sensory receptors mw %% (+

Question 1

Sensory receptors

Answer 1

• Are nerve endings, often with specialized non-neural structures
• They are transducers that convert different forms of energy into frequency of Action Potentials (APs).

Question 2

Terminology

Answer 2

• Sensory modality – a type of stimulus activating a particular receptor: eg. touch.
• An adequate stimulus is the type of energy a receptor normally responds to
• Sensory receptors are highly sensitive to one specific energy form but activated by other intense stimuli (poke in the eye - “see stars”)

Question 3

Types of sensory receptors

Answer 3

• Mechanoreceptors: stimulated by mechanical stimuli - pressure, stretch, or deformation. Detect many stimuli
• Proprioceptors: are mechanoreceptors in joints and muscles that signal information related to body or limb position
• Nociceptors: respond to painful stimuli - tissue damage and heat

Question 4

Cutaneous Mechanoreceptors and Proprioceptors

Answer 4

• Are good examples of the principles of peripheral sensory processing
• Transduction in ALL sensory receptors involves opening or closing of ion channels
• An adequate stimulus causes a graded membrane potential change called a a generator potential
• The adequate stimulus in cutaneous mechanoreceptors and proprioceptors is - membrane deformation.
• This activates stretch-sensitive ion channels causing ion flow across the membrane.

Question 5

The receptor potential is graded to stimulus intensity

Answer 5

• A stimulus – causes local current to flow to the part of the membrane with voltage-gated ion channels.
• This generates action potentials (APs).
• The larger the stimulus, the larger the receptor potential and the HIGHER THE FREQUENCY of APs in a sensory nerve.
• This is frequency coding of stimulus intensity

Question 6

Mechanoreceptors and APs

Answer 6

• For some mechanoreceptors: if the stimulus persists – APs persist
• Often we are interested in novel and changing events
• Aware of putting on our clothes, after that their mechanical stimulation is not important - until we take them off!

Question 7

ADAPTATION

Answer 7

• Some mechanoreceptors ADAPT to a maintained stimulus and only signal change – eg. the onset of stimulation.
• It responds only to a change or a novel event.

Question 8

Different receptors and extents of adaptation

Answer 8

• Rapidly/Moderately-adapting receptors include Pacinian corpuscles
• Slowly-adapting receptors.
• Nociceptors which are free nerve endings detecting painful stimuli - do not adapt.

Question 9

Features of the Pacinian corpuscle

Answer 9

• A myelinated nerve with a naked nerve ending
• enclosed by a connective tissue capsule of layered membrane lamellae separated by fluid (like an onion)

Question 10

How does the Pacinian corpuscle respond?

Answer 10

1. A mechanical stimulus deforms the capsule and nerve ending
2. This stretches the nerve ending and opens ion channels
3. Na⁺ influx causes local depolarisation – a generator potential
4. APs are generated and fire at the myelinated nerve

Question 11

The Pacinian corpuscle and adaptation

Answer 11

• Shows rapid adaptation
• Fluid redistribution in the capsule – rapidly dissipates stimulus - removes mechanical stretch of nerve ending – APs stop firing.
• Withdrawal of the stimulus - capsule springs back - AP fire again
• Detects ON and OFF phases of a mechanical stimulus

Question 12

Importance of lamellae

Answer 12

Capsule intact:

• Normal, rapidly adapting ON/OFF response

Capsule removed:

• Bare nerve ending loses much of adaptation
• Continues to produce a receptor/generator potential

The non-neural accessory structure is critical to how this sensory receptor works – in general they enhance sensory function

Question 13

Sensory receptors and receptive fields

Answer 13

• Possess receptive fields: the particular region of the sensory space (e.g., the body surface, or the visual field) in which a stimulus will modify the firing of that neuron.​

Question 14

Our ability to tell 2 points on the skin apart

Answer 14

Depends on 2 things:

1) receptive field size
2) neuronal convergence

• it is determined by a 2 point discrimination test.
• So convergence and a large secondary receptive field indicates a relatively insensitive area.

Question 15

What is acuity?

Answer 15

The ability to locate a stimulus on the skin and differentiate it from another closeby.

Question 16

How can it be so precise?

Answer 16

Lateral Inhibition is important

• Information from neurons with sensory receptors at the edge of a stimulus is strongly inhibited, compared with information from the centre of the stimulus.
• The contrast between relevant and irrelevant information is enhanced
• Lateral inhibition occurs in the spinal cord for cutaneous information.

Question 17

Componenets of proprioceptors

Answer 17

• Muscle spindles – monitor muscle length and rate of change of muscle length and so they control reflexes and voluntary movements.
• Golgi tendon organs - monitor tension on tendons
• Tension is produced by muscle contraction, so monitoring muscle tension.
• Joint receptors - monitor joint angle, rate of angular movement and tension on the joint.

Question 18

Functions of proprioceptors

Answer 18

1. They send sensory information to allow the brain to control voluntary movement.
2. The muscle spindles and Golgi tendon organs provide the sensory information that drives spinal cord reflexes.
3. They provide sensory information to perceive limb and body position and movement in space = kinaesthesia.

Question 19

Contractile skeletal muscle fibres

Answer 19

• Most are extrafusal muscle fibres
• However, a few specialized intrafusal fibres with specialized sensory and motor innervation are contained within a capsule
• Form what is called a muscle spindle

Question 20

Structure of muscle spindle (pic)

Answer 20

Question 21

Structrure of Golgi tendon

Answer 21

• The top one is muscle spindle structure
• The bottom one is Golgi tendon

Question 22

Intrafusal fibre types

Answer 22

1. Nuclear bag fibres - bag shaped and nuclei collected together
2. Nuclear chain fibres - nuclei lined up in a chain.

Question 23

Functions of the components in muscle fibres

Answer 23

• Primary endings from Ia afferent nerves wrap around the centre of intrafusal fibres : form annulospiral endings
• Secondary endings from type II afferents form flower-spray endings.
• The ends contain contractile sarcomeres
• The central area has no contractile material.

Question 24

What causes the contractile ends to strech?

Answer 24

• Gamma (γ) motor neurones innervate the contractile ends of intrafusal fibres
• So when they fire, the ends shorten, but the central area does not , therefore it gets stretched

Question 25

How does spindle information contribute to perception of body position and movement?

Answer 25

* Joint movement is organized by groups of muscles working in opposition ie. agonists and antagonists (eg. biceps and triceps) * When agonist contracts, antagonist relaxes and the joint moves.

Question 26

Golgi tendon organ (GTO)

Answer 26

* Nerve endings of GTO mingle with the tendon bundles at ends of muscles. * They are **stretch receptors and monitor stretch** of tendon. * Tendons are fairly **inelastic**, so passive stretch does not affect them much (unlike muscle spindles). Muscles have to develop tension by contracting to stretch the tendons.

Question 27

The organization of muscle proprioceptors

Answer 27

A)Muscle spindles lie in **parallel** and **GTO’s in series** with the extrafusal muscle fibres B)Isometric muscle contractions **increase tension** in GTOs and Ib sensory axons fire. But activated muscles stay the **same length** (isometric), so 1a afferents do not fire.

Question 28

What is the relevance of the gamma motor innervation of the muscle spindles?

Answer 28

* If it was not present, then when muscle contracts, muscle spindle would be **floppy** and spindle discharges could **stop** * The brain would not be informed about muscle length * A lack of information flow to the brain about muscle length could prevent use of that muscle

Question 29

The solution

Answer 29

So simultaneously: * **α-motor** neurones fire causing **extrafusal** fibre contraction * **γ-motor** neurones fire causing **intrafusal** fibre **_ends_** to contract and so this **stretches** the central sensory elements * This **restores tension** and resets the sensitivity of the central sensory part of intrafusal fibres, to a new muscle length