The General and Special Senses

“We do not see things as they are; we see things as we are.”

--Anais Nin

Anais Nin, French author1903 - 1977

How do we sense the world around

us?

Characteristics of Sensory Receptors

• They are all energy transformers, transforming various stimuli (light, pressure waves, chemicals etc.) into action potentials

• All have sensitivity ranges...

Sensory Adaptation

• Why does a hot bath feel cool so soon?

• Why do the cats smell awful when you first walk into the Anatomy Lab?

• And why do you continue to hear my voice no matter how long I talk?

Sensory Adaptation

Phasic Receptors Tonic Receptors

rapid adaptation slow or no adaptation

cease firing if strength of a continuous stimulus remains constant

continuous signal transmission for duration of stimulus

allows the body to ignore constant unimportant information

monitoring of parameters that must be continually evaluated

Sensory AdaptationsPhasic Receptors Tonic Receptors

odor light

touch sound

temperature

Referred painmaps Worthy of Note:

• Law of Specific Nerve Energies: Stimulation of a specialized sensory nerve fiber produces only one type of sensation

• Adaptive value: what matters most is… change

Taste

• Papillae have taste buds on the outer surface (fungiform papilla) or in the troughs of the papilla (foliate, vallate and filliform papilla).

• Each papilla has anywhere from 2 to 15 taste buds on it.

• Taste buds regenerate every 10 days.

To see your papillae easily, drink some milk and look at your tongue in a mirror.

Taste buds:

• Salt (over most of the tongue)

• Na+ ions trigger the taste bud

• Sour (sides of tongue)

• H+ ions trigger the taste bud

• Bitter (back of the tongue)

• Sweet (tip of the tongue)

• Umami - glutamate and some nucleotides

• Water receptors in the pharynx

Smell and Taste

The sense of taste is strongly connected to olfaction. Many “taste deficiencies” are due to a decreased ability to smell. It has been shown that age-related decreases in taste are usually not due to problems with taste buds, but due to decreased olfactory input.

Olfaction

• Several million bipolar sensory neurons in a pseudostratified epithelium

• Over 1000 genes responsible for the proteins needed (some say 3% of our genes involved in olfaction - largest group of related genes discovered so far)

• Sensory input goes directly to the cerebral cortex, WITHOUT being first routed through the thalamus

Variables of Sound

• What pitch is that?

• How loud is that?

• Where is that sound coming from?

Properties of Sound• Measured in:

• Hz = cycles (waves) per second

• High frequency = high pitch

• Low frequency = low pitch

• Human range: 20 - 20,000 Hz

• Amplitude of sound waves (in decibels, dB).

• High amplitude = loud sounds

• Low amplitude = soft sounds

The Organ of Corti

Coding for Pitch• which hair cells are stimulated along the cochlea

Coding for Amplitude (Loudness)

• the degree of bending of the hair cells

Coding for Direction

• which ear receives the sound first

Hearing Impairments

• Conduction Deafness

• Nerve or sensory deafness

Equilibrium

• Vestibular apparatus = semicircular canals + otolith organs

• Otolith organs provide information regarding linear acceleration

• utricle: horizontal acceleration

• saccule: vertical acceleration

• Semicircular canals ! rotational acceleration

Accommodation

• Is the ability to keep in focus an object as it changes it's distance to and from the eye

• Relaxed ciliary muscle = lens thin and focussed for distant vision

• Contracted ciliary muscle = lens thick for close vision

• Presbyopia = loss of close vision due to poor contraction of ciliary muscles.

Acuity = Resolving Power(“nearsightedness”)

(“farsightedness”)

The Retina

Neural Pathways to the Brain

Amazing Neural Processing