Sensory Perception: A Summary

AP BiologySpring 2011

Stimulus: form of energy that activates receptor endings of a sensory neuron

Sensations: conscious awareness of stimulus Perception: understanding of what a

sensation means

Mechanoreceptors: reseptors stimlate by physical stimuli, such as pressure, touch, stretch, sound, or motion

Thermoreceptors: respond to either heat or cold and help maintain body temperature

Chemoreceptors: detect chemical energy of substances dissolved in a fluid bathing them Gustatory (taste) receptors and olfactory

(smell) receptorsOsmoreceptors: detect changes in the

solute levels of some body fluid

Photoreceptors: detect differences in energy of visible light and UV light

Pain receptors: respond to excess heat, pressure, or specific classes of chemicals released from damaged or inflamed tissuesMany different groups

All sensory receptors transduce stimulus energy into action potentials

Brain assesses each stimuli by which nerve pathways are carrying action potentials, the frequency of action potentials traveling on each axon in the pathway, and the number of axons recruited by the stimulus

Sensory Adaptation: a diminishing response to an ongoing stimulusEx. The pressure exerted by a sock

Example of how we make use of sensory pathwaysFigure 35.2, page 601

Somatic sensations: arise in the cerebrum’s outer layer of gray matter, the cerebral cortex.

Some body parts have more sensory acuity (more neurons)Fingers, thumbs, lips

Sensation of touch, pressure, cold, warmth, and pain

Free Nerve Endings: unmyelinated or thinly myelinated, branched endings of sensory neurons in skin and internal tissuesThermoreceptos, mechanorecptors, and pain

receptorsAdapt slowly to stimualtion Different subpopulations respond to different

stimuli

Encapsulated receptors: detect somatic sesations, 4 types 1. Meissner’s Corpuscle

Fingertips, lips, eyelids, nipples, genitals Adapts very slowly to low-frequency vibrations

2. Bulb of KrauseThermoreceptor, activated at 20 degrees C or lower,

below 10 degrees C contributes to painful freezing sensations

3. Ruffini endings Detect steady touch and pressure, and temp.’s above

45 degrees C 4. Pacinian Corpuscle

Sensitive to fine textures, occurs in dermis, near freely movable joints, and in some internal organs, sensitive to rapid pressure changes brought on by touch and vibrations

Stretch receptors in muscle spindle fibers Increase firing rate as muscle stretchesSignal brain about positions of the body’s

limbs

Pain: perception of a tissue injury

Somatic pain: response to signals from pain receptors in skin, skeletal muscles, joints, tendonsSuperficial somatic pain arises at or near skin

surface , sharp or pricking often does not last long

Deep somatic pain arises deeper in skin or muscles or joints, diffuse and lasts longer

Visceral pain: associated with internal organs Is response to high chemical stimulation,

muscle spasm, muscle fatigue, excessive distension of gut, inadequate blood flow to organs, and other abnormal conditions

Cells injured release chemicals that activate pain receptors

Bradykinins: open floodgates for histamine, prostaglandins, and other participants in inflammatory response

Signals from pain receptors enter spinal cord and cause release of a neuromodulator, substance P, which activates neurons that can signal the sensory cortex

Assess the intensity and type of pain Different signals rouse body and mediate

emotional responses Release natural opiates- endorphins and

enkephalinsLower pain preception

Influence pain tolerance:Emotional states, culture factors, possibly age

(older people handle pain better)Hyperalgesia: intense or long lasting pain

leads to this condition where pain is amplifiedBad sunburn and hot shower

Referred pain: perception of visceral sensations as somatic sensations Ex. Heart attack

Makes mistake because of nervous system constructionSensory inputs of skin and certain internal organs enter same segments of spinal cordSkin encounters more pain than organs, so brain may interpret most sensory input as arriving from skin

Phantom pain: amputees sense presence of a missing body part as if it were still there

Severed sensory nerves continue to respond to the amputation

Brain projects pain back to missing part, past the healed region

Chemoreceptors become activated by binding molecules of a substance that is dissolved in the fluid bathing them

Stimulus triggers signals that travel along nerves through thalamus, signals end in cerebral cortex Perception takes shape and fine tuning

Also reaches limbic system, integrates emotional states and with stored memories

Olfactory Receptors: fire off signals when exposed to water-soluble or volatile (easily vaporized) chemicals

Receptor axons lead into one of two olfactory bulbs

In these small brain structures, axons synapse with cells that sort out scent Then, information flows along olfactory tract to cerebrum, where further processed

Use olfactory cues to navigate, find food, communicate

Pheromones: signaling molecules secreted by one individual that change the social behavior of other individuals of its species

Vomeronasal organ: reptiles, most mammals (not primates), cluster of sensory cells that detect pheromones Humans have reduced version

Taste receptors: found on antennae, legs, tentacles, or inside mouth Chemoreceptors

located in our surface of mouth, throat, and upper part of tongue (taste buds)

5 main sensations: sweet, sour, salty, bitter, umani (savory taste)

Vestibular apparatus: one each ear, consist of two sacs (utricle and saccule) along with 3 semicircular canals Sacs and canals interconnect into a continuous,

fluid-filled system where mechanoreceptors are stimulated when you move

Organ of Dynamic Equilibrium: inside bulging semi-circular canal, gelatinous mass which hair cells project Any rotation of head displaces fluid Hydrostatic pressure exerted by moving fluid

shifts gelatinous mass, which bends hair cells Bending causes signals to flow from sensory

neurons to vestibular nerve, which carries signals about motion to brain

Organ of Static Equilibrium: inside each utricle and saccule, send messages to brain about how head is oriented relative to ground Thick membrane rests on hair cells that project

upward from floor of sac Membrane contains mass of crystals, weigh it

down Head upright: weighted membrane presses

down on hair cells, bends them slightlyIf posture changes or speed up or slow down

movement, position of membrane above hair cells will shift

Brain also evaluates input from receptors in skin, joints, tendons

Integration of all information allows control of eye muscles that keep visual fields in focus even with movement

Helps maintain awareness of body’s position and motion in space

Vertigo: sensation that world is moving or spinning around Stroke, inner ear infection, loose particles in

semicircular canals Arises from conflicting sensory inputs

Motion Sickness: from mismatched signals Passengers in a vehicle, confusion of if in

motion (outside vehicle) or stationary (inside vehicle)

Amplitude: loudness (intensity), measure in decibels

Frequency: number of wave cycles per second, more cycles per second higher the frequency and pitch

Water readily transfers vibrations to body tissues

Sound spreads out in air Outer ear: adapted for gathering sounds

from air Pinna: folded flap of cartilage, sheathed in

skin, projects from side of headAuditory canal leads from pinna to middle ear

Middle ear: amplifies and transmits air waves to inner ear Eardrum: thin membrane, vibrates fast in

response to pressure waves Behind drum is air-filled cavity and small

bones: hammer, anvil, stirrupBy interacting bones transmit force of sound

waves from eardrum to a smaller surface (oval window)

Oval window: elastic membrane in front of inner ear

Inner ear: has vestibular apparatus, cochleaCochlea: pea sized, fluid-filled structure,

transduction of waves of sound into action potentials occurs here

Sound waves make stirrup vibrateMiddle ear bone pushes against oval windowTransmits pressure waves to fluid in 2 of 3

cochlear ducts (scala vestibuli and scala tympani)

Waves end at another membrane, round window (bows inward and outward in response)

Third cochlear duct sorts out pressure waves Its basilar membrane wall, is stiff and narrow

near oval window, then broadens and becomes more flexible deeper in coil High pitched sounds make stiff, narrow part of

cochlear duct vibrateLow pitched sounds make flexible part vibrate

Acoustical organ: attached to one surface of basil membrane

Organ of Corti: has arrays of hair cells (acoustical receptor with modified cilia at one end)

Cilia bend when pressure moves basilar membrane

Mechanical energy transduced into action potentials that reach brain

Damage of hairs results in hearing loss

Vision: requires eyes and image perception in brain centers that can interpret patterns of visual stimulation

Eyes: sensory organs that contain a tissue of many densely packed photoreceptors

Photoreceptors: contain pigment molecules that can absorb photon energy, which can be converted to excitation energy in sensory neurons

Ciliary photoreceptors: plasma membrane around the cilium develops into the photosensitive surface Cnidarians, some flatworms, vertebrates

Rhabdomeric photoreceptors: photosensitive surface develops from microvilli around cilium Most flatworms, mollusks, annelids,

arthropods, echinoderms

Photoreceptors dispersed though integument (protective area)

Ocellus: simplest eye, photoreceptors project into these pigmented sports or shallow cups of integument Can determine direction of light sourceUse light as cue for orientation, predators, bio-

clocks

Visual Field: part of outside world that eye sees

Eye lens: helps image formation, transparent structure, bends all light rays from a given point in the visual field so they converge onto photoreceptors

Cornea: helps sharpen images, transparent cover that directs light rays onto lens

Simple arthropod eye: one lens for all photoreceptors Spiders

Compound eye: many closely packed rhabdomeric units Some have thousands of units of a sort called

ommatidia, inside each is a photoreceptor with rhabdomeric microvilli; visual pigment rhodopsin, embedded in membrane

Visual mosaic: each unit samples small part of visual field

Crustaceans and insects

Camera eyes: most complexLight enters dark chamber through pupilAn opening in ring of contractile tissue called

iris Behind pupil, lens focus light on retina (tissue

with many photoreceptors) Axons of sensory nerves converge to form optic

tract One tract from each eye extends to brain Cephalopods: octopuses and squids

Wall of eyeball (three layers)

Sensory Tunic (inner layer)

Retina: absorbs, transduces light energy

Fovea: increases visual acuity

Vascular Tunic (middle layer)

Choroid: blood vessels nutritionally support wall cells, pigments prevent light scattering

Ciliary Body: muscles control lens; shape; fine fibers hold lens upright

Iris: adjusting iris controls incoming light

Pupil: serves as enterance for light

Start of optic Nerve: carries signals to brain

Fibrous Tunic (outer layer)

Sclera: protects eye ball (white of eye)

Cornea: focuses light

Interior of eyeball

Lens Focuses light on photoreceptors

Aqueous humor Transmits light, maintains pressure

Vitreous body Transmits light, supports lens and eyeball

Form an inverted, reversed image

When ciliary muscle contracts, lens bulges, bending light rays from a close object so that they become focused on the retina

When muscle relaxes, lens flattens, focusing light rays from a distant object on the retina

Species that move about in night or dimly lit areas need to intercept as much of the available light as possible

Large pupils let in more light Large irises can be dilated more to let in more light

In between retina and choroid is pigmented epithelium

Anchored to epithelium is rod and cone cellsCiliary photoreceptors

Rod and cone cells transduce photon energy into action potentials, messages by which brain cells monitor the visual field

Rod Cells: detect very dim light, basis for coarse perception of movement across visual field Most abundant outside fovea, small circular

region near centre of retina Has rhodopsin pigments Membrane folding and high density of

pigments increases odds of intercepting photons

Cone cells: detect bright light, basis of sharp vision and colour perception

Sense of colour and daytime vision starts when red, green, and blue cone cells, each with different kind of visual pigment, absorb photons

Fovea has greatest density of cone cells Basis of greatest visual acuity (most precise

discrimination between any 2 points in visual field)

Distinct types of sensory neurons lie above rods and cones

These neurons receive, process, and start to integrate signals that arise from transduction of photon energy

Input from about 125 million rods and cones converge on the retinal neurons known as bipolar cells

Humans have 11 types of bipolar neurons- 10 for cones, and 1 for rods

Sort out objects that are lighter than darker ones in visual field’s background

Information also flows laterally among amacrine cells and horizontal cells

Messages converge on 1 million ganglion cells

These are output neurons; axons are start of an optic nerve that carries action potentials to the brain

Before a transduced signal leaves the retina, neurons start integrating and processing it

Humans have 2 optic nerves, 1 from retina in each eye

Each optic nerve delivers signals concerning a stimulus from the left visual field to the right cerebral hemisphere And from right visual field

to left hemisphere

Optic nerve axons end in layered brain region- lateral geniculate nucleus

Each layer has map corresponding to receptive fieldsDeals with one kind of visual stimulus: form,

movement, depth, colour texture After early processing signals reach different parts of visual cortexIntegration organizes action potentials and produces visual sensations

Colour blindness: Cone cells fail to develop

Focusing problems: Astigmatism: unevenly curved corneas, cannot

properly bend all incoming light rays to same focal point

Myopia (nearsightedness): horizontal axis of eyeball is longer than vertical axis, or ciliary muscle responsible for adjusting lens contracts too strongly Outcome: images of distant get focused in front of retina

instead of on it Hyperopia (farsightedness): eyeballs vertical

axis is longer than horizontal Outcome: light rays from closeobjects get focused

behind retina