Sensing and Vision

• Sensing is how sensory cells translate stimuli into action potentials for the nervous system to integrate.
• General sensory receptors are modified nerve endings of sensory neurons.
• Touch is a general somatic sense that detects pressure, pain, temperature, and tension.
• Special senses are vision, smell, taste, hearing, and equilibrium.
• Special senses use special sensory receptor cells in sensory structures (like the ear and eye) or epithelial structures (like taste buds and olfactory epithelium).
• Color blindness is sex-linked.
• There are 3 types of colorblindness: black-white, red-green, and blue-yellow. The most common is red-green and least common is black-white.
• With partial color blindness, the person still sees the color, but all the colors are darker shades.

Vision

• Vision is the dominant human sense.
• Light is electromagnetic waves.
• Photoreceptors in the eyes convert light energy into electrical energy (APs) that travel to the brain.

Touch

• Sensory receptors for touch are mechanoreceptors, which detect vibration, pressure, stretch, and touch.
• There are 6 types of mechanoreceptors in the skin:
  • Meissner's Corpuscles detect indentation and slipping of objects.
  • Ruffini's Corpuscles detect stretch.
  • Hair Follicles detect light touch.
  • Pacinian Corpuscles detect vibration.
  • Merkel's Disks create understanding of structure and texture.
  • Krause's Corpuscles detect pleasant, light tactile sensations.

Eye Structure

• The eye is surrounded by protective fat and the bony orbit within the skull.
• Accessory structures:
  • Eyebrows keep sweat and sunlight out of eyes.
  • Eyelids and eyelashes trigger reflexive blinking to keep eyes moist.
  • Lacrimal apparatus (tear gland) consists of the lacrimal gland that produces tears and drains secretions.
  • Extrinsic eye muscles: 6 on each eyeball control eye movement
• The eye is internally hollow with fluids (humors) that hold its shape
• Three layers make up the wall of the eyeball:
  • Fibrous Layer: the outermost layer that mainly consists of the sclera (white part), an anchoring site for extrinsic eye muscles, and includes the cornea, a window that lets light in.
  • Vascular Layer: the middle layer that includes the choroid, which supplies blood, and suspensory ligaments/ciliary zonule, which encircles and holds up the lens.
  • Intrinsic eye muscles (focus lens):
    • Ciliary body: a ring of muscle tissue around the lens
    • Iris: (colored part), a ring of smooth muscle between the cornea and lens that contracts/expands to change pupil size. The pupil allows light in, so the iris controls the amount.
  • Inner layer (retina):
    • Outer pigmented layer: pigment cells that help absorb light so it doesn't scatter.
    • Inner neural layer: contains neurons and neuroglia that create pathways for light.
    • Millions of photoreceptors convert light energy into APs that travel through the optic nerve to the thalamus and then the visual cortex in the brain. There are 2 types:
      • Rods: register black and white
      • Cones: detect fine detail and color. -Rods are for vision/Cones are for color
• The fovea centralis provides the sharpest, clearest vision. The optic disc is where the optic nerve exits.
• Inside the eye:
  • Lens: a convex, transparent disc that focuses light and projects it onto the retina.
  • Vitreous humor: a clear gel that fills the posterior segment behind the lens.
  • Aqueous humor: a clear fluid that fills the anterior segment in front of the lens.

Chemical Senses

• Chemical senses include smell (olfaction) and taste (gustation).
• Chemoreceptors detect molecules in the air (dissolved in nasal membranes) and food (dissolved in saliva).
• Smell bypasses the brain's filter, making it unique among the senses.

Smell (Olfaction)

• Odor: gaseous molecules
• AP’s are used for action potentials
• The molecules hit the olfactory epithelium in the roof of the nasal cavity, which has olfactory sensory neurons.
• Nose hairs filter out some of the molecules.
• Molecules bind to receptors which leads to APs down the olfactory nerve into the ethmoid bone, and eventually to the olfactory bulb that connects to the brain.
• Travels down the olfactory tract to the olfactory cortex.
• Signals are sent to the frontal lobe for identification and the limbic system for emotional processing.

Taste (Gustation)

• Taste Buds: sensory organ of taste (mainly inside papillae on the tongue)
  • Gustatory epithelial cells: taste receptor cells
  • Basal epithelial cells: stem cells that make new gustatory epithelial cells.
• Sensory receptor cells send APs to the gustatory cortex of the brain, releasing digestive enzymes.

Hearing & Balance

• The ear is the sensory organ for hearing and balance (equilibrium).
• Ears receive mechanical waves and convert them to nerve signals.
• Sound creates vibrations in the air that hit the eardrum, causing tiny bones (auditory ossicles) to move internal fluid against a membrane.
• This triggers tiny “hair” cells to stimulate neurons leading to APs in the brain.
• Losing inner ear hairs means they do not grow back.

Structure of the Ear

• The ear has 3 parts: outer/external, middle, and inner.
  • The outer & middle ear are for hearing & the inner ear is for hearing & maintaining equilibrium
• The external (outer) ear includes:
  • Pinna/auricle (part we see made of cartilage)
  • External acoustic meatus (auditory canal)
    • Its function is to catch sound waves, channeling them deeper into the ear.
• The middle ear:
  • Tympanic cavity: serving as a relay station between the outer and inner ear.
  • The sound waves collide with the tympanic membrane (eardrum), creating a connective tissue boundary between the external and middle ear
  • 3 tiny bones (auditory ossicles): malleus (hammer), incus (anvil), and stapes (stirrup).
• Their function is to amplify sound waves so they are stronger when they get to inner ear.
  • The auditory ossicles conduct vibrations to the oval window.
    • This aids in moving the fluid closer to the inner ear
  • Eustachian tube: A passage from the middle ear to the pharynx, aiding in equalizing pressure around the eardrum
• The inner ear:
  • Labyrinth: bony and membranous.
• The divisions contain fluid allowing them to conduct sound vibrations so we can hear and respond to changes in equilibrium
• The function is to turn physical vibrations into electrical impulses (APs) to travel to the brain
  • 3 structures comprise the inner ear's bony labyrinth:
    • Vestibule: a structure maintains balance and head movements.
      • They trigger fluid to stimulate hair cells, sending APs through the vestibular nerve.
    • Semicircular canals: aid in maintaining balance when the head rotates.
    • Cochlea: Contains hair cells that vibrate at different frequencies.
• This stimulates the organ of Corti to send APs through the cochlear nerve to the auditory cortex in the brain.