95 terms

The Somatosensory System

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somatosensation
sensory information from the skin(superficial or cutaneous) or from the musculoskeletal system
sensory info from the skin
touch(superficial pressure and vibration), pain, and temperature
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
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
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
sensory info processed unconsciously
-at the spinal level in local neural circuits
-at the cerebellum to adjust movements and posture
sensory info is also processed
in the cerebrum which involves sensation and perception
peripheral sensory receptors
located at the distal end of a peripheral neuron and classified by the stimulus they respond to or their response time
mechanoreceptors
respond to mechanical deformation of the receptor by touch, pressure, stretch, or vibration
chemoreceptors
respond to substances released by cells
thermoreceptors
respond to heating or cooling
nocioceptors
respond to damage or potential damage resulting in pain sensation- found with each of the other types
tonic receptors
a sensory nerve ending that responds as long as a stimulus is present
phasic receptor
a sensory nerve ending that adapts to a constant stimulus and stops responding- i.e. stretch on the bladder
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
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
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
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
coarse touch
free nerve endings throughout skin- responds to crudely localized touch or pressure, and sensations of tickle and itch
nocioceptors
free nerve endings responsive to stimuli perceived as pain
thermal receptors
free nerve endings respond to warmth or cold within a temperature range that is not damaging to the tissue
dermatome
the area of skin innervated by axons from cell bodies in a single dorsal root
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
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
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
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
golgi tendon organs
respond to tension in the tendon caused by both active and passive stretch- sensitive to light changes
joint receptors
respond to mechanical deformation of the capsule and ligaments
-types- Ruffini's endings, Paciniform corpuscles, Ligament receptors, and free nerve endings
Ruffini's endings
in the joint capsule- signal extremes of joint range, more passive range
Paciniform corpuscles
respond to movement
Ligament receptors
signal tension
Free nerve endings
stimulated by inflammation
Pathways to the brain(ascending, somatosensory pathways)
-Conscious Relay Pathways
-Divergent Pathways
-Unconscious Pathways
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
Divergent Pathways
convey info not somatotopically organized to many areas of the brain
Unconscious Pathways
bring unconscious proprioceptive and movement related info to the cerebellum
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
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
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
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
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
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
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
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
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
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
Info carried in the 4 spinocerebellar tracts
contribute to automatic movements and postural adjustments and does not reach consciousness
analgesia
absence of pain in response to stimuli that normally would be painful
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
Indications for further testing of somatosensation
-any complaints of sensory abnormality or loss
-non-painful skin lesions
-localized weakness or atrophy
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)
Limitations of somatosensory evaluations
-Require conscious awareness & cognition
-Do not evaluate how somatosensation is used in movements
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
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
Sensory Ataxia
lesion of the proprioceptive pathway (located in peripheral sensory nerves, dorsal roots, dorsal columns of spinal cord & medial lemnici)
Incoordination not due to weakness
Three types:
1. Sensory
2. Vestibular
3. Cerebellar
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
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
Order of sensory loss
1.Conscious proprioception and discriminative touch
2. Cold
3. Fast pain
4. Heat
5. Slow pain
Relief of compression
allows sensory recovery in reverse
*demyelination of axons, most severely affects proprioception & vibratory sense
Spinal Cord Lesions
Can be caused by trauma to the cord, diseases, infection
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
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
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
postherpetic neuralgia
varicella zoster(shingles) that is severe or untreated= severe pain persisting more than 1 month
Brain Region Lesions
Usually cause a mix of ipsilateral & contalateral signs except in the upper midbrain where all sensory loss is contralateral
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
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
Cerebral Region Lesions
-Thalamic lesions
-Somatosensory cortex lesions
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
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
Sensory extinction (sensory inattention)
sensation loss only evident in bilateral testing
Peripheral sensitization
nociceptors awaken in response to injury or ischemia excessively react to the stimuli
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
Antalgic gait
modified gait in response to lower limb injury - characterized by a shortened stance on the affected side
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
Pain Matrix
Includes parts of the brainstem, amygdala, hypothalamus, thalamus & areas of the cerebral cortex + nociceptors
-Creates multiple aspects of the pain experience
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
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
Cognitive-evaluative
meaning the person ascribes to the pain
- antinociception (top down inhibition of pain)
- pronociception (biological amplification of pain signals)
Chronic Pain
3 Types:
-nocioceptive chronic pain
-neuropathic pain
-chronic pain syndrome
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
Chronic Pain Syndrome
pain that persists longer than 6 months after normal healing would have been expected - example is chronic low back pain
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
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
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
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
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
chronic low back pain(chronic pain syndrome)
aching pain, muscle guarding, abnormal movement, disuse 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
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