descending motor pathways Flashcards
what are the descending motor pathways divided into?
upper and lower motor neurons
what are interneurons?
They are not part of the two neuron chain in DMPs but they can influence it
where do a) corticospinal b)corticobulbar/corticonuclear and c) extrapyramidal go?
a) cortex to spinal cord
b) cortex to brainstem
c) originating in different regions of the brainstem
what are three pathways of the corticospinal tract?
anterior, lateral and corticonuclear fibres
where do UMNs originate?
in the cerebrum or subcortical structures
what do UMNs do?
they influence LMN activity, modify local reflex activity, superimpose more complex patterns of movement
where are the cell bodies of UMN found?
in the cortex or upper structures of the CNS
what happens with UMN in the subcortical or cortical regions?
these regions send UMNs to LMNs to the muscle that they wish to move
where do the lower motor neurons originate?
from the lower brainstem or the ventral grey horn of the spinal cord - where the cell bodies are
what are the functions of the LMNs?
they are peripheral nerves to motor end plates/neuromuscular junctions, spinal nerves to reach muscles
how do LMNs leave in the head?
as cranial nerves
how does the LMN axon leave the SC?
through the ventral rootlets/root - efferent - CNS to PNS
what are the four divisions of the extrapyramidal pathway?
tecto, rubro, reticulo and vestibulospinal
where are the cell bodies for the corticospinal pathway?
they are in the pre central gyrus
how does the corticospinal tract work?
UMN will descend from the precentral gyrus through the internal capsule via the cerebral peduncles in the midbrain and the ventral pons. It will eventually reach the pyramidal level where it will cross the midline and enter the corticospinal tract in lateral columns of white matter. It will then enter grey matter to synapse with the LMN for the muscle
how are the tracts organised?
somatotopically - legs medial, trunk and then arm laterally
what is the corticonuclear tract?
starts in the cortex - precentral gyrus - internal capsule - peduncles - specific cranial nerve nuclei - muscles
what are the similarities in corticonuclear and spinal pathways?
cerebral cortex - pre central gyrus - internal capsule - brainstem/spinal cord
how do the cortico nuclear and spinal pathways differ?
at the brainstem - fibres go wither to corticonuclear which is cortex to brainstem for cranial nerves or to corticospinal which is cortex to spine for spinal nerves
where do primary synapse with secondary neurons in the corticonuclear and spinal tracts?
nuclear - cranial nerve nuclei in brainstem
spinal - anterior horn of grey matter of spinal cord
what are the positions of the parts of the internal capsule?
anterior - between the head of the caudate nucleus and the lentiform nucleus
genu - where it bends
posterior - between the lentiform nucleus and the thalamus
where do descending fibres all travel?
in the posterior limb of the internal capsule somatotopically
where do the motor fibres of the face travel in the IC?
in the genu
what is the somatotopic organisation of the IC?
arms are anterior, then trunk and legs posterior
where are the cerebral peduncles?
connected to the IC and is in the ventral part of the midbrain
do the ascending sensory fibres use the IC?
yes - connect to the thalamus first before the cortex and do not connect to the peduncles
what is the somatotopic organisation of the crus cerebri and peduncles?
lateral posterior is the legs
where do fibres travel after the peduncles?
into the basal pons and down the medulla using the pyramids
where do the descending fibres cross?
they decusate at the level of the decussation of the pyramids
what is the connection at the pons between cerebellar hemispheres?
transverse pontine fibres
where do the fibres travel after decussation?
recollect at medulla and cross as caudal part and then travel in the lateral cortical spinal tract in the lateral white column of the cord
why is there bilateral control of some muscles?
not all fibres cross - 15% do not and travel ipsilasterally to their respective level - some ipsilateral but mostly contralateral limb control
fibres that cross later travel where?
anterior cortical spinal tract in the ventral white columns of the cord
where do UMN contact the second order neurons?
they contact the LMN which are second order at the ventral grey horn contralaterally
how do LMNs leave the spinal cord?
they leave via the ventral rootlets through segmental spinal nerves
what muscles are controlled ipsilaterally and contralaterally?
contra - limb muscles
ipsi - axial muscles
what happens with ipsilateral control?
fibres do not decusate at pyramids they continue down and at suitable level they contact LMN that project to both sides of the spinal cord
where can lesions form?
in the upper or the lower motor neurons
what is poliomyelitis?
it is infection of the cell bodies of LMNs resulting in a atrophy or waste
what can peripheral nerve injury result in?
lesion in the ventral grey horn of the spinal cord in LMN
what can a lesions of the LMN neurons result in?
flaccid paralysis of affected muscles - weakness
diminshed or absent tendon reflexes at the level of the lesion
muscle wasting
muscle weakness
hypotonia
fasciculations or fibrillations
why are reflexes affected?
although the sensory fibre may work to detect the stimuli, the motor cannot
dimished - hyporeflexia
absent - areflexia
why does wasting occur?
not being used so degenerates
what is hypotonia?
reduced level of contraction at any given time
what are fasciculations/fibrillations and what is the difference?
they are abnormal spontaneous contractions
fibrillations are rapid and irregular and fasciculations are twitches
how can you get a UMN only lesion?
lesion in the cerebral hemisphere or as they descend to lateral white column of the spinal cord e.g. through a stroke
what is the initial result of a UMN lesion?
flaccid paralysis of the oppsite/contralateral limbs
loss of tendon reflexes as not contact to LMN
what will happen after several days or weeks?
motor function recovers but there is hypertonia - increased contraction
what is the LT result of an UMN lesion?
there is increased, brisk spinal reflex below the lesion - hyperreflexia
spastic paralysis of the involved muscles - weak and non compliant
loss of fine motor control so permanent inability to carry out fine movements of the hand and feet
what is a sign of UMN lesions?
Babinski reflex
the foot is stroked with a sharp small object from front to back
normally results in flexion of toes
positive means that there will be extension of the large toe coupled with fanning of other toes
why are other functions usually ok with UMN lesions?
other pathways appear to take over most corticospinal functions - movement will originate elsewhere in the cortex
axial muscle groups also have supply from other side so are spared
what is the order of cranial nuclei in the midbrain from top to bottom?
occulomotor, trochlear, abducens
what is the order of cranial nuclei in the pons and medulla top to bottom?
pons - trigeminal and then facial
medulla - glossopharyngeal and vagus, accessory and hypoglossal
what do the UMNs do in the corticonuclear tract?
they influence the LMNs in the cranial nerves
where do the UMNs for the corticonucler pathway originate?
more laterally within the lateral cortex/pre central gyrus
what is different in the CNT to the CST?
in the CNT the innervation of the LMNs is largely bilateral - only two exceptions
what is the innervation of head and neck muscles?
it is bilateral except for the lower facial and extrinsic tongue muscles - contralateral control
what is the facial nerve for predominantely?
facial expression muscles
what is the medial bump?
the facial nucleus - this is a colliculus and the motor fibres from CNVII wrap around the nucleus of 6 and leave via the cerebellarmedullary junction
how is the upper face stimulated?
UMN from the genu to level of the facial nuclei - ipsilateral and contralateral projection (same for 3,4,6,4,9,10,11). This stimulates a LMN from both sides to go to both sides of the upper face
how is the lower face stimulated?
UMN sends axon via IC to contralateral facial nucleus only (same for 12) and then LMN to lower portion of face
how will damage to the facial nerve itself present?
all the fibres come together and travel as one nerve - therefore there will be paralysis of the whole of one side of the face
how will unilateral damage to corticonuclear fibres present?
deprives the lower half of the opposite facial motor nucleus so paralysis of the lower half of the face on the side opposite to lesion
how will unilateral damage to corticonuclear fibres present in UMN?
contralateral lower quadrant weakness - angle of the mouth
how will unilateral damage to corticonuclear fibres present in LMN?
ipsilateral half of face - orbicularis oculi and facial muscles, angle of mouth, unable to close eyes, cannot elevate eyebrows
where will deviation of tongue be in upper motor neuron lesion?
to the contralateral side - there is not UMN from same side therefore weakness in that muscle so deviates that way
where will deviation of tongue be in lower motor neuron lesion?
hypoglossal nerve itself is paralysed and therefore ipsilateral
what is innervation of the extrinsic muscles of the tongue?
it is contralateral
what is the function of extrinsic muscles of the tongue?
change the shape and direction of it
what are other pathways for?
they come from other parts of the cortex and they control other aspects that are non voluntary
what is the reticulo pathway for?
for modifying voluntary movements and breathing and consciousness
what is the vestibulo pathway for?
posture and stimulation further down pathways
what is the rubro pathway for?
stimulating LMNs in spinal cord and controlling muscular tone
what structures, other than the cerebral cortex can control muscles?
the vestibular system, the reticular formation, the tectum, red nucleus and the basal ganglia
what is the reticular formation?
it is the pons and the medulla
what is the course of the reticulospinal pathway?
from the reticular formation to the SC
what is the vestibular nuclei?
the pons and the rostral medulla
what is the course of the vestibulospinal pathway?
from the vestibular nuclei to the SC
what is the course of the rubrospinal pathway?
from the red nucleus in the midbrain to the spinal cord
