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Sensory Receptors Flashcards

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
Nociceptors and adaption
these are free nerve endings detecting painful stimuli - do not adapt. it is important not to ignore painful stimuli.
26
The Pacinian corpuscle is the
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)
27
How does the Pacinian corpuscle respond
- 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
28
the Pacinian corpuscle shows rapid
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
29
If lamellae are removed
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
30
Sensory receptors possess
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
31
our ability to tell 2 points apart on the skin is measured by the
two point discrimination test This ability depends on two things 1) receptive field size 2) neuronal convergence
32
sensory neurons with neighbouring receptive fields may exhibit
neuronal convergence = multiple presynaptic neurons input on a smaller number of post-synaptic neurons (3 onto 1).
33
Convergence of primary sensory neurons allows
simultaneous sub-threshold stimuli to sum at the secondary neuron, forming a large secondary receptive field (dotted patch of skin) and initiating APs.
34
So lots of convergence and a large secondary receptive field indicate
a relatively insensitive area.
35
2-point discrimination test -
distance between points adjusted until you just perceive 2 points rather than one. lips are more sensitive than back and limbs
36
High acuity is when
two signals go to brain
37
Low acuity is when
one signal goes to brain. | can b e caused by high levels of convergence
38
acuity =
The ability to locate a stimulus on the skin and differentiate it from another close by
39
How do we Locate a stimulus so precisely?
Lateral Inhibition is important. | in tertiary neurons, inhibition of lateral neurons enhances preception of stimulus.
40
when stimulus is fired say through pin on skin, primary neuron response is
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
Lateral Inhibition
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
Lateral inhibition is widespread in the
spinal cord and important in pathways with high precision information eg touch and skin hair movement.
43
Proprioceptors include:
- Muscle spindles - Golgi tendon organs - Joint receptors
44
what do joint receptors do?
monitor joint angle, rate of angular movement and tension on the joint.
45
what do Golgi tendon organs do
monitor tension on tendons | - tension is produced by muscle contraction, so monitoring muscle tension.
46
what do muscle spindles do
monitor muscle length and rate of change of muscle length - they control reflexes and voluntary movements.
47
Proprioceptors do three things
- 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
kinaesthesia =
perception of limb and body position and movement in space
49
Most contractile skeletal muscle fibres are
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
central region of muscle spindles lacks
myofibrils
51
golgi tendon organ is surrounded by
capsule.
52
golgi tendon organ contains
- collagen fibres - afferent neurons - sensory neurons
53
muscle spindles contain
- sensory neurons to CNS - Gamma motor neurons from CNS - muscle spindles - extrafusal fibre (outside and thick) - intrafusal fibre (inside and thinner)
54
γ motoneurones are smaller in diameter than
the alpha (α) motoneurones that innervate the extrafusal muscle fibres
55
Gamma (γ) motoneurones innervate and
cause contraction of the contractile ends of the intrafusal fibres
56
There are two kinds of intrafusal fibre
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
Primary endings from Ia afferent nerves wrap around
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
Secondary endings from type II afferents form
flower-spray endings
59
The ends of intrafusal fibres contain
contractile sarcomeres - BUT the central area has no contractile elements.
60
Muscle stretch stimulates the
spindle stretch receptors Stretch sensitive ion channels open, creating local generator potential, this causes regenerative action potentials (APs) in the afferent fibres.
61
Resting AP frequency depends on
the length Lo
62
During stretch from L0 to L1,
increase of AP frequency is proportional to velocity of stretch (the slope of the line)
63
increase of AP frequency at new steady state
causes L1 > L0
64
spindle stretch | So the difference between 1 and 3 informs about
muscle length
65
spindle stretch : | AP frequency at 2 informs about
rate of change of length
66
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
67
agonist muscle is stretched then
contraction occurs. opposite changes in length happen in the antagonist - stretching the agonist increases and - shortening the agonist (contracting it) reduces spindle discharge
68
So spindle and joint receptor information together inform the brain about
joint position.
69
Golgi tendon organ (GTO) monitors
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
golgi tendon organ: | muscle contraction increases
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
The organization of muscle proprioceptors
- Muscle spindles lie in parallel | - GTO’s in series with extrafusal muscle fibres.
72
what is the relevance of the gamma motor innervation of the muscle spindles?
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
α motor neurone fires
extrafusal muscles contract/shorten
74
No γ motor activity
so spindle becomes slack and goes “off air” – it is no longer reporting muscle length.
75
γ motor neuron activation
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
If α motor neuron fires without γ
1a spindle sensory firing would decrease when muscle shortens
77
But when both α AND γ motor neurones fire together
both the muscle and the muscle spindle shorten together | there is no drop off in 1a firing during contraction
78
alpha-gamma coactivation is the norm for
voluntary movements. alpha motoneurones are activated causing contraction gamma motoneurones are activated in parallel to maintain spindle sensitivity.
79
Muscle, tendon and joint proprioceptors
inform the brain on movements and position of our body in space. Act automatically to control movements via spinal cord reflexes.

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