lec 9 - motor tracts of the spinal cord Flashcards
Descending tracts
composed of upper motor neurons from the cerebral cortex and brainstem that descend and control the activity of lower motor neurons in the brainstem and spinal cord
Corticospinal tracts
> Lateral corticospinal tract
> Anterior/Ventral corticospinal tract
Other descending motor tracts
Corticobulbar tract
Rubrospinal tract
Vestibulospinal tracts
Tectospinal tract
Reticulospinal tracts
The primary motor cortex is in the _____ in the ___ lobe
precentral gyrus
frontal
Primary Motor Cortex =
Allows for execution of motor plans, motor actions, and motor behaviors
Upper motor neurons (UMNs) that form the descending tracts arise from the primary motor cortex
The motor homunculus is the map of ____
the body on the primary motor cortex
The ____ and the ____ contain a much larger area in the frontal lobe and are responsible for designing the motor plan based on the proprioceptive information it received from the thalamus and from the primary somatosensory cortex.
supplementary motor area
premotor cortex
The neurons that control lower limb muscles =
> located in the medial aspect of the primary motor cortex
> near the interhemispheric fissure
the motor neurons that control upper limb muscles =
> located in the more superior, and slightly lateral, aspect of the primary motor cortex
> face and hands represent a large portion of the primary motor cortex, as they require many motor neurons to control them and allow for the fine motor skills executed by them
It is important to note that the motor neurons in the primary motor cortex are _____
upper motor neurons
> descend in the brainstem and spinal cord
synapse with lower motor neurons, which exit the CNS and use the PNS to reach skeletal muscles in the body
UMNs synapse on ____
LMNs, rather than on muscles directly
UMNs originate from the cerebral cortex and brainstem
UMNs in the cerebral cortex send motor commands to LMNs in the spinal cord and brainstem involved in voluntary movements
UMNs in the brainstem send motor commands to LMNs in the spinal cord involved in reflexive movements
The pyramidal neurons in the precentral gyrus (primary motor cortex) =
> command motor actions on the contralateral side of the body
> considered upper motor neurons (UMNs)
> they do not synapse on muscles = synapse on lower motor neurons (LMNs) to instruct the LMNs how to cause muscle contraction
The UMNs in the cerebral cortex have two functions:
> Send motor commands to the LMNs in the spinal cord to cause movement of muscles in the trunk and limbs via spinal nerves
> Send motor commands to the LMNs in the brainstem to cause movement of muscles in the head and neck via cranial nerves
Upper Motor Neurons (UMNs)
Corticospinal:
UMNs that originate in the cerebral cortex and innervate LMNs in the spinal cord
Spinal nerves carry LMNs to skeletal muscles in the trunk and limbs
Upper Motor Neurons (UMNs)
Corticobulbar:
UMNs that originate in the cerebral cortex and innervate LMNs in the brainstem
Cranial nerves carry LMNs to skeletal muscles in the head and neck
LMN cell bodies are in the ____
ventral horn of the spinal cord
> their axons travel in spinal nerves to reach skeletal muscles
Lower Motor Neurons (LMNs) =
> originate in the brainstem and spinal cord and leave the CNS to travel in the PNS to synapse on skeletal muscles and release acetylcholine to cause muscle contraction.
> release acetylcholine (ACh) at neuromuscular junctions (NMJs) to cause skeletal muscle contraction
> action of LMNs is controlled and regulated by UMNs
Alpha and Gamma LMNs
⍺-motor neurons = large LMNs that innervate extrafusal skeletal muscle fibers
𝛾-motor neurons = smaller LMNs that innervate intrafusal muscle fibers
Lower Motor Neuron (LMN) Signs =
Damage or injury to LMNs in ventral horn of spinal cord, ventral rootlets, ventral roots, or peripheral nerves causes LMN signs
LMN signs include hyporeflexia or areflexia, hypotonia, fasciculations/ spasms, muscular atrophy, and weakness
Upper Motor Neuron (UMN) Signs =
Damage or injury to the UMNs in the brain, brainstem, or spinal cord causes UMN signs
UMN signs include hyperreflexia, hypertonia, and weakness
Key clinical test: Babinski’s test
weakness
UMN lesion - yes
LMN lesion - yes
atrophy
UMN lesion - no
LMN lesion - yes
fasciculations
UMN lesion - no
LMN lesion - yes
reflexes
UMN lesion - increased
LMN lesion - decreased
tone
UMN lesion - increased
LMN lesion - decreased
Areflexia:
lack of reflexes
Hyporeflexia:
diminished reflexes
Hyperreflexia:
exaggerated reflexes
Paralysis:
no movement
Paresis:
muscle weakness
Atonia:
lack of muscle tone
Hypotonia:
decreased muscle tone
Hypertonia:
increased muscle tone
Atrophy:
degeneration of cells (muscular atrophy = degeneration of muscle cells)
Amyotrophic Lateral Sclerosis (ALS)
> ALS/Lou Gehrig’s disease
> gradual, progressive degeneration of both UMNs and LMNs
> eventually leading to respiratory failure and death
> most cases are idiopathic
genetic mutation
Amyotrophic Lateral Sclerosis (ALS)
Initial symptoms:
> weakness or clumsiness, which begins focally then spreads to involve adjacent muscle groups
> painful muscle cramps, twitching, spasticity, gradual worsening muscle weakness over time, and fasciculations
> difficulty with ambulation, speaking, swallowing, breathing
On exam, both UMN and LMN signs are observed
Amyotrophic Lateral Sclerosis (ALS)
Treatment
disease modifying agents to prolong survival in early stages and extend time until tracheostomy needed
> riluzole and edaravone
symptom management
muscle relaxants
Medial motor pathways:
synapse on medial LMNs in the ventral horn
Control axial/proximal muscles for postural movements
Lateral motor pathways:
synapse on lateral LMNs in the ventral horn
Control distal limb muscles for fine, volitional movements
Lateral corticospinal tract =
site of origin: primary motor cortex, other frontal and parietal area
site of decussation: pyramidal decussation at cervicomeduallary junction
levels of termination: entire cord (predominantly at cervical and lumbosacral enlargements)
function: voluntary movements and fine motor control of distal limb muscles
> contralateral movement
Anterior corticospinal tract =
site of origin: primary motor cortex and supplementary motor area
site of decussation: -
levels of termination: cervical and upper thoracic cord
function: on the ipsilateral side of the injury/damage - bilateral axial and girdle muscles
Lateral Corticospinal Tract
Pathway:
1) UMN cell bodies in 1° motor cortex
2) UMN axons descend in corona radiata > internal capsule > cerebral peduncles/basis pedunculi in midbrain > longitudinal fibers in pons > pyramids of medulla
3) UMNs decussate in pyramids of medulla (motor decussation)
4) UMNs descend in contralateral lateral corticospinal tract in lateral column/funiculus of spinal cord
5) UMNs synapse on lateral LMNs in ventral horn of spinal cord
Because the UMNs decussate at the pyramids of the medulla, if there is damage to the UMNs rostral/superior to the pyramidal decussation, then the clinical deficits will be:
on the contralateral side of the damage/injury
If there is damage to the UMNs or lateral corticospinal tract caudal/inferior to the pyramidal decussation, then the clinical deficits would be:
on the ipsilateral side of the injury/damage
Anterior Corticospinal Tract
Pathway:
1) UMN cell bodies in 1° motor cortex
2) UMN axons descend in corona radiata > internal capsule > cerebral peduncles/basis pedunculi in midbrain > longitudinal fibers in pons > pyramids of medulla
3) UMNs descend in ipsilateral anterior corticospinal tract
4) UMNs synapse on medial LMNs in right and left ventral horns
The lateral corticospinal tract contains about ___ of the fibers, while the ventral corticospinal tract contains ___ of fibers.
90%
10%
Once the UMN of the ventral corticospinal tract reaches its designated level, it gives off two branches:
> right branch goes into the right ventral gray horn
> left branch goes into the left ventral gray horn
> each branch synapses with a LMN on the right and left side of the body, respectively
Botch UMNs and LMNs are tonically active =
there is equal innervation to the axial and postural muscles on the left and right sides of the body as well as for the ventral and dorsal aspects of the body
Due to the bilateral innervation of the ventral corticospinal tract, damage to this tract =
will not result in major clinical deficits since the contralateral tract provides sufficient drive to maintain posture
The tonic activity of the UMNs and LMNs also allows us to ____
maintain posture constantly
Rubrospinal tract:
controls movement of contralateral limbs
Stimulates flexor and inhibits extensor muscles and helps maintain muscle tone
Vestibulospinal tracts:
balance and head and neck positioning
Tectospinal tract:
coordinate head and eye movements toward auditory and visual stimuli
Reticulospinal tracts:
automatic posture and gait-related movements
Upper Cervical Spinal Cord tracts =
Ascending tracts:
> gracile tract
> cuneate tract
> dorsal spinocerebellar tract
> anterolateral system
Descending tracts:
> lateral corticospinal tract
> ventral corticospinal tract
Lower Cervical Spinal Cord tracts =
Ascending tracts:
> gracile tract
> cuneate tract
> dorsal spinocerebellar tract
> anterolateral system
Descending tracts:
> lateral corticospinal tract
> ventral corticospinal tract
Thoracic spinal cord tracts =
Ascending tracts:
> gracile tract
> cuneate tract
> dorsal spinocerebellar tract
> anterolateral system
Descending tracts:
> lateral corticospinal tract
> ventral corticospinal tract
Lumbar spinal cord tracts =
Ascending tracts:
> gracile tract
> anterolateral system
Descending tracts:
> lateral corticospinal tract
> ventral corticospinal tract
Sacral spinal cord tracts =
Ascending tracts:
> gracile tract
> anterolateral system
Descending tracts:
> lateral corticospinal tract
