Physio. Ch10 Part 2 Special Senses

Hair cells of the ear

• Mechanoreceptors

• 3,500 sit in the ear on a basilar membrane

• Responsible for changing sound waves into nerve impulses

• Apical surface has hair like projections called stereocillium that bend

Stereocilia hearing

• Part of hair cells, located in the cochlea

• Hair like projections that bend when pushed against tectorial membrane

• Bend towards tallest stereocillium causes K channels to close, depolarization

- Neurotransmiter is released and action potential fires

• Bend away from tallest stereocillium causes K channels to open, hyperpolarization

- Neurotransmiter stopped from releasing and no action potential fires

Sound Intensity

• The further a Stereocillium bends the more intense the sound is

- Further bending causes more neurotransmitter releases

Sound Pitch/frequency

• Regulated by what hair cells are stimulated

- Higher frequency stimulate stiffer hair cells at the base of the cochlea

Vestibular apparatus

• Part of the inner ear

• Helps to maintain balance and equilibrium

• Detects head movement and acceleration to do this

- Vestibule detect static equilibrium and linear acceleration of the head

- Semicircular ducts detect angular rotation of the head

Stereocilia equalibrium

• Part of hair cells, located in the vestibule and semicircular

• Bend due to changes in head/body acceleration

- Opens mechanical gated ion channels, depolarizing cells and sending action potentials

Gustatory chemoreceptors

• Chemical receptors in the tastebuds

- 50, 150 per taste bud

• Tastants interact with receptors to cause sensation of taste

Tastants

• Chemicals that interact with Gustatory chemoreceptors to produce taste

• Only 4 exist

- Salty

- Sour

- Sweet and Umami

- Bitter

Olfactory chemoreceptors

• Chemical receptors

• Responsible for smell

• Highly sensitive receptors

• Act through GPCR pathways that cause depolarization

Photoreceptors

• Receptors that change light energy into electrical energy

• Only 750mV to 350mV can exit these receptors (visible light spectrum)

Reflection

• Light waves strike and bounce off an object

• We can only see light that reflects

Refraction

• The way that light waves bend when they pass through something

• The density of the object makes a big difference here

- Cornea and lense do this in the eye before the retina is reached

Accommodation

• The ability of the lens to focus light from an object on the retina as the distance between the object and the eye decreases

• Accomplished by thickening and thinning the lens

Ciliary Muscle

• The muscle the surrounds the lens and powers accommodation

• Attached to the lens by zonular fibers

- When contracts the zonular fibers are slackened

- When relaxed the zonular fibers are tightened

Zonular fibers

• Fibers that connect the ciliary muscle to the lens

• When tight the lens is flattened

• When slackened the lens is rounder

Near objects

• Light rays coming in are more spread out

• Lens needed to be rounded to account for this

- Ciliary muscle contracts, slackening zonular fibers

Far objects

• Light rays coming in are less spread out

• Lens needed to flatter to account for this

- Ciliary muscle relax, tightening zonular fibers

Iris

• The colored part of the eye

• Contracts to protect the eye from excessive light

- Controlled by parasympathetic nervous system

• Dilates to allow the eye to take in more light

- Controlled by sympathetic nervous system

Myopia

• Pathophysiology corrected by a concave lens

• Commonly referred to as “near sightedness”

• The lens / cornea are to strong for the eyeball

- Distant images are brought to a point of focus in front of the retina, making distant objects appear blurry

Hyperopia

• Pathophysiology corrected by a convex lens

• Commonly referred to as “far sightedness”

• The lens / cornea are to weak for the eyeball

- Near images are not brought to a point of focus before reaching the retina, making near objects appear blurry

Astigmatism

• Pathophysiology corrected by an uneven lens

• The lens / cornea are irregularly curved

• Creates several points of focus in the retina

Concave lense

• A lense that causes light rays to spread out when it is passed through

Convex lense

• A lease that causes light rays to come to a point when it is passed through

Visual Acuity

• Sharpness of the vision dependent on resolving power

Resolving power

• The ability to tell two closely spaced objects apart

Snellen Eye Chart

• The eye doctor chart

• Exam to be done from 20 feet away

Retina

• Located in the back of the eye

• Responsible for taking in light rays and deciphering them

- Uses photoreceptors to do this

Photoreceptors

• Receptors located in the retina that change visual light energy into electrical energy

• Subdivided into rods and cones

Rods

• Used for vision in low light situations

• Sees in black and white

• Communicate with bipolar cells cells

• Uses Rhodopsin as its photopigments

Cones

• Used for vision in high light situations

• Sees in color

• Communicates with bipolar cells

• Uses L opsin, M opsin, S opsin as their photopigments

- Used in long, medium, short cones respectively

Bipolar cells

• Cells that are the bridge between photoreceptors and ganglion cells

Ganglion cells

• Receive information from bipolar cells and transmit it down their axons to the brain

Phototransduction

• Conversion of light energy into electrical signals, carried out by photoreceptors

Photopigments

• Also called “retinal”

• Light absorbing portion of photoreceptor

• Made from vitamin A

• Four different types

- Rhodopsin

- L opsin

- M opsin

- S opsin

Membranous disks

• Outer segment of photoreceptors

• Contain retinal and opsin (photopigments)

- Uses G-protein transduction

Phototransduction of Light

• Step 1: Rhodopsin molecule is activated by light

• Step 2: Retinal and Opsin dissociate

• Step 3: Opsin activates transducin

• Step 4: Transducin’s alpha subunit activates phosphodiesterase (PDE)

• Step 5: PDE facilitates degradation of cGMP, causing Na to stop coming in and hyperoplarization of photoreceptor

- cGMP activates Na channels

Rhodopsin molecule

• Found in the retinal

• Activated by light

• Activation causes dissociation of retinal and opsin

Opsin

• Found in photoreceptor

• Disassociates from retinal when Rhodopsin is activated by light

Transducin

• Found in photoreceptor

• Activated by opsin

• Alpha subunit activates phosphodiesterase (PDE)

Phosphodiesterase (PDE)

• Activated by Transducin

• Facilitates degradation of cGMP

cGMP

• Degraded by Phosphodiesterase (PDE)

• Opens Na channels

- Causes the release of neurotransmitter glutamate

Gutamate

• Inhibitory neurotransmitter

• Inversely related to levels of cGMP

Dark conditions

• Na channels that are active, membrane potential -35 to -45mV

• Causes depolarization that travels down the membrane and opens Ca channels

- Causes more glutamate release

• Glutamate stops bipolar cells from stimulating ganglion cells

Light conditions

• Na channels that are inactive, membrane potential -70mN

• Causes hyperpolarization that travels down the membrane and closes Ca channels

- Causes less glutamate release

• Bipolar cells stimulating ganglion cells

Color vison

• Colors are perceived based on patterns of activation of the different

cones

Red-green most common

• Most common form of color blindness

• Affects men more because its genetically linked

- 6% of men

Ishihara charts

• Charts used to detect color blindness

Visual Fields

• Retina divided into nasal and temporal sides

• Right visual field projected onto left side of retina

- Left visual field projected onto right side of retina

Optic chiasm

• X shaped nerve pathways behind the eyes

• Allows for the nasal side of each eye to cross over

- Nasal retinal nerves run to left side of brain

- Temporal retinal nerves run to right side of brain

Neural Pathways for Vision

• Visual cortex processes the information coming from the retina as action potentials in ganglion cell axons

Pupillary reflex

• Contraction of pupils

• Parasympathetic innervation

Fovea

• The sight of highest visual acuity