The Somatosensory System

Unconscious Pathways

bring unconscious proprioceptive and movement related info to the cerebellum

Spinal Cord Lesions

Can be caused by trauma to the cord, diseases, infection

Spinoreticular Tract(divergent)

-carries slow, aching pain -originates in the body and enters via the dorsal root ganglion -crosses at the spinal level where it enters -ends in the reticular formation in the brainstem(which modulates arousal, attention, and sleep/waking cycles)- thus severe pain disrupts sleep

Spinolimbic Tract(divergent)

-carries slow, aching pain -originates in the body and enters via the dorsal root ganglion -crosses at the spinal level where it enters -passes though the thalamus -ends in the amygdala, basal ganglia, cerebral cortex, and hypothalamus- leads to autonomic and affective responses to pain

Spinomesencephalic Tract(divergent)

-carries slow, aching pain- originates in the body and enters via the dorsal root ganglion -crosses at the spinal level where it enters -ends in the superior colliculus and periaqueductal gray areas in the midbrain -contributes to turning eyes and head toward the source of pain and to activating descending pain control system

Somatosensory cortex lesions

-contralateral decrease or loss of discriminative sensations -These include conscious proprioception, 2 point discrimination, stereognosis, localization of touch & pinprick (nociceptive) stimuli -These require cortical processing

speed of relay and processing of info is based on

-diameter of axon -degree of axon myelination -number of synapses in the pathway: info will travel faster the fewer synapses it has to pass through

somatosensory peripheral neurons have

-distal axons conducting info from receptor to cell body - cell bodies located either outside the spinal cord in the dorsal root ganglia or outside the brain in the cranial nerve ganglia -proximal axons projecting from the cell body into the spinal cord or brainstem

sensory endings

-primary endings- annulospiral endings of afferents wrap around the central region of each intrafusal fibers -secondary endings- flower-spray endings of afferents end on nuclear chain fibers

Trigeminal Lemniscus(conscious relay)

-transmits discriminative touch, temperature, and fast pain from the face -originates at the face then moves to the pons -travels via the trigeminal nerve to the caudal medulla where it crosses and ascends to the ventral posteromedial(VPM) nucleus of the thalamus -ends in the primary somatosensory cortex of the postcentral gyrus

Dorsal Column/ Medial Lemniscus System(conscious relay)

-transmits info about discriminative touch, conscious proprioception, and stereognosis -enters the spinal cord via dorsal root -ascends ipsilaterally in the dorsal column via fasciculus gracilis(lower limb info) or fasciculus cuneatus(upper limb info) -fasciculus gracilis synapse in the nucleus gracilis and fasciculus cuneatus synapse in the nucleus cuneatus - then ascend to the ventral posterolateral(VPL) nucleus in the thalamus -ends in the primary somatosensory cortex of the postcentral gyrus

Rostrospinocerebellar Tract(unconscious relay- internal feedback)

-transmits proprioception and movement from the cervical spinal cord -originates in the body to the ventrolateral gray matter of the spinal cord -travels ipsilaterally to the cerebellum via both the inferior and superior cerebellar peduncles -internal feedback -impairment of this tract can be tested with or without vision

Anterior Spinocerebellar Tract(unconscious relay- internal feedback)

-transmits proprioception and movement from the thoracolumbar spinal cord -originates in the body to the nucleus proprius of the dorsal horn -crosses at the spinal level where it enters - ascends contralaterally in this tract to enter the cerebellum via the superior cerebellar peduncle and going to both sides of the cerebellum -internal feedback

Cuneocerebellar Pathway(unconscious relay- high fidelity)

-transmits proprioception and movement info from the arms and upper body -originates in the arms and upper body and travels via the posterior columns to synapse in the lateral cuneatus nucleus(in the medulla) -forms this tract and enters the cerebellum via ipsilateral inferior cerebellar peduncle

Posterior SpinocerebellarPathway(unconscious relay- high fidelity)

-transmits proprioception and movement info from the legs and lower body -originates in the legs and lower body and travels in the dorsal column to synapse in the nucleus dorsalis(Clark's nucleus) located in the medial dorsal horn of the cord from T1-L2 -forms this tract and remains ipsilateral into the cerebellum via the inferior cerebellar peduncle

Trigeminoreticulolimbic Pathway(divergent)

-transmits slow, aching pain from the face -fibers in the trigeminal nerve synapse in the reticular formation then project into the intralaminar nuclei and onto areas in the cerebral cortex similar to the spinolimbic tract

SpinothalamicTract(anterior and lateral) (conscious relay)

-transmits temperature, fast pain, and coarse touch -enters via the dorsal root -crosses at the spinal level where is enters -travels contralaterally to the VPL of the thalamus -ends in the primary somatosensory cortex of the postcentral gyrus

In posterolateral medulla or lower pons

ipsilateral loss of pain & temperature from the face + contralateral loss of pain & temperature from body - discriminative touch & proprioception intact

Basic Somatosensory Input Pathway

-receptors in periphery receive mechanical, chemical, or thermal stimulation -if the receptor potentials reach threshold, an action potential is produced -the action potential is carried along a peripheral axon to a soma in the dorsal root ganglion -the action potential goes along the proximal axon into the spinal cord -info then ascends via white matter to parts of the brain

Complete somatosensory evaluation measures

-sensitivity(ability to detect a specific stimuli) and thresholds(least amount of stimulation that can be perceived) for stimulation for each conscious sensations to map peripheral nerve distribution(identifying area of skin innervated by a single peripheral nerve)

analgesia

absence of pain in response to stimuli that normally would be painful

chronic low back pain(chronic pain syndrome)

aching pain, muscle guarding, abnormal movement, disuse syndrome

Relief of compression

allows sensory recovery in reverse *demyelination of axons, most severely affects proprioception & vibratory sense

Nociceptive Chronic Pain

due continued stimulation of nociceptive receptors -from cancer or RA - can lead to primary hyperalgesia in which injured tissue has excessive hypersensitivity to stimuli & is a mechanism of protection of injured tissue -involve divergent tracts

Neuropathic Pain

due to abnormal activity within the nervous system - example is phantom limb pain

Motivational-affective

effects of pain on emotions & behavior including increased arousal (which inhibits sleep) & avoidance - travels in spinolimbic & spinoreticular tracts to the medial & intralaminar nuclei of thalamus & limbic system

Nerve Conduction Studies (NCSs)

evaluate the function of peripheral nerves (large diameter fibers with myelin) using surface electrodes to record the electrical activity that results when nerves are stimulated - looks at distal latency, amplitude of the evoked potential & conduction velocity

thermal receptors

free nerve endings respond to warmth or cold within a temperature range that is not damaging to the tissue

nocioceptors

free nerve endings responsive to stimuli perceived as pain

coarse touch

free nerve endings throughout skin- responds to crudely localized touch or pressure, and sensations of tickle and itch

subcutaneous fine touch receptors

have large receptor fields and transmit on afferents -Pacinian corpuscles- respond to touch and vibration -Ruffini's corpuscles- sensitive to stretch of skin

superficial fine motor receptors

have small receptive fields and transmit on afferents -Meissner's corpuscles- sensitive to light touch and vibration -Merkel's disks- sensitive to pressure -Hair follicle receptors- respond to displacement of hair

sensory info is also processed

in the cerebrum which involves sensation and perception

Ruffini's endings

in the joint capsule- signal extremes of joint range, more passive range

Thalamic lesions

in ventral posterolateral (VPL) & posteromedial (VPM) nucleus of the thalamus: decreased or lost sensation in contralateral body or face - rare to have pain

Episodic Tension-type headaches(chronic pain syndrome)

include mild to moderate pain (usually bilateral) lasting 30 minutes to 7 days - not aggravated by physical activity - no nausea or vomiting - may include photophobia or phonophobia but not both

crossed analgesia

indicates that a single lesion can cause pain sensation to be lost on the side of the face ipsilateral to the lesion and on the opposite side of the body

Sensory Ataxia

lesion of the proprioceptive pathway (located in peripheral sensory nerves, dorsal roots, dorsal columns of spinal cord & medial lemnici)

peripheral sensory receptors

located at the distal end of a peripheral neuron and classified by the stimulus they respond to or their response time

Complete transection

loss of all sensation in the dermatomesone or two levels below the level of lesion & loss of voluntary motor control below the lesion

gamma motor neurons(efferents)

maintain sensitivity of the spindle throughout the range of muscle lengths -dynamic axons- end on nuclear bag intrafusal fibers -static axons- end on both nuclear bag and nuclear chain intrafusal fibers

Cognitive-evaluative

meaning the person ascribes to the pain - antinociception (top down inhibition of pain) - pronociception (biological amplification of pain signals)

Chronic Pain Syndrome

pain that persists longer than 6 months after normal healing would have been expected - example is chronic low back pain

sensory info from the musculoskeletal system

proprioception(muscle stretch, tendon tension, joint position, and deep vibrations in both static position sense and kinesthetic or movement sense) and pain

nocioceptors

respond to damage or potential damage resulting in pain sensation- found with each of the other types

thermoreceptors

respond to heating or cooling

joint receptors

respond to mechanical deformation of the capsule and ligaments -types- Ruffini's endings, Paciniform corpuscles, Ligament receptors, and free nerve endings

mechanoreceptors

respond to mechanical deformation of the receptor by touch, pressure, stretch, or vibration

Paciniform corpuscles

respond to movement

chemoreceptors

respond to substances released by cells

golgi tendon organs

respond to tension in the tendon caused by both active and passive stretch- sensitive to light changes

Hemisection

results in Brown-Sequard syndrome = ipsilateral loss of voluntary motor control, conscious proprioception & discriminative touch + contralateral loss of pain & sensation - all below level of lesion

Sensory extinction (sensory inattention)

sensation loss only evident in bilateral testing

somatosensation

sensory information from the skin(superficial or cutaneous) or from the musculoskeletal system

muscle spindle

sensory organ in the muscle- respond to changes in muscle length and velocity of length change -components- muscle fibers, sensory endings, and motor endings

Ligament receptors

signal tension

sensory info from the skin

touch(superficial pressure and vibration), pain, and temperature

Indications for further testing of somatosensation

-any complaints of sensory abnormality or loss -non-painful skin lesions -localized weakness or atrophy

sensory info processed unconsciously

-at the spinal level in local neural circuits -at the cerebellum to adjust movements and posture

Free nerve endings

stimulated by inflammation

Types of muscle/joint pain

-Superficial pain - encourages withdrawal (movement to escape the source of pain) -Deep pain - occurs after tissue damage to encourage rest of the damage tissue

Pathways to the brain(ascending, somatosensory pathways)

-Conscious Relay Pathways -Divergent Pathways -Unconscious Pathways

Neuropathy (Peripheral Nerve Lesions)

-Dysfunction or pathology of one or more peripheral nerves - caused by trauma or disease -Complete severance - lack of sensation in the distribution of the nerve, motor & reflex loss, maybe pain -Compression - affects large myelinated fibers first

Red Flags for Low Back Pain

-No improvement with bed rest -Age > 50 -Previous or current history of cancer -Unexplained weight loss -Severe trauma -History of osteoporosis -Substance abuse -Fever or chills -Recent skin or urinary infection -Immunosuppression -Corticosteroid use -Symptoms unrelated to activity -Sciatica -Leg pain worsens with standing/waling(sitting helps) -Urinary &/or bowel incontinence or retention -Lower limb weakness

Referred Pain

-Pain perceived as arising from somewhere in a site different from the actual site Examples: -Cardiac pain referred to chest & left side of arm (dermatomes T1 - T4) -Gallbladder pain referred to right upper abdomen & scapula (dermatomes T6 - T8) *Very important to identify referred pain to avoid misdiagnosis & allow for appropriate referrals

Neuropathic pain symptoms

-Paresthesia - non-painful abnormal sensation, often described as pricking & tingling -Dysesthesia - painful abnormal sensation, including burning/shooting & aching sensations -Allodynia - sensation of pain in response to normally non-painful stimuli -Secondary hyperalgesia - excessive sensitivity to stimuli that are normally mildly painful in the injured tissue

Limitations of somatosensory evaluations

-Require conscious awareness & cognition -Do not evaluate how somatosensation is used in movements

Cerebral Region Lesions

-Thalamic lesions -Somatosensory cortex lesions

Order of sensory loss

1.Conscious proprioception and discriminative touch 2. Cold 3. Fast pain 4. Heat 5. Slow pain

Chronic Pain

3 Types: -nocioceptive chronic pain -neuropathic pain -chronic pain syndrome

red flags for headache

Caused by excessive pressure (i.e. hydrocephalus, tumor) -Headache present at wakening -Pain triggered by coughing, sneezing, or straining -Vomiting (may be indicative of migraine) -Worse when lying down Caused by serious intracranial disease (i.e. tumor encephalitis, meningitis) -Progressive worsening over days & weeks -Neck stiffness & vomiting (meninges irritation) -Rash & fever (bacterial meningitis or Lyme disease) -History of cancer, HIV infection Caused by hemorrhage -Headache following head injury -Abrupt onset *Headache + onset of paralysis or altered consciousness - needs attn*

Sensory Ataxia

Conscious Proprioception: impaired Conscious Vibratory Sense: impaired Standing balance: worse with eye closed

Cerebellar Ataxia

Conscious Proprioception: normal Conscious Vibratory Sense: normal Standing balance: poor with eyes open or closed

Pain Matrix

Includes parts of the brainstem, amygdala, hypothalamus, thalamus & areas of the cerebral cortex + nociceptors -Creates multiple aspects of the pain experience

Incoordination not due to weakness

Three types: 1. Sensory 2. Vestibular 3. Cerebellar

Brain Region Lesions

Usually cause a mix of ipsilateral & contalateral signs except in the upper midbrain where all sensory loss is contralateral

phasic receptor

a sensory nerve ending that adapts to a constant stimulus and stops responding- i.e. stretch on the bladder

tonic receptors

a sensory nerve ending that responds as long as a stimulus is present

receptive field

area of the skin innervated by a single afferent neuron -distally the receptive fields are smaller and have a greater density of receptors -proximally the receptive fields are larger

Varicella Zoster (shingles)

caused by an infection of the dorsal root ganglion with varicella-zoster virus - itching, burning, tingling, & pain in one dermatome (along one peripheral nerve) or trigeminal cranial nerve - lasts 1 to 4 weeks

Complex Regional Pain Syndrome(CRPS) (pain matrix malfunction)

chronic syndrome of pain, vascular changes, & atrophy in a regional distribution - also known as causalgia, Sudeck's atrophy, sympathetically maintained pain, reflex sympathetic dystrophy

intrafusal fibers

connect to extrafusal fibers outside the spindle- contract only at their ends as central regions cannot contract -2 types- -nuclear bag fibers- clump of nuclei in the central region -nuclear chain fibers- nuclei arranged in single file

In medial medulla or lower pons

contralateral loss of pain in face + contralateral loss of discriminative touch & conscious proprioception in body

In posterolateral upper pons or midbrain

contralateral sensory loss in face (except proprioception) & all contralateral loss in body

Info carried in the 4 spinocerebellar tracts

contribute to automatic movements and postural adjustments and does not reach consciousness

Divergent Pathways

convey info not somatotopically organized to many areas of the brain

Conscious Relay Pathways

conveys high-fidelity somatotopically arranged info to cerebral cortex- transmits discriminative touch and proprioceptive info ipsilaterally, and discriminative pain and temperature contralaterally

Antalgic gait

modified gait in response to lower limb injury - characterized by a shortened stance on the affected side

Peripheral sensitization

nociceptors awaken in response to injury or ischemia excessively react to the stimuli

Migraine(chronic pain syndrome)

syndrome includes headache, nausea, vomiting, extreme sensitivity to light & sound, dizziness, & cognitive disturbances - some do not include headache - some are preceded by an aura

Fibromyalgia(pain matrix malfunction)

tenderness of muscles & adjacent soft tissues, stiffness of muscles, & aching pain - the painful area shows a regional rather than dermatomal or peripheral nerve distribution

Somatosensory Evoked Potentials (SEPs)

test transmission of sensory info in both peripheral nerves & CNS pathways - stimulate the skin over a peripheral nerve & record the resulting electrical activity from the skin at the cervical spinal cord & from the scalp over pirmary somatosensory cortex

Discriminative

the ability to localize the site, timing, & intensity of tissue damage or potential damage - travels in the spinothalamic tract & processed in somatosensory & insular cortex

dermatome

the area of skin innervated by axons from cell bodies in a single dorsal root

postherpetic neuralgia

varicella zoster(shingles) that is severe or untreated= severe pain persisting more than 1 month