Studied by 1 personsomatic sensation
enables our body to feel, to ache, to sense hot or cold, and to know what the body is doing
receptors are distributed throughout body rather than being concentrated at small, specialized location
propioception
sense of body position
skin
protects, prevents evaporation of body fluids, largest sensory organ
hairy and glabrous (hairless) skin
mechanoreceptors
most of sensory receptors, unmyelinated axons sensitive to bending, stretching, pressure, or vibration; monitor skin contact, pressure in heart and blood vessels, stretching of digestive organs and urinary bladder and force against teeth
pacinian corpuscle
deep in dermis, long as 2 mm and 1 mm in diameter; 2500 Pacinian corpuscles with highest densities in fingers; large receptive fields that cover entire fingers or half a palm; respond quickly but stop firing as stimulus continues; most sensitive to vibrations at 200-300 hz
ruffini’s endings
found in both hairy and glabrous skin; large receptive fields that cover entire fingers or half a palm; generate a more sustained response during a long stimulus
meissner’s corpuscles
1/10 sizes of pacnian corpuscles, located in ridges of glabrous skin (raised parts of fingerprints); small receptive fields; respond quickly but stop firing as stimulus continues; respond best to 50 Hz but also 1-10 Hz
merkel’s disks
nerve terminal and flattened, non neural epithelial cell; small receptive fields; generate a more sustained response during a long stinulus
krause end bulbs
border regions of dry skin and mucous membrane (lips + genitals), nerve terminals look like knotted balls of string
hair
part of sensitive receptor system; grow from follicles that are innerved by the terminations of single axons that either run parallel to it or wrap around the follicle; details of innervation differ on types of hair follicles, but in all, the bending of hair causes a deformation of follicle and surrounding skin tissues which deforms nerve endings; mechanoreceptors can be slowly adapting or rapidly adapting
pacinian corpuscle mechanism
capsule compressed → energy transfer to nerve terminal → deformed membrane → mechanosensitive channels open → current flow generates depolarizing receptor potential → action potential in axon
maintained pressure inhibits deformation of axon terminal and the action potential stops until the pressure is no longer applied
unmyelinated axon terminals
have mechanosensitive ion channels that convert mechanical force into change in ionic current by altering gating or change increasing/decreasing channel opening
two point discrimination
higher density of mechanoreceptors
fingertips enriched with receptors with small receptive fields
more brain tissue devoted to sensory information of fingertips than elsewhere
special neural mechanisms devoted to high-resolution discriminations
primary afferent axons
information from somatic sensory receptors to spinal cord/brain stem, enter spinal cord through dorsal roots, cell bodies lie in dorsal root ganglia; varying diameters and type of sensory receptor attached
Aα axons
thickest diameter, myelinated, fastest, proprioceptors of skeletal muscle
Aβ axons
myelinated, second fastest, mechanoreceptors of skin
Aδ axons
myelinated, third fastest, fast sharp pain, temperature
C axons
nonmyelinated, slow; temperature, slow dull pain, itch
spinal nerves
cervial: C 1-8
thoracic: T 1-12
lumbar: L 1-5
sacral: S 1-5
dermatome
area of skin innervated by right and left dorsal roots of a single spinal segment; delineate sets of bands on body surface correlated with skin sensation
dorsal root cuts
corresponding dermatome on one side of body doesn’t lose all sensation because adjacent dorsal roots innervate overlapping areas; cutting three adjacent dorsal roots causes loss in sensation in one dermatome
cauda equina
spinal nerves stream down within lumbar and sacral vertebral column (spinal cord ends around third lumbar vertebrae); filled with CSF in dura sack
lumbar puncture: collection of CSF, needle inserted into CSF-filled cistern at the midline
spinal gray matter
inner core, divided into dorsal horn, intermediate zone, and ventral horn
second-order sensory neurons: receive sensory input from primary afferents, lie within dorsal horns
spinal white matter
thick, outer covering divided into columns
dorsal column-medial lemniscal pathway
information about touch or vibration of skin
large dorsal root axon → ipsilateral dorsal column nuclei → medial lemniscus → medulla, pons, midbrain (no longer ispilateral) → ventral posterior (VP) nucleus in thalamus → primary somatosensory cortex S1
conservation of information
information is altered every time it passes through synapses in brain; strength might be changed, inhibition, cortex output may influence cortex input
trigeminal nerves
somatic sensation of face, enters brain at pons
two twin trigeminal nerves break up into three peripheral nerves that innervate face, mouth areas, outer two-thirds of tongue, and dura mater
supplemented with facial, glossopharyngeal and vagus nerves to sense skin around ears, nasal areas, and pharynx
trigeminal touch pathway
sensory axons of trigeminal nerve → ipsilateral trigeminal nucleus → decussate and project into medial VP nucleus in thalamus → somatosensory cortex
primary somatosensory cortex
(S1; areas 1, 2, 3a and 3b)
post central gyrus
area 3b
primary somatic sensory cortex; receives dense inputs from VP nucleus, responsible to somatosensory stimuli only, lesions impair somatic sensation, and provokes somatic sensory experiences with electrically stimulated; more concerned with sense of body position rather than touch
area 1
texture information from area 3b
area 2
size and shape information from area 3b
somatosensory cortex pathway
thalamic inputs to S1 terminate mainly in layer IV, which then project to cells in other layers; S1 neurons with similar inputs and responses are stacked vertically into columns that extend across cortical layers with alternating columns of rapidly adapting and slowly adapting sensory responses
somatotopy
mapping of sensations on body’s surface onto a structure in the brain; roughly resembles a body with legs and feet at the top of the postcentral gyrus and its head at the opposite, lower end of the gyrus
homunculus
little man in the brain
represents quantity of brain dedicated to sensation of body parts - not scaled to human body proportions
somatotopic maps
map is not always continuous but can be broken up: hand separates head and face
map is not scaled: mouth, tongue, and fingers have larger input density and more important sensory input; trunk, arms and legs have smaller input density
map varies among species: whiskers take up large areas of S1 in rodents
not limited to a single map, somatic sensory system has several maps of body
cortical map plasticity
cortical maps are dynamic and adjust depending on amount of sensory experience; map plasticity is widespread in the brain
phantom limb
stimulation of skin regions whose somatotopic representations border those of missing limb (phantom arm when face is stimulated) because original areas are now activated from stimulation in a different part of body
posterior parietal cortex
neurons with large receptive fields, hard to characterize elaborate stimulus preferences; concerned with somatic sensation, visual stimuli, movement planning, and attentiveness; perception and interpretation of spatial relationships, accurate body image, learning of tasks involving coordination of body in space
neglect syndrome
part of body or part of world is ignored or suppressed, and its existence is denied; most common following damage to right hemisphere and usually improve or disappear with time
ex. someone insists amputated leg is in bed, falls on floor, is not recognized as part of his body
nociceptors
free, branching, unmyelinated nerve endings that signal that body tissue is damaged or at risk of being damaged
present in most body tissues: skin, bone, muscle, internal organs, blood vessels, heart, meninges, but not brain
contain ion channels receptors stimulated by mechanical stimulation, extreme temperature, oxygen deprivation and certain chemicals
mechanical nociceptors
reacts to strong pressure through change in membrane (stretch, bend, or force between channels, extracellular proteins or intracellular cytoskeletal components) OR release of second messengers that regulate ion channels
thermal nociceptors
reacts to extreme hot/cold through the opening of heat sensitive ion channels
chemical nociceptors
respond to histamine, proteases, ATP, K+ ion channels that bind to specialized gated ion channels
proteases
enzymes that digest proteins; break down abundant extracellular peptide kininogen to form bradykinin. Bradykinin binds to receptors that activate ionic conductances in sone nociceptors
pain
perception of irritating, sore, stinging, aching, throbbing, miserable, or unbearable sensations arising from the body
nociception
sensory process that provides signals that trigger pain
pain and nociception may happen independently of each other
hyperalgesia
reduced threshold for pain, an increased intensity of painful stimuli, or spontaneous pain
primary hyperalgesia
occurs within damaged tissue
secondary hyperalgesia
hypersensitivity in tissues surrounding a damaged area
substance P
CNS mechanisms: activation of mechanoreceptors Abeta by light touch may evoke pain due to communication between touch and pain pathways in spinal cord
inflammation
response of body tissues in attempt to eliminate injury and stimulate healing process; characterized by pain, heat, redness, and swelling
histamine
mast cells, part of immune system, are activated by exposure to foreign substances, which bind to surface receptors on nociceptors AND causes blood capillaries to become leaky, leading to swelling and redness at injury site
inflammatory soup
neurotransmitters (glutamate, serotonin, adenosine, ATP), peptides (substance P, bradykinin), lipids, (prostaglandins, endocannabinoids), proteases, neurotrophins, cytokines, chemokines, and ions
trigger inflammation and modulate excitability of nociceptors, increasing the sensitivity to thermal or mechanical stimuli
bradykinin
directly depolarizes nociceptors; long-lasting intracellular
prostaglandins
generated by enzymatic breakdown of lipid membrane; don’t elicit pain but increase nociceptor sensitivity
aspirin and NSAIDS used to treat hyperalgesia because they inhibit enzymes that synthesize prostaglandins
substance P
synthesized by nociceptors and is released by axon branches following activation of another axon branch; causes vasodilation and histamine release, increasing sensitization and may cause secondary hyperalgesia
referred pain
information from viscera and cutaneous nociceptors mixed in spinal cord
visceral nociceptor activation perceived as cutaneous sensation
ex. heart attack causes pain in chest and left arm, appendicitis pain in abdominal wall near naval
pain afferent neurotransmitter
glutamate; contain substance P which is released by high-frequency trains of action potentials and are necessary for moderate to intense pain
touch pathway
specialized structures in skin; swift, myelinated Aβ fibers; terminate in deep dorsal horn
ascends ipsilaterally
pain pathway
free nerve endings; slow with thin lightly myelinated Aδ and C fibers; Aδ and C fibers branch, run in zone of Lissauer and terminate in substantia gelatinosa
ascends contralaterally
spinothalamic pain pathway
spinothalamic fibers → spinal cord → medula, pons, midbrain without synapsing → thalamus → along medial lemniscus but remains separate
medial lemniscal pathway
axons of second-order neurons decussate at BRAIN → spinothalamic tract → ventral surface of spinal cord
trigeminal pain pathway
small-diameter fibers in trigeminal nerve → second-order sensory neurons in spinal trigeminal nucleus of brain stem → trigeminal lemniscus → thalamus
region of pathway effect
spinothalamic tract and trigeminal lemniscal axons synapse over a wider region of thalamus than those of medial lemniscus; some terminate in VP nucleus but touch and pain still remain segregated, other spinothalamic axons end in small intralaminar nuclei of thalamus
gate theory of pain
certain neurons of dorsal horns, which project an axon up spinothalamic tract are excited by large-diameter sensory axons and unmyelinated pain axons; projection neuron is inhibited by an interneuron and interneuon is excited by sensory axon and inhibited by pain axon
activity in pain axon alone maximally excites projection axon, allowing nociceptive signals to rise to brain, but if the large mechanoreceptive axon fires concurrently, the interneuron is activated and suppresses nociceptive signals
descending regulation
dorsal horn of spinal cord → raphe nuclei of medulla → periaqueductal gray matter (PAG) of midbrain
periaqueductal gray matter
zone of neurons in midbrain suppresses pain when stimulated electrically; modulates flow of nociceptive information in spinal cord upon input from brain structures related to emotion
endorphins
endogenous morphine-like substances that bind to opioid receptors; highly concentrated in areas that process and modulate nociceptive information; suppress glutamate release and inhibit through hyperpolarization of postsynaptic membranes
sound
audible variations in air pressure (compressed and rarefied air); all sound propagates at same speed but varies by frequency and intensity
frequency
number of compressed or rarefied patches of air that pass our ears each second (hertz Hz)
pitch
high or low tone, determined by frequency
intensity
amplitude, difference in pressure between compressed and rarefied patches, determines volume (high intensity = loud sounds)
outer ear
pinna to tympanic membrane
pinna
outer ear, helps collect sounds from a wide area; more sensitive to sounds from ahead rather than behind - localize sounds
cartilage covered by skin
auditory canal
2.5 cm inside skull, entrance to internal ear
middle ear
tympanic membrane, ossicles, muscles
tympanic membrane
eardrum, conical in shape
moves ossicles in response to sound waves
ossicles
attached to eardrum, series of bones, transfer movements of tympanic membrane to oval window; amplify sound waves 20x due to greater force on oval window than tympanic membrane and smaller surface area of oval window than tympanic membrane
higher pressure needed to move fluid in cochlea than the air outside
malleus
hammer shaped, attaches to tympanic membrane and forms rigid connection with incus
incus
anvil shaped, between malleus and stapes
stapes
stirrup shaped, flexible connection with incus, flat bottom portion (footplate) moves in and out like a piston at the oval window to transmit sound vibrations to cochlea
estuachian tube
air in middle ear continuous with nasal cavities (valve usually keeps it shut)
equalizes pressure between atmosphere and ear
attenuation reflex
when tensor tympani and stapedius muscles contract, chain of ossicles becomes more rigid and sound conduction diminishes; greater at low frequencies and loud sounds
adapts ear to continuous sound at high intensities; protects inner ear from loud sounds; activated when we speak to diminish hearing of our own voices
tensor tympani muscle
anchored to bone in cavity of middle ear and attaches to malleus
stapedius muscle
fixed anchor of bone and attaches to stapes
inner ear
apparatus medial to oval window
oval window
second membrane covers hole in skull
cochlea
snail shaped; fluid behind oval window, transforms physical motion of membrane into neural response
reisnner’s membrane
separates scala vestibuli from scala media
basilar membrane
separates scala tympani from scala media
organ of corti
contains auditory receptor neurons
hair cells
contain 10-300 stereocilia; not neurons and do not generate action potentials; sandwiched between basilar membrane and reticular lamina; synapse onto neurons in spiral ganglion within modiolus
rods of corti
span across hair cells and basilar membrane and provide structural support
tectorial membrane
hangs over organ of corti
perilymph
fluid in scala vestibuli and scala tympani; like CSF: low K+ and high Na+; soundwaves cause perilymph to flow between scala vestibuli and scala tympani
endolymph
fluid in scala media; like intracellular fluid: high K+ and low Na+; causes basilar membrane to bend
stria vascularis
active transport process that establishes difference in ion content; absorbs sodium from and secretes potassium into endolymph
endocochlear potential
endolymph has electrical potential 80 mV more than perilymph; enhances auditory transduction
helicotrema
point where scala vestibuli and scala tympani become continuous at apex
basilar membrane
separates scala tympani from scala media
membrane wider at apex than base
stiffness of membrane decreases from base to apex
flipper-like
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