Sensory Receptors 2

Question 1

What are proprioceptors?

Answer 1

Mechanoreceptors that signal body or limb position.

Question 2

Name three components of proprioceptors.

Answer 2

Muscle spindles.
Golgi tendon organs.
Joint receptors.

Question 3

What do muscle spindles monitor and control?

Answer 3

Monitor muscle length and rate of change of muscle length.

Control reflexes and voluntary movement.

Question 4

What do golgi tendon organs do?

Answer 4

Monitor tension on tendons (tension is produced by muscle contraction, so monitors muscle tension also).

Question 5

What do joint receptors monitor?

Answer 5

Joint angle, rate of angular movement and tension on the joint.

Question 6

Name the three things that proprioceptors are responsible for doing.

Answer 6

Sending sensory information to the brain to control voluntary movement.
Muscle spindles and golgi tendon provide sensory information for spinal cord reflexes.
They provide sensory information to perceive limb and body position and movement in space (kinaesthesia).

Question 7

What are most contractile skeletal muscle fibres?

Answer 7

Extrafusal muscle fibres.

Question 8

What are intrafusal muscle fibres?

Answer 8

They have a specialised sensory and motor innervation contained in a capsule (forming a muscle fibre).

Question 9

How do muscle spindles lie in relation to muscle fibres?

Answer 9

Muscle spindles lie parallel with muscle fibres.

Question 10

Describe the structure of the golgi tendon.

Answer 10

A capsule with sensory neurones wrapped round collagen fibres, attached to extrafusal muscle fibres.

Question 11

Describe the structure of a muscle spindle.

Answer 11

Intrafusal fibre with afferent nerves forming annulospiral endings around the centre, and different afferent nerves forming flower-spray endings at either side.

Question 12

What are the two kinds of intrafusal fibre?

Answer 12

Nuclear bag fibres and nuclear chain fibres.

Question 13

What are nuclear bag fibres?

Answer 13

Bag shaped and nuclei collected together.

Question 14

What are nuclear chain fibres?

Answer 14

Nuclei lined up in a chain.

Question 15

Primary endings from Ia afferent nerves spiral round where on the intrafusal fibres to form what kind of ending?

Answer 15

Centre to form annulospiral endings.

Question 16

Secondary endings from type II afferents form what kind of ending?

Answer 16

Flower-spray endings.

Question 17

Where on the intrafusal fibres can contractile sarcomeres be found?

Answer 17

Ends contain contractile sacromeres, central area has no contractile material.

Question 18

What kind of motorneurone innervates ends of intrafusal fibres (causing contraction)?

Answer 18

Gamma motorneurones.

Question 19

What kind of motorneurone innervates extrafusal muscle fibres (causing contraction)?

Answer 19

Alpha motorneurones.

Question 20

How do gamma motorneurones compare in size with alpha motorneurones.

Answer 20

Gamma are smaller in diameter.

Question 21

What does muscle stretch stimulate?

Answer 21

The spindle stretch receptors.

Question 22

Explain how APs are generated in spindle stretch receptors.

Answer 22

Stretch sensitive ion channels open, creating local generator potential, this causes regenerative action potentials in afferent fibres.

Question 23

What does the resting AP frequency in muscle spindle depend on?

Answer 23

The length at rest, L0.

Question 24

During a muscle stretch (L0 to L1) what is the increase in AP frequency proportional to?

Answer 24

Velocity of stretch.

Question 25

If the muscle is stretched to a greater length, what will happen to the frequency of action potentials fired?

Answer 25

It will increase.

Question 26

How is joint movement organised?

Answer 26

By groups of muscles working in opposition i.e. agonist and antagonists. When the agonist contracts, antagonist relaxes and joint moves.

Question 27

Information from what two places informs the brain about joint position?

Answer 27

Spindles and joint receptors.

Question 28

How does stretching and contracting a muscle spindle affect AP firing frequency?

Answer 28

Stretching - increases AP frequency.

Contraction - decreases AP frequency.

Question 29

How do muscles stretch tendons?

Answer 29

Tendons are relatively elastic, so passive stretch doesn’t really affect them. Muscle must develop tension by contracting to stretch tendons.

Question 30

How do golgi tendon organs lie relative to muscle fibres?

Answer 30

They are in series with muscle fibres.

Question 31

What kind of afferent neurone goes from the golgi tendon organ to the CNS?

Answer 31

Ib neurone.

Question 32

What effect do isometric muscle contractions have?

Answer 32

They increase tension in the GTOs so Ib sensory axons fire, but activated muscles stay same length so Ia afferents don’t fire.

Question 33

Why is gamma motor innervation of muscle spindle fibres important?

Answer 33

If it wasn’t present when muscle contracts, spindle would be floppy and spindle discharges could stop.
Brain would not be informed about muscle length - could prevent use of that muscle.

Question 34

How does the gamma motorneurone stop the muscle spindle going floppy?

Answer 34

It activates contraction at poles of muscle spindle, so it shortens to match length of shortening muscle - keeping spindle active and transmitting to the brain.

Question 35

What would happen if an alpha motor neurone fires without a gamma motor neurone firing?

Answer 35

Muscle contracts/shortens, but spindle stays same length, so sensory 1a firing decreases.

Question 36

What is the term used to describe the action of gamma motor neurones causing contraction at ends of muscle spindle to match length of contracting muscle to maintain spindle sensitivity to stretch?

Answer 36

alpha-gamma coactivation (this is the norm for voluntary movements).

So gamma motor neurones are activated in parallel to maintain spindle fibre sensitivity.

Question 37

What is sensory neuronopathy?

Answer 37

A rare condition resulting in loss of all proprioception. Causes patients to lose the sense of their bodies in space and motor control. Can result from loss of large myelinated fibres due to an autoimmune attack on sensory neurones in the dorsal root ganglia.