Lecture 16 - spinal circuits

Question 1

What type of arrangement is the motor system in?

Answer 1

hierarchical

Question 2

What type of motoneurons control the wrists and hands?

Answer 2

dorsal lateral

Question 3

What type of motoneurons control parts of the arms and shoulder?

Answer 3

ventral lateral

Question 4

What type of motoneurons control the neck?

Answer 4

ventral medial

Question 5

What are central pattern generators?

Answer 5

assemblies of neurons that produce a particular path of activity that control movements of limbs and walking

Question 6

In development what develops first, middle and last?

Answer 6

CPGs first
sensory feedback second
descending inputs last

Question 7

Describe the development of the spinal cord.

Answer 7

• neuro stem cells organise in a straight line
• they do this by diffusing down from the top of the body of one type of molecule, competing with others to form gradients across
• this then determines which transcription factors they express
• this instructs the cell what protein to make

Question 8

What are the 4 different types of CPG neurons and what type are they?

Answer 8

1. V0 - commissural
2. V1 - inhibitory
3. V2 - glutamatergic V2a and inhibitory V2b
4. V3 - excitatory commissural

Question 9

What is each of the CPG neurons axon projection in the embryonic spinal cord?

Answer 9

V0 = rostrally
V1 = rostrally and ipsilaterally
V2 = ipsilaterally and caudally
V3 = caudally

Question 10

How could you alter the movement of an animal?

Answer 10

• Different types of interneurons are specified by different transcription factors
• Certain genes can be killed to alter movement by removing neurons (knock out)
• If both the excitatory and inhibitory neurons are knocked out the animals can only do one type of movement
• If only one neuron is knocked out, then the mice could die before birth

Question 11

What does it mean when activity is happening on one side of the body?

Answer 11

inhibitory neurons are working on the other side of the body to inhibit it and then there is switchover

Question 12

Describe the action of walking (spinal circuits).

Answer 12

• involves activity within one side of the spinal cord changing
• out of phase activity in phlexor and extensors so they act one after the other.
• they inhibit the flexor motor neurons and at the same time inhibit the cells that are inhibiting the motor neurons from being active

Question 13

Describe the stepping reflex in newborns.

Answer 13

• disappears around 6 weeks
• gradually replaced by voluntary walking behaviour
• hierarchy not established
• stepping driven by sensory feedback and CPGs - sets off local circuits in limbs
• this disappears then re-emerges as corticospinal influence is established (descending control)

Question 14

What are the two types of interneurons from the V1 family?

Answer 14

1. renshaw cells
2. Ia inhibitory interneurons

Question 15

Describe renshaw cells.

Answer 15

• recurrent inhibition
• inhibits the activity of motoneurons to allow activity to go from one group of motoneurons to another

Question 16

Describe what Ia inhibitory interneurons do.

Answer 16

• send an axon to muscle spindles to detect stretching in muscles and they send a process in the spinal cord
• they connect with the motor neuron that innervates the muscle that which the sensory neuron has derived its info.
• also innervates Ia inhibitory interneurons that inhibit the other muscle group

Question 17

What are the 4 types of sensory fibres entering the CNS via the spinal cord?

Answer 17

1. A alpha
2. A beta and A delta
3. A gamma
4. C fibres

Question 18

What role do A alpha sensory fibres play?

Answer 18

type Ia from muscle spindle = monitor velocity of muscle stretch

type Ib from Golgi organ = monitor force of contraction

Question 19

What role do A beta and A delta sensory fibres play?

Answer 19

type 2 fibres from muscle spindle = monitor length of muscle and position in space

Question 20

What role do A gamma sensory fibres play?

Answer 20

touch, pain, heat and vibration

Question 21

What role do C fibres play?

Answer 21

pain, heat

Question 22

Describe the flexor reflex.

Answer 22

• multi synaptic pathway
• not as fast as the stretch reflex
• c fibres are slow and their sensation arrives after the hand has been pulled away

Question 23

Describe the flexor and crossed extensor reflex.

Answer 23

• requires more control than the flexor reflex
• neurons sending information to the other side of the body to make sure that balance isn’t lost
• for example if you stand on lego it make sure you only pick the leg that is in pain not both of them

Question 24

What are long propriospinal interneurons?

Answer 24

long propriospinal interneurons reciprocally connect the cervical and lumbar spinal cord and contribute to locomotor movement (rodents).

Question 25

Describe descending PINs.

Answer 25

- form a complex bilateral system with excitatory and inhibitory components to mediate inter limb coordination and to relay info to CPG. - most originate from laminae VII-VIII and the deep dorsal horn. - vast majority of descending PINs are excitatory both on the ipsilateral or contralateral side, the small inhibitory population terminates ipsilaterally.

Question 26

Descrive ascending PINs.

Answer 26

- form a powerful ipsilateral excitatory pathway from the rostral lumbar cord to motoneurons controlling proximal forelimb muscles. - originate mostly from the intermediate grey matter in the lumbar spinal cord and preferentially project ipsilaterally. - they project to the intermediate grey matter and the ventral horn throughout the length of the cervical spinal cord.

Question 27

Describe transcortical (slow) reflex pathways.

Answer 27

- go by the cortex - info comes into the spinal cord - goes through white matter pathways to neurons in the medulla - goes into the brain cortex - gives us control over what we do in terms of reflexes (don't have to remove hand from pain if we don't want too)

Question 28

What represents ascending pathways carrying impulses to a subconscious level?

Answer 28

spinocerebellar tracts

Question 29

What are the 6 pathways into the spinal cord and their functions?

Answer 29

1. lateral corticospinal tract - voluntary control of distal musculature 2. anterior corticospinal tract - voluntary control of Proximal musculature 3. reticulospinal tracts - regulate flexor reflexes and initiate patterned activity 4. rubrospinal tract - excitation of flexor muscles 5. tectospinal tract - orientation to visual stimuli 6. vestibulospinal tract - balance