Functional Hierarchy of Motor System Flashcards
What is in control of spinal reflexes?
Brainstem nuclei
What are the brainstem nuclei controlled by?
Cerebral cortex
Basal Ganglia
Cerebellum
These control the brainstem nuclei and instruct voluntary nucleus
Give an account of the function of the following descending systems:
Motor cortex
Braistem centres
Basal Ganglia
Cerebellum
Motor cortex: Planning, initiating and directing voluntary movements
Basal ganglia feeds into the motor cortex (caudate nucleus and putamen feed into the globus pallidus and the substantia nigra, the latter two then feed into the thalamus before being transmitted to the motor cortex)
Basal ganglia is responsible for gating proper initiation of movement
Brainstem: Basic movements and postural control
Cerebellum: feeds into the brainstem, sensory motor coordination of ongoing movement
What is the effect of lower motor neurone lesions?
Lower Motor Neurones at spinal levels directly innervate muscles to initiate reflex and voluntary movements
So lesions cause flaccid paralysis and muscle atrophy
What regulates alpha motor neurone activity?
Upper Motor Neurones in the brainstem or cortex synapse with multiple lower circuit neurones to regulate a motoneurone activity
What do upper motor neurones from the cortex control?
Complex spatiotemporal skilled movements
Brainstem is responsible for basic movement and postural control
What do upper motor neurone lesions cause?
Lesions cause spasticity, some paralysis, may be transient.
Lesions in many posture-regulating pathways - spastic paralysis
Corticospinal lesions - weakness (paresis), rather than paralysis
Which motoneurones are proximal shoulder muscles and finger muscles attached to?
- proximal shoulder muscle are mapped to medial motoneurones
- finger muscles are mapped to much more lateral motoneurones
What is the spinal cord descending input?
The spinal cord receives descending input via neurons in the brainstem AND direct cortical input via Corticospinal / Pyramidal tract.
Where does vestibular sensory input enter?
Informs us of balance - enters at the brainstem
Movements in response to visual, olfactory, auditory, emotional, intellectual cues result in nerve entry to what structure?
Cortex
What sensory organ is responsible for the stretch reflex?
Muscle spindle
What spinal cord segment is responsible for the following reflexes?
Biceps jerk, Triceps jerk, Patellar tendon, Achilles tendon reflex
Biceps jerk C6 Triceps jerk C7 Patellar tendon L4 Achilles tendon reflex - S1
What condition may result in a loss of ankle jerk?
Diabetes
What is the sensory organ for the flexor reflex?
Noiceceptors in skin muscles and joints
How does the flexor reflex work?
- Increased activity in the flexor muscles of the affected part via a number of excitatory interneurones.
- At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited
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FLEXOR REFLEX - ipsilateral flexion in response to pain
How do we prevent falling over when the noiciceptor is activated, which in turn has activated our flexor and inhibited out ipsilateral extensor?
The contralateral limb extends via:
- several excitatory interneurones which cross the spinal cord and excite the contralateral extensors
- at the same time, via several interneurones, there is inhibition of the contralateral flexors
this helps to maintain an upright posture by extending the limb to bear the body weight.
- sensory information ascends to the brain in the contralateral spinothalamic tract.
Why do noiciceptor fibres conduct more slowly than muscle spindle afferents?
•Nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly
What organ is involved in excessive load to a muscle?
Golgi tendon organ
What causes inhibition of relfexes?
Reflexes can be over-ridden with conscious thought
How many synapses does an alpha motor neurone receive?
Over 10,000
Many are from descending cortical excitatory and inhibitory inputs, with continual integration of EPSPs and IPSPs
How can the golgi tendon organ inhibition of a flexor be inhibited?
Descending voluntary excitation of a motoneurones can override the inhibition from the GTOs and maintain contraction.
How can the stretch reflex be over-ridden?
Strong descending inhibition hyperpolarizes α-motoneurones and the stretch reflex can not be evoked.
What are y-motoneurones dependant on?
Entirely dependant on descending pathways
When do muscles become spastic?
During high y-motoneurone activation of muscle spindles, the muscles become resistant to stretch
What is the clinical relevance of the stretch reflex?
Allows assessment of the integrity of the whole spinal cord
Allows localisation of a problem (reflexes evoked above, but NOT below given level may localise a problem eg. segmental trauma to the spine.)
Which reflex is highly localised and involves alpha motoneurones in only one or two spinal segments?
Which reflex involves sensory fibre input which is diffuse and spreads through several spinal segments?
Stretch reflex
Withdrawal reflex (the more powerful the pain stimulus, the greater the spinal spread and so the larger the response - known as FACILITATION)
When does facillitation occur?
When there are several similar inputs or even several diverse inputs
Similar: Several pain fibre inputs
Diverse: Stretch reflex + pain input - greatly exaggerated response
How does facilitation happen during the presence of diverse inputs?
Pain fibre input facilitates the action of the muscle spindles by maintaining the a-motoneurones in a more depolarised state
What is the babinski sign?
Normally, stroking lateral aspect of the sole causes plantar flexion, downward curling of the big toe, only if the corticospinal tract is intact and functioning normally.
Damage or disruption to CS tract causes extension and not flexion, toes fan up and out = Babinski sign
Happens in children in children under 1 year old - motor system isn’t fully developed
and after epileptic seizures - transient cortical function disruption
What is spinal transection?
Complete tear in the spinal cord
What is spinal shock?
Spinal shock is a combination of areflexia/hyporeflexia and autonomic dysfunction that accompanies spinal cord injury
Loss of supra-spinal exitation
What are the features of spinal shock when reflexes start to return back to normal?
often reflexes are exaggerated - light touch evokes powerful withdrawal reflex of the whole limb
extensor reflexes can be so exaggerated that body weight can be supported briefly
clonus – stretch causes oscillatory muscle contraction/relaxation
it is very distressing for spinal cord injured patients
