Biol 66: Chapter 10 (vestibular, auditory, gustation, olfaction, balance)

Sensory Receptors

• Cells (neurons/modified epithelial cells) that receive sensory information (stimulus) from environment

• Transduce different energy forms into graded potentials that initiate action potentials

  • AFFERENT SENSORY INPUT TO CNS

Mechanoreceptors

Respond to mechanical stimuli, like touch or pressure

Thermoreceptors

respond to cold/warmth

Photoreceptors

Respond to light

Chemoreceptors

respond to binding of particular chemicals

Nociceptors

respond to painful stimuli

Receptor Potential

Graded potential in sensory receptors in response to environmental stimulus

• Transduction involves opening of ION CHANNELS

• If depolarization at initial segment of axon → reaches threshold → gated ion channels open → AP generated

Sensory Adaptations

Decrease in receptor sensitivity (responsiveness) during maintained stimulation; Phasic or Tonic receptors

Phasic

AKA fast-adapting receptors: respond with a burst of activity when stimulus is first applied but quickly adapt to stimulus by decreasing response

• EX: pressure when seated on a chair; odor, temperature, taste

Tonic

AKA slow-adapting receptors: maintain a high firing rate as long as stimulus is applied

• persistent action potentials

• EX: receptors in joint and muscle that maintain posture; pain

Somatic Sensation

• touch

• pressure

• pain

• temperature

• senses of posture and movement

Vestibular System

• structures are in inner ear

• sensation of

  • head position

  • head movement (angular acceleration in 3 dimensions)

  • linear acceleration

Vestibular system detects changes in the motions and position of head through:

Bending of hair cells (modified epithelial cells that are sensory receptors) in otolith organs and semsicircular canals results in afferent activity

Otolith Organs

2 otolith organs/maculae

• saccule and utricle

• sense linear acceleration with respect to gravity

• each sensor has a mass of OTOLITHS (calcium carbonate crystals) on top of GELATINOUS SUBSTANCE

Semicircular Canals

3 fluid-filled → endolymph

• sense angular acceleration of the head in the three dimensions of space (X-Y-Z) to maintain balance

• each canal has a crista (sensory organ, in ampulla)

• crista has a gelatinous mass (cupula) on top → pushed by endolymph movement

  • cupula bending stimulates hair cells

Chemosensation

• chemoreceptors: respond to chemical stimuli

• taste/gustation

• smell/olfaction

Taste bud

• comprised of 50-100 specialized epithelial cells called TASTE CELLS that undergo APS and synapse with sensory neurons

• taste cells → transduce chemicals

Salty

Na+ through ion channel → depolarization → opens Ca2+ channels → neurotransmitter released → sensory neuron stimulated

Sour

H+ through ion channel (and other effects) → opens Ca2+ channels → neurotransmitter released → sensory neuron stimulated

Sweet + Umami

binds to membrane receptors for sweet + umami → sugar or amino acids → G-protein → second messenger → Close K+ channels → depolarization → NT → sensory neuron stimulated

Olfaction Process

1. Odorants → stimulate olfactory sensory neurons (bipolar) by binding to membrane receptors (proteins in the cilia of olfactory sensory neurons)

2. axons of sensory neurons synapse onto olfactory bulb of brain

3. olfactory tract (grouping of axons) carries AFFERENT information from bulb to other brain areas for perception to occur

   • 10,000 odors are coded by variations in binding of odorants to 380 different membrane receptors → unique patterns of activity that brain interprets to perceive an odor

Sound

Results from vibration of gas, liquid, or solid molecules

Sound waves

Zones of atmospheric rarefaction (low pressure) and compression (high pressure)

• cause movement of auditory structures → transduced into action potential

Intensity

Amplitude of sound wave, determines loudness

Frequency

Number of cycles per second of the sound wave, determines pitch (higher frequency = higher pitch)

Audition

1. Pinna + external auditory canal → focus sound waves on → tympanic membrane → rocks malleus, incus, and stapes (ossicles) → oval window → ripples in cochlear fluid → vibrations in basilar membrane

   1. shearing between basilar + tectorial membran (in organ of corti) → bend hair cells in organ of corti

   2. depolarize (ion channels open) → AP in sensory neurons → NT to CNS

Organ of Corti

where auditory transduction occurs; basilar membrane + tectorial membrane

Apical cochlea

low frequency/pitched sounds cause large vibrations at the top

Basal cochlea

High frequency/pitch components of complex sounds cause large vibrations at bottom

Tonotopic

arranged by frequency

Light

• reflected off objects

• wave-like properties

Wavelength

• distance between two peaks (nm)

• corresponds to color

• 400-700 nm visible spectrum

Three Layers (Tunics) of the Eye

• Fibrous

• Choroid

• Retina

Fibrous Tunic

• outer connective tissue

  • sclera

  • cornea

Sclera

White; attachment of muscles that move eyeballs

Cornea

anterior region of sclear; clear; transmission of light

Choroid

• beneath sclear

  • pupil

  • iris

  • uvea

  • ciliary muscle

Pupil

anterior opening for light entry into the eye

Iris

pigmented muscle around pupil; pupillary dilation + constriction

Uvea

blood vessels (in eye)

Ciliary muscle

lens accomodation → changes lens shape to focus image on retina

Retina

• posterior eye; extension of brain

  • photoreceptors

  • fovea

  • optic nerve

  • blind spot

Fovea

• small region in retina w/highest concetration of cones

• greatest VISUAL ACUITY (resolution)

Optic Nerve

• myelinated axons of ganglion cells in retina

• afferent signals from eye to brain

Blind spot

• exit spot for optic nerve

• no photoreceptors

Accomodation

• changing of lens shape to focus light on retina

• process of eye’s ability to change its plane of focus

• far vision → flattened lens

• near vision → rounded lens

Visual transduction

• occurs in RETINA, based on images focused there by CORNEA + LENS

Inverted image

• due to lens optics → images on retina are upside down + only small fraction of object’s actual size

Lens flatten

• distant objects

• ciliary muscle relaxes

• muscle places tension on suspensory ligament

Lens round

• distance decreases

• ciliary muscles contract

• reduce tension on suspensory ligament

Emmetropia

• normal vision

• no correction

• rays focus on retina

Hyperopia

• farsightedness

• rays focus BEHIND retina

• convex lens corrects

Myopia

• nearsightedness

• rays focus IN FRONT of retina

• concave lens corrects

Astigmatism

• rays do not focus

• uneven lens corrects