L18 Muscle Reflexes: Control of Posture Flashcards Preview

Physiology > L18 Muscle Reflexes: Control of Posture > Flashcards

Flashcards in L18 Muscle Reflexes: Control of Posture Deck (21)
Loading flashcards...

Learning Outcomes (for general perusal)

•To develop the story of motor control  away from the motor unit, to include the spinal reflexes and their part in overall skeletal muscle control.

•To show how the stretch and inverse stretch reflex maintain and signal muscle force, length and acceleration

•To describe some of the other more complex reflexes that maintain posture


The Reflex Arc 

  1. What is it?
  2. What does it consist of?

  1. the basic unit of spinal cord integration
  2. a sense organ, an afferent neuron leading to the dorsal horn of the spinal cord (control centre), where it synapses either directly with the effector neuron or (in more complex reflexes) with interneurons. The effector neuron then leaves the spinal cord via the ventral root or corresponding motor cranial nerve to innervate the relevant muscle. 


  1. Where does the STRETCH REFLEX originate from?
    1. What does it control?
  2. Where does the INVERSE STRETCH REFLEX originate from?
  3. What are the more compex reflexes?

  1. Muscle Spindle
    1. controlling and signalling muscle LENGTH
  2. Golgi tendon organ
    1. controlling and singnalling muscle FORCE
  3. crossed extensor reflex, postural and locomotory reflexes


​What is the primary length tranducer in the muscle?

the muscle spindle


Length: Muscle Spindle

  1. How is the muscle spindle arranged?
  2. What is it composed of?
  3. Describe the afferent fibres
  4. What are the contractile ends of the spindles innervated by?

  1. in parallel with the extrafusal fibres and so is ideal to signal muscle length and stretch to the brain
  2. specialised muscle fibres - Type Intrafusal Fibres (skeletal muscle is type extrafusal fibres) with contractile ends and non-contractile centres
  3. Come from the spindle centres, large, myelinated and classed as type Ia and type II fibres
  4. γ – efferent nerves. Small diameter, unmyelinated nerves


Length: Muscle Spindle Fibre

​Within one single small spindle there are a number of different types of fibre, some signaling rapid change, some signaling length.  Together they provide the brain with a second to second picture of muscle length and how its changing. 


Name the three types of fibre in the spindle

Dynamic Nuclear Bag Fibres

Static Nuclear Bag Fibres

Nuclear Chain Fibres



Length: Muscle Spindle Fibre

​What are the following? 


  1. Dynamic Nuclear Bag Fibres
  2. Static Nuclear Bag Fibres
  3. Nuclear Chain Fibres

1. Dynamic Nuclear Bag Fibres (Bag1 Fibres) 

  • Innervated by fast conducting Ia efferents
  • signal rapid changes in muscle length (velocity), firing maximally when the muscle is being rapidly stretched (dynamic response). 
  • rapidly adapting receptors like (pacinian corpuscle)

2. Statuc Nuclear Bag Fibres (Bag2 Fibres)

  • sends both type Ia and more slowly conducting type II afferents
  • signals muscle length with its discharge frequency being proportional to absolute length of a muscle

3. Nuclear Chain Fibres

  • role = signal absolute muscle length.  Together these provide the static response of the muscle spindle


Length: Muscle Spindle Fibre

  1. What is the efferent supply of the Muscle spindle bag and nuclear chain fibres?
  2. What do the fibres in your answer to part 1 do?
  3. What does this prevent?

  1. The γ – efferents
  2. maintain the sensitivity of the spindle in the face of muscle shortening.  When the main α nerve of the muscle activates the extrafusal fibres that contract the muscle, the γ fibres fire and shorten the spindle too (α/γ.co activation).  
  3. This prevents muscle contraction unloading the spindle and rendering it neurally silent.



Length: Muscle Spindle Fibre

  1. Which fibres detect velocity?
  2. Which fibres detect length?

  1. Dynamic Nuclear Bag fibres (Bag1)

    •Innervated by Ia fast conducting nerves

    •Fast adapting

    •Phasic response

  2. Static Nuclear bag fibres (Bag2) and nuclear chain fibres

    •Innervated mainly by type II, slower conducting than Ia

    •Slowly adapting

    •Static response


The Stretch Reflex

  1. What is the stretch reflex?
  2. What aids the dampening effect?
  3. How are muscle length and velocity signalled?

  1. Centrally the afferents from the bag and chain fibres directly innervate the muscle being stretched so it contracts against the stretch, damping rapid changes in muscle length, smoothing movement
  2. Reciprocal innervation of the antagonistic muscle via an inhibitory interneuron in the spinal cord means that it is inhibited too
  3. signalled centrally by ascending tracts from the spindles (dorsal and spinocerebellar tracts).



Outline the Stretch Reflex

•Monosynaptic reflex

•Muscle contracts against stretch

•Reciprocal innervation

•Smooths and damps muscle movement

•Signals length and velocity of muscles centrally (spinocerebellar and dorsal tracts)


What is force transduced into nerve signals by?

Golgi Tendon Organ

which converts force exerted by the muscle into nerve signals regulating it via a spinal reflex and sending information on contractile force centrally


Force : Golgi Tendon Organ

  1. What is the golgi tendon organ?
  2. What is it's arrangement and where is it?
    1. What does this mean?
  3. What do they innervate the main a nerve by?
  4. What is it's function?

  1. consisting of a net of knobbly nerve endings embedded within the fascicles of a tendon
  2. in series with the muscle in relatively non-distensible tendon
    1. golgi tendon organs fire more in response to muscle contraction (force) than to muscle stretch
  3. via an inhibitory interneuron, thus a reflex inhibition within the muscle can be caused
  4. thought to be the signal feeding back force information to the supraspinal systems controlling movement, enabling us to finely modulate not just length of a muscle but also force exerted
    1. Originally the GTO was thought to serve a protective function in preventing overcontraction of muscles


Outline the Inverse Stretch Reflex

•Reflex inhibition of α output to muscle

•Originally thought to prevent overcontraction

•Now part of a sophisticated control system sending information about force centrally

•Enable us to control muscle force finely as well as length




  1. What reflex is elicited with a patellar hammer clinically?
  2. What information can it give, how is this useful?
  3. How does 'reinforcement' help when attempting to elicit the stretch reflex?
  4. What may increased gamma discharge to muscle spindles bring about?

  1. muscle stretch reflex
  2. about nervous system excitability, and conduction velocity in peripheral nerves) helps differentially diagnose cerebellar disease and peripheral nerve problems
  3. by increasing γ discharge in the entire motor pool through the activity of a large muscle group “Jendrassiks manoeuvre". Generally increasing excitability.
  4. sensitises them and may set up an 8 Hz physiologic tremor in the muscle.  This is because an over-amplified and under damped system will always oscillate - CLONUS. The Stretch reflexes are allowed to proceed uninhibited.



  1. In whom can CLONUS be seen?
  2. Who else might this be seen in?

  1. Upper Motor Neuron lesions when the descending inhibitory input to γ fibres is absent. Increased spindle sensitivity
  2. Amytrophic lateral sclerosis, multiple sclerosis, spinal cord lesions and hepatic encephalotpathy.



  1. What is the clasp-knife effect?

  1. When muscles are hypertonic (over excitable stretch reflexes), we often see the stretch reflex abruptly change to the inverse stretch reflex
    1. in a patient with an UMN lesion and concomitant spastic paralysis, passive flexion of the arm is resisted by the triceps stretch reflex,  the reflex, all of a sudden being replaced by an inverse stretch response when the tendon is stretched causing the resistance to flexion to suddenly collapse, much like closing a swiss army knife



​Historic Note 


19th century  historic note; Grand Paralysis of the Insane in the end stages of syphillis.  Dorsal columns attacked (Tabes dorsalis)


Don't revise


More Complex Reflexes

there are many far more complicated reflexes integrated on a spinal level

Name the more complicated reflexes

Flexor Reflex - where stimulation of a flexor muscle inhibits the antagonistic extensor muscle on the same limb

Crossed Extensor Reflex - In the opposing limb, the muscles reflexively act the opposite way, with extensors contracting and flexors being inhibited

Positive Supporting Reflex - where the leg extends to push down on a finger touching the sole of the foot.  The foot can actually “follow” the finger, even in the absence of supraspinal influences. (Supraspinal reflexes normally override it in conscious humans) Loss of this reflex in UMN lesions

Cord Righting Reflex- pattern generators within the spinal cord setting alternate contractions of flexors and extensors in a precisely timed and rhythmic fashion.  This can occur in the absence of supraspinal influence.  It seems that the brain simply “switches on” the oscillations in the spinal pattern generator.  The presence of rhythmically oscillating spinal circuits was first proposed to the Physiological Society 1912 by T Graham Brown, then working for the legendary CS Sherrington





What does it involve?

  • proprioception
  • sensation from the soles of the feet
  • visual inputs
  • vestibular function

all of which input results in the neck muscles keeping the head up, the trunk muscles keeping the centre of gravity above the base and the leg muscles pressing against the floor so that we don’t fall over


Summing Up (for general perusal)

  • A group of simple spinal reflexes maintain muscle length and force automatically
  • These reflexes are tonically active, and may be assessed clinically.  Theyre often accentuated in Upper Motor Neuron lesions
  • There are also more complicated spinal reflexes that carry out surprisingly complicated tasks (eg. walking!

Decks in Physiology Class (46):