Muscle stretch reflexes and proprioception

Question 1

Types of muscle receptors

Answer 1

Muscle spindles and Golgi tendon organs

Question 2

Muscle spindles signal

Answer 2

Stretch

Question 3

Golgi tendon organs signal

Answer 3

Tension produced by muscle contraction

Question 4

Joint mechanoreceptors consist

Answer 4

Larger fibres signal joint position

Smaller Aδ fibres are most active at the extremes of movement and are protective.

Question 5

Muscle spindles are situated

Answer 5

In the fleshy part of muscles

Question 6

Golgi tendon organs are situated

Answer 6

In tendons at the ends of a muscle

Question 7

Are muscle spindles encapsulated?

Answer 7

Yes

Question 8

How do muscle spindles lie with extrafusal fibres?

Answer 8

Specialised intrafusal fibre in parallel with extrafusal fibres

Question 9

Muscle spindle structure and innervation

Answer 9

Specialised intrafusal muscle fibers the central part of which is non-contractile and contains the nuclei.

Large diameter myelinated sensory nerve fibres wrapped around the noncontractile part of the muscle the ends of which are sensitive to stretch of the intrafusal fibre

Small diameter gamma motor nerve fibres that innervate the contractile ends of the intrafusal fibres

Question 10

Two types structural types of intrafusal fibres

Answer 10

Nuclear chain fibres

Nuclear bag fibres

Question 11

Nuclear chain fibres abundance and structure

Answer 11

Nuclear chain fibres (variable numbers per spindle), named because their nuclei are aligned in a single row
(chain) in the centre of the fibre

Question 12

Nuclear bag fibres abundance and structure

Answer 12

Nuclear bag fibres (2-3 per spindle) - named because their nuclei are collected in a bundle in the middle of the fibre.

Question 13

Two functional types of intrafusal fibers and their function

Answer 13

Static: Non-adapting (or very slowly adapting); these measure the length of the spindle at any instant.

Dynamic: Rapidly (though not completely adapting); these measure the rate of change of length

Question 14

Nuclear bag fibre function

Answer 14

Nuclear bag fibres can be static

Only nuclear bag fibres are dynamic, the contractile ends of the dynamic bag fibres are more viscous so stretching occurs in the central nuclear part.

Question 15

Nuclear chain fibre function

Answer 15

All nuclear chain fibres are static

Question 16

Main sensory nerves sending afferents from stretch receptors in the muscle

Answer 16

Myelinated A-alpha and A-beta fibres.

Question 17

A-alpha/type I afferent (fastest fibre) records

Answer 17

From the centre of all fibres.

These afferents report dynamic, rate of change, as well as static information

Question 18

Gamma (γ) motor nerves supply

Answer 18

The dynamic and the static intrafusal fibres to modulate the activity of these fibres

Question 19

A-beta/type II afferent records

Answer 19

Only from static bag and chain fibres

Therefore primarily reports spindle/ muscle length.

Question 20

Type Ia and Type II afferents in a state of rest

Answer 20

Both fibres fire at regular intervals

Question 21

Activity in type Ia and II fibres when muscle stretches

Answer 21

When antagonistic muscle contracts (thus analysed muscle is stretched), a large volley of action potentials is sent through the Ia fibres.

There is also a slight increase in static II fibres as the static fibres reach a new level of stretch

Question 22

Activity in type Ia and II when stretch is maintained

Answer 22

When the muscle remains in state of stretch, soon the Ia fibres adapt and firing reduces to the same as the static fibres.

Question 23

Basis of the stretch reflex

Answer 23

The Ia afferent makes excitatory monosynaptic connections with the alpha motor neurons to the muscle.

Question 24

Knee jerk reflex spinal level

Answer 24

Quadriceps (knee jerk) L2-4 patella tendon

Question 25

Function of the stretch reflex

Answer 25

Resists stretching of a muscle and maintains its length. This is important for posture (standing upright), holding a heavy object still in one hand etc. When deviation from the intended position is detected, the muscle contracts to correct this

Question 26

How is the stability of he stretch reflex improved ?

Answer 26

The stretch reflex recruits and inhibits other motor neurons to improve the stability of the motor response. The stretch reflex doesn’t only act on the same muscle but across several motor pools to ensure a locally co-ordinated motor response In addition to stimulating the homonymous muscle, the Ia afferent stimulates the motor neurons supplying the synergist muscles. Inhibits (via Ia inhibitory interneuron) the motor neurons supplying the antagonist muscles. This is called “reciprocal inhibition”.

Question 27

The myotactic (stretch) reflex is a purely monosynaptic reflex

Answer 27

False (other muscles activated and inhibited)

Question 28

How do descending and recurrent pathways affect the stretch reflex?

Answer 28

Corticospinal tract: signalling voluntary movement, therefore inhibits the stretch reflexes Vestibulospinal tract: primarily engages the extensor anti-gravity muscles Reticulospinal tract: modulates the intensity of reflex via gamma motor neurons

Question 29

How is fatigue prevented in a recurrently stimulated muscle?

Answer 29

Recurrent processes engage the Renshaw inhibitory neurons which inhibit the firing muscle to prevent fatigue.

Question 30

Role of gamma motor nerves in modulation of stretch

Answer 30

When muscle is stretched (sustained), there is tension in the muscle and thus Ia firing from the spindles. Without gamma motor neurons when muscle contracts, unload the spindle, and this leads to absence of firing via the spindle When gamma motor neuron stimulated: innervate the contractile ends of the intrafusal fibre, contracts in parallel with the extrafusal fibres so continual reporting of contractile information (feedback from Ia)

Question 31

The stretch reflex short latency component

Answer 31

The monosynaptic connection predominates in the short latency component (M1) and is involved primarily with axial and proximal muscle control.

Question 32

The stretch reflex long latency component

Answer 32

The long latency (M2) is slower than monosynaptic but shorter than voluntary reaction time. It involves the cerebral cortex and is involved mostly in fine voluntary distal limb movements.

Question 33

Jaw jerk reflex

Answer 33

(CN V)

Question 34

Biceps reflex | Brachioradialis reflex

Answer 34

C5/C6 | C6

Question 35

Extensor digitorum reflex | Triceps reflex

Answer 35

C6/C7

Question 36

Ankle jerk reflex

Answer 36

S1/S2

Question 37

Golgi tendon structure

Answer 37

The body of the organ is made up of braided strands of collagen (intrafusal fasciculi) that are less compact than elsewhere in the tendon and are encapsulated. The capsule is connected in series with a group of muscle fibers at one end, and merge into the tendon proper at the other. Each capsule is about 1 mm long, has a diameter of about 0.1 mm,

Question 38

Golgi tendon organ innervation and activation

Answer 38

Perforated by one or more afferent type Ib sensory nerve fibers (Aɑ fiber). Stretching the tendon compresses and stimulates the nerve

Question 39

Ib afferent response when activated

Answer 39

The Ib afferent provides feedback inhibition onto the homonymous motorneurons via a spinal interneuron (i.e. the opposite to the muscle spindle stretch reflex) The muscle contraction stretches the GTO If excessive load is placed on the muscle, the GTO reflex causes relaxation - thereby protecting the muscle.

Question 40

In locomotion when are flexors and extensors recruited?

Answer 40

Extensors – recruited during stance phase, ‘anti-gravity’ function Flexors – recruited during swing phase, voluntary movement

Question 41

Proprioceptive ascending pathways terminate in

Answer 41

Area 3a of S1.