Motor systems - Motor control basics

Question 1

what is voluntary brain control of muscles via?

Answer 1

α motoneurones in the spinal cord

Question 2

how would you describe Reflex control of muscles?

Answer 2

autonomous and hard-wired into motoneurone circuits at each segmental spinal level.

Question 3

how do Brainstem nuclei control spinal reflexes?
give an example?

Answer 3

integrate them into higher order reflexes that control posture and balance

e.g., descending inputs from vestibulospinal and reticulospinal tracts regulate trunk and limb muscle reflexes.

Question 4

what do brainstem nuclei receive?

Answer 4

inputs about voluntary movements from higher centres – from 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

1) Descending control pathways,
2) Basal ganglia,
3) Cerebellum
4) Local spinal cord/brain stem circuits

Question 6

The spatial (‘somatotopic’) map of your body musculature in the spinal cord - medio-laterally applies to what?

Answer 6

(applies to arms and legs)

Question 7

The spatial (‘somatotopic’) map of your body musculature in the spinal cord - proximally applies to what?

Answer 7

Proximal (shoulder/hip) muscle are mapped to medial motoneurones

Question 8

The spatial (‘somatotopic’) map of your body musculature in the spinal cord - distally applies to what?

Answer 8

Distal (finger/toe) muscles map to more lateral motoneurones

Question 9

what do muscles do along the long axis of spinal cord?

Answer 9

map spatially

Question 10

how would you describe higher brain centres?

Answer 10

functionally interdependent and control different aspects of voluntary movements.

Question 11

where does the spinal cord receive descending input from?

Answer 11

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

Question 12

does sensory input enter at all levels of the spinal cord?

Answer 12

Sensory input is crucial and enters at all levels

Question 13

describe sensory input of the spinal cord?

Answer 13

Spinal cord - proprioceptors, touch, pain etc

Question 14

describe sensory input of the brain stem?

Answer 14

vestibular system informs about balance

Question 15

describe the sensory input of the cortex?

Answer 15

movements in response to visual, olfactory, auditory, emotional, intellectual cues.

Question 16

what does damage to sensory inputs (at spinal level) result in?

Answer 16

paralysis as if the motoneurones themselves had been damaged.

Question 17

what may cause loss of all proprioception?

Answer 17

Viral infection
Autoimmune attack on sensory neurones in the dorsal root ganglia (DRG). Lost large myelinated fibres = sensory neuronopathy (very rare).

Question 18

what is the simplest spinal reflex?

Answer 18

stretch reflex

Question 19

example of the stretch reflex?

Answer 19

patellar tendon (knee-jerk) reflex

Question 20

what is the stretch reflex?

Answer 20

knee jerk follows a sharp tap to an inelastic tendon

Question 21

describe the mechanism of the stretch reflex?

Answer 21

Tendon transmits force to the muscle fibres (more elastic than tendons) and so are more able to stretch.

Stretch activates 1a afferent sensory nerves in the muscle spindle :

Increases the number of action potentials in 1a afferents projecting through the dorsal horn into the spinal cord.

causing rapid contraction of the AGONIST muscle.

Because muscles use agonists and antagonists to move joints - another spindle branch also connects (indirectly) with the ANTAGONIST muscles.

So, as the AGONIST muscle contracts, ANTAGONIST muscle relaxes (enabling agonist joint movement)

Question 22

how do spindle sensory afferents divide?

Answer 22

Spindle sensory afferents divide to make 3 branches/ connections (two in spinal cord, one with the brain)

Question 23

what type of reflex is the stretch reflex?

Answer 23

MONOSYNAPTIC REFLEX – one synapse, no interneurones involved

Direct contact to α-motoneurones in the stretched muscle : causing rapid contraction of the AGONIST muscle.

Question 24

what also makes up the spindle afferetn connection?

Answer 24

inhibitory interneurons

Question 25

what do inhibitory interneurons allow?

Answer 25

↓ activation of α-motoneurones to the antagonist muscle - which then relaxes this is called RECIPROCAL INHIBITION.

Question 26

where does the spindle afferent branch travel to tell the brain about muscle legnth?

Answer 26

Spindle afferent branch also travels up the dorsal columns to thalamus and somatosensory cortex

Question 27

where does the stretch reflex also exist?

Answer 27

exists in arms as well as legs Muscle stretch - stimulates muscle spindles Causes reflex muscle contraction – muscle shortens to previous length

Question 28

describe the mechanism of the inverse stretch reflex?

Answer 28

Caused by 1b afferent nerves from the Golgi tendon organs (GTO) – which monitor muscle tension. (Golgi = anatomist who discovered them). Muscle contracts and shortens – pulls on tendon and (sensory) 1b afferent nerves from the GTOs ↑ firing (higher AP frequency). Causes 3 things : Activation of inhibitory interneurones to the agonist muscle = decrease in contraction strength. Activation of excitatory interneurones to antagonist muscles. As for spindles, information about muscle tension ascends spinal dorsal columns to somatosensory cortex.

Question 29

why is inverse stretch reflex also known as the clasp knife reflex?

Answer 29

It is like an automatic knife action (clasp knife, USA) Greatly increasing tension in tendon leads to a collapse of resistance So, like a spring-loaded knife opening up, joint extends - Protective mechanism to prevent muscle damage

Question 30

describe the mechanism of the golgi tendon organ reflex?

Answer 30

Weightlifting - agonist quadriceps contracted - pulls hard on the tendon - GTO afferents (blue) ↑ firing - synapse with inhibitory interneurone - reduces motoneurone firing (red) – agonist muscle inhibited, so relaxes rapidly GTO afferents (blue) – also synapse with excitatory interneurone - ↑ activation of antagonist motor neurone (purple axons) – antagonist contracts rapidly Avoids muscle contracting so hard tendon is ripped from the bone Reflex is polysynaptic and protective

Question 31

describe the mechanism of the flexor/withdrawal reflex?

Answer 31

Question 32

what is the circuitry if the flexor reflex?

Answer 32

if you only withdrew your limb from a tack and nothing else, you fall over (NOT protective!!). to prevent this, contralateral limb extends via:- CROSSED EXTENSOR REFLEX (contralateral extension to prevent falling over). 3) excitatory interneurones cross the spinal cord, excite contralateral extensors 4) Other interneurones cross the spinal cord, synapse with inhibitory neurones and inhibit contralateral flexors Helps maintain upright posture by extending the limb opposite the flexed one - to bear the shift in body weight. 5) Sensory information ascends to the brain in the contralateral spinothalamic tract. Basic circuitry is similar to that used for walking Hardwired at spinal cord level.

Question 33

Spinal cord circuitry for Flexor & Crossed Extensor reflex

Answer 33

Question 34

holding an ever increasing load?

Answer 34

Question 35

holding excessive load?

Answer 35

Question 36

how is the GTO reflex overridden?

Answer 36

Each α motoneurone receives >10,000 synapses (red below) Many are from the thalamus and cortex. Cause excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). Some make direct contact, most via interneurones Each α motoneurone integrates these signals. i.e. Net effect at cell body is summed (total excitation minus total inhibition). Also, membrane potentials at synapses in distant dendrites have less effect than those nearer/on the cell body So: when holding something heavy but very important, you think - “I don’t want to drop this” - Descending voluntary excitation of α motoneurones overrides the inhibition from the GTOs and maintains muscle contraction Blocks the GTO reflex

Question 37

how can you override the stretch reflex?

Answer 37

Question 38

how can you override the withdrawal reflex?

Answer 38

Question 39

what is the clinical relevance of reflexes?

Answer 39

important for assessing integrity of the whole spinal cord circuit : - afferent nerves, balance of synaptic inputs to the motoneurones, the motoneurones, neuromuscular junctions and muscles. Can help localisation of spinal level of a problem - Evoked above, but NOT below a spinal level may localise a problem, e.g. segmental trauma to the spine. - Stretch reflex: spindle input is highly localised and affects only α motoneurones at same or adjacent spinal segments