Senses

Sensing is defined as the process where sensory cells translate various stimuli into action potentials.
- The types of stimuli include chemical, electromagnetic, and mechanical stimuli.
- These action potentials can be integrated by our nervous system.
General sensory receptors are typically modified nerve endings of sensory neurons present in our bodies.
Touch is classified as a general (somatic) sense, which relates to detecting pressure, pain, temperature, and tension through a variety of general sensory receptors.

Special senses include vision, smell, taste, hearing, and equilibrium.
- They utilize special sensory receptor cells located in sensory organs, such as the ear and the eye, or epithelial structures, such as taste buds and olfactory epithelium, which are located in the head.

Vision is considered the dominant sense in humans.
Light is described as electromagnetic waves.
Photoreceptors in our eyes, which are the sensory organs of vision, convert light energy into electrical energy (action potentials).
- These converted action potentials travel to the brain for processing.

The eye is surrounded by protective fat and housed in the bony orbits of the skull.

Eyebrows: Function to keep sweat and sunlight out of the eyes.
Eyelids and Eyelashes: Trigger reflexive blinking, which helps keep the eyes moist.
Lacrimal Apparatus: Comprises the lacrimal gland, which produces and secretes tears, and ducts that drain these secretions.
Extrinsic Eye Muscles: There are six extrinsic muscles on each eyeball that control eye movement:
- Lateral Rectus: Controls lateral movement.
- Superior Rectus: Elevates and turns the eye medially.
- Medial Rectus: Controls medial movement.
- Inferior Rectus: Depresses and turns the eye medially.
- Inferior Oblique: Elevates and turns the eye laterally.
- Superior Oblique: Depresses and turns the eye laterally.

The eye is internally hollow and contains fluids (humors) that hold the shape of the eye.
The eyeball wall is constructed from three layers:
- Fibrous Layer: The outermost layer includes:
- Sclera: The white part of the eye, serves as an anchoring site for extrinsic eye muscles.
- Cornea: Acts as the window that allows light to enter the eye.
- Vascular Layer: The middle layer includes:
- Choroid: Supplies blood to all layers of the eye.
- Ciliary Body: A ring of muscle tissue around the lens.
- Iris: The colored part of the eye comprised of smooth muscle that controls the size of the pupil.
- Inner Layer: Known as the retina, which has two layers:
- Outer Pigmented Layer: Contains pigment cells to absorb light and prevent scattering.
- Inner Neural Layer: Contains numerous neurons and neuroglia to create pathways for light.

Millions of photoreceptors are present in the retina, converting light energy into action potentials that are transmitted through the optic nerve to the thalamus and then to the visual cortex in the brain.
There are two types of photoreceptors:
- Rods: Responsible for detecting black and white.
- Cones: Detect fine detail and color.
The optic disc is where the optic nerve exits the eye, and the fovea centralis is regarded as the area providing the sharpest and clearest vision.

Lens: A convex transparent disc that focuses incoming light on the retina.
Vitreous Humor: A clear gel that fills the posterior segment of the eye behind the lens.
Aqueous Humor: A clear fluid filling the anterior segment in front of the lens.

Chemical senses include smell (olfaction) and taste (gustation) and rely on chemoreceptors to detect molecules in air and food.
- Smell detects molecules dispersed in the air (dissolved in fluids from nasal membranes).
- Taste detects molecules dissolved in saliva from food.

Odor refers to gaseous molecules we perceive as smell.
These molecules interact with the olfactory epithelium located at the roof of the nasal cavity which is composed of olfactory sensory neurons.
- Nose hairs filter some of the molecules.
When molecules bind to receptors, action potentials are generated down the olfactory nerve, traveling to the ethmoid bone and then to the olfactory bulb, which connects to the brain.
- The sensory signals travel down the olfactory tract to the olfactory cortex, and then to the frontal lobe for identification and along the emotional pathways in the limbic system.

Gustatory Epithelial Cells: Taste receptor cells detect taste.
Basal Epithelial Cells: Stem cells that regenerate gustatory epithelial cells, especially after injuries such as burning the tongue.
Taste Buds: The sensory organ for taste, primarily found within papillae (the bumps on the tongue).
Taste receptor cells generate action potentials that signal the gustatory cortex of the brain, resulting in the release of digestive enzymes to break down food, converting it into energy.

The ear serves as the sensory organ for hearing and balance (equilibrium).
Ears can receive mechanical waves and convert them into nerve signals understood by the brain.
Sound vibrations create air pressure changes that hit the eardrums, causing small bones (auditory ossicles) to move fluid against membranes, stimulating hair cells, which trigger action potentials sent to the brain.

The ear is composed of three parts:
1. Outer/External Ear: For sound reception
2. Middle Ear: Functions in hearing
3. Inner Ear: Involved in both hearing and maintaining equilibrium

Pinna (Auricle): The visible part of the ear made of cartilage.
External Acoustic Meatus: The auditory canal that channels sound waves deeper into the ear, leading to the tympanic membrane (eardrum).
The tympanic membrane is a connective tissue boundary between the external and middle ear.

The middle ear, also termed the tympanic cavity, acts as a relay station between the outer and inner ear.
Contains three tiny bones known as the auditory ossicles:
- Malleus (Hammer)
- Incus (Anvil)
- Stapes (Stirrup)
The function of the ossicles is to amplify sound waves before they reach the inner ear.
Vibrations conducted by the ossicles travel to the oval window, setting the inner ear fluid in motion.
The Eustachian Tube connects the middle ear to the pharynx, assisting in equalizing pressure around the eardrum.

The inner ear comprises a labyrinth (both bony and membranous), filled with fluid that helps in conducting sound vibrations and maintaining equilibrium.
Its primary function is to convert physical vibrations into electrical impulses (action potentials) for transmission to the brain.
Components in the inner ear:
- Semicircular Canals: Help in maintaining balance during head rotations.
- Cochlea: Houses hair cells that vibrate at various frequencies, stimulating the organ of Corti, which sends action potentials through the cochlear nerve to the auditory cortex in the brain.
- Vestibule: A key structure for maintaining balance, as head movements stimulate fluid which then activates hair cells, sending action potentials through the vestibular nerve to the brain.