Special Senses Physiology

Vision

• The ability to see.

• This is because the photoreceptors in the eye or the sensory cells are depolarized at rest.

• When activated, they are hyperpolarized.

• Unique to the visual system.

Optical Component

• Focuses the visual image on the receptor cells.

• Front part of the eye.

Neural Component

• Transforms the visual image into a pattern of graded and action potentials.

• Back of the eyeball.

Sclera

• Membrane surrounding the eyeball.

• The extra ocular muscle is attached.

Cornea

• The region where the sclera becomes clear at the front region of the eye.

• Light waves refract and (hopefully) converge in the area where photoreceptors are packed.

• The cornea is static and cannot change - light passes through and refracts a certain amount.

Iris

• Gives eyes colour.

• Innervated by the autonomic NS.

• Aperture that regulates the amount of light.

Extra Ocular Muscle

• Responsible for eye movements, looking up and down, or side to side.

Pupil

• The hole that allows light to pass though into the back of the eye.

• Size is regulated by the iris.

• Is constricted or dilated by contraction of the smooth muscle of the iris.

  • SNS → Dilate.

  • PSNS → Constrict.

Lens

• Works together with the cornea to focus on the visual image on the retina.

• The shape and size of the lens can be changed.

Zonular Fibers

• Attracted to the lens are these little fibers.

• These fibers are attached to the ciliary muscles.

Ciliary Muscles

• Can either be released or they can be contracted.

• They change the shape of the lens.

Retina

• Located behind the lens against the back of the eye.

• Where the photoreceptors are found.

  • Rods and cones.

Rods

• Activated in low light conditions; very sensitive.

• They are monochromatic.

Cones

• Activated when there is more light present and responsible for colour vision.

Retinal Ganglion Cells

• Activated by rods and cones.

• Comprises optic nerve and transmits visual information to the cortex.

• Taking information back towards the brain.

• Cortex is made up of the axons of retinal ganglion cells.

Optic Nerve

• Leaves through the back of the eyeball and heads back towards the thalamus.

Aqueous Humour

• The gap between the lens and the cornea.

• Filled with a gelatinous-like fluid.

Vitreous Humour

• Behind the lens where there’s a large gap.

• Filled with another type of gelatinous fluid.

Refraction

• Light bends when passing between media of different densities, changing the object's apparent position.

• Image inversion occurs in the eye, where the lens focuses light upside down on the retina.

Lens Focusing Mechanism

• When objects are close, corneal refraction alone cannot focus the image on the retina.

• The image forms behind the retina, resulting in a blurry image.

• The ciliary muscle contracts, surrounding the lens.

• This makes the lens shorter and fatter, increasing its refractive power.

• The increased refraction focuses the image properly onto the retina.

Image in Focus (Relaxed Ciliary Muscles)

• Light reflects off and towards the eye, refracting a certain amount.

• The ciliary muscles relaxed with tension on the zonular fibers, flattened lens.

• Light rays from distant objects are nearly parallel.

• When the image hits the cornea, the image is reconstructed on the retina.

Image Out of Focus (Relaxed Ciliary Muscles)

• An object gets closer to the eye.

• As a result, the refraction at the cornea is insufficient to allow the image to form.

  • Instead, it forms on the back of the retina.

• The ciliary muscles are relaxed and the light rays from near objects diverge.

Image in Focus (Constricted Ciliary Muscles)

• To see the image clearer, there is a firing of parasympathetic nerves, the ciliary muscles constrict, the zonular fibers slack, and the lens becomes rounder.

  • This increases the amount of refraction.

• Lens becomes spherical.

Accommodation

• The eye's ability to automatically adjust its focus.

• It involves the lens changing shape, becoming more rounded for near vision and flatter for far vision.

• Typically goes away at 45 years old.

• The ciliary muscles breakdown and the lens is no longer contractable.

Presbyopia

• loss of lens elasticity, reducing near vision accommodation.

• Caused mainly by aging and ciliary muscle changes.

• People without reading glasses hold objects farther away to see clearly.

• They rely solely on corneal refraction, as accommodation is impaired.

Myopia

• Eye is too long for the lens’s focusing power.

• Excessive refraction causes the image to focus in front of the retina.

• Can see near objects, but not far.

• Treated with glasses (concave lens) or laser eye surgery.

Hyperopia

• The eye is too short for the lens’s focusing power.

• Insufficient refraction causes the image to focus behind the retina, making near objects blurry.

• Can see distant objects clearly, but not up close.

• Treated with glasses (convex) or laser eye surgery (not as successful).

Astigmatism

• Surface of lens or cornea is not smoothly spherical.

• Can be corrected with glasses or complex laser surgery.

Glaucoma

• Damage to the retina from raised intraocular pressure, typically due to aqueous humour build-up.

• Poor drainage causes pressure on the lens, which pushes against the vitreous humour, transmitting force to the retina and photoreceptors, leading to damage.

• There is no reliable treatment for glaucoma.

Cataracts

• Age-related clouding of the lens; happens later than presbyopia.

• Lens cells die and debris accumulates within them.

• A grain forms in the lens, reducing the ability to see clearly.

Cataracts Treatment

• Treated by removing the lens, cleansing it with a microscopic vacuum, and replacing it with a silicone or artificial lens.

• The ciliary muscles and zonular fibers cannot properly activate or innervate the lens for accommodation.

  • People with cataracts already have reduced accommodation due to presbyopia.

Interneurons

• Horizontal, bipolar, and amacrine cells.

• Take information from the photoreceptors and transfers the information to retinal ganglion cells that make up the optic nerve.

• Information from the retina is taken back to the lateral geniculate nucleus, where they synapse and travel back to the cortex.

Photoreceptor

• The photoreceptor is depolarized at rest or when light is absent.

• The photoreceptor is hyperpolarized when light is present or in response to a stimulus.

• The photoreceptor is more negative when light is present than when light is not present.

Phototransduction (No light) - Step 1

• When no light is present, cGMP is generated by guanylyl cyclase.

• The enzyme, guanylyl cyclase, converts GTP to cGMP.

• cGMP binds to its receptor on the cation channel.

  • ∴ Cation channel opens; Na⁺ and Ca²⁺ flows into the cell.

Phototransduction (No light) - Step 2

• The photoreceptor depolarizes as the Na⁺ and Ca²⁺ are entering the cell.

• When no light is present, the photoreceptors are relatively depolarized.

• The membrane potential is -35mV when there is no light present.

Phototransduction (Light) - Step 1

• The discs of the cones contain a photopigment, which has a chromophore called retinal.

• When light hits the photopigment, retinal changes from cis to trans.

• This activates cGMP phosphodiesterase, an enzyme that breaks down cGMP into GMP.

Phototransduction (Light) - Step 2

• When light is present, a pathway is activated which reduces the amount of cGMP.

• cGMP decreases, making the cGMP-gated Na⁺/Ca²⁺ channels close

  • No Na⁺/Ca²⁺ influx → less positive charge inside

• ∴ The photoreceptor is hyperpolarized when light is present.

Neural Pathways in Vision

• Light signals are converted into APs through interactions between photoreceptors, bipolar cells, and ganglion cells.

• Photoreceptors and bipolar cells only undergo graded responses due to a lack of voltage-gated channels; ganglion cells are the first to fire APs.

• Photoreceptors connect to bipolar and ganglion cells via ON- and OFF-pathways.

Differences in ON vs. OFF Pathways

• ON-pathway bipolar cells spontaneously depolarize in the absence of input and have inhibitory glutamate receptors (metabotropic).

• OFF-pathway bipolar cells hyperpolarize without input and have excitatory glutamate receptors (ionotropic).

Similarities in ON vs. OFF Pathways

• In the absence of light, photoreceptors are depolarized, and glutamate is released onto bipolar cells.

• When light strikes, photoreceptors hyperpolarize due to decreased cGMP (from activation of cGMP-dependent phosphodiesterase), closing cation channels and reducing glutamate release.

ON-Pathway (In Darkness)

• Photoreceptors are depolarized as cGMP opens cation channels (Na⁺, Ca²⁺ influx).

• They release glutamate, which binds inhibitory metabotropic receptors on bipolar cells, causing hyperpolarization and reduced neurotransmitter release to ganglion cells.

• Ganglion cells are not stimulated to fire APs.

ON-Pathway (In Light)

• Photoreceptors hyperpolarize → less glutamate → ON-bipolar cells depolarize, releasing excitatory neurotransmitter onto ganglion cells.

• Ganglion cells depolarize and fire APs to the brain.

OFF-Pathway (in Darkness)

• Glutamate binds ionotropic receptors on bipolar cells, opening cation channels and causing depolarization.

• Bipolar cells release excitatory neurotransmitter, stimulating ganglion cells to fire APs.

OFF-Pathway (In light)

• Reduced glutamate → bipolar cells hyperpolarize → decreased excitation of ganglion cells → APs inhibited.

Co-Existence of ON & OFF Pathways

• The coexistence of ON and OFF pathways enhances contrast detection and image resolution, especially at edges and borders.

Visual Pathways - Lateral Field

• Information from the lateral half of the visual field strikes the nasal region of the retina.

• It travels via retinal ganglion cell axons to the optic chiasm, where the two optic nerves meet.

• At the optic chiasm, the information crosses to the contralateral side.

• It then synapses at the lateral geniculate nucleus (LGN).
  Neurons from the LGN relay the information to the visual cortex.

Visual Pathways - Temporal Field

• Information goes to the temporal region of the retina.

• It travels back to the optic chiasm, but does not cross the optic chiasm.

  • It stays on the same side, or the ipsilateral side.

• It foes to the ipsilateral geniculate nucleus, where is synapses.

• The information goes to the visual cortex in the occipital lobe.

Hearing

• Hearing relies on the physics of sound and the physiology of the ear, auditory nerves, and brain regions that process sound.

• Sound energy travels through molecules in a medium—usually air.

• Speech is the movement of air molecules.

• Without molecules, there is no sound.

Tuning Fork

• Striking a tuning fork sets nearby air molecules into motion, creating zones of compression and zones of rarefaction.

• These alternating high- and low-pressure areas form oscillations in the air, producing sound.

• The pitch depends on where the tuning fork is struck.

Zones of Compression

• Where the air molecules are tightly packed or closer together.

Zones of Rarefraction

• Regions where there are relatively few air molecules.

Amplitude

• Determined by how many air molecules are within one of the zones of compression.

• The difference between pressure of molecules in the zones of compression and rarefraction.

• Amplitude = Loudness.

Frequency

• Determined by the distance between the zones of compression.

• The faster the vibration, the hgiher the pitch.

• Number of cycles per second = frequency = pitch.

Pressure vs. Time

• The number of cycles per second represents the frequency (length).

• The amplitude (width) represents the loudness of the sound.

External Auditory Canal

• Funnels the zones of compression and zones of rarefaction towards the middle and inner ear.

• The sound travels along the external auditory canal and hits the eardrum.

Tympanic Membrane

• It vibrates in and out as air molecules push against it,

• The “in and out” of the eardrum will be consistent with the frequency and amplitude of the sound.

  • Increased frequency = eardrum moves quickly.

  • Decreased frequency = eardrum moves slowly.

Malleus

• The malleus is attached to skeletal muscles and the eardrum.

• A small bone in the middle ear which transmits vibrations of the eardrum to the incus.

Incus

• Attached to the malleus.

• A small anvil-shaped bone in the middle ear, transmitting vibrations between the malleus and stapes.

Stapes

• Attached to the stapedius muscle and the incus.

• A small stirrup-shaped bone in the middle ear, transmitting vibrations from the incus to the inner ear.

Tensor Tympani Muscle and Stapedius Muscle

• The tensor tympani and stapedius muscles can contract to protect the ear from sustained loud sounds by dampening movement of the middle ear bones.

• However, they do not protect against sudden loud noises (like a bang), as they can't contract quickly enough in time.

Cochlear Divisions

• The cochlea is divided into 3 compartments.

  • Top compartment: Scala vestibuli.

  • Middle compartment: Cochlear duct.

  • Bottom compartment: Scala tympani.

Cochlear Fluid Movement

• Sound waves entering the external auditory canal move the eardrum, which in turn moves the middle ear bones.

• The stapes pushes against the oval window, causing perilymph to move toward the end of the cochlear duct (helicotrema).

• Goes down through the cochlear tract—from the scala vestibuli to the scala tympani.

• This downward movement activates the sensory receptors.

Scala Vestibuli

• Full of perilymph (fluid), which has the same composition of the CSF.

• Is the superior most duct of the cochlea duct.

Scala Tympani

• The inferior most duct of the cochlea.

• It is filled with perilymph.

Cochlear Duct

• Where the sensory receptors for the auditory system are located.

• Filled with endolymph.

• A fluid-filled tube within the cochlea of the inner ear.

Organ of Corti

• A specialized sensory epithelium that allows for the transduction of sound vibrations into neural signals.

  • Located within this organ are hair cells.

Hair Cells

• The sensory receptors for the auditory system are located in the cochlear duct on the organ of Corti.

• They have stereocilia protruding from their apical surface.

• There are two anatomically distinct groups:

  • A single row of inner hair cells

  • Three rows of outer hair cells

Vestibulocochlear Nerve

• Takes auditory information from the ear towards the brain.

Stereocilia

• Extend into the endolymph and transduce pressure waves caused by fluid movement in the cochlear duct into receptor potentials.

• Projects into the tectorial membrane.

Basilar Membrane

• Supports hair cells, serves as the base layer of the organ of Corti, and propagates sound vibrations that allow the brain to interpret sounds.

• Outer hair cells are attached to the membrane at the bottom.

Interpreting Sound - Mechanical Movement

• Perilymph movement causes the basilar membrane to move up and down.

• This motion makes hair cells move back and forth.

• Step one.

Interpreting Sound - Interaction

• Stereocilia on the outer hair cells are embedded in the stationary tectorial membrane.

• Movement of the basilar membrane causes bending of stereocilia, leading to mechanical deformation.

• Step two.

Interpreting Sound - Frequency

• Different regions of the basilar membrane vibrate maximally in response to different frequencies.

• Hair cells at the point of peak vibration undergo the greatest deformation.

• Step three.

Interpreting Sound - Interpretation

• This deformation is converted into signals sent to the CNS.

• The brain interprets the pattern of hair cell stimulation as a specific sound frequency.

• This allows different sound frequencies to be detected along the organ of Corti.

• Step four.

AP - Depolarization

• The hair cells move back and forth when the basilar membrane moves.

• In the inner ear, K⁺ concentration is higher outside the hair cell—opposite of typical neurons, where K⁺ is higher inside at rest.

• Step one.

AP - Stereocilia

• When stereocilia bend toward the tallest bundle member, mechanically-gated cation channels open, and K⁺ flows into the hair cell down its gradient.

• As K⁺ enters, the hair cell depolarizes, generating a graded potential and triggering the release of glutamate onto afferent neurons.

• Step two.

AP - Activation

• If enough glutamate is released to bring these afferent neurons to threshold, APs are generated in the vestibulocochlear nerve and travel to the brain.

  • ∴ Activation of the auditory system.

• Step three.

AP - Slack

• Bending of the hair cells in the opposite direction closes the channels, allowing the cell to rapidly repolarize.

• Last step.

Neural Pathways in Hearing

• Cochlear nerve fibers synapse with interneurons in the brainstem.

• From the brainstem a multineuron pathway transmits information through the thalamus to the auditory cortex in the temporal lobe.

Hearing Aids

• Amplifier placed in auditory canal which activates existing auditory “machinery.”

• For people whose auditory machinery in the ear is not as sensitive; eardrum doesn’t move as well.

Cochlear Implants

• An externally located audio sensor receives sound input and activates electrodes that physically stimulate the cochlear nerve.

• The receiver on the outside of the head bypasses the outer, middle, and inner ear, delivering sound information directly.

• It electrically stimulates the vestibulocochlear nerve in a way that preserves sound features for accurate processing by the cortex.