L3 - Somatosensation Flashcards
(142 cards)
1
Q
What is the difference between perception and sensation?
A
In perception, the body is consciously aware and receives interpreted information (more precise and localised)
Sensation is any stimuli of which the body is aware (not necessarily consciously so)
2
Q
In the process of sensation, what are the three main levels of neural integration?
A
- Receptor level; sensory receptors
- Circuit level; all ascending pathways
- Perceptual level; neural connections in the cerebral cortex
3
Q
What is meant by the term ‘adequate stimuli’ in relation to sensory receptors?
A
The form of energy the receptor is most responsive to; e.g. temperature receptors might be most responsive to temperatures above 30 degrees
4
Q
Define transduction
A
The conversion of stimulus energy into information that can be processed by the CNS
5
Q
Describe how most sensory receptors transduce signals (generally speaking)
A
The opening or closing of ion channels converts mechanical, chemical, thermal energy into a change in membrane potential to produce an action potential
6
Q
What is meant by the “adaptation of sensory receptors”
A
That there can be tonic/phasic (rapidly/slowly) adapting receptors - which have changes in response to a long-lasting stimuli
7
Q
Describe phasic sensory receptors
A
8
Q
Describe tonic sensory receptors
A
9
Q
List the 5 somatic tactile sensations
A
- Touch
- Pressure
- Vibration
- Itching
- Tickle
10
Q
Name and describe the two subcategories of touch (and no, I do not mean the kind your uncle did to you when you were 9)
A
- Crude touch: The ability to perceive that something has touched the skin
- Discriminative touch: Provides location and texture of source
11
Q
What is pressure?
Answer for the physical kind please, not the feelings this degree stirs up
A
Pressure is a sustained somatic tactile sensation over a large area
12
Q
What is vibration?
A
Rapidly repetitive tactile somatic sensory signals
13
Q
What is itching?
What is the medical term for itching?
A
Pruritus
It is the chemical stimulation of free nerve endings
14
Q
What is a tickle?
A
Stimulation of somatic tactile sensory free nerve endings
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A
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22
Q
Describe receptive fields
A
23
Q
Many tactile sensations can be traced to 5 main types of mechanoreceptors.
Name them
A
- Meissner corpuscle
- Merkel discs
- Ruffini corpuscle
- Pacinian corpuscle
- Hair root plexus
24
Q
Where are Meissner corpuscles located?
A
The dendrites are enclosed in connective tissue in the dermal papillae, near the epidermis
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What do Meissner corpuscles detect?
Discriminative touch and vibration but **not** fine details (e.g. shape) in the hand. They are more useful for listing how much contact the hand has with something, useful for handgrip control
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When do Meissner corpuscles mainly generate impulses and what kind of receptors are they (tonic/phasic)?
What size would their receptor fields be?
Mainly generate impulses upon onset of touch and removal/cessation of touch - thus are rapidly adapting/phasic
Small receptor fields
27
Where are Meissner corpuscles mainly located?
Hairless areas; e.g. fingertips soles of feet, nipples, eyelids, genitalia
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This is a big card, take the time to explain the answer so it makes intuitive sense
Discuss the features of the **Meissner corpuscle** and why they suit it.
**Include**:
Location with skin
Type of thing picked up
Purpose
Phasic/tonic and generation of impulses
Receptor field size
Main locations in the body
This cheeky monkey has its **dendrites enclosed in CT** within the **dermal papillae**, very close to the epidermis. The epidermis is avascular and mostly dead SSE for resistance to abrasion, so its location in the dermal papillae has it as close as possible to where the touch will occur, thus meaning it can pick it up more accurately.
I will be using the hand as an example, but it exists in hairless locations on the body
It senses **discriminative touch and vibration, but not fine details (e.g. in the hand, more useful for handgrip control)**. What does this mean? It means it can tell you whether specific parts of the hand have made contact with an object, but not the characteristics of that object.
Think about it, this allows for a basic and highly specific, "**hey brain, it's mister Meissner here, I feel like I'm touching something**" - very useful to, for instance, develop a picture of how much of the hand is touching the object prior to picking it up.
Fitting this purpose, the **main impulses are generated at the onset and cessation of touch** (on/off), either yes it's there or no it's not. This is a rapidly adapting system, and thus is phasic, as it's not concerned with how long we've been touching something but rather more dichotomously that we are or aren't touching it.
The **receptor fields** will be **small**. Why would you want small? If the receptor field was large and said 'yes, I'm touching this" it gives very little information about how much surface area is in contact with the object. E.g, if there was a metre receptor field, that might just mean that a single cm is touching, or all 100, the information is too broad. Consider instead if the 100 cm receptor field were broken down into 20 smaller receptor fields of 5 cm; then the same 1 cm of contact would only trigger 1 receptor field out of the 20, allowing for a much more representative sense of the degree of contact.
These receptors are found in hairless sections of the body. Why would this make sense? We have evolved from apes which had hair all over their bodies, so they wisely use their hairs to sense a lot of their movements. We have certain areas where zero (or next to zero hair grows) and still need to know what's happening in these areas. That's where the Meissner corpuscles come in. Correspondingly, they are located in **hairless locations, fingertips, soles of feet, nipples, eyelids, genitalia.**
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Merkel disc
Unencapsulated, located in the dermal papillae.
Modified free nerve ending
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Meissner corpuscle
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Where do Merkel discs pick up touch from?
Its flattened dendrites touch the cells of the stratum basale of the epidermis
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Are Merkel discs tonic or phasic?
Phasic - slowly adapting
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Do Merkel discs have small or large receptive fields?
Small
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What type of sensation do Merkel discs pick up?
What % of the hand's receptors do they make up?
Discriminative touch/fine detail
Make up 25% of the receptors in the hands
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Discuss the features of Merkel discs and why they are useful
Modified free nerve ending mechanoreceptor, flattened (maybe why they're called discs?) dendrites reach out and touch the cells of the stratum basale of the epidermis, so nice and close to the external surfaces where the compression will be strongest.
Fires continuously as long as the stimulus is detected, therefore it is tonic/slowly adapting receptor
Used in discriminative touch (AND THEREFORE WILL HAVE SMALL RECEPTOR FIELD), as well as fine detail
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Which two mechanoreceptors are superficial?
Meissner corpuscles and Merkel discs
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Where are Ruffini corpuscles located?
Found deep in the dermis and hypodermis of the skin (particularly in the soles of the feet)
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Ruffini corpuscle
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What is the etymology of corpuscle?
Small body
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What do Ruffini corpuscles detect? (3)
Heavy touch
Continuous touch (and thus are tonic)
Pressure/stretching of skin
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What impact will the large receptor fields of Ruffini corpuscles have on sensation?
It will be hard to localise a specific point of pressure (RC mainly present in the deep dermis of the foot)
We don't need to have much detail in the foot, it is the thing keeping us upright but it isn't involved in manipulation of objects etc like the hand is
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Describe Ruffini corpuscles
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Pacinian corpuscle
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Which type of mechanoreceptor is visible to the naked eye?
Pacinian corpuscles (1 mm)
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What type of mechanoreceptor detects deep-pressure, or high-frequency vibration, or fine texture by moving fingers?
Pacinian corpuscles
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Describe the features of Pacinian corpuscles
Large and visible to the naked eye. Dendrite enclosed within an onion-like connective tissue capsule
Senses deep pressure (obviously, given it itself is deep), high-frequency vibration, and fine texture by moving hand over objects - off/on detected as a pattern which tells the brain about the texture. It has a phasic response as it is most closely associated with touch/stop touch (onset and when removed)
The receptive field is large (if it's deep, it's going to get input from all over anyway, so this makes sense).
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Pacinian corpuscles
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Describe hair root plexus
Phasic
Free nerve endings found around hair follicles
Detects movement of hair, fine touch
Phasic/rapidly adapting, only at onset
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Given that all fingers have the same relative number of Merkel discs in the fingertips, why is the index finger often preferred?
The Homonculus has a bigger area of the cortex dedicated to the index finger
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Explain how textural detail is produced from mechanoreceptors, as well as listing which mechanoreceptors are involved
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What is haptic perception?
Identifying a 3D object without vision (in hand)
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What three elements are needed to identify something haptically?
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Where does the dorsal column medial lemniscal pathway decussate?
After reaching the gracilis and cuneatus nuclei in the caudal medulla
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Where are cold receptors located?
In the stratum basale of the epidermis
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Where are warm receptors located?
In the dermis
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How do thermal receptors respond? (Phasic/tonic)
What is an everyday example of this?
Phasic at first, but then tonic at a low frequency
When hopping into the shower, it feels really hot at first, but despite the temperature not changing, the sensation of heat gradually lessens, even to the point of having to turn up the hot. The temperature is constant, but the initial phasic response of the thermal receptors conveying a lot of heat gradually gives way to low frequency tonic impulses of heat sensation
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We can perceive changes in our skin temp by what minimum difference?
0.01 degree Celcius
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Most heat receptors are labelled by what abbreviation?
TRPV
Transient receptor potential channels, of the vanilloid subtype
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What are the two abbreviations for cold receptors?
Trpm and Anktm
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Each thermal receptor contains \_\_\_\_
Except some ____ receptors, which?
Each thermal receptor contains only one type, except some cold receptors also contain Trpv1 heat receptors
61
Nociceptor free nerve endings are found in every tissue of the body, except?
The brain
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There are three kinds of nociceptors, what are they?
Of these, which is myelinated?
1. Mechanical (only responds to strong pressure)
2. Thermal (also respond to chemical)
3. Polymodal (mainly respond to chemical but can respond to all)
Only the mechanical nociceptors are myelinated, even then only thin myelin.
63
Describe what would trigger fast pain and how fast/via what the pain signal is transmitted via
* Occurs rapidly after a stimulus, ya know, why it's called fast pain. About 0.1 seconds after the event
* Can be triggered by a sharp pain like a needle puncture or a cut
* Larger myelinated A-delta (III) nerve fibres, with a conduction velocity of 5-30 m/s carr signal
* Is not felt in deeper tissues
* Usually stops before healing complete
64
Describe what would trigger slow pain, how it is conducted, and via what
* Begins slowly and increases in intensity
* Sent via smaller unmyelinated C (IV) fibres, conduction velocity of 0.5-2 m/s
* Aching or throbbing pain
* In both superficial and deep tissues
* Persistent and difficult to treat
* Ceases once homeostasis is restored
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How many neurons are there in a pain pathway?
3
Receptor neuron is primary, ascending tract to thalamus is secondary, thalamus to cortex is tertiary
66
What Brodmann's areas are included in the postcentral gyrus (which is within the parietal lobe)?
Brodmann's areas 1, 2, 3a, and 3b
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The relative sizes of the cortical areas depicted in the sensory homunculus are proportional to? (2)
1. the number of sensory receptors
2. the sensitivity of each part of the body
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What is experience-dependent plasticity?
Cortical representative areas of a body part can become larger if it is used more often
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Where does the 3b section of the postcentral gyrus get its information from?
What kind of information comes to this section?
Where is this information forwarded to?
Receives dense input from the ventral posterior part thalamus
Neurons are responsive to somatosensory stimuli and no other sensory stimuli
3b projects to area 1 about texture
3b projects to area 2 about size and shape
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Where does the 3a section of the postcentral gyrus get its information from?
What kind of information comes to this section?
3a receives dense input from the ol' thalamus about proprioception.
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Posterior parietal lobe (Brodmann's numbers 5 & 7)
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Which part of the brain puts together all the separate sensory info, in addition to vision, for interpretation?
The posterior parietal lobe
| (the somatosensory association area)
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Where does 3b part of the primary send its somatosensory information? What information to where?
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What is stereognosis?
Stereo = shape
Gnosis = knowledge
Shape-knowledge
the mental perception of depth or three-dimensionality by the senses, usually in reference to the ability to perceive the form of solid objects by touch.
76
What information is carried in the Dorsal Medial Lemniscal Pathway? (5)
1. Proprioception
2. Vibration
3. Discriminative touch (2 point)
4. Weight discrimination
5. Stereognosis
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The dorsal column medial lemniscal pathway is divided into?
Fasciculus Gracilis: Lumbar/sacral levels
More medial, this comes in first so naturally will be more central
Fasciculus Cutaneous: thoracic/cervical levels
Not present until higher up as pertains to the upper limbs, present more laterally
78
List the receptors with A-alpha fibre types
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List the receptors with A-beta fibre types
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List the receptors with A-delta fibre types
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List the receptors with C fibre types
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Where does the dorsal column medial lemniscal pathway decussate?
Within the caudal medulla after the FON have synapsed onto the SON in the nucleus gracilis and the nucleus cuneatus.
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The DCML pathway is divided into two subsections, what are they and where do they synapse?
Both subsections travel in the dorsal column
The fasciculus gracilis synapses with the gracilis nucleus in the medulla (both more medial)
The fasciculus cuneatus synapses with the cuneatus nucleus in the medulla (more lateral)
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What type of information is carried in the DCML pathway? (5)
1. Proprioception
2. **Discriminative touch**
3. **Vibration**
4. **Weight discrimination**
5. **Stereognosis**
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Where do the FON synapse with SON in the DCML pathway?
In the medulla in the nucleus gracilis and nucleus cuneatus
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What common area of the spinal cord do the fasciculus gracilis and the fasciculus cuneatus travel in?
The dorsal column
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In what tract do the SON of the DCML pathway travel through?
The medial lemniscus
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Where do the SON synapse with the TON in the DCML pathway?
After travelling through the medial lemniscus, the SON synapse with TON in the thalamus
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Where do the TON of the DCML pathway go?
Send out thalamic fibres to reach the somatosensory cortex
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The DCML pathway travels _____ in the spinal cord
ipsilaterally
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The DCML pathway travels _____ in the brainstem and brain
Contralaterally
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What three sensory pathways directly bring information through the spinal cord to the cerebellum?
1. Anterior spinocerebellar tract
2. Posterior spinocerebellar tract
3. Cuneocerebellar tract
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The following three tracts all contain information from the same type of receptors:
1. Anterior spinocerebellar
2. Posterior spinocerebellar
3. Cuneocerebellar
What is the main form of information it receives? What are the receptors? What fibre types are they carried in?
All carry proprioceptive info
All receive information from:
* Muscle Spindles (A-alpha)
* Golgi tendon organs (A-alpha)
* Joint capsule mechanoreceptors (A-beta)
96
Which 2 tracts carrying information to the cerebellum do not decussate?
What is interesting about the 3rd tract that does decussate?
Both posterior spinocerebellar tract and cuneocerebellar tract do not decussate; both also enter the inferior cerebellar peduncles
Anterior spinocerebellar tract does decussate (immediately at the level it enters), travelling up the spinal cord contralaterally, through the brainstem until it reaches the midbrain, where it turns around and enters the superior cerebellar peduncles. Whilst in the superior cerebellar peduncles, most of the neurons decussate a second time and thus enter the cerebellum ipsilaterally
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What spinal regions does the posterior spinocerebellar tract carry information for?
Lower thoracic, lumbar and sacral regions
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Where do the FON synapse with SON in the posterior spinocerebellar tract? How does it get to this point?
If the FON is below L2, then it must ascend the SC within the fasciculus gracilis until it reaches the level of L2.
At L2 and above, the FON synapses in **Clarke's** **nucleus** in the spinal cord.
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Where does the posterior spinocerebellar tract decussate?
It does not decussate, mate
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Where does the SON of the posterior spinocerebellar tract travel?
Leaves from Clarke's nucleus and ascends the SC ipsilaterally in **lateral fasciculus** and **brainstem**, entering the cerebellum through the **cerebellar peduncle**
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Where does the cuneocerebellar tract decussate?
It does not decussate, mate
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What spinal regions does the cuneocerebellar tract carry information for?
Cervical levels only
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Describe the pathway of the FON to the SON of the cuneocerebellar tract
FON travel with DCML pathway neurons in the **fasciculus cuneatus**, synapsing with SON in the **lateral cuneate nucleus** in the **medulla**
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Where do SON travel from and to in the cuneocerebellar tract?
SON begin in the lateral cuneate nucleus in medulla, ascending ipsilaterally into the cerebellum via the inferior cerebellar peduncle
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The anterior spinocerebellar tract is concerned primarily with?
The trunk and lower limb
There may be one for upper limb but little is known about it
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Where do FON synapse with SON in the anterior spinocerebellar tract?
In the dorsal horn shortly after entering
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Where does the anterior spinocerebellar tract decussate?
Upon immediately entering the dorsal horn, and a second time (for most neurons) within the superior cerebellar peduncle
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Where do SON travel in the anterior spinocerebellar tract?
After FON synapses with SON in dorsal horn, the SON of anterior spinocerebellar tract crosses contralaterally and travels up SC and brainstem until it reaches the midbrain, where it turns around and enters the superior cerebellar peduncle (and most neurons decussate again, returning to ipsilateral) before entering the cerebellum
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What is the main information type carried by the spinothalamic tract?
Pain and temperature
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Where do the FON of the spinothalamic tract synapse with SON?
Enter the SC through Lissauer's tract, synapsing with the SON in the substantia gelatinosa (in the dorsal horn) and deep posterior horn layers
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What is Lissauer's tract?
The pathway through which the spinothalamic tract enters the spinal cord
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What part of the spinal cord is frequently cut in intractable pain?
Lissauer's tract
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What is the substantia gelatinosa?
The location where the FON of the spinothalamic tract synapse with SON
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Where does the spinothalamic tract decussate?
After the FON synapse with the SON in the substantia gelatinosa, about 1 to 2 segments above the spinal cord entry; travelling contralaterally in the anterolateral fasciculus
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In the spinothalamic tract, there is an orderly arrangement of the upper limb and lower limb travelling within the anterolateral fasciculus, which is more anterior?
the upper limb travels in the SC's anterolateral fasciculus more anteriorly than the lower limb
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Explain where the SON travel and synapse with TON in the spinothalamic tract
The SON starts in the substantia gelatinosa and decussates up 1 to 2 segments to enter the anterolateral fasciculus; ascending contralaterally in the SC (with upper limb more anterior than lower limb), synapsing with TON in the thalamus
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Where do the TON in the spinothalamic tract go?
Project from the thalamus to the somatosensory cortex, plus other cortical areas like the insula and anterior cingulate gyrus (the affective aspect of pain)
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Where is core temperature detected by?
Receptors in the anterior hypothalamus (by monitoring temp of blood)
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How does the hypothalamus receive information about the peripheral temperature?
Via **spinohypothalamic tract** from peripheral temperature receptors
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What nerve fibre type carries cold sensation?
A-delta
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What nerve fibre type carries hot temperature?
C fibre
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What hypothalamic nuclei responds to core and peripheral temperature information?
How does it effect a response?
The posterior nuclei
Stimulates or inhibits the sympathetic part of the ANS
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What are the three main pathways of descending control of pain?
1. Cerebral cortex - descending pathway
2. Periaqueductal grey (PAG) - in midbrain
3. Reticular formation - in pons/medulla
Notice how there is only one in each section
127
Describe the cortical control of pain
Via descending motor tract - **corticospinal (pyramidal) tract**
* modifies the activity of all ascending systems
* Prominent in the control of conscious and reflex response to noxious stimuli (e.g. withdrawing hand from hot stove)
* Under intense emotional experiences, subjective awareness and defensive reflexes can be suppressed
* **terminates in the dorsal horn**
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What is the PAG and where is it found?
Periaqueductal Grey Matter
Found in midbrain surrounding the cerebral aqueduct
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Stimulation of the PAG in humans can cause?
Amelioration of intractable pain
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How does the PAG receive information about noxious stimuli?
Ascending pathways send info about noxious stimuli to PAG
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How does the PAG get information about behavioural state-pain?
Hypothalamus-cortex sends info to PAG
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How does the PAG lessen the experience of pain?
Where does it project to?
Projects to the **raphe nuclei** in **medulla** and **pons** and **reticular formation**.
These, in turn, project to the relevant dorsal horn in the spinal cord that is experiencing pain, releasing serotonin and adrenalin
Small, enkephalin containing inhibitory interneurons suppress the transmission of pain through the spinothalamic tract
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How do inhibitory interneurons stimulated by the PAG (via the raphe nuclei and reticular formation), suppress the transmission of pain?
They release enkephalin
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What hormone inhibits the transmission of pain in the spinothalamic tract?
Enkephalin
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Is this degree worth all this pain?
No
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Describe the relationship between the PAG and reticular formation, and their role in stimulating inhibitory interneurons
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What neurotransmitter convey the signal of pain in the dorsal horn?
Substance P and glutamate
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What neurotransmitter does enkephalin decrease the release of?
Glutamate from the primary afferent neuron
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What neurotransmitters stimulate the inhibitory interneuron to release enkephalin?
Adrenaline (though noradrenaline in the picture on the next slide) and serotonin
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Describe how enkephalin blocks the transmission of pain in the DH
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Why does referred pain occur?
