Functional hierarchy of the Motor system

Question 1

How does the brain voluntary control muscles?

Answer 1

via alpha motoneurones in the spinal cord - circuits at each segmental spinal level

Question 2

What division of the NS does reflex control of muscles come under

Answer 2

PNS - autonomic

Question 3

Descending inputs from which tracts regulate trunk and limb muscle reflexes?

Answer 3

vestibulospinal and reticulospinal

Question 4

Brainstem nuclei recieve control inputs about voluntary movements from higher centres too. Give examples

Answer 4

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

Question 5

What are the 4 systems that control movement?

Answer 5

descending control pathways

basal ganglia (found deep within cerebral hemispheres)

cerebellum

local spinal cord/brain stem circuits

Question 6

More proximal muscles like the shoulders and arms lie where in the spinal cord?

Answer 6

more medially

Question 7

More distal muscles like the legs lie where in the spinal cord?

Answer 7

more laterally

Question 8

The spinal cord receives descending input via what? and direct cortical input via what?

Answer 8

the brainstem and direct cortical input via the corticospinal 9pyramidal) tract

Question 9

Where does sensory input enter

Answer 9

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 10

what is the result of damage to sensory inputs (at spinal level)

Answer 10

paralysis as if the motoneurones themselves had been damaged (as sensory info isn’t being sent to brain so brain doesn’t know what muscles to move)

Question 11

what is the simplest reflex found in all muscles?

Answer 11

the stretch reflex

Question 12

best example of the stretch reflex?

Answer 12

knee-jerk reflex when you hit the patellar tendon

uses info from muscle spindles which monitor muscle length

Question 13

Describe the knee-jerk reflex

Answer 13

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 which increases the number of APs in afferent nerves (towards CNS) projecting through the dorsal horn into the spinal cord.

spindle sensory afferents divide and make 3 types of connections

Question 14

what are the 3 types of connections that spindle sensory afferents divide into?

Answer 14

1. Many directly activate the alpha-motoneurones to the stretched muscle
2. Sensory fibres from the stretched spindle also connect indirectly with and influence the antagonist muscles.
3. Spindle afferent information also ascends in the dorsal columns - somatosensory cortex - tells brain about length of muscles

Question 15

directly activating the alpha-motoneurones to the stretched muscle cause what

Answer 15

rapid contraction of the agonist muscle - monosynaptic reflex, no interneurons involved

Question 16

When the agonist muscle contracts what happens to the antagonist muscle?

Answer 16

it relaxes (stretches)

this is called reciprocal inhibition - it happens because spindle afferents connect with and activate inhibitory interneurones which decrease activation of alpha-motoneurones to the antagonist muscle - which then relaxes it

Question 17

How is the stretch reflex a classic negative feedback loop?

Answer 17

Antagonist muscle stretch (relaxation) - stimulates muscle spindles which causes reflex muscle contraction – muscle shortens to previous length

Question 18

what is the inverse stretch reflex called?

Answer 18

golgi tendon organ or clasp-knife reflex

Question 19

Describe the golgi tendon organ

Answer 19

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 increases firing of APs

this causes -
1. Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength.

Activation of excitatory interneurones to antagonist muscles.

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

Question 20

Why do we have a clasp-knife reflex?

Answer 20

This reflex is polysynaptic and protective

It prevents muscles contracting so hard that the tendon insertion is torn away from the bone

Question 21

What is the flexor (withdrawal) reflex and what does it do?

Answer 21

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.

Question 22

How does the flexor reflex happen?

Answer 22

Increased sensory APs from pain receptors causes:

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

FLEXOR REFLEX - ipsilateral (same side) flexion in response to pain

however if you withdraw limb from something and do nothing else you would fall over - need contralateral limb extension

Question 23

Describe how contralateral limb extension works in order to stop you from falling over when you do the flexor reflex

Answer 23

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.

Sensory information ascends to the brain in the contralateral spinothalamic tract - tells the brain what is going on

Question 24

True or false: the flexor and crossed extensor reflex is far slower than the stretch reflex

Answer 24

true

Question 25

What happens if you put an excessive load on a muscle?

Answer 25

the GTO reflex is activated and the load is dropped rapidly however depending on what the load is you can control it via voluntary input from the CNS

ie if it was rocks then you would drop it but if it was a baby then you wouldn’t