exam 1 - sensory neuro

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1

Plato

Existence follows a hierarchy of reality. Even an Idea is a truly real entity. Our perceptions of reality are ‘real’, even if constructed or lacking in fullness. (Real as in extant, not real as in fully true/correct)

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René Descartes

“cogito ergo sum” – I think, therefore, I am. • First answered what prefigures today’s “brain in a jar” problem.

First highlights the dichotomy between sensation and perception

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Sensation

external. process through which our sense organs receive external physical stimuli and translate those signals into a neural event. AKA raw data.

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Perception

internal. process by which the brain processes, organizes, selects, integrates, and interprets neural signals generated by sensory organs.

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what does a sensory system contain?

sensory organ itself, neural pathways that transmit info to the brain, and brain regions that process that information & generate perceptions and behaviors.

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where does a sensory system provide information from?

outside world, organs inside the body, from memory and imagination

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what does it mean when it’s said that sensation isn’t just “outside”?

there are special and organic sensory systems.

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what are special sensory systems?

sight, hearing, touch, balance (vestibulocochlear), taste, smell

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what are organic sensory systems?

pain, itch, thirst, hunger, moistness, fatigue, sleepiness, oxygen level, and more

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psychophysics

relationship between a physical stimulus and the resulting sensation.

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psychometric function

describes the relationship between stimulus and stimulus response (as a neural output or a behavior).

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psychophysics: what are the attributes of a stimulus?

intensity, spatial properties, temporal properties, and difference limen

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intensity

physical strength of the stimulus. (how many photons, decibels, mBar of pressure)

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spatial properties

Where the stimulus originates, directionality, movement, vibration.

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temporal properties

order of events, temporal separation between events, duration, frequency

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difference limen

Discrimination between small differences in a stimulus’ properties -or- the ability to discriminate between two different, but similar stimuli (e.g. light at 580 or 590 nm registers as slightly different shades of red)

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detection threshold

the minimum amount of a stimulus required to elicit a response in the target system.

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perceptual threshold

The filter a sensory transmission must pass to be processed by the brain (perceived) and/or evoke awareness.

A stimulus may be perceived, but without awareness. That is a subliminal signal. Not all physical stimuli CAN cause awareness. (not considered subliminal unless awareness is possible.)

Pheromones, Head movements ,Magnetic fields

ex. sound wave hits ear (audible frequency?) - ear transmits signal (is it loud enough?) - brain processes signal (any other sounds?) - sound is heard (perception + awareness) (is it salient?)

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subliminal signal

stimulus perceived but w/o awareness

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range fractionation

different cells have different thresholds for firing, over a range of stimulus intensities

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adaptation - a phenomenon of sensation

Prior exposure to a condition can heighten or dampen activity of a sensory system

Adaptation mechanisms are specific to the system.

Usually as a result of continuous exposure to a stimulus.

Depends on stimulus parameters

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habituation - a process of perception

Prior exposure to a condition can heighten or dampen activity of a sensory system. Habituation happens at the level of the brain as a result of repeated exposure to the same or similar stimulus

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Spatial integration and Receptive fields

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

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How do events integrate across time?

Depending on duration of the event, and the frequency of the event (time between events), distinct events may or may not be perceived as discrete events.

Think frames per second/refresh rate

Closeness of events in time can accentuate or dull perception of another stimulus . E.g. certain sound frequencies are augmented if heard in close succession.

This can be a strictly temporal effect, or an additive effect depending on the sensory modality

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masking

The phenomenon of one stimulus attenuating another.

A sound might block out perception of another sound if the timing or frequency is right.

A slight electrical stimulation of the skin (suitable to cause tingling) dulls the sense of pressure and pain.

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backward masking

affects “threshold” of sensory processing system

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forward masking

affects threshold of sensory organ

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cnidarian

basic neural net with no central processing

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Protostomes

Ventral nerve cord with associated ganglia. Less modulation of synapses. Segmentation of body plan. True Head & brain

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Chordates

Dorsal hollow nerve tube. True head and brain. More synaptic modulation. Segmented body plan.

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Evolution of the nervous system

Chordate brain evolution: segments in head grow larger and specialize to accommodate processing of sensory inputs clustered around the mouth and coordinate food seeking/survival.

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all sensory processing begins in…

receptor cells

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A receptor is a cell or specialized neuron that responds to a particular form of energy which is converted to a …

graded potential

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If the potential reaches the neurons threshold, some receptors can generate an…

action potential

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Other receptors are not neurons (photoreceptors, cochlear hair cells, and vestibular hair cells) and can …..

NOT produce action potentials

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Coding

patterns of action potentials in a sensory system

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Receptors Use Multiple Strategies to Encode Intensity

Strategy:

1. A single neuron can convey stimulus intensity by changing the frequency of its action potentials

2. Multiple neurons can act in parallel – as the stimulus strengthens, more neurons are recruited

3. Different neurons respond to different ranges (range fractionation)

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what do ion channels do?

are proteins that span the bilayer to allow ions to pass in and out through passive diffusion:

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+ Leak channels

stay open all the time

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+ Gated channels

open and close in response to:

voltage changes (voltage-gated)

molecules binding (ligand-gated)

mechanical action

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ion pumps

are proteins that span the membrane to allow ions to pass in and out but require energy to do so

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what is the process of the action potential?

At the axon hillock:

  1. Voltage-gated Na+ channels open in response to initial depolarization

  2. More voltage-gated channels open and more Na+ ions enter until membrane potential hits +40 mV

  3. Voltage-gated Na+ channels inactivation gate closes

  4. As inside of cell becomes more positive, voltage-gated K+ channels open

  5. K+ moves out of the cells

  6. Polarity overshoots beyond the resting potential but restored as K+ channels close

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somatosensation

Diverse range of sensations: Touch, pressure, vibration, limb position, heat, cold, itch, pain

Transduced by receptors in skin or muscles to CNS

Specialized, functionally distinct circuits for each submodality

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Cutaneous mechanoreceptors

fine touch, vibration, pressure

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Proprioception

Our ability to sense the position of our own limbs and other body parts in space

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Free nerve endings

pain, temp, course touch

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Somatosensory afferents convey information from…

the skin surface to central circuits.

  1. psuedounipolar neuron

  2. bipolar neuron

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Dermatomes

The territory innervated by each spinal nerve.

Maps of innervation arising from each dorsal root ganglion and its spinal nerve.

Defined in patients suffering from shingles or after surgical interruption

Individual differences

Some overlap (more precise for pain)

Helpful in determining area of spinal damage

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Ia, Ib, II afferents

largest and fastest, supply sensory receptors to muscles for proprioception

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Aβ afferents

smaller, convey touch

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Aδ and C afferents

small and slow, pain and temperature

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Receptive fields

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

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Receptive fields and two-point discrimination threshold

smaller areas like the fingers/palm have a lower threshold in detecting two different points whereas areas like the back, belly, and thighs have a higher threshold

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Slowly adapting afferents

Generate sustained discharge during ongoing stimulus • Provide spatial info: size and shape of stimulus

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Rapidly adapting afferents

Fire rapidly when a stimulus is first presented • Fall silent with continual stimulation • Convey changes (stimulus movement)

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Sensory transduction

Process of converting energy of stimulus into an electrical signalSomatic sensory afferents

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Somatic sensory afferents

Stimulus alters permeability of cation channels (positively charged ions) in afferent nerve endings • Generates a receptor potential: Depolarize (an action potential in the afferent fiber)

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Mechanoreceptor properties - MERKEL

25% of the mechanoreceptors in the hand

Especially in the fingertips

Highest spatial resolution of all sensory afferents

Texture and preception - edges/points/corners/curvature

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Mechanoreceptor properties - MEISSNER

40% of the mechanoreceptors in the hand

Closest to the skin surface

Textured objects that move across the skin

motion detection; grip control

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Mechanoreceptor properties - PACINIAN

10-15% of the mechanoreceptors in the hand

Detect vibrations transmitted through held objects

Skilled use of tools (using a wrench, writing, cutting bread with a knife)

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Mechanoreceptor properties - RUFFINI

20% of the mechanoreceptors in the hand

Skin stretch - tangential force; hand shape; motion direction

Least understood Internally generated stimuli: Movement of fingers

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Touch dome

a ring of Merkel cell complexes.

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Circumferential endings

Sensitive to hair deflection. Connected to Aβ (both types), Aδ,

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Longitudinal lanceolate endings

Connected to Aβ (both types), Aδ, and C fibers (longitudinal only). -associated with gentle or sensual touch.

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stereognosis

the mental perception of depth or three-dimensionality by the senses, usually in reference to the ability to perceive the form of solid objects by touch.

harder to tell things apart on wrist then on finger tips

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Afferent responses to Braille

Experiment comparing responses of different afferents as fingertip moves across Braille lettering

Each dot = action potential recorded from a mechanoreceptor

Move fingertip from left to right and then repeat moving down finger to engage multiple receptive fields (red dots)

even though the individual can’t see, the brain is still creating a picture for them to understand

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Social Touch (C-fibers) - infancy

Helps to develop bond of closeness between parents and children (causes release of neurotransmitters that facilitate bonding)

Helps to reinforce learning in young children (recognition of self vs. other).

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Social Touch (C-fibers) - social wellbeing

Causes feelings of closeness with others.

Biases autonomic nervous system to calm/restive states. (cuddling, hugging, etc)

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Social Touch (C-fibers) - sensual touch

an example of perceptual gating.

Sensual touch is perceived as pleasant when – In the appropriate space (erogenous zones) – Is wanted/expected – Is from an individual the perceiver is attracted to

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Proprioceptors -“receptors for self”

Information about the position of limbs & other body parts in space

think of guy who had trouble picking up mug and trying to get up from wheelchair

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Low threshold mechanoreceptors provide this information (about position of limbs and other body parts in space)

Muscle spindles, Golgi tendon organs, Joint receptors

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Mechanoreceptors specialized for proprioception - MUSCLE SPINDLES

Signal changes in muscle length

Sensory afferents are coiled around the intrafusal muscle fibers. When muscle is stretched, the tension activates the nerve endings, triggering an action potential

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Primary endings (group Ia afferents) - muscle spindles

LARGEST myelinated sensory axons have rapidly adapting responses to changes in muscle length. Transmit information about limb dynamics: velocity and direction of movement.

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Secondary endings (group II afferents) - muscle spindles

produce sustained responses to constant muscle lengths. Static position of limbs.

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Mechanoreceptors specialized for proprioception - Golgi tendon organs

Signal changes in muscle tension. These Group Ib afferents are distributed among the collagen fibers that form the tendons.

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Mechanoreceptors specialized for proprioception - Joint receptors

Relay finger position for range of motion protection. Mechanoreceptors in and around the joints.

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muscle-tendon unit

muscle spindle and golgi tendon organ.

muscle length, velocity of change of muscle length, and active muscle tension

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joint

ruffini ending, pacinian ending, golgi ending

low and high load tension and compression loads throughout entire ROM

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fascia

ruffini ending and pacinian ending

low and high tesion loads during joint movement

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skin

hair follicle receptor, ruffini ending, painian ending, merkel ending, meissner ending

superficial tissue, deformation/stress, compression during joint movement

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central pathway - tactile from body

DORSAL COLUMN-MEDIAL LEMNISCAL PATHWAY

1st order neurons -

info goes up ipsilaterally in spinal cord to medulla (dorsal columns)

→ (fasilicus gracilis → gracile nucleus - LOWER LIMBS Imedial bundle)) and (fasciculus cuneatus → cuneate nucleus - UPPER LIMBS/TRUNK AND NECK (lateral bundle))

2nd order neurons -

internal arcuate fibers cross (decussate) the midline → form medial lemniscus

synapse @ ventral posterior lateral nucleus (VPL) of the thalamus

3rd order neurons -

VPL neurons send axons to synapse in somatosensory cortex (SI and SII)

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trigeminal nerve - cranial nerve V

includes 3 regions: opthalamic, maxillary, and mandibular

opthalamic = skin touch from forehead to certain extent in the back of the head

maxillary = nose/top of mouth/upper lip

mandibular = sense of touch from jaw/lower lip/cheek

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central pathway - tactile from face

TRIGEMINOTHALAMIC PATHWAY

1st order neurons -

cell bodies in trigeminal ganglia (CN V)

nerve has 3 subdivisions = opthalamic, maxillary, mandibular

enter brainstem @ pons → synapse on trigeminal brainstem complex; different nuclei process different stimuli submodalities (principal and spinal nuclei)

2nd order neurons -

decussate → ascend as trigenminal lemniscus

synapse @ ventral posterior medial nucleus (VPM) of the thalamus

3rd order -

VPM neurons send axons to synapse in somatosensory cortex (SI)

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central pathways - proprioception

SPINOCEREBELLAR TRACT

(travel w axons in dorsal column w some differences)

1st order neurons -

bifurcate into ascending/descening branches (dorsal/ventral horns)

lower limbs innervate Clarke’s nucleus in the medial dorsal horn

2nd order neurons -

travel to medulla → ipsilateral cerebellum by the dorsal spinocerebellar tract

send collaterals → synapse in proprioceptive neurons of the dorsal columun nuclei

(som) 3rd order neurons -

decussate → medial lemniscus (w fibers from cutaneous mechanoreceptors) → VPL thalamus

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ventral posterior lateral nucleus (VPL)

body

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ventral posterior medial nucleus (VPM)

face

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Somatotopic maps

the foot, leg, trunk, forelimbs and face are represented in a medial to lateral arrangement

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Homunculus

(“little man”) illustrates the proportion of representation in cortical processing

Facial expression, speaking and hand use require lots of cortical circuitry

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Areas of SI

3b and 1: respond to cutaneous stimuli

3a = proprioceptors

2 = tactile and proprioception

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cortical area function - 3b

obligatory 1st step in cortical processing

Lesions produce profound deficits in all forms of tactile sensations

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cortical area function - area 1

Lesions produce inability to discriminate texture of objects

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cortical area function - area 2

Lesions produce inability to discriminate size and shape of objects

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Functionally distinct cortical columns

Vernon Mountcastle proposed that neurons with similar response properties might cluster in functionally-distinct columns

Letters/numbers match stimulus on finger to electrode placement in cortex

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Somatosensory cortex plasticity

Cortical circuits are capable of reorganization in adults

Effect of peripheral lesions (ie cutting nerve to hand, amputation of digit) on cortical maps

Immediately following digit 3 lesion, the corresponding cortical region is unresponsive

After a few weeks, the unresponsive area becomes responsive to stimulation of neighboring regions of skin

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Cortical representations change with experience

Monkey trained to use specific digits for a particular task that is repeated many times

Functional representation of those digits expands at the expense of other areas

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Intracortical microstimulation of human somatosensory cortex

Ultimate goal: to create a sensory neuroprosthesis to restore tactile sensation

Goal of this paper: to determine how S1 stimulation is perceived

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1st order

dorsal root

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2nd order

brainstem

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3rd order

thalamus

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Topographical arrangement persists in

primary cortex

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