Descending Tracts in the Spinal Cord Flashcards
What is the corticospinal tract the main tract for?
nearly all voluntary muscle activity
Where does the corticospinal tract originate?
the precentral gyrus in the frontal lobe (area 4)
Where are the cell bodies of the corticospinal tract located?
at the giant pyramidal neurons of Betz (100µm in diameter)
Where do axons of the corticospinal tract dessucate?
in the pyramids in the medulla oblongata
Where are neurons that supply the leg and foot located?
on the medial aspect of the hemisphere
Where are neurons that supply the hand and face located?
on the lateral aspect of the hemisphere
Describe the pathway of axons of the corticospinal tract
- pass down through the internal capsule into the cerebral peduncles in the midbrain
- continue down the brainstem to reach the pyramids in the medulla oblongata
- cross to the opposite side and continue as the lateral corticospinal tract in the spinal cord
- at each level of the spinal cord, axons peel off and enter the ventral horn
- terminate by synapsing with lower motor neurons
What are the ventral corticospinal tract axons?
axons that do not cross over the medulla and continue down on the same side to enter white columns of the spinal cord before crossing lower down
What percentage of all corticospinal fibres do not cross at all?
3%
Where do axons that do not cross at all end?
in synapses on the ipsilateral side as the motor cortex
What do the lateral and ventral corticospinal tracts consist primarily of?
a single, uninterrupted neuron called upper motor neurons
Where do upper motor neurons synapse?
on lower motor neurons
How do lower motor neurons send their axons out of the spinal cord?
via the ventral roots into peripheral nerves to supply voluntary muscles
What do suppressors of the corticospinal tract do?
prevent the lower motor neurons from over-discharging, e.g. they prevent excessive reflex contraction of a muscle during the stretch reflex
What happens if the suppressors of the corticospinal tract are damaged?
the lower motor neurons are freed from their control and fire excessively in response to reflex stimuli or discharge spontaneously
Describe the pathway of the rubrospinal tract
from the red nucleus in the midbrain to lower motor neurons in the cervical part of the spinal cord to supply upper limb muscles
Describe the pathway of the vestibulospinal tract
from vestibular nuclei to lower motor neurons in the spinal cord to supply the axial musculature
What is the axial musculature?
neck and back muscles responsible for posture
Describe the pathway of the reticulospinal tract
from the brainstem reticular formation to lower motor neurons in the spinal cord to supply the axial musculature
What is the most common site of injury of lesion to the upper motor neuron?
the cerebral hemisphere before pyramidal decussation
What is the most common cause of injury to the upper motor neuron?
an artery becoming occluded which deprives the neurons of their blood supply and they die (cerebrovascular accident/stroke)
What happens if damage to the upper motor neuron is above the pyramidal decussation?
the signs and symptoms will be seen in the muscles on the opposite side of the body
What happens if damage to the upper motor neuron is below the pyramidal decussation?
the ensuing paralysis will be on the same side as the damage
How can a person who is paralysed on one side of the body make crude movements of the trunk?
uncrossed fibres along with some of the crossed ones, supply the muscles of the trunk
What does regaining function of muscles after a stroke involve?
plasticity of the cortex; neurons can take on new functions with training
How can reflexes still be elicited with an upper motor neuron lesion?
the lower motor neurons are not affected, and the reflex arc is complete
What can happen when the lower motor neurons can discharge spontaneously without motor neuron modulation?
the muscles contract strongly (spasticity)
What does upper and lower motor neuron paralysis tend to be respectively?
- upper - spastic
- lower - flaccid
What is the Babinski reflex?
when the sole of the foot of a healthy person is stroked in a heel-to-toe direction, the toes curl downwards however in a patient with an upper motor neuron lesion, the toes will fan out in the opposite direction
When does lower motor neuron lesion occur?
when the cell bodies of the ventral horn are selectively destroyed (e.g. by poliomyelitis) or when a nerve to a muscle is cut
What are characteristics of flaccid paralysis?
muscles unable to contract so they become soft, flabby and atrophic
What happens if the cell bodies are destroyed in a lower motor neuron lesion?
the axons cannot regenerate, and the paralysis is permanent
What happens if the nerve is cut in a lower motor neuron lesion?
the part of the axon attached to the cell body can regenerate and some of the functions may be restored
Give examples of muscles commonly weakened by polio (lower motor neuron lesion)
- shoulder muscles
- muscles behind the arm
- hip muscles
- thumb muscles
- muscles that straighten the knee
- muscles that lift the foot
