neurobiology 205 - exam three

somatic sensation

somatic 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