Sensory Systems Overview

Sensory Neurons

• Sensory neurons are abundantly found throughout the body.

  • Examples: Tactile receptors, chemoreceptors, thermoreceptors (temperature receptors in the skin).

Special Senses

• Special senses are properties of the head, including concentrated sensory organs.

• Sensory perceptions unique to the head:

  • Vision

  • Equilibrium

  • Hearing

  • Olfaction (Smell)

  • Gustation (Taste)

Olfactory Sensation

• Olfactory neurons (part of cranial nerve I) are bipolar neurons with a single axon and dendrite.

• Olfactory cilia contain receptors for odoriferous hydrocarbons.

• Molecules must dissolve in mucus to bind to receptors, creating an action potential.

• Structure of Olfactory Epithelium:

  • Bipolar neurons

  • Olfactory stem cells

  • Supporting epithelial cells (simple columnar epithelium)

Mucous Membranes

• Mucous membranes composed of an epithelial layer and underlying connective tissue (lamina propria).

  • Found in various systems: olfactory, respiratory, digestive, urinary.

• Olfactory mucosa is a specific example.

Vision

• Vision is arguably the most essential sense; 70% of sensory receptors are in the eye.

• The occipital lobe processes about 40% of cerebral cortex activity for visual processing.

• Eye Structure:

  • Anterior and posterior cavities each containing different humors:

  • Anterior: filled with aqueous humor (circulating fluid).

  • Posterior: filled with vitreous humor (non-circulating, jelly-like substance).

• Eye Layers:

  • Outer fibrous layer (sclera and cornea)

  • Middle vascular layer (choroid, ciliary body, iris)

  • Inner neural layer (retina)

Radiation of Light

• Light passes: cornea → pupil → lens and is refracted to focus on the fovea centralis (high concentration of cones, best vision).

• The eye maps photons and transforms light energy into action potentials.

Retina Structure

• Comprises three main layers:

  • Photoreceptors: rods (grayscale, low acuity) and cones (color, high acuity).

  • Bipolar cells: relay information from photoreceptors to ganglion cells.

  • Ganglion cells: their axons form the optic nerve, relaying info to the brain.

• Fovea centralis: greatest concentration of cones; best visual acuity.

• Optic disc: blind spot due to absence of photoreceptive cells.

Visual Processing

• Photoreceptor cells are backward-facing, which inverts the image, but the brain corrects this inversion.

• The pigmented layer provides protection and nutrients and prevents light scattering.

Color Perception

• Visible light is within a specific spectrum; colors correspond to wavelengths of light.

• Human eyes can see red (400nm), green, and blue frequencies but not ultraviolet.

Accommodation Reflex

• Controlled by the ciliary muscles which contract to bulge the lens for near vision or relax for distance.

• Pupil response adjusts based on light levels (dilates in low light, constricts in bright light).

Hearing Mechanics

• Ear Structure: External, middle, and inner ears.

  • External: auricle (pinna), external acoustic meatus.

  • Middle: tympanic membrane, ossicles (malleus, incus, stapes).

  • Inner: cochlea (hearing) and vestibular system (equilibrium).

Cochlear Function

• The cochlea transforms sound waves into mechanical vibrations in the perilymph and endolymph.

• Hair cells in the cochlea detect vibrations and convert them into action potentials sent to the brain.

• Frequency Processing: Depends on where the basilar membrane vibrates within the cochlea (further for low frequency, closer for high frequency).

Disorders

• Glaucoma: increased intraocular pressure.

• Cataracts: clouding of lenses.

• Astigmatism: misshapen cornea or lens impacting light refraction.

• Color blindness: usually red-green color deficiency, more common in males due to the X chromosome.