Descending Pathways Flashcards
What does the CST do? What type of neurons are the CST cells?
helps with conscious movement/control of fine movements
they are called “upper motor neurons”
This tract arises from the motor cortex (precentral gyrus), passes through the medullary pyramids, and terminates in the spinal cord.
Where does CST begin?
area 4 which is precentral gyrus (next to area 312)
Give an overview of the CST pathway.
the CST starts in area 4
travels in the cerebral peduncle
crosses in the medullary pyramid
travels in the lateral funiculus, and synapse on anterior horn cells (Some fibers don’t cross—10%.
Cells of the cst are termed “upper motor neurons”
The axons arising from the cortex converge in the corona radiata and then descend through the internal capsule, crus cerebri in the midbrain, pons, and medulla. A majority (about 90%) of the fibers cross to the contralateral side at the juncture of the medulla and spinal cord, forming the lateral corticospinal tract, which descends to all levels of the spinal cord and terminates in the spinal gray matter of both the dorsal and ventral horns. The remaining fibers do not cross at the juncture of the medulla and spinal cord; these uncrossed fibers constitute the anterior corticospinal tract. The fibers in this tract descend through the spinal cord and, ultimately, cross over at different segmental levels to synapse with anterior horn cells on the contralateral side.
Describe the crossing of CST fibers in the medulla.
a majority of corticospinal fibers cross to the contralateral side in the caudal medulla (pyramidal decussation) and descend as the lateral corticospinal tract and the remaining fibers descend ipsilaterally as anterior corticospinal tract
How is the CST organized?
Where are the cells of origin functionally associated with the arm and leg located?
This somatotopic organization is called a cortical homunculus.
The corticospinal tract is somatotopically organized throughout its entire projection. In brief, the cells of origin functionally associated with the arm are located in the lateral convexity of the cortex, whereas the cells of origin functionally associated with the leg are located along the medial wall of the hemisphere.
Describe motor neurons. What do they innervate? What are they/what are they called?
Where is the phrenic nucleus found?
Where is the spinal accessory nucleus found?
Motor neurons (IX) innervate skeletal muscle, are large alpha motor neurons, are termed lower motor neurons and “the final common pathway”
The phrenic nucleus is found in cervical ventral horn (C3,4,5 keeps the diaphragm alive )
The spinal accessory nucleus is found at C1-C6
the spinal accessory and phrenic nuclei occupy the same area in the high cervical cord, and also have upper motor neurons synapsing on their motor neurons.
Where does the CST synapse?
The cst synapses on the large anterior horn cells of the medial and lateral motor nuclei in the ventral horn of the spinal cord
Describe the following CST terminations:
Dorsal horn
Intermediate gray
Ventral horn
dorsal horn - modulate sensory afferents, from somatosensory cortex
intermediate gray- gross, rapid movements
ventral horn- independent digit movements, primarily controls flexors
What are cells of origin called in CST?
Betz cells or pyramidal cells area 4 (some from 312)
Describe corona radiata and the “internal capsule”
As the cst axons first spew out of the pyramidal cells they form a circular or crown like shape known as the “corona radiata”
As the long axons of the pyramidal cells descend between the thalamus and the putamen and globus pallidus they are called the “internal capsule”.
What is between the crus and central portion of the midbrain?
What can loss of its cells lead it?
one can observe black cells between the crus and the central portion of the midbrain. This is the substantia nigra which contains dopamine producing cells which, when lost, can lead to Parkinson’s disease.
What do upper motor neuron lesions result in? Lower motor neuron injury?
Upper motor neuron lesions result in spasticity, exaggerated reflexes, and some hemiplegia.
(lower motor neuron injury …anterior horn cell or total nerve legion - leads to flaccid paralysis)
What are anterior horn cells and their axons referred to as?
What happens if you cut them?
The anterior horn cells plus their axons are termed “lower motor neurons”—cut these and you get flaccid paralysis, hyporeflexivity, and fasciculations
Describe a motor unit.
alpha motor neuron plus the skeletal muscle fibers it innervates
small motor units result in precise movements (ex: flexor digitorum profundus)
large motor units result in axial or coarser movements (gluteus maximus)
Describe the pathology of ALS.
destroys the anterior horn cells and the pyramidal cells (lower motor neuron signs domination).
wipes out CST (can see paucity of axons)
