44.1 How Do Sensory Organs Convey Information to the Brain?

• The ability to sense a change in the environment depends on two processes:

  • Transduction, the conversion of an external stimulus to an internal signal in the form of action potentials along sensory neurons.

  • Transmission of the signal to the central nervous system (CNS).

• The first process, transduction, requires a sensory receptor cell specialized for converting light, sound, touch, or some other signal into an electrical signal.

• Sensory receptors are membrane proteins 1at are categorized by type of stimulus:

  • Mechanoreceptors respond to distortion caused by pressure.

  • Photoreceptors respond to particular wavelengths of light.

  • Chemoreceptors detect specific molecules.

  • Thermoreceptors detect changes in temperature.

  • Nociceptors sense harmful stimuli such as tissue injury.

  • Electroreceptors detect electric fields.

  • Magnetoreceptors detect magnetic fields.

44.2 Mechanoreception: Sensing Pressure Changes

• Animals have a variety of mechanisms for mechanoreception-the detection of pressure changes.

• The organ, known as a statocyst, is lined with mechanoreceptor cells and contains a small calcium-rich particle that normally rests on the bottom of the organ.

• Hair cells are mechanoreceptor cells, named for their stiff outgrowths called stereocilia.

• Many hair cells also have a single kinocilium, a true cilium that contains a 9 + 2 arrangement of microtubules.

• The mammalian ear is the site of two important mechanosensations: hearing，the sensation produced by the wavelike changes in air pressure called sound, and equilibrium, the sensation of orientation, rotation, and acceleration.

• The number of pressure waves that occur in 1 second is the frequency of the sound, reported in units called hertz (Hz), or cycles per second.

• When you hear different sound frequencies, you perceive them as different pitches.

• One of these bones, the stapes, vibrates against a membrane called the oval window, which separates the middle ear from the inner ear.

• Hair cells, forming rows in the middle chamber, are embedded in tissue at sits atop the basilar membrane.

• In addition, the hair cells' stereocilia touch yet another, smaller surface called the tectorial membrane.

• Next to the cochlea are two other inner ear structures-the semicircular canals and vestibule-that allow us to maintain equilibrium.

• The vestibule contains two dense clusters of hair cells called maculae (singular: macula) that detect linear acceleration and orientation.

• Vertigo is a medical condition that makes affected people feel as if they are spinning or their environment is.

44.3 Photoreception: Sensing Light

• The organs involved in photoreception range from simple light-sensitive eyespots in flatworms to the sophisticated, image-forming eyes of vertebrates, cephalopod mollusks, and arthropods.

• Insects have compound eyes composed of hundreds or thousands of light-sensing columns called ommatidia.

• In contrast, the simple eye-a structure with a single lens that focuses incoming light onto a layer of many receptors cells-evolved independently.

• The front of the sclera forms the cornea, a transparent sheet of connective tissue.

• The iris is a pigmented, round muscle just inside the cornea.

  • The iris can contract or expand to control the amount of light entering the eye.

• The pupil is the hole in the center of the iris.

• Light enters the eye through the cornea and passes through the pupil and a curved, clear lens.

• Early anatomists established that the photoreceptors in vertebrate eyes come in two distinct types: small rod-shaped cells and cone-shaped cells, called rods and cones, respectively.

• The membranes contain large quantities of a transmembrane protein called opsin; each opsin molecule is associated with a molecule of the pigment retinal.

• The two-molecule complex found in rod cells is called rhodopsin.

44.4 Chemoreception: Sensing Chemicals

• Chemoreception occurs when chemicals bind to chemoreceptors and thereby initiate action potentials in senso neurons.

• The sense of taste, called gustation, and the sense of smell, called olfaction, originate in chemoreceptors.

• In humans, the chemoreceptor cells that sense taste are clustered in uctures known as taste buds.

• Glutamate triggers the sensation called umami, which is the meaty taste of the molecule monosodium glutamate (MSG).

• Airborne molecules that convey information about food or 1e environment are called odorants.

• Axons from these neurons project to the olfactory bulb, the part of the brain where olfactory signals are processed and interpreted.

• Bombykol was the first chemically characterized pheromone, meaning a chemical that is secreted into the environment and that affects the behavior or physiology of animals of the same species.

44.5 Other Sensory Systems

• Animals detect heat energy by thermoreception and adjust their behaviors or physiological processes, such as shivering and sweating, in response.

• Electrogenic fishes have specialized organs near their tails that generate electric fields stronger than those of regular nerves or muscles.

Magnetoreception has been described in many groups of organisms, including bacteria, fungi, invertebrates, and all vertebrate classes.