Neuroscience Exam 2

what does the somatosensory system detect

touch, pressure, vibration, limb position (proprioception), heat, cold, itch, and pain

somatosensory afferent neurons are …

psuedounipolar; cell body is midway along the path of the axon, not at the beginning

in somatosensory afferent neurons where do the action potentials begin and travel to

begin at receptor endings, turn into acons that travel to CNS; AP bypasses cell body

where do afferent somatosensory neurons reside

dorsal root ganglion (spinal cord) and trigeminal ganglia (brainstem)

sensory transduction

converting energy in a stimulus into an electrical signal

steps to trigger an action potential in somatosensory afferent neurons

stimulus causes cation channels in afferent fiber endings to open, channel openings create receptor potential, if large engough receptor potential triggers AP

what are mechanorecpetive afferent fiber endings encapsulated by

mechanoreceptor cells - specialized cells that help tune responses to a particular somatic feature

what do mechanoreceptor cells allow for

greater sensitivity to stimuli

what are nerves that lack mechanoreceptor cells called

free nerve endings

afferent properties: touch and proprioception vs pain and temperature

larger diameter and myelinated vs smaller diameter and little to no myelination

receptive field

area of skin over which stimulation causes a significant change in firing rate of action potentials; largely realted to branching area of fiber ending

two point discrimination

minimum distance between two tactile points on skin that is percieved as being from two distinct points

what determines the two point discrimination threshold

receptor density and field size

T/F: tactile spatial acuity vaires sustantially across body

true

rapidly adapting afferent somatosensory neurons

fire rapidly at beginning of stimulus, then cease with continued stimulation

what do rapidly adapting afferent somatosensory neurons convey

information about changes in ongoing stimulation (ex. putting on clothes and not feeling them as day goes on)

slowly adapting afferent somatosensory neurons

fire rapidly at beginning of stimulus, then steadily at a lesser rate with continued stimulation

what do slowly adapting afferent somatosensory neurons convey

information about static spatial attributes like size and shape (ex. feeling continuous weight of a heavy object)

what are the differences in sensitivity to somatosensory submodalities (temperature, light touch, vibration, etc.) determined by

types of ion channels and mechanoreceptor cells associated with afferent

glabrous skin

hairless skin of mammals predominantly found on palms, feet, and lips

what is glabrous skin specialized for

disciminative touch → use with object recognition and to provide feedback when gripping and reaching (sensations from touching an object alone are often enough to form a “neural image” and identify it)

4 types od mechanoreceptive afferents in glabrous skin

merkel cells, meissner corpuscles, pacinian corpuscles, ruffini corpuscles

merkel cells characteristics

slowly adapting, located in epidermis at border of dermis, mechanosensitive ion channels in the membrane of the fiber surrounded by merkel cell

which cell has the highest spatial acuity

merkel cells - highly sensitive to points, edges, and curvature

meissner corpuscles characteristics

rapidly adapting, located in dermis at border with epidermis, made of several layers (center contains 2-6 afferent fibers)

T/F: merkel cells are 4 times more sensitive than Meissner corpuscles

False; Messierners corpuscles are more sensitive with less spatial acuity

what are meissners corpuscles highly sensitive to

low frequency (3-40 Hz) vibrations - ex. vibrations of textured object slipping from hand

pacinian corpuscles characteristics

rapidly adapting, located deep in dermis or subcutaneous layer, made of several concentric layers of membranous around 1 afferent fiber

what are pacinian corpuscles most sensitive to

mechanoreceptor afferents, can detect deflections as small as 10 nanometers (very large receptive fields) - suited for detecting high frequency vibrations transmitted through grasped objects

ruffini corpuscles characteristics

slowly adapting, located deep in skin and in tendons and ligaments, parallel to strech lines in skin

what are pacinian corpuscles sensitive to

cutaneous stretch produced by finger movement - thought to provide information about finger and hand position

hairy skin

the 90% of skin of mammals outside of the palms, lips, and feet (includes regions with very little hair)

what does hairy skin transmit

information on discriminative touch and non-discrimminative affective touch (touch that evokes emotional response)

what tactile recpetors are in hairy skin

ruffini and pacinian corpuscles

which mechanoreceptive afferents innervate hair follicles

merkel cells innervate touch domes surrounding follicles

what wraps around the base of the follicle

circumferential endings and longitudinal lanceolate endings

what are the types of proprioceptive mechanoreceptors

muscle spindles, golgi tendon organs, joint receptors

intrafusal vs extrafusal muscle fiber function

sense muscle stretch vs produce force of muscle contraction

muscle spindles activation

muscle stretch → stretch of intrafusal fibers → activation of mechanically gated ion channels in afferent endings

what afferents innervate muscle spindles

group 1a: rapidly adapting (signal limb dynamics) and group II: slowly adapting (signal static position)

would big muscles with coarse movements like the quads have more or less density of muscle spindles than small muscles with fine movements like the eyes

lesser density of muscle spindles

how are golgi tendon organs arranged and what innervated them

in series with a tendon and group of 10-20 extrafusal muscle fibers; group Ib afferents (fast)

what do golgi tendon organs signal changes in

muscle tension

joint receptors location

mechanorecpetors in and around joints - activated at extremes of joint position (protective role) when near the limits of normal range of motion

primary somatosensory cortex (S1)

somatosensory neurons in thalamus mainly project to postcentral gyrus in parietal lobe

brodmann’s area

neighboring cortical regions also process somatosensory input

what was the result when scientests recorded with electrode from one point in cortex and would touch different areas of the body until a neural response was evoked

each brodmann’s area contains a complete somatotopic map

T/F: cortical representation of somatosensory input shows partial segregation of tactile and proprioceptive processing

true

plasticity of the cerebral cortex due to lack of stimulation

immediately after peripheral lesion (ex. removing a finger), corresponding cortical area is unresponsive - after weeks the same brain area is responsive to stimulation of body areas neighboring the lesion

plasticity of the cerebral cortex due to overstimulation

monkey brain trained on a task that requires it to use digits 2,3, and 4 - after training the cortical area devoted to those digites expand at the expense of other corical areas

nociceptors

somatosensory afferents with free nerve endings that transduce painful stimuli into electrical signals; slow conduction

what do nociceptors innervate

skin, deep tissue, and internal organs

when are nociceptors vs thermoreceptors activated

extreme stimulus levels vs lower stimulus levels

what are the two responses to pain

sharp, localized 1st pain and a delayed, diffuse, longer-lasting 2nd pain

what fibers eliminate 1st pain vs 2nd pain

A gamma vs C

A gamma fiber types

I: respond to dangerously intense chemical or mechanical stimulation; II: respond to dangerously intense thermal stimulation

C fibers

most respond to all types of nociceptive stimuli, some respond to noxious heat, chemical irritant, acidic substances, or cold

human threshold for painful heat applied to skin vs cold applied to skin

43 C (110 F) vs 15 C (59 F)

what type of channels are sensitive to temperature

transient receptor potential

T/F: channels are typically only sensitive to one modalitiy of nociceptive stimuli

False; example - acids elicit a “burning” sensation, perhaps by activating TRPV1, which mainly transduces noxious heat

what happens when TRPs (transient receptor potential) activated

allow influx of Na+ and Ca2+ → depolarization

T/F: several channels are known to be activated by exogenous chemicals

true - TRPM8 activated by menthol and TRPV1 activated by capsaicin

central pain pathway steps (4)

1. cell bodies of nociceptors and thermoreceptive afferents in dorsal root ganglia alongside spinal cord

2. afferents enter spinal cord through dorsal root, split into ascending and descending collaterals, then synapse on neurons in dorsal horn of nearby spinal segments

3. dorsal horn neurons decussate, then ascend spinal cord through lateral column

4. dorsal horn neurons synapse on various targets in brainstem and thalamus

anterolateral system

bundle of axons of dorsal horn neurons traveling up the lateral column of the spinal cord; contains informationon pain, temperature, non-discriminative affective touch, itch

2 parallel central pain pathways

sensory-discriminative aspects of pain and affective-motivational aspects of pain

sensory-discriminative pathway

location, intensity, and quality of painful stimuli; nociceptive dorsal horn projects to thalamus, then project to somatosensory cortex

affective-motivational pathway

fear and anxiety, unpleasent feeling, “fight or flight” reaction; nociceptive dorsal horn neurons project to various brainstem areas, plus a thalamic relay to cingulate and insular cortices

hyperalgesia

the phenomenon where stimuli in the area of an injury that would normally be considered slightly painful are percieved as very painful; cut on finger and areas by it are sensitive to pain

causes of hyperalgesia

peripheral and central sensitization

peripheral sensitization

“inflammatory soup” of substances released by nociceptors or immune cells in areas of damaged tissue, they interact with receptors or ion channels on nociceptors and augment their response

central sensitization

increase in the excitability of dorsal horn neurons following high level of activation of nociceptive afferents; several different mechanisms including enhancement of synaptic responses due to activation of NMDA receptors in the dorsal horn

modulating the transission of pain to the brain

local modulation at the level of the spinal cord and modulation via descending projections

local modulation at the level of the spinal cord

tactile afferents with receptive fields that overlap with nociceptive afferents inhibit transmission of the pain signal by dorsal horn neurons via an inhibitory interneuron

modulation via descending projections

in dorsal horn, an interneuron synapses on the presynaptic terminals of nociceptive C fibers, inhibiting excitatory transmission to dorsal horn neurons

the placebo effect

the beneficial effect following administration of a sham pharmeceutical substance

how does the placebo effect work

mediated by the activation of descending projections (opiod-based) based on the anterolateral system

retina

innermost layer at posterior eye containing photoreceptors that transduce light stimuli into electrical signals

pathway of light through the eye

cornea → aqueous humor → pupil → lens → vitreous humor → retina

cornea

highly transparent tissue at the front of the eye; highly refractive → serves to bend the path of light

lens

highly transparent tissue; refractive, but less so than cornea; refractive power can be dynamically altered by changing the thickness of the lens

pupil

opening in the middle of iris; can dilate or constrict to alter amount of light iluminating the retina

aqueous humor

clear fluid in front of lens; supplies nutrients and oxygen to lens and cornea

vitreous humor

clear, gelatinous substance behind lens; maintains shape of eye; contains phagocytes that remove cellular debris that could obstruct light passage

what do the cornea and lens serve to do

focus light from one point in the visual field onto one point in the retina (one-to-one mapping)

pupillary light reflex

contracts pupillary constrictor muscle (sphinctor), decreases pupil size → decreases amount of light entering the eye

accommodation

reflexive changing of the shape of lens to increase or decrease refraction; bringing near or far objects into focus on the retina

what is accommodation controlled by

edinger-westphal nucleus; triggers reflexive contraction or relaxation of the ciliary muscles, which is attached to lens by zonule fibers

in the retina how many basic classes of neurons are arranged in a well-defined, layered circuit

5

photoreceptors

neurons that transduce light energy into electrical signal

photoreceptor components

inner segment contains nucleus, outer segment houses membranous discs containing photopigment (where phototransduction occurs)

photopigment types

rods and cones

main, feed-forward information flow

photoreceptors → bipolar cells → ganglion cells; axons of ganglion cells comprise the optic nerve

lateral information flow

horizontal cells ←> photoreceptor/bipolar cell synapse; role in adapting response to different background light intensities

what is the first strange thing about phototransduction

light stimulation does not cause action potentials in photoreceptors, but causes a graded change in membrane potential with a corresponding change in the release of synaptic vesicles; retinal cells are very close → less need for axons, which are useful for transmitting signals over long distances

what is the second strange thing about phototransduction

light stimulation causes membrane hyperpolarization, not depolarization; at rest, membrane is depolarized → voltage gated Ca2+ channels near synapse activated → high rate of release of neurotransmitter; in response to light, membrane is transliently hyperpolarized → transient closing of Ca2+ channels → transient decrease in NT release

photopigment

retinal molecule enclosed by opsin; light causes a double bond in retinal to break, changing it from cis to trans isomer which is its active state

different types of opsin

rhodopsin (rods) and cone ospins (cone)

phototransduction cascade (7)

1. light activates rhodospin (G-protein-coupled receptor)

2. rhodospin activates transducin (G-protein)

3. transducin activates a phosphodiesterase (effector protein)

4. phosphodiesterase hydrolyzes the cGMP (2nd messenger)

5. cGMP levels decrease in cytosol of outer segment

6. less cGMP causes closing of cGMP-gated ion channels-these channels are permeable to Na+ and Ca2+

7. decrease of inward current → hyperpolarization

signal amplification in the phototransduction cascade

1 activated rhodopsin → closure of 200 channels → 1 mV of hyperpolarization