Proprioception

Question 1

what are proprioceptors?

Answer 1

Somatosensory receptors in muscles and joints.

Question 2

What are the two type of muscle spindle afferent proprioceptors? And what do they respond too?

Answer 2

Type 1a - velocity dependant and type II - stretch dependant

Question 3

Type Ia and type II are enclosed in what?

Answer 3

The fusiform (they are intrafusal fibres)

Question 4

Give a general structure of intrafusal fibres.

Answer 4

Central stretchy, not contractile portion, with contractile poles.

Question 5

What happens to intrafusal fibres when the muscle is passively stretched?

Answer 5

Afferents from type 1a and type II send signals proportional to stretch to alpha motor neurones on synergist muscles. This triggers a reflex contraction - returns muscles to original length. stretch reflex contraction.

Question 6

Function of this stretch reflex contraction?

Answer 6

ensures legs contract enough to support your weight. ‘simple reflex arc’ which helps maintain muscles at a set length.

Question 7

What mechanisms prevent this stretch reflex contraction from occurring with every movement? (3)

Answer 7

Gamma motor neurones, reciprocal inhibition and pre synaptic inhibition.

Question 8

Outline how gamma motor neurones work?

Answer 8

During intentional movement, both gamma and alpha motor neurones fire at the same time. Alpha motor neurones act on extrafusal muscle, gamma act on intrafusal fibres, contracting the length of the poles, leaving the central elastic portion (with the neurones) unchanged therefore no afferents fire, meaning there is no stretch reflex contraction.

Question 9

How can gamma motor neurones fine tune motor control?

Answer 9

By increasing pole tension, they can increase sensitivity of contraction.

Question 10

What inputs on spindle afferents?

Answer 10

BOTH local and descending pathways. from upper motor neurones

Question 11

What is the role of inhibitory fibres and how do they achieve this (type 1a afferents)?

Answer 11

To prevent antagonist muscle contraction by surpassing the 1a afferents to alpha motor neurones on the antagonist.

Question 12

How do reciprocal inhibitory neurones work?

Answer 12

use NT glycine to act on fast ligand gates channels to hyper polarise the Alpha motor neurone to make it less responsive to input (non selective, rapid and transient)

Question 13

How do pre synaptic inhibitory interneurones work?

Answer 13

Local application of GABAb agonists on the same metabotropic pre synaptic receptors - reduce transmission release. This stops type 1a mediated hyper reflex.
Selective and longer lasting.

Question 14

How are the inhibitory neurones activated?

Answer 14

By upper motor neurones

Question 15

When are inhibitory neurones activated?

Answer 15

When free movement is needed. and by local circuits during cyclical or rhythmic movement.

Question 16

How do these stretch receptors (intrafusal fibres) correct voluntary movement if it is not following the intended trajectory?

Answer 16

If intrafusal contraction is faster than extrafusal contraction - if a load is being applied, then the central portion of the intrafusal fibres will fire afferents, as the poles of the intrafusal will shorted more than the extrafusal- stretching the central portion. This boosts alpha motor neurone activity- recruiting more muscles and the extrafusal fibres will catch up.

Question 17

how can the sensitivity of the stretch reflex be modulated?

Answer 17

By changing background activity levels of gamma motor neurones.

Question 18

What is resistance to low velocity stretch known as?

Answer 18

Hypertonia

Question 19

What are the mechanical properties of Type 1a nuclear bag intrafusal muscle.

Answer 19

Two poles which are flexible in response to low velocity stretch, therefore stretch with passive stretch, Rapid extension stiffens the poles meaning no extension, and the central ‘bag’ of nuclei have to stretch instead- firing type 1a afferents

Question 20

How is the monosynaptic stretch reflex (type 1a afferent reflex) tested?

Answer 20

Tendon tap. - loss of descending inhibition and followed by upper motor neurone lesion- this causes hyper reflexia. With monosynaptic reflexes this causes spasticity.

Question 21

How are Type II nuclear chain intrafusal fibres arranges and what is their role?

Answer 21

Arranged in neat rows of nuclei through the central portion, they are activated in proportion to the amount of stretch of the intrafusal muscle fibre.

Question 22

How is the disynaptic stretch reflex (type II afferent reflex) tested?

Answer 22

Flexing and extending a joint- loss of descending inhibition and upper Motor neurone causes hyper refelxia, in disynaptic neurones this causes hypertonia.

Question 23

What are Golgi tendon organs?

Answer 23

sensitive nerve endings of type 1 afferents which bind between collagen bundles in muscle

Question 24

What is the role of Golgi tendon organs?

Answer 24

monitor tension generated by muscle contraction and activate afferents- both positive and negative.

Question 25

What neurones do type 1b Golgi tendon afferents excite?

Answer 25

non reciprocal inhibitory and excitatory interneurons.

Question 26

Outline type 1b afferents excitation of non reciprocal inhibitory interneurons

Answer 26

Suppress activity of their own muscles alpha motor neurones. This is present in antigravity muscles - the fusiform spindles ensure the muscle contracts enough to maintain posture, whilst the Golgi tendon ensures ther is not too much muscle tension.

Question 27

Outline type 1b afferents excitation of excitatory interneurones.

Answer 27

1b afferents from leg extensors excite excitatory neurones- this increases alpha motor neurone activity mainly of the anti gravity muscles. this is an active reflex and works under dynamic condition such as walking