Lecture 11 Senses
T. Irving 01/07/97
Goals of this Section:
Students should be able to:
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
Sensory receptors
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
Sensory Stimulation and Adaptation
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
Pain receptors
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
Stretch receptors (ìPropioreceptorsî)
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
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
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.
We can taste four main categories of sensation
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
Structure of the eye:
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
Focusing elements in the eye
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 contains the photoreceptive rods and cones
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
Two different kinds of photoreceptor cells
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
Photopigments
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
Cone cells are responsible for color vision
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
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
Optic Nerve
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"
How the eye forms images
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
Focusing Disorders: Near, farsightedness and Astigmatism
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
The Ear and Hearing
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
Outer 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
Middle Ear
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
Inner Ear
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 transduction
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
The inner ear and balance
Two kinds of senses related to balance:
Dynamic and static equilibrium
Dynamic equilibrium:
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
Static equilibrium
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.
Deafness
Conduction deafness
Congenital defects of infections can cause ossicles to fuse preventing sound from getting to the inner ear.
Nerve deafness
Cilia in the Organ of Corti get worn away with either normal aging or continuous exposure to loud sound (> ~120 dB)