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Science for Medicine > Sensory Receptors > Flashcards

Sensory Receptors Flashcards

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1
Q

Sensory receptors are

A

nerve endings - often with specialized non-neural structures.

Transducers – converting different forms of energy into frequency of Action Potentials (APs)

they inform the CNS about the internal and external environment

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2
Q

sensory modality

A

stimulus type activating a particular receptor: eg. touch, pressure, pain, temperature, light

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3
Q

an adequate stimulus

A

type of energy to which a receptor normally responds

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4
Q

sensory receptors

A

highly sensitive to one specific energy form but are activated by other intense stimuli - poke in the eye, “see stars”

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5
Q

Types of sensory receptors:

A
  • Mechanoreceptors
  • Proprioceptors
  • Nociceptors
  • Thermoreceptors = Detect cold and warmth
  • Chemoreceptors = detect chemical changes eg pH
  • Photoreceptors = respond to particular wavelengths of light
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6
Q

Mechanoreceptors

A

stimulated by mechanical stimuli
- pressure, stretch, deformation.

Detect many stimuli
- hearing, balance, blood pressure - also skin sensations of touch and pressure

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7
Q

Proprioceptors

A

are mechanoreceptors in joints and muscles.

They signal information about body or limb position

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8
Q

Nociceptors

A

respond to painful stimuli, tissue damage and heat

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9
Q

Cutaneous Mechanoreceptors and Proprioceptors are good examples of

A
  • principles of peripheral sensory processing

- sensory receptor transduction involves ion channels opening or closing

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10
Q

an adequate stimulus causes a

A

graded membrane potential change,

- a receptor potential or generator potential (a few mV)

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11
Q

membrane deformation is

A

the adequate stimulus in cutaneous mechanoreceptors and proprioceptors

this activates stretch-sensitive ion channels – so ions flow across the membrane and change the membrane potential locally.

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12
Q

A stimulus triggers

A

ions to flow through the membrane locally.

When depolarisation reaches the area with voltage-gated ion channels (first node) - action potentials start firing

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13
Q

electrode at position 1 and 2 measure

A 
change in membrane potential
electrode 1 (receptor membrane) measures Receptor potentia
electrode 2 (node of ranvier) Action potentials
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14
Q

in sensory nerve a larger stimulus causes

A
  • larger receptor potential
  • higher frequency of action potentials

this is called frequency coding of stimulus intensity

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15
Q

Skin is packed with different receptors for touch

- merkel receptors

A

sense steady pressure and texture

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16
Q

Skin is packed with different receptors for touch

- meissner’s corpuscle

A

responds to flutter and stroking movements

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17
Q

Skin is packed with different receptors for touch

- pacinian corpuscle

A

senses vibration

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18
Q

Skin is packed with different receptors for touch

- Ruffini corpuscle

A

responds to skin stretch

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19
Q

Skin is packed with different receptors for touch

- sensory nerves

A

carry signals to spinal cord

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20
Q

Skin is packed with different receptors for touch

- free nerve endings of nocieptor

A

responds to noxious stimuli

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21
Q

for some mechanoreceptors: if the stimulus persists

A

APs persist

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22
Q

ADAPTATION is when

A

some mechanoreceptors ADAPT to a maintained stimulus and only signal change – eg. the onset of stimulation

eg- We are aware of putting on our clothes, after that - continuous mechanical stimulation is not important - until we take them off!

different receptors show different extents of adaptation

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23
Q

Rapidly/Moderately-adapting receptors include

A
  • Pacinian corpuscles

- Meissner’s corpuscles

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24
Q

Slowly-adapting receptors include

A
  • Merkel’s discs

- Ruffini endings

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25
Q

Nociceptors and adaption

A

these are free nerve endings detecting painful stimuli - do not adapt.

it is important not to ignore painful stimuli.

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26
Q

The Pacinian corpuscle is the

A

best understood mechanoreceptor

  • Comprises a myelinated nerve with a naked nerve ending
  • enclosed by a connective tissue capsule of layered membrane lamellae
  • each layer separated by fluid (a bit like a spongy onion)
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27
Q

How does the Pacinian corpuscle respond

A
  • A mechanical stimulus deforms the capsule and nerve ending
  • This stretches the nerve ending and opens ion channels
  • *Na+ influx causes local depolarisation - a generator/receptor potential *
  • APs are generated and fire where myelination begins
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28
Q

the Pacinian corpuscle shows rapid

A

adaptation.
First: Mechanical stimulus deforms capsule - nerve ending is stretched - ion channels open - local depolarisation causes generator potential - APs fire - brain detects stimulus ON.

Next: rapid fluid redistribution in capsule dissipates stimulus laterally
- vertical force causes mechanical stretch of nerve ending stops and so APs stop firing.

As stimulus is withdrawn - capsule springs back - AP fire again

Detects ON and OFF phases of mechanical stimulus

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29
Q

If lamellae are removed

A

much of the adaptation is lost.
The function of this sensory receptor depends on the non-neural accessory structure - the capsule - it enhances sensory function

Capsule intact
- Normal, rapidly adapting ON/OFF response

Capsule removed

  • bare nerve ending loses much of adaptation
  • So it continues to produce a receptor/generator potential
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30
Q

Sensory receptors possess

A

receptive fields.
a somatic sensory neuron is activated by stimuli in a specific area called the receptive field
so a touch-sensitive neuron in the skin responds to pressure within a defined receptive field

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31
Q

our ability to tell 2 points apart on the skin is measured by the

A

two point discrimination test

This ability depends on two things

1) receptive field size
2) neuronal convergence

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32
Q

sensory neurons with neighbouring receptive fields may exhibit

A

neuronal convergence = multiple presynaptic neurons input on a smaller number of post-synaptic neurons (3 onto 1).

33
Q

Convergence of primary sensory neurons allows

A

simultaneous sub-threshold stimuli to sum at the secondary neuron, forming a large secondary receptive field (dotted patch of skin) and initiating APs.

34
Q

So lots of convergence and a large secondary receptive field indicate

A

a relatively insensitive area.

35
Q

2-point discrimination test -

A

distance between points adjusted until you just perceive 2 points rather than one.

lips are more sensitive than back and limbs

36
Q

High acuity is when

A

two signals go to brain

37
Q

Low acuity is when

A

one signal goes to brain.

can b e caused by high levels of convergence

38
Q

acuity =

A

The ability to locate a stimulus on the skin and differentiate it from another close by

39
Q

How do we Locate a stimulus so precisely?

A

Lateral Inhibition is important.

in tertiary neurons, inhibition of lateral neurons enhances preception of stimulus.

40
Q

when stimulus is fired say through pin on skin, primary neuron response is

A

proportional to stimulus strength.
In secondary neurons, pathway closest to stimulus inhibits neighbours.
In tertiary neurons, inhibition of lateral neurons enhances preception of stimulus.

41
Q

Lateral Inhibition

A

receptors at edge of a stimulus are more strongly inhibited Lateral inhibition “sharpens or cleans up” sensory information

  • receptors at edge of a stimulus are more strongly inhibited than receptors near centre
  • enhances the contrast between relevant and irrelevant information
  • allows precise location to a single hair movement
42
Q

Lateral inhibition is widespread in the

A

spinal cord and important in pathways with high precision information eg touch and skin hair movement.

43
Q

Proprioceptors include:

A
  • Muscle spindles
  • Golgi tendon organs
  • Joint receptors
44
Q

what do joint receptors do?

A

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

45
Q

what do Golgi tendon organs do

A

monitor tension on tendons

- tension is produced by muscle contraction, so monitoring muscle tension.

46
Q

what do muscle spindles do

A

monitor muscle length and rate of change of muscle length - they control reflexes and voluntary movements.

47
Q

Proprioceptors do three things

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

(balance system in inner ear contributes too)

48
Q

kinaesthesia =

A

perception of limb and body position and movement in space

49
Q

Most contractile skeletal muscle fibres are

A

extrafusal muscle fibres

Few specialized intrafusal fibres have their own sensory and motor innervation and are contained within a capsule
- they form a muscle spindle

Muscle spindles lie in parallel with muscle fibres

50
Q

central region of muscle spindles lacks

A

myofibrils

51
Q

golgi tendon organ is surrounded by

A

capsule.

52
Q

golgi tendon organ contains

A
  • collagen fibres
  • afferent neurons
  • sensory neurons
53
Q

muscle spindles contain

A
  • sensory neurons to CNS
  • Gamma motor neurons from CNS
  • muscle spindles
  • extrafusal fibre (outside and thick)
  • intrafusal fibre (inside and thinner)
54
Q

γ motoneurones are smaller in diameter than

A

the alpha (α) motoneurones that innervate the extrafusal muscle fibres

55
Q

Gamma (γ) motoneurones innervate and

A

cause contraction of the contractile ends of the intrafusal fibres

56
Q

There are two kinds of intrafusal fibre

A

nuclear bag fibres
- bag shaped with nuclei collected together
nuclear chain fibres
- nuclei lined up in a chain.

remember that muscle fibres are multinucleate

57
Q

Primary endings from Ia afferent nerves wrap around

A

the centre of intrafusal fibres : they form annulospiral endings
so when they fire - the two ends contract and shorten - but the central area does not - it therefore gets stretched out

58
Q

Secondary endings from type II afferents form

A

flower-spray endings

59
Q

The ends of intrafusal fibres contain

A

contractile sarcomeres - BUT the central area has no contractile elements.

60
Q

Muscle stretch stimulates the

A

spindle stretch receptors

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

61
Q

Resting AP frequency depends on

A

the length Lo

62
Q

During stretch from L0 to L1,

A

increase of AP frequency is proportional to velocity of stretch (the slope of the line)

63
Q

increase of AP frequency at new steady state

A

causes L1 > L0

64
Q

spindle stretch

So the difference between 1 and 3 informs about

A

muscle length

65
Q

spindle stretch :

AP frequency at 2 informs about

A

rate of change of length

66
Q

Joint movement is organized by

A

groups of muscles working in opposition ie. agonists and antagonists (eg. biceps and triceps)
- when agonist contracts, antagonist relaxes and the joint moves

67
Q

agonist muscle is stretched then

A

contraction occurs.
opposite changes in length happen in the antagonist

  • stretching the agonist increases and
  • shortening the agonist (contracting it) reduces spindle discharge
68
Q

So spindle and joint receptor information together inform the brain about

A

joint position.

69
Q

Golgi tendon organ (GTO) monitors

A

muscle tension.
Nerve endings of GTO mingle with the tendon bundles at ends of muscles.
- They are stretch receptors and monitor stretch of tendon.

Tendons are inelastic, so passive stretch does not affect them much (unlike muscle spindles).
- Muscles have to develop tension by contracting to stretch the tendons

70
Q

golgi tendon organ:

muscle contraction increases

A

the tension in the tendons

  • this stretches the nerve endings of the GTO and
  • initiates APs in the group 1b afferent fibre from the GTO
  • GTOs lie in series with the muscle fibres
71
Q

The organization of muscle proprioceptors

A
  • Muscle spindles lie in parallel

- GTO’s in series with extrafusal muscle fibres.

72
Q

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

A

If they didn’t exist, when extrafusal muscles contract and shorten, the muscle spindles would stay the same length, it would become floppy and stop firing APs
- so the brain would not be informed about muscle
length
- this could limit use of that muscle

73
Q

α motor neurone fires

A

extrafusal muscles contract/shorten

74
Q

No γ motor activity

A

so spindle becomes slack and goes “off air” – it is no longer reporting muscle length.

75
Q

γ motor neuron activation

A

contracts the poles of the muscle spindle, so it shortens to match the shortening of the muscle.
This keeps the spindle active and “on air” - transmitting information to the brain

76
Q

If α motor neuron fires without γ

A

1a spindle sensory firing would decrease when muscle shortens

77
Q

But when both α AND γ motor neurones fire together

A

both the muscle and the muscle spindle shorten together

there is no drop off in 1a firing during contraction

78
Q

alpha-gamma coactivation is the norm for

A

voluntary movements.
alpha motoneurones are activated causing contraction
gamma motoneurones are activated in parallel to maintain spindle sensitivity.

79
Q

Muscle, tendon and joint proprioceptors

A

inform the brain on movements and position of our body in space.
Act automatically to control movements via spinal cord reflexes.

Science for Medicine (16 decks)

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