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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