Somatosensation

 

An adequate stimulus is that form of stimulation to which a receptor has the lowest threshold. 

 

Touch is the sense by which we determine:

1. size

2. shape

3. texture

 

Cutaneous Receptors

• Mechanoreceptors

  • Meissner's corpuscle (texture, slow vibrations)

  • Pacinian corpuscle (deep pressure, fast vibrations)

  • Ruffini's end organ (sustained pressure)

  • Merkel's disc (sustained touch and pressure)

  • Free nerve endings

• Thermoreceptor

• Bulboid corpuscles (stretch)

• Chemoreceptor

 

Mechanoreceptors

 

Detect stimulus, send impulses along the sensory nerves that enter the dorsal roots of the spinal cord.

The axons connecting touch receptors to the spinal cord are large myelinated fibres that convey information from the periphery towards the cerebral cortex extremely rapidly.

Generally, they are linked to collagen-fibre networks within the capsule. Ion channels are situated near these networks.

In sensory transduction, the afferent nerves transmit through a series of synapses in the central nervous system, first in the spinal cord or trigeminal nucleus, depending on the dermatomic area concerned.

One pathway then proceeds to the ventrobasal portion of the thalamus, and then on to the somatosensory cortex.

 

Pacinian and Meissner corpuscles, Merkel’s disks and Ruffini endings sense different aspects of touch.

All these receptors have ion channels that open in response to mechanical deformation, triggering action potentials that can be recorded experimentally by fine electrodes.

cutaneous receptors are a part of primary afferents; others are not

receptors potentials are graded, small amplitude , passively conducted

 

Type 1

• superficial, lie at the boundary of epidermis and dermis

• small RFs

• well-defined boundaries

• Meissner’s corpuscles

• Merkel’s discs

• shape, texture

 

Meissner's corpuscle 

• adapt quickly and so respond best to rapidly changing indentations (ex.vibration/flutter)

• RF smaller than pacinian

• codes the velocity by which the skin is displaced

• Hand grip control

 

Merkel's disc

• In burns, Merkel endings are most commonly lost

• called hair disks in hairy skin

• sustained touch and pressure

• pressure

• Fine detail

 

Type 2

• deep in the dermis

• large RFs

• badly defined edges

• Ruffini corpuscles

• Pacinian corpuscles

 

Pacinian corpuscle 

• adapt quickly and so respond best to rapidly changing indentations (ex.vibration/flutter)

• larger receptive field compared to meissner

• afferents respond to the skin acceleration

 

Ruffini's end organ 

• slowly adapting

• slowly changing indentations

• frequency of firing is directly proportional to skin indentation by a mechanical force

• codes the skin position

 

NOM

type I receptors more directly concerned w/ form & texture perception > type 2

Meissner&Merkel highest density in fingertips, lips, tongue, least in the trunk

greater density=greater somatotopic maps.

convergence differs with receptor; Merkel’s disc (aff) receives 2-7 receptors, 1:1 for Pacinian corpuscles 

The Pacinian corpuscles can respond to skin indentation as small as 1 µm. The force is transmitted via the corpuscle to deform the neurite within. stretch-sensitive Na+ channels open and depolrize, repolarize. The Human skin is either hairy or glabrous.

hairy skin lower density of Merkel’s disks and in possessing the two additional kinds of mechanoreceptor closely related with hairs as shown in table below

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a- hairy skin only. polymodal bare nerve ending: heat, intense mechanical forces, chemicals in times of tissue damage, poorly localized burn pain.

Itch recpetors have c afferent fibres and respond to histamine

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hyperalgesia & Allodynia

 

Neurologists measure sensitivity by determining the patient’s two-point threshold, the distance between two points on the skin necessary in order for the individual to distinguish two distinct stimuli from just one. This method involves touching the skin with calipers at two points. Not surprisingly, body with greatest Number and Distribution of receptors has greatest acuity. Threshold is lowest on the fingers and lips.

somatosensory receptors have receptive fields. The size of mechanoreceptors' receptive fields in a given area determines the degree to which detailed stimuli can be resolved: the smaller and more densely clustered the receptive fields, the higher the resolution. ​

 

Ex. the cornea is 100x more sensitive to painful stimuli than are the soles of the feet.

Ex.The fingertips are good at touch discrimination, but the torso is not.

 

                                  two points/single receptive field = unable to feel the two separate points

                          two points span more than a single RF = both will be felt

                                                        less tactile acuity = larger receptive fields

                                             High density of receptors =higher number of sensory nerves

       

Touch is also involved in the active control of movement

cup decreasing pressure on fingertips=grab reflex

 

                         

 

Thermoreceptors

codes absolute and relative changes in temperature, primarily within the innocuous range.

Temperature receptors show adaptation.

slowly adapting

temp percep relies on comparing responses of warm and cold R's

warmth receptors are thought to be unmyelinated C-fibres

cold have both C-fibers and thinly myelinated A delta fibers

 

Cold, warmth and pain are detected by thin axons with “naked” endings, which transmit more slowly.

 

stimulus for a warm receptor is warming, results in an increase in their action potential discharge rate.

Cooling results in a decrease in warm receptor discharge rate.

Warm receptors respond to temperature increases of greater than 0.1oC in the range from 30 to 43°C.

 

For cold receptors their firing rate increases during cooling and decreases during warming. 

Cold receptors respond to temperature decreases of greater than 0.1oC in the range from 35 to 15°C.

 

Some cold receptors also respond with a brief action potential discharge to high temperatures, i.e. typically above 45°C, and this is known as a paradoxical response to heat.

 

A special form of thermoreceptor is found in some snakes, the viper pit organ and this specialized structure is sensitive to energy in the infrared part of the spectrum.

 

In the cornea cold receptors are thought to respond with an increase in firing rate to cooling produced by evaporation of lacrimal fluid 'tears' and thereby to elicit a reflex blink.

 

Temperatures likely to damage an organism are sensed by sub-categories of nociceptors that may respond to noxious cold, noxious heat or more than one noxious stimulus modality (i.e., they are polymodal).

 

The nerve endings that respond to cooling are :

moderate density in the skin

high density in the cornea, tongue, bladder, and facial skin

 

The speculation is that lingual cold receptors deliver information that modulates the sense of taste; i.e. some foods taste good when cold, while others do not.

 

Chemoreceptors

 

 

 

Pathway Anatomy

 

webs of sensory nerve cell endings wrapped around the base of hairs ->Hair plexus

 

 

 touch receptors -> sensory nerves -> spinal cord -> thalamus -> SSC

 

Detect stimulus, send impulses along the sensory nerves that enter the dorsal roots of the spinal cord.

axons connecting touch receptors to the SC are Ad, convey info from periphery towards the cerebral cortex extremely rapidly.

sensory neurons are linked to collagen-fibre networks within the capsule. Ion channels are situated near these networks.

In sensory transduction, the afferent nerves transmit through a series of synapses in the central nervous system, first in the SC (body) or trigeminal nucleus (face).

One pathway then proceeds to the ventrobasal portion of the thalamus, and then on to the somatosensory cortex.

 

 

nerves cross midline

relay stations for touch in the medulla and the thalamus

before projecting on to the SSC1

 

Cross-talk between sensory and motor systems begins at the first relays in the spinal cord, including proprioceptive feedback on to motor neurons, and it continues at all levels of the somatosensory system. The primary sensory and motor cortices are right beside each other in the brain.

 

1. The posterior (dorsal) column AKA medial lemniscal pathway carries and processes discriminative touch and proprioceptive information from the body.

• discriminative touch and proprioceptive information

• prop and touch afferents never merge

 

2. sensory trigeminal pathway carries and processes discriminative touch and proprioceptive information from the face

• Consequently, it is the cranial homologue of the MLP. 

• discriminative touch and proprioceptive information

 

3. neospinothalamic pathway carries and processes sharp, pricking pain and dropping temperature (cool/cold) information from the body

• well localized

• sensations are the short lasting “fast” or “first” pain 

• somatotopic representation (allows for accurate localization of the painful stimulus)

 

4. spinal trigeminal pathway

• crude touch, pain and temperature information from the face

• homologue of archi-, paleo- and neo-spinothalamic pathways

• sharp, cutting pain information are segregated from those carrying dull, burning pain and deep aching pain information

 

Cuneate fasciculus & Gracile fasciculus have their own somatotopic organization (MLP)

MLP the midline @ Medulla

 

Brain imaging is also starting to produce insights about cortical plasticity by revealing that the map of the body in the somatosensory area can vary with experience.

thermo-Lissauer's tract

There are relay stations for touch in the medulla and the thalamus, before projection on to the primary sensory area in the cortex called the somatosensory cortex. The nerves cross the midline so that the right side of the body is represented in the left hemisphere and the left in the right.

 

 

Graphesthesia

Blind Braille readers have an increased cortical representation for the index finger used in reading

string players an enlarged cortical representation of the fingers of the left hand.

 

Skin is sensitive enough to measure a raised dot that is less than 1/100th of a millimetre high – provided you stroke it as in a blind person reading Braille

 

One active area of research asks how the different types of receptor contribute to different tasks such as discriminating between textures or identifying the shape of an object.

 

Functional brain imaging suggests that the identification of textures or of objects by touch involves different regions of cortex.

 

Brain imaging is also starting to produce insights about cortical plasticity by revealing that the map of the body in the somatosensory area can vary with experience.

 

Nociception

 

pain receptors provides little information about the nature of the stimulus

Ex. there is little difference between the pain due an abrasion and a nettle sting.

 

The ancient Greeks regarded pain as an emotion not a sensation.

 

• Nociceptor

  • Thermal 

    • activated by noxious heat or cold at various temperatures

    • First discovered TRPV1 threshold heat pain temperature of 42 °C

    • Other temperature in the warm–hot range is mediated by more than one TRP channel

    • Diff temp=particular C-terminal domain

    • cool stimuli are sensed by TRPM8 channels

    • C-terminal domain differs from the heat sensitive TRPs for cool receptors

  • Chemical

    • Chemical nociceptors have TRP channels that respond to a wide variety of spices.

    • Most responsive: Capsaicin. others: environmental irritants like acrolein, a World War I chemical weapon and a component of cigarette smoke.

    • Apart from these external stimulants, chemical nociceptors have the capacity to detect endogenous ligands, and certain fatty acid amines that arise from changes in internal tissues.

    • Like in thermal nociceptors, TRPV1 can detect chemicals like capsaicin and spider toxins.

    • Aδ fibres

  • Mechanical 

    • Respond to excess pressure or mechanical deformation

    • also respond to incisions that break the skin surface

    • processed as pain by the cortex

    • frequently have polymodal characteristics

    • it is possible that some of the transducers for thermal stimuli are the same for mechanical stimuli. 

    • Aδ fibres

  • Sleeping/silent

    • Although each nociceptor can have a variety of possible threshold levels, some do not respond at all to chemical, thermal or mechanical stimuli unless injury actually has occurred.

    • These are typically referred to as silent or sleeping nociceptors since their response comes only on the onset of inflammation to the surrounding tissue.

  • Polymodal

    • Many neurons perform only a single function, therefore neurons that perform these functions in combination are given the classification "polymodal."

 

 

 

Crossing over of duties:

TRPA1 appears to detect both mechanical and chemical changes

 

 

Nociceptors include receptors that respond to :

• heat above 46*C

• to tissue acidity

• to capsaicin, garlic, and wasabi, & can produce pain.

• to chemical stimuli that cause itch.

 

Ex. Histamine is such a nociceptor, can be rel'd bug bites or allergies.

 

Tissue injury =release of numerous chemicals (damage & inflammation)

 

 

 

Prostaglandins enhance the sensitivity of receptors to tissue damage and ultimately can induce more intense pain sensations.

 

Prostaglandins also contribute to the clinical condition of allodynia, in which innocuous stimuli can produce pain, as when sunburned skin is touched.

 

Inflammatory mediators (ex. bradykinin, serotonin, prostaglandins, cytokines, and H+) are released from damaged tissue and can stimulate nociceptors directly.

 

Primary Sensitisation: reduce the activation threshold of nociceptors 

Persistent injury= persistent pain=hypersensitivity

 

phenomenon of enhanced pain is called hyperalgesia.

• There is a lowering of the pain threshold

• an increase in the intensity of pain

• sometimes both a broadening of the area over which pain is felt

• or even pain in the absence of noxious stimulation

 

Hyperalgesia involves sensitisation of the peripheral receptors as well as complex phenomena at various levels of the ascending pain pathways.

 

These include the interaction of chemically mediated excitation and inhibition.

 

The Hyperalgesia observed in chronic pain states results from the enhancement of excitation and depression of inhibition.

 

Much of this is due to changes in the responsiveness of the neurons that process sensory information.

 

Temperatures likely to damage an organism are sensed by sub-categories of nociceptors that may respond to noxious cold, noxious heat or more than one noxious stimulus modality (i.e., they are polymodal).

 

Can use brain scans to determine how pain is processed to help test the efficacy of pain treatments. 

 

Pain messages can be suppressed by systems of neurons that originate within the gray matter in the brainstem.

 

These descending systems suppress the transmission of pain signals from the dorsal horn of the spinal cord to higher brain centers.

 

Some of these descending systems use endorphins

 

Recent findings indicating that endorphins act at multiple opioid receptors in the brain and spinal cord have had important implications for pain therapy.

 

For example, scientists began studying how to deliver opioids into the spine after discovering a dense distribution of opioid receptors in the spinal cord horn. After a technique for delivering opioids into the spine was used successfully in animals, such treatments were begun in humans; the technique is now common in treating pain after surgery.

 

Action potentials in the nociceptive nerves entering the spinal cord initiate automatic protective reflexes, such as the withdrawal reflex.

 

Modern imaging tools are used to help scientists better understand what happens in the brain when pain is experienced.

 

One finding is that no single area in the brain generates pain; rather, emotional and sensory components together constitute a mosaic of activity leading to pain.

 

Subjects immersed their hands in painfully hot water and were then subjected to hypnotic suggestion of increased or decreased pain intensity or pain unpleasantness. Using positron emission tomography (PET), it was found that during changes in experienced pain intensity there was activation of the somatosensory cortex, whereas the experience of pain unpleasantness was accompanied by activation of the anterior cingulate cortex.

 

Interestingly, when people are hypnotized so that a painful stimulus is not experienced as unpleasant, activity in only some areas of the brain is suppressed, showing that the stimulus is still experienced.

 

The setting in which the injury occurs (childbirth/car accident), as well as the emotional impact, also determine our overall response to the experience.

 

Traditional Chinese Medicine uses a procedure called "acupuncture" for the relief of pain. This involves fine needles, inserted into the skin at particular positions in the body along what are called meridians, which are then rotated or vibrated by the person treating the patient. They certainly relieve pain but, until recently, no one was very sure why. Forty years ago, a research laboratory was set up in China to find out how it works. Its findings reveal that electrical stimulation at one frequency of vibration triggers the release of endogenous opoiods called endorphins, such as met-enkephalin, while stimulation at another frequency activates a system sensitive to dynorphins. This work has led to the development of an inexpensive electrical acupuncture machine (left) that can be used for pain relief instead of drugs. A pair of electrodes are placed at the "Heku" points on the hand (right), another at the site of pain.

 

Two classes of peripheral afferent fibres respond to noxious stimuli:

1. Relatively fast myelinated fibres, called Αδ fibres and

2. Very fine, slow, non-myelinated C fibres

Both sets of nerves enter the spinal cord, where they synapse with a series of neurons that project up to the cerebral cortex.

 

Primary Afferent Fibres

 

Aβ fibres

Aδ fibres

• initial reflex response 

C fibres

• C fiber nociceptors

  • responsible for the second, burning pain

• C fiber warming specific receptors

  • responsible for warmth

• ultra-slow histamine-selective C fibers

  • responsible for itch

• tactile C fibers

  • sensual touch

  • includes CT fibres, also known as C low-threshold mechanoreceptors (CLTM), which are unmyelinated afferents found in human hairy skin, and have a low mechanical threshold < 5 milliNewtons. They have moderate adaptation and may exhibit fatigue on repetitive stimulation and "afterdischarges" for several seconds after a stimulus.

• C mechano- and metabo- receptors in muscles or joints

  • responsible for muscle exercise, burn and cramp

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Key function of pain is the inhibition of activity (for rest and healing).

 

The first such modulatory mechanism to be discovered was the release of endogenous analgesics.

 

Animal experiments have revealed that electrical stimulation of brain areas such as the aqueductal gray matter causes a marked elevation in the pain threshold and that this is mediated by a descending pathway from the midbrain to the spinal cord.

 

A number of chemical transmitters are involved including endogenous opioids such as met-enkaphalin.

 

The pain-killer morphine acts on the same receptors at which some of the endogenous opioids act.

 

thermal noci's activated by high/low skin temp, activate ad fibres

glutamate and peptides(subs P)

released from central and peripheral terminals of noc afferents

peripheral release; responsible for changes in blood flow adn histamine secretion that causes redness, heat, swelling characteristic of neurogenic inflammation

blocking mediators for excite or sensite nociceptors can produce analgesia,

ex. bradykinin receptors antagonists NSAIDs (aspirin) inhibit production of prostas and vallinoid receptor ligans act on ion channels that transduce noxious heat

therefore, aspirin : inhib production of prostas & inhib vallinoid receptor ligands

 

Substances which help stop an itch include the active ingredients:

• Antihistamines, ex. diphenhydramine 

• Corticosteroids, ex. hydrocortisone topical cream; see topical steroid

• Counterirritants, ex. mint oil, menthol, or camphor[16]

• Crotamiton (trade name Eurax) is an antipruritic agent available as a cream or lotion, often used to treat scabies. Its mechanism of action remains unknown.

• Local anesthetics, ex. benzocaine topical cream (Lanacane)

 

 

Nociception Pathways

Two parallel ascending pathways, which deal with:

1. The localisation of pain (similar to the pathway for touch)

2. The other responsible for the emotional aspect of pain

This second pathway projects to quite different areas than the somatosensory cortex, including the anterior cingulate cortex and the insular cortex.

During changes in experienced pain intensity there was activation of the somatosensory cortex, whereas the experience of pain unpleasantness was accompanied by activation of the anterior cingulate cortex.