functional hierarchy of the motor system Flashcards
how are muscles directly controlled?
via alpha motoneurones
what controls spinal reflexes?
brainstem nuclei
what are spinal reflexes integrated into?
higher order reflexes
what do higher order reflexes control?
posture and balance
what do trunk and limb muscles involved with higher order reflexes receive input from?
vestibulospinal and reticulospinal tracts
what controls brainstem nuclei?
cerebral cortex
basal ganglia
cerebellum
what 4 systems control movement?
descending control pathways
basal ganglia
cerebellum
local spinal cord and brainstem circuits
what do LMNs do?
directly innervate muscles to initiate reflexes and vountary movements
what would a LMN lesion cause?
flaccid paralysis and muscle atrophy
where are LMNs located?
cranial and spinal levels
where are UMNs located?
brainstem or cortex?
what do UMNs do?
synapse with multiple lower circuit neurones to regulate alpha motoneurone activity
control complex spatiotemporal skilled movements
what wold an UMN lesion cause?
spasticity
some paralysis
what would happen if there were many lesions in posture regulating pathways?
spastic paralysis
what would corticospinal lesions cause?
weakness rather than paralysis
what is the mediolateral map of the spinal cord?
proximal muscles are mapped to medial motoneurones and distal muscles are mapped to much more lateral motoneurones
where does the spinal cord receive descending input from?
brainstem
where does the spinal cord receive direct cortical input?
pyramidal tract
what level does sensory input enter the nervous system?
all levels
what does damage to sensory input result in?
paralysis as if the motoneurones had been damaged
what type of sensory input would occur at the spinal cord?
proprioceptors
touch
pain
what type of sensory input would occur at the brainstem?
vestibular systems inform about balance
what is the simplest segmental reflex?
stretch reflex
what crucial cord segments make up the biceps and triceps jerk?
C6 and C7 respectively
what crucial cord segment makes up the patellar tendon reflex?
L4
what crucial cord segment makes up the achilles tendon reflex?
S1
what is the significance of testing reflexes?
helps to detect level of spinal cord damage. can evoke reflexes above but not below the level of damage
what may impaired reflexes be a sign of?
nerve damage
in the flexor withdrawal reflex, how does sensory information ascend to the brain?
ascends in the contralateral spinothalamic tract
why is the flexor withdrawal reflex much slower than the stretch reflex?
- because there are several interneurons in the pathway each of which have a small synaptic delay
- nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly
what reflex activates in the case of an excess load being placed on a muscle?
golgi tendon organ reflex
can reflexes be consciously over ridden?
yes
what causes alpha motoneurones to override the inhibition from the golgi tendon organs?
descending voluntary excitation of said alpha motoneurones
how can the stretch reflex be overridden?
strong descending inhibition hypepolarizes aplha motoneurones and the stretch reflex cannot be evoked
what does the activity of gamma motoneurones depend on?
entirely on descending pathways
what does high gamma motoneurone activation of muscle spindles result in?
it causes muscles to become extremely resistant to stretch and thus become spastic
what does facilitation do?
increases the effects of sensory inputs
what is the babinski sign?
when damage or disruption to the corticospinal tract causes extension of the toes when the lateral aspect of the sole is stroked
when can the babinski sign be seen when nerves are not damaged?
in children less than 1 year old - motor system not fully developed
after epileptic seizures - transient cortical function disruption
what happens in spinal transection?
immediate loss of sensory and autonomic effects- loss of bowel, bladder and sexual regulation
what is spinal shock?
loss of supraspinal excitation and reflexes not evoked for 2-6 weeks - then gradual return of reflexes
what often happens in spinal shock when reflexes return?
reflexes are very exaggerated - light touch evokes powerful withdrawal reflex of the whole limb
what is clonus?
stretch causes oscillatory muscle contraction/relaxation, very distressing
