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Sensory PhysiologySensory PhysiologySensory Physiology

By By Dr. Carmen Dr. Carmen RexachRexach

PhysiologyPhysiologyMt. San Antonio CollegeMt. San Antonio College

Transduction

• Ability to transform the energy of sensation into the energy of nerve impulses

epidermis

Meissner corpuscle

Categories of sensory receptors• Type of stimulus energy transduced

– chemoreceptors– photoreceptors– thermoreceptors– mechanoreceptors– nociceptors

• Type of sensory information– proprioceptors– cutaneous skin receptors– special senses

Tonic and phasic receptors• Tonic

– relatively constant firing during stimulation– greater stimulus = greater firing frequency– do not adapt

• Phasic– quick response– decreased rate of firing if stimulus is

maintained– adaptation

pain

Wrist watch

Law of Specific Nerve Energies

• Stimulation of a sensory neuron will always give the sensation of the sensory modality specific to that neuron

Generator potential• Graded potentials which generate

an action potential in response to sensory stimulus– results of transduction of

stimulatory signal– connectors between stimulus and

the transmission to the central nervous system

Cutaneous sensations

• Touch, pressure, hot, cold, pain• specialized vs. naked dendrites• two neural pathways

– pressure & proprioceptors– hot/cold/pain receptors

Neural pathway for pressure

receptors

stimulus Myelinatedafferent neurons

Dorsal column of the spinal cord(ipsilateral)

Medulla oblongata (decussation of pyramids)

Medial lemniscus2nd order neurons

Thalamus3rd order neuron

Postcentral gyrussensory cortex

• Medial lemniscus, or Reil’s band, is a sensory pathway for proprioceptors or touch receptors

• Carries signals from nucleus gracilis and nucleus cuneatus in the medulla to the thalamus

Neural pathway for pain

receptors

Stimulus Unmyelinatedafferent fibers

2nd order interneuronsin spinal cord

Lateral spinothalamictract

3rd order neuronsin thalamus

Postcentralgyrus

Receptive fields and acuity• Area of skin above each sensory receptor

which changes the firing rate of the neuron when stimulated.

• Two point discrimination and lateral inhibition

Large receptor field, low densitySmall receptor fields with high density

Overlapping stimulation between neighboring receptive fieldsprovides general information about the location of a stimulus.

Lateral inhibition “sharpens contrast” in the pattern of action potentials received by the CNS.

Somatosensory areas in the cortex of the brain are anatomically organized in relation to the source of information, with larger areas dedicated to parts of the body that process fine discriminations.

Somatosensory cortex

Vestibular apparatus and equilibrium

• Two parts in the inner ear• Sense of balance or equilibrium

Anterior canal

ampullaVestibular nerve

cochleautricle

sacculeLateralcanal

Posteriorcanal

Otolith Organs• Utricle and saccule

– Detect linear acceleration of the head– Head position relative to gravity (which

way is up)otoliths

Otolithic membrane

Supporting cellsHair cellsSensory nerve fibers

Sensory hair cells

Resting Increase Decrease

depolarization hyperpolarization

Semicircular canals• Rotational acceleration• Components

– Ampula and cupula– 3 semicircular canals at 90o angles to

each other• Receptors:

– hair cells (stereocilia + 1 kinocilium)• Mechanisms

– stereocilia bend in direction of kinocilium– membrane is depolarized CNVIII

Sensory hair cells

• Ampula: – bulge– contains hair cells

• Cupula: – gelatinous membrane– projects into

endolymph– hair cells bend when

endolymph moves

Cupula

supportcells

Cristaampularis

Sensory nerve fibers

Hair cells

Hearing

• Sound waves• Frequency: measured in hertz (Hz)

– distance between peaks– higher frequency = higher pitch

• Intensity: measured in decibles (dB)– loudness of sound– amplitude of sound waves

Pathway of sound through the ear

Earcanal

pinna

Temporal bone

malleus

incus

Semicircular canals

Tympanicmembrane

stapes

Middleear

Eustachiantube

cochlea

Cochlear nerve

Hearing

helicotrema Cochlea

Cochlearduct

Scala vestibuli

Scala tympaniRound window

Middle ear cavityTympanic membrane

External auditory canal

malleus

incus

stapes

The vibrations in the stapes are transmitted to the oval window,which sets off the ripples in the cochlear fluid.

Ripples in the cochlear fluid are transmitted to basilar membrane, which moves in response.

Ripples in the cochlear fluidcause the rasping of the tectorial membraneacross the hair cells,altering ion movementsinto those cells, andincreasing NT release.

Organ of Corti

SEM of hair cells of Organ of Corti

Neural pathway for hearing

Medial geniculate body (thalamus)

Auditory cortex

thalamus

Cochlear nucleus

From Organ of CortiVestibulocochlearnerve

Medullaoblongata

midbrain

Inferior colliculus

Conduction deafness vs. sensory deafness

• Conduction deafness– impairment of movement of sound

waves to oval window– cannot hear at all frequencies

• Sensory deafness– impairment of transmission of nerve

impulses from cochlea to the auditory cortex

– can hear some pitches more than others

Tests for deafness

Damaged Organ of Corti

Vision: Parts of the eye

Pathway of light through the eye

• Visible light (400-700nm) conjunctiva cornea anterior chamber pupil lensvitreous chamber retina hyperpolarization of the rods & cones

action potential impulse into optic nerve optic chiasmathalamus visual cortex of the occipital lobe

Refraction

• Bending of light as it passes from the density of one medium to the next

• Light refracts at:– cornea (greatest refractive index

between air and cornea, 43 diopters)– lens (refractive power can vary from 13

to 26 diopters due to elasticity = accommodation)

– retina

Accommodation• Ability of the eyes to keep image focused

on retina as distance between the eyes and the object changes

• Depends on lens elasticity

Near vision Far vision

Ciliary muscle contracts

Zonular fibers relax

Lens becomes lemon shaped

Zonular fibers tighten

Ciliary muscle relaxes

Lens becomes flat

The contraction state of the ciliary muscles determines theamount of tension that the zonular fibers exert on the lens:

contracted = lower tension and more rounded lens,relaxed = higher tension and more flattened lens.

Visual acuity

• Emmetropia = normal vision (20/20)• Ametropia = imperfect refraction in

which focus is not on retina– Myopia = nearsighted; concave lens– Hyperopia = farsighted, convex lens– Astigmatism = irregularity of surface

• Presbyopia = “old eyes”; convex lens

Astigmatism

• Asymmetry in radii of curvature of different meridians of cornea, lens, or retina

• Vision is blurred• Can be corrected with lenses

Corrective glasses and contact lensesalter the location of image focus to correct for structural abnormalities

Structure of the retina

Ganglion and amacrine cells

Bipolar cells

Horizontal cells

Human retina contains

120 million rods

1 million cones

Retinal cells• Ganglion cells and amacrine cells

– Action potentials• photoreceptors, bipolar cells, horizontal

cells– graded potentials– Photoreceptors = rods and cones

• dark current– more positive than most neurons – constant influx of Na+

Rods• Rhodopsin

– transmits blue/red; absorbs green– absorption maximum = 500 nm (green)

• Composition– scotopsin

• protein– retinal (retinene)

• light absorbing pigment molecule derived from Vitamin A

dark adaptationgradual increase in photoreceptor sensitivity

Rhodopsin• Catalyzes the only light sensitive reaction in vision• Light striking the rod causes scotopsin to change

shape result in detachment of scotopsin from retinal = bleaching!

• Hyperpolarization leads to an action potential in optic nerve

Light strikesrods

Rods bleachdue to change in

ion permeability--depolarizes neurons

Action potential

Photoisomerization leads to phototransduction

Isomerization of photopigments

Leads to split of transducin*

Activates phosphodiesterase

Activates cGMP

Na+ channels close

hyperpolarization

Decrease release of inhibitory NT

Action potential

*Note: transducin is a G-protein (α,β,γ subunits!)

Cones

• Visual acuity– greater than rods– less sensitive to light

• Trichromatic theory of color vision– blue, green, red – retinene and associated protein

• fovea centralis– 1:1 ratio with the ganglion cells

Each of the three types of cones has a photopigmentthat absorbs light in a specific range of wavelengths.In dim light, only rods respond.

Photoreceptor activation and action potential

Rods/conesrelease

inhibitory NT (glutamate)

ata constant

rate

In the dark

Photoreceptoractivation

Decrease in NTrelease

hyperpolarization

Activation of the ganglion cells

Stimulation of bipolar cells

Action potential in optic nerve

Neural pathways from the retina

Outside tract from each sideto lateral geniculate body

of the thalamus

Decussation in theoptic chiasma

Geniculostriate system70-80% axons to striatecortex of occipital lobe

Tectal system20-30% axons to superiorcolliculus: eye and body

movements

Optic nerve

Movements of the eyes are tightly regulated by skeletal muscles whose neural controls are influenced by head position and operated inways that assure convergent image formation.

Gustatory transduction involves the interaction of tastant molecules in saliva with the receptor cells in the taste buds onthe papillae of the tongue; these receptor cells undergoonly graded potentials during gustatory transduction.

Taste

Olfactory transduction involves the interaction of odorant molecules in nasal mucus with receptors on the ciliated endings of olfactory neurons.

Olfactory neurons rarely persist more than two months;stem cells undergo mitosis and differentiation to assure that there is no loss of ability to smell.

Smell