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somatosensation

the "body senses", deals with processing stimuli

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3 separate but interactive systems of somatosensation

exteroceptive system
proprioceptive system
interoceptive system

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skin

largest sensory organ, heaviest organ in the body, function is to prevent fluid from escaping, protection from dirt and pathogens, and report info about stimuli it comes in contact with

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two types of skin

hairy and glabrous (hairless skin)

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two layers of skin

epidermis and dermis

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epidermis layer of skin

outermost layers, including several layers of dead skin cells

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dermis layer of skin

inner layers, including matrices of glands, blood vessels, muscle, and hair

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hypodermis layer of skin

layer of connective adipose tissue

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mechanoreceptors

receptors that respond to mechanical stimulation such as pressure and stretching, change in conformation leads to opening of ion channels and membrane depolarization

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4 basic types of mechanoreceptors

Meissner Corpuscles
Pacinian Corpuscles
Merkel's Disks
Ruffini's endings

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Meissner Corpuscles

respond to light tapping (touch) and flutter, found on hairless skin (lips, fingertips, palms, nipples), small receptive field

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Pacinian Corpuscles

respond to pressure and vibration, large receptive field (intestines and genitalia), located deep within dermis in both skin types

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Merkel's Disks

sensitive to fine detail and mechanical stimulation, small receptive fields (ridges of fingertips), located in outer layer of skin (epidermis)

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Ruffini's endings

respond to stretching of skin, proprioception, large receptive field, found on both types of skin (hairy and non-hairy)

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touch information travels:

FAST (due to heavy myelination which increases speed of electro-transmission)

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review: dorsal vs. ventral

dorsal= sensory
ventral= motor

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mechanoreceptor cell bodies are located in the:

Dorsal Root Ganglion (DRG)

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sensory information enters the spinal cord through the:

dorsal horn

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properties of the spinal nerves

there are 30 spinal nerves, each nerve has a dorsal root and a ventral root, enters spinal cord through notch in vertebrae

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touch acuity

the ability to distinguish a separation between two closely adjacent stimuli applied to the skin, "two point" discrimination

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touch information travels:

FAST (due to heavy myelination which increases speed of electro-transmission)

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dermatome

the area of skin innervated by right and left dorsal roots of a single spinal segment

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what happens to adjacent dorsal roots in innervation

adjacent dorsal roots slightly overlap in innervation

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review: grey matter vs. white matter

grey matter contains cell bodies, white matter is myelination for axons

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sensory information travels in 2 paths

1. to interneurons in dorsal horn= rapid reflex response
2. up spinal cord to dorsal columns all the way to the brain for conscious perception of the stimuli

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asomatognosia

the failure to recognize parts of one's own body (neglect syndrome, right hemi damage), result of damage to somatosensory association cortex

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DCML "Dorsal Column-Medial Lemniscus Pathway"

the ascending touch pathway:
-ascend ipsilaterally through dorsal column nuclei in medulla
-decussate at level of medulla, info transmitted contralateral
-ascend white matter tract
-synapse in ventro-posterior (VP) thalamus
-project to primary somatosensory cortex

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cortical mapping of somatosensation

founded by Wilder Penfield, mapped out S1 using electrical circuits on the brain to determine where different parts of the body were mapped

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primary SS cortex names

post central gyrus= primary somatosensory cortex: S1, 3b, parietal lobe

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homunculus

concept of a topographical map of the body in the SS cortex, is "little man" in Latin, body part mapping is not proportional or continuous

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corresponding area in the cortex is proportional to:

sensitivity of that body region

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different regions of skin have different sensitivities due to:

variation in receptor type and density and differences in amount of brain tissue dedicated to sensory input from that area= "importance"

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Phantom Limb

compelling perception that an amputated limb or organ is still attached to the body and functioning, can be frustrating, painful, itchy, feel on fire, previously considered a psychological disorder

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explanation of phantom limb:

rewiring of the brain to adjacent area, it is a result of reorganization of S1 cortical map, proven by fMRI activation of cortical regions devoted to missing limb (ex: activation of area devoted to amputated arm when stimulate face)

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collateral sprouting

new branching of undamaged axons to vacant area (can be good, bad, or neither)

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neuroplasticity

neurons from adjacent body parts in S1 grow into vacant synapses and relay info to association cortices

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reason for plasticity

cortical reorganization occurs due to lack of input in adjacent areas (proven through owl monkey study with amputated finger)

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association somatosensory cortex

S1 -> somatosensory association cortex (posterior parietal lobe)= bilateral info from both sides of body is processed

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damage to somatosensory association cortex results in:

astereognosia or asomatognosia

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astereognosia

inability to recognize objects by touch, result of damage to somatosensory association cortex

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asomatognosia

the failure to recognize parts of one's own body (neglect syndrome, right hemi damage), result of damage to somatosensory association cortex

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thermoreceptors

receptors that respond to certain temperatures and changes in temperature, free nerve endings, both warm and cold receptors

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warm receptors

fire to increase in temperature or steady high temperature, located deep within the dermis, Action Spectrum (86-118F)

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cold receptors

fire to decrease in temperature or steady low temperature, located close to the surface, action spectrum (68-113F)

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response to extreme temperatures:

nociceptors that respond to extreme temperatures, related to pain, non-selective ion channels activated by heat > 43C, some cold receptors have cation channels too

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nociceptors

free nerve endings activated by stimuli that have potential to cause tissue damage (i.e. pain)

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location and activation of nociceptors

distributed throughout the skin, muscles, joins, and connective tissue (none in brain), some activated by pressure in the joints, some activated by chemicals released from damaged tissue (puncture)

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pain and temperature cell bodies live in:

DRG

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axons enter spinal cord through:

dorsal root

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afferents divide into 2 branches: (for pain/temp)

travel a bit up spinal cord in zone of lussauer, or synapse on a secondary neuron that decussates and travels up spinothalamic tract to brain

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pain and temperature path (spinothalamic pathway)

-info enters the dorsal root and synapses on secondary neurons
-secondary neuron decussates immediately and ascends the spinal cord in the spinothalamic tract
-synapses in the ventral posterior (VP) nucleus of the thalamus
-projects to primary somatosensory cortex

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pain information travels:

SLOWER than touch info (due to less myelination on axons)

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2 types of afferent fibers for pain/temp:

small and myelinated- faster response to pain (sharp pain), then small and unmyelinated- slower response to pain (dull ache)

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spinothalamic pathway

path pain and temperature information takes to get from stimuli to S1

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differences between DCML and spinothalamic paths

DCML- (touch), synapses on medulla, doesn't cross midline until midbrain. Spinothalamic- (pain and temp), synapses upon entering spinal cord, crosses midline almost immediately

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Brown-Sequard Syndrome

constellation of sensory and motor signs following damage to one side of spinal cord (because pain and touch signals cross at different areas)

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descending modulation of pain

electrical stimulation of the periaqueductal gray (PAG) has analgesic effects

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PAG as modulation of pain

PAG receives info from brain regions related to emotion, and strong emotion= PAG excitation through disinhibition

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opitates:

act on receptors for endogenous opioids/endorphins

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acupuncture

system of complementary medicine that involves pricking the skin or tissue with needles, used to alleviate pain

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placebo effect

measurable, observable, or felt improvement in health or behavior not attributable to medication or treatment

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why does acupuncture or placebo effect work?

this type of pain relief block pain by u-opioid receptor blockers, activation of endogenous opioids