Functional Hierarchy of the Motor System

Question 1

What part of the spinal cord allows brain control of muscles?

Answer 1

Alpha motorneuones

Question 2

What exerts control over spinal reflexes and what do they do?

Answer 2

Reflex control of muscle in autonomous and so it is hardwired into motoneurone circuits at each segment, but the brainstem nuclei exert control over spinal reflexes and integrate them into higher order reflexes controlling posture and balance

Question 3

What does the brainstem nuclei receive control input from?

Answer 3

• Cerebral cortex (motor and premotor cortex and supplementary areas)
• Basal ganglia
• Cerebellum

Question 4

What are the four system which control movement?

Answer 4

1. Descending control pathways
2. Basal ganglia
3. Cerebellum
4. Local spinal cord/brainstem circuits

Question 5

How do the higher brain centres control voluntary movement?

Answer 5

Higher brain centres are functionally interdependent and control different aspects of voluntary movements. The spinal cord receives descending input via the brainstem and the corticospinal (pyramidal) tract.

Question 6

What are the different levels that sensory input enters?

Answer 6

1. At the spinal cord in the form of proprioceptors, touch, pain etc
2. At the brainstem the vestibular system informs about balance
3. At the cortical level – we make movements in response to visual, olfactory, auditory, emotional and intellectual cues

Question 7

What occurs is there is damage to sensory inputs (at spinal level)?

Answer 7

Paralysis as if the motoneurones themselves have been damaged.

Question 8

What is the stretch reflex?

Answer 8

• Found in all muscles – i.e. patellar tendon reflex

* It used information from muscle spindles which monitor muscle length

Question 9

How does the stretch reflex work?

Answer 9

1. Follows a sharp tap to an inelastic tendon
2. Force in transmitted to muscle fibres as they are more elastic and so are more able to stretch
3. Stretch activate sensory nerves in the muscle spindle increases number of APs in afferent nerves projecting through the dorsal horn into the spinal cord

Question 10

What is a monosynaptic reflex?

Answer 10

Spindle sensory afferents directly activate alpha motoneurons (efferent) to the stretch muscle causing rapid contraction of the AGONIST muscle

No interneurons are involved

Question 11

What is reciprocal inhibition?

Answer 11

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

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

This happens because spindle afferents connect with and activate inhibitory interneurons which decrease activation of alpha-motoneurones to the antagonist muscle which then relaxes (recip. inhibition)

Question 12

Name three way spindle sensory afferents divide and make 3 types of connections

Answer 12

• Monosynaptic reflex
• Reciprocal inhibition
• Ascend in the dorsal columns and make connections in the somatosensory cortex to tell the brain about the length of muscles

Question 13

Why is the stretch reflex a negative feedback loop?

Answer 13

Muscle stretch stimulate muscle spindles and causes muscle contraction and then muscle shortens to previous length

Question 14

Describe the inverse stretch reflex (golgi tendon organ)

Answer 14

Caused by afferent nerve 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 increasing firing of APs

Question 15

How does the inverse stretch reflex work?

Answer 15

Increase tension of tension increases firing in 1b sensory nerves:

1. Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength
   a. Agonist muscle relaxes
2. Activation of excitatory interneurons to antagonist muscles
   a. Antagonist muscle contracts
3. Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex

Question 16

How is the inverse stretch reflex protective?

Answer 16

This reflex is polysynaptic (disynaptic) and protective – its prevents muscles contracting so hard that the tendon insertion is town away from the bone

Question 17

Describe flexor (withdrawal) reflex - with crossed extension

Answer 17

Flexor or withdrawal reflexes use information from pain receptors (nociceptors) in skin, muscles and joints – they are polysynaptic and protective

They withdraw part of the body away from the painful stimulus and in towards the body – so they flex the affected part.

Ipsilateral flexion in response to pain

Question 18

How do flexor (withdrawal) reflexes work?

Answer 18

Increased sensory APs from pain receptors cause:
1. Increase activity in the flexor muscles of the affected part via a number of excitatory interneurons.

2. At the same time, via a number of excitatory and inhibitory interneurons, the antagonistic extensors are inhibited

Question 19

What else occurs in the flexor (withdrawal) response other than ipsilateral flexion?

Answer 19

If limb is withdrawn, and nothin else is done, you would fall over:

To prevent this, the contralateral limb extends via:
• Excitatory interneurons, which cross the spinal cord, excite the contralateral extensors
• At the same time, via interneurons, there is inhibition of the contralateral flexors

This helps maintain an upright posture by extending limb to bear the body weight

Question 20

Via what tract does sensory information trade to the brain?

Answer 20

Contralateral spinothalamic tract

Question 21

Why is the flexor and crossed extensor reflex much slower than the stretch reflex?

Answer 21

There are several interneurons in the flexor pathway, each with a small synaptic delay, and the nociceptive sensory fibres have smaller diameter than muscle spindle afferents and so conduct more slowly.

Question 22

Can the GTO reflex be overridden?

Answer 22

When load is excessive – GTO reflex is activated and load is dropped rapidly. However, if the load was precious or it would be painful for you to drop, it can be overridden. The GTO reflex can be over-ridden by voluntary input from the CNS.

Descending voluntary excitation of alpha motorneurones can override the inhibition from the GTOs and maintain contraction.

Question 23

How can the stretch reflex be overridden?

Answer 23

Strong descending inhibition hyperpolarises alpha motoneurones and the stretch reflex cannot be evoked

So when testing reflexes, subjects may need to be distracted to prevent voluntary effects on the reflex responses

So the absence of stretch reflex responses in an uncooperative patient may not be evidence of peripheral nerve damage

Question 24

What are alpha motoneurons?

Answer 24

Lower motor neurons of the brainstem and spinal cord. They innervate muscle fibers of skeletal muscle and are directly responsible for initiating their contraction