CNS lesions only
What are the signs and symptoms of damage to upper motor neurones?
Weakness
Hypertonia, increased muscle tone
Hyper-reflexia, enhanced reflexes
Extensor plantar reflex called Babinski sign (note this is normal in babies)
Why does damage to upper motor neurones cause hypertonia?
Loss of overall inhibitory effect on lower motor neurones
Increased activity of lower motor neurones
Why does damage to upper motor neurones cause hyper-reflexia?
Loss of overall inhibitory effect on lower motor neurones
More excitable lower motor neurones
How long do the signs and symptoms of damage to upper motor neurones take to develop? Why?
After a few days or a week
Due to spinal shock
What is spinal shock?
Refers to how loss of innervation of lower motor neurones initially decreases their activity and excitability, this is spinal shock
But afterwards, their activity and excitability increases to above normal levels
What does spinal shock give similar signs and symptoms to?
Lower motor neurone lesion
Where is the cell body of lower motor neurones located?
For lower motor neurones supplying the trunk and limbs, ventral horn of grey matter of spinal cord
For lower motor neurones supplying the head and neck, brainstem motor nuclei
Where does the axon of lower motor neurones travel?
For lower motor neurones supplying the trunk and limbs, in peripheral nerves
For lower motor neurones supplying the head and neck, in cranial nerves
What is the function of lower motor neurones?
Stimulate muscles to contract
What types of neurones are involved in a spinal reflex?
Primary sensory neurone
Lower motor neurone
Inhibitory interneurone
What stimulates the patellar reflex arc?
Stretch of the quadriceps muscle stimulates muscle spindles in it
What carries sensory information from muscle spindles to the spinal cord in the patellar reflex arc?
Primary sensory neurones
What do primary sensory neurones involved in the patellar reflex arc do in the spinal cord?
Synapse onto lower motor neurones supplying the quadriceps muscle and stimulate them
Synapse onto inhibitory interneurones supplying the hamstring muscle and stimulate them
What does stimulating the lower motor neurones in the patellar reflex arc do?
Stimulates the quadriceps muscle to contract
What does stimulating the inhibitory interneurone in the patellar reflex arc do?
Inhibits lower motor neurone supplying hamstring muscle
Prevents hamstring muscle contracting and opposing the quadriceps muscle
What is the purpose of deep tendon reflexes?
Maintain posture
What causes damage to lower motor neurones?
CNS lesions damage the cell body and proximal axon
PNS lesions damage the axon
What are the signs and symptoms of lower motor neurone damage?
Weakness
Hypotonia, decreased muscle tone
Areflexia, absence of deep tendon reflexes
Fasciculation, uncoordinated muscle contraction
Wasting
Why does damage to lower motor neurones cause wasting?
Loss of trophic factors secreted from from the lower motor neurone onto the muscle membrane
How long does wasting take to occur with lower motor neurone damage?
Weeks to months
