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How the cochlea distinguishes pitch. Different frequencies of pressure waves cause different regions of the basilar membrane, which vary in width and stiffness, to vibrate, stimulating particular hair cells and sensory neurons. This is perceived on the auditory area of the cerebral cortex as different pitches of sound.

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Chemoreceptors in an insect. The antennae of the male silkworm moth Bombyx mori are covered with sensory hairs which have chemoreceptors that are sensitive to the sex pheromone released by the female.

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Transduction in the cochlea.

Vibrations of the stapes against the oval window produce pressure waves in the perilymph of the cochlea, and causes the basilar membrane to vibrate, stimulating hair cells.

The hair cells depolarizes their membranes, initiating action potentials.

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1. Percussion waves in the air created by vibrating objects cause the tympanic membrane to vibrate.
   
2. The bones of the middle ear (malleus, incus, stapes) transmit the vibrations to the oval window on the cochlea ("snail").
   
3. The vibrations cause waves in the perilymph of the cochlea.
   
4. Hair cells on the basilar membrane send action potentials via the auditory nerve to the brain.
   

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Pit vipers such as rattlesnake have a pair of infrared receptors which detect infrared radiation (heat) emitted by live prey.

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Some migrating animals, such as these beluga whales, apparently sense Earth’s magnetic field and use the information for orientation.

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Organs of equilibrium in the inner ear.
The semicircular canals, arranged in three planes, detect angular movements.		The cilia of the hair cells project into a gelatinous cap called the cupula bathed in endolymph.	/th>	When the head moves, inertia presses the endolymph against the cupula, bending the cilia and triggering action potentials.

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The sclera is the white outer layer of connective tissue.

At the front, the sclera becomes the transparent cornea, letting light into the eye.

The iris regulates amount of light entering the pupil, the hole in the center of the iris.

The iris is part of the choroid, a thin layer of pigmented cells.

The lens focuses light on the retina.

The retina lies inside the choroid and contains photoreceptors.

The fovea is the center of the visual field with a high concentration of cone cells.

Signals leave the eye at the optic disk (blind spot ) to the optic nerve.

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Focusing in the mammalian eye. Contraction of the ciliary muscles thickens the lens and bends light more sharply, focusing on near objects (accommodation).

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Sensory receptors in human skin. Receptors in the epidermis include naked dendrites and hair movement receptors in the dermis. Other receptors in the dermis are encapsulated by connective tissue.

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The interaction of muscles and skeletons in movement. Back–and–forth movement of a body part is generally accomplished by antagonistic muscles. This arrangement works with either an endoskeleton or an exoskeleton.

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The “head” of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere, using ATP for muscle-sacromere.html: 49_28SkeletMuscleStructB.jpg
The myofibrils are composed to two kinds of myofilaments: thin actin filaments and thick myosin filaments arranged in repeating sarcomeres.

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During contraction, the thick and thin filaments slide past each other, shortening the muscle fiber .

The muscle contracts when stimulated by a motor neuron.

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A skeletal muscle consists of a bundle of muscle fibers, which is a bundle of myofibrils.

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Sensory receptors in human skin. Receptors in the epidermis are naked dendrites, as are hair movement receptors wound around the base of hairs in the dermis. Most other receptors in the dermis are encapsulated by connective tissue.

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Sensory reception for motion in vertebrates. The bending of cilia in hair cells, in response to movement of the surrounding fluid, controls release of neurotransmitters to sensory neurons, which conduct action potentials to the CNS.
Hair cells detect motion in the inner ear and in lateral line systems of fishes and amphibians.

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Cellular organization of the vertebrate retina.

Each bipolar cell receives information from photoreceptors on several rods or cones, and each ganglion cell from several bipolar cells.

Horizontal and amacrine cells integrate information across the retina.

Action potentials are then propagated to the brain by the optic nerve via the optic disk (blind spot).

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Rods contain the pigment rhodopsin, which changes shape when it absorbs light, a process referred to as “bleaching.” Cones contain either red, green, or blue-sensitive opsin. Rods and cones make synapses with bipolar cells.

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Smell in humans. Odorant molecules dissolved in mucus bind to specific receptor proteins in the plasma membrane of chemoreceptors, triggering action potentials. Neurons in the olfactory bulb receive these action potentials and their axons form a bundle called the olfactory nerve.

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A bat using sonar to locate its prey.
Some bats use ultrasound to echolocate prey .

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The receptor cells for taste in humans are modified epithelial cells organized into taste buds, which are scattered in several areas of the tongue and contain many chemoreceptors.

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Sensory transduction by a sweetness receptor.

Binding of a sugar molecule to a receptor cell initiates a signal transduction pathway involving cyclic AMP and protein kinase A.

K⁺ channels in the membrane close, and the membrane depolarizes.

Voltage-gated calcium Ca²⁺ channels open, and Ca²⁺ diffuses into the receptor cell.

Synaptic vesicles release neurotransmitters, sending signals to the sensory neuron.

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Humans possess four major taste perceptions — sweet, sour, salty, and bitter. A fifth, called umami , is elicited by the amino acid glutamate.

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Neural pathways for vision.

The two optic nerves meet at the optic chiasm, where they cross to the opposite side of the brain and synapse with interneurons in the lateral geniculate nuclei of the thalamus.

Sensations are relayed to the primary visual cortex in the occipital lobe .