Special Senses

Special Senses - Cranial Nerves

  • I Olfactory nerve - sensory for smell

  • II Optic nerve - sensory for vision

  • III Oculomotor nerve - motor fibers to eye muscles

  • IV Trochlear - motor fiber to eye muscles

  • V Trigeminal nerve - sensory for the face; motor fibers to the face

  • VI Abducens nerve - motor fibers to eye muscles

  • VII Facial nerve - sensory for taste; motor fibers to the pharynx

  • VIII Vestibulocochlear nerve - sensory for balance and hearing

  • IX Glossopharyngeal nerve - sensory for taste; motor fibers to pharynx

  • X Vagus nerves - sensory and motor fibers for pharynx, larynx, and viscera

  • XI Accessory nerve - motor fibers to neck and upper back

  • XII Hypoglossal nerve - motor fibers to tongue

Touch - General Sense

Conscious awareness of incoming sensory information is called sensation. 

Touch - Technically not a special sense. It’s a general sense. Located in the dermis and the subcutaneous tissue. Exhibit varying degrees of intricacy. (scale of organism)

  1. Nociceptors - Trauma

    1. Respond to heat, mechanical, stress, and chemicals - associated with tissue damage

    2. Most concentration in skin

  2. Thermoreceptors

    1. Respond in changes in temperature

    2. In dermis, skeletal muscles, liver, and hypothalamus

    3. Cold receptors > warm receptors

  3. Mechanoreceptors

    1. Respond to physical distortion of cell membrane (e.g.: stretching, twisting, compression)

    2. Baroreceptors sensitive to internal pressures: blood pressure, lung stretch, digestive tract tension 

    3. Proprioceptors monitors of muscle stretch 

    4. Tactile receptors - touch, pressure, vibration

      1. Merkel cells - function as touch receptors in association with sensory nerve endings

      2. Meissner’s corpuscles (or tactile corpuscles) are a type of nerve ending, responsible for sensitivity to light touch

  4. Chemoreceptors

    1. Respond to small concentration changes of specific molecules (chemicals)

    2. Internal chemoreceptors monitor blood composition (e.g. Na, pH, pCO2)

    3. Found within aortic and carotid bodies

    4. Very important for homeostasis

Photoreception - process by which the eye detects light energy

The Eye and Vision

  1. Vision is our dominant sense; 70% of our body’s sensory receptors are found in the eye

  2. Accessory Structures of the Eye

    1. Eyebrows are short, coarse hairs overlying the supraorbital margins of the eye that shade the eyes and keep perspiration out.

    2. Eyelids (palpebrae), eyelashes, and their associated glands help to protect the eye from physical danger as well as from drying out.

    3. Lacrimal glands, which secretes a dilute saline solution that cleanses and protects the eye as it moistens it, and ducts that drain excess fluid into the nasolacrimal duct.

    4. Ciliary and Meibomian Glands - secrete acidic sweat to kill bacteria, lubricate eyelids, and moisten with oils 

    5. Conjunctiva is a transparent mucous membrane that lines the eyelids and the whites of the eyes. It produces a lubricating mucus that prevents the eyes from drying out.

    6. Sclera - white connective tissue - anchors and protects

    7. Cornea - Allows for light to pass through (refracts, or bends, light slightly)

    8. Iris - The colored part of the eye, involuntary muscles to dilate pupil

    9. Pupil - central opening of the iris

      1. Regulates the amount of light entering the eye during:

        1. Close vision and bright light - pupils constrict

        2. Distant vision and dim light - pupils dilate 

        3. Changes in emotional state - pupils dilate when the subject matter is appealing or requires problem-solving skills

  3. Inner Eye Parts

    1. Aqueous humor

      1. A plasma like fluid that fills the anterior segment

      2. Supports, nourishes, and removes waste

    2. Lens

      1. Avascular, biconcave, transparent, flexible structure that can change shape to allow precise focusing of light on the retina.

        1. Refracts light greatly

    3. Victrous humor

      1. Gel-like substance behind the lens

      2. Keeps the eye from collapsing

  4. Photoreception in the Retina (back of the eye)

    1. Photoreception is the process by which the eye detects light energy

      1. Photoreceptors are modified neurons that structurally resemble tall epithelial cells.

      2. Rods are highly sensitive and are best suited to night vision/dim light. Help with peripheral vision

      3. Cones are less sensitive to light and are best adapted to bright light and color vision.

      4. Photoreceptors contain a light-absorbing molecule called retinal (vitamin A sub).

      5. Ganglion cell axons:

        1. Run along the inner surface of the retina

        2. Leave the eye as the optic nerve

      6. The optic disc:

        1. Is the site where the optic nerve leaves the eye

        2. Lacks photoreceptors (the blind spot)

  5. Retina - layer where photoreceptors and glial cells capture light and convert it into electrical signals. 

    1. Leave at the optic nerve

    2. Binocular vision - two eyes to see

The movement of each eyeball is controlled by six extrinsic eye muscles that are innervated by the abducens, oculomotor, and trochlear nerves.

The Ear: Hearing and Balance

Physiology of Hearing

  • Ear - Houses two senses

    • Hearing (interpreted in the auditory cortex of the temporal lobe)

    • Equilibrium (balance) (interpreted in the cerebellum)

  • Properties of Sound

    • Sound - pressure disturbance (alternating high and low pressure) produced by a vibrating object and propagated by the molecules of the medium.

    • Frequency is the number of waves that pass a given point in a given time.

    • Amplitude, or height, of the wave reveals a sound’s intensity (loudness).

Auditory processing involves perception of pitch, detection of loudness, and localization of sound.

Anatomy of the Ear

  1. Outer (external) ear

    1. Pinna (auricle) - collects sound

    2. External auditory canal - channels sound inward

    3. Cerumen - ear wax

  2. Middle ear (tympanic cavity)

    1. Two tubes are associated with the inner ear

      1. The opening from the auditory canal is covered by the tympanic membrane (eardrum)

      2. The auditory tube connecting the middle ear with the throat (eustachian tube)

        1. Allows for equalizing pressure during yawning or swallowing

        2. This tube is otherwise collapsed

    2. Three bones span the cavity. Vibrations from eardrum move the malleus. These bones transfer and amplify sound to the inner ear.

      1. Malleus (hammer)

      2. Incus (anvil)

      3. Stapes (stirrup)

  3. Inner ear

    1. Also known as osseous labyrinth - twisted bony tubes within temporal bone

      1. Includes sense organs for hearing and balance - Contains the vestibule, the cochlea, and the semicircular canals

        1. Filled with perilymph (extracellular fluid)

        2. Cochlea - channels new vibrations through perilymph; transfers Organ to Corti

        3. Organ to Corti - contains stereocilia (hair like cells) that move due to the waves in the perilymph

        4. Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

Equilibrium and Orientation

The equilibrium sense responds to various head movements and depends on input from the internal ear, vision, and information from stretch receptors of muscles and tendons.

  • Equilibrium has two functional parts

    • Static equilibrium - in the vestibule

    • Dynamic equilibrium - in the semicircular canals

Rotation of the head causes endolymph within the semicircular canal to push against the cupula covering the hair cells, resulting in bending of their stereocilia and the initiation of a nerve impulse.

Maculae - sensory receptors for static equilibrium. Information from the balance receptors goes directly to reflex centers in the brain stem, rather than to the cerebral cortex.

The Chemical Senses: Taste to Smell

Chemoreceptors - both senses compliment each other and respond to many of the same stimuli

  1. The Olfactory Epithelium and the Sense of Smell

    1. Olfactory epithelium receptors are in the roof of the nasal cavity. Neurons have long cilia. Chemicals must be dissolved for detection. 

    2. To smell a particular odorant, it must be volatile (can be vaporized or transition from liquid to gas) and it must be dissolved in the mucus/fluid coating the olfactory epithelium that stimulates the olfactory receptors.

    3. Axons of the olfactory receptor cells synapse in the olfactory bulbs, sending the impulses down the olfactory tracts to the temporal lobe, thalamus, hypothalamus, amygdala, and other members of the limbic system.

    4. Olfactory discrimination - although difficult to describe, the number of different odors recognizable is immense.

  2. Gustatory Buds and the Sense of Taste

    1. Taste buds, the sensory receptor organs for taste, are located in the oral cavity, on tongue, soft palate, and cheeks.

    2. Physiology of taste 

      1. For a chemical to be tasted it must be dissolved in saliva, move into the taste pore, and contact a gustatory hair (papillae)

      2. Each taste sensation appears to have its own special mechanism for transduction.

      3. Like smell, chemicals must be volatile enough to be tasted

    3. Afferent fibers carrying taste information from the tongue are found primarily in the facial nerve and glossopharyngeal cranial nerves.