Sensory Systems

General Function: input information to the CNS.

Sensory adaptation is a reduction in sensitivity due to a constant stimulus. (That smell in a grandparent's house that you notice at first and then 'goes away'...)

General Senses:
Temperature - Thermoreceptors detect cold and warmth. Adapt quickly.

Pain - Found in skin, joints, and walls of blood vessels. Two Types:
  1. fast pain: myelinated fibers carry this sharp pain signal to the CNS quickly.
  2. Slow Pain: unmyelinated fibers carry this burning or aching pain to the CNS slowly.

Referred pain is pain felt in a different area of the body than that which is damaged (a heart attack frequently is felt as referred pain to the left arm and/or jaw).

Touch - Many sub-types (beyond the scope of this course). Detect light touch or deep pressure. Baroreceptors detect changes in pressure (air pressure in the lungs, as well as liquid pressure in the blood and bladder).

Proprioception - proprioceptors detect position of the body.

Chemical - Chemoreceptors detect chemical concentrations (detectors exist for checking dissolved CO2 in blood).

Special Senses:
Olfaction (smell) --> Nose
Gustation (taste) --> Tongue
Vision --> Eyes
Hearing (and Equilibrium) --> Ear

The Nose
Olfactory glands produce a mucus that covers the exposed ends of the Olfactory Receptor Cells. Chemicals, breathed in through the nose, dissolve in the mucus layer and bind (lock-and-key mechanism) with Olfactory Receptor Cells, producing action potentials.The brain may identify odors by comparing the pattern of action potentials an odor stimulates to past patterns.

The Tongue

Taste buds are similar to Olfactory Receptors Cells, in that chemicals in food/drink dissolve in saliva in the mouth and bind
(lock-and-key mechanism) with taste buds.

There are four primary taste sensations:
The picture on the right has significant controversy surrounding it. All areas (except the middle) of the tongue have taste buds that can detect the four primary taste sensations. It seems as though the areas on the picture above deal with sensitivity. The anterior tip of the tongue detects sweets faster than the back of the tongue.

The tongue can also detect lipids (fats and oils).

The sense of smell and taste are linked. This means that the sense of taste is much stronger when our olfactory organs are fully functional (for example, no stuffy nose).

The Eye
The external/visible structures of the eye:

Palpebra: Eyelid serves to spread tears and protect the eye.
Sclera: Fibrous protective outer coating of the eye. Does not allow light in.
Iris: Contractile colored ring that controls the amount of light that enters the eye.
Pupil: The space where light enters the eye.
Lacrimal gland: Produces tears that prevent bacterial/fungal colonization of the eye.
Lacrimal duct: Duct by the nose that drains tears off the surface of the eye.

Internal anatomy of the eye:

Lens: can change shape to focus the eye on nearby or far off objects.
Vitreous Humor: Clear jelly that helps maintain the eye's spherical shape.

Retina: Found on posterior part of the eye, contains photoreceptors that detect light and process signals:
  • Rods: Detect any light. Very sensitive.
  • Cones: Detect colors of light. Not sensitive (for example you need lots of light to activate your cones)
  • Ganglion Cells: Receive input from many photoreceptors. Converge data to allow the brain to detect motion.
  • Fovea: Point of highest concentration of photoreceptors (and cones). The eye is aimed to allow light to fall on the fovea.
  • Optic Disk: Where neurons 'dive' down into the optic nerve. No photoreceptors here, therefore a blind spot exists.
Optic Nerve: Sends visual sensory messages to the brain.

The Ear

Pinna: Outer part of the ear, helps direct sound into the auditory canal.
Auditory Canal: A tube that allows the transmission of sound to the tympanum.
Tympanum: A lightweight membrane that is stretched across the auditory canal. The tympanum vibrates when sound hits it.
Auditory Tube (aka the Eustacian Tube): Allows air to move in and out from the throat. Helps equalize pressure on the inside of the space in the middle ear. Allows the tympanum to not be bend in or out due to pressure imbalances.
Auditory Ossicles: Three bones of the ear (Incus, Maleus, Stapes) that vibrate when the tympanum vibrates.
Cochlea: Snail-shaped organ that has a canal running through it filled with mucus and sensory hairs. Vibrations from the auditory ossicles are transmitted as shockwaves in the mucus. These shockwaves bend sensory hairs (the higher the pitch the shorter the hair the sound bends) which, in turn, send action potentials to the brain.
Semi-Circular Canals: Contain mucus and sensory hairs. Also have otoliths (small crystals). As the head turns the otoliths press against sensory hairs (otoliths have more inertia and accelerate more slowly than the mucus that surrounds them). These hairs send action potentials that the brain processes for balance information.