10/23 - Sensory System - Peripheral Receptors

Question 1

“soma”

Answer 1

“soma” = “body,” in Greek

Question 2

The cell bodies of all neurons involved in primary processing of sensory information are located ______ .

Answer 2

in a GANGLION located somewhere in the periphery.

Question 3

For the body, the cell bodies of all neurons involved in primary processing of sensory information are located _____ .

Answer 3

DORSAL ROOT GANGLIA.

Question 4

For structures in the head. the cell bodies of all neurons involved in primary processing of sensory information are located ______ .

Answer 4

in ganglia related to each cranial nerve.

For example, neurons carrying sensations from the face will be in the TRIGEMINAL GANGLION.

Question 5

Somatosensory System

Answer 5

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.

Question 6

Somatosensory System:

4 Senses

Answer 6

Touch
Body Position
Pain
Temperature

Divide into 2 subgroups:
Touch & Body Position
Pain & Temperature

Question 7

largest organ in body

Answer 7

Skin = largest organ in body (3000 mm2, 6ft person)

All of the skin is innervated, but to varying degrees

Question 8

The Mechanical Senses

Answer 8

Touch (Somatosensory)

Proprioception

Question 9

Thermal

Answer 9

TEMPERATURE

Question 10

Chemical

Answer 10

Pain
Itch

Itch is often considered a category of pain.

Question 11

Different forms of energy elicit distinct types of somatosensory perception

Answer 11

Same type of physical energy can elicit different sensations depending on type & location of receptor & stimulus intensity.

Question 12

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.

Answer 12

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.

Question 13

Mechanoreceptors

Answer 13

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.

Question 14

Proprioceptors

Answer 14

low threshold

can detect small changes in muscle tension

Question 15

Receptor

Answer 15

located on primary afferent ending itself

stretch sensitive ion channel

Question 16

Cell body

Answer 16

in DRG or Trigeminal ganglion

Question 17

DRG NEURON

Peripheral process:

Answer 17

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)

Question 18

DRG NEURON

Central process

Answer 18

Enters through dorsal root and courses directly to posterior column where they ascend to the brainstem forming the fasciculus gracilis or fasciculus cuneatus.

Question 19

What sensory axons are the largest sensory axons?

Answer 19

Proprioception axons

Consequently, they are also the fastest conducting.

Question 20

Trigeminal Ganglion

Answer 20

Large structure that contains sensory neurons that send axons into the 3 divisions of the trigeminal nerve:
V1, V2, and V3.

Question 21

V1 of the Trigeminal

Answer 21

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.

Question 22

V2 of the Trigeminal

Answer 22

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.

Question 23

V3 of the Trigeminal

Answer 23

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.

Question 24

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.

Answer 24

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.

Question 25

Peripheral somatosensory nerve fibers vary in size

Answer 25

Specialization starts in the periphery. Fibers of different diameter are associated with different submodalities.

Question 26

A alpha | Aα

Answer 26

Group I Largest in axon diameter of the 4 types. Proprioceptors of Skeletal Muscle Primary receptors of muscle spindle. Golgi tendon organ.

Question 27

A beta | Aβ

Answer 27

Group II Mechanoreceptors of Skin Touch Secondary receptors of muscle spindle. All cutaneous mechanoreceptors.

Question 28

A delta | Aδ

Answer 28

Group III Free nerve endings of touch, pain, and pressure. Nociceptors of neospinothalamic tract. Cold thermoreceptors.

Question 29

C

Answer 29

Group IV Nociceptors of paleospinothalamic tract Warmth receptors

Question 30

MECHANORECEPTORS

Answer 30

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.

Question 31

What are mechanoreceptors sensitive to?

Answer 31

Receptors that are sensitive to physical distortion bending, stretching, pressure, vibration.

Question 32

Where are mechanoreceptors found?

Answer 32

In addition to the skin, they are present in organs such as the heart, intestines, bladder, and even teeth.

Question 33

How do mechanoreceptors respond to stimuli?

Answer 33

Sensory receptors transduce physical stimulus into electrical signal called the receptor or generator potential in response to skin distortion.

Question 34

What components are mechanoreceptors composed of?

Answer 34

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.

Question 35

glabrous skin

Answer 35

external skin that is naturally hairless. It is found on the ventral portion of the fingers, palmar surfaces of hands, soles of feet, lips.

Question 36

What 2 Mechanoreceptors do NOT have a layered structure associated with it.

Answer 36

Ruffini and Merkel

Question 37

What 2 Mechanoreceptors are situated superficially in the skin.

Answer 37

Merkel’s disks & Meissner’s corpuscles Because of this superficial placement, they have limited branching, and are densely packed.

Question 38

Ruffini endings

Answer 38

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.

Question 39

Merkel cells

Answer 39

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.

Question 40

Because pressure spreads out as move away from source ___ and ___ can be activated by stimuli arising from more distant stimuli.

Answer 40

Ruffini endings and | Pacinian corpuscles

Question 41

What 2 Mechanoreceptors are situated deeper in the skin.

Answer 41

Pacinian corpuscles & ruffini’s endings deep Because they are deep, they can also be sparse.

Question 42

Which 2 have Mechanoreceptors small Receptive Fields.

Answer 42

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.

Question 43

receptive field revisited....

Answer 43

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.

Question 44

The ____ the receptive field the greater the discrimination of two points.

Answer 44

The smaller the receptive field the greater the discrimination of two points.

Question 45

Two-Point Discrimination

Answer 45

Measure of spatial resolution

Question 46

Areas with the finest resolution:

Answer 46

1. Have higher density of mechanoreceptors in the skin | 2. Are enriched with receptors with a Small receptive field.

Question 47

Receptors with layered capsules are ______, due in large part to the mechanical properties of the capsules.

Answer 47

Receptors with layered capsules are Rapidly Adapting, due in large part to the mechanical properties of the capsules.

Question 48

Merkel’s Disc

Answer 48

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.

Question 49

Pacinian Corpuscle: | Layering

Answer 49

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.

Question 50

ADAPTATION

Answer 50

Most receptors become less sensitive (i.e., fire fewer action potentials) during the course of a maintained stimulus.

Question 51

Slowly Adapting

Answer 51

Very little change in firing rate during the course of a maintained stimulus. Respond best to unchanging stimuli. Detect static position.

Question 52

Rapidly Adapting

Answer 52

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.

Question 53

Sensory receptors adapt to sustained inputs.

Answer 53

Some adapt slowly, others more rapidly.

Question 54

Convey information on static position, like pressure.

Answer 54

Slowly adapting

Question 55

convey information about the rate of change or movement of a stimulus (e.g. vibration)

Answer 55

Nociceptors are an exception to the rule. They adapt not at all or minimally to maintained stimuli.

Question 56

Exception to the rules of ADAPTATION

Answer 56

Rapidly adapting

Question 57

Pacinian Corpuscle: | Adaption

Answer 57

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.

Question 58

Meissner’s corpuscles: | Adaption

Answer 58

Meissner’s corpuscles are also rapidly adapting. They respond best to low frequency stimulation.

Question 59

Meissner’s

Answer 59

``` 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.

Question 60

Velocity

Answer 60

Meissner’s

Question 61

Displacement

Answer 61

Merkel & Ruffini Ending

Question 62

Acceleration

Answer 62

Pacinian

Question 63

Merkel

Answer 63

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

Question 64

Pacinian:

Answer 64

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

Question 65

Ruffini Ending

Answer 65

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.

Question 66

Form perception

Answer 66

Discrimination of the shape of grasped objects (stereognosis) Resolution of fine spatial detail (e.g., 2-pt discrimination) Texture perception

Question 67

“REAL WORLD” FUNCTIONS OF CUTANEOUS MECHANORECEPTORS

Answer 67

Form perception Control of grip Perception of remote events; i.e., events transmitted through objects touching body, such as manipulating tools

Question 68

Nonencapsulated

Answer 68

Merkel endings | Free nerve ending

Question 69

Layered capsule

Answer 69

Pacinian corpuscle Meissner corpuscle These also have rapid adaption because of their layered capsule.

Question 70

Thin capsule

Answer 70

Ruffini ending

Question 71

High sensitivity (i.e., low force threshold) and exquisite spatial resolution due to small receptive field?

Answer 71

Merkel mechanoreceptors

Question 72

Fine tactile discrimination; grip control?

Answer 72

Meissner’s corpuscles

Question 73

Underlie tactile form perception?

Answer 73

Merkel mechanoreceptors

Question 74

Monitors slippage of objects along the surface of the skin, allowing modulation of grip on an object.

Answer 74

Ruffini Ending

Question 75

This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense of and control of finger position and movement:

Answer 75

Ruffini Ending

Question 76

Underlie the perception of vibration as well as distant events, e.g., forces exerted during tool use:

Answer 76

Pacinian corpuscle

Question 77

Though these receptors have poor spatial resolution, they possess extreme sensitivity to REALLY minute subcutaneous displacements (most sensitive ~10nm):

Answer 77

Pacinian corpuscle

Question 78

Most rapidly adapting so respond to detecting rapidly changing mechanical stimuli (i.e. vibration)

Answer 78

Pacinian corpuscle

Question 79

Good spatial resolution combined with exquisite sensitivity to rapid displacement of skin at surface; minute amount of indentation can be detected.

Answer 79

Meissner’s corpuscles

Question 80

Accessory structures?

Answer 80

Merkel endings

Question 81

Nociceptors

Answer 81

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

Question 82

Nociceptors Restated

Answer 82

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.

Question 83

The function of A delta versus C nociceptors are distinctive: Size & conduction velocity

Answer 83

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.

Question 84

Aδ Fibers

Answer 84

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

Question 85

Aδ Fibers: | Response properties

Answer 85

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

Question 86

C Fibers

Answer 86

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

Question 87

C Fibers

Answer 87

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

Question 88

C Fibers: | Response properties

Answer 88

Often POLYMODAL: mechanical, heat, chemical VARIED: different combinations of modalities (some are modality-specific), different collections of chemical sensitivities There are several different types

Question 89

Endogenous:

Answer 89

Originate from within the body, like neurotransmitters

Question 90

Transduction In Nociceptors

Answer 90

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+)

Question 91

Nociceptors Definition

Answer 91

A receptor with free nerve endings specifically sensitive to noxious stimuli of one or more types. All have thinly myelinated or unmyelinated axons.

Question 92

TRPV1 KO mice

Answer 92

TRPV1 KO mice show reduced behavioral responses to heat, but not to mechanical stimuli… CH “C heat” : Missing in TRPv1 KO’s

Question 93

CONCLUSIONS: PAIN TRANSDUCTION | and TRPV1

Answer 93

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

Question 94

SENSITIZATION

Answer 94

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.

Question 95

Hyperalgesics

Answer 95

feel more pain earlier than controls

Question 96

Hyperalgesia

Answer 96

stimuli that were perceived as mild are now perceived as intense

Question 97

Allodynia

Answer 97

tenderness – stimuli that were not painful, now induce pain

Question 98

Sensitization

Answer 98

increased response common, especially with tissue injury Decreased threshold Increased response Spontaneous activity

Question 99

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…

Answer 99

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

Question 100

Mechanisms of Peripheral Sensitization

Answer 100

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

Question 101

Trigeminal Neuralgia

Answer 101

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.

Question 102

MOST COMMON VESSEL that causes problems in Trigeminal Neuralgia.

Answer 102

MOST COMMON VESSEL IS SUPERIOR CEREBELLAR ARTERY.

Question 103

Treatment of Trigeminal Neuralgia

Answer 103

Trigeminal neuralgia usually can be treated pharmacologically, but a number of surgical treatments are available if absolutely necessary. These include: 1. 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. 2. 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. 3. 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.