Functional Hierarchy of Motor System

Question 1

What is in control of spinal reflexes?

Answer 1

Brainstem nuclei

Question 2

What are the brainstem nuclei controlled by?

Answer 2

Cerebral cortex

Basal Ganglia

Cerebellum

These control the brainstem nuclei and instruct voluntary nucleus

Question 3

Give an account of the function of the following descending systems:

Motor cortex

Braistem centres

Basal Ganglia

Cerebellum

Answer 3

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

Question 4

What is the effect of lower motor neurone lesions?

Answer 4

Lower Motor Neurones at spinal levels directly innervate muscles to initiate reflex and voluntary movements

So lesions cause flaccid paralysis and muscle atrophy

Question 5

What regulates alpha motor neurone activity?

Answer 5

Upper Motor Neurones in the brainstem or cortex synapse with multiple lower circuit neurones to regulate a motoneurone activity

Question 6

What do upper motor neurones from the cortex control?

Answer 6

Complex spatiotemporal skilled movements

Brainstem is responsible for basic movement and postural control

Question 7

What do upper motor neurone lesions cause?

Answer 7

Lesions cause spasticity, some paralysis, may be transient.

Lesions in many posture-regulating pathways - spastic paralysis

Corticospinal lesions - weakness (paresis), rather than paralysis

Question 8

Which motoneurones are proximal shoulder muscles and finger muscles attached to?

Answer 8

• proximal shoulder muscle are mapped to medial motoneurones
• finger muscles are mapped to much more lateral motoneurones

Question 9

What is the spinal cord descending input?

Answer 9

The spinal cord receives descending input via neurons in the brainstem AND direct cortical input via Corticospinal / Pyramidal tract.

Question 10

Where does vestibular sensory input enter?

Answer 10

Informs us of balance - enters at the brainstem

Question 11

Movements in response to visual, olfactory, auditory, emotional, intellectual cues result in nerve entry to what structure?

Answer 11

Cortex

Question 12

What sensory organ is responsible for the stretch reflex?

Answer 12

Muscle spindle

Question 13

What spinal cord segment is responsible for the following reflexes?

Biceps jerk, Triceps jerk, Patellar tendon, Achilles tendon reflex

Answer 13

Biceps jerk C6 Triceps jerk C7 Patellar tendon L4 Achilles tendon reflex - S1

Question 14

What condition may result in a loss of ankle jerk?

Answer 14

Diabetes

Question 15

What is the sensory organ for the flexor reflex?

Answer 15

Noiceceptors in skin muscles and joints

Question 16

How does the flexor reflex work?

Answer 16

1. Increased activity in the flexor muscles of the affected part via a number of excitatory interneurones.
2. At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited

●

FLEXOR REFLEX - ipsilateral flexion in response to pain

Question 17

How do we prevent falling over when the noiciceptor is activated, which in turn has activated our flexor and inhibited out ipsilateral extensor?

Answer 17

The contralateral limb extends via:

3. several excitatory interneurones which cross the spinal cord and excite the contralateral extensors
4. 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.

5. sensory information ascends to the brain in the contralateral spinothalamic tract.

Question 18

Why do noiciceptor fibres conduct more slowly than muscle spindle afferents?

Answer 18

•Nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly

Question 19

What organ is involved in excessive load to a muscle?

Answer 19

Golgi tendon organ

Question 20

What causes inhibition of relfexes?

Answer 20

Reflexes can be over-ridden with conscious thought

Question 21

How many synapses does an alpha motor neurone receive?

Answer 21

Over 10,000

Many are from descending cortical excitatory and inhibitory inputs, with continual integration of EPSPs and IPSPs

Question 22

How can the golgi tendon organ inhibition of a flexor be inhibited?

Answer 22

Descending voluntary excitation of a motoneurones can override the inhibition from the GTOs and maintain contraction.

Question 23

How can the stretch reflex be over-ridden?

Answer 23

Strong descending inhibition hyperpolarizes α-motoneurones and the stretch reflex can not be evoked.

Question 24

What are y-motoneurones dependant on?

Answer 24

Entirely dependant on descending pathways

Question 25

When do muscles become spastic?

Answer 25

During high y-motoneurone activation of muscle spindles, the muscles become resistant to stretch

Question 26

What is the clinical relevance of the stretch reflex?

Answer 26

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.)

Question 27

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?

Answer 27

Stretch reflex

Withdrawal reflex (the more powerful the pain stimulus, the greater the spinal spread and so the larger the response - known as FACILITATION)

Question 28

When does facillitation occur?

Answer 28

When there are several similar inputs or even several diverse inputs

Similar: Several pain fibre inputs

Diverse: Stretch reflex + pain input - greatly exaggerated response

Question 29

How does facilitation happen during the presence of diverse inputs?

Answer 29

Pain fibre input facilitates the action of the muscle spindles by maintaining the a-motoneurones in a more depolarised state

Question 30

What is the babinski sign?

Answer 30

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

Question 31

What is spinal transection?

Answer 31

Complete tear in the spinal cord

Question 32

What is spinal shock?

Answer 32

Spinal shock is a combination of areflexia/hyporeflexia and autonomic dysfunction that accompanies spinal cord injury

Loss of supra-spinal exitation

Question 33

What are the features of spinal shock when reflexes start to return back to normal?

Answer 33

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