David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu C – Il corpo umano Sensory Systems

How do sensory cells convert stimuli into action potentials? How do sensory systems detect chemical stimuli? How do sensory systems detect mechanical forces? How do sensory systems detect light? Sensory Systems

Sensory cells transduce physical and chemical stimuli into neuronal signals. Sensory transduction: begins with a receptor protein that can detect a specific stimulus. The receptor protein opens or closes ion channels in the membrane, changing the resting potential. Sensory Systems - How do sensory cells convert stimuli into action potentials?

Sensory receptor proteins can be: ionotropic - ion channels themselves or directly affect ion channels (mechanoreceptors, thermoreceptors, electrosensors); metabotropic - affect ion channels through G proteins and second messengers (chemoreceptors, photoreceptors). Sensory Systems - How do sensory cells convert stimuli into action potentials?

Sensory receptor cells transduce energy of a stimulus into a change in membrane potential. Receptor potential generates action potentials in the receptor cell, or causes release of a neurotransmitter. Sensory Systems - How do sensory cells convert stimuli into action potentials?

Stimulating a sensory cell produces a receptor potential

Sensory Systems - How do sensory cells convert stimuli into action potentials? Sensation depends on which part of the CNS receives the sensory messages. Intensity of sensation is coded as the frequency of action potentials. Some sensory cells transmit information to the brain about internal conditions, but there is no conscious sensation.

Some sensory receptor cells are organized with other cells in sensory organs. Sensory systems include sensory cells, associated structures, and neuronal networks that process the information. Sensory Systems - How do sensory cells convert stimuli into action potentials?

Adaptation: diminishing response to repeated stimulation. Enables animals to ignore background conditions but remain sensitive to changing or new stimuli. Some sensory cells don’t adapt (e.g., mechanoreceptors for balance). Sensory Systems - How do sensory cells convert stimuli into action potentials?

Chemoreceptors: receptor proteins that bind various ligands; responsible for taste and smell. Also monitor internal environment, such as CO 2 levels in blood. Sensory Systems - How do sensory systems detect chemical stimuli?

Olfaction is the sense of smell. Olfactory sensors are embedded in epithelial tissue at top of nasal cavity (in vertebrates). Axons extend to the olfactory bulb in the brain, dendrites end in olfactory hairs on the nasal epithelium. Odorant: a molecule that binds to a receptor protein on the olfactory cilia. Olfactory receptor proteins are specific for particular odorants. Sensory Systems - How do sensory systems detect chemical stimuli?

Olfactory receptors communicate directly with the brain

Gustation is the sense of taste. Taste buds: clusters of chemoreceptors Human taste buds are embedded in the tongue epithelium, on the papillae. Some fish have taste buds on the skin; duck-billed platypus has taste buds on the bill. Sensory Systems - How do sensory systems detect chemical stimuli?

Taste buds are clusters of sensory cells

Tongue epithelium is shed and replaced at a rapid rate. Taste bud cells last about 10 days. Neurons form new synapses with new taste bud cells as they are formed. Humans taste salty, sour, sweet, bitter, and umami: savory, meaty taste from receptors for amino acids. Full complexity of taste involves both gustatory and olfactory receptors. Sensory Systems - How do sensory systems detect chemical stimuli?

Mechanoreceptors: sensory cells that respond to mechanical forces. Physical distortion of the plasma membrane causes ion channels to open, this changes membrane potential and leads to action potentials. The skin has diverse mechanoreceptors. Merkel’s discs: adapt slowly; continuous information. Meissner’s corpuscles: adapt quickly: information about change. Sensory Systems - How do sensory system detect mechanical forces?

The skin feels many sensations

Ruffini endings: deeper, adapt slowly, react to vibrating stimuli of low frequencies. Pacinian corpuscles: deep, adapt rapidly, react to vibrating stimuli at high frequencies. Sensory Systems - How do sensory system detect mechanical forces?

Mechanoreceptor cells constantly provide information on position of limbs and stresses on muscles and joints. Essential for maintaining posture and coordinating movement. Muscle spindles are mechanoreceptors in muscle cells: stretch receptors. When muscle is stretched, action potentials are generated in neuron. CNS adjusts strength of contraction to match load on muscle. Golgi tendon organ: in tendons and ligaments. Provides information about the force generated by muscle; prevents muscle tearing. Sensory Systems - How do sensory system detect mechanical forces?

Stretch receptors

Sensory Systems - How do sensory system detect mechanical forces? Auditory systems use mechanoreceptors to convert pressure waves to receptor potentials. Human ears: pinnae collect sound waves and direct them to the auditory canal. The tympanic membrane covers the end of the auditory canal and vibrates in response to pressure waves.

Sensory Systems - How do sensory system detect mechanical forces? Structures of the human ear

Sensory Systems - How do sensory system detect mechanical forces? Middle ear: air filled cavity, open to the throat via the eustachian tube, it equilibrates air pressure between middle ear and the outside. Ossicles: malleus, incus, stapes: transmit vibrations of tympanic membrane to the oval window. The ossicles translate vibration of tympanic membrane to smaller movement but greater force at the smaller oval window. Movement of oval window is translated into pressure changes in the fluid-filled inner ear.

Sensory Systems - How do sensory system detect mechanical forces? Inner ear includes the cochlea (tapered and coiled chamber composed of three parallel canals separated by two membranes), Reissner’s membrane, and basilar membrane. Organ of Corti sits on the basilar membrane: transduces pressure waves into action potentials. Hair cells: mechanoreceptors in organs of hearing and balance. They have microvilli called stereocilia. Bending of stereocilia open or close ion channels. When the plasma membrane is depolarized, neurotransmitters are released.

Sensory Systems - How do sensory system detect mechanical forces? Hair cells have mechanosensors on their stereocilia

Sensory Systems - How do sensory system detect mechanical forces? Mammalian inner ear has three semicircular canals at angles to each other; sense position and orientation of head. The vestibular apparatus has two chambers that sense position of head and acceleration.

Sensory Systems - How do sensory system detect mechanical forces? Organs of equilibrium (part 1)

Sensory Systems - How do sensory system detect mechanical forces? Organs of equilibrium (part 2)

Sensory Systems - How do sensory system detect light? Photosensitivity: sensitivity to light. A range of animal species from simple to complex can sense and respond to light. All use the same pigments: rhodopsins. Rhodopsin: opsin (a protein) and a light- absorbing group; it sits in plasma membrane of a photoreceptor cell.

Sensory Systems - How do sensory system detect light? In vertebrate eyes, the retinal and opsin separate, this is called bleaching. Rod cell: one type of vertebrate photoreceptor, which release neurotransmitters from base of cell that synapses with a neuron. Outer segment is a stack of plasma membrane discs packed with rhodopsin.

Sensory Systems - How do sensory system detect light? Eye anatomy

Sensory Systems - How do sensory system detect light? Sclera: tough connective tissue, becomes transparent cornea on front. Iris (pigmented): controls amount of light reaching photoreceptors; opening is the pupil. Lens: crystalline protein; focuses image – allows accommodation; changes shape. Retina: photoreceptor layer.

Sensory Systems - How do sensory system detect light? Staying in focus

Sensory Systems - How do sensory system detect light? Vertebrate photoreceptors: rod cells and cone cells. Fovea: area where cone cell density is highest Cone cells have low sensitivity to light; rod cells are responsible for night vision.

Sensory Systems - How do sensory system detect light? Rods and cones

Sensory Systems - How do sensory system detect light? The retina has five layers of cells. Light must pass through all layers before it is captured by rhodopsin. Light not captured is absorbed by a layer of pigmented epithelial cells. Layer at front consists of ganglion cells—their axons form the optic nerve. Photoreceptor cells are connected to ganglion cells via bipolar cells.

Sensory Systems - How do sensory system detect light? The retina (part 1)

Sensory Systems - How do sensory system detect light? The retina (part 2)