Functional Hierarchy of the Motor System

Question 1

How does voluntary brain control of msucles happen?

Answer 1

voluntary brain control of muscles is via alpha motoneurones in the spinal cord

Question 2

What is in control of the reflex contorl of muscles?

Answer 2

reflex control of muscles is autonomous

It is hard-wired into motoneurone circuits at each segmental spinal level

Question 3

brainstem nuclei themselves receive control inputs about voluntary movements from what higher centres?

Answer 3

the cerebral cortex (motor, premotor and supplementary motor cortex), the basal ganglia and the cerebellum

Question 4

What 4 systems contorl movements?

Answer 4

descending control pathways

basal ganglia

cerebellum

local spinal cord/brain stem circuits

Question 5

What motor neurons from the spinal cord control medial and lateral muscles?

Answer 5

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

Question 6

How does the spinal cord recieve descending input?

Answer 6

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

the spinal cord receives descending input via the brainstem AND direct cortical input via the Corticospinal (Pyramidal) tract

Question 7

Sensory inpput is cruical, what levels does it enter at?

Answer 7

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 cortical level, we make movements in response to visual, olfactory, auditory, emotional, intellectual cues

Question 8

What can damage to sensory inputs at the spinal level elad to?

Answer 8

paralysis if the motorneurones themself have been damaged

Question 9

Apart form the proprioceptive system, what else can give information on where the parts of the body are?

Answer 9

the visual system

Question 10

What is a spinal reflex?

Answer 10

those which act through the spinal cord

building blocks for movements

Question 11

What is the simplest reflex that is found in all muscles?

Answer 11

strech reflex

Question 12

What is an example of the strech reflex?

Answer 12

patellar tendon or knee-jerk reflex

Question 13

What is the process of the strech reflex being initiated?

Answer 13

follows a sharp tap to an inelastic tendon

force is transmitted to the muscle fibres – they are more elastic than tendons and so are more able to stretch

stretch activates the sensory nerves in the muscle spindle

this increases the number of APs in afferent nerves projecting through the dorsal horn into the spinal cord

Question 14

in the strech reflex, the spindle sensory afferents divide and make 3 types of connection, what are they?

Answer 14

1. Many directly activate the a-motoneurones to the stretched muscle causing rapid contraction of the AGONIST muscle - this is a MONOSYNAPTIC REFLEX, no interneurones are involved, it is the only such connection known
2. the sensory fibres 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 interneurones which decreases activation of a-motoneurones to the antagonist muscle - which then relaxes. This is called RECIPROCAL INHIBITION

3. 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 15

What type of feedback loop is the strech reflex?

Answer 15

Negative feedback loop

Muscle stretch stimulates muscle spindles which causes reflex muscle contraction - muscle shortens to previous length

Question 16

What causes the inverse strech reflex?

Answer 16

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

Question 17

How are APs fired from the GTO?

Answer 17

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

Question 18

What do APs from the GTOs cause?

Answer 18

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

Question 19

What happens in the golgi-tendon organ reflex and why?

Answer 19

agonist muscle is inhibited and relaxes rapidly

antagonist muscle is activated leading to it contracting

this reflex is polysynaptic and protective

it prevents the muscle contracting so hard that the tendon insertion is torn away form the bone

Question 20

Why is the GTo reflex sometimes called the clasp-knife reflex?

Answer 20

because greatly increasing tension in tendon leads to a collapse of resistance

like a spring-loaded knife opening up

it is a protective mechanism to prevent muscle damage

Question 21

Flexor (Withdrawal) reflex - with ___________

Answer 21

crossed extension

Question 22

What does the flexor reflex use information from?

Answer 22

Flexor or withdrawal reflexes use information from pain receptors (nociceptors) in skin, muscles and joints

Question 23

What is the flexor reflex?

Answer 23

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 24

How does the flexor reflex occur?

Answer 24

Increased sensory APs from pain receptors cause:

1. 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

Question 25

What is the cuircity of the flexor withdrawl reflex?

Answer 25

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 26

If you withdraw the limb form the stimulu and do nothing else what would happen and therefore what is done about this?

Answer 26

if you withdraw limb from a tack and do nothing else, you would fall over – which is NOT protective!

to prevent this the contralateral limb extends

Question 27

How does the contralateral limb extend during the flexor withdrawl reflex?

Answer 27

1. Several excitatory interneurones which cross the spinal cord excite the contralateral extensors
2. 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.
3. Sensory information ascends to the brain in the contralateral spinothalamic tract

Question 28

What is the Spinal cord circuitry for flexor and crossed extensor reflex?

Answer 28

one leg flexes to withdraw from the painful stimulus (on left)

the other extends to maintain balance because of shifted weight

there are several interneurons in the pathway - each with a small synaptic delay

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

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

Question 29

Can reflexes be over-ridden consciously?

Answer 29

Ask someone to hold an increasing load. The load stretches the muscle and its spindles, causing reflex contraction to restore the arm position (stretch reflex)

When load is excessive, GTO reflex is activated and load is dropped rapidly but if the load was not blocks, but a child, would you inevitably drop it because of the Golgi tendon reflex?

NO!

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

Question 30

How can the inhibition from the GTOs be over ridden and contraction maintained?

Answer 30

one α motoneurone receives >10,000 synapses (in red below)

many are from the brain: descending cortical excitatory and inhibitory signals so continual integration of EPSPs and IPSPs

So when holding something heavy but very important, descending voluntary excitation of a motoneurones can override

Question 31

Can the strech reflex also be overridden?

Answer 31

strong descending inhibition hyperpolarizes α-motoneurones and the stretch reflex can not 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