Sensory SystemJohn Paul L. Oliveros, MD, DPPS

Section AGeneral principles

General Principles

Awarenesss of our external and internal world is brought about by neural mechanisms that process afferent information

Stimulus energy receptor potentials (graded potentials) action potentials (Nerve fibers)

Sensory system› Part of the nervous system that consists of

sensory receptors Neural pathways Processing areas of the brain

Sensory information› Information processed by a sensory system› May or may not lead to conscious awareness of the stimulus

Sensation› Sensory information that reaches consciousness

Perception› A persron’s understanding of the sensation’s meaning

Receptors Sensory Receptors

› Initiates neural activity at the border betwee the nervoussystem and the outside world

› Change stimulus energy (pressure, temperature, light, soundwaves, etc)

› Can either be: Specialized ending s of afferent neurons Separate cellthat affect the ends of

afferent neurons

Stimulus› Energy that impinges upon and

activates a sensory receptor Stimulus transduction

› The process by which stimulus is transformed into an electrical response

Adequate stimulus› The type of energy to which a receptor

responds in normal functioning› Receptors respond best to only a very

narrow range of stimulus energy (lowest threshold)

Receptor Potential Transduction process in all sensory

receptors involve the opening and closing of ion channels that receive information about the outside world

Receptor potential› A change in the membrane

potential on a specialized receptor membrane

› It is a Graded potential separate receptors:

graded potential causes release of neurotransmitter

Receptors on afferent neuons: A local current must flow to a part of

an axon that can produce an action potential

1st node of Ranvier Part of myelinated afferent neurons

capable of producing action potentials

Receptor Potential Graded potential magnitude

determines action potential frequency but not action potential magnitude

Factors controlling receptor potential magnitude› Stimulus strength› Rate of change of stimulus

strength› Temporal summation of

successive receptor potentials› Adaptation

Decrease in receptor sensitivity Results in decrease in frequency

of action potentials in an afferent neuron despite maintenance of the stimulus at a constant strength

Neural pathways in sensory system Sensory pathway

› A bundle of parallel 3-neuron chains

Sensory units› A single afferent

neuron with all its receptor endings

Receptive field› Portion of the body

that when stimulated leads to activity in a particular afferent neuron

Ascending pathways

Central processes› Part of afferent neurons

that enter the brain or spinal cord and synapse with interneurons

2nd order neurons› Interneurons that

synapse with afferent neurons

› Synapse with 3rd, 4th, etc interneurons until the cerebral cortex is reached

Ascending pathways Specific ascending pathways

› Ascending pathways in the brain and spinal cord that carry information about single types of stimuli

› Somatosensory cortex Lies in the parietal lobe of the brain

behind the junction of the parietal and frontal lobes

Where information from somatic recepotrs are transmitted

Information from skin, skeletal muscles, tendon and joints

› Visual cortex At the occipital lobe Where spefic pathways from the eyes

transmit

› Auditory cortex Where specific pathways from the

ears transmit Loacted at the temporal lobe

Ascending pathways

Nonspecific ascending pathways› Activated by sensory

units of several different types

› Signal general information

Polymodal neurons› 2nd order neurons that

respond to inputs from several afferent neurons, each activated by a different stimulus

Association Cortex and Perceptual Processing Cortical Association Areas

› Areas of the brain outside the primary cortical sensory areas but are adjacent to them

› Elaborates perception information from the primary sensory cortical areas

› Regions closests to the primary sensory cortical areas process information in fairly simple ways and serves basic sensory function

› Regions farther from the primary sensory cortical areas process information in more complicated ways Arousal Attention Memory Language Emotional and motivational

significance (frontal lobe/ limbic system)

Association Cortex and Perceptual Processing Factors that affect perception

1.Afferent information is influenced by sensory receptor mechanisms and by processing of the information along afferent pathways

2. Factors such as emotions, personality, experience and social background can influence perceptions so that 2 persons can witness the same events and yet perceive them differently

3. Not all informationentering the CNS give rise to conscious sensation* e.g. carotid/aortic bodies

4. We lack suitable receptors for many energy forms* x-ray, radio and TV waves

5. Damaged neural networks may give rise to faulty perceptions * phantom limb phenomenon

6. Some drugs alter perceptions* drugs* diseases

In summary:

*3 processes needed for perception to occur1. transducing stimulus energy into action potentials by receptor2. transmitting data through the CNS3. Interpreting the data

• 3 iportant organizational principles of the sensory system

• 1. there is heirarchical processing of afferent information along individual pathways

• 2. information is processed by parallel pathways, each of which handles a limited aspect of neural signals generated by the sensory transducers

• 3. information at each stage along the pathway is modified by “topdown” influences serving emotions, attention, memory and language

Primary sensory coding

The sensory system codes 4 aspects of a stimulus› Stimulus type› Intensity› Location› duration

Stimulus type› AKA stumulus modality

(temp, sound, pressure)› Submodalites:› A receptor type is

particularly sensitive to one stimulus modality (adequate stimulus) Due to the signal

transduction mechanisms and ion channels in the receptor’s plasma membrane

e.g. Vision receptors have pigments whose shape is transformed by light

Primary sensory coding

Stimulus intensity› Distinguishing intensity

Frequency of action potentials Inc. Stimulus strengthinc. Receptor potential inc. Action

potential frequency single receptor Other receptors of the same neuron

Recruitment Calling in of receptors on additional afferent neurons

Primary sensory coding

Stimulus location› Factors:

Main factor: Site of the stimulated receptor

amount of convergence of neuronal input in ascending pathways: inversely related to acuity/precision

Size of the receptive field covered by a receptor

Overlap of nearby receptive fields

Primary sensory coding

Primary sensory coding Lateral inhibition

› More important in localization than the different sensitivites of receptors throughout the receptor field

› Information from afferent neurons whose receptors are at the edge of the stimulus is inhibitted compared to information from the stimulus’ center

› Increases contrast between relevant and irrelevant information

› May occur at any levels of the pathway but mostly on the early stages

Primary sensory coding Stimulus duration

› Receptors differ in the way they respond to a constantly maintained stimulus adaptation

› Rapidly adapting receptors: Important in signaling rapid

change On response On-off response

› Slowly adapting receptors: Maintain response at or near

the initial level of firing regardless of the stimulus duration

For prolonged events (posture)

Central control of afferent information Reticular formation

and cortex: main control

Section BSpecific Sensory Systems

Somatic sensation Somatic sensation:

› Skin› Muscles› Bones› Tendons› joints

Activation gives rise to sensations› touch, › Pressure› Warmth› Cold› pain › awareness of the position of the parts

and their movement Each sensation has a specific receptor

type Information enters both specific and

non-specific pathways› Specific pathways cross to the

opposite side of the brain (somatosensory cortex

Somatic sensation

Somatic sensation

Somatosensory cortex Endings of axons of the specific pathways are

grouped according to the location of the receptors giving rise to the pathways

Touch-pressure

Skin mechanoreceptors› Rapidly adapting

receptors Touch Movement vibration

› Slowly adapting receptors pressure

Sense of posture and movement Receptors for

postures and movement› Muscle-spindle

stretch receptors› Vision and

vestibular organs› Mechanoreceptors

in joints, tendons, ligaments, skin

Kinesthesia› Sense of

movement at a joint

Temperature

Thermoreceptors› Warmth receptors

Respond to temp between 30c-43c

Increase discharge rate upon warming

› Cold receptors Stimulated by small

decrease in temperature

Pain Nociceptors

› Detect stimulus that causes tissue damage

› Respond to intense mechanical deformation, excessive heat, and many chemicals (several secreted by damaged cells)

Hyperalgesia› Increased sensitivity to painful

stimuli› Last for hours after the stimulus is

over Referred pain

› Sensation of pain is experienced at a site other than the injured/diseased part

› Due to activation of intrneurons by incoming nociceptive afferents

› Visceral and somatic afferents often converge in the same interneurons in the pain pathway

Analgesia› Selective suppresion of pain without

effects on consciousness or other sensation

Stimulation-produced analgesia› Electrical stimulation

Transcutaneous electric nerve stimulation› Electrodes are placed on the surface

of the skin above the painful site or nerves leading from it

› Stimulation of non-pain, low threshold fibers leads to inhibition of neurons in the painful pathway

Acupuncture› Needles are introduced into specific

parts of the body to stimulate afferent fibers which causes analgesia

› It ivolves endogenous opiod neurotransmitters

Vision

Light› Receptors of the eyes are

only sensitive to visible light

› Wavelength Distance between 2

successive wave peaks of the electromagnetic radiation

› Frequency Measured in hertz (cycles per

second) Varies inversely with

wavelength

› Visible spectrum Between 400-700nm Light of different wavelength

is percieved as colors

Vision Optics of vision

› Retina Focuses the image being viewed Thin layer of neural tissue lining the

back of the eyeball Contains

Rods Cones ‘neurons

› Lens and cornea Optical system that focus the impinging

light rays into an image upon the retina Surface are curved to bend light rays

coming from different directions and focus them into a single point at the retina

› Fovea centralis Area in the retina with the greatest

visual clarithy Area where light rays from the

cornea/lens are focused Image is upside down and reversed

right to left

Vision

Optics of vision

Vision Optics of vision

› Accomodation Process of focusing and adjusting

image on the retina Cornea

Greater part of focusing image on the retina

Lens Adjustments for distance made by

changing its shape

› Cilliary muscles Controls the shape of the lens Stimulated by parasymphatetic Sphincter like and draws lens towards

it as it contracts Accomodation for viewing near objects

› Zonular fibers Attaches the ciliary muscles to the lens

Pulls lens to flatten it to focus distant objects

Relaxes to make lens more spherical to focus near objects

Vision

Accomodation› Include the following

mechanisms To view near objects

Moving of lens slightly towards the back of the eye

Turn the eyes inward and towards the nose (convergence)

Constrict the pupil To view far objects

Opposite of the above mechanisms

Vision Abnormalities

› Presbyopia Normal part of aging process Increasing stiffness of the lens making

accomodation difficult

› Cataract Opacity of the lens Usually due to changing color due to age

› Nearsightedness / myopia Unable to see distant objects clearly Eyeball is too long Far images focus at a point in front of the

retina

› Farsightedness / hyperopia Eye is too short Near objects are focused behind the retina Near vision is poor

› Astigmatism Lens and cornea doesn’t have a smoothly

spherical surface

› Glaucoma Aqueous humor is formed faster than it is

removed Increase intraocular pressure Leading cause of irreversible blindness Axons of the otic nerve die

Vision Iris

› Controls the amount of light entering the eye

› Ringlike pigmented muscular tissue

› Color is not significant› Sympathetic innervation

Radial muscles contract Pupils enlarge

› Parasymphatetic innervation: Sphincter muscles contract Pupils contract

Pupils› Whole in the center of the

iris› Where light enters the eye

Vision

Photoreceptor cells

vision

Photoreceptor cells› Rods

Extremely sensitive to very low levels of illumination

› Cones Less sensitive and

respond to bright light

Choroid› Pigmented layer

behind the retina› Absorbs light and

prevents reflection back to the rods and cones

Photopigments› Inside photoreceptors› Absorb light› 4 types:

Rods (1): rhodopsin) Cones (3)

› Each contains: Opsin

Group of integral proteins that surround and binds a chromophore molecule

Differs in each of the 4 photopigments

Light filters differently in each photopigments and thus absorbs light most effectivly at different spectrum

Chromophore Light sensitive part of the

photopigment Same in all 4 photopigments A derivative of vit. A (retinal)

Vision

Photoreceptors

Vision Light retinal changes shape

photoreceptor hyperpolarization decrease release of neurotransmitter (glutamate) hyperpolarization of bipolar cell

In the dark:› Retinal has resting shape› Photoreceptor cell partially

depolarized› More neurotransmitter is

transmitted Dark adaptation

› Temporary blindness when one steps into a darkend room from bright sunlight

› At brighlight: rhodopsin completely activated

› At dark: at least 10mins needed to restore rhodopsin to resting state

Neural Pathways of vision

Vision

Color vision› The colors we

perceived are related to the wavelengths of light that are reflected, absorbed, or transmitted by the pigments in the objects of our visual world

› White light Mixture of all colors

› Blacklight Absence of all light

› Begins with the activation of the photopigments of the cone receptor cells (red, green, blue)

Vision

Color vision› Ganglion cells

general brightness: receives input from all 3 colors

Opponent color cells: Code for specific color Excitatory input from one

type of receptor and inhibitory from another

› Color blindness AKA color deficiency Lack red or green pigments

entirely or have them in abnormal form

Trouble perceiving red vs green

Vision Eye movement

› Controlled by 6 skeletal muscles

› 2 basic movements Fast movements

AKA saccades small jerking movements Rapidly bring eye from one

fixation point to another Allow search for visual field Prevent adaptation Move during certain periods of

sleep (watching visual imagery of dreams)

Slow movements Involve in tracking visual

objects moving throught the visual field

Compensation during movements of the head

Hearing

Sound› Sound energy:

Medium: gaseous, liquid, or solid medium

Vibration of the mediums’ molecules

Vibrating objects can serve as a sound source

› Sound wave: Zones:

Zones of compression Molecules close together Pressure is increased

Zones of rarefaction Molecules are far apart Pressure is less

Sound wave (cont)

Consists of rapidly alternating pressures

Amplitude Determined bydifference

between the 2 zones Related to the loudness of

the sound Frequency

Number of zones in a given time

Determines the pitch we hear

Keenly audible frquency: 1000-4000hz

Audble frquency: 20-40,000hz

Hearing

Hearing Sound transmission in the ear

› External auditory canal Help amplify and direct sound

› Tympanic membrane Vibrate at the same frequency of

the sound waves

› Middle ear cavity Air filled cavity in the temporal

bone

› Auditory/eustachian tube Connects the middle ear to the

pharynx Exposes the middle ear to

atmospheric pressure Normally close but opens during

yawning, sneezing, swallowing to equal middle ear pressure to atmospheric pressure

Pain during sudden change of altitude because of pressure difference between middle ear and atmosphere

Hearing

Middle ear› Sound waves amplified

by chain of bones that act as pistons and couple the motions of the tympanic membrane to the oval window Malleus Incus stapes

› Force of sound waves transferred from tympanic membrane to oval window

Hearing

Hearing Inner Ear/Cochlea

› Fluid filled, spiral shaped passage in the temporal bone

› Where the receptors cells are located

› Cochlear duct Fluid filled membranous tube Follows the cochlear spiral Divides the cochlea lengthwise

› Scala vestibuli On side of cochlear duct and

ends on the oval window

› Scala tympani Below the cochlear duct and

ends on the round window

› Oval window Separates inner ear from

middle ear

Basilar membrane› Forms one side of the cochlear

duct› Organ of corti

Sits on the basilar membrane Contains receptor cells

Hair cells Mechanoreceptors with hairlike

stereocilia Transform pressure waves in the

cochlea into receptor potentials Movements of basilar membrane

stimulate hair cells Tectorial membrane

Move in relation to haircells Bend sterocilia to open ion

channels Efferent nerve fibers

From brainstem Dampen response for protectionr

Afferent neurons Forms cochlear portion of cranial

nerve VIII

Hearing

Neural pathways of hearing› Cochlear N. brainstem

interneurons multineuron pathway thalamus auditory cortex

› Hearing aids Amplify incoming sounds

› Cochlear implants Used when there is extensive

damage Restore functional hearing Directly stimulate the

cochlear nerve with tiny electric currents

Bypass the cochlea

Vestibular system Vestibular apparatus

› Series of fluid filled membranous tubes that connect with each other and with the cochlear duct

› Contains hair cells that detect changes in the motion and position of the head

› Consists of: 3 semicircular canals Utricle saccule

› Labyrinth Bony canals of the inner ear

that contains the vestibular apparatus and cochlea

Vestibular system Semicircular canals

› Detect angular acceleration during rotation of the head along 3 perpendicular axes Nodding head up and down

(yes) Turning head from side to side

(no) Tipping the head so ear touches

shoulder

› Receptor cells Also contains hairlike stereocilia

› Cupula Gelatinous mass that ensheaths

the stereocilia

› Ampulla Slight bulge in the wall of each

duct

Vestibular system Utricle and saccule

› Receptors: Mechanoreceptors with

stereocilia Utricle: horizontalposition Saccule: vertical position Otoliths:

Tiny calcium carbonate stones Embedded in gelatinous

substance together with stereocilia

Makes gelationous substance heavier than surrounding fluid

Moves according to the force of gravity

› Provide information on: Linear acceleration

Up and down Back and forth

Changes in head position in relation to gravity

Vestibular system Vestibular information and dysfunction

Hair cell vestibular branch of cranial N. VIII brainstem multineuronal pathway vestibular centers of parietal lobe

3 uses of vestibular information:1. Control eye muscles to fix eye in the

same point in spite of head movement

› nystagmus: large jerky back and forth movement of the eye in response to unusual vesdtibular input

2. To maintain upright posture

3. To provide conscious awareness of the position and acceleration of the body

Vertigo:› Illusion of movement (usually

spinning)› Accompanied by feelings of nausea

and lightheadedness› Occurs when there is a mismatch in

the information from the various sensory systems

› e.g. Looking down from the building Motion sickness

› Unfamiliar patterns of linear and rotational acceleration are experienced and adaptation to them has not occured

Meniere’s disease› Involves the vestibular system› Episodes of abrupt and severe

dizziness, ringing of ears, bouts of hearing loss

› Due to increase fluid pressure in the membranous duct sytem of the inner ear

Chemical senses

Receptors: chemoreceptors

Taste› Tastebuds

Specialized organs for taste

10,000 + present

› 4 basic groups Sweet Sour Salty Bitter

› Pathways end up in mouth region of the somatosensory cortex

Chemical Senses Smell

› 80% of flavor of food is contributed by smell

› Odor of a substance is related to its chemical structure

› Olfactory receptor cells Lie in the olfactory epithelium

in upper part of the nasal cavity

Specialized afferent neurons With single enlarged dendrite

that extends to the surface of the epithelium

Cilia: Processes of dendrites Non-motile Bath in mucus Containreceptor proteins for

olfactory stimuli

Odorant 1000 or so different plasma

membrane odorant receptor types

Axons (cranial N. I) olfactory bulbs olfactory cortex (limbic system)

Limbic system:› Emotional behavior› Fodd getting behavior› Sexual behavior

Olfactory discrimination› Increased in hunger› More keen in women› Smokig decrease sensitivity› Decreases with age› Decreases with nasal congestion

Chemical senses

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