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NMS sem2 > W2: Somatosensory Systems > Flashcards

W2: Somatosensory Systems Flashcards

1
Q

What does a primary sensory afferent axon split into?

A

One part that innervates = the primary afferent. E.g., innervates a patch of skin.
The other side goes to the dorsal horn of the spinal cord where it’s cell body resides in the dorsal root ganglion.

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2
Q

What is the area called that the primary sensory afferent innervates?

A

The area is the receptive field for that individual neurone.

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3
Q

Why is it called a ‘primary’ afferent?

A

Because it is the first neurone in a signalling pathway which is taking information from the periphery to the brain.

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4
Q

What happens when a stimulus comes into a receptive field?

A

Neurone is activated and will send information into the CNS dorsal horn. This axon splits.
Information can either go upto the brain or go onto a second order neurone in the dorsal horn, which then sensory an axon upto the brain.

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5
Q

Where does sensory information coming up through the spinal cord reach in the brain?

A

Thalamus = part of the brain that receives all sensory information

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6
Q

Where are action potentials generated in primary sensory neurones and why is this unusual?

A

In primary sensory neurones, the APs are generated at the peripheral nerve ending. This is unusual because normally APs are generated at the axon hillock (cell body).

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7
Q

What is the most common neurotransmitter for primary sensory neurones?

A

All primary sensory neurones are excitatory. So neurotransmitter is commonly glutamate.

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8
Q

What is the relationship between axon diameter and propagation speed?

A

The bigger the axon the faster the propagation of an action potential

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9
Q

What is the relationship between myelin sheath and propagation speed?

A

The more myelin sheath it has, the faster the propagation of action potentials.

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10
Q

Describe Aalpha axons

A

Group I
Large in diameter with fastest speed of propagation
Sensory receptors found in proprioceptors of skeletal muscle

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11
Q

Describe Abeta axons

A

Group II
Intermediate diameter and myelin sheath and speed
Sensory receptors found as mechanoreceptors of skin

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12
Q

Describe Adelta fibres

A

Group III
Smaller than Abeta but still intermediate diameter, speed and myelin sheath
Sensory receptors for pain and temperature

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13
Q

Describe C axons

A

Group IV
Small diameter with no myelin, and a slow speed of propagation
Sensory receptors detect temperatures pain and itch

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14
Q

What does each nerve axon innervate?

A

A specific receptive field within its dermatome

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15
Q

Do receptive field sizes vary?

A

Receptive field sizes vary, determining precision of localisation. The smaller the receptive field, the more localised the stimulus has to be for that neurone to be activated.

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16
Q

What is 2 point discrimination?

A

It means that you are activating 2 individual neurones by applying 2 points onto the skin, and vary the distance between the two points until you are activating 2 separate neurones and applying the stimulus into 2 separate receptive fields.
Illustrates the degree of innervation of different parts of the body, e.g., back is not highly innervated by lots of neurones compared to the thumb etc.

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17
Q

What information goes to the somatosensory cortex?

A

All of the sensory information coming from neurones innervating.

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18
Q

What is the sensory homunculus?

A

The volume of brain given to each area of the body innervated, mapped on a diagram to size.

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19
Q

What is a dermatome?

A

An area of skin that is innervated by afferent axon fibres. Signalling all its sensation via a single nerve from a single spinal nerve root.

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20
Q

How does a stimulus get converted into an action potential?

A

Sensory receptor detects the stimulus. The neural membrane of the nerve ending has ion channels = transduction channels. If the stimulus is sufficient to open a channel, positive ions will cross the membrane to depolarise the cell, causing a graded receptor potential/membrane potential. The larger the stimulus the larger the graded potential produced, and when this reaches threshold an action potential is generated.

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21
Q

What does stimulus intensity determine?

A

Determines the rate of firing of the action potential, NOT SIZE. Firing frequency can vary.

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22
Q

Can threshold potential vary?

A

Yes depending on the nerve ending there are different stimuli and different thresholds to be reached.

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23
Q

What is a slow or non-adapting sensory receptor?

A

There is continuous firing of action potentials when a stimulus is present.
Important when maintaining information about a stimulus is valuable, e.g., amount of stretch or pain

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24
Q

What is a fast adapting sensory response?

A

Constantly changing stimulus is required. Useful when it is important to signal a change in stimulus, and stop paying attention to no longer important stimuli.
E.g., tactile receptors
Stimulus increases to a plateau. There is higher rate of firing at the change in stimulus and once it is maintained, AP firing decreases or stops due to adaptation of the receptor.

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25
Q

What are cutaneous sensory receptors?

A

Different types of receptors or nerve ending that innervate the skin.

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26
Q

Give examples of cutaneous sensory receptors

A

Mechanoreceptors, thermoreceptors, nociceptors

27
Q

What are the 4 major types of cutaneous mechanoreceptors?

A

Superficial - merkel’s receptor, meissner’s corpuscle

Deep - Ruffini’s corpuscle, pacinian corpuscle

28
Q

Where are the cutaneous mechanoreceptors found?

A

Tactile receptors at the end of an Abeta fibre

29
Q

Describe the density of Merkel’s receptors

A

High density in the epidermis of digits and around mouth
Lower density on glabrous skin
Very low density in hairy skin

30
Q

Are Merkel’s receptors slow or fast adapting?

A

Slow adapting

31
Q

What do Merkel’s receptors respond to and what do they perceive?

A

Sustained light touch
They respond to initial skin indentation and sustained pressure upto several seconds in duration
Perceive form and texture

32
Q

What is the apparatus of a Merkel’s receptor?

A

Specialised keratocyte

33
Q

Where are Meissner’s corpuscles found?

A

In the papillary dermis (just below papillary ridges)

34
Q

Are Meissner’s corpuscles slow or fast adapting?

A

Rapidly adapting
Constantly changing stimulus is required, e.g., to detect putting on clothes but not wearing them. E.g., involved in adjustment of grip force when objects are lifted.

35
Q

What do Meissner’s corpuscles respond to?

A

Light touch and vibration

36
Q

What do Ruffini’s (bulbous) corpuscles respond to?

A

Particularly lateral movement or stretching of skin.

Responds to deep touch and stretch.

37
Q

What is the apparatus of a Ruffini’s corpuscle?

A

Apparatus is a network of collagen fibres. Collagen fibres move across each other by stretch and cause the ion channels to open.

38
Q

What are Ruffini’s corpuscles involved in?

A

Involved in monitoring grasped object slippage and the reflex adjustment of grip force.

39
Q

Where are Pacinian corpuscles found?

A

In the deeper layers of the dermis

40
Q

Are Pacinian corpuscles slow or fast adapting?

A

Rapidly adapting. Needs a stronger stimulus e.g., deep touch or poke or high frequency vibration

41
Q

What is the apparatus of a Pacinian corpuscle?

A

Fully encapsulated nerve ending. ‘Onion’ structure deforms to take up distortion due to a mechanical stimulus (pressure).

42
Q

What does the hair follicle receptor respond to?

A

Responds to light touch, but activation in dermis

Detects hair deflection

43
Q

What is the apparatus of the hair follicle receptor?

A

Axon/nerve fibre is wrapped around the hair follicle

44
Q

Is the hair follicle receptor slow or fast adapting?

A

Rapidly adapting. A constantly changing stimulus is required.

45
Q

What is the apparatus of cutaneous thermoreceptors?

A

They are bare nerve endins

46
Q

Are cutaneous thermoreceptors slow or fast adapting?

A

Slow adapting sensory receptors (they will eventually adapt)

47
Q

What do cutaneous thermoreceptors detect?

A

Sensitive to changes in temperature, not an indication of absolute temperature. Compares signals from the warm and cold receptors. Ion channels open in response to temperature change.

48
Q

What is the family of thermoreceptor channels called?

A

Transient Receptor Potnetial family, TRP

49
Q

What are TRPM8 channels?

A

Cold channels open 10-38°C; maximum at 25°C (also opened by menthol)

50
Q

What are TRPV3/4 channels?

A

Warm channels. Open at 29-45°C with a maximum at 45°C.

51
Q

How is temperature perceived?

A

Individual nerve endings expressing one channel or another will be activated to a greater or lesser extent at a particular temperature. 2 neurones will be activated but the amount will determine how much of a signal it will send to the brain, which will decide what the temperature actually is.

52
Q

What fibres are cold receptors found on?

A

C and Abeta

53
Q

What fibres are warm receptors found on?

A

C fibres

54
Q

What are the 2 types of cutaneous nociceptors?

A 
High threshold mechanoreceptors (Abeta) - well localised pricking pain
Polymodal nociceptors (C) - sensitive to mechanical stimulus, damaging heat (above 46) and noxious chemicals. A poorly localised burning pain.
55
Q

Describe cutaneous nociceptors (apparatus, slow or fast, high or low threshold)?

A

Bare nerve endings
Non-adapting/slow sensory receptors
High threshold with an adequate stimulus capable of causing damage

56
Q

What does proprioception provide information about?

A

Joint position, muscle length, muscle movement, accelerate, tension/force

57
Q

Muscle spindle - what are extrafusal fibres?

A

Fibres that generate the tension or force of movement when the muscle contracts

58
Q

Muscle spindle - what are infrafusal fibres?

A

In parallel to extrafusal fibres. Within the fibrous capsule.

59
Q

Relationship between firing in muscle spindle and muscle tone

A

Firing contributes to muscle tone. The degree of contraction of the muscle is maintained by the firing of the muscles.

60
Q

What does the muscle spindle being stretch sensitive mean?

A

When there is an increase in stretch there is an increase in the firing of action potentials. Since the muscle spindle is in parallel to the muscle, it is sensitive to length.

61
Q

What is the position of the Golgi tendon organ in relation to the muscle?

A

In series with the muscle.

62
Q

Describe the lemniscal pathway (dorsal columns) - type of fibres, information, mode of information?

A

Large sensory Abeta fibres
Touch, vibration, 2 point discrimination, proprioception
Information can go into the spinal cord but mainly goes straight to the brain - quick

63
Q

Describe the spinothalamic pathway (anterolateral tracts) - type of fibres, type of information, mode of information?

A

The second order neurones use these tracts to send information
Small sensory Adelta and C fibres
Pain and temperature (some touch)
Potential for modification of information in spinal cord before being sent upto the brain or potential for the brain to modify the signal before it gets sent up, so more potential for changing.

NMS sem2 (14 decks)

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