Functional Hierarchy of the Motor System

Question 1

What is the 4 systems that control movement?

Answer 1

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

Question 2

How does the spinal cord receive descending input?

Answer 2

Via the brainstem

Question 3

How does the spinal cord receive direct cortical input?

Answer 3

Via the corticospinal (pyramidal) tract

Question 4

What is sensory input supplied by at the spinal cord?

Answer 4

Proprioceptors

Question 5

What informs the brainstem about balance?

Answer 5

Vestibular system

Question 6

What is movement in response to at the cortical level?

Answer 6

Visual, olfactory, auditory, emotional, intellectual cues

Question 7

What happens if there is damage to sensory inputs at spinal level?

Answer 7

Paralysis if motoneurones are damaged

Question 8

What information does the stretch reflex use?

Answer 8

Information from muscle spindles which monitor muscle length

Question 9

What happens in a stretch reflex?

Answer 9

a sharp tap to an inelastic tendon leads to the force being transmitted to muscle fibres which are more able to stretch
Stretch activates sensory nerves in muscle spindle
Increases APs in afferent nerves projecting through dorsal horn into spinal cord
Spindle sensory afferents divide and make 3 types of connections

Question 10

What are the 3 types of connections made after division of the spindle sensory afferents?

Answer 10

1. Activate a-motoneurones to stretched muscle = rapid contraction of agonist muscle (monosynaptic reflex) - no interneurones involved
2. Sensory fibres from spindle connect indirectly and influence antagonist muscles - allows antagonist to relax when agonist contracts = reciprocal inhibition
3. Afferent information ascends in dorsal columns = connections in somatosensory cortex = tells brain about muscle lengths

Question 11

What is inverse stretch reflex?

Answer 11

Caused by afferent nerves from Golgi tendon organs which monitor muscle tension
Muscle contracts and shortens = pulls strongly on tendon and 1b sensory nerves from the GTOs increased firing of APs

Question 12

What does the inverse stretch reflex cause?

Answer 12

Activation of inhibitory interneurones to agonist muscle
Activation of excitatory interneurones to antagonist muscles
Info about muscle tension ascends in dorsal columns to somatosensory cortex

Question 13

What is the importance of the clasp-knife reflex?

Answer 13

Protective mechanism to prevent muscle damage

Question 14

What are flexor reflexes?

Answer 14

Use info from pain receptors in skin, muscles and joints

Polysynaptic + protective

Question 15

How does the flexor reflex work?

Answer 15

Increased sensory APs from pain receptors cause:

1. increased activity in flexor muscles
2. Antagonistic extensors are inhibited

Question 16

What prevents a person from falling over when withdrawing limb from pain?

Answer 16

Contralateral limb extends by:

• excitatory interneurones cross spinal cord excite contralateral extensors
• inhibition of the contralateral flexors
• sensory info ascends to brain in contralateral spinothalamic tract

Question 17

What happens if excessive load is placed on a muscle?

Answer 17

GTO reflex is activated and load is dropped to protect muscle

Question 18

What is the exception for GTO reflex?

Answer 18

Can be over-ridden by voluntary input from the CNS
e.g. if carrying a child
descending voluntary excitation of a motoneurones override inhibition from GTOs and maintain contraction of muscle

Question 19

How can stretch reflex be overridden?

Answer 19

Strong descending inhibition hyperpolarises a-motoneurones and stretch reflex cannot be evoked