Lecture 9 - General Principles of Sensory Processing, Touch, and Pain

Perception vs Reality

perception is a construct

Sensory processing

sensory receptor organs detect energy or substances

• answers the question: “what type of stimulus was that?”

• begins in receptor cells

• sensory information processing is selective and analytical

Sensory receptor organs

organs specialized to detect a certain stimulus

• very diverse - both within and across species

Receptor cells

within the organ to convert the stimulus into an electrical signal - transmitted back to the CNS

Adequate stimulus

the type of stimulus to which a sensory organ is particularly adapted - e.g., light for your eye

How can we classify sensory systems?

in terms of the type, the speciic modality, and the adequate stimuli

Types of sensory system

• mechanical

• visual

• thermal

• chemical

• electrical

• magnetic

Mechanical modality

• touch

• pain

• hearing

• vestibular

• joint

• muscle

Mechanical adequate stimuli

• touch - contact with or deformation of body surface

• pain - tissue damage

• hearing - sound vibrations in air or water

• vestibular - head movement and orientation

• joint - position and movement

• muscle - tension

Visual modality

seeing

Visual adequate stimuli

seeing - visible radiant energy

Thermal modality

• cold

• warmth

Thermal adequate stimuli

• cold - decrease in skin temperature

• warmth - increase in skin temperature

Chemical modality

• smell

• taste

• common chemical

• vomeronasal

Chemical adequate stimuli

• smell - odorous substances dissolved in air or water

• taste - substances in contact with the tongue or palate

• common chemical - changes in CO2, pH, osmotic pressure

• vomeronasal - pheromones in air or water

Electrical modaltiy

electroreception

Electrical adequate stimuli

electroreception - differences in density of electrical currents

Magnetic modality

magnetoreception

Magnetic adequate stimuli

magnetoreception - orientation of Earth’s magnetic field

Sensory systems

have a restricted range of responsiveness

• ex. the frequency range for hearing, which varies with species

• can influence each other

All senses use the…

same type of energy in transmitting signals - action potentials - doesn’t matter if it’s a visual stimulus (light) or auditory (sound)

Labeled lines

this concept says that the brain recognized distinct senses because action potentials travel along separate nerve tracts - logical conclusion - couldn’t have visual and auditory stimuli using same axons

Sensory tranduction

the conversion of electrical energy from a stimulus into a change in membrane potential in a receptor cell

Generator potentials

local changes in membrane potential of receptor cell induced by stimuli - initiate nerve impulses

Pacinian corpuscle

skin receptor that detects vibration

• a stimulus to the corpuscle produces a graded electrical potential

• when the potential is big enough, the receptor reaches threshold and generates an action potential

Coding

patterns of action potentials in a sensory system that reflect a stimulus

• a single neuron can convey stimulus intensity by changing the frequency of its action potentials

When a neuron hits its maximum firing, how do you indicate stronger stimuli?

multiple neurons can act in parallel — as the stimulus strengthens, more neurons are recruited

Range fractionation

takes place when different cells have different threshold for firing, over a range of stimulus intensities

• important because individual cells often cannot reflect the entire range of a stimulus

How do we know the location of the stimulus?

somatosensory system

Somatosensory system

detects body sensations, including touch and pain

• stimulus location is determined from the position of the activated receptors - system inputs map into a representation of the body’s surface in your cortex

Adaptation

the progressive loss of response to a maintained stimulus

Tonic receptors

show slow or no decline in action potential frequency

Phasic receptors

display adaptation and decrease frequency of action potentials

Purpose of adaptation?

important for showing CHANGES in environment

Other ways to control information

• accessory structures, such as eyelids

• central modulation of sensory modulation

Central modulation of sensory modulation

higher brain centers suppress some sensory inputs and amplify others

Each sensory system has distinct…

sensory pathway (i.e., neurons connecting to other neurons, connecting to other neurons, etc.) in the brain, and passes through stations during processing

Most pathways pass through…

regions of the thalamus

Pathways terminate in…

the cerebral cortex

Receptive field

the space in which a stimulus will alter a neuron’s firing rate

• in the periphery

• easy to imagine with the visual system — but it’s true for all the systems

differ in size, shape, and response to types of stimulation

• can be examined experimentally

Neurons all along the sensory pathway have…

receptive fields, but the receptive fields change as you move higher and higher in the sensory pathway

Initial receptor cell

relatively small receptive field - cell fires (more) when stimulus occurs in its receptive field

In contrast (to initial receptor cell)

when you move to higher-level neurons (e.g., cortical cells), you often get larger receptive fields (due to convergence of input from multiple sensory cells

At higher levels…

you also get receptive fields that often have a center-surround system

• stimulation of center produces opposite effect of stimulation of the surround

• creates a sharper contract in the sensation of the stimulus

Receptive fields in the cortex

• primary sensory cortex

• secondary sensory cortex

Primary sensory cortex

exists for each modality

• S1 or somatosensory 1

• receives touch information from the opposite side of the body

Secondary sensory cortex

receives its main input from the primary cortical area for that modality

• also called nonprimary sensory cortex

• S2 or somatosensory 2

• maps both sides of the body

Different patterns of representation in somatosensory cortex

• some animals have a different pattern

• the nose of the star-nosed mole is an organ for touch, and its somatosensory cortex responds to input from the star

Association areas

areas in the brain show a mixture of inputs from different modalities

Polymodal cells

allow for intersensory interactions

Synesthesia

a condition in which a stimulus in one modality creates a sensation in another

• ex. a person may perceive colors when looking at letters, or a taste when hearing a tone

Touch

• really made up of different kinds of skin receptors

• part of the somatosensory system

Four tactile receptors perceive touch

• pacinian corpuscles

• meissner’s corpuscles

• merkel’s discs

• ruffini’s endings

Pacinian corpuscles

vibration, fast-adapting

Meissner’s corpuscles

touch, fast-adapting

Merkel’s discs

touch, slow-adapting

Ruffini’s endings

stretch, slow-adapting

How does somatosensory information get to the brain?

• the dorsal column system delivers touch information to the brain

• receptors send axons via the dorsal column of the spinal cord where they synapse on dorsal column nuclei in the brainstem

• Axons from neurons in the medulla cross the midline, and go to the thalamus

Dermatome

a strip of skin innervated by a particular spinal room

• adjacent dermatomes overlap a small amount

Somatosensory brain regions

cells are arranged by the body surface plan

• brain regions reflect the density of body innervation

Receptive fields

can be changed by experience

• cortical map represents the innervation of a body region

If the nerve to the body region is severed, the cortical area will shrink

What happens if a body region is removed?

• the cortical area for adjacent body regions will expand

• stimulation of specific body regions will expand their cortical representation

  • doesn’t mean neurons moved

Pain

an unpleasant experience associated with tissue damage

Congenital insensitivity to pain

an inherited syndrome — can still discriminate touch

Tells us that pain and touch

are separate systems

Why have pain?

people without ability to feel pain — frequently injured, often die young

• pain helps us to withdraw from its source, engage in recuperative actions, and to signal others

Nociceptors

peripheral receptors that respond to painful stimuli

Free nerve endings

in the dermis

• have specialized receptor proteins

respond to temperature changes, chemicals, and pain

nociception

pain reception

Capsaicin

the “hot” in chili peppers

The receptors that binds capsaicin is the…

transient receptor potential vanilloid type1 (TRPV1) or vanilloid receptor 1

• normally detects painful heat

• a different receptor respond to warm heat

on C fibers

TRP2

differs from TRPV1

• detects even higher temperatures

• does not respond to capsaicin

• Is found on Aδ fibers

Aδ fibers

larger myelinated axons that register pain quickly

C fibers

thin unmyelinated axons that conduct slowly, producing lasting pain

Cool-menthol receptor 1 (CMR1)

responds to menthol and to cool temperatures—located on C fibers

Anterolateral or spinothalamic system

transmits the sensations of pain and temperature

• free nerve endings synapse on spinal neurons in the dorsal horn

• pain information crosses the midline in the spinal cord, before ascending to the thalamus

Pain information is integrated in the…

cingulate cortex

• different subregions are activated if a person is experiencing the pain or is empathizing with another

Analgesia

the loss of pain sensation

Opioids

drugs that control pain

Opioid-peptides

the endogenous neurotransmitters in the brain

Three classes of endogenous opioids

endorphins, enkephalins, and dynorphins

Opioid receptors

respond to opiates or opioids

Periaqueductal gray

an area in the midbrain involved in pain perception

• pain is not just signaling from the periphery — it is also modulated by the central nervous system, including the brain itself

Descending pain modulation system

allows our CNS and even our brain to inhibit incoming pain signals

Transcutaneous electrical nerve stimulation (TENS)

delivers electrical pulses to the skin

• TENS relieves pain by stimulating the nerves around the source of the pain — not entirely sure of mechanism

Naloxone

an opioid antagonist that can block the analgesic effect of TENS — tells us TENS depends on endogenous opioid release

Placebo

can sometimes relieve the pain, even though it is an inert substance

Acupuncture

relieves pain by inducing endorphin release

• “fake” acupuncture works as well as “real” acupuncture