Lecture 13 & 14 - Somatosensory System Flashcards
What are the 2 types of sensory systems? Do both of them have CNS and PSN components?
- General 2. Special YUP
What is another name for our general sensory systems?
Somato sensory systems
What are the 2 general sensory systems?
- Touch/proprioception 2. Temperature/pain
What is proprioception?
Sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement
What are the 5 special sensory systems and each of the stimulus they transduce?
- Gustatory: water soluble molecules 2. Olfactory: odorant molecules 3. Visual: electromagnetic radiation 4. Auditory: mechanoreceptive stimulus 5. Vestibular: mechanoreceptive stimulus
What is the main role of sensory systems?
Transduces stimuli into electrical signals
By what is pain and temperature sensed?
Free nerve endings
By what is touch/vibration sensed on the skin?
4 main types of mechanoreceptors:
- Meissner corpuscle
- Pacinian corpuscle
- Merkel’s corpuscle
- Ruffini endings
How are the special senses distributed throughout the body?
Discretely distributed receptors
How are the general senses distributed throughout the body?
Broadly distributed receptors
Do the special senses have unimodal or polymodal receptors? What does this mean?
Unimodal receptors: one type of receptor for each sense
Do the general senses have unimodal or polymodal receptors? What does this mean?
Polymodal receptors mostly: each receptor has multiple functions
What 5 stimuli can somatosensory receptors perceive?
- Pressure 2. Vibration 3. Temperature 4. Cellular distress 5. Other chemical signals
Where are afferent somatosensory neurons found in the PNS? 2 places
- Spinal nerves 2. Cranial nerves
What are the 2 specialized terminal ends of afferent somatosensory neurons?
- Mechanoreceptors 2. Nocireceptors
What type of receptors are mechanoreceptors? What senses do they perceive?
Mechanically gated ion channels Touch/vibration and proprioception
Are mechanoreceptor terminated afferent axons myelinated?
YUP
What type of receptors are nociceptors? What senses do they perceive?
Polymodal receptors Temperature and pain
Are nociceptor terminated afferent axons myelinated?
NOPE or very thinly
What are the 4 CNS structures that play a role in the somatosensory systems?
- Dorsal horns 2. Brainstem 3. Thalamus 4. Cortex
What is this?
Pacinian corpuscle
What is this?
Free nerve ending
Where are nerve endings located in the skin?
Superficial dermis
What is this? Numbers?
Nerve ending
- Dermis
- Epidermis
Which is close to skin surface: dermis or epidermis?
Epidermis
What is a receptive field test?
Stimulus probe is placed on the hand and electrical signals along the median nerve in the arm are recorded
How do the stimuli recorded in the receptive field tests evolve overtime? What does this demonstrate?
Bursts of APs that slow over time ⇒ touch adaptation
Meissner’s corpuscles:
- Location?
- Encapsulated or not?
- Structure?
- Size of receptive field?
- What type of sense does it perceive?
- Touch adaptation speed?
- Receptive field test result?
- Superficial dermis
- Encapsulated
- Single branched axon women into a connective tissue sheath
- Small receptive field
- Light superficial touch
- Rapid touch adaptation
- Small AP burst that decreases quickly
What’s this?
Meissner’s corpuscle
Merkel’s disks
- Location?
- Encapsulated or not?
- Structure?
- Size of receptive field?
- What type of sense does it perceive?
- Touch adaptation speed?
- Receptive field test result?
- Extreme superficial dermis
- Unencapsulated
- Contain several mitochondria and branch outward away from the epithelial tissue
- Small receptive field
- Sustained light pressure
- Slow touch adaptation
- Large AP burst that slowly decreases
Pacinian corpuscles:
- Location?
- Encapsulated or not?
- Structure?
- Size of receptive field?
- What type of sense does it perceive?
- Touch adaptation speed?
- Receptive field test result?
- Deeper in dermis than Meissner’s corpuscles and Merkel’s disks
- Encapsulated
- Branched axon in the center of the multi layer encapsulation = red onion appearance
- Large receptive field
- Deep touch
- Rapid touch adaptation
- Large AP burst with rapid decrease
Ruffini’s endings
- Location?
- Encapsulated or not?
- Structure?
- Size of receptive field?
- What type of sense does it perceive?
- Touch adaptation speed?
- Receptive field test result?
- Deeper in dermis than Meissner’s corpuscles and Merkel’s disks
- Encapsulated
- Inner bundle of branched axons with one large capsule around it
- Large receptive field
- Sustained deep touch
- Slow touch adaptation
- Large burst of APs that decrease slowly
What is this (yellow part)?
Merkel’s disks
What is this on the right?
Pacinian corpuscle
What’s this?
Ruffini’s endings
What’s this?
Ruffini’s endings
What is the 2 point discrimination? What 2 factors is it dependent upon?
At various parts of the body we can distinguish 2 points very close together (eg: digits), whereas at other parts of the body (eg: back and calves) sensations must be further from one another to be distinguishable
Dependent on:
- Receptive field size: the smaller, the greater acuity
- Receptor density: the higher the density the greater the acuity
What is lateral inhibition? How does this affect two point discrimination? What does it explain?
Individual cell sends APs via afferent neurons and inhibits this in neighboring lateral axons
This is what allows 2 point discrimination and explains why there is a more robust signal in the center of the receptive field
What are the 3 proposed theories to explain how mechanoreceptors work?
- The membrane bound channels open because they are spread apart when tissue is stretched
- The membrane bound channels open because they are anchored to proteins which pull them apart
- The membrane bound channels open because there is a second messenger system that facilites this in response to mechanical stimuli
Are free nerve endings encapsulated?
NOPE
What are the smallest somatosensory fibers?
Nocireceptor terminated fibers
What are the 3 types of unimodal nociceptors?
- Mechanoreceptive
- Chemoreceptive
- Thermoreceptive
Are most nociceptors unimodal or polymodal?
Polymodal
What receptors are sensitive to temperature? Aka thermoreceptive nociceptors?
Transient receptor potential ion channels (TRP channels)
How many human TRP channels are there?
28
How many families of TRP channels are there? What are the 3 most important ones? What are they classified by?
6 families
3 most important:
- Akyrin TRP channels (TRPA)
- Melastatin TRP channels (TRPM)
- Vanilloid TRP channels (TRPV)
Classified by temperature range
What chemicals do TRP channels also respond to?
Chemicals such as allicin, menthol, camphor, and capsaicin
How many transmembrane domains do TRP channels have?
7
What are chemoreceptive nociceptive receptors sensitive to?
- NTs
- Chemicals released during cellular damage
What is an example of 5 chemicals released during cellular damage?
- Histamine
- Prostaglandins
- Substance P
- Bradykinin
- K+
What does substance P on chemoreceptive nociceptors cause?
Hyperalgesia = abnormally heightened sensitivity to pain
How do NSAID’s work?
They prevent the stimulation of chemoreceptive nociceptors by prostaglandins by preventing their production:
they inhibit cyclooxygenase 2 which converts arachidonic acid into prostaglandins and thromboxanes
Describe the pathway of somatosensory information from somatosensory receptors to the CNS.
Sensory receptor → Dorsal root ganglion → Dorsal root → Dorsal horn of the spinal cord → CNS
What are afferent fibers classified according to?
- Size
- Level of myelination
What are the 4 types of afferent fibers?
- A alpha
- A beta
- A delta
- C
A alpha afferent fibers:
- Speed (in m/s)
- Level of myelination
- Function
- 80-120 m/s
- Highly myelinated
- Proprioreceptors of skeletal muscle
A beta afferent fibers:
- Speed (in m/s)
- Level of myelination
- Function
- 35-75 m/s
- Moderate myelination
- Mechanoreceptors of skin
A delta afferent fibers:
- Speed (in m/s)
- Level of myelination
- Function
- 5-30 m/s
- Low myelination
- Pain and temperature perception
C afferent fibers:
- Speed (in m/s)
- Level of myelination
- Function
- 0.5-2 m/s
- No myelination
- Temperature, pain, and itch perception
What are the 2 types of pain? What type of afferent fiber is each felt by?
- First pain: A delta fibers
- Second pain: C fibers (persisting pain)
Does the somatosensory system display dermatomes?
YUP
What are the 3 ascending somatosensory pathways? Where is each located? What sensation(s) does each PRIMARILY perceive?
- Dorsal column-medial lemniscus pathway: body → touch and proprioception
- Spinothalamic pathway: body → pain and temperature
- Trigerminal pathway: face → all types
Describe the dorsal column-medial lemniscus pathway.
Afferent fibers in dorsal root ganglion → dorsal root axons in dorsal column: cuneate fasciculus (upper limbs) + gracile fasciculus (lower limbs) → synapse in dorsal column nuclei in medulla (cuneate and gracile) → internal arcuate fibers with decussation → medial lemniscus (through pons) → synapse in ventral posterior nucleus of thalamus (VPN) → synapse in primary somatosensory cortex (S1)
What does nociception refer to?
Pain + temperature
Describe the spinothalamic pathway.
Afferent fibers in dorsal root ganglion → dorsal root axons synapse in dorsal horn of spinal cord → decussation at spinal cord level → lateral spinothalamic tract (through medulla) → synapse in ventral posterior nucleus of thalamus (VPN) → synapse in primary somatosensory cortex (S1)
Describe the trigeminal pathway.
3 branches of trigeminal nerve → trigeminal ganglion → synapse trigeminal nucleus in pons → decussation at the pons → trigeminothalamic tract → synapse in ventral posterior nucleus of thalamus (VPN) → synapse in primary somatosensory cortex (S1)
From what side of the body or face does S1 receive input form?
Contralateral side
At which point of the 3 ascending somatosensory pathways do we become conscious of the somatosensory input?
S1
Where is S1 located?
Parietal lobe, caudal to the postcentral gyrus
What Brodman areas does S1 contain?
- 1
- 2
- 3a
- 3b
Where is the secondary somatosensory cortex? What is another name for it? What Brodman areas does it contain?
Posterior parietal cortex (PPC)
Caudal to S1
- 5
- 7
What is a homunculus? 2 types? 3 main categories?
Neurological “map” of the anatomical divisions of the body. Sensory and motor types
- Trunk/extremities
- Face
- Interior of mouth/abdominal organs
What is the main somatosensory receptors on rodents’ noses? How are these innervated?
Whiskers = vibrissae
Each has 1 somatosensory barrel leading up to S1
What are the 4 defining characteristics of the primary sensory system?
- Receives dense direct input from thalamus
- Neurons are unisensory
- Lesions produce unisensory impairment
- Electric stimulation evokes unisensory perception
What is phantom limb syndrome? What is it due to?
This occurs when a patient has an amputated extremity but still feels a vivid/painful impression that the limb is still present.
Due to a reorganization of the cortical map (homunculus) where neighboring regions of S1 expand to cover the area that corresponded to that limb
Where is the olfactory complex?
Base of forebrain
Where is the auditory cortex?
Temporal love
Where is the visual cortex located?
Occipital lobe
What is agnosia? What is it due to?
What can it cause?
Inability to attend to/recognize contralateral stimuli despite sensory and motor systems being intact (no memory loss) due to damage to the PPC
Can cause ADD
What are the 2 types of agnosias? 2 names for each
- Tactile = astereognosis: inability to identify an object by touch only
- Visual agnosia = neglect syndome: inability to identify an object by vision only
On what 5 parts of the NS do opoids act?
- Forebrain
- Midbrain
- Brainstem
- Spinal cord dorsal horns
- Peripheral nerves
Can our thoughts control pain? Explain.
YUP
Perception of pain is subject to central modulation: top down influence and is context dependent (eg: emotions and activities can influence pain levels)
What is the gate theory of pain? How does this work?
Non-painful input (like rubbing) closes the “gates” to painful input, which prevents and suppresses the pain sensation from traveling to the CNS
A beta fiber from skin mechanoreceptors stimulates an inhibitory interneuron that connects to a second-order C afferent neuron
What is congenital analgesia? What is it due to?
Mutation in the SCN9A gene which is responsible for the nociceptive VG-Na+ channels → inability to feel pain and a person can end up not realizing they have caused tissue damage. This is very dangerous!!
How do the gracile fasciculus and the cuneate fasciculus differ?
Cuneate mediates both vibratory and position sense
Gracile only mediates position
Does the spinothalamic tract have tactile function?
Yes but way less fine resolution
By what mechanoreceptor is vibration sensed?
Pacinian corpuscles
What sensory receptors
What mechanoreceptors senses proprioception in distal joints?
Ruffini receptors
