what are receptive fields?
sensitive areas that have more sensitive areas with smaller receptor fields
-ex. hands, lips, tongue, face, feet
long vs. short axon
- long = depolarization near stimulus –> action potential –> conduction into CNS (ex. somatic, visceral, olfactory)
- short = depolarization near stimulus –> synapse to primary afferent –> conducted into CNS (ex. taste, photo, hair)
where are synapses located?
- somatosensory = synapses in the CNS
- hair and photo = synapses on peripheral nerve endings of vestibular cochlear nerve or retinal interneurons
special somatosensory receptors
- meisnner - tactile shape
- merkel - tactile indentations
- hair - tactile in hairy skin
- Ruffini - stretching and shapes
- pacinian - vibrations
- muscle spindles - proprioception
types of nociceptors
- Adelta - encapsulated; pinprick
2. C-polymodal - free nerve ending; tissue damage
Meissner’s corpuscle
- fingers, palms, soles, toes
- touch and low frequency vibration
- ex. movement across skin
merkel receptors
- lips, genitalia
- shape and texture of non-moving objects touching skin
pacinian corpuscle
- hands, feet, nipples
- high frequency vibrations
- breast feeding
Ruffini corpuscle
wide distribution - skin stretch
where are the cell bodies of proprioception and tactile sensation located?**
dorsal root ganglion
function of T6
- below T6 - fasciculus gracilis –> innervate legs
- above T6 - fasciculus cuneatus –> innervate arms
different positions of neurons
- 1st order - dorsal root ganglion
- 2nd order - in medulla
- 3rd order - VPL of the thalamus
afferent vs. efferent myotatic/stretch reflex
- afferent - intrafusal muscle fibers; stretch sensation
- efferent - extrafusal muscle fibers; contract to counteract the stretch
posterior spinocerebellar tract
- proprioception from muscle spindles and Golgi tendons
- does NOT cross (decussate)
anterior spinocerebellar tract
- proprioception and cutaneous info.
- crosses twice (spinal cord and pons)
cerebellar ataxia
uncoordinated gate –> cerebellum not processing proprioceptive stimuli
Friedreichs ataxia
neurodegenerative –> affect spinocerebellar tracts
- lack of upper limb coordination
- wide gate; wheel chair early adult
- autosomal recessive - FXN gene
what is fast and acute pain conducted by?
carried to CNS by myelinated Adelta fibers
-Adelta also carry cold stimulus
what is slow and chronic pain conducted by?
carried to CNS by unmyelinated C fibers
-C fibers also carry heat stimulus
neospinothalamic tract
- pain signals to brain
- fibers cross in lateral funiculus
anterolateral cordotomy
lesioning spinothalamic tract to interrupt pain transmission
periaquaductal gray
surrounds aquaduct and is origin of a descending pain-control pathway
-start of controlling how you feel pain
role of enkephalins
inhibit NT release from central process of nociceptive dorsal root ganglion neurons
what does stimulation of descending pain pathways result in?
analgesia
capsaicin vs. menthol
capsaicin - binds to heat channels priming them
menthol - binds to cold channels priming them
neospinothalamic tract injury
injury –> loss of pain and thermal sensation on contralateral side inferior to the level of the lesion
primary somatosensory cortex or thalamic lesion - left side lesion
loss of contralateral sensation for vibration, position, sense, touch, pain, temp., on entire right side of body
lateral pontine or medullary lesion - left side lesion
- loss of sensation of pain and temp of ipsilateral face
- loss of sensation of pain and temp of contralateral body
medial medullary lesion
affects posteromedial column
- no affect on face sensation
- loss of contralateral sensation to body
distal symmetrical polyneuropathy
most distal parts of limbs affected
-starts distally works proximally
isolated nerve lesion
stabbing most common cause
posterior column medial lemniscal pathway
vibration, sense, touch, position
anterolateral pathway
pain and temp.
hyperalgesia
enhancement of pain sensation from tissue damage and release of endogenous chemicals
- activate or lowers nociceptor threshold
- treat with aspirin/NSAIDs
allodynia
pain sensitization, sun burn, sore throat
- caused by thermal, mechanical, chemical
- symptom of fibromyalgia, migraines, neuropathies
phantom limb pain
overactive dorsal horn neurons on same side as amputated limb - pain from amputated limb
causalgia
sympathetic dystrophy syndrome
-burning sensation from increased sympathetic efferent activity - peripheral nerve injury
neuralgia
suicide pain - can be debilitating and severe
-ex. trigeminal neuralgia
paresthesia
sensation with no apparent cause
-caused by nerve compression or PVD
meralgia paresthetica
numbess/tingling of outside thigh
-compression of pants on lateral cutaneous nerve
thalamic pain syndrome aka dejerine-roussy syndrome
lesions of posterior thalamus causing chronic pain
-occurs after stroke
referred pain
pain from deep visceral structures felt on surface
- mistake location of pain
- ex. myocardial ischemia radiating to arms/wrists
2 point discrimination
help detect early stage neuropathies
-fingertips and toes have small receptor fields
