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-Yr2 Neuro > 7. *Sensory Pathways > Flashcards

7. *Sensory Pathways Flashcards

1
Q

What is a modality? How does information from a modality travel to the brain?

A

A type of stimulus (heat, cold, touch etc). There are specialised receptors that will receive information from the modalities autonomic receptors will carry this to the brain.

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2
Q

What are the three main categories of sensory fibres?

A

A betas, A deltas and C

There are also A alphas are which are to do with proprioception

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3
Q

Outline the characteristics of each type of sensory fibre

A
  • The A-beta fibers are very fast transducing fibers that are large and myelinated. Mainly involved in transmitting mechanical stimulation
    • The A-delta fibers are also myelinated so are fast transducing but are slightly less fast than the A-betas so they transmit fast pain signals and temperature
    • C fibers are very slow transducing as they don’t have any myelination and so are involved in the achy pain that you experience
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4
Q

What do all the sensory fibres join to make?

A

• Together these fibers build up a peripheral nerve. The same nerve carries all these different types of stimulation (pain, temperature etc.) and the type of stimulation determines which type of fiber carries it.

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5
Q

What is special about the nerve endings of sensory axons?

A

• Within these nerve terminals you have individual axons which have modified terminals themselves. These will be specific for different modalities
○ E.g. C fibers are unmyelinated and have fee nerve endings that are specific for heat. Mechanoreceptors have encapsulated nerve endings

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6
Q

What are sensory receptors?

A

Transducers that convert energy from the environment into neuronal action potentials

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7
Q

What is the absolute threshold?

A

The strength of a stimulus that produces a detectable response 50% of the time

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8
Q

What happens once a strong enough stimulus has been given?

A
  • Once a strong enough stimulus has been given, you will generate a generator potential in the nerve terminal causing the nerve ending to become depolarized. If this is strong enough then it will send a chain of action potentials along the nerve to the spinal cord
    • So generator potential creates action potential which is then propagated and neurotransmitters are released in the spinal cord
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9
Q

What are TRP channels?

A
  • The temperature receptors that are present on free nerve endings are known as TRP channels (transient receptor potential channels)
    • Many TRP channels have been discovered which are activated to different stimuli (such as the first one that was discovered was activated by capsaicin) and they are also activated at different temperatures
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10
Q

What are the different types of TRP channels and what are their functions?

A
  • The TRP channels are TRP 1-4 and they range from a damaging level of heat to very tepid temperatures
    • There are also cold activated channels which are called TRPM8 channels
    • So there are different receptors that are activated for different temperatures and they pass a different type of sensory information on
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11
Q

What are the different types of mechanoreceptors and what type of stimulus do they respond to?

A

Meissner’s corpuscle - fine discriminative touch
Merkel cells - light touch and superficial pressure
Pacinian corpuscles - deep pressures, vibration and tickling
Ruffin endings - continuous pressure or touch and stress

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12
Q

How are different mechanoreceptors specific to different types of stimuli?

A

Due to adaptation which is how the body understands what is going on by how the brain is perceiving the information

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13
Q

What are tonic receptors? Give an example of a tonic receptor

A
  • Tonic receptors – do not adapt/adapt very slowly
    • They will keep sending an impulse to the brain as long as the stimulus is present so the brain knows that the stimulus is still there
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14
Q

Give an example of a tonic receptor

A

Merkel cells slowly adapt to allow fine touch to be perceived so if you are being stroked, these receptors will be firing the whole time to let your brain know you are being stroked

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15
Q

What are the characteristics of physical receptors?

A

• Phasic receptors detect a change in stimulus strength.
They show a trail of action potentials at the start and at the end so it can be seen that they are adapting quickly. It is the body understanding when a change is taking place and they are also called movement or rate receptors

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16
Q

Give an example of a type of phasic receptor

A

• Pacinian corpuscle receptors are an example of this as a sudden pressure excites the receptor and so a signal is sent and when the pressure is released another signal is sent out. This is phasic adaptation

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17
Q

What are somatosensory dermatoses?

A

Look at where in the body the stimulus is occurring

Each SS dermatome is specific to a spinal nerve

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18
Q

What is a receptive field?

A

An area on the skin which causes the activation of a single sensory neurone when activated

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19
Q

Where would you find small receptive fields and where would you find larger ones?

A
  • Small receptive field on the arm which allows detection of fine touch in a small area which is needed for precise perception. This is even more so on the fingers
    • However on the back there are very large receptor fields so cells can detect changes in a much wider area
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20
Q

What would the size of the receptive field be indicative of?

A

How sensitive the part of the body is reflects the size of the receptive field. The more smaller the receptive field the more densely packed neurons are there and the more sensitive it is

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21
Q

What is two point discrimination? How is the size of the receptive field linked to this?

A
  • Ability to distinguish two points as being separate
    • When two points are placed in a small receptive field, there is a high chance they will be perceived as two points and not one. This is because the area will be densely packed with neurones.
    • Where the receptive field is larger, two points are often recognised as one (such as on the back)
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22
Q

What are nociceptors? What type of fibres are they?

A

These are involved with pain. They are mainly A-delta fibers so are fast as they are myelinated
They can also be made of C-fibres

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23
Q

What are the two different type of A delta fibres?

A

• A-delta fibers come in two different types
○ type 1 are A-delta mechanoheat receptors which transfer noxious (harmful) mechanical and heat stimuli
○ Type 2 are A-delta mechanoreceptors and are involved in transmitting only noxious mechanical stimuli

24
Q

How are C-fibres involved with nociception?

A

C-fibers are unmyelinated so slowly transduce dull, persistent or second pain
These are polymodal so respond to different modalities. There are only one type of c fibers.

25
Q

How is the dorsal root involved in signal transduction?

A
  • When the signal is being transmitted from the periphery to the spinal cord it first comes across the dorsal root
    • The cell bodies of primary afferent neurones are located in the dorsal root ganglia (for neurones from the body) and for the face they are located in trigeminal ganglia
    • Dorsal root ganglia is the period just before you enter the spinal cord. Spinal nerves will come through the intervertebral foramen into the spinal cord (so they have come from the periphery to the CNS)
26
Q

Outline the general organisation of the dorsal horn

A

Found in the spinal cord, organised into rexed laminae (1-7) Different fibers terminate in different laminae

27
Q

What is the significance of the different rex laminae in the dorsal horn?

A
  • Laminae 1 and 2 are also referred to as the superficial dorsal horn and is where pain and temperature fibers terminate
    • Laminae 3 to 6 are also referred to as the superficial dorsal horn and this is where innocuous mechanical stimuli fibers terminate
28
Q

What is the main neurotransmitter released in the dorsal horn?

A

Glutamate is released from presynaptic to postsynaptic neurone within the dorsal horn. This then transmits to the brain
Glutamate is excitatory

29
Q

What is the function of interneurones in the dorsal horn?

A

• The dorsal horn isn’t just made up of simple neurones but there are many interneurones which modulate the pain and help transmit mechanical stimulation throughout the horn

30
Q

What is lateral inhibition and why is it important?

A
  • Lateral inhibition – focusing on one point and removing all the background stimuli that we don’t want to perceive.
    • Because receptive fields can overlap, if we don’t have lateral inhibition the stimulus received can be more diffuse
31
Q

Where does lateral inhibition happen?

A

Dorsal horn

32
Q

What inhibits any peripheral inputs that are not important in the spinal cord?

A

The interneurones in the spinal cord

33
Q

What is the gate control theory?

A
  • In the dorsal horn, sensory information is open to modulation
    • The theory suggests that you can modulate a pain response in the dorsal horn by activating an A-beta fiber. This is the one that is involved with innocuous mechanical signals (so just a brush, not harmful). This makes sense because when you hurt yourself you often rub it better as you are stimulating AB fibers which inhibit any pain fibers that have been stimulated
34
Q

What are the two main sensory areas in the cortex? Where are they located?

A
  • S1 is the primary somatosensory cortex - in the postcentral gyrus
    • S2 is the second somatosensory cortex – usually just receives input from S1. Located in the parietal operculum
    • Posterior parietal cortex – spatial awareness of the body
35
Q

What are the two main sensory pathways that bring sensory information to the brain?

A 
Dorsal column pathway
The spinothalamic (anterolateral) pathway
36
Q

What type of sensory information does the dorsal columns pathway transmit?

A

• Transmits innocuous stimuli
○ Fine discriminative touch (two point discrimination)
○ Vibration

37
Q

How does sensory information enter the dorsal column pathway?

A

• AB fibers enter the dorsal horn and then through interneurones and second order synaptic transmission they enter the dorsal column pathway
• Somatotopic organisation :
○ information can go from the lower limbs (so below T6) and travel ipsilaterally along the gracile tract. This is for lower limbs
○ For upper limbs (above T6) , they travel ipsilaterally along the cuneate tract

38
Q

What happens to nerve fibres once they have entered the dorsal columns?

A

Fibres in the gracile tract have their first synapse in the gracile nucleus
Fibres in the cuneate tract will have their first synapse in the cuneate nucleus
Both these nuclei are found in the medulla so first order neurones from the dorsal pathway terminate in the medulla

39
Q

What happens to second order neurones in the dorsal pathway? Where do they terminate?

A
  • Second order neurones cross the midline (decussate) in the caudal medulla and they form the contralateral medial lemniscus tract (basically a tract from the medulla to the thalamus)
    • Axons from the second order neurones terminate in the ventral posterior lateral nucleus of the thalamus. Neurones from lower parts of the body such as the limbs terminate more laterally in the VPL
40
Q

Where do third order neurones go from and to in the dorsal column pathway?

A

This is the final part of the pathway wherethe 3rd order neurones from the VPL nucleus of the thalamus project onto the somatosensory cortex

41
Q

What are the pathways involved in the spinothalamic tract?

A

• Spinothalamic is also called anterolateral as there are two pathways involved in it. They each have their own tract
○ Lateral pathway – pain and temperature sensations
○ Anterior pathway – crude touch

42
Q

What happens to 1st and 2nd order neurones of the spinothalamic tract?

A
  • 1st order neurones (primary afferents) will terminate in the dorsal horn
    • 2nd order will decussate (cross over) immediately forming the spinothalamic tract (unlike in the dorsal column tract where they decussate in the medulla). They also terminate in the ventral posterior lateral nucleus of the thalamus
43
Q

What are the main differences between the spinothalamic and the dorsal columns pathway?

A

• Spinothalamic:
○ Transmits pain, temperature and coarse touch
○ Cross in the spinal cord
• Dorsal column:
○ Transmits light touch, vibration and 2-point discrimination
○ Cross over in the brain stem

44
Q

What is the spinoreticular tract?

A

It is the area of the brain that is involved in the emotional response of pain

45
Q

What areas of the brain respond to pain?

A

Mainly it is the S1 and S2 cortex that show activity when the body is in pain but  the three other cortex (insula cortex, anterior cingulate cortex and prefrontal cortex) also show a response to the pain
The amygdala, brain and spinal cord also respond to the pain

46
Q

How is quantitative sensory testing carried out?

A

Temperature – you can find out the different threshold values for different temperatures
Vibrations and brush – you can see how sensitive patients are to these
These are used to see if the sensory NS is sensitised or if there is any pathology

47
Q

What happens in an anterior spinal cord lesion?

A

This is a partial spinal cord injury where the anterior spinal artery is blocked so there is damage to the bottom half of the spinal cord. 
The spinothalamic tract is not working so there is a loss of pain and temperature below the lesion on both sides. However, the dorsal column tract is intact so they will retain light touch and vibration below the level of the injury. 
However, if the entire spinal cord was damaged you wouldn’t have any sensation below the level of the lesion

48
Q

What is an electrical perceptual threshold test?

A

Semi-automated method where you send a current to the skin and the patient either feels something or not
It is a good method for diagnosing a spinal cord injury by looking  at where the sensations are felt and which nerves/tracts relate to which particular area

49
Q

Define pain

A

Unpleasant sensory and emotional experience associated with actual or potential tissue damage

There is both a sensory and emotional component to pain. So for example, if you hit your thumb with a hammer, it will hurt and you’ll also have a negative emotional feeling to go with it.

50
Q

What are the different types of pain?

A 
• Nociceptive — tissue damage, typically 
Acute (e.g. skin cut) 
•Muscle — lactic acidosis, ischaemia (e.g. 
stretching, fibromyalgia) 
• Somatic — well-localised (e.g. 
inflammation, infection) 
• Visceral — deep, poorly localised (e.g. 
stomach, colon, IBS) 
•Referred — from an internal 
organ/structure (e.g. Angina) 
•Neuropathic — dysfunction of the 
nervous system
51
Q

What is nociceptive pain?

A

Pain that arises from actual or threatened damage to non-neural tissue and is due to activation of nociceptors. This term is used to describe pain occurring with a normally functioning nervous system such as tissue damage and inflammation

52
Q

What is neuropathic pain?

A

Pain that is initiated or caused by a primary lesion or disease affecting the somatosensory system. This type of pain is typically chronic

53
Q

What is chronic pain?

A

Pain that lasts at least 6 months

54
Q

Give some examples of neuropathic pain

A 
• Radicular low back pain (sciatica) 
• Diabetic neuropathy 
• Post herpetic neuralgia 
• Post-surgical pain 
• HIV-induced neuropathy 
• Chemotherapy induced neuropathy 
• Complex Regional Pain Syndrome 
(CRPS)
55
Q

What happens in pain of the sciatica?

A

Spinal nerves become compressed in the intervertebral foramen so you get pain in the leg

56
Q

What happens in complex regional pain syndrome?

A

You develop pain around the whole body which spreads spontaneously

57
Q

Define the terms allodynia and hyperalgesia, sensitization, hypoalgesia and paraesthesia

A

• Allodynia: pain due to a stimulus that does not normally provoke pain (e.g. brushing the skin of someone with peripheral neuropathy)
• Hyperalgesia: increased pain from a stimulus that normally provokes pain (e.g. pressing a skin that has been damaged by sunburn)
• Sensitization: increased responsiveness of nociceptive neurons to their
normal input
• Hypoalgesia: Diminished pain in response to a normally painful stimulus
• Paraesthesia: abnormal sensation, whether spontaneous or evoked

-Yr2 Neuro (7 decks)

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