Vertebrate Physiology

This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

Sensory Physiology

Sensory receptors are specific to the characteristic they are sensing ie temperature, pressure, chemicals, movement sound sight etc.

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.
odor, touch, temperature

Tonic receptors produce a constant signal (AP frequency) while a stimulus is applied.
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 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.

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.
Trichromatic color vision relies on cones which 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 in humans.
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.

Humans can detect about 1500 different wavelengths. The red cones receive maimum stimulation at 565 nm, the green at 530 nm and the blue at 440 nm. Wavelengths in between these stimulate more than one type of cone at different intensities and the brain interprets the combination of these signals as different colors.

Birds, reptiles and fishdiffer from mammals they can be trichromatic, tetrachromatic or pentachromatic. Most mammals are dichromatic, having only blue and green cones. Trichromatic color vision seems to have evolved again in primates and is present in old world monkeys, apes and humans. It is thought that the green opsin gene duplicated and diverged and one changed to the red opsin gene (located on the X chromosome of humans).


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This webpage was created by Peter King. Please contact the the author with comments at
Last edited January 10, 2011.
copyright Peter King.