Describe in general receptors in the skin

Name the various chemoreceptors, state their location, anatomy and mechanism of action

Describe the anatomy of the eye giving the function of each part

Describe the receptors for vision and tell how they work

Describe common vision disorders

Describe the anatomy of the ear and the function of each part

Describe causes of deafness

May be relatively simple:

pressure, temperature, touch, pain, stretch, chemical stimuli

Or take the form of more complex organs:

Eye, ear, tongue, nasal cavity

Some receptors are constructed from the dendrites of sensory neurons

Some receptors (pain, blood pressure) are simply unspecialised dendrites

Other receptors have dendrites that have various structural modifications

Many are specialized epithelial cells that synapse with sensory neurons

Stimulation causes a local depolarization in the dendrites

If it exceeds a threshold value, an action potential is produced

On -off pulses in these neurons is interpreted as the appropriate type of information in the brain in specialized sensory regions for each of the senses (hearing, vision etc.)

Continuous stimulation of many receptors causes adaptation

i.e. after a while receptors cease to respond

Pressure receptors in skin show adaptation

Pain receptors do not

Complex senses like vision have more complex patterns of adaptation

Respond to excessive pressure, chemicals, extremes in temperature

Impulses from these neurons are interpreted as pain in the CNS

Pain originating from trauma in internal organs sometimes appears to come from the he skin or from skeletal muscles remote from the organ

Called "referred pain"

Neurons from pain receptors travel in the same spinal nerves that carry impulses from the skin.

Heart attacks often manifest themselves as pain in the left shoulder or the arm

Muscles have contain specialized "spindle fibers" which are modified muscle cells wrapped with the dendrites of sensory neurons

Stretching causes stimulation of these neurons which transmit impulses to the CNS

The CNS in turn, initiates muscle contractions which allow us to walk and maintain posture

Tendons and joints have similar structures which prevent over-stretching

Smell receptors detect chemicals in the air and in the food we eat

Much of what we interpret as taste is actually smell

Specialized olfactory receptors are located in epithelial cells that line about a 1 square inch area on the roof of the nasal cavity

~10 million of these cells

Olfactory hairs are projections that project into the mucus layer of the nasal cavity

Olfactory hairs contain receptors for various chemicals that dissolve in the mucus

Chemicals bind to these receptors causing local depolarization in the receptor cells which synapse with sensory neurons in a projection of the brain called the olfactory bulb.

Even though we can distinguish about 10,000 different odors, we appear to have only about 1,000 different kinds of receptors in olfactory cells.

Not known how this is achieved

Taste receptors are located in structures called taste buds

Located within numerous small projections called papillae that cover the surface of the tongue

Taste buds mainly around front back and sides of tongue, relatively few in middle

Taste receptors have microvilli called taste hairs located on the exposed surface

Taste hairs contain the receptors that bind to chemicals found in saliva.

Synapse with sensory neurons that transmit impulses to the brain.

sweet, sour, salty, and bitter

Receptors for chemicals that give these taste sensations located specific parts of the tongue

Some complex tastes are do to stimulating more than one of these at once

More sensations we interpret as tastes are due to vaporization of chemicals within food particles that diffuse to the olfactory receptors in the nose

Eyeball is roughly spherical ~ 2.5 cm diameter

Two fluid filled chambers containing aqueous humor (front) and vitreous humor (back) respectively separated by the lens

The sclera is a tough white connective tissue layer that surrounds and protects the eyeball

Inside the sclera is a layer of darkly pigmented cells called the choroid

Pigments absorb scattered light reducing blur and flair

contains blood vessels that nourish the other tissues of the eye

The iris and ciliary body are specialized regions of the choroid layer

At the front of the eye, the sclera becomes the transparent cornea through which light enters the eye

The cornea is where most of the focusing of incoming light takes place

The iris, which lies behind the cornea, consists of smooth muscles and contains pigments that give us eye color

Relaxing and contracting of these smooth muscles cause the pupil to open and close

These smooth muscle are controlled by the autonomic division of the PNS

Ciliary muscles are smooth muscle attached by fine ligaments attached to the lens

Responsible for accommodation, the process of "fine tuning" the focus so that the eye can focus at both near and far objects but not at the same time

The retina is the innermost layer making up the eyeball

It contains three layers of cells:

rods and cones

bipolar cells

ganglion cells

Light is absorbed and transduced in the rods and cones.

The bipolar cells conduct the electrical signals to the ganglion cells

Bipolar cells and ganglion cells make interconnections with many different rods and cones

Allows for processing of the image before it goes to the brain

Contrast detection, motion detection among other phenomena

Rods function in dim light and are not sensitive to color

Rods do not have the same acuity as cones

Three different kinds of cones

Each kind is sensitive to either blue, green or red light.

Rods and cones contain many stacked membranous disks which contain photopigments

i.e. molecules that absorb light and undergo a chemical transformation that ultimately results in a electrical stimulus

In rod cells, the disk membranes contain an integral membrane protein called rhodopsin

Consists of protein opsin and a derivative of Vitamin A called retinal

Since it absorbs light also called a pigment

Light is absorbed in a double bond within the retinal molecule causing a cis-trans isomerization

This causes a temporary separation of retinal from opsin which ultimately results in a change in conductivity of the cell

When enough opsin is split, impulses are transmitted to the bipolar cells.

If exposed to bright light, all rhodopsin in rods breaks apart

It takes a while for our eyes to adapt to bright light. During this time rhodopsin is recombining

All cones contain retinal, but have variations of opsin-like proteins which cause it to preferentially absorb red, green or blue light

Perception of color depends on the relative proportions of red, green, or blue cones that are stimulated and send impulses to the visual centers in the brain

Cones yield more detailed images than rods but are much less sensitive to light

Cones are concentrated in one part of the retina called the fovea

This area has the highest visual acuity

Color blindness can result from lack of pigment in particular cones

Red or green deficiency most common

This form affects 8% of white males, 1 % of females. much rarer in other races

Sensory neurons from the ganglionic cells in the retina leave the eye at one place, the optic nerve.

The place where these neurons leave the eye has no rods or cones ---> "blind spot"

Cornea does most of the bending of the light rays from far objects ( > 20 feet away) so that they converge on the retina to form a clear image

Closer objects cannot be focused by the cornea, but the lens can become rounder by action of muscles in the ciliary body

Since close vision requires work on the part of the eye, long periods of reading, looking at computer screens etc. can result in eyestrain

Myopia occurs when the eyeball is too long and rays come to a focus in front of the retina so that distant objects are blurry

Divergent (concave) lenses correct this problem

Hyperopia occurs when the eyeball is too short and light rays from nearby objects come to a focus behind the retina

Converging (convex) lenses can correct this problem

Presbyopia is caused by a weakening of the ciliary muscle with age so that the eye lens no longer bulges enough to bring nearby objects into focus

Astigmatism results from irregularities in the cornea so that not all light rays from all angles focus at the same point on the retina.

Can be also be corrected by (complicated) lenses

Sound travels in waves of vibrating air molecules

Things that create sound waves create air pressure changes that alternatively compress and decompress the surrounding air

This disturbance propagates as a traveling wave at the speed of sound (340 m/s)

The ear needs to convert these air pressure changes to nerve impulses

The ear is a complex organ consisting of the outer ear, middle ear, and the inner ear

The outer ear consists of a structure called the pinna which connects to the auditory canal ending in the tympanic membrane (eardrum)

These structures direct sound waves to the eardrum causing it vibrate

The middle ear is an air-filled chamber within the temporal bone of the skull

Vibrations in the tympanic membrane are transmitted via three tiny bones called the hammer, anvil, and stirrup (maleus, incus, stapes)

These bones transmit vibrations from the tympanic membrane to a small membrane on the inner ear called the oval window

The middle ear is connected to the upper pharynx by the Eustachian tube.

This allows equalization of air pressure on either side of the eardrum with changes in altitude

The inner ear consists of a structure called the cochlea located within the temporal bone.

Cochlea consists of three fluid filled compartments arranged in a spiral

Tympanic canal, vestibular canal, and cochlear canal

The cochlear canal bounded on the bottom by the basilar membrane

Hair cells attached to the basilar membrane rub against the tectoral membrane

Hair cells, basilar membrane and tectoral membrane collectively called the organ of Corti

Sound waves are transmitted to the oval window by the stirrup

Waves propagate through the vestibular canal and the tympanic canal.

The round window at the end of the tympanic canal bulges to absorb the pressure

The basilar membrane will move up and down in response to the sound wave

Cilia rubbing against the tectoral membrane generate impulses transmitted to the brain by the auditory nerve.

Cilia at different locations sensitive to different frequencies.

Near the base, high frequencies, near the tip low frequencies

High volume or intensity of sound causes the basilar membrane to vibrate with a greater amplitude causing more and more frequent nerve impulses from the hair cells

Semicircular canals contain structures called ampulla

During angular or rotational motion fluid in the semicircular canals flow over ampulla distorting gelatinous material stimulating the hair cells

When the body is at rest, otoliths (small crystals CaCO₃ ) rest on the gelatinous material and hair cells not stimulated.

If the body moves either horizontally or vertically, hair cells are stimulated.

Congenital defects of infections can cause ossicles to fuse preventing sound from getting to the inner ear.

Cilia in the Organ of Corti get worn away with either normal aging or continuous exposure to loud sound (> ~120 dB)