These notes are intended to accompany lectures in Human Physiology by Dr. Peter King at Francis Marion University, Florence, SC 29502.
Sensory receptors are specific to the characteristic they are sensing ie temperature, pressure, chemicals, movement sound sight etc. They can be modified afferent neurons or specialized sensory cells that communicate with afferent neurons via a chemical synapse.
Four functional receptor types are
1. chemoreceptors
2. photoreceptors
3. thermoreceptors
4. mechanoreceptors
The thing they have in common is that they change the polarization
of the cell and may eventually cause an action potential.
Sensory nerve endings act in a similar way as dendrites.
Response to a stimulus is local and graded and contributes toward
changing a cell toward or away from threshold.
While similar to an EPSP they are called receptor or generator
potentials
Receptor response fall into 2 categories, phasic and tonic
receptors
Phasic receptors send APs in quick sensation when first
stimulated but soon reduce the AP frequency even if the stimulus
continues. They adapt to the stimulation.
Examples?
odor, touch, temperature
Tonic receptors produce a constant signal (AP frequency)
while a stimulus is applied.
Examples?
photoreceptors, mechanoreceptors
Cutaneous Sensations
Touch, pressure, heat, cold and pain are stimuli picked up by
different sensory neurons.
Heat, cold and pain are picked up by naked nerve endings in the
skin.
There are more cold receptors than hot receptors and the hot receptors
are deeper.
Temperature inhibits or excites each receptor.
Pain sensory neurons (nociceptors) are free nerve endings that
pick up tissue damage.
Axons may be myelinated or not.
Neurotransmitter is substance P (11 aa polypeptide) in the CNS
Touch is picked up by naked dendritic endings wrapped around hair follicles and by other expanded endings such as Ruffini endings and Merkel's discs
Taste and Smell
Taste ands olfactory receptors react to molecules dissolved in
fluid and are both classified as chemoreceptors.
They are somewhat integrated and we often confuse one sensation
with the other.
Taste receptors are modified epithelial cells with long microvilli.
Cells are clustered together and microvilli extend through a pore
to the external environment.
Stimulation causes depolarization and release of neurotransmitter
that stimulates the associated neuron.
Most taste buds in humans are found on the tongue and are innervated
by the glossopharyngeal nerve (IX) and the facial nerve (VII).
Olfactory sensors are dendritic endings of bipolar neurons.
They connect to neurons in the olfactory bulb.
These neurons are unique in thaat they undergo mitosis. They have
a lifespan of about 2 months.
Dendrites end in a knob with cilia. Molecules bind to receptors
on the cilia and cause a depolarization by opening ion channels.
Cascading enzymes amplify the effect and the human nose can
detect a billionth of an ounce of perfume in air.
Neural connection is direct to the limbic system in the forebrain.
Important in memory and emotions.
Over 1000 genes have been recognized as coding for olfactory receptor
proteins.
10,000 different odors can be distinguished.
Vestibular apparatus and cochlea
Our sense of balance and our hearing are controlled by sensors
in the ear.
The sensory cells are similar in these 2 organs.
They are modified epithelium cells called hair cells.
Hair cells have 20 - 50 cell membrane projections called
stereocilia and one true cilium (kinocilium) connected
by fibers.
When the sterocilia push toward the kinocilium the cell is excited,
and inhibited if movement is reversed.
The vestibular apparatus picks up movement with hair cells
that detect movement of fluid in semicircular canals. A
gelatinous membrane, the cupola, covers the hairs and is
moved by the endolymph.
Hairs of some cells in the utricle and saccule are imbedded in
a gel containing heavy otoliths that pick up acceleration
and give us a sense of gravity
Sound
Sound is also received by hair cells.
Sound waves in the air depress the tympanic membrane.
In the inner ear (area between the tympanic membrane and the cochlea)
3 bones (malleus, incus and stapes) transfer the
tympanic movement mechanically to the oval window of the
cochlea.
Movement of the oval window cause waves in the fluid (perilymph)
in the cochlea.
Movement of fluid in the Scala vestibuli and the Scali tympani
cause depression of a vestibular and basilar membrane swhich separates
another fluid-filled (endolymph) compartment, the cochlear duct.
Stereocilia on hair cells in the cochlea duct are displace with
depression of the membranes.
Different frequencies of sound waves in air cause stimulation
of different hair cells.
Sight
There are 2 types of photoreceptors in the eyes, rods and
cones located on the retina.
These neurons contain photopigments that breakdown when hit by
photons.
Cones sense color vision in bright light and rods provide black
and white vision in low light.
Rods contain the photopigment rhodopsin which breaks down
to retinene and opsin.
The breakdown of rhodopsin changes the ionic permeability of
the cell membrane which causes changes in cell polarization.
Cones have retinene combined with 3 different photopsins
that absorb photons with different wavelengths corresponding to
blue, green and red.
There are approx. 120 million rods and 6 million cones, but
only about 1.2 million neurons in the optic nerve of each eye.
4000 cones in the fovea connect to about 4000 ganglion cells,
giving greater visual acuity in the that area.
Single ganglion cells outside the fovea may receive input from
a large number of rods.
This provides greater light sensitivity but lower acuity.
This webpage was created by Peter King. Please contact the
the author with comments at pking@fmarion.edu.
Last edited July 20, 2010.
http://people.fmarion.edu/pking/humanphys/senses.html
copyright Peter King.