Descending Pathways (handout based) Flashcards
Originate in: Cerebral cortex, Cerebellum & Brainstem
Upper Motor Neurons
Are entirely within the CNS; Form descending tracts
Upper Motor Neurons
Begin in CNS; Made up of alpha motor neurons (A-α)
Lower Motor Neurons
From anterior horns of spinal cord
Lower Motor Neurons
From brainstem cranial nerve nuclei
Lower Motor Neurons
Make up spinal and cranial nerves
Lower Motor Neurons
Classified according to where they synapse in the ventral horn (UMN); Innervate postural and girdle muscles
Medial activation system
Classified according to where they synapse in the ventral horn (UMN); Associated with distally located muscles used for fine movements
Lateral activation system
Classified according to where they synapse in the ventral horn (UMN); Facilitate local reflex arcs
Nonspecific activating system
the most important descending tract
Corticospinal tract
descends through all levels of the neuraxis except the cerebellum
Corticospinal tract
arises primarily from area 4, and 6, 3,1 and 2
Corticospinal tract
passes through the internal capsule, the cerebral peduncle, the basis pontis, and the pyramids of the medulla oblongata
Corticospinal tract
in the caudal medulla, about 75 to 90% of the fibers decussate to form the lateral CST in the lateral funiculus of the SC
Corticospinal tract
about 8 % of fibers remain uncrossed and form the anterior CST (Türck’s bundle) in the anterior funiculus
Corticospinal tract
Fibers in the anterior CST decussate at segmental spinal levels
Corticospinal tract
roughly about 98 percent of fibers in the pyramidal tract are crossed; remaining 2 percent remain ipsilateral and form the tract of Barnes
Corticospinal tract
fibers terminate on motor neurons in the lateral part of the ventral horn that supply the distal limb musculature
Lateral corticospinal tract
fibers terminate on motor neurons in the medial part of the ventral horn that supply the neck, the trunk, and the proximal limb musculature
Anterior corticospinal tract
constitutes the largest component of the cortically originating descending fiber system
Corticopontocerebellar tract
has been estimated to contain approx 19M fibers, in contrast to the pyramidal tract, which contains approx 1 M
Corticopontocerebellar tract
originates from wide areas of the cerebral cortex, but primarily from the primary sensory and motor cortices, and descends in the internal capsule, cerebral peduncle, and basis pontis, from which its fibers project on pontinenuclei
Corticopontocerebellar tract
it plays a role in the rapid correction of movement
Corticopontocerebellar tract
lesions result to ataxia that occurs contralateralto frontal or temporal lobe pathology
Corticopontocerebellar tract
Innervates the head; Most fibers terminate in reticular formation near cranial nerve nuclei
Corticobulbar tract
Corticobulbar tract
face areas of the cerebral cortex ➡️ genu of the internal capsule ➡️cerebral peduncle (occupy a dorsolateralcorner of the CST segment as well as a small area in the medial part of the base of the peduncle) ➡️ basis pontis(where they intermix with corticospinalfibers) and pyramid but do not reach the spinal cord
The majority of cranial nerve nuclei receive bilateral cortical input
Corticobulbar tract
Direct corticostriateprojections reach the neostriatumvia the internal and external capsules
Corticostriate fibers
from the prefrontal cortex, cingulate gyrus, olfactory cortex, hippocampus, and septal area
Corticohypothalamic fibers
reach the hypothalamus via the internal capsule
Corticohypothalamic fibers
considered by some as an indirect CST -because of the similarity in the site of termination of both tracts, and because the red nucleus receives an input from the cortex
Rubrospinal tract
function of this tract is not well understood; it is believed to play a role in the turning of the head in response to light stimulation
Tectospinal tract
neurons of origin in the superior colliculuscross in the dorsal tegmentaldecussationand descend throughout the neuraxis to occupy a position in the anterior funiculusof the cervical spinal cord
Tectospinal tract
Fibers of this tract terminate on neurons in laminaeVI, VII, and VIII
Tectospinal tract
from the lateral vestibular nucleus, descends ipsilaterally in the pons, medulla, and spinal cord, where it occupies a position in the lateral funiculus
Lateral VestibulospinalTract
terminates on interneurons in laminaeVII and VIII, with some direct terminations on alpha motor neuron dendrites in the same laminae
Lateral Vestibulospinal Tract
facilitates extensor motor neurons and inhibits flexor motor neurons
Lateral vestibulospinal tract
Origin in the medial vestibular nucleus
Medial vestibulospinal tract
fibers join the ipsilateral and contralateral medial longitudinal fasciculus, descend in the anterior funiculus of the cervical cord segments, and terminate on neurons in laminaeVII and VIII
Medial vestibulospinal Tract
plays a role in controlling head position
Medial vestibulospinal tract
The neurons of origin of these tracts are located in the reticular formation of the pons and medulla oblongata
Reticulospinal tract
they modify sensory activity through their interaction with spinothalamic neurons in the dorsal horn
Reticulospinal tract
originate predominantly from the hypothalamus
Descending Autonomic Pathway
are small-caliber fibers that follow a polysynaptic route and are scattered diffusely in the anterolateralfuniculusof the spinal cord
Descending autonomic pathway
project on neurons in the intermediolateralcell column
Descending autonomic pathway
Lesions of this system result in autonomic disturbances
Descending Autonomic Pathway
Serotonergicfibers from the raphenucleus of the medulla oblongata
Descending Monoaminergic Pathways
Noradrenergic fibers from the nucleus locus ceruleusin the rostralponsand caudal midbrain
Descending monoaminergic pathway
Enkephalinergic fibers from the periaqueductal gray matter in midbrain
Descending Monoaminergic pathway
descend in the lateral and anterior funiculi both ipsilateral and contralateral to their site of origin
Descending monoaminergic pathway
descend in the lateral and anterior funiculiboth ipsilateraland contralateralto their site of origin
Descending Monoaminergic pathway
The CST facilitates _______ motor neurons and inhibit _______ motor neurons
Flexor; extensor
The CST descends through all levels of the neuraxis except the __________.
Cerebellum
The CST passes through the ____________ of the internal capsule
posterior limb
The anterior CST is also referred to as __________.
turck’s bundle
The remaining 2% of the CST that remains uncrossed throughout its course is referred to as the ___________.
tract of Barnes
The __________________ constitutes by far the largest component of the cortically originating descending fiber system
corticopontocerebellar tract
Considered as indirect CST
Rubrospinal tract
is believed to play a role in the turning of the head in response to light stimulation
Tectospinal tract
___________________ facilitates extensor motor neurons and inhibits flexor motor neurons
Lateral Vestibulospinal tract
