Functional Hierarchy of the Motor System

Question 1

what controls the voluntary brain control of muscles?

Answer 1

via alpha motoneurones in the spinal cord - they are the “final common path”

Question 2

true or false:

reflex control of muscles is autonomous

Answer 2

true

Question 3

true or false:

brainstem nuclei exert control over spinal reflexes and integrate them into higher order reflexes to control posture and balance

Answer 3

true

Question 4

where does brainstem nuclei themselves receive control impulses from?

Answer 4

brainstem nuclei themselves receive control inputs about voluntary movements from higher centres - the cerebral cortex (motor, premotor and supplementary motor cortex), the basal ganglia and the cerebellum

Question 5

what are the four systems that control movement?

Answer 5

descending control pathways, basal ganglia,
cerebellum
local spinal cord/brain stem circuits

Question 6

what are the upper neurons systems that control movement?

Answer 6

descending control pathways
basalganglia
cerebellum

Question 7

what are the lower motor nueron systems that control movement?

Answer 7

local spinal cord/brain stem circuits

Question 8

what is involved in the descending control pathways?

Answer 8

motor cortex- planning, initiating and directing voluntary movements

brainstem centres- basic movements and postural control

Question 9

what does the basal ganglia system do?

Answer 9

gating proper initiating of movement and feeds into the motor cortex of descending pathway

Question 10

what does the cerebellum system do?

Answer 10

sensory motor coordination of ongoing movement and feeds into the brainstem centres of the descending pathway

Question 11

what do the descending systems feed into?

Answer 11

local circuit neurons and motor neuron pools

Question 12

what do sensory inputs feed into?

Answer 12

local circuit neurons who feed into motor neuron pools which feed into skeletal muscles

Question 13

what is the final common path?

Answer 13

motor neuron pools

Question 14

where is the spacial map of body musculature in the spinal cord? and what does it apply to?

Answer 14

medio-laterally (applies to arms and legs)

Question 15

what muscles are mapped to medial motoneurones?

Answer 15

proximal shoulder muscle

Question 16

what type of myscles map to more lateral motoneurons?

Answer 16

distal finger muscles

Question 17

How are Higher brain centres described?

Answer 17

Higher brain centres are functionally interdependent and control different aspects of voluntary movements.

Question 18

where does the spinal cord receieve descending input from?

Answer 18

via the brainstem AND direct cortical input via the Corticospinal (Pyramidal) tract.

Question 19

where does sensory input enter?

Answer 19

at the spinal cord in the form of proprioceptors, touch, pain etc
at the brainstem the vestibular system informs about balance
at cortical level. We make movements in response to visual, olfactory, auditory, emotional, intellectual cues.

Question 20

spindle sensory afferents divide and make 3 types of connections, what are they?

Answer 20

Many directly activate the -motoneurones to the stretched muscle causing rapid contraction of the AGONIST muscle

Because muscles use agonists and antagonists to move joints-the sensory fibres from the stretched spindle also connect indirectly with and influence the antagonist muscles

Spindle afferent information also ascends in the dorsal columns and makes connections in the somatosensory cortex to tell the brain about length of muscles

Question 21

Many directly activate the -motoneurones to the stretched muscle causing rapid contraction of the AGONIST muscle.

What type of reflex is this? and why?

Answer 21

this is a MONOSYNAPTIC REFLEX, no interneurones are involved, it is the only such connection known.

Question 22

muscles use agonists and antagonists to move joints -
the sensory fibres from the stretched spindle also connect indirectly with and influence the antagonist muscles.

What action does this have? and why?

Answer 22

So, when the AGONIST muscle contracts,
the ANTAGONIST muscle relaxes (stretches).

This happens because spindle afferents connect with and activate inhibitory interneurones which decrease activation of -alpha motoneurones to the antagonist muscle - which then relaxes
this is called RECIPROCAL INHIBITION.

Question 23

what type of reflex is a classic negative feedback loop?

Answer 23

stretch reflex

Question 24

what is the inverse stretch reflex?

Answer 24

caused by afferent nerves from the Golgi Tendon Organs (GTO) – which monitor muscle tension

muscle contracts and shortens – this pulls strongly on the tendon and 1b sensory nerves from the GTOs firing of APs

Question 25

what does the inverse stretch reflex cause?

Answer 25

Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength.

Activation of excitatory interneurones to antagonist muscles.

Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex.

Question 26

how does the flexor reflex protect you?

Answer 26

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

FLEXOR REFLEX - ipsilateral flexion in response to pain.

Question 27

what is the circuitry of flexor withdrawal reflex?

Answer 27

Small diameter Aδ nociceptive fibres triggering pain enter cord

they branch and activate interneurons in several spinal segments

which activate α motoneurones controlling all the flexor muscles of the affected limb

Question 28

how does the contralateral limb extend?

Answer 28

Several excitatory interneurones which cross the spinal cord 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.

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

This basic hard wired circuitry is similar to that for walking

Question 29

why is : the flexor and crossed extensor reflex is far slower than the stretch reflex

Answer 29

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

Question 30

Can The GTO reflex can be over-ridden by voluntary input from the CNS?

Answer 30

yes

Question 31

how much synapses does one alpha motoneuron receive?

Answer 31

> 10,000 synapses