Lecture 22: Motor control 2

Question 1

What are the 4 synaptic inputs into the alpha motoneurons which act as the Final common path

Answer 1

1. Descending tracts, ie corticospinal/pyramidal tract
2. spinal interneurons (excitatory and inhibitory),
3. propriospinal neurons (coordinate upper and lower limbs),
4. Afferent fibres (type 1a) from muscle spindles

Question 2

How do sensory systems help to control the motor system before and during movement.

Answer 2

Before the movement, CNS must have info on current position of body part and length of its fibres.
During movement continuous info about muscle state, its position and speed its changing is sent back.
The sensory receptors can initiate reflexes or inhibit muscle contraction as well

Question 3

What are the 4 main sensory receptors that are help to control movement

Answer 3

1. Muscle spindles which monitor speed of length change
2. Golgi tendon organ which monitor muscle tension
3. Nociceptive (pain) receptors in the skin
4. Joint receptors (monitor position, hyperextension/hyperflexion)

Question 4

What are the 5 elements of (spinal) reflexes

Answer 4

1. What are the receptors activated
2. What are the afferent (sensory) fibres that go to the brain
3. What are the central synaptic relays
4. Efferent (motor ) fibres are axons of motoneurons
5. Effectors = (skeletal muscle)

Question 5

What is the stretch/ myotatic/ tendon/ monosynaptic reflex evoked by (what triggers its receptor)

Answer 5

From the muscle spindles by stretch or vibration of the muscle. They are very sensitive- a tap on the tendon. This causes a depolarisation to threshold which starts an AP down afferents

Question 6

What are the 5 components of the stretch reflex

Answer 6

1. Receptor: annulo-spiral’ endings in muscle spindles (mechanoreceptor)
2. Afferents: 1a afferent- fastest
3. Synaptic relay: Glutamergic excitatory synapses on alpha motoneurons - one synapse
4. Efferents: axons of alpha motoneurons (average myelinated speed)
5. Effectors: homonymous or synergistic muscles (also reflex inhibition of antagonistic muscles)

Question 7

What is reciprocal inhibition in stretch reflex

Answer 7

Due to additional synapses of 1a afferent collateral on the 1a inhibitory interneurons that release GABA, the motorneurons innervating antagonist muscles are inhibited as well

Question 8

Why is it important to test multiple stretch reflexes

Answer 8

Testing for symmetry of response to see if there is a blockage of the path

To identify which level of the spinal cord or brainstem a lesion has occured

Question 9

One more collateral branch of the 1a afferents from stretch receptor excite neurons which form which tract

Answer 9

Spinocerebellar tract

Question 10

What is the relationship between gamma and alpha motoneurons

Answer 10

Gamma motoneurons are usually activated before alpha but they are both coactivated.

Question 11

What do the gamma motoneurons actually do

Answer 11

They help to contract the intrafusal fibres on the end of the muscle spindle when the extrafusal muscle fibres are contracting, and this allows the stretch receptors to keep firing during muscle contraction because otherwise it would be all relaxed and then the afferent sensory info about length of muscle wouldn’t be sent

Question 12

What evokes the inverse myotatic reflex/ reverse myotatic reflex

Answer 12

The golgi tendon organs, embedded in the tendon, innervated by 1b afferent fibres carrying information about tension of muscle.

Question 13

What are the components of the reverse myotatic reflex

Answer 13

1. Receptor : Golgi tendon organ
2. Afferent: 1b fibres 3. Central synaptic relay: glutamergic excitement of 1b inhibitory interneuron which releases GABA to 4. alpha motoneuron to the 5. same muscle

(also some excitation of antagonist muscles)

Question 14

What is the purpose of the reverse myotatic reflex

Answer 14

Relaxation of the muscle helps to:

• Protection of muscles from overloading
• Compensation for muscle fatigue
• Helps to maintain muscle tension in optimal range

Question 15

What is the purpose of the flexion/flexor/withdrawal reflex

Answer 15

Evokes a fast withdrawal of limb from nociceptive stimulus

Question 16

What are the components of flexion/withdrawal reflex

Answer 16

1. receptor: nociceptors
2. afferents: type 3 and 4 fibres that is not supper fast
3. Synaptic relays : They synapse to excitatory interneurons which excite flexor alpha motoneurons which project to multiple spinal segments
   - they also have afferents which activate inhibtory interneurons of extensor muscles.
4. Efferent: axons of MN
5. Effector : flexor muscles

Question 17

What reflex helps to maintain balance in the foot when nociceptive receptors are being activated

Answer 17

Crossed extension reflex is caused by afferent flexion reflex synapsing with interneurons on the other side of the spinal cord which in the other limb will have activation of extensors and inhibit flexors which supports the body from falling.