Reflexes and Their Modulation

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Questions and Answers

What initiates the withdrawal reflex?

Mechanical stimulation of the skin

Motor neuron activation

Stretch receptor signals

Nociceptor activation (correct)

What happens to the motor neurons of extensor muscles during the withdrawal reflex?

They are excited by interneurons

They are activated

They remain unchanged

They are inhibited (correct)

How do crossed extensor reflexes differ from withdrawal reflexes?

They occur at the same time and involve opposite limbs (correct)

They do not involve excitatory interneurons

They are slower in response than withdrawal reflexes

They are triggered by non-painful stimuli

What is the role of the sensorimotor cortex in transcortical stretch reflexes?

It modifies the action of spinal reflexes Signup and view all the answers

Which type of reflex involves more complex circuits than simple withdrawal responses?

Automatic behaviors Signup and view all the answers

What might happen to voluntary actions in the presence of reflex abnormalities?

They can become erratic or impaired Signup and view all the answers

What influences reflex sensitivity in relation to postural stability?

The amount of stretch encountered Signup and view all the answers

What is the role of 'preparatory set' in postural control?

It is the motor system's readiness to execute a specific response. Signup and view all the answers

In the experiments assessing reflexes, how did reflex amplitude change?

It changed based on ongoing muscle activity and direction of perturbation. Signup and view all the answers

What experimental setup was used to examine reflex responses to instability?

Participants seated with hands attached to a robot providing stable and unstable support. Signup and view all the answers

What type of reflexes are believed to contribute to postural stability in complex 3D situations?

Spinal reflexes and possibly cortical reflexes Signup and view all the answers

How do reflexes respond to non-symmetric stability situations?

They adjust reflex responses based on the direction of instability. Signup and view all the answers

What early changes occur in response to postural errors?

Changes in co-contraction levels Signup and view all the answers

Which variable does NOT influence the modulating of reflex responses?

The level of coordination between limbs Signup and view all the answers

What distinguishes transcortical reflexes from their spinal equivalents?

They can be flexibly tuned to task stability requirements. Signup and view all the answers

Which strategy is least suitable for correcting standing posture during small perturbations on stable surfaces?

Stepping strategy Signup and view all the answers

What is a key feature of anticipatory postural adjustments (APAs)?

They enable independent limb movement during self-induced perturbations. Signup and view all the answers

Which of the following best describes automatic reactions in postural control?

They are modified according to the context of the perturbation. Signup and view all the answers

What role does the brainstem play in reactive postural control strategies?

It mediates faster responses than those controlled voluntarily by the cortex. Signup and view all the answers

How does the cortex influence the stretch reflex circuit during voluntary actions?

It reduces the size of stretch reflexes through descending input. Signup and view all the answers

What has been demonstrated regarding pre-movement changes in neural activity?

They result in increased excitability of task-relevant neural circuits. Signup and view all the answers

In the context of postural stability, what role does preparatory set play?

It enables early stabilizing actions following a significant stimulus. Signup and view all the answers

What was concluded regarding the impact of motor cortex inhibition on reflex modulation?

It eliminates reflex modulation but not StartReact responses. Signup and view all the answers

What happens to stretch reflex sensitivity when stability is reduced?

Stretch reflex sensitivity decreases. Signup and view all the answers

How does co-contraction relate to environmental stability?

Co-contraction increases as environmental stability is reduced. Signup and view all the answers

Which region showed larger long latency stretch reflexes without TMS in an unstable situation?

Biceps brachii showed larger responses. Signup and view all the answers

What did the hypothesis regarding co-contraction suggest?

Co-contraction and stretch reflex sensitivity increase as stability diminishes. Signup and view all the answers

What effect does TMS have on reflex responses in relation to postural stability?

TMS does not suppress stabilizing StartReact responses. Signup and view all the answers

What is suggested about reflex role in stability with regards to upper limbs?

Reflexes play a larger role in upper limbs compared to lower limbs. Signup and view all the answers

Which hypothesis concerning motor cortex inhibition and reflex modulation is proposed?

Inhibition eliminates reflex modulation in varying stability. Signup and view all the answers

Study Notes

Reflexes

Reflexes are automatic responses to stimuli, involving neural circuits within the spinal cord.
Examples include the withdrawal reflex and the crossed extensor reflex.
The withdrawal reflex is initiated by pain receptors (nociceptors) and causes the withdrawal of a limb from a painful stimulus.
The crossed extensor reflex is a simultaneous response where the opposite limb extends to provide stability during the withdrawal reflex.

Reflex Modulation

Reflexes can be modulated by various factors, including the context of movement and the level of voluntary control.
Transcortical stretch reflexes involve signals from stretch receptors reaching the sensorimotor cortex and modifying the reflex response.
Preparatory set refers to the readiness of the motor system to generate a desired action or response, potentially influencing reflex expression.
Changes in co-contraction, or the simultaneous activation of opposing muscle groups, are important for postural stability, with levels increasing as environmental stability decreases.

Automatic Postural Reactions

Automatic postural reactions are complex responses to postural perturbations, involving multiple muscles and occurring too late to be reflexes.
These reactions are generated in the brainstem and are likely driven by feedback or feedforward mechanisms.
Anticipatory postural adjustments (APAs) are feed-forward responses to expected perturbations, occurring before a movement to enhance stability.

Reflex & Voluntary Interactions

The cortex can regulate lower motor circuits and modulate reflexes.
Voluntary muscle activation can inhibit reflexes, demonstrating cortical-spinal interactions.
Muscle synergies, the coordinated activation of muscle groups, may simplify the complex control of movement.

Reflex Abnormalities

Spasticity, an involuntary increase in muscle tone, often results from reduced inhibition of stretch reflexes due to reduced cortical input.
This hyper-reactive stretch reflex leads to velocity-dependent resistance to movement and can cause pain.
Abnormal reflexes are often the result of damage to the central nervous system, disrupting the normal regulation of reflex pathways.

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Description

This quiz covers the automatic responses known as reflexes, including key examples like the withdrawal reflex and crossed extensor reflex. It also explores how reflexes can be influenced by factors such as context, voluntary control, and co-contraction. Test your understanding of these fundamental neurological concepts!