Lecture 22: Short and long-loop reflexes and control of sensory feedback

Question 1

Take home message:

Answer 1

Long-latency reflexes
possess most or all of the functional
capabilities of voluntary actions

Question 2

Short latency reflexes

Answer 2

Give spinal proprioceptive feedback
quickest of the three (20-50ms)

the difference in timing is due to upper vs lower limbs, lower has farther to travel

It is a muscles response to stretch with a latency of 25-50ms also called a spinal stretch reflex or a MONOSYNAPTIC stretch reflex

SLR recruit only spinal circuits and have simple processing capabilities

SLR scales in two ways:
1. Amplitude of SLR scale with amount of background activity
- Quad contracted vs quad relaxed, contracted is a background noise leading to a father kick
2. SLR scales with amount and rate of muscle stretch
-kick faster and longer cause of properties of muscle spindle, more ap, more neurotransmitter, more force

SLR mostly driven by rate and amount of motion cause by protuberance

Question 3

Voluntary Epoch/Feedback

Answer 3

Long latency + the visual feedback, longest of the three 120s+ with spinal, cortical and visual feedback

Question 4

Polysynaptic or Long latency feedback/Epoch

Answer 4

Long-latency response (LLR: 50-105 ms): spinal and cortical proprioceptive feedback

It is also often called the ‘long-loop’, ’long-latency’ or transcortical’ reflex

similar to SLR but Long latency involves a transcortical feedback loop that recruits structures in the cerebral cortex

Voluntary actions and LLRs recruit the same sensory and motor areas of the brain. Recruits sensory and motor circuits in the brain and show flexible goal directed processing. Flexible = can change reflex based on goal

LLRs increase when the goal is to resist a mechanical protuberance (higher emg output)

Higher emg output when smaller target

Question 5

Use of Sensory Feedback to Control and Correct Movements - Arm posture example

Answer 5

Desired State: Goal of the
task. In this example, maintain
the arm in a fixed target. Feedback about actual position of hand

Motor Plan: Maintain the arm in the target

Executive= cerbral cortex and cerebellum

Motor Command: Activity in limb motor circuits (cortical or subcortical) to maintain the hand in the target

Executive (Feedback Controller)
processes sensory information, selects and programs appropriate response

Effector: The Effector is the component of the peripheral motor system (i.e., leg, arm) that generates the desired
motor response

Question 6

Error occurs

Answer 6

when there is an error, how to get back to desired state

Expected state is the intended or planned movement

Error is based on difference between
‘expected’ (hand in target) and ‘actual’
position of the hand (hand in target or not)

Error signal used to adjust
ongoing movement

Closed-Loop or Feedback Control System: Movement control mechanism that uses sensory feedback to detect
errors and correct voluntary motor actions

Comparator: Compares the actual and expected state of the body. An error is any difference between actual and expected hand position

Question 7

Summary:

Answer 7

Summary:
1. SLRs allow rapid, stimulus dependent responses to a mechanical perturbation

2. LLRs are slower, polysynaptic responses that enable rapid, task-
   dependent control of sensory feedback to support voluntary motor actions