43.1
Overview of Sensory Receptors
- Sensory receptors are specialized cells, or the endings of sensory neurons, that detect a stimulus and convert it into an electrical signal the nervous system can use. Watch The Sensory System on YouTube, from Bozeman Science
- Sensory transduction is that conversion step. The stimulus opens or closes ion channels, producing a receptor potential — a graded potential whose size is proportional to stimulus strength. Watch The Sensory System on YouTube, from Bozeman Science
- If the receptor potential reaches threshold, the sensory neuron fires action potentials, which are all-or-none. So strength cannot be encoded in spike size — it is encoded in spike frequency and in how many receptors get recruited. Watch Peripheral somatosensation | Organ Systems | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Classified by the energy they detect: mechanoreceptors (touch, pressure, stretch, sound, gravity), chemoreceptors (taste, smell, blood chemistry), thermoreceptors (heat/cold), photoreceptors (light), electroreceptors (electric fields), magnetoreceptors, and nociceptors (tissue damage = pain). Watch Peripheral somatosensation | Organ Systems | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Classified by location: exteroceptors monitor the external environment; interoceptors monitor internal conditions.
- Important interoceptors: proprioceptors (muscle spindles sense stretch, Golgi tendon organs sense tension) tell you where your limbs are without looking; baroreceptors in the aortic arch and carotid sinus sense blood pressure. Watch Neurology | Spinal Cord: Stretch Reflex | Muscle Spindle on YouTube, from Ninja Nerd
- The labeled line principle: a given sensory pathway always produces the same sensation regardless of how it is stimulated, because the brain interprets the source, not the signal. Pressure on the eye is perceived as light.
- Sensory adaptation — receptors stop responding to an unchanging stimulus. Phasic (fast-adapting) receptors signal change: light touch, smell. Tonic (slow-adapting) receptors keep reporting: pain, proprioception, blood pressure. Watch Sensory adaptation | Processing the Environment | MCAT | Khan Academy on YouTube, from khanacademymedicine
43.2
Mechanoreceptors: Touch, Pressure, and Body Position
- Skin receptors sit at different depths and adapt at different speeds, which is why touch feels layered. Watch Overview of Sensation and Meissner's Corpuscle | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Meissner corpuscles — shallow, fast-adapting, light touch and fine texture. Dense in fingertips and lips. Watch Overview of Sensation and Meissner's Corpuscle | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Merkel cells — shallow, slow-adapting, sustained light touch, edges and shapes.
- Pacinian corpuscles — deep, onion-like layered capsule, very fast adapting, deep pressure and vibration. The layers are why they respond only to a change in pressure.
- Ruffini endings — deep, slow-adapting, skin stretch and sustained pressure.
- Free nerve endings — no capsule; pain (nociception) and temperature. Watch Peripheral somatosensation | Organ Systems | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Two-point discrimination — how far apart two points must be to feel like two — measures receptor density. Fingertips resolve a couple of millimetres; the back needs centimetres.
- Muscle spindles lie parallel to muscle fibers and detect stretch; they drive the knee-jerk stretch reflex. Golgi tendon organs sit in tendons and detect tension, protecting against overload. Watch Neurology | Spinal Cord: Stretch Reflex | Muscle Spindle on YouTube, from Ninja Nerd
43.3
Hearing, Vibration, and Balance
- All of these run on one cell type: the hair cell, a mechanoreceptor topped with stereocilia. Watch 2-Minute Neuroscience: The Cochlea on YouTube, from Neuroscientifically Challenged
- Bending the stereocilia toward the tallest opens ion channels → depolarization → more neurotransmitter released. Bending away closes them → hyperpolarization. So hair cells signal direction, not just presence. Watch 2-Minute Neuroscience: The Cochlea on YouTube, from Neuroscientifically Challenged
- Fish and aquatic amphibians have a lateral line system: hair cells in neuromasts, capped by a gelatinous cupula, detect water movement and pressure waves — a sense of "distant touch". Watch Ancestral & Weird Senses: Crash Course Zoology #8 on YouTube, from CrashCourse
- Outer ear: the pinna funnels sound down the auditory canal to the tympanic membrane (eardrum). Watch Hearing & Balance: Crash Course Anatomy & Physiology #17 on YouTube, from CrashCourse
- Middle ear: three ossicles in order — malleus (hammer) → incus (anvil) → stapes (stirrup) — transfer vibration to the oval window. Amplification happens because force from the large eardrum is concentrated onto the much smaller oval window. The Eustachian tube equalizes pressure with the throat. Watch Hearing & Balance: Crash Course Anatomy & Physiology #17 on YouTube, from CrashCourse
- Inner ear: the cochlea, a coiled tube of three fluid chambers — scala vestibuli, scala media (cochlear duct), and scala tympani. The round window bulges to release the pressure wave. Watch 2-Minute Neuroscience: The Cochlea on YouTube, from Neuroscientifically Challenged
- The organ of Corti sits on the basilar membrane; its hair cells brush against the overlying tectorial membrane when the membrane vibrates, and that shearing is what bends the stereocilia. Watch 2-Minute Neuroscience: The Cochlea on YouTube, from Neuroscientifically Challenged
- Pitch is coded by place (tonotopy). The basilar membrane is narrow and stiff at the base → resonates to high frequencies; wide and flexible at the apex → low frequencies. Loud sounds simply bend hair cells further, raising firing rate. Watch Hearing & Balance: Crash Course Anatomy & Physiology #17 on YouTube, from CrashCourse
- Hearing loss from loud noise is hair-cell death, and hair cells do not regenerate in mammals.
- Balance uses the vestibular apparatus: the utricle and saccule contain otoliths (calcium carbonate crystals) on a gelatinous membrane that shifts with gravity and linear acceleration; the three semicircular canals, each in a different plane, contain a cupula in the ampulla that deflects with rotational acceleration. Watch 2-Minute Neuroscience: Vestibular System on YouTube, from Neuroscientifically Challenged
- Both hearing and balance travel on cranial nerve VIII, the vestibulocochlear nerve. Watch 2-Minute Neuroscience: Vestibular System on YouTube, from Neuroscientifically Challenged
43.4
Chemoreception: Taste and Smell
- Chemoreception is the oldest and most universal sense — even bacteria do it.
- Taste buds sit in papillae on the tongue. Each bud holds many receptor cells with microvilli exposed to dissolved chemicals. Watch Taste & Smell: Crash Course Anatomy & Physiology #16 on YouTube, from CrashCourse
- Five basic tastes: sweet, sour, salty, bitter, umami (savory/glutamate). The old "tongue map" of separate zones is wrong — all regions detect all five. Watch Taste & Smell: Crash Course Anatomy & Physiology #16 on YouTube, from CrashCourse
- Mechanism split worth memorizing: salty (Na+) and sour (H+) act directly on ion channels. Sweet, bitter, and umami act through G-protein-coupled receptors using the G protein gustducin and a second-messenger cascade.
- Bitter detection is protective — many plant toxins are bitter — which is why humans have many bitter receptor genes but only a couple for sweet.
- Olfactory receptors are bipolar neurons in the olfactory epithelium at the roof of the nasal cavity. Odorants must dissolve in mucus to be detected. Watch 2-Minute Neuroscience: Olfaction on YouTube, from Neuroscientifically Challenged
- Mammals have roughly 1000 olfactory receptor genes (Buck and Axel, Nobel Prize 2004) and each neuron expresses one receptor type. Neurons with the same receptor converge on the same glomerulus in the olfactory bulb, giving a combinatorial code — a few hundred receptors distinguish thousands of smells. Watch 2-Minute Neuroscience: Olfaction on YouTube, from Neuroscientifically Challenged
- Smell is the exception to thalamic relay: olfactory signals reach the olfactory cortex and limbic system directly, without a thalamic relay first. That direct limbic link is why smells trigger memory and emotion so strongly. Watch 2-Minute Neuroscience: Olfaction on YouTube, from Neuroscientifically Challenged
- Many vertebrates also have a vomeronasal organ (Jacobson's organ) dedicated to pheromones.
- Most of what you call "flavor" is smell — which is why food is bland when you are congested. Watch Taste & Smell: Crash Course Anatomy & Physiology #16 on YouTube, from CrashCourse
43.5
Vision: Photoreceptors
- Eye designs: eyespots in flatworms detect light direction only; compound eyes in arthropods are built of many ommatidia, each sampling one point, producing a mosaic image excellent at detecting motion; single-lens eyes in vertebrates and cephalopods focus a true image — a classic case of convergent evolution. Watch Ancestral & Weird Senses: Crash Course Zoology #8 on YouTube, from CrashCourse
- Vertebrate eye path of light: cornea → aqueous humor → pupil (sized by the iris) → lens → vitreous humor → retina. Watch Vision: Crash Course Anatomy & Physiology #18 on YouTube, from CrashCourse
- Focusing (accommodation) is done by ciliary muscles changing lens shape in mammals — contracting rounds the lens for near objects. Watch Vision: Crash Course Anatomy & Physiology #18 on YouTube, from CrashCourse
- Retina landmarks: the fovea is a cone-packed pit giving sharpest vision; the optic disc, where the optic nerve exits, has no photoreceptors and is the blind spot. The choroid nourishes the retina and the sclera is the tough white outer coat. Watch Vision: Crash Course Anatomy & Physiology #18 on YouTube, from CrashCourse
- The vertebrate retina is "inside out": light passes through ganglion cells and bipolar cells before reaching the rods and cones at the back. Signal then travels back out: photoreceptor → bipolar cell → ganglion cell → optic nerve. Watch Photoreceptors (rods vs cones) | Processing the Environment | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Rods: over 100 million per eye, extremely light-sensitive, no color information, concentrated in the periphery — night and peripheral vision. Pigment is rhodopsin = retinal (from vitamin A) bound to the protein opsin. Watch Photoreceptors (rods vs cones) | Processing the Environment | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Cones: about 3 million, need bright light, give color and high acuity, densest in the fovea. Three types tuned to roughly blue (~420 nm), green (~530 nm), and red (~560 nm) — color is computed from the ratio of their responses. Watch Photoreceptors (rods vs cones) | Processing the Environment | MCAT | Khan Academy on YouTube, from khanacademymedicine
- Phototransduction — the step most exams target: light isomerizes 11-cis retinal to all-trans retinal → activates the G protein transducin → activates phosphodiesterase → breaks down cGMP → cGMP-gated Na+ channels close → the photoreceptor hyperpolarizes → it releases less glutamate. In the dark the cell is depolarized and releasing steadily. Light turns the signal DOWN, not up. Watch 2-Minute Neuroscience: Phototransduction on YouTube, from Neuroscientifically Challenged
- Visual pathway: optic nerve → optic chiasm (where nasal fibers cross, so each hemisphere sees the opposite visual field) → lateral geniculate nucleus of the thalamus → primary visual cortex in the occipital lobe. Watch Vision: Crash Course Anatomy & Physiology #18 on YouTube, from CrashCourse
- Two forward-facing eyes give overlapping fields → binocular vision and depth perception, typical of predators; laterally placed eyes give a wide field for spotting predators.
- Red–green color blindness is X-linked recessive, so it is far more common in males. Vitamin A deficiency causes night blindness because retinal cannot be made. Watch 2-Minute Neuroscience: Phototransduction on YouTube, from Neuroscientifically Challenged
43.6
Other Vertebrate Sensory Systems
- Infrared / heat sensing: pit vipers, pythons, and boas have pit organs between eye and nostril that image infrared radiation and can detect differences of a fraction of a degree — they hunt warm prey in total darkness. Watch Ancestral & Weird Senses: Crash Course Zoology #8 on YouTube, from CrashCourse
- Electroreception: sharks and rays use the ampullae of Lorenzini, jelly-filled canals that detect the tiny electric fields of muscle activity in hidden prey. Some fish also generate their own field and read distortions in it (electrolocation). Watch Ancestral & Weird Senses: Crash Course Zoology #8 on YouTube, from CrashCourse
- Magnetoreception: many migrating birds, sea turtles, and some fish orient to Earth's magnetic field, in some species using crystals of magnetite. Watch Ancestral & Weird Senses: Crash Course Zoology #8 on YouTube, from CrashCourse
- These are all reminders that the human sensory world is only a subset of what is detectable.