Anatomy of the Human Eye

Vision

• The cornea is the clear front surface of the eye that contributes to light focusing.
• The iris is the colored part of the eye; controls pupil size based on light.
• The pupil is the opening in the iris center, which allows light to enter the eye and focus on the retina.
• The sclera is the white of the eye providing structural support and protection.
• The conjunctiva lines the sclera and secretes mucus and tears.
• Aqueous humor is a clear fluid between the cornea and the lens.
• The lens is a converging and biconvex transparent structure that focuses light on the retina.
• The lens is attached to the ciliary body by ligaments.
• Vitreous humor is a jelly-like substance of proteins and collagen that fills the space between the lens and the retina.
• Vitreous humor maintains the spherical shape of the eye.
• The retina is the light-sensitive tissue at the back of the eye with photoreceptors.
• The retina contains ganglion, bipolar, and photoreceptor cells (rods and cones)
• The retina converts light into electric signals.
• The optic nerve transmits electrical signals from the retina to the brain.
• Light goes through the cornea and pupil which then adjust in size.
• The lens focuses the light onto the retina.
• Retinal photoreceptor cells (rods and cones) convert light into electrical signals.
• Rods help handle low-light vision, and cones handle color and detail.
• Electrical signals are transmitted through the optic nerve to the brain.
• The occipital lobe in the brain processes the signals, creating a visual image.
• The cornea and lens refract light rays to focus them onto the retina, which is essential for clear vision.
• Rods are sensitive to light, enabling vision in dim conditions, but do not perceive color.
• Cones function best in bright light and provide color vision and sharp detail.
• Rods are for low-light vision, while cones are for bright-light and color vision.
• Rods have high sensitivity to light, while cones have low sensitivity.
• Rods do not enable color vision, while cones enable red, green, and blue color vision.
• Rods have low acuity (blurry vision), while cones have high acuity (sharp vision).
• Rods are primarily in the periphery of the retina, while cones are concentrated in the fovea.
• There are approximately 120 million rods and 6 million cones.
• Rods work best in dim light and cones work best in bright light.
• Rods are better at motion detection compared to cones.
• Cones, bipolar neurons, and ganglion cell axons form the optic nerve.
• The optic nerve carries signals from the retina to the optic chiasm & optic tract.
• The signals then travel to the thalamus, specifically the lateral geniculate nucleus (LGN).
• Signals undergo optic radiations from the LGN and are sent to the primary visual cortex.
• Light triggers hyperpolarization in the photoreceptor.
• Hyperpolarization reduces neurotransmitter release from the photoreceptor.
• Reduced neurotransmitter release leads to depolarization of the bipolar cell.
• Depolarization of the bipolar cell increases neurotransmitter release.
• Increased neurotransmitter release excites the ganglion cell, which fires action potentials.
• Action potentials are sent to the brain via the optic nerve.
• Ganglion cell axons form the optic nerve, carrying signals to the lateral geniculate nucleus (LGN) in the thalamus.
• Optic nerves cross at the optic chiasm sending information from each visual field to the opposite brain hemisphere.
• LGN neurons send signals via optic radiations to the primary visual cortex (V1) in the occipital lobe.
• V1 is the first cortical area for visual processing and is essential for conscious vision.
• Visual information moves from the striate cortex (V1) to the extrastriate cortex and then to the visual association cortex.
• The extrastriate cortex has specialized regions for orientation, movement, color, and depth.
• V1 sends information to V2, which then sends the information to other parts of the brain.
• The ventral pathway (parvocellular input) handles shape details.
• The ventral pathway (magnocellular input) handles movement.
• A mixed magnocellular/parvocellular path handles brightness, color, and some shape.
• The visual cortex processes visual information through the "what" and "where" streams.
• The "what" stream (ventral stream) handles object recognition and identification.
• The "where" stream (dorsal stream) handles spatial processing and motion detection.

Audition

• The outer, middle, and inner ear each play a crucial role in processing sounds.
• The outer ear includes the auricle and ear canal, collects sound waves, and amplifies them, which then causes the eardrum to vibrate.
• The middle ear contains the malleus, incus, and stapes, which amplify sound waves, with the stapes bone attaching to the oval window.
• The Eustachian tube equalizes pressure between the outside air and the middle ear.
• The inner ear contains the cochlea, which transforms vibrations into electrical impulses.
• The electrical impulses then travel along the auditory nerve to the brain.
• The cochlea contains 25,000 nerve endings set into motion by fluid movement.
• The vestibular organ in the inner ear is responsible for balance.
• Frequency, measured in Hertz (Hz), determines the pitch of a sound.
• Amplitude, measured in decibels (dB), determines the loudness of a sound.
• Timbre is the unique quality of a sound that distinguishes it from others.
• The pinna (outer ear) collects sound waves and funnels them into the auditory canal.
• Sound waves travel through the auditory canal to the tympanic membrane (eardrum), causing it to vibrate.
• The vibrations of the eardrum are transmitted to the ossicles (malleus, incus, and stapes).
• The ossicles amplify the sound.
• The stapes pushes against the oval window of the inner ear.
• The cochlea is a spiral-shaped, fluid-filled structure also contained in the inner ear.
• The basilar membrane vibrates in response to fluid movement inside the cochlea.
• Tiny hair cells, located on the basilar membrane, bend as the membrane vibrates.
• This bending converts mechanical vibrations into electrical signals.
• Audition is the process of converting sound waves into electrical signals and then processing them in the brain.
• The auditory nerve carries the electrical signals from the cochlea to the brain.
• The signals travel through the brainstem, the thalamus (medial geniculate nucleus, MGN), and to the auditory cortex.
• The auditory cortex in the temporal lobe interprets the signals as sounds.
• The vestibular system, located in the inner ear, controls balance and spatial orientation.
• The vestibular system consists of the semicircular canals and the otolith organs/vestibular sacs (utricle and saccule).
• The semicircular canals are filled with fluid and detect rotational movements.
• The otolith organs detect linear acceleration and gravity.
• Three canals at the base of which is the ampulla.
• Ampulla houses the crista, the sensory epithelium.
• Crista contains hair cells (sensory receptors), which are similar to those in the cochlea.
• Cilia of these cells are embedded in a gelatinous structure called the cupula.
• The cupula detects head movements and changes in orientation.
• Vestibular and cochlear nerves (hearing) are branches of the vestibulocochlear nerve.
• Most vestibular nerve axons synapse in the vestibular nuclei in the medulla.
• Some axons go directly to the cerebellum, spinal cord, medulla, and pons.
• Cortical projections are responsible for feelings of dizziness, nausea, and vomiting.
• Projections to brainstem nuclei that control neck muscles maintain head uprightness.
• The vestibular system directly controls eye movements to compensate for head movement, ensuring stable vision.
• The vestibular nerve carries signals from the vestibular system to the brainstem.
• The signals are then relayed to various brain regions, including the cerebellum.
• The pathway also projects to the thalamus and then the cerebral cortex.
• The cochlea converts sound vibrations into electrical signals via the basilar membrane.
• Different frequencies of sound cause different parts of the basilar membrane to vibrate, allowing the ability to distinguish different pitches.

Smell & Taste

• Smell relies on chemoreceptors, which are specialized neurons that detect airborne chemical molecules (odorants).
• Olfactory epithelium is located in the nasal cavity and contains olfactory receptor neurons (ORNs) embedded in the nasal mucosa.
• Each ORN expresses one type of olfactory receptor protein.
• Binding of odorants activates G protein-coupled receptors, increasing cAMP, opening ion channels and causing depolarization.
• The axons of ORNs converge to form the olfactory nerve (cranial nerve I).
• The olfactory nerve synapses in the olfactory bulb.
• Within the olfactory bulb, ORN axons synapse with mitral cells in glomeruli.
• Mitral cells refine and amplify olfactory signals and form the olfactory tract.
• The olfactory tract projects to the piriform cortex, amygdala, and entorhinal cortex.
• These regions are involved in odor identification, emotional responses, and odor-related memory.
• Progenitor cells (POC stem cells) in the olfactory epithelium continuously regenerate ORNs.
• Nasal mucosa inflammation can cause transient anosmia by blocking odorants from reaching ORNs.
• Smell is highly sensitive and influenced by concentration, context, and individual differences.
• The brain can recognize and discriminate thousands of odors, adapt to constant odors (olfactory adaptation) and analyze complex odor mixtures.
• Taste relies on chemoreceptors located in taste buds.
• Taste buds are located on the tongue, soft palate, and pharynx and contain gustatory cells.
• Gustatory cells have microvilli that project into taste pores.
• Taste buds also contain basal stem cells that replenish gustatory cells.
• Taste molecules must dissolve in saliva to reach taste pores.
• Salt is detected by Na+ ion channels.
• Sour is detected by H+ ions.
• Sweet is detected by G protein-coupled receptors that bind sugars.
• Bitter is detected by G protein-coupled receptors that bind various bitter compounds.
• Umami is detected by G protein-coupled receptors that bind glutamate (protein).
• Binding of taste molecules triggers depolarization of gustatory cells, releasing neurotransmitters.
• Gustatory signals are carried by the facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves to the gustatory nucleus.
• From the gustatory nucleus, signals travel to the thalamus and then to the gustatory cortex in the insular cortex.
• The gustatory cortex processes taste information.
• Taste preferences are influenced by genetics, experience, and cultural factors.
• The mouth also has thermoreceptors & nocioreceptors (chili=pain)
• Smell contributes significantly to flavor perception.
• Taste preferences can be modified through learning and experience.
• Odorants activate chemoreceptors in the olfactory epithelium.
• ORNs transmit signals via CN I to the olfactory bulb.
• Mitral cells in the olfactory bulb refine signals.
• Signals are sent to the piriform cortex, amygdala, and entorhinal cortex.
• POC stem cells regenerate ORNs.
• Nasal mucosa inflammation causes transient anosmia.
• The brain recognizes, adapts to, and analyzes odors.
• Taste molecules dissolve and diffuse to taste pores.
• Gustatory cells detect salt (Na+), sour (H+), sweet, bitter, and umami.
• Signals are transmitted via CN VII, IX, and X to the gustatory cortex.
• Taste preferences are influenced by genetics, experience, and culture.
• Smell significantly contributes to flavor.
• Taste preferences can be learned.
• Both use chemoreceptors.
• Both are heavily integrated with the brain.
• Both are extremely important for survival.

Skin and Tactile Sensation

• The skin is the largest organ, with multiple layers enabling the perception of touch, temperature, and pain.
• The epidermis is the outermost layer and primarily made up of keratinocytes and provides a protective barrier.
• The dermis is below the epidermis, containing connective tissue, blood vessels, hair follicles, and sensory receptors.
• The hypodermis is the deepest layer, with adipose tissue providing insulation and cushioning.
• Mechanoreceptors respond to mechanical stimuli like pressure, vibration, and stretch.
• Meissner's corpuscles are located in the dermal papillae for sensitivity to light touch and low-frequency vibrations (rapidly adapting).
• Pacinian corpuscles are deep in the dermis and hypodermis that are sensitive to deep pressure and high-frequency vibrations (rapidly adapting).
• Merkel's disks are in the basal epidermis and are sensitive to sustained light touch and pressure (slowly adapting).
• Ruffini endings are in the dermis and are sensitive to skin stretch (slowly adapting).
• Thermoreceptors detect temperature changes and respond to both warmth and cold.
• Chemoreceptors respond to chemical stimuli (e.g., itch, irritation).
• Nociceptors detect tissue damage and pain.
• Bending of the nerve ending in the Pacinian corpuscle causes mechanical pressure which deforms the membrane.
• This deformation stretches the corpuscle's membrane, opening mechanically gated ion channels.
• Influx of ions depolarizes the sensory neuron, and if threshold is reached, an action potential is generated.
• The action potential travels along the sensory nerve fiber to the central nervous system.
• Thermoreceptors respond to changes in skin temperature which trigger internal and external responses to maintain body temperature.
• The lateral spinothalamic tract transmits pain and temperature information.
• Sensory neurons enter the spinal cord, synapse in the dorsal horn, and cross to the opposite side.
• They then ascend to the thalamus and project to the somatosensory cortex.
• Sensory neurons enter the spinal cord, ascend in the dorsal columns, and synapse in the brainstem.
• They cross to the opposite side, ascend to the thalamus, and project to the somatosensory cortex.
• Pain originates from noxious stimuli such as mechanical, thermal, and chemical stimuli.
• Chemical mediators such as prostaglandins, serotonin, substance P, potassium, and histamine are released from damaged cells.
• Nociceptors are sensory nerve endings that detect painful or noxious stimuli.
• C fibers are unmyelinated, small, and conduct impulses slowly and respond to thermal, mechanical, and chemical stimuli and produce dull, diffuse, aching, burning, and delayed pain.
• A-delta fibers are myelinated, larger, and conduct impulses rapidly and respond to mechanical (pressure) stimuli and produce sharp, localized, fast pain.
• Primary sensory neurons conduct painful sensations from the periphery to the dorsal root of the spinal cord.
• Secondary sensory neurons transmit painful sensations to the thalamus.
• Tertiary sensory neurons transmit painful sensations from the thalamus to the somatosensory areas of the cerebral cortex.
• The spinothalamic tract is a major central pain pathway from the spinal cord to the thalamus that transmits sensory information related to pain, temperature, and crude touch.
• First-order neurons have cell bodies in the dorsal root ganglion, splitting into peripheral and central branches.
• Second-order neurons have cell bodies in the Rexed laminae, decussate, and ascend in the spinothalamic tract to the thalamus.
• Third-order neurons have cell bodies in the thalamus and project to the primary somatosensory cortex.
• Noxious stimuli is detected by nociceptors.
• Nerve impulses travel to the spinal cord.
• Impulses are shunted to the brain via nerve tracts in the spinal cord and brainstem.
• The brain then processes the pain sensation and initiates a motor response to cease the action causing the pain.
• The gate control theory suggests that a neurological gate in the spinal cord controls the transmission of pain signals to the brain.
• Nerve gates in the spinal cord determine whether the pain messages pass through easily or are blocked.
• Large-diameter fibers (A-beta fibers) carry non-painful touch sensations and inhibit pain signals, closing the gate.
• Small-diameter fibers (A-delta and C fibers) carry pain signals and open the gate.
• Psychological factors influence the gate.
• Afferent nerve fibers carries pain signals from the periphery to the spinal cord.
• The dorsal horn is where the gate is located and the first synapse occurs.
• A second-order neuron transmits signals from the spinal cord to the thalamus.
• The thalamus relays signals to the cortex, where pain is consciously perceived.
• A first order neuron is the afferent nerve fiber.
• A second order neuron is the neuron from the dorsal horn to the thalamus.
• A third order neuron is the neuron from the thalamus to the cortex.
• Nociceptive pain is the normal type of pain arising from actual or potential tissue damage from a specific stimulus.
• Neuropathic pain results from damage to the nervous system itself and can be chronic and difficult to treat.
• Nociceptive pain is caused by tissue damage, while neuropathic pain is caused by nerve damage.
• Nociceptive pain is a response to a specific stimulus, while neuropathic pain can occur without any external stimulus.
• Nociceptive pain is typically sharp and localized, while neuropathic pain can be burning, shooting, or tingling.
• Fast-twitch fibers produce rapid, powerful contractions but fatigue quickly.
• Slow-twitch fibers produce less forceful contractions but are more resistant to fatigue.
• Intermediate fibers have characteristics of both fast- and slow-twitch fibers.
• Slow-twitch and intermediate fibers are relied upon for nonstrenuous activities.
• Slow-twitch fibers are aerobic, while fast-twitch fibers are anaerobic.
• Proprioceptors are sensory receptors that provide information about body position, movement, and muscle tension.
• Muscle spindles detect muscle stretch, while Golgi tendon organs detect muscle tension.
• Joint kinesthetic receptors detect joint angle and movement.
• Proprioceptive information travels along ascending pathways to the brain.
• The information is processed at the circuit level (spinal cord) and perceptual level (cortical sensory centers).
• Reticular formation and medulla, thalamus, somatosensory cortex, cerebellum, and motor cortex is where information arrives.