lOMoARcPSD|14985576
LEARNING OBJECTIVES
WEEK 4
Sensory Systems
Predict how photoreceptors change in response to light exposure.
Photoreceptors hyperpolarize in response to light, meaning that they are usually depolarized
due to leaky Na+ channels that results in sodium always entering the cells even during “rest”.
Photoreceptors include rods and cones. Looking at the diagram, A is in the presence of no light
and B is in the presence of light. In A: GPCR is not active, and cGMP is still connected to the
sodium channel, keeping it open. In B: GPCR is active, attaching to the effector, causing cGMP
go be converted to GMP and results in the sodium channel being closed.
Explain how sensory information is encoded in action potentials.
In vision: Rods and cones do not fire action potentials, they only release neurotransmitters.
Bipolar cells also do not fire action potentials, they only regulate the amount of
neurotransmitters being released in response to rod and cone input. However, ganglion cells do
, lOMoARcPSD|14985576
fire action potentials by the optic nerve to the visual cortex.
In hearing: Nothing in hearing pathway fires action potential, but the bending of the stereocilia
in the direction of kinocilia releases neurotransmitters that will cause the auditory cortex to fire
action potentials. The hair cells (which have stereocilia on them) cause sensory neurons to send
actional potentials to the auditory cortex.
Explain how the processes of vision and hearing occur.
Vision: Light enters the eye through the cornea, which then travels to the pupil (the amount of
light let inside the pupil is governed by the iris), which then travels to the lens, which then the
leans and cornea bend light rays to focus them on the retina. The retina is made up of many
layers of cells which then continue the process of vision. The rods and cones are at the very
back of the eye, and since the optic nerve begins at the front and ends at the back there is a
spot in the eye where there are no light-sensitive cells, creating a blind spot. So, rods and
cones, as photoreceptors, hyperpolarize in response to light and do not fire action potentials.
Then, rods and cones synapse onto bipolar cells, which also do not fire action potentials but will
adjust release of neurotransmitters in response to rod and cone input (note: bipolar cells are
still neurons, however they don’t fire action potentials, only graded potentials). These cells then
synapse onto ganglion cells, which do fire action potentials that transmit to the optic nerve,
which then transmits to the visual cortex. Other cells involved in this process are amacrine and
horizontal cells—amacrine cells connect ganglion and bipolar cells, while horizontal cells
connect bipolar and photoreceptors.
Hearing: Sound waves (mechanoreceptors) enter the outer ear, contacting the pinna and going
through the ear canal, being received by the tympanic membrane (eardrum). Then, sound is
amplified in the middle ear by three small bones called malleus, incus, and stapes. The stapes
then transmits energy of vibrations to the inner ear, causing even more amplification.
Specifically, it transmits energy to the oval window of the cochlea, where the transfer of sound
vibration to fluid pressure waves occurs (as endolymph is present in the inner ear). The cochlea,
as well as containing endolymph fluid, is divided into the upper and lower canal, which are
separated by the basilar membrane. Vibration of the oval window causes fluid pressures waves
in the canals, and then is mechanically received by hair cells in the organ of Corti. This causes
stereocilia to bend through the movement of the basilar membrane. The stereocilia are
connected on one side to a tectorial membrane, and the other side to the basilar membrane.
When the basilar membrane pushes in due to pressure waves, it causes sound vibrations that
then causes hair cells to depolarize and release neurotransmitters. When the basilar membrane
goes back down, it causes hair cells to repolarize. Neurotransmitters then cause sensory
neurons to fire action potentials that are sent to the auditory cortex.
More on the basilar membrane: at the apex, the basilar membrane is the most flexible, so that
is where low-frequency sounds stimulate hair cells. At the base of the basilar membrane, it
receives high-frequency sound.
Evaluate the molecular effect of a stimulus on a sensory pathway.
Predict how changes in a sensory pathway will affect sensory perception.
LEARNING OBJECTIVES
WEEK 4
Sensory Systems
Predict how photoreceptors change in response to light exposure.
Photoreceptors hyperpolarize in response to light, meaning that they are usually depolarized
due to leaky Na+ channels that results in sodium always entering the cells even during “rest”.
Photoreceptors include rods and cones. Looking at the diagram, A is in the presence of no light
and B is in the presence of light. In A: GPCR is not active, and cGMP is still connected to the
sodium channel, keeping it open. In B: GPCR is active, attaching to the effector, causing cGMP
go be converted to GMP and results in the sodium channel being closed.
Explain how sensory information is encoded in action potentials.
In vision: Rods and cones do not fire action potentials, they only release neurotransmitters.
Bipolar cells also do not fire action potentials, they only regulate the amount of
neurotransmitters being released in response to rod and cone input. However, ganglion cells do
, lOMoARcPSD|14985576
fire action potentials by the optic nerve to the visual cortex.
In hearing: Nothing in hearing pathway fires action potential, but the bending of the stereocilia
in the direction of kinocilia releases neurotransmitters that will cause the auditory cortex to fire
action potentials. The hair cells (which have stereocilia on them) cause sensory neurons to send
actional potentials to the auditory cortex.
Explain how the processes of vision and hearing occur.
Vision: Light enters the eye through the cornea, which then travels to the pupil (the amount of
light let inside the pupil is governed by the iris), which then travels to the lens, which then the
leans and cornea bend light rays to focus them on the retina. The retina is made up of many
layers of cells which then continue the process of vision. The rods and cones are at the very
back of the eye, and since the optic nerve begins at the front and ends at the back there is a
spot in the eye where there are no light-sensitive cells, creating a blind spot. So, rods and
cones, as photoreceptors, hyperpolarize in response to light and do not fire action potentials.
Then, rods and cones synapse onto bipolar cells, which also do not fire action potentials but will
adjust release of neurotransmitters in response to rod and cone input (note: bipolar cells are
still neurons, however they don’t fire action potentials, only graded potentials). These cells then
synapse onto ganglion cells, which do fire action potentials that transmit to the optic nerve,
which then transmits to the visual cortex. Other cells involved in this process are amacrine and
horizontal cells—amacrine cells connect ganglion and bipolar cells, while horizontal cells
connect bipolar and photoreceptors.
Hearing: Sound waves (mechanoreceptors) enter the outer ear, contacting the pinna and going
through the ear canal, being received by the tympanic membrane (eardrum). Then, sound is
amplified in the middle ear by three small bones called malleus, incus, and stapes. The stapes
then transmits energy of vibrations to the inner ear, causing even more amplification.
Specifically, it transmits energy to the oval window of the cochlea, where the transfer of sound
vibration to fluid pressure waves occurs (as endolymph is present in the inner ear). The cochlea,
as well as containing endolymph fluid, is divided into the upper and lower canal, which are
separated by the basilar membrane. Vibration of the oval window causes fluid pressures waves
in the canals, and then is mechanically received by hair cells in the organ of Corti. This causes
stereocilia to bend through the movement of the basilar membrane. The stereocilia are
connected on one side to a tectorial membrane, and the other side to the basilar membrane.
When the basilar membrane pushes in due to pressure waves, it causes sound vibrations that
then causes hair cells to depolarize and release neurotransmitters. When the basilar membrane
goes back down, it causes hair cells to repolarize. Neurotransmitters then cause sensory
neurons to fire action potentials that are sent to the auditory cortex.
More on the basilar membrane: at the apex, the basilar membrane is the most flexible, so that
is where low-frequency sounds stimulate hair cells. At the base of the basilar membrane, it
receives high-frequency sound.
Evaluate the molecular effect of a stimulus on a sensory pathway.
Predict how changes in a sensory pathway will affect sensory perception.
