LIFESCI 3K03 Topic 5 Part III: Spinal Interneurons

Central pattern generators

spinal interneurons

- most input to alpha motor neurons mediated by a complex network of spinal interneurons
- this intricate network of interneurons can design simple to highly complex patterns of movement based on input from primary sensory axons, descending axons from brain, collaterals of lower motor neuron axons, other interneurons

reciprocal inhibition

reflex arc using spinal interneurons to support simple monosynaptic reflexes (e.g. stretch reflex)
- contraction of one muscle accompanied by relaxation of its antagonish muscle

withdrawal reflex

reflex arc used to withdraw limb from aversive stimulus
- excitatory input for multiple ipsilateral motor units

crossed-extensor reflex

reflex arc for extensors and flexors on opposite side
- excitatory and inhibitory for ipsilaterial and contralateral motor units

what are central pattern generators

- circuitry for many rhythmic movements resides in spinal cord
- complex network of sensory, motor, and interneurons
- evidence from animal spinal transections
- measured bursts of rhythmic and coordinated activity arising for input that was constant

what are central pattern generators controlled by?

- controlled by two "half-centers" of spinal neurons
- mutually inhibiting halves that produce alternating bursts of flexor and extensor activity
- rhythmic activity will continue as long as input exists
- circuitry for walking resides in the lumbar/sacaral spinal cord
- Something in spinal cord generating pattern
- Continous input from brain
- Interneuron job to start and stop motion
- Inhibit alternatively
- Extensors: quads
- Flexors: hamstrings
- When A fires, B inhibits, vice versa

seperating central pattern generators

rhythm generators - setting the timing of patterns to be generated
pattern generators - sending the required pattern of muscles activations

what initiates and adapts CPGs?

descending control (input from brain) initiates and adapts CPGs
- can influence both rhythm and pattern genereation neural pools

gait retraining

changing the mechanics of how you walk or run
- easy to override motor patterns or CPGs
- very difficult to "rewrite" them
- Can retrain someone to move differently
- Walk/on toes, hard to rewire i.e. when running for bus

sensory information and CPGs

sensory information may also influence BOTH rhythm and pattern
stepping in cats
- proprioception
- amount of stretching can regulate the stance phase
- cats can match speed of treadmill, through CPG alone (after trained/electrical/drug stim)
- Cats have limited use of back legs, overtime can get back legs to start walking again

- sensory information from the skin
- stimulus to dorsal side of paw causes pattern with increased flexion to avoid a tripping hazard
- only impacts during swing

explain this diagram

- patterns are continually adjusted from higher and lower inputs
- Central input from brain, muscle spindles provide CPGs
Rhythm example
- Can edit how fast you’re walking/running
Pattern example
- Generating walking pattern, see puddle and avoid puddle, tweaks to pattern
- Injury --> brain says it hurts, change walking pattern
- How quickly, speed, extensor flexor: rhythm
- Directly firing extensor and flexor: pattern

explain this diagram

- Descending input from brain, neurons are rhythm generators which exist in spinal cord
- Pattern generators connect to alpha motor neuron  go to muscle