Spinal Reflexes, Proprioceptors, CPGs

Segmental Reflex

SC reflex

-The neural circuit for the reflex is contained within the SC

Components of a Segmental Reflex

- Primary sensory afferent

- One or more interneurons (only the 1 a if monosynaptic)

- Motor pools

What is meant by the 1a afferent being monosynaptic?

- The 1 A is from the muscle spindle

- Doesn't need an interneuron (connects straight to the motor neuron, INs give the opportunity to say no)

- Ie why you don't have control over the stretch reflex

Synaptic Inputs to Motor Pools

When an axons enters a motor pool to synpase on an alpha MN, that axon typically produces collateral branches that diverge to affec most of the MNs in the motor pool

- If an axon connects to a MN in a MP, it will connect to more than half of the MNs

How is the size principle used in reflexes?

Determines which MNs respond

- Axons entering a MP do not choose specific MNs

Do spinal afferents (reflexes) turn on alpha and gamma MNs?

No

- Only alpha

Stretch Reflex

Quick m stretch causes burst of APs from 1A afferents

How does the stretch reflex work?

1A afferents synapse directly on alpha MNs (makes powerful EPSPs)

- Results in contraction of homonymous motor pool and the antagonist is simultaneously inhibited

The stretch reflex is homeostatic for ___?

Muscle length

- Works in both directions

- Negative feedback for muscle length

How does the stretch reflex inhibit the antagonist?

- The 1A afferent also synpases on the 1A inhibitory IN

- The IN releases glycine (inhib NT) onto antagonist MNs

1A afferents account for most ___ input?

Excitatory

In LL mm, stretch reflex input can account to ___% of the excitatory drive to the MP during gait?

20-40

(when walking up a hill = more stretch = 1A will respond more)

Even though the 1A synapse is monosynaptic, how can it be regulated?

- It can't be turned off, but it can be turned down or up

- INs synapsing on the axon terminal can attenuate the calcium influx and increase or decrease the amount entering the postsynaptic cell

Alpha-Gamma Coactivation

The process of alpha and gamma MNs functioning simultaneously during most movements

Importance of Alpha-Gamma Coactivation (2)

1. Preserves and regulates 1A fring rate whether m is shortening or lengthening

2. Maintains the ability of the stretch reflex to have influence

Do 1A and 1B afferents affect gamma MNs?

No

What determines gamma MN activity?

Spinal circuits and descending motor control systems

1As care about ___?

Velocity

Group 2 cares about ___?

Muscle length

What does the tonic stretch reflex originate from?

Group 2 spindle afferents (respond to m length)

Tonic Stretch Reflex

Excitatory effect on homonymous MNs

- Mediated by Group 2 INs

- Also recruits synergists throughout the limb

How does the tonic stretch reflex work?

1. Group 2 afferents synapse on Group 2 Ins

2. The Group 2 INs synapse on multiple locations

- Homonymous mm

- Agonists

- Synergists above and below the joint

Dynamic Gamma

Increase the influence of bag fibers

- Velocity

Static Gamma

Increase the influence of chain fibers

- M length

Group 1A

Wrap around chain and bag fibers

Group 2

Wrap around chain fibers only

Muscle Synergies

Motor control term describing coordinated activation of groups of mm

- Whole limb movements w/out using a ton of neuro space/energy

How does muscle synergy work?

Muscle, joint, and cutaneous receptors deliver info to MNs in SC via INs that across multiple joints

Is the tonic stretch reflex normal?

No

Increased ___ makes the tonic stretch reflex strong during walking?

Static gamma drive

GTO Autogenic Inhibition During Rest

1B Afferent Affect

- Disynaptic inhibition through the 1B-IN (inhibitory interneuron)

- Negative feedback for force

GTO Autogenic Inhibition During Walking

1A afferent effect

- Disynaptic facilitation through the 1B-EN (excitatory interneuron)

- Brief + feedback

- Compensatory inhibition to synergists above and below equalizes whole limb force

- Only been observed in stance phase

Flexor Withdrawal with Crossed Extension

Results from A-delta nociceptor activation

- Fast and myelinated

How does the flexor withdrawal with crossed extension work?

Primary afferents activate second order sensory neurons in the dorsal horn which project to another set of INs in lamina 7

FWCE Ipsilateral Effects

The INs project to excitatory and inhibitory INs that in turn project to MNs in a reciprocal pattern

- Excite ipsilateral flexors

- Inhibit ipsilateral extensors

FWCE: The last IN is always ___?

IPSP

FWCE: Contralateral Effects

The lamina 7 INs decussate and project to contralateral excitatory and inhibitory INs that in turn project to MNs on the contralateral side of the SC

- Inhibit contralateral flexors

- Excite contralateral extensors

Effects of Cutaneous Stimulation on mm At Rest

- Facilitate mm under stimulated area

- Inhibits their antagonists

(skin pressure increases muscle activation through Group 2 INs)

Ruffini Organs

May contribute to the tonic stretch reflex through group 2 pathways

Golgi-Mazzoni (pressure receptors)

Appear to inhibit mm acting around a joint

- Sprained knee = mm produce less force

All movement is a result of interactive control from the (4)?

1. MSK systems

2. SC systems

3. Brainstem/cerebellar systems

4. Cortical systems

H-Reflex

Monosynaptic reflex elicited by electrically stimulating a n

- (can stimulate 1As without stimulating all alpha MNs)

- Sensory and motor components live together in peripheral nn

Central Pattern Generators

Neuronal circuits in the lumbar SC the elicit stepping movements on the LLs

- Control life sustaining motor patterns

Each LL has a ___

CPG

- Cycles of 2 CPGs are coordinated so that when one leg flexes the other extends

Control of Walking

- Stimulation of the mesencephalic locomotor region (MLR) activates descending pathways within the brainstem

- These activate neurons in the flexor and extensor components of the CPG on each side of the SC

How do metabotropic receptors affect CPGs?

Set the neurons into a bursting mode, such that once activated, they remain active with a sustained burst until reset by inhibition

- Occurs bc neuron is held depolarized

Rhythm Generator

Intrinsic oscillation in spinal INs generates the rhythm to match step cycle duration

Pattern Generators

Spinal networks also used for flexor withdrawal and reciprocal inhibition distribute the rhythmic drive to the appropriate MPs

___ systems activate the CPG and guide behavior?

Supraspinal

Aspects of CPG that are Under Supraspinal Control (5)

1. Activation and speed (reticulospinal)

2. Form and direction (reticulospinal)

3. Balance (vestibulospinal)

4. Coordination w other activities (corticospinal)

5. Navigation (corticospinal)

How do sensory inputs influence the pattern generator?

- Stretch to the hip flexors helps initiate the flexor burst to start swing phase

- Force on the ankle and knee extensors prolongs stance phase through + feedback

- Painful stimuli or obstacles can modify the pattern

SC Stumbling Corrective Reactions

Obstale contacted by paw produces a stereotyped response to lift paw over obstacle and continue swing phase

- If obstacle remains, heigh of paw during swing begins to increase

Stumbling corrective reactions are stronger with intact ___?

Brainstem and cerebellum

SC Adaptations to Training

- Kinematics and EMG of spinal locomotion improve w training

- Hypersensitivity to pain associated w SCI may be partially ameliorated by gait training

(SC injury increases sensitivity to pain, gait training can improve)

Pain Avoidance Response

Even with a fully transected SC, over the course of a few mins a rat SC learns to keep the paw out of water in contact results in shock