Somatosensory Receptors Flashcards
(79 cards)
1
Q
What are sensory receptors?
A
- specialised cells providing the central nervous system with information about the world outside & inside our body
-respond to several types of stimuli
2
Q
What do sensory receptors do?
A
- convert stimuli into action potentials through the process of transduction, that carry information to the central nervous system
- central nervous system interprets this information as various sensations
3
Q
What are the somatic senses?
A
- pain
- temperature
- touch
- pressure
- vibration
- proprioception
4
Q
What are the special senses?
A
- vision
- hearing
- smell
- taste
- equilibrium
5
Q
What do primary Afferent fibres do?
A
- carrying their action potentials away from receptors
6
Q
The larger the diameter of the axon & the amount of insulating myelin….the…..
A
- faster the conduction velocity
7
Q
1st order neurones - primary Afferent:
A
- nerves with receptor endings
- cell bodies = in dorsal root ganglia
- enter spina cord via dorsal roots
8
Q
1st order neurones synapse with 2nd order neurones, what do these do?
A
- travel to the brain via two primary ascending tracts
9
Q
What do the 3rd order neurone do?
A
- connect to the cerebral cortex
10
Q
How are primary Afferent neurones classified?
A
- conduction velocity & their diameter
- there are four types of
11
Q
1st type of primary Afferent neurone:
A
A-alpha:
Diameter = 12-20um
Conduction velocity = 70-120m/s
Role & receptors = muscle spindle, Golgi tendon organ, touch and pressure
- fastest conduction velocity
12
Q
2nd type of primary Afferent neurone:
A
A-beta:
Diameter = 5-14um
Conduction velocity = 30-70m/s
Role & receptors = touch, pressure, vibration
13
Q
3rd type of primary Afferent neurone:
A
A-sigma:
Diameter = 2-7um
Conduction velocity = 12-30m/s
Role / receptors = touch & pressure, pain & temperature
14
Q
4th type of primary Afferent neurone:
A
C
Diameter = 0.5-1um
Conduction velocity = 0.5-2m/s
Role / receptors = pain and temperature
- unmyelinated
- slowest conduction velocity = slow pain & temperature
15
Q
Pathway to the spine:
A
- Afferent information from the receptor travels to the brain by ascending pathways
- the primary Afferent nerve enters the spine via the dorsal root, the cell body for the nerve is outside the spinal cord
- in the dorsal root ganglion
- slower axons travel to lamina I and III, in more dorsal regions
- faster conducting axons travel into deeper ventral lamina = III-IX
- these can synapse with secondary neurones that travel up the cord to the brain
- or they can ascend without synapsing
16
Q
What are dermatomes?
A
- an area of skin sensation associated with a particular spinal level of Afferent entry
17
Q
The sensory areas in the skin in a dermatome associated with a particular spinal level can be seen in what disease?
A
- shingles
18
Q
How does dermatomes and shingles work:
A
- herpes zoster virus resides in the primary Afferent cell bodies
- in a dorsal root ganglion of a particular spinal nerve
- the virus can become active, travels down the primary Afferent
- to cause pain and blistering in the ends of the Afferent nerves
- these specific receptor regions then become visible
19
Q
Why is referred pain important?
A
- Afferents from several organs enter the spinal cord at specific levels,
- along with somatosensory input from the dermatomes for that level of entry
- these organs don’t have somatotrophic representation in the cortex
- when activity is stimulated, they are interpreted as pain coming from somatosensory regions of dermatomes
20
Q
What is an example of referred pain?
A
- narrowed arteries to cardiac tissue result in hypoxic myocardium
- afferents along the myocardium are stimulated
- they enter the spinal cord at levels T1-T4
- this is interpreted as pain arising from dermatomes associated with T1-T4
= chest and inner part of left arm
21
Q
What are the two main tracts / columns / sensory pathways through the spinal cord?
A
- spinothalamic tract
- dorsal column
22
Q
Where do sensory axons go?
A
- up the spinal cord
- ascending
- in discrete pathways
23
Q
Where do motor axons go?
A
- down the spinal cord
- descending
- in different discrete pathways
24
Q
What does the spinothalamic tract do?
A
- carries information from temperature, pain, crude touch & pressure receptors
25
What do the 2nd order of neurones do in the spinothalamic tract?
- after synapsing in the spine, the secondary neurone crosses in the spine
- to ascend contralaterally in the spinothalamic tract to the thalamus of the mid brain
26
What does the information going through the spinothalamic tract have?
- poor spatial discrimination
- transmitted with slower conduction velocities
- 0.5-40m/s in smaller fibres
27
What happens after the third synapse in the spinothalamic tract?
- the tertiary neurone travels to the sensory cortex
- the sensation becomes conscious
28
What does the dorsal colum do?
- carries information from proprioreceptors, fine touch and discrimination
29
What is the conduction velocity in the dorsal column?
- 40-100m/s
30
What happens in the dorsal column?
- primary afferents do not synapse
- they ascend ispilaterally
- they do not cross in the spine
- they ascend to dorsal column nucleus in the brainstem
- this synapses with the 2nd neurone, that crosses to the other side within the medulla
- where it synapses with the 3rd neurone in the thalamus, then to the sensory cortex
31
Where does the somatosensory information arrive in the somatosensory cortex?
- sensory cortical areas
- in the post central gyrus of the parietal lobe
- where activity results in conscious sensation
32
What does the cortex show and why?
- somatotrophic representation
- because different parts of the body are represented in different parts of the cortical areas
33
What does the amount of cortex given to processing a particular sensation mean?
- reflects the density of receptors
- sensitivity of that region
34
What is the sensory homonculus?
- the map of sensations associated with different areas of the cortex
- more cortex is dedicated to processing sensitive areas
= fingers, lips, tongue
35
What is it important to remember?
- somatic sensations from one side of the body
- are processed by the cortex on the opposite side of the brain
36
An example of a somatosensory defect:
- if a specific peripheral nerve is cut by trauma
- sensation will be lost from the receptive fields associated with the primary Afferent axons, in the damaged nerve
37
How does more widespread damage of primary afferent nerves happen?
- as a result of neuropathy
- brain / spine lesions
38
Gradual damage to primary afferent nerves by:
- trauma
- nerve root damage
- neuropathy
- what are these due to?
- diabetes
- multiple sclerosis
- chronic excessive alcohol consumption
- chemotherapy
39
What do these common causes lead to?
- pins and needles
- numbness
- modalities of sensation are affected
40
How else can the primary afferent neurones be damaged?
- the spinal root is damaged by trauma or tumour
- the areas of skin where the dermatome is affected, can point out which spinal root is affected
41
What is the most common type of ascending tract damage?
- bilateral spinal damage
42
What is bilateral spinal damage?
- causes sensory loss of all modalities below the level of the lesion
43
What happens if a unilateral lesion occurs?
- rare
- some senses are lost on the side of the lesion
- other senes can be lost on the opposite side
44
What is somatosensory damage within the brain caused by?
- stroke
- damaging the cortex
- damaging the tracts from the thalamus to the cortex
45
What does somatosensory damage to the brain cause?
- causes sensory loss from opposite sides of the body
- example = brain bleed
- can potentially cause paralysis if motor areas are affected
46
What is pain?
- a sensation that indicates damage or potential damage to body tissues
- detected by a specific receptor
- NOT just an intense stimulation of other receptor types
47
Pain receptors have free nerve endings that respond to:
- chemical associated with tissue damage or inflammation = potassium, prostaglandins, ATP, adenosine, 5-HT
- mechanical stress associated with tissue damage
- temperatures above 43 degrees, or temperatures below 18 degrees
48
How pain is handled in the spine can be modified by:
- other neural activity
- inflammatory mediators
- drugs
49
What is the axon type that pain receptors can be attached to?
- slow pain = conducted by slow conduction velocity axons = type C
- fast pain = conducted by fast conduction velocity axons = type A
50
What is somatic pain?
- superficial = from pain receptors in the skin
- deep = from pain receptors in underlying muscles, joints & bones
51
What is visceral pain?
- pain from other internal organs
52
What are causes of visceral pain?
- ischaemia
- inflammation
- excess stretching or contraction of hollow organs
- difficult to localize
53
What is referred pain?
- stimulation of pain receptors from visceral organs
- first order nerves synapse with second order nerves from somatic nerves
- entering the spine at that level
- the brain interprets sensation as if arising from the somatic Afferent nerves
54
What is fast pain?
- starts and stops quickly
- localised, identifiable, distinctive
- only relevant to skin
- conducted by alpha-sigma fibres = myelinated
55
What is slow pain?
- diffuses
- aching / burning / throbbing
- persists for hours or days
- superficial or deep structures
- conducted by C fibres = unmyelinated
56
Central pathways in pain perception 1:
1. Pain receptor is stimulated
2. Action potentials travel along the primary afferent nerves to the spinal cord
3. This triggers spinal reflexes, such as a limb withdrawal - controlled by neuronal circuits in the spinal level
4. Does not rely on descending inputs from the brain
57
Central pathways in pain perception 2:
- second order nerves ascend the cord in the spinothalamic tract and synapse in the thalamus
58
Central pathways in pain perception 3:
1. Pathways from the thalamus lead to reticular formation, increasing arousal and awareness
2. Then relay to the hypothalamic defence area to initiate autonomic responses associated with pain
3. Pupillary dilation, pallor, urination, sweating, nausea
4. Cardiovascular responses = bradycardia, hypotension
5. Pathways from the reticular formation also access the sensory cortex, pain becomes a fully conscious sensation
59
Central pathways in pain perception 4:
1. The sensory cortex is also accessed by a more direct path from the thalamus
2. Sensory cortical activation allows localisation of the pain in the parietal cortex
3. The frontal lobe contributes to the distress caused by
60
What are nociceptors?
- bare nerve endings
- dense distribution in skin
- receptive to specific stimuli
- some are multimodal
- prostaglandins sensitise them
- prostaglandin inhibitors = analgesic
61
What are nociceptors activated by?
- potentially damaging stimulus
= trauma, heat, tissue damage
62
What happens when nociceptors are activated?
- action potentials are sent in the primary afferent nerves
- C or A
- to the spinal cord
- they synapse with second order nerves that ascend into the spinothalamic tract to the brain
- pain becomes conscious when it reaches the sensory cortex
63
Can ascending pain signal be altered?
- several ways
64
What happens as part of the inflammatory response?
- chemicals such as prostaglandins and kinins are released from mast cells and white blood cells
- they sensitise the nociceptor and make it more responsive
65
What does the primary afferent nerve then do?
- A type
- on entering the spinal cord, may send off a branching axon
66
What does sending off a branching axon do?
- synapses with an inhibitory interneurone, exciting it
- this interneurone synapses between the peripheral nociceptor and the ascending second order neurone
67
What happens after it synapses?
- the transmitter released is inhibitory
- reduces the transmitter released from the primary Afferent nerve
- this is pre synaptic inhibition
- this closes the gate
- limiting the pain signals passing
68
How else can the same interneurone be accessed?
- by primary afferent nerves from mechanoreceptors
- A beta type
- rubbing the skin activates it
- so it stops pain signals, as you are rubbing it better
69
What happens when the pain signal reaches the hypothalamus, which is part of the limbic system?
- a descending pathway is activated
- this can access the inhibitory interneurone
- limits the transmission of pain
70
What neurotransmitters are involved in the descending pathway?
- endorphins
- they bind to several opioid receptor subtypes
- serotonin as well
71
What is analgesia?
- parts of pain pathways that can be interrupted to reduce pain
72
1. What happens when pain afferents can access inhibitory neurones in the spinal cord?
- when excited, inhibits the transmission of the synapse between the first order Afferent & second order neurone
- reducing pain at the spinal level
73
2. At the level of the receptor: what are the analgesic drugs that reduce pain by inhibiting the production of prostaglandins?
- paracetamol
- aspirin
- NSAID
- inhibit cyclo-oxygenate enzymes producing the prostaglandins
- reduce inflammation & fever, as they inhibit prostaglandins in the hypothalamus
74
3. At the level of the peripheral nerves:
- local anaesthetics are used to block afferent pain signal in the primary afferent nerve
75
4. At the level of the central nervous system:
- these agents can be injected into the spine for a spinal block
- to block pain signals at this point
76
5. What happens when stimulation of A-beta nerves from mechanoreceptors happens?
- stimulate the inhibitory interneurone in the spinal cord
- when its active, will inhibit the transmission of pain from the primary afferent nerve to the second order nerve ascending the cord
- blocking pain transmission at a spinal level
77
6. How else can this inhibitory neurone be accessed?
- limbic pathway, descending
- once the pain signal has ascended to the brain and is processed there
78
7. What drugs can interfere with pain pathways in the brain?
Opiates = inhibit pain pathways in the brain and spinal cord by presynaptic inhibition
- opioid & serotonin receptors
- ketamine is an example
79
What does general anaesthesia do for pain?
- switches off the cortex
- so no pain is perceived while unconscious
