Study Notes: Special Senses (Taste, Smell, Sight, Hearing, Balance)

The Special Senses

Overview of Special Senses

  • Special senses include:
    • Taste (gustation)
    • Smell (olfaction)
    • Sight
    • Hearing
    • Balance
  • Special sensory receptors are localized to specific regions, primarily in the head.
  • Unlike general sensory neurons, the receptors for special senses are not free nerve endings but are specialized receptor cells.

The Chemical Senses: Taste and Smell

  • Taste (gustation): Refers to the sensation of flavor perceived when substances interact with the taste buds.
  • Smell (olfaction): The ability to detect chemicals in the air, facilitating the sense of smell.
  • Chemoreceptors: The receptor types that respond to chemical stimuli, essential for both taste and smell.

Taste – Gustation

  • Taste Receptors:
    • Found within taste buds, primarily located on the surface of the tongue.
    • Most taste buds are located within the papillae of the tongue (e.g., fungiform and vallate papillae).
  • Taste buds consist of a collection of 50-100 epithelial cells that are responsible for detecting taste stimuli.

Taste Bud Structure

  • Components of a Taste Bud:
    • Fungiform Papillae: Mushroom-shaped structures on the tongue surface.
    • Vallate Papillae: Large papillae arranged in a V-shape at the back of the tongue.
    • Gustatory Epithelial Cells: The cells responsible for taste sensation, embedded in the taste bud.
    • Taste Pore: The opening through which gustatory hairs project to interact with taste substances.
    • Gustatory Hair: The sensory surface of the gustatory epithelial cells that interacts with dissolved chemicals.

Taste Sensation and the Gustatory Pathway

  • Four Basic Qualities of Taste:
    1. Sweet
    2. Salty
    3. Sour
    4. Bitter
  • A fifth taste, umami, associated with savory substances (often described as “deliciousness”).
  • Myth of the Taste Map: The idea that different regions of the tongue respond to specific tastes is a myth; all taste modalities can be elicited from any area containing taste buds.
  • There are no structural differences among taste buds in their ability to taste various flavors.

Gustatory Pathway

  • Taste information is transmitted to the cerebral cortex primarily through:
    • Facial Nerve (VII): Anterior tongue (gustatory fibers).
    • Glossopharyngeal Nerve (IX): Posterior tongue (gustatory fibers).
    • Vagus Nerve (X): Taste fibers from the epiglottis and lower pharynx.
  • Once taste information is received, sensory neurons synapse in the medulla's solitary nucleus, which transmits signals to the thalamus, and then to the parietal cortex for processing.

Olfactory Receptors

  • Anatomy of Olfactory Receptors:
    • Olfactory Epithelium: The sensory surface in the nasal cavity responsible for the sense of smell.
    • Olfactory Cilia: Hair-like structures that detect odorant molecules.
    • Olfactory Bulb: The brain structure that processes olfactory information.
    • Mitral Cell: The output cell located in the olfactory bulb, involved in transmitting olfactory signals.
  • Pathway of Olfaction:
    • Air inhaled containing odor molecules passes through the nasal conchae and interacts with the olfactory epithelium, leading to signal transmission along the olfactory tract.

The Eye and Vision

  • The eye is considered the visual organ, with approximately 70% of all sensory receptors located within it.
  • Cerebral Cortex and Vision: About 40% of the cerebral cortex is dedicated to processing visual information.
  • Anatomy of the Eyeball:
    • Components protect and support the photoreceptors while gathering, focusing, and processing light into images.
    • The eye includes three major tunics:
    1. Fibrous Tunic: Provides shape and anchors eye muscles.
    2. Vascular Tunic: Contains blood vessels and controls shape of the lens.
    3. Sensory Tunic (Retina): Contains photoreceptors necessary for detecting light.

The Fibrous Tunic

  • Sclera: The white, opaque part of the eye that covers most of the fibrous tunic. It provides structural support and serves as an anchor for eye muscles.
  • Cornea: The transparent anterior sixth, allowing light to enter the eye while aiding in focusing.

The Vascular Tunic

  • Choroid: Vascular, darkly pigmented membrane, covering the posterior five-sixths of the vascular tunic, supplying blood to the eye.
  • Ciliary Body: A thickened ring of tissue that encircles the lens. Consists of smooth ciliary muscles affecting lens shape.
  • Iris: The visible colored part of the eye, which controls pupil size by contraction or dilation of the sphincter and dilator pupillae muscle.

The Retina

  • Photoreceptors:
    • Two main types:
    1. Rod Cells: More sensitive to light, enabling vision in low-light conditions.
    2. Cone Cells: Responsible for color vision and functioning best in bright light.
  • The retina is comprised of multiple layers, with photoreceptors at the outermost level, followed by bipolar cells, and then ganglion cells, which relay information to the optic nerve.

Internal Chambers and Fluids

  • Posterior Segment: Cavity filled with vitreous humor, supporting the retina and helping maintain intraocular pressure.
  • Anterior Segment: Contains aqueous humor, which is continuously renewed and provides nutrients to the lens and cornea.
  • Aqueous Humor Formation: Aqueous humor is created via blood filtration at the ciliary processes and flows from the posterior chamber through the pupil into the anterior chamber, getting reabsorbed into the bloodstream via the scleral venous sinus.

The Eye as an Optical Device

  • Lens: A biconvex structure that focuses light onto the retina.
    • Myopic eye (nearsighted) focal point is in front of the retina.
    • Hyperopic eye (farsighted) focal point is behind the retina.

Cataract

  • A cataract occurs when there is inadequate nutrient delivery to parts of the lens. Possible causes include congenital factors, exposure to sunlight, smoking, and certain medications.

Visual Pathways

  • Visual information is primarily processed in the cerebral cortex, responsible for conscious perception of sight.
  • The pathway begins with photoreceptor activation in the retina and the optic nerve carrying signals to various brain regions.

Anatomical Pathway Details

  • The visual pathway includes routes from the retina to the optic nerve, then to the optic chiasma where information from each eye is integrated, then to the lateral geniculate nucleus of the thalamus, and finally to the primary visual cortex located in the occipital lobe.

The Ear: Hearing and Equilibrium

  • The ear serves as the receptor organ for both hearing and equilibrium, divided into three main regions:
    1. Outer Ear: Responsible primarily for capturing sound.
    2. Middle Ear: Acts in sound amplification.
    3. Inner Ear: Processes sound and aids in balance.

The Outer Ear

  • Comprises the auricle (pinna) that directs sound and the external acoustic meatus leading to the tympanic membrane, the boundary between external and middle ear.

The Middle Ear

  • A small air-filled cavity within the temporal bone containing the ear ossicles (malleus, incus, stapes) and the pharyngotympanic (auditory) tube that connects to the pharynx.
Ear Ossicles
  • The smallest bones in the body:
    • Malleus: Attached to the eardrum.
    • Incus: Between the malleus and stapes.
    • Stapes: Vibrates against the oval window, transmitting sound to the inner ear.

The Inner (Internal) Ear

  • Known as the labyrinth, it consists of a bony labyrinth and a membranous labyrinth. The former contains three parts: semicircular canals, vestibule, and cochlea.
  • The membranous labyrinth is a series of sacs and ducts filled with endolymph, while the bony labyrinth is filled with perilymph.

Cochlea

  • The cochlear duct contains receptors for hearing and features the organ of Corti as the receptor epithelium.
    • Sound waves vibrate the tympanic membrane, causing the ossicles to amplify and transmit pressure onto the oval window, resulting in wave movement through cochlear fluid and the corresponding excitation of hair cells.

Equilibrium and Auditory Pathways

  • The equilibrium pathway transmits information about head movement and position predominantly to lower brain centers, while the ascending auditory pathway relays information from cochlear receptors to the cerebral cortex.