neuro unit 2

receptive field

area of skin surface over which stimulation results in a significant change in firing rate of action potentials

spatial accuracy

ability to discern the presence of 2 tactile stimuli separated by a small distance

smaller receptive fields = higher spatial accuracy

pseudounipolar neurons

have soma and axon but no dendrites

axon is split in 2 directions: one towards SC, one towards skin/muscle

somatosensory system afferents

why do pseudounipolar neurons not have dendrites

no need to sense chemical stimuli from environment

mechanoreceptors

respond to mechanical stimulation

stretch activated channels

channels open with deformation or stretch of skin or muscle

allows influx of Na or Ca that depolarize afferents and cause action potentials

receptor potential

change in membrane potential of receptor afferent by cation influx (na and ca)

graded potentials w/ strength of stimulus (more stretch = higher potential)

slow adapting receptors

continue to respond throughout entire duration of stimulus

fast adapting receptors

quickly stops responding to stimulus

merkel receptors

slow adapting, valleys of fingerprints, 25% of mechanoreceptors in hand, high spatial resolution and accuracy, smallest receptive field, sensitive

meissner corpuscle

fast adapting, peaks of fingerprints, 40% of mechanoreceptors in hand, high sensitivity, larger receptive field, and lower spacial resolution than Merkel cells

pacinian corpuscle

fast adapting, deep in tissue, 10% of mechanoreceptors in hand, high sensitivity, huge receptive field, low spatial resolution

ruffini afferents

slow adapting, deep in tissue, 20% of mechanoreceptors in hand, sensitive to stretching during movement, large receptive field, low spatial accuracy

how do we detect stimuli of different intensities?

1. frequency of action potential firing

2. number of sensory neurons recruited

proprioceptors

sensory input from muscles to spinal cord

class of mechanoreceptors

muscle spindles

AP firing rate provides info on muscle length

stretching muscle fibers activate mechanoreceptors on sensory afferents

golgi tendon organs

AP firing rate provides info about force of muscle contraction

contracting muscle fibers activate mechanoreceptors on sensory afferents

MS vs GTO

both fire APs during passive stretch

during muscle contractions muscle spindles decrease AP firing rate and golgi tendons increase AP firing rate

homunculus

map of little man that is proportional to apatial acuity across the body

nociception

the encoding and processing of noxious stimuli by CNS and PNS

(only sensory processing)

pain

an unpleasant sensory and emotional experience associated with actual or potential tissue damage

(sensory and higher brain processing)

noxious stimuli

stimuli that are actually or potentially damaging to tissue

analgesia

relief from pain

nociceptors

pain receptors

have free nerve endings

2 categories: A-delta and C fibers

express channels and receptors to transduce mechanical, thermal and chemical stimuli

A-delta fibers

lightly myelinated; only mechanical and thermal pain stimuli (not chemical); fast, intense; first pain

C fibers

not myelinated; mechanical, thermal, and chemical pain stimuli; throbbing, chronic pain; second pain

thermal/mechanical pain stimuli

detected through TRPs; influx of Ca++ an/or Na+ when activated, causing depolarization, triggering APs

chemical pain stimuli

detected by TRP channels and GPCRs

somatic nociceptive pain

dense innervation (supply of nerves), localized pain; can determine where it comes from

thermal, chemical, and mechanical nociceptors

protective; positive process

visceral nociceptive pain

sparser innervation (supply of nerves), diffuse, referred pain; don’t know exactly where the pain is

mostly mechanical nociceptors

protective; positive process

hyperalgesia

increased response to pain; inflammation

inflammation pain

causes release of prostaglandins which interact with pain receptors to decrease depolarization and AP firing threshold (causes more AP to fire and more pain to be felt)

neuropathic pain

due to direct damage of receptors or spinal cord and brain regions that process pain

not protective; not helpful/ serves no purpose

nociceptive pain vs neuropathic pain

nociceptors send signals signaling pain vs damage to actual nerves

referred pain

first order neurons: sensory neurons

second order neurons: neurons in spinal cord that first order neurons synapse on

second order neurons not specific to somatic or visceral inputs which results in confusion of the interpretation of source of pain

gate control theory of pain

rubbing a painful area to make it feel better

interneurons(inhibit second order neurons from transmitting pain) are usually inhibited by C fibers but touch fibers stimulate interneurons to keep them working

opiate receptors

pre and postsynaptic, metabatropic

opiates on presynaptic neuron

reduced NT release, reduced excitation of postsynaptic neuron

opiates on postsynaptic neuron

cause hyperpolarization of postsynaptic neuron, making it less likely to respond to pain afferent

cornea

transparent tissue

aqueous humor

produced by vascular part of ciliary body

poor drainage can cause glaucoma

lens

where focusing of vision happens

transparent tissue

protein build up on lens causes cataracts

vitreous humor

gelatinous substance, contains phagocytes that clean up debris

cornea function

majority of light refraction, not adjustable

lens function

adjustable light refraction to focus at various distances

zonal fibers hold lens in place

ciliary muscles shape lens

pupil function

narrows light path

controlled by iris muscle

retina regions

optic disc, macula, fovea

optic disc function

in/out point

retinal axons leave

no photoreceptors/ blind spot

macula

high acuity

yellowish pigment, filters UV light

site of macular degeneration

fovea

center of macula

exactly where you’re looking (highest visual acuity)

mostly cone photoreceptors

myopia

nearsightedness (can’t see far)

light converges too soon, in front of lens

hyperopia

farsightedness (can’t see close)

light converges too late, behind lens

emmetropia

normal vision

light converges right on the lens

retinal pigment epithelium

contains melanin that prevents backscatter of light

remove and recycle photoreceptor discs

photoreceptor maintenance

discs removed and recycled by retinal pigmented epithelium

discs at tip phagocytosed by epithelial cells

recycled discs reenter at the base

photoreceptors

rods and cones

changes to light current

graded potentials (no AP)

interneurons

only make connections in retina

light “on” or “off” signals

horizontals, bipolar, and amacrine cells

grade potentials (no AP)

projection neurons

generate action potentials to send info to brain

only cell w AP bc only cell that leaves retina

ganglion cells

photoreceptors

do not fire APs - graded potentials

depolarize when light decreases

hyperpolarize when light increases

photoreceptor role

transduce light to graded potential receptors and release glutamate to bipolar cells

bipolar cells role

sign conserving or sign inverting operation

produce graded potentials and release glutamate to ganglion cells

ganglion cells role

generate APs to bring info to CNS

horizontal cells role

mediate interactions between cell groups

outer segment of photoreceptors

contain cGMP Na+/Ca++ channels that depolarize the cell; channels open/close in response to cGMP levels

inner segment of photoreceptors

contain potassium leak channels that hyperpolarize the cell

synapse of photoreceptors

graded glutamate release based on net change in potential between inner and outer segments

the more depolarized the synapse, the more Ca++ comes in to release NT

dark vs light glutamate release

in the dark, guanylyl cyclase makes cGMP that depolarizes the cell and continues the release of glutamate normally

light causes change in r(something) which releases G proteins, turning on phosphodiesterase enzymes that turn cGMP into GMP, less cGMP means less activation of Na/Ca channels to release glutamate

ON/OFF cells

on cells depolarize with more light

off cells depolarize with less light

rods

more sensitive to dim light

more numerous than cones in the retina

cones

specialized for color and high visual acuity

high concentration in fovea

sensitive to bright light

distribution of rods and cones

rods more in periphery of retina, cones more in center of retina

ospins of cones

respond better to some photon wavelengths of colors than others (optimal but not exclusive)

cones express 1 of 3 ospins

pathway from eye to brain

visual field → retina → optic nerve → optic chiasm → optic tract → thalamus → optic radiations → visual cortex in occipital lobe

optic nerve vs chiasm vs tract

optic nerves has axons from ONE eye; chiasm and tract have axons from BOTH eyes

high spatial acuity means…

most area in the visual cortex

“what” pathway

ventral

object identification and recognition

“where/how” pathway

dorsal

object location

can identify objects but have hard time interacting with them

agnosias

visual-form: inability to recognize objects

color: inability to recognize colors

face: inability to recognize faces

outer ear

boost sound pressure with vibrations

middle ear

sound amplification by bones

inner ear

sensory transduction

cochlea

inner ear

contains organ of corti

where main transduction happensa

auditory pathway

sound waves → tympanic membrane/ eardrum → malleus → incus → stapes → oval window → vestibular canal → basilar membrane → stereocilia → K+ ions → Ca++ ions → NTs → APs

frequency map

vibration of basilar membrane is maximal at sound frequency

membrane goes from high to low pitch

activation of hair cells

movement of tectorial membrane from basilar membrane vibration causes force that bends stereocilia of the hair cell

sensory transduction in hair cells

movement of hair bundles by tectorial membrane opens stretch activated K+ channels that depolarize hair cell

endolymph

surrounds stereocilia of the hair cell

high in K+ concentration

cochlear duct

perilymph

surrounds cell body of hair cell

low in K+ concentration

found in tympanic canal

conductive vs sensorineural hearing loss

middle ear; ex, ruptured eardrum vs inner ear and trouble transducing signal; ex, tinnitus

vestibular system functions

processes sensory information

• static position

• velocity, acceleration, and direction of movement

mediates rapid automatic behaviors

multisensory integration

vestibular hair cells

similar to auditory cells

difference is they release NT at rest, movement of hair bundle can increase or decrease NT release, have kinocilium (taller, thinner stereocilia)

kinocilium movement

movement of bundle towards kinocilium depolarizes the cell

movement of bundle away from kinocilium hyperpolarizes the cell

depolarization activates Ca2++ channels that trigger high levels of glutamate release

vestibular hair cells at rest

low tension that holds stretch channels slightly open allowing low depolarization, triggering low levels of glutamate release

tilt vs acceleration

response lasts the duration of the tilt (either inc or dec firing rate whole time) vs transient response that lasts until inertia is overcome (inc firing rate until otolithic membrane catches up then firing rate drops)

vestibular pathway

semicircular canal detects motion → otoconia and gel layer move → vestibular hair cells move → K+ ions → Ca++ ions → NTs → APs

semicircular canals

rotational acceleration only

chemosensory systems

detects chemicals in the environment

olfactory, gustatory, and trigeminal systems

olfactory system

processes info about odorants

strong connection w limbic system; reason for strong connections between smell, emotion, and memories

olfactory epithelium

site of sensory transduction

odors held by mucus produced by Bowman’s gland

ORNs bind odors on their cilia and produce APs

ORNs

expresses 1 type of odor receptor protein; 1 receptor 1 odorant

G Protein-Coupled Receptors (GPCRs)

can be degraded by exposure to things in enviro but are regenerated by basal cells (stem cells found in epithelium) every 6-10 weeks