Transduction

• Stimulus is changed into electrical signal

• Different types of stimuli– mechanical deformation– chemical– change in temperature

• Warmth, cold, nociceptors

– electromagnetic • Rods and cones in the retina

Sensory systems

• All sensory systems mediate 4 attributes of a stimulus no matter what type of sensation – modality– location– intensity– timing

Receptor Potential

• Membrane potential of the receptor

• A change in the receptor potential is associated with opening of ion (Na+) channels

• Above threshold as the receptor potential becomes less negative the frequency of AP into the CNS increases

Labeled Line Principle

• Different modalities of sensation depend on the termination point in the CNS– type of sensation felt when a nerve fiber is

stimulated (e.g. pain, touch, sight, sound) is determined by termination point in CNS

– labeled line principle refers to the specificity of nerve fibers transmitting only one modality of sensation

Adaptation

• Slow-provide continuous information (tonic)-relatively non adapting-respond to sustained stimulus– joint capsul– muscle spindle– Merkel’s discs

• punctate receptive fields

– Ruffini end organ’s (corpusles)• activated by stretching the skin

Adaptation

• Rapid (Fast) or phasic

• react strongly when a change is taking place

• respond to vibration – hair receptors 30-40 Hz– Pacinian corpuscles 250 Hz– Meissner’s corpuscles- 30-40 Hz– (Hz represents optimum stimulus rate)

Mechanoreceptors

• Information transmitted to the brain from mechanoreceptors in fingers allows us to:– feel the shape & texture of objects– play musical instruments– type on computer keyboards– palpate and perform adjustments– perform a multitude of tasks using our hands

• Tactile information is fragmented by receptors & must be integrated by the brain

Tactile information

• The ability to recognize objects placed in the hand on the basis of touch alone is one of the most important complex functions of the somatosensory system. (Gardner & Kandel)

• Tactile information obtained from palpation is crucial in the practice of chiropractic.

Stereognosis

• The ability to perceive form through touch– tests the ability of dorsal column-medial

lemniscal system to transmit sensations from the hand

– also tests ability of cognitive processes in the brain where integration occurs

Receptors in skin• Most objects that we handle are larger than

the receptive field of any receptor in the hand

• These objects stimulate a large population of sensory nerve fibers– each of which scans a small portion of the object

• Deconstruction occurs at the periphery

• By analyzing which fibers have been stimulated the brain reconstructs the pattern

Tactile

• No single sensory axon or class of sensory axons signals all relevant information

• Spatial properties are processed by populations of receptors that form many parallel pathways

• CNS constructs a coherent image of an object from fragmented information conveyed in multiple pathways

Mechanoreceptors• Rapidly adapting cutaneous

– Meissner’s corpuscles in glabrous (non hairy) skin• signals edges

– Hair follicle receptors in hairy skin – Pacinian corpuscles in subcutaneous tissue

• Slowly adapting cutaneous– Merkel’s discs have punctate receptive fields

• senses curvature of an object’s surface

– Ruffini end organs activated by stretching the skin• even at some distance away from receptor

Somatic Sensory Cortex

• Receives projections from the thalamus

• Somatotopic organization (homoculus)

• Each central neuron has a receptive field

• size varies in different areas of skin

• lateral inhibition can aid two point discrimination

Somatosensory Cortex

• Two major pathways– Dorsal column-medial lemniscal system

• Most aspects of touch, proprioception

– Anterolateral system• Sensations of crude touch, nociception, temperature,

tickle, itch and sexual sensations

Somatosensory Cortex (SSC)

• Inputs to SSC are organized into columns by submodality– cortical neurons defined by receptive field &

modality• some columns activated by rapidly adapting

Messiner’s, others by slowly adapting Merkel’s, still others by Paccinian corp.

– most nerve cells are responsive to only one modality e.g. superficial tactile, deep pressure, temperature, nociception

Somatosensory cortex

• Brodman area 3, 1, 2 (dominate input)– 3a-from muscle stretch receptors (spindles)– 3b-from cutaneous receptors– 2-from deep pressure receptors– 1-rapidly adapting cutaneous receptors

• These four areas are extensively interconnected (serial & parallel processing)

• Each of the 4 regions contains a complete map of the body surface

Somatosensory Cortex• Detailed features of a stimulus are

communicated to the brain

• in early stages of cortical processing the dynamic properties of central neurons and receptors are similar (eg rapidly adapting cutaneous receptors connected to rapidly adapting 2nd and 3rd order neurons)

• in the later stages of cortical processing the central nerve cells have complex feature detecting properties and integrate various sensory inputs

Somatosensory Cortex

• 3 different types of neurons in BM area 1,2 have complex feature detection capabilities– Motion sensitive neurons

• respond well to movement in all directions but not selectively to movement in any one direction

– Direction-sensitive neurons• respond much better to movement in one direction than in

another

– Orientation-sensitive neurons• respond best to movement along a specific axis

Other Somatosensory Cortical Areas

• Posterior parietal cortex (BM 5 & 7)– BM 5 integrates tactile information from

mechanoreceptors in skin with proprioceptive inputs from underlying muscles & joints

– BM 7 receives visual, tactile, proprioceptive inputs

• intergrates stereognostic and visual information

– Projects to motor areas of frontal lobe– sensory initiation & guidance of movement

Secondary SSC (S-II)

• Secondary somatic sensory cortex (S-II)– located in superior bank of the lateral fissure– projections from S-1 are required for function

of S-II– projects to the insular cortex, which innervates

regions of temporal lobe believed to be important in tactile memory

Sensory innervation of Spinal joints

• Tremendous amount of innervation with cervical joints the most heavily innervated

• Four types of sensory receptors– Type I, II, III, IV

Type I mechanoreceptors

• Outer layers of joint capsul

• fire at a degree proportional to joint movement or traction

• low threshold

• dynamic-fire with movement

• slow adapting

• tonic effects on lower motor neuron pools

Type II Mechanoreceptors

• Deeper layers of joint capsul

• low threshold

• rapidly adapting

• completely inactive in imobilized joints

• functions in joint movement monitering

• phasic effects on lower motor neuron pools

Type III Mechanoreceptors

• Recently found in spinal joints

• very high threshold

• slow adaptation

• joint version of Golgi tendon organ

Type IV receptors

• Nociceptors

• very high threshold

• completely inactive in physiologic normal joint

• activation with joint narrowing, increased capsul pressure, chemical irratation

Pain & Analgesia

• Noxious Insults to Body stimulate Nociceptors

• Nociceptors are activated by:– Mechanical Stimuli– Thermal Stimuli– Chemical Stimuli

Sensations of Pain

• Pricking

• Burning

• Aching

• Stinging

• Soreness

Pain vs. Nociception

• Nociception-reception of signals in CNS evoked by stimulation of specialized sensory receptors (nociceptors) that provide information about tissue damage

• Pain-perception of adversive or unpleasant sensation that originates from a specific region of the body

Perception of Pain

• All perception involves an abstraction and elaboration of sensory inputs

• highly subjective nature of pain is one the factors that makes it difficult to define and treat clinically

Pain

• Conspicuous sensory experience that warns of danger

• Chronic pain is a massive economic problem- in US more than 2 million people are incapacitated by pain at any give time

• Drives most chiropractic practices

Nociceptors

• Least differentiated of all sensory receptors

• Can be sensitized by tissue damage– hyperalgesia

• repeated heating

• axon reflex may cause spread of hyperalgesia in periphery

• sensitization of central nociceptor neurons as a result of sustained activation

Sensitization of Nociceptors

• Potassium from damaged cells-activation

• Serotonin from platelets- activation

• Bradykinin from plasma kininogen-activate

• Histamine from mast cells-activation

• Prostaglandins & leukotriens from arachidonic acid-damaged cells-sensitize

• Substance P from the 1o afferent-sensitize

Nociceptive pathways• Fast• A delta fibers• glutamate• neospinothalamic• mechanical, thermal• good localization• sharp, pricking• terminate in VB

complex of thalamus

• Slow• C fibers• substance P• paleospinothalamic• polymodal/chemical• poor localization• dull, burning, aching• terminate; RF

– tectal area of mesen.

– Periaqueductal gray

Nociceptive pathways

• Spinothalamic-major – neo- fast (A delta)– paleo- slow (C fibers)

• Spinoreticular

• Spinomesencephalic

• Spinocervical (mostly tactile)

• Dorsal columns- (mostly tactile)

Cardinal signs of inflammation

• Rubor-redness

• Calor-heat

• Tumor-swelling

• Dolar-pain

Pain Control Mechanisms

• Peripheral• Gating theory

– involves inhibitory interneruon in cord impacting nocicep. projection neurons

• inhibited by C fibers

• stimulated by A alpha & beta fibers

• TENS

• Central• Direct electrical + to

brain -> analgesia• Nociceptive control

pathways descend to cord

• Endogenous opiods

Endogenous opioids

• Periaquedutal gray– enkephalin projections to Raphe

• Raphe N.– serotonin projections to the cord

• Inhibitory interneurons in cord– release enkephalin which can cause presynatic

inhibition of incoming C fibers and A delta fibers

Pain control

• Endogenous opioid peptides and receptors are located at key points in the pain modulatory system

Surgery to Alleviate Pain

• Over the years surgical intervention to treat pain has been tried at every level of the nervous system from the primary afferent fiber to the cortex

• Procedures not very sucessful

• Pain can return with new sensations often unlike anything the patients have felt before– spontaneous aching, shooting pain, numbness, cold,

heaviness, burning, etc.

Surgery to alleviate pain (cont)

• Central pain syndromes often cause more distress than the pain the operation was intended to relieve

• Many instances of chronic pain result from spontaneous lesions to central sites in nociceptive pathways

• cases of intractable pain resulting from vascular damage to CNS (Dejerine & Roussy)

Headache

• Referred pain to surface of head• Intracranial origins

– meningitis• inflammation of meninges

– migraine• vasocontraction/vasodilatation

– irritation of meninges• e.g. abuse of alcohol• constipation

Headache (cont.)• Extracranial origins

– Muscle spasm• connective tissue bridges between muscle & dura in

upper cervical spine

– Irritation of nasal passages and/or sinuses– eye disorders– cervical joint dysfunction

• spill over of signals from cervical joints (C2) to nucleus of CN V

– traction of dura• mandibular branch of CN V has a recurrent or

meningeal branch which innervates part of dura

Relationship of Cervical Spine to HA • CN V sensory innervation of most of head &

face. (Three divisions).

• CN V nucleus of termination extends all the way down to level of C2.

• Some of cervical joint afferents synapse directly in CN V nuclei.

• C2 afferents synapse both in dorsal horn (DRG) & CN V nuclei.

• Overlap between CN V & C2 can cause headache associated w/ cervical dysfunction

Muscle Receptors• Muscle contain 2 types of sensory receptors

– muscle spindles respond to stretch• located within belly of muscle in parallel with

extrafusal fibers (spindles are intrafusal fibers)

• innervated by 2 types of myelinated afferent fibers– group Ia (large diameter)

– group II (small diameter)

• innervated by gamma motor neurons that regulate the sensitivity of the spindle

– golgi tendon organs respond to tension• located at junction of muscle & tendon

• innervated by group Ib afferent fibers

Muscle Spindles

• Nuclear chain

• Nuclear bag– dynamic– static

• A typical mammalian muscle spindle contains one of each type of bag fiber & a variable number of chain fibers ( 5)

Muscle Spindles

• sensory endings– primary-usually 1/spindle & include all branches

of Ia afferent axon• innervate all three types

• much more sensitive to rate of change of length than secondary endings

– secondary-usually 1/spindle from group II afferent• innervate only on chain and static bag

• information about static length of muscle

Golgi tendon organ (GTO)

• Sensitive to changes in tension

• each tendon organ is innervated by single group Ib axon that branches & intertwines among braided collagen fascicles.

• Stretching tendon organ straightens collagen bundles which compresses & elongates nerve endings causing them to fire

• firing rate very sensitive to changes in tension

• greater response associated with contraction vs. stretch (collagen stiffer than muscle fiber)

CNS control of spindle sensitivity• Gamma motor innervation to the spindle

causes contraction of the ends of the spindle– This allows the spindle to shorten & function

while the muscle is contracting– Spindle operate over wide range of muscle length

• This is due to simultaneously activating both alpha & gamma motor neurons during muscle contraction. (alpha-gamma coactivation)– In slow voluntary movements Ia afferents often

increase rate of discharge as muscle is shortening

CNS control of spindle sensitivity

• In movement the Ia afferent’s discharge rate is very sensitive to variartions in the rate of change of muscle length

• This information can be used by the nervous system to compensate for irregularities in the trajectory of a movement & to detect fatigue of local groups of muscle fibers

Summary

• Spindles in conjunction with GTO’s provide the CNS with continuous information about the mechanical state of a muscle

• For virtually all higher order perceptual processes, the brain must correlate sensory input with motor output to accurately assess the bodies interaction with its environment