Cortical Map and Plasticity

Michael Merzenich

American; detailed “brain maps in various sensory systems”; cortical plasticity; helped engineer a version of cochlear implant; FastForWord and Posit Science

Wilder Penfield

Canadian; mapped sensory and motor areas of patients; sensory and motor cortex topographically organized; stimulation of other areas of cortex triggered; one of the originators of the “homunculus”

central sulcus

separates motor cortex and sensory cortex

motor cortex

decisions to move

sensory cortex

information from body is received

homunculus

“little man”; primary somatosensory cortex and primary motor cortex are topographically organized

Vernon Mountcastle

discovered functional organization of cerebral cortex

columnar organization of cortex

all neurons in a vertical cross section of cortex respond to the same sensory signal; neurons are organized in six layers; neurons of a cortical column respond together

David Hubel and Torsten Wiesel

• co- Nobel prize laureates 1981

• famous for discoveries outlining the neuronal processing that occurs within the visual cortex

  • determined critical period for visual information from each eye to map onto cerebral cortex

ocular dominance columns

temporal windows during development in which environmental factors influence the formation of synaptic connection and circuit function

effects of monocular deprivation experiment and result

• sew right eye of kitten shut during first year of life

  • pattern of ocular dominance columns was altered: neurons in columns that would normally respond to right eye respond to left eye

Konrad Lorenz

critical period in animal behavior; imprinting with geese

Michael Merzenich monkey experiment

• mapped cortical area in somatosensory cortex of fingers of monkey

• “use it or lose it” - mapped sensory cortex before and after manipulation of digit 3

  • map for digit 3 disappears and other maps move into the cortical real estate

• “neurons that fire together wire together” - merged two fingers so they respond two the same sensory information

  • cortical maps for digits 3 and 4 merged to form a single map

  • now touching a single point on either of the sewn fingers stimulates the map

sound

change in pressure of air

how does sound enter the ear?

sound hits eardrum/tympanic membrane which is connected to ossicles (malleus, incus, stapes); auditory system

function of auditory/cochlear nerve

send information to brain

cochlea

where sensory information is located [inner ear]

perilymph

fluid that fills the vestibular and tympanic duct

endolymph

fluid that fills the cochlear duct (scala media)

which membranes surround the hairs cells

tectorial membrane

hair cells

basilar membrane

oval window

products movement of perilymph, which in turn products movement of endolymph

organ of corti

where sound is sensed

speedal ganglion

contains neurons, sends information to hair cells

hair cells

sensory neurons of the inner ear, located in organ of corti

describe what happens when endolymph moves and stereocilia bends towards kinocilium

• stretch-activated channels activate

• depolarization of hair cells

• K+ enters

• Ca+2 voltage-gated channels activated

• neurotransmitters released into synapse

  • sufficient activity hits threshold >> action potential

how are the cochlea and auditory cortex organized?

topographically; high frequencies near base and low frequencies near apex

types of peripheral hearing loss

conductive and sensorineural

conductive hearing loss

damage in outer or middle ear

sensorineural hearing loss

damage in inner ear (most commonly, hair cells)

cochlear implants

treatment option for people with severe to profound sensorineural hearing loss; small, complex, electronic device that can help to provide a sense of sound

how does a cochlear implant work?

electrodes inserted in cochlea, electrodes stimulated by microphone, which stimulates neurons; electrical impulses stimulate dendrites of acoustic neurons