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Perception

• Deals with general senses, special senses, and lab 4.

Outline

• Introduction covers how sensory receptors work in general.
• Touch is discussed briefly.
• Special senses covered in detail include smell, taste, vision, hearing, and equilibrium.
• Brief discussion of pain, many senses will be tested in Lab 4.

Sensory Reception Intro

• Sight, smell, taste, hearing, and equilibrium are special senses.
• Receptors are specialized cells limited to the head and innervated by cranial nerves.
• Touch is a general sense.
• Sensory receptors are cells that respond to specific stimuli and send information to the CNS and can be classified via location, stimuli, or receptor type.

Receptors: Classified by type of stimulus

• Chemoreceptors respond to chemicals in solution.
• Thermoreceptors are sensitive to temperature changes.
• Nociceptors respond to painful stimuli from tissue damage.
• Mechanoreceptors are sensitive to mechanical force and mechanically gated ion channels.
• Photoreceptors respond to light energy.

How sensory receptors get stimulated

• Energy from a stimulus produces a change in a receptor protein.
• The receptor protein is usually a membrane protein on the sensory receptor cell
• Changing the receptor protein changes a membrane property called "receptor potential".
• This might open or close an ion channel, altering the membrane potential and the likelihood of an action potential.

Sensory Transduction

• Sensory transduction is converting stimulus energy into a receptor potential, possibly leading to an action potential.
• Transducers convert energy from one form to another.
• Sensory receptors convert sensory stimuli into nerve impulses.
• In sensory receptors, a signal from outside the cell is transduced to be transmitted throughout the body as electrical signals.

Receptor cells: Two main mechanisms of action

• Direct effect on membrane potential directly causes the opening or closure of ion channels which changes the membrane potential and affects neurotransmitter release.
• Indirect effect on membrane potential binds to a receptor protein, activating a second messenger, triggering a cascade of events that change the membrane potential and affecting the release of neurotransmitter.

Adaptation - see 15.3

• Adaptation is the decreased sensitivity to a continued stimulus.
• Adaptation can be central or peripheral, and receptors can be tonic or phasic.

Touch

• There are many different receptors, each giving different types of information!
• Free nerve endings respond to touch, pressure, pain, and temperature.
• Root hair plexus is made up of free nerve endings stimulated by hair movement.
• Tactile discs are fine touch and pressure receptors, sensitive to shape and texture.
• Bulbous corpuscles are sensitive to pressure and distortion of the deep dermis.
• Lamellar corpuscles are sensitive to deep pressure and high-frequency vibration.
• Tactile corpuscles are sensitive to fine touch, pressure and low-frequency vibration.

Special senses

• More specialized in terms of their structure and nerve endings.
• Located only in specific parts of the body.
• The five special senses are smell (olfaction), taste (gustation), vision, hearing, and balance (equilibrium).

Olfaction: Anatomy review

• The olfactory epithelium consists of pseudostratified ciliated columnar epithelium with olfactory receptor cells.
• The olfactory receptor cells have olfactory dendrites exposed to the external environment and are stimulated by airborne odorant molecules.

Olfaction

• Receptors are chemoreceptors.
• Mucus acts as a solvent to dissolve airborne odorants.
• Odorant binds to a specific receptor, triggering a series of events inside the receptor cell that leads to the opening of Na+ channels using an indirect effect.
• Receptor cells are neurons with many different types of receptors.
• Olfactory receptor cells are unique, as they exposed to the external environment and have a short lifespan.

Gustation: Anatomy review

• The sensory receptor organ is the taste bud, found on papillae
• Receptor cells are in the epithelia of papillae (50-250 taste buds per papilla).
• A taste bud contains gustatory (taste) cells, basal cells, and supporting cells.
• Primary tastes are sweet, salty, sour, umami, and bitter.

Gustation (taste)

• Tastants dissolve in saliva, enter the taste pore, and stimulate the taste cells (chemoreceptors).
• Different mechanisms exist for different tastes: salty, sweet, sour, bitter, and umami.
• Some types of taste stimuli have direct effects, while others have indirect effects.

Vision: Anatomy review

• Vision is a complex organ, with some photoreceptive cells.
• The wall has three layers: fibrous, vascular, and inner (where photoreceptors are).
• The internal cavity is filled with fluids (humors).
• The lens divides the cavity into anterior cavity (filled with aqueous humor).

Vision: Intro

• The eye functions like a camera to capture visual information.
• There are receptors to respond to light (photoreceptors).
• Humans perceive light in the visible spectrum.
• Light is bent (refraction) via the cornea and the lens to focus an image on the retina.
• Improper bending results in images falling out of focus.

Vision: Photoreceptors

• The two types of photoreceptors are rods and cones -Rods: for vision in low light -Cones: colour vision (in bright light), and visual acuity (found in macula, esp. the fovea – no rods here)
• Both contain outer segments that are packed with discs, which contain visual.

Rod and Cone cells

• Outer Segment is photoreceptor and sensitive to light pigments
• Next is the Inner segment, which then connects to the Synaptic Ending

Vision: Averted vision: Andromeda galaxy!

• Used in low-light settings for higher visual resolution.

Role of the lens: near vs. distance vision

• For Close Vision: The Ciliary Muscle is Contracted and the Lens is Rounded to allow focus to be on the Fovea
• For Distant Vision: The Ciliary Muscle is Relaxed and the Lens is Flattened as the body relaxes
• Most people need reading glasses as the proteins in their eyes and lenses lose elasticity due to age

Role of the lens: refractive problems

• Myopia or Nearsightedness is caused by the eyeball being too deep or the Resting Curvature being too great which projects images in front of the retina
• This condition is fixed by equipping the user a Diverging/Concave Lens
• Hyperopia or Farsightedness is caused by the eyeball being too shallow or the lens being too flat. The Ciliary Muscle must contract to focus
• Hyperopia is caused by an inelastic lens and can be fixed with a Convex Lens

Hearing

• Consists of sound waves being transferred to the eardrum, which triggers three bones called Stapes, Incus and Maillecus.
• From there the vibrations are transferred to the Cochlea which relays it to the brain

Hearing: anatomy review

• Structures of the external ear gather sound waves and funnel them in towards the tympanic membrane.
• Vibrations of the tympanic membrane trigger vibrations of the ossicles (malleus, incus, stapes) of the middle ear.
• The stapes transmits the vibrations to the perilymph-filled vestibular duct of the inner ear.

Hearing: anatomy review (cont'd)

• Sound waves arrive at the eardrum causing displacement of the bones.
• Vibration of the basilar membrane causes hair cells to vibrate against the tectorial membrane to relay information about the intensity of stimulation to the CNS via the cranial nerve VII

Hearing: Anatomy review

• Movement of the tympanic membrane causes displacement of the Auditory ossicles which vibrate the stapes located on the oval window of the ear
• The Basilar Membrane releases hair to vibrate against the surface of the tectorial membrane which causes vibration of the hair cells to activate
• All transferred to the Cochlea which then relays the signal to the CNS by Cranial Nerve VIII

Hearing

• Receptors are hair cells in the Organ of Corti which is in the inner ear
• Stereocilia on each hair cell are linked together to cause vibrations
• When the stereocilia move it causes inner cell membrane K+ channels to open.

The role of stereocilia in sound transduction

• The bundle of Stereocilia is connected to the Tectorial membrane through a Tip Link
• When vibration occurs, the Tip Links Stretch resulting in a mechanical reaction that activates the ion channels on on the hair cells
• K+ can enter to depolarize the hair cell.

Hearing: pitch and volume

• Pitch is related to the frequency of sound waves.
• Different frequencies cause vibrations of different parts of the basilar membrane stimulate different cells, which are interpreted as different pitches
• Volume is related to a Sound waves amplitude -Larger waves (greater amplitude) = louder sound -Larger waves (with more energy) cause more noise

Equilibrium: Receptor cells

• Includes the semicircular ducts (anterior, posterior and lateral)
• Uses Utricle and Utricle to transmit sensory information.

Equilibrium quick anatomy review

• The vestibular apparatus contains the semicircular canals and vestibule of inner ear.
• Semicircular canals provide information on movement in different directions.
• Utricle and saccule give info on linear acceleration, plus position of head relative to gravity.

Pain

• Indicates Potential or actual tissue damage and activates Nocireceptors by chemical released by damaged cells
• Sensory Input may overlap and confuse different stimuli

Projection pathways for pain

• Nocireceptors detect painful Stimuli and relays this the the Dorsal Root which connects with two new Neurons to the primary and somesthetic cortexes.
• Info is then transferred up by the second Neuron which is then relayed by the Thalamus .
• The relayed signal ends up in the Reticular System

Spinal gating of pain signals

• The is essentially the some information but simplified
• Some Info is relayed to the Thalamus and Spinalthalamic Tract.

Home message

• Sensory receptors will always transduce sensory inputs
• The types of receptors always interact to deliver the appropriate results to any given stimuli