L14 - descending pathways Flashcards
which tracts are descending
lateral corticospinal
ventral corticospinal
extrapyramidal tracts
types of neurones involved in descending pathways
upper
lower
interneuron
origin of UMN
cerebral and subcortical structures (basal ganglia and other centres in the brainstem)
origin of LMN
brainstem and spinal cord (ventral grey horn)
role of interneuron
- Provides opportunities for motor neurones to be modified
- important in reflex arcs
what type of neurones are LMN
peripheral nerves
where do LMN terminate
motor end plates / neuromuscular junctions
how to LMN exit the spinal cord
- cell bodies take origin in the ventral grey horn
- exit via the ventral route
- efferent
pyramidal tracts
descending pathways pass through pyramids in the medulla
corticobulbar/corticonuclear
descending pathways which originate in the cortex and stop at the cranial nerve nuclei
corticonuclear pathway - facial nerve
- cerebral cortex
- precentral gyrus
- internal capsule
- brainstem / spinal cord at a specific nucleus associated with a cranial nerve
- UMN innervates muscles of face and makes contact with cell body of LMN
- LMB leaves brainstem via facial nerve
- innervate muscles of facial expression
where to UMN and LMN synapse (lateral corticospinal tract)
contralateral ventral grey horn
corticospinal pathway
- UMN travels down spinal cord
- via pyramids in medulla
- enter corticospinal tracts
- descend in the lateral white column of the spinal cord
- at level of muscle, they jump into the ventral grey horn to synapse with LMN
- LMN uses peripheral spinal nerve to send axons and innervate the specific skeletal muscle
location of anterior limb of IC
between head of caudate and lentiform nucleus
location of posterior limb of iC
between thalamus and lentiform nucleus
location of fibres of corticospinal tract in IC
Face = GENU
Arms, trunk, legs = POSTERIOR LIMB
innervation of IC
deep perforating arteries
brainstem path of descending fibres
- start in cortex
- funnel down via post. limb of IC
- continuous with cerebral peduncles in midbrain
- travel through ventral pons
- regroup in the medulla and majority of them cross at the decussation of pyramids to travel contralaterally
what % of fibres cross at decussation of pyramids
85%
what type of innervation does the anterior corticospinal tract provide
bilateral
what type of innervation does the posterior corticospinal tract provide
contralateral
what happens to the 15% of fibres which do not cross at the level of the pyramids
- descend ipsilaterally down the spinal cord
- some join the anterior corticospinal tract and supply muscles
- some cross and contact the LMN at the level of the muscle
function of anterior corticospinal tract
axial musculature
function of lateral corticospinal tract
limb musculature
where do UMN and LMN synapse in anterior corticospinal tract pathway
inspilateral ventral grey horn
mechanisms of LMN lesions
peripheral nerve injury
infection
consequences of LMN lesions
- flaccid paralysis of muscles involved
- diminished or absent tendon reflexes at level of lesion
- muscle wasting
- muscle weakness
- hypotonia
- fasciculation/fibrillation
hyporeflexia
diminished tendon reflexes
areflexia
absent tendon reflexes
hypotonia
decreased muscle tone
fasciculation
spontaneous muscle twitches
fibrillaiton
rapid spontaneous muscle contractions
UMN injury
due to lesion to cerebral hemisphere or as they descend to lateral white column of the spinal cord or corticospinal tract
initial affects of UMN injury
- Flaccid paralysis of opposite limbs
- Loss of tendon reflexes
long term affects of UMN
- increased spinal reflex below lesion
- loss of fine motor control
- hypotonia
why would axial muscle groups be spared is there was an UMN injury
- Axial muscles are supplied bilaterlal
- If you have an UPM lesion on one side, it does not matter so much as the other side takes over and innervates the muscles
how to test presence of pathological reflexes
Babinski sign
Babinski sign
- stroke sole of foot with sharp object from front to back
- normal plantar response = toes curl down
- extensor plantar response = toes fan
innervation of LMN in corticonuclear pathway
largely bilateral
- fibres from RHS of precentral gyrus innervate muscles in both RHS and LHS of face
why are UMN or corticouclear pathways not as severe as corticospinal pathways (on the whole)
innervation if bilateral
injury to one will be okay as other side can still innervate it
which cranial nerves do not receive bilateral innervation
7
12
CN7 - facial nerve innervation
- only upper facial nucleus receives bilateral innervation
- lower facial nucleus is supplies contralaterally
CN12 - hypoglossal innervation
only innervated contralaterally
facial colliculus
Due to the abducens nucleus, behind which we find the arching fibres / internal genu of the facial nerve
corticonuclear input to facial motor nuclei - UPPER FACIAL NUCLEUS
- UMN sends axon via the genu of IC
- UMN sends bilateral projections to both sides of upper facial nucleus
- synapses with LMN in upper facial nucleus and innervates both sides of upper face
corticonuclear input to facial motor nuclei - LOWER FACIAL NUCLEUS
- lower face region of pre-central gyrus sends UPM to the contralateral lower facial nucleus
- LMN supplies lower quadrant of contralateral side of face
injury to RHS IC genu presentation
- upper face okay due to bilateral innervation
- lower left side of face is weak
what has happened if there is a weak lower and a weak upper face
Bell’s Palsy
indicates damage to facial nerve itself or lower LM lesion
lesion to left hypoglossal nerve, which way does the tongue deviate
to the left
lesion to RHS CN12 fibres in cortex / IC
tongue deviates to left
as left hypoglossal nerve not receiving innervation
UMN lesion fo CN12
deviation to contralateral side of lesion
LMN lesion for CN12
peripheral hypoglossal nerve itself is paralysed
pathway for CN12
- UMN from pre-central gyrus supplies contralateral hypoglossal nuclei
- LMN travel via hypoglossal nerve to intrinsic muscles of tongur
examples of subcortical structures which play a role in fine tuning information
- basal ganglia
- tectum, red nucleus
- reticular formation
- vestibular system
extrapyramidal pathways
subcortical structures which play a role in fine tuning information which do not originate in the cortex
reticulospinal pathway
- reticular formation to spinal cord
- voluntary movement. breathing/ consciousness
reticular formation
pons and medulla
vestibulospinal pathway
- vestibular nuclei to spinal cord
- controls posture
rubrospinal pathway
- red nucleus to spinal cord
- controls muscle tone
location of red nucleus
midbrain (tectum)
location of vestibular nucleis
pons and rostral medulla
location of anterior corticospinal tract
ventral white column (compared to other tracts which are beside the white column)
