General Principles of Sensory Processing & PERCEPTION

How the nervous system receives information from the outside world

receives information from 6 senses

(Chemosenses, taste (Gustation) and smell (olfaction), touch, vision, and inner ear senses, sound (audition) and proprioception (vestibular) systems, somatosensory

To understand how to respond to our world - we need to get information from the world - hence

We need to understand our sensory systems

what and how we perceive our environment is

constrained by our biology

● need mechanisms to get information about the environment to our nervous system

sensory receptor organs

organs specialized to receive a particular type of stimuli

⧫ converts (transduces) physical energy in the environment into changes in membrane potential

the sensory receptors (or receptor cells)

specialized cells that respond to a particular energy or substance in the environment (internal or external)

Sensory receptors help transduce the energy coming in

to something our brain can understand, namely neural energy or membrane potentials

some sensory receptor cells have axons are either

bipolar or unipolar neurons

the structure of the sensory receptor cell determines

the kind of stimuli or form of energy it can respond to

sensory receptor cells are all in the peripheral nervous system

where the information is being detected, and their job is to transduce the signal into changes in membrane potentials

Most sensory receptor cells contain a _ (the part that extends into the PNS) and a_ (the part that extends into the CNS)

peripheral process, central processes

Some sensory receptor cells are Bipolar neurons

olfactory system, retina of the eye, and in the ganglia of the vestibulocochlear nerve

In the somatosensory system are | Unipolar neurons (pseudo-unipolar) with 2 caveats

1) humans don't have true unipolar cells have pseudo-unipolar neurons- sensory neurons with cell bodies located in spinal dorsal root ganglion and cranial nerve ganglia. They are called pseudo-unipolar because developmentally they originate as bipolar neurons and subsequently become unipolar.)

2) unipolar (pseudo-unipolar) cells have no axon hillock (cause there is only one neurite coming out that goes in both directions), end in the periphery has voltage-gated Na+ and K+ channels so it can generate an action potential

some sensory cells are free-nerve endings

axons that terminate without any specialized cell or end organ ( detect pain or temperature)

Transduction

process of taking physical energy of some kind and converting it to the energy into membrane potentials

TRP (Transient Receptor Potential) Channels or "TRiP" channels

superfamily of cation channels within the membrane, play a role in most of our sensory systems where they act as sensors. (have a role in taste, vision, olfaction, hearing, touch, as well as thermal and osmosensation)

about 28 of TRP channels classified into 6 subfamilies in the mammalian system

TRP are ion channels called

cation-selective pores

TRP channels are

generally non-selective

TRP channels will let through

Na+, Ca+, Mg+ across the plasma membrane

TRP channels formed by

subunits with six transmembrane domains

TRP channels activated by a variety of different stimuli

small molecules tetrahydrocannabinol and menthol, by mechanical stresses and by G-coupled receptor proteins

Adequate Stimulus in that each sensory receptor

responds to a different type of energy

adequate stimulus

type of stimuli for which the sensory receptor is particularly sensitive

absolute threshold

minimal intensity required for the detection of the stimulus 50% of the time

Absolute threshold is useful for

assessing how sensitive we are to faint stimuli or our minimal detection for the perception

difference between the adequate stimulus and the absolute threshold

that the adequate stimulus is at the level of the receptor cell (what it can detect)

the absolute threshold is

what YOU can detect using that system

specific receptive field of a sensory receptor

region of space that influences the activity of a given sensory neuron (could increase or decrease activity)

the part of the skin that excites this sensory neuron the most is the

center of the receptive field = center is excitatory

if area that surrounds that center is touched the activity is inhibited

surround is inhibitory

if you touch outside of the receptive field

nothing happens

auditory system receptive fields are based on the | location in the cochlea (in the inner ear) that is stimulated by a range of frequencies

gustatory system (taste) receptive fields are

placed on tongue or in the nose that is excited by specific neurons

visual system this is the location

within our visual field

Receptive fields of the olfactory system

are not well known

Somatosensory information first sent to the

spinal cord → various parts of the brainstem (midbrain, medulla, pons) → thalamus (processed here) → primary sensory area of the cortex

Somatosensory information from our peripheral nerves spinal cord

and then up to the brain stem

Somatosensory information from our face and neck to

brainstem from the cranial nerves (this information misses the spinal cord)

Information from taste, smell, vision, and audition goes through

brainstem from the cranial nerves (this information misses the spinal cord) to thalamus primary sensory cortical areas, in most cases information gets sent out from the primary areas.

exception is the olfactory system which goes directly to the to the cortex (so it skips the thalamus)

each modality has its own distinct pathway

efferent fibers that are modulating the incoming information based on past experiences

receptor (generator) potentials

Transduction starts with changes in the membrane potential in the membrane around the sensory receptors in the sensory organ

Changes in membrane potentials are known as

generator or receptor potentials (generator potentials resemble EPSPs)

Generator potentials occur in sensory receptors

step between receiving the energy stimuli and the initiation of an action potential

Generator potential can be generated a number of ways depending

on the receptor cell being stimulated, if the potential change is big enough it will cause an axon potential

Example of generator potential

receptor cell receives information in form a change in pressure or depression in skin →the receptor then transduces that change in pressure into a change in membrane potential or a receptor potential (this is the fundamental difference between a receptor potential in the PNS and a graded potential in a CNS neuron

the receptor potential is something that is first transduced, graded potentials

do not need to be transduced both are membrane potentials)

The transduction of sensory information into receptor potentials and then into changes in neuronal firing requires

a code

All sensory systems get the same basic information from the stimuli

transduced sensory information into receptor potentials (so the receptor potentials are the basic information) which is then converted/translated into something the brain understands to be a particular type of stimulus

Different receptors respond

to different energies

Modality (type)

special receptors are found to be differentially sensitive to different forms of energy

Modality is based on the

anatomy of the receptor cells and what kind of energy stimulates them

messages from the different senses all use action potentials

brain recognizes different modalities because each sends action potentials along separate nerve tracts

Labeled Lines and this relates to the fact that a set of receptors that is

selectively sensitive to a given type of stimulus has a specific pattern of connections in the CNS

Labeled lines are set up for

particular sensory experiences. (example: you don't use visual pathways for auditory stimuli)

The brain can localize a stimulus to

a particular place on the body

lateral inhibition (e.g. you detect a pencil point on the back of the hand as a point, not a stimulus of the whole hand)

if the pencil point stimulates the center of the receptive field then the area around the center will be inhibited as will neighboring cells with fibers in the surrounding area of that center field.

The net result --- within the CNS --- area of sensation is less than the area that is actually stimulated, due to the areas of inhibition of activity: lateral inhibition increases contrast between strong and weak signals

The one exception to lateral inhibition is in the

auditory system- it uses differences in sound frequencies to determine the location of stimuli

Due to the all-or-none nature of the action potential, information about intensity from a single receptor is carried only in the

rate of discharge

Stimulus intensity is encoded in two ways

frequency coding and recruitment

frequency coding

firing rate of sensory neurons increases with increased intensity but this only works to a certain point

Recruitment

number of primary afferents responding to a stimulus increases. Kicks in when a single neuron has reached its peak firing rate

Example: muscle fibers = as the stimulus gets stronger more fibers are brought in so the response is larger, in this case it is a muscle contraction

So, the number and frequency of receptors responding gives you

information on the intensity of the stimulus

Typically sensory receptors show adaptation

slow and progressive loss of response if the stimulus is continuously applied- Important for "tuning out" of stimuli so brain is not overloaded with information

application of a weak stimulus

lots of action potentials (Aps) at first - but, then they decrease

Application of a stronger stimulus

more APs - but, they still diminish in number after a while

no matter what the intensity, If a stimulus persists for a long time

, the number of AP's decreases- the frequency decreases

duration of the sensation (how long we perceive the stimulus) is a function of

how fast we adapt to the stimulus

This occurs because the nervous system is more interested in

changes in stimuli. You get more information when there is a Change than when there is not a change and that is necessary for survival

Tonic (slowly adapting) receptors

show a slow or nonexistent decline in the frequency of nerve impulses as stimulation is maintained

Phasic (rapidly adapting) receptors

show a rapid decrease in frequency of nerve impulses even with sustained stimulation. This is good for detecting on and off

The sensory regions of the cortex play a critical role

in the conscious perception of stimuli

Each of the sensory areas of the cortex has "association areas"

which play a role in the perception of stimuli

A perception is a combination of simple sensory "impressions"

accompanied by an interpretation based on past experiences

Sensations and perceptions are

subjective

In order to perceive a stimulus the nervous system must process information from

many neurons

The response of neurons at successive levels is

more complex and the information has to be integrated

Neurons at higher levels respond to

more and more complex stimuli

The adequate stimulus is

different in successive levels of the labeled lines (of the pathways)

for example: higher up in the cortex are neurons that respond to specific aspects of stimuli: such as stimuli that move, or stimuli that move only one direction, or stimuli oriented along a specific axis

Detection of specific features of the stimulus is a property of

higher cortical neurons

The size of the receptive field becomes

larger as we move up each level of processing

The increase in receptive field size appears to be

important for the more complex stimuli

In order to perceive form, it is better to get

sensory information from more than one point on the hand

The different sub-modalities

converge on one common neuron

The information from various types of somatosensory receptors, as well as information from other sensory systems are found to

converge in association areas of the cortex

Try the apple and potato test: take a crunchy apple and a potato and cut similar sized cubes of each. Then have a friend or family member close their eyes and hold their nose and taste each one. They will likely have a hard time telling which is which. Why? Because, we get information from more than just our gustatory system, if we can see and smell the thing it sets up expectations about what we are about to perceive- so all of these things work together in our higher association areas

FECHNER'S AND & WEBER'S LAW

The most important information that is sent to the brain is "change in a stimulus". It is important for us to be able to detect differences in stimuli that are well above the adequate threshold

As a way of measuring this difference threshold a German scientist and philosopher, Gustav Fechner, proposed the

just noticeable difference (JND)

The JND is not a fixed quantity

it is roughly proportional to the magnitude of the stimulus

To describe the relationship between the stimulus magnitude and our ability to detect difference in the stimuli, a German physiologist - Ernst Weber - came up with a formula known as

Weber's Law . . .which allows for the quantification of the perception of change in a given stimulus. It is a linear relationship in the threshold and the difference threshold

Weber's Law Equation calculates that for somatosensory stimuli (and this holds for other sensory stimuli) - the JND is

2%. Meaning, if there is about a 2% difference or change in the stimuli being presented we should perceive that difference