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The Somatosensory System Flashcards

1
Q

What is the somatosensory system?

A

The ability of the body to sense its environment

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2
Q

What are primary sensory neurones?

A

They are the first neurone of a three neurone pathway that takes signals from the periphery to the brain. All primary sensory neurones are excitatory.

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3
Q

What is the structure/form of a primary neurone?

A

The cell body is located in the dorsal root ganglion (outside CNS). A single axon splits into two parts.

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4
Q

Describe the pathway of the primary axons.

A

One part innervates the skin, creating a receptive field. The other part enters the dorsal horn of the spinal cord, where it splits: one branch goes directly to the brain, and the other synapses with a second order neurone.

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5
Q

What happens to the axon that synapses in the dorsal horn?

A

It synapses with the cell body of second order neurones in the dorsal horn, sending an axon up to the brain, eventually reaching the thalamus.

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6
Q

Where is the action potential initiated in a sensory neurone?

A

At the peripheral nerve ending, unlike generic neurones where it is generated in the axon hillock.

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7
Q

Describe the stimulation of a primary sensory neurone.

A

A stimulus adequate for the primary sensory neurone generates an AP at the peripheral nerve ending, which propagates along the axon to the dorsal horn.

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8
Q

What is a receptive field?

A

A patch of skin innervated by the peripheral end of a specific axon.

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9
Q

Which tract contains the axons going straight to the brain?

A

Dorsal column.

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10
Q

Which tract contains the axons that synapse in the spine?

A

Spinothalamic tract.

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11
Q

What type of neurones are primary neurones?

A

They are all excitatory; ‘primary inhibitory’ neurones do not exist.

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12
Q

What is the main neurotransmitter released at the synapse between the primary and secondary neurone?

A

Excitatory neurotransmitter, usually glutamate.

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13
Q

What is an ‘adequate stimulus’?

A

A stimulus specific to a neurone that activates it, generating an AP.

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14
Q

What differentiates the types of primary neurones?

A

They are highly specialised for particular stimuli, differentiated by the speed of AP conduction velocity.

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15
Q

What physical features dictate the speed/velocity of an AP?

A

Degree of myelination of the axon and thickness of the axon.

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16
Q

What are the four different types of primary/afferent axons?

A

A-alpha (proprioceptors, high conduction velocity), A-beta (mechanoreceptors, medium conduction velocity), A-delta (temperature and pain, low conduction velocity), C (temperature, pain, and itch, lowest conduction velocity).

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17
Q

Which axon has the fastest conduction velocity?

A

A-alpha due to its large diameter and heavy myelination.

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18
Q

Which axon picks up temperature and pain?

A

A-delta.

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19
Q

Which axon has the lowest conduction velocity?

A

C due to its smallest diameter and lack of myelination.

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20
Q

Are all receptive fields equal?

A

No, receptive fields vary in size.

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21
Q

What is the distance between two receptive fields based on?

A

It varies in different parts of the body based on the number of neurones innervating that patch of skin.

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22
Q

What is a ‘2-point discrimination’ test?

A

It tests receptive field size by applying two points to the skin to activate two individual neurones.

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23
Q

How does a ‘2-point discrimination test’ work?

A

By applying two distinct points to the skin, if two separate points are recognized, it indicates stimulation of two separate receptive fields.

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24
Q

What does the size of the receptive field determine?

A

It determines the precision of localisation of a stimulus.

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25
How does the size of a receptive field relate to the activation of cortex?
More neurones innervating a patch of skin (small receptive field) require more brain area to process the information.
26
What is a dermatome?
An area of skin innervated by afferent axon fibres, signaling sensation via a single nerve from a single spinal nerve root.
27
What is a stimulus?
Energy being applied to the receptor.
28
What is signal transduction?
Energy (e.g. mechanical, touch, temperature) applied to the skin is converted into action potentials by sensory receptors.
29
What is required of a stimulus for it to activate a sensory receptor?
1. Be the right type (adequate) of stimulus (e.g. mechanical, thermal, etc.). 2. Reach the threshold before a signal is actually sent.
30
How does your neurone transduce a physical stimulus into a signal?
The nerve ending has a membrane with ion channels called transduction channels. The application of the stimulus needs sufficient energy to open these channels, allowing positively charged ions to enter the cell, resulting in depolarization and a receptor potential. Whether a signal is sent depends on the threshold value, and the firing rate of action potentials (APs) is proportional to stimulus strength.
31
What is a graded receptor potential?
A graded receptor potential is the amount of ion channel opening and ion influx, and therefore the amount of depolarization, is proportional to the amount of stimulus applied. ## Footnote For example, with a small stimulus, you get a small receptor potential; with a larger stimulus, you get a bigger receptor potential.
32
What are the adaptations of sensory responses?
There are two types of sensory receptors for different stimuli: slow adapting (tonic) receptors, which continuously fire APs when the stimulus is present, and fast adapting (phasic) receptors, which are most activated during a change in stimulus but decrease firing when the stimulus is maintained.
33
What type of stimuli do slow adapting (tonic) receptors pick up?
These receptors mainly pick up painful stimuli or the amount of stretch at a tendon/skin, which is important for maintaining information about a stimulus for survival.
34
What type of stimuli do fast adapting (phasic) receptors pick up? Give an example.
They respond to a changing stimulus and are important for picking up stimuli that need initial awareness but not continuous attention. ## Footnote Example: Tactile (touch) receptors, such as feeling a shirt when first worn but not after it settles on the skin.
35
What is a tonic receptor?
A tonic receptor is slow-adapting and detects the strength of a stimulus
36
What is a phasic receptor?
A phasic receptor is fast-adapting and detects how fast the stimulus changes.
37
What are the three types of cutaneous sensory receptors?
1. Mechanoreceptors: touch, pressure, stretch, vibration 2. Thermo-receptors: hot, cold 3. Nociceptors: noxious stimulation (pain)
38
What do cutaneous mechanoreceptors respond to?
They are tactile receptors that pick up information on texture, pressure, and vibration.
39
What type of axon do cutaneous mechanoreceptors have?
They are at the end of Aβ fibres.
40
What determines the structure of mechanoreceptors?
The structure determines function, indicating its location (superficial vs deeper in tissue).
41
What are the four major types of mechanoreceptors?
1. Merkel’s receptor (disk) - superficial 2. Meissner’s corpuscle - superficial 3. Ruffini’s corpuscle (ending) - deeper 4. Pacinian corpuscle - deeper
42
Where are Merkel’s receptors mostly found?
High density in the epidermis of digits and around the mouth; lower density on glabrous skin; very low density in hairy skin.
43
What type of sensory receptor are Merkel’s receptors?
They are slowly adapting receptors.
44
What is the adequate stimulus for Merkel’s receptors?
They respond to sustained light touch.
45
What is a Merkel cell?
A specialized keratocyte apparatus that responds to initial skin indentation and sustained pressure, providing perception of form and texture.
46
Where are Meissner’s corpuscles found?
They are found in the papillary dermis.
47
What type of sensory receptor are Meissner’s corpuscles?
They are rapidly adapting sensory receptors.
48
What is the adequate stimulus for Meissner’s corpuscles?
They pick up light touch and vibration.
49
Where are Ruffini’s corpuscles found?
They are found in the deeper layer of skin (dermis).
50
What is the adequate stimulus for Ruffini’s corpuscles?
They respond to lateral movement or stretching of skin and deep touch.
51
Where are Pacinian corpuscles found?
They are found in the deeper layers of the dermis.
52
What type of sensory receptor are Pacinian corpuscles?
They are rapidly adapting sensory receptors.
53
What is the adequate stimulus for Pacinian corpuscles?
They require a stronger stimulus, such as deep touch, poke, or high-frequency vibration.
54
What receptors are involved in the adjustment of grip force?
Meissner’s and Ruffini’s corpuscles are involved in the adjustment of grip force.
55
Why do we drop things as we get older?
Older individuals have fewer Meissner’s and Ruffini’s corpuscle neurons, which can affect grip force.
56
What is the structure of cutaneous thermo-receptors?
They are bare nerve endings with no specialized apparatus at the end of the axon.
57
What makes cutaneous thermo-receptors highly specialized?
They express ion channels that open in response to temperature.
58
What type of sensory receptor are cutaneous thermo-receptors?
They are slowly adapting sensory receptors.
59
What are the two types of cutaneous thermo-receptor neurons?
1. Warmth receptors 2. Cooling receptors
60
What do cutaneous thermo-receptors sense?
They are sensitive to changes in temperature but poor indicators of absolute temperature.
61
What are thermo-receptor ion channels called?
They are called the Transient Receptor Potential (TRP) family.
62
What types of channels are TRP channels?
They are non-specific cation channels.
63
What is TRPM8?
A cold channel that opens between 10-38°C and is maximally active at 25°C; also opened by menthol.
64
What is TRPV3/4?
Warm channels that open between 29-45°C and are maximally active at 45°C.
65
What are cutaneous nociceptors?
They are bare nerve endings that are non-adapting sensory receptors.
66
What is the threshold for cutaneous nociceptors?
They have a high threshold and will not fire unless the stimulus is sufficiently large to damage tissue.
67
What are the two types of cutaneous nociceptors?
1. High threshold mechanoreceptors (sharp pain) 2. Polymodal nociceptors (dull, burning pain)
68
What transducer channel is found at the end of C fibres?
TRPV1, which is opened by heat and capsaicin.
69
What do hair follicle receptors respond to?
They respond to light touch and detect hair deflection.
70
What is the structure of hair follicle receptors?
The axon of a neuron wraps around the hair follicle.
71
What type of sensory receptor are hair follicle receptors?
They are rapidly adapting receptors.
72
Which cutaneous receptors are mechanoreceptors?
A (Meissner's corpuscle), B (Pacinian corpuscle), C (Ruffini’s), E (Merkel discs).
73
What are the cutaneous mechanoreceptors?
A: Meissener corpuscle, B: Pacinian corpuscle (lamellated), C: Ruffini’s, E: Merkel discs (tactile) (touch)
74
What is proprioception?
Proprioception detects the mechanical status of the musculoskeletal system, providing information about joint position, muscle length, muscle movement, acceleration, and tension/force being applied to different parts of your body.
75
What are the sensory apparatus that provide proprioceptive information?
Muscle spindle and Golgi tendon organ.
76
What type of axons are the primary afferents in the muscles?
Group 1 axons: These large diameter, highly myelinated axons propagate action potentials quickly, allowing for rapid proprioceptive feedback.
77
What is the muscle spindle?
The muscle spindle is a specialized sensory apparatus used for detecting muscle length and acceleration around a joint.
78
Where are muscle spindles found?
Muscle.
79
What is the classification of the axon in muscle spindles?
Group 1a sensory axons afferents wrapped around the central portion of the intrafusal fibres.
80
How are muscle spindles positioned in muscles?
They are positioned in parallel to other muscle fibres.
81
What happens when muscle spindles are stretched?
If the intrafusal fibres are stretched, an action potential will be fired in 1a axons of primary sensory neurones, contributing to muscle tone.
82
Where is the Golgi tendon organ located?
At the junction of muscle and tendon.
83
What group of axons innervates the Golgi tendon organ?
Innervated by Group 1b sensory afferents.
84
How is the Golgi tendon organ positioned in muscles?
Positioned in series with muscle, making it sensitive to muscle tension generated by contraction.
85
What are the two main pathways that sensory information travels towards the brain?
Lemniscal Pathway (Uses Dorsal Columns) and Spinothalamic Pathway (Uses Anterolateral Tracts).
86
Which part of the primary sensory axon is the dorsal column?
The primary sensory neurone axon splits in the dorsal horn, with one branch going straight up to the brain via the dorsal column.
87
Describe the pathway of the dorsal column.
Primary neurone synapses with secondary in the medulla; secondary neurone axon reaches the thalamus, and information is sent to the primary somatosensory cortex in the parietal lobe via third order neurone.
88
What type of axons are in the dorsal column?
Large sensory A-β fibres.
89
What information is carried via the dorsal column?
Sensitive to touch, vibration, two-point discrimination, proprioception.
90
Describe the spinothalamic pathway.
The other branch of the axon goes into the dorsal horn of the spinal cord, synapses onto second order neurone which sends information up to the brain, reaching the thalamus and then the primary somatosensory cortex.
91
What type of axons are in the spinothalamic pathway?
Small sensory A-δ and C fibres.
92
What information is carried via the spinothalamic pathway?
Sends information about pain and temperature (some touch).

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