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Sensory Physiology
The general purpose of the sensory systems is to convert a specific type of
stimulus into electrical energy the CNS can interpret. Each of the
sensory systems are unique in how they allow us to interact with our
environment.
• Somatosensation
• Vision
• Audition
• Gustation
• Olfaction
•Sensory receptors detect change in the environment.
•Stimulation of sensory receptors stimulates afferent impulse to the
CNS.
•The brain or spinal cord will interpret the afferent impulse and
respond accordingly.
Sensory Receptors Types
• Mechanoreceptors : respond to physical deformation
such as vibration, pressure, stretch and tension
• Thermoreceptors : respond to heat and cold
• Photoreceptors: Located in the eyes respond to light
• Chemoreceptors : respond to chemical change in the
body such a taste smell and body fluid composition
• Nociceptors : respond to pain
• Proprioceptors: gives your body a sense of position
and movements
– Located in joint capsules, muscles and tendons.
Receptive Fields
• A large receptive field will have reduced sensory acuity. The smaller
the receptive the greater the sensory acuity. How does this relate to
cortical mapping.
Overlapping Receptive Fields
• Overlapping stimulation between
neighboring receptive fields
provides general information about
the location of a stimulus.
• The brain is able to determine
contrast between the neurons.
Because
neuron B
is firing at the
highest freq.,
it inhibits A
and C via
inhibitory
pathways to
a greater
extent than
A and C
inhibit B.
Lateral inhibition “sharpens contrast” in the
pattern of action potentials received by the CNS,
allowing a finer resolution of stimulus location.
CNS activity can screen out certain types of sensory
information by inhibiting neurons in the afferent pathway.
Dermatomes
• Area of the skin that is supplied by a single nerve root
Primary Sensory Cortex
The Eye
• The eyes have binocular
vision (we see with both
eyes)
• Sclera is the white of the
eye provides protection
and sight for muscular
attachment.
• The Iris contracts and
dilates to allow light into
the eye
• Ciliary muscle attach to
the lens allow us to focus
• Extra ocular eye muscles and nerves
Extra Ocular Eye Muscles
Movements of the eyes are tightly regulated by
skeletal muscles whose neural controls are
influenced by head position and operated in
ways that assure convergent image formation.
The Eye
Retina
• Light gets focused here. Photoreceptors are a specialized receptors
that convert light waves into electrical impulses.
– Rods allow for night and peripheral vision. Make up a
majority of the retina. (view objects in with peripheral vision)
– Cones : color vision and acuity. In highest concentration of in
the fovea. ( turn our heads towards objects)
– Optic disc is where the blood vessels enter and optic nerve
exits the retina to project to the cortex.
The Retina
Images formed on the retina are upside down and
are only a small fraction of the object’s actual size.
Things in the upper visual field project to the inferior retina
Accommodation
•
•
•
•
The convex shaped lens will refract
light medially.
Objects that are closer strike the lens
at a greater angle (refraction)
– This will project the image past
the retina.( out of focus)
The PNS contacts ciliary muscles
which results in the lens to become
shorter and more convex in shape.
– The more convex lens refracts
the light medially at a greater
angle allowing it to focus on the
retina.
Presbyopia :near vision is dependant
on the flexibility of the lens
– With age the lens becomes
stiffer making it more difficult to
see objects up close
ANS and Pupils
• Sympathetic stimulation
causes radial muscles to dilate
the pupils
– The lens becomes less
convex reducing the angle
of refraction
• Parasympathetic stimulation
causes circular muscles to
contract causing constriction
of the pupil
– This makes the lens more
convex increasing the
angle of refraction.
Light Transmission
• Rods and cones function as
Photoreceptors.
– Absorb light are located on
the back of the retina.
• Bipolar ,Amacrine and
Horizontal cells work as
interneurons.
• Retinal Ganglion cells (RGC)
are the projection neurons that
go to the brain.
– RGC associated with cones
provide information about
visual clarity and contrast.
– RGC on the peripheral
retina associated with rods
project information about
movement
Photoreceptors
•
The cones have a 1:1 ratio to
projection neurons. This
allows us to see both color
and contrast.
– You turn your head to
focus light on the fovea of
the receptor
• Rods are more numerous and
tend to converge on
horizontal cells. Because you
have many rods projecting to
only one horizontal cell; your
cortex losses the ability to
discriminate.
– Objects are easier to see
if you view the in your
peripheral vision.
Interneurons of the Eye
• Bipolar cells work as
interneurons that synapse
with the photoreceptors
and retinal ganglion cells
• Horizontal cells: Many
Rods converge on them.
• Amacrine and horizontal
cells are interneurons that
inhibit lateral bipolar cells
to create contrast. This
increases visual acuity.
Visual Pathways
• Temporal field (peripheral
vision):
– Is projected to the medial
retina.
– Optic nerve fibers will cross
at the optic chiasm a travel
as optic tracts to the opposite
LGN.
• Nasal field: (binocular vision)
– Project to the lateral retina
of the eye.
– These fibers continue to the
thalamus in the same side.
The Ear
Outer, Middle and Inner Ear
•
Outer Ear
•
Middle Ear
•
Inner Ear
– auricle
• focuses sound into the auditory canal towards the tympanic
membrane (ear drum)
– auditory (Eustachian) tube connects to nasopharynx
• equalizes air pressure on tympanic membrane
– ear ossicles
• malleus
• incus
• Stapes
– stapedius and tensor tympani muscles attach to stapes and malleus
– cochlea
• organ of sound reception
– vestibular apparatus
• semicircular ducts, utricle and saccule
– organs of equilibrium and balance
Anatomy of Inner Ear
Sound Production
• The ear funnels sound through the external auditory meatus.
• These waves cause the tympanic membrane to vibrate.
Ear Ossicles
• The tympanic cavity
contains three small
bones: the malleus,
incus, and stapes
– Transmit
vibratory
motion of the
eardrum to the
oval window
– Dampened by
the tensor
tympani and
stapedius
muscles
Physiology of Hearing - Middle Ear
• Tympanic membrane
– has 18 times area of oval window
– ossicles transmit enough force/unit area at oval window to
vibrate endolymph in scala vestibuli
• Tympanic reflex – specific muscles controlled by cranial nerves
(V3) and (VII) are designed to protect the hearing apparatus.
– tensor tympani m. tenses tympanic membrane
– stapedius m. reduces mobility of stapes
• best response to slowly building loud sounds
• occurs while speaking
– Gun shots or entering a car with the radio on loud can damage the
ear because this mechanism doesn’t have time to adapt.
Stimulation of Cochlear Hair Cells
• Vibration of ossicles causes vibration of basilar
membrane under hair cells.
– as often as 20,000 times/second
Anatomy of Cochlea
•
A fluid filled tube divided into 3
segments
– Scala media (cochlear duct)
• spiral organ (organ of
Corti)
– contains hair cells
that detect sound
– filled with
endolymph (ECF)
– Scala vestibuli and Scala
tympani
• filled with perilymph
•
The vibrations in the stapes are
transmitted to the oval window,
which creates ripples (vibrations) in
the cochlear fluid of the scala
vestibuli
Spiral Organ
Spiral Organ
• Stereocilia of hair cells
attach to gelatinous
tectorial membrane
• Inner hair cells
– hearing
• Outer hair cells
– adjust cochlear
responses to different
frequencies
– increase precision
Cochlear Tuning
• Increases ability of cochlea to receive some sound
frequencies
• Outer hair cells contract reducing basilar membranes
freedom to vibrate
– fewer signals from that area allows brain to
distinguish between more and less active areas of
cochlea
• Pons has inhibitory fibers that synapse near the base
of IHCs
– increases contrast between regions of cochlea
Auditory Processing Centers
Auditory Pathway
Equilibrium
• Control of coordination and balance
• Receptors in vestibular apparatus
– semicircular ducts contain crista
– saccule and utricle contain macula
• Static equilibrium – perceived by macula
– perception of head orientation
• Dynamic equilibrium
– perception of motion or acceleration
• linear acceleration perceived by macula ( moving in a car)
• angular acceleration perceived by crista ( turning head)
Macula
•
•
•
•
•
Made up of the saccule and
utricle
Require gravity to work.
Utricular hairs respond to
horizontal movement(
driving)
Saccular hairs respond to
vertical movement ( elevator)
This bends the stereocilia and
one kinocilium resulting in a
specific direction would result
in stimulation of the
vestibulocochlear nerve
Saccule and Utricle
•
Referred to as the gravitational
receptors.
– hair cells with stereocilia
and one kinocilium buried
in a gelatinous otolithic
membrane
– Otoliths embedded in the
membrane (CaC03 add to
the density which increases
inertia and enhance the
sense of gravity and motion
Semicircular Channels
•
•
•
Crista ampullaris :consists of
hair cells buried in a mound
of gelatinous membrane (one
in each duct)
Orientation causes ducts to
be stimulated by rotation in
different planes
The combination of 6
channels can provide sensory
feedback in all planes of
motions.
– Each channel signals 2
direction. (Forward and
backwards)
– This gives us
information regarding
acceleration and
deceleration.
Crista Ampullaris - Head Rotation
•
•
As head turns, endolymph
lags behind, pushes
cupula, stimulates hair
cells
This results in stimulation
of the vestibular division
on the vestibulocochlear
nerve
Bending of stereocilia in opposite directions has
opposite effects on their membrane potentials.
Vestibular Projection Pathways