Somatic Sensory System Flashcards
These are non-selective cation channels
Mechanosensitive ion channels (mechanoreceptors)
Released to signal moderate to intense pain
Substance P
Makes pain worse through spread of pain signaling molecules
Hyperalgesia
Bring info to brain stem or spinal cord
Primary afferent axons
When these are deformed, mechanosensitive ion channels open, depolarizing receptor potential
Pacinian corpuscle
What type of threshold do mechanosensitive ion channels have?
Low mechanical threshold. Doesn’t take a lot to open
Caused by varicella zoster virus which causes chicken pox. Infection of dorsal root ganglion that remains for life and infects along dermatomes
Shingles
Sensation in face carried through these
Trigeminal nerves (CN 5)
The NT for nociceptor neurons in the spinal cord
Glutamate
Block glutamate release and hyperpolarize neurons. Placebo affect mediated through these
Endogenous opioids
The cortical somatotopy map is called this
Homunculus
These are segregated in the VP nucleus of the thalamus
Touch and pain
Dull, longer pain produced by c fibers
Second pain
Projections to large areas of the cortex, more so than mechanosensory pathways
Pain projections (spinothalamic and trigeminal)
This depolarized nociceptors, thus inflammatory responses which lead to its release are painful
Histamine
How is touch info transmitted into the spine?
At specific levels depending on the area of the body from with the stimulus originates (build a somatotopic map)
Powerful analgesics, also produce mood changes, nausea, drowsiness, mental stupor, etc. Bind to opioid receptors, widely expressed in pain pathways
Opioids
Multiple types of these are expressed in one neuron sensitive to it unlike light and odorants
Pain sensors in pair sensitive neurons
Cells with similar characteristics exist in these in S1
Vertical columns
Multiple maps exist here
S1
Extreme heat or cold
Thermal
Where is crossing over in the dorsal column medial lemniscal pathway?
Medial lemniscus
A sensory instrument different than the other mechanoreceptors
Hair
Caused by a number of conditions like allergies, infections, cancer, iron deficiency, liver disease, stress
Itch
Enter spinal cord same as cutaneous, from the same level of the spine in the same dorsal root ganglion
Visceral nociceptor axons
Detects pain and temp, decussate immediately and run in ventrally in the tract, dont synapse until thalamus, don’t communicate with medial lemniscus (but close by)
Spinothalamic pathway
Have small receptive fields (Mechanoreceptors)
Meissners corpuscles and Merkels discs
Have varying diameters and size which are correlated with type of receptor
Primary afferent axons
Studies have recently shown this could occur in humans as seen in non human primates
Somatosensory plasticity
SLIDE 65
SLIDE 65
Rat facial whiskers that are important sensory instruments that were used to demonstrate the plasticity of the somatosensory system
Vibrissae
Primary afferents projecting to the brain
A beta neurons
Model for how stimulation of touch pathway can reduce pain
Gate theory of pain (Melzack and Wall)
Lactic acid production leads to this build up in extracellular fluid
H
Protects, prevents evaporation. Can detect dot only .006 mm x .04 mm
Touch, skin
Synapse on second order sensory neurons and also send processes to the brain
Large A beta touch neurons
Can make nociceptors more sensitive, not made by them
Prostaglandins
Can adapt firing rates after long stimuli
Thermoreceptors
SLIDE 20
SLIDE 20
Can be hairy or glabrous
Skin
These also travel in the dorsal column along with A beta neurons
Second order axons
These depolarize when stretched or deformed
Mechanically gated channels
What do adjacent dorsal roots innervate?
Overlapping areas
This map is not continuous or to scale, recent studies show that the genitals map to a location more consistent with body location
Cortical somatotopy
Type of inhibitory interaction that can take place in dorsal column nuclei and thalamic nuclei that promotes contrast enhancement
Later inhibition
This travels through the medulla, pons, and midbrain to the ventral posterior (VP) nucleus of the thalamus in the dorsal column medial lemniscal pathway
Medial lemniscus
These aren’t simply relays, they alter info as it passes through
Dorsal column nuclei and thalamic nuclei
Free endings of unmyelinated C fibers and lightly myelinated alpha delta neurons
Nociceptors
Cross talk between these pathways occurs in the spinal cord
Pain and touch
Most are polymodal, respond to thermal, chemical, mechanical (multiple signals)
Nociceptors
Enter the spinal cord at dorsal roots
Primary afferent axons
In mice, these are each represented in S1 by a barrel or group of neurons
Mouse facial whiskers
These are slow or fast adapting (Mechanoreceptors)
Hair follicles
Inability to recognize objects by touch (posterior parietal cortex)
Astereoagnosia
Naloxone is antagonist (theoretically inc pain)
Endogenous opioids
Second order neurons that decussate and connect to thalamus via trigeminal lemniscus
Trigeminal pain pathway
Layer 4 neurons of S1 project to these
Other layers
Different ones of these detect different stimuli
Touch receptors
How are mechanosensitive ion channels opened?
With a force from inside the membrane, from extracellular proteins, from intracellular proteins, etc
Some pain meds can cause this, pain can also suppress it
Itch
Can be invoked by stimulating regions whose S1 representations border those of the lost limb
Phantom limb
Have large receptive fields (Mechanoreceptors)
Pacinian corpuscles and Ruffinis endings
Varies 20 fold across the body with fingertips being the most sensitive
Two point discrimination
Cause damage and are sensed as pain
temp extremes
Changes based on loss or increased use and can use functional imaging to study in humans
Somatotopic map
The ascending pain pathway
Spinothalamic pathway
These are overrepresented in the somatosensory map in mice, reflecting the use and importance of whiskers
Barrels
Small diameter fibers in trigeminal nerve synapse on neurons in spinal trigeminal nucleus in Brian stem
Trigeminal pain pathway
SLIDE 69
SLIDE 69
Vary in stimulus frequencies, pressures, receptive field sizes, each has a preferred stimulus
Mechanoreceptors of the skin
Have more mechanoreceptors, small receptive fields, more brain power, high resolution mechanisms
Fingertips
Par of body or world ignored (posterior parietal cortex)
Neglect syndromes
How does somatic info enter the spinal cord?
At the appropriate level (somatotopic mapping)
What are the 4 groups of spinal segments?
Cervical (c) 1-8
Thoracic (t) 1-12
Lumbar (l) 1-5
Sacral (s) 1-5
Sensory proprioceptors of skeletal muscle
A alpha
Warm receptors to these neurons only
C fibers
These project to dorsal horn and depress nociceptive neuron activity in descending pain control pathways
Raphe (serotonergic) neurons
Types of pain perceived without nociceptor activation
Chronic pain, emotional pain
SLIDE 49
SLIDE 49
Strong pressure
Mechanical
Some spots of the skin are sensitive to these, but not both
Hot or cold
Primary afferent axons with high mechanical thresholds
A delta and c
Fast, sharp pain due to a delta fibers
First pain
Slowest, small, unmyelinated afferent axons
C fibers
The columns of the spinal cord contain
Fibers
Sensory pain, temperature, and itch
c
Since dorsal roots innervate overlapping areas, what would need to happen to lose all sensation from an area?
All the dorsal roots would need to be cut
Areas 5 and 7 with large receptive fields and elaborate stimulus preferences. Integrates with visual, attention, movement
Posterior parietal cortex
SLIDE 11
SLIDE 11
Electrical stimulation here causes sensation
S1
How many distinct TRP channels?
6
Above this temp, pain receptors active, not thermorecepotors
55 C
Synapse on ipsilateral trigeminal nucleus then decussate, then go to thalamus, then S1
Trigeminal touch pathway
Dorsal horn contains
Sensory neurons
These are part of the touch system
Hairs
What rule does cortical somatotopy follow?
Use it or lose it. More use = more representation
Has some sensation from CN 7, 9, 10 and have large diameter sensory nerves
Trigeminal touch pathway
Sensory mechanoreceptors of skin
A beta
Paraventricular and periaqueductal gray matter (PAG) neurons synapse on Raphe nucleus, suppress pain
Descending pain control pathways
Can trigger itch
Histamine
Highly responsive to somatosensory input, but no other senses
S1
Thalamus projects to these S1 areas
3a (body position) and 3b
Ventral horn contains
Motor neurons
SLIDE 45
SLIDE 45
The first demonstration of cortical use dependent plasticity
Mouse whiskers (vibrissae)
Posterior parietal cortex areas that also process touch info but more deeper analysis with less mapping
Posterior parietal cortex areas 5 and 7
SLIDE 63
SLIDE 63
Lesions here impair somatic sensation
S1
These axons also synapse in intralaminar nuclei of the thalamus
Spinothalamic axons
Prolonged stimulation of these causes decrease in receptor potential, release reverses this
Pacinian corpsucle
How do A beta neurons travel to the brain in the dorsal column medial lemniscal pathway?
On the ipsilateral side through the dorsal column (touch and limb position)
Mapped by S1 stimulation (Penfield) or by recording from S1 neurons after somatic stimulation, maps similar. This was done in awake patients undergoing brain surgery, there are no pain receptors in the brain
Cortical somatotopy
Histamine binding activates these itch channels which are also activated by heat and capsaicin
TRPV1
SLIDE 72
SLIDE 72
Inputs from these spread over larger region of thalamus than medial lemniscus
Spinothalamic tract and trigeminal lemniscus
Skin receptor that can be seen with the naked eye
Meissner’s corpuscle
These are fast adapting, the quickly stop paying attention to stimulus (Mechanoreceptors)
Meissners and Pacinian
Stimulate interneurons, excites it which means it releases more inhibitory NT to block pain
Gate theory of pain
In mice, these are adjusted depending on whisper use or if a whisker is removed
Responses and anatomy
Activated by methanol or below 250 C
TRPM8
Cross talk between these two in the spinal cord produces referred pain
Viscera and skin
Cold receptors to these neurons
A delta and c fibers
Are in most tissues like bone, meninges, but not brain
Nociceptors
Thalamic neurons send processes to the primary somatosensory cortex (S1) in this pathway
Dorsal column medial lemniscal pathway
Has immediate crossing over
Spinothalamic pathway
Carries pain ion from head, face, and neck
Trigeminal pain pathway
Differences in firing rates of thermoreceptors are most pronounced during and right after these
changes
What does the CNS not know about the stimulus that causes thermoreceptors to fire
It doesn’t know what kind of stimulus it is
Is a conscious awareness, perception
Pain
Are innervated by free nerve endings; bending of the hair causes changes in AP firing rate
Follicles
Number of these receptors and channels but not well understood
Itch
Intermediate zone contains
Interneurons
How does lateral inhibition work?
Amplification or inhibition by neighboring cells increases contrast enhancement to make it easier for brain to decode
SLIDE 61
SLIDE 61
What happens with a digit is stimulated by a spinning disk in monkeys?
The region of the cortex devoted to input from that digit expanded
These are slow adapting and pay attention to the stimulus longer (Mechanoreceptors)
Merkels and Ruffinis
Sensory pain and temperature
A delta
A one to one relationship exists between these two things
Spinal segments and dermatomes
Mechanoreceptors with nerve terminal, flattened epithelial cell
Merkel’s discs
Well prescribed in maps, info enters spine at specific places
Dermatomes
Primary afferent axons with low mechanical threshold
A beta
Different from touch in that stimuli have potential to cause tissue damage
pain
Contain C fibers and carry sensory info to spinal cord and brain stem
Primary afferent axons
These rest like pain pathways
Thermoreceptors
What does the selectivity of a mechanoreceptor axon depend on?
Structure of the ending
Are like bands on body surface, adjacent dorsal roots innervate overlapping areas
Dermatomes
What happens when a digit is removed from a monkey?
The neurons representing that digit in S1 responded to adjacent digits
Central circuits not understood for itch, this is a peripheral NT we know of
Glutamate
Spinal nerves pass through these in the vertebral column
Notches
What do other mechanoreceptors not named do?
Detect blood pressure, bladder extension, etc.
Show more cortex devoted to fingers of the left hand
Violinists
This happens in our brains to an extend due to many experiences
Remapping
These branch out in zone of Lissauer and synapse on substantial gelatinosa neurons
Primary afferents of nociceptor neurons
Receives input from VP of thalamus
S1
SLIDES 77 and 78
SLIDES 77 and 78
Carry touch info
Dorsal columns
This shows that receptive fields form orderly map, as in retina or auditory system
Cortical somatotopy
Oxygen deprivation, some chemicals, temperature extremes, mechanical stress
damaging stimuli (pain)
This is widespread in the brain (visual, auditory, motor)
Plasticity
How many pairs of dorsal and ventral roots in spinal cord?
30
Depletes substance P from nerve endings, but this is not thought to be its mechanism of action. It reduces pain by reducing the function of pain fibers through several mechanisms
Capsaicin
Innervate face, mouth, dura matter, tongue
Trigeminal nerves (CN 5)
Detect temp with specific membrane mechanisms
Thermoreceptors
Free, unmyelinated axons that detect pain
Nociceptors
Increased intensity, reduced threshold, spontaneous pain
Hyperalgesia
Activated above 430 C and capsaicin
TRPV1
These determine perception
Skin thermoreceptors
Formed form 20-70 layers fo connective tissue (viscous liquid in between) with nerve terminal in middle
Pacinian corpuscle
Cross talk here can produce referred pain, the feeling of pain in a different location from the actual body part with nociceptor signaling
Spinal cord
The size of this is determined by density and importance of input (hand, face, tongue, and lips are overrepresented)
Size of cortical sensory region
These contain temp sensitive neurons that help maintain stable body temp
Hypothalamus and spinal cord
Sensory process, can be controlled by higher centers, to detect pain
Nociception
Synapse on second order sensory neurons within the dorsal horn
Primary afferent axons
Like pain in that transmitted through thin axons (C fibers) and can be triggered by multiple stimuli
Itch
Area of skin innervated by both dorsal roots of single spinal segment
Dermatomes
Each has unmyelinated axon branches with mechanosenstive ion channels
Mechanoreceptors of the skin
SLIDE 29
SLIDE 29
Perception and interpretation of spatial relationships, body image, coordinated body movements. Somatosensory integrated especially with visual system, with others too
Posterior parietal cortex
Has 6 layers with thalamic input to layer 4
S1
Have a different pathway than mechanoreptors
Nociceptors (pain)
Can be as bad as chronic pain but carried by different axons
Chronic itch
These axons connect to dorsal column nuclei at medulla spinal cord junction
Dorsal column axons
pathways for touch and vibration different from pain and temperature pathway
Dorsal column medial lemniscal pathway
All sensory systems have this
Parallel processing of information
This has alternation columns of neurons receiving input from slowly adapting or fast adapting receptor input
S1
What type of signaling occurs through the molecules in pain?
Diffusible signaling
Endorphins are these, also expressed in pain pathways, in PAG etc produce analgesia
Endogenous opioids
What do different types of mechanoreceptors have?
Different preffered stimuli
Most touch info processing in the cortex, located in parietal lobe, S1, Brodmanns 3B
Somatosensory cortex
Pain and touch synapse on different neurons here
VP nucleus in thalamus
Above this temp, normal thermoreceptors don’t respond and pain receptors do
430 C
These are thought to encode the touch responsive mechanosensitive ion channels
Piezo genes
A single one of these can give info about intensity, position, and duration of stimulus
Single receptor
3 other types of mechanoreceptors
Pacinian corpuscle
Ruffini’s endings
Meissner’s corpuscles
SLIDE 14
SLIDE 14
Opiate receptors are present on these neurons
Itch detecting neurons
A combination of receptors is firing during various temp ranges. The brain has to decode this population of information to give us our conscious perception of a specific temp
Population coding
Largest sensory organ
Touch
Made by nociceptors and can make them more sensitive
Bradykinin, substance P
Part of S1 that process somatosensory info
1, 2, 3a
These cell bodies exist in dorsal root ganglion
Nociceptive neurons in the spinal cord
This influences input via descending tracts
Cortex
Dorsal column axons ascend in this after crossing over
Medial lemniscus
Some channels open in response to proteases (ATP, K, bradykinin, H) - what are all these?
Pain signals
Reduced by A beta fiber activity (rubbing or stimulating touch pathways)
Pain
S1 area 3b connects to these areas proven by lesions
1 (texture information)
2 (size and shape information)
Mechanoreceptor with nerve terminal like balls of string
Krasue end bulbs
