10/23 - Sensory System - Peripheral Receptors Flashcards
“soma”
“soma” = “body,” in Greek
The cell bodies of all neurons involved in primary processing of sensory information are located ______ .
in a GANGLION located somewhere in the periphery.
For the body, the cell bodies of all neurons involved in primary processing of sensory information are located _____ .
DORSAL ROOT GANGLIA.
For structures in the head. the cell bodies of all neurons involved in primary processing of sensory information are located ______ .
in ganglia related to each cranial nerve.
For example, neurons carrying sensations from the face will be in the TRIGEMINAL GANGLION.
Somatosensory System
Collective category for all sensations that are NOT vision, hearing, taste, olfaction or vestibular sense of balance (i.e., “Special Senses”)
Receptors are distributed throughout the body – external & internal.
Somatosensory System:
4 Senses
Touch
Body Position
Pain
Temperature
Divide into 2 subgroups:
Touch & Body Position
Pain & Temperature
largest organ in body
Skin = largest organ in body (3000 mm2, 6ft person)
All of the skin is innervated, but to varying degrees
The Mechanical Senses
Touch (Somatosensory)
Proprioception
Thermal
TEMPERATURE
Chemical
Pain
Itch
Itch is often considered a category of pain.
Different forms of energy elicit distinct types of somatosensory perception
Same type of physical energy can elicit different sensations depending on type & location of receptor & stimulus intensity.
Different forms of physical energy: mechanical, thermal, and chemical give rise to distinct types of precepts which group into broad categories known as submodalities: touch, proprioception, thermal, pain & itch, and arguably certain visceral sensations are all different submodalities.
For example, gentle mechanical stimuli give rise to touch sensations, like tapping, stroking or pressure, whereas very intense mechanical stimuli can give rise to pain.
Mechanoreceptors
low threshold
can detect minute forces impinging on skin (exteroceptive)
Our ability to sense touch relies on mechanoreceptors.
These receptors can detect very minute forces impinging on the skin.
Proprioceptors
low threshold
can detect small changes in muscle tension
Receptor
located on primary afferent ending itself
stretch sensitive ion channel
Cell body
in DRG or Trigeminal ganglion
DRG NEURON
Peripheral process:
Large diameter, heavily myelinated axons carry information from muscle spindles/joint receptors (proprioception)
Slightly smaller diameter, myelinated axons, carry information from cutaneous receptors (touch, vibration)
DRG NEURON
Central process
Enters through dorsal root and courses directly to posterior column where they ascend to the brainstem forming the fasciculus gracilis or fasciculus cuneatus.
What sensory axons are the largest sensory axons?
Proprioception axons
Consequently, they are also the fastest conducting.
Trigeminal Ganglion
Large structure that contains sensory neurons that send axons into the 3 divisions of the trigeminal nerve:
V1, V2, and V3.
V1 of the Trigeminal
Sensory innervation of orbit including eyeball.
Touch, pressure, pain. Also area around orbit externally including forehead.
Some trigeminal ganglion sensory neurons send their axons into V1. These axons enter the orbit above the muscles that elevate the eyelid or move the eye. They carry the sensory information from the orbit as well as the eye itself.
Think of touching the eye with your finger or putting in a contact lens. Also, putting drops in your eyes. The touch and cool sensations are carried to the CNS via branches of V1.
V2 of the Trigeminal
Sensory innervation of area around zygomatic arch, nasal cavity and upper teeth.
Some trigeminal ganglion sensory neurons send their axons into V2. These axons leave the skull through a separate opening and innervate structures such as the upper teeth, the area of the face over the zygomatic arch, the nasal cavity, and the hard and soft palates.
V3 of the Trigeminal
Sensory innervation area around mandible and chin, including lower teeth and tongue.
Some trigeminal ganglion sensory neurons send their axons into V3 which leaves through another opening.
This branch innervates the teeth in the mandible and the skin over the cheek, the tongue (general sensory only) and the chin.
This branch of the trigeminal nerve also contains axons that innervate muscles of mastication.
The neurons giving rise to these axons are located within the brainstem.
In addition to the peripheral processes (V1, V2, V3), the trigeminal ganglion cells give rise to a ____ that enters the CNS carrying sensory information from different regions of the face.
In addition to the peripheral processes (V1, V2, V3), the trigeminal ganglion cells give rise to a Central Process that enters the CNS carrying sensory information from different regions of the face.
Peripheral somatosensory nerve fibers vary in size
Specialization starts in the periphery.
Fibers of different diameter are associated with different submodalities.
A alpha
Aα
Group I
Largest in axon diameter of the 4 types.
Proprioceptors of Skeletal Muscle
Primary receptors of muscle spindle.
Golgi tendon organ.
A beta
Aβ
Group II
Mechanoreceptors of Skin
Touch
Secondary receptors of muscle spindle.
All cutaneous mechanoreceptors.
A delta
Aδ
Group III
Free nerve endings of touch, pain, and pressure.
Nociceptors of neospinothalamic tract.
Cold thermoreceptors.
C
Group IV
Nociceptors of paleospinothalamic tract
Warmth receptors
MECHANORECEPTORS
Receptors that are sensitive to physical distortion – bending, stretching, pressure, vibration.
In addition to the skin, they are present in organs such as the heart, intestines, bladder, and even teeth.
Consist of unmyelinated distal segment that is often surrounded by a fibrous capsule or related to some other specialization. The axons have mechanosensitive ion channels which depend on stretching or changes in tension for activation.
Sensory receptors transduce physical stimulus into electrical signal called the receptor or generator potential in response to skin distortion.
What are mechanoreceptors sensitive to?
Receptors that are sensitive to physical distortion
bending, stretching, pressure, vibration.
Where are mechanoreceptors found?
In addition to the skin, they are present in organs such as the heart, intestines, bladder, and even teeth.
How do mechanoreceptors respond to stimuli?
Sensory receptors transduce physical stimulus into electrical signal called the receptor or generator potential in response to skin distortion.
What components are mechanoreceptors composed of?
Consist of unmyelinated distal segment that is often surrounded by a fibrous capsule or related to some other specialization.
The axons have mechanosensitive ion channels which depend on stretching or changes in tension for activation.
Because they are unmyelinated, they are easily stimulated.
glabrous skin
external skin that is naturally hairless.
It is found on the ventral portion of the fingers, palmar surfaces of hands, soles of feet, lips.
What 2 Mechanoreceptors do NOT have a layered structure associated with it.
Ruffini and Merkel
What 2 Mechanoreceptors are situated superficially in the skin.
Merkel’s disks & Meissner’s corpuscles
Because of this superficial placement, they have limited branching, and are densely packed.
Ruffini endings
profusely branched & interspersed among collagen fibers of the extracellular matrix-
The lack of a layered structure, combined with the limited elasticity of collagen gives rise to the Slow Adapting nature of these receptors.
Merkel cells
simple cup shaped endings
They are so-named becuase they are closely associated with a specialized epi cell, MC, whose function is unknown.
Their relatively “naked” state thought to account for Slow Adapting nature.
Because pressure spreads out as move away from source ___ and ___ can be activated by stimuli arising from more distant stimuli.
Ruffini endings and
Pacinian corpuscles
What 2 Mechanoreceptors are situated deeper in the skin.
Pacinian corpuscles & ruffini’s endings deep
Because they are deep, they can also be sparse.
Which 2 have Mechanoreceptors small Receptive Fields.
Merkels and Meissner’s have small Receptive Fields due to a limited spread of their terminal endings.
They are densely packed in this skin so that they are sensitive to fine spatial details.
receptive field revisited….
Each sensory receptor has a receptive field.
This conveys information about the location of a stimulus.
For a cutaneous receptor, the receptive field is the area of skin where endings are located.
Receptive fields vary in size.
The ____ the receptive field the greater the discrimination of two points.
The smaller the receptive field the greater the discrimination of two points.
Two-Point Discrimination
Measure of spatial resolution
Areas with the finest resolution:
- Have higher density of mechanoreceptors in the skin
2. Are enriched with receptors with a Small receptive field.
Receptors with layered capsules are ______, due in large part to the mechanical properties of the capsules.
Receptors with layered capsules are Rapidly Adapting, due in large part to the mechanical properties of the capsules.
Merkel’s Disc
The ending is an expansion of a terminal of a sensory fiber, which is applied to the base of a specialized cell called a Merkel cell.
Pacinian Corpuscle:
Layering
PC is a more extreme version of a layered ending.
Often described as “onion-like” in cross-section.
Layers: thin epi cells with fluid in between.
Even more effective than Ruffini at rapidly displacing force.
ADAPTATION
Most receptors become less sensitive (i.e., fire fewer action potentials) during the course of a maintained stimulus.
Slowly Adapting
Very little change in firing rate during the course of a maintained stimulus.
Respond best to unchanging stimuli.
Detect static position.
Rapidly Adapting
Very quick decrease in firing rate during the course of a maintained stimulus.
Respond best to changing stimuli.
Detect change and movement of stimuli.
Rapid adaptation allows receptor to respond to high frequency stimulation. i.e., vibration.
Sensory receptors adapt to sustained inputs.
Some adapt slowly, others more rapidly.
Convey information on static position, like pressure.
Slowly adapting
convey information about the rate of change or movement of a stimulus (e.g. vibration)
Nociceptors are an exception to the rule. They adapt not at all or minimally to maintained stimuli.
Exception to the rules of ADAPTATION
Rapidly adapting
Pacinian Corpuscle:
Adaption
The capsule of a Pacinian corpuscle is not needed for mechanoreception, however, it is essential for fast adaptation.
Adaptation rate is slowed in the absence of a capsule.
Meissner’s corpuscles:
Adaption
Meissner’s corpuscles are also rapidly adapting.
They respond best to low frequency stimulation.
Meissner’s
Small Receptive Field Fast Adapting Superficial Velocity, Touch Encapsulated
Good spatial resolution combined with exquisite sensitivity to rapid displacement of skin at surface
Minute amount of indentation can be detected.
Fine tactile discrimination; grip control.
Velocity
Meissner’s
Displacement
Merkel & Ruffini Ending
Acceleration
Pacinian
Merkel
Small Receptive Field
Superficial
Slow Adapting
Displacement
High sensitivity (i.e., low force threshold) and exquisite spatial resolution due to small receptive field
Underlie tactile form perception
Pacinian:
Large Receptive Field
Fast Adapting
Deep
Acceleration
Though these receptors have poor spatial resolution, they possess extreme sensitivity to REALLY minute subcutaneous displacements (most sensitive ~10nm)
Most rapidly adapting so respond to detecting rapidly changing mechanical stimuli (i.e. vibration)
Underlie the perception of vibration as well as distant events, e.g., forces exerted during tool use
Ruffini Ending
Large Receptive Field
Slow Adapting
Deep
Displacement
This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement.
Monitoring slippage of objects along the surface of the skin, allowing modulation of grip on an object.
Form perception
Discrimination of the shape of grasped objects (stereognosis)
Resolution of fine spatial detail (e.g., 2-pt discrimination)
Texture perception
“REAL WORLD” FUNCTIONS OF CUTANEOUS MECHANORECEPTORS
Form perception
Control of grip
Perception of remote events; i.e., events transmitted through objects touching body, such as manipulating tools
Nonencapsulated
Merkel endings
Free nerve ending
Layered capsule
Pacinian corpuscle
Meissner corpuscle
These also have rapid adaption because of their layered capsule.
Thin capsule
Ruffini ending
High sensitivity (i.e., low force threshold) and exquisite spatial resolution due to small receptive field?
Merkel mechanoreceptors
Fine tactile discrimination; grip control?
Meissner’s corpuscles
Underlie tactile form perception?
Merkel mechanoreceptors
Monitors slippage of objects along the surface of the skin, allowing modulation of grip on an object.
Ruffini Ending
This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement:
Ruffini Ending
Underlie the perception of vibration as well as distant events, e.g., forces exerted during tool use:
Pacinian corpuscle
Though these receptors have poor spatial resolution, they possess extreme sensitivity to REALLY minute subcutaneous displacements (most sensitive ~10nm):
Pacinian corpuscle
Most rapidly adapting so respond to detecting rapidly changing mechanical stimuli (i.e. vibration)
Pacinian corpuscle
Good spatial resolution combined with exquisite sensitivity to rapid displacement of skin at surface; minute amount of indentation can be detected.
Meissner’s corpuscles
Accessory structures?
Merkel endings
Nociceptors
Mechanoreceptors do not code painful sensations (dynamic range does not extend to the noxious)
Separate population of fibers code for painful sensations
Free nerve endings:
In contrast to some mechanoreceptors, nociceptors are not associated with specialized structures in skin.
Some are myelinated, some are not.
Multiple types: Aδ & C
Nociceptors Restated
Although mechanoreceptors just discussed respond to more intense forces with higher firing rates, it’s important to appreciate the fact that these mechanoreceptors do not code painful sensations.
Likewise, there are thermal receptors that respond to cooling and warming and that show changes in firing rate with changes in temperature, but these receptors do not code painful burning sensations or the pain that is elicited by intense cold.
This is because the dynamic ranges of these receptors do not extend into the noxious range– thus a separate set of primary afferent somatosensory fibers codes for painful sensations.
The function of A delta versus C nociceptors are distinctive:
Size & conduction velocity
Aδ (A Delta): The First Pain; sharp, intense initial pain
C fiber: The Second Pain; the dull, aching pain that follows the initial stimulus and pain.
Aδ Fibers
Perception: “fast, sharp, well-localized pain”
~13% (cutaneous nociceptors)
Small: (1-5um), lightly myelinated
Conduction velocity: 5-30 m/s
Na+ channels: mainly typical voltage gated
Response properties:
Noxious mechanical stimuli
Some also to noxious heat
It was previously thought that they did not respond to chemical stimuli, but…
many DO respond to endogenous inflammatory mediators
but probably NOT to exogenous irritants, e.g. capsaicin
Aδ Fibers:
Response properties
Noxious mechanical stimuli
Some also to noxious heat
It was previously thought that they did NOT respond to chemical stimuli, but…
- many DO respond to Endogenous inflammatory mediators
- but probably NOT to Exogenous irritants, e.g. capsaicin
C Fibers
Perception: “slow, dull or burning, poorly localized” pain
~87% (cutaneous nociceptors)
Tiny (0.2 - 1.5um), unmyelinated
Conduction velocity: 0.5 - 1.2m/s
Voltage-gated Na+ channels
Response properties:
often POLYMODAL: mechanical, heat, chemical
VARIED: different combinations of modalities (some are modality-specific), different collections of chemical sensitivities
C Fibers
Perception: “slow, dull or burning, poorly localized” pain
~87% (cutaneous nociceptors)
Tiny (0.2 - 1.5um), unmyelinated
Conduction velocity: 0.5 - 1.2m/s
Voltage gated Na+ channels
Response properties:
often POLYMODAL: mechanical, heat, chemical
VARIED: different combinations of modalities (some are modality-specific), different collections of chemical sensitivities
C Fibers:
Response properties
Often POLYMODAL: mechanical, heat, chemical
VARIED: different combinations of modalities (some are modality-specific), different collections of chemical sensitivities
There are several different types
Endogenous:
Originate from within the body, like neurotransmitters
Transduction In Nociceptors
6-transmembrane receptor (member of TRP “transient receptor potential” family)
Activated by multiple stimuli: capsaicin, low pH, heat
NOT mechanical!
Loop region lines pore; non-specific cation channel (Ca++>Na+)
Nociceptors
Definition
A receptor with free nerve endings specifically sensitive to noxious stimuli of one or more types. All have thinly myelinated or unmyelinated axons.
TRPV1 KO mice
TRPV1 KO mice show reduced behavioral responses to heat, but not to mechanical stimuli…
CH “C heat” : Missing in TRPv1 KO’s
CONCLUSIONS: PAIN TRANSDUCTION
and TRPV1
TRPV1 important in nociceptive transduction of painful heat stimuli
TRPV1 not SOLE nociceptive receptor; in TRPV1 KOs… Although responses to 1 class of exogenous irritants are virtually eliminated; TRPV1 does NOT mediate mechanical nociception
Recent data suggests that TRPV1 restricted to subset of C fibers LACKING MECHANICAL SENSITIVITY
SENSITIZATION
Response of nociceptors is plastic, i.e., changes over time
Adaptation (decreased response)
Sensitization (increased response) more common, especially with tissue injury
Decreased threshold
Increased response
Spontaneous activity
Perceptual correlates
Allodynia (tenderness) – stimuli that were not painful, now induce pain
Hyperalgesia – stimuli that were perceived as mild are now perceived as intense
Spontaneous pain
Chemicals released in the damaged tissue, including K+ ions from injured cells, serotonin from platelets, and assorted proteins (e.g., bradykinin, histamine) from multiple sources, sensitize nociceptive endings in the damaged skin, producing hyperalgesia.
Hyperalgesics
feel more pain earlier than controls
Hyperalgesia
stimuli that were perceived as mild are now perceived as intense
Allodynia
tenderness
– stimuli that were not painful, now induce pain
Sensitization
increased response
common, especially with tissue injury
Decreased threshold
Increased response
Spontaneous activity
Until recently, the sensitizing action of inflammatory mediators was thought to be short-lived and quickly resolved when inflammation resolved.
However, it is now clear that inflammation can have surprisingly long-lasting effects.
This is called “hyperalgesic priming…
This graph plots the amount of hyperalgesia or increased sensitivity to paw pinch, comparing a normal paw to one that has been treated with an inflammatory stimulus, carrageenan… pay particular attention to the x-axis scale– you are looking at weeks
Mechanisms of Peripheral Sensitization
Mechanisms varied and complex
Changes in receptor sensitivity
Changes in ion channel expression
Changes in neurotransmitter expression; e.g., upregulation of peptides
Changes in nerve fiber morphology
Trigeminal Neuralgia
Trigeminal neuralgia (also called tic douloureux) is a condition characterized by brief attacks of excruciating pain, usually less than a minute in duration, in the distribution of one (or sometimes more than one) division of the trigeminal nerve. Between attacks, there are no significant sensory abnormalities.
Most cases appear to be caused by compression of the trigeminal nerve by a blood vessel or tumor, resulting in demyelination and aberrant activity in trigeminal nerve fibers; subsequent changes in the sensory nuclei where these fibers terminate are likely to be involved as well.
RECENT STUDY: 92% OF PATIENTS WITH PAIN WERE FOUND TO HAVE NEUROVASCULAR COMPRESSION.
MOST COMMON VESSEL IS SUPERIOR CEREBELLAR ARTERY.
MOST COMMON VESSEL that causes problems in Trigeminal Neuralgia.
MOST COMMON VESSEL IS SUPERIOR CEREBELLAR ARTERY.
Treatment of Trigeminal Neuralgia
Trigeminal neuralgia usually can be treated pharmacologically, but a number of surgical treatments are available if absolutely necessary. These include:
- Expose the lateral brainstem, move the artery away from the nerve and insert a block to prevent contact. This causes no disruption in nerve function.
- Lesioning the involved nerve root and/or the trigeminal ganglion (e.g., transction, radio frequency disruption). The destructive procedures have a serious disadvantage, in that the patient loses all tactile sensibility, in addition to pain, in the area. Also lose corneal blink reflex.
- Section the spinal trigeminal tract slightly caudal to the obex, thus removing the afferent input to the caudal nucleus. Tactile sensibility remains intact, and the corneal blink reflex is usually preserved.
The fact that this operation abolishes pain sensation over one entire half of the face is a major piece of evidence that the caudal part of the spinal trigeminal nucleus deals with pain and that afferents from all 3 divisions of the trigeminal nerve extend at least into the caudal medulla.
