The Senses

Lecture Outline

I.What are sensory receptors?

A.Sensory receptors change environmental stimuli into nerve impulses.

  • Refer to Table 33.1 for a list of the different types of environmental stimuli

  • In order to be detected, an environmental stimulus must be strong enough to open ion channels in the membranes of receptor cells, thus depolarizing them and initiating a generator potential
  • A generator potential may initiate an action potential in the sensory neurons that synapse with the sensory receptor, and the impulse is transmitted to the brain
  • Impulses conducted to specific sensory areas of the cerebral cortex produce conscious sensations
  • The special senses include sight, hearing, taste and smell
  • General somatic senses include receptors for heat, cold, pain, and pressure
  • Receptors that sense the internal environment are located within the walls of blood vessels
  • Receptors that provide information about body position and movement are located in the muscles, tendons, joints, and inner ear

II.How do animals sense their internal and external environments and their position in space?

  • Receptors within blood vessels and organs sense the internal environment.
  • Temperature-sensitive neurons in the hypothalamus, act as the body’s thermostat, constantly monitoring the temperature of the blood
  • Homeostatic mechanisms include shivering or sweating, contracting or dilating the blood vessels in the skin, and an increase or decrease in cellular respiration (which produces heat)
  • Other receptors located in the walls of arteries are sensitive to the levels of carbon dioxide and oxygen in the blood, as well as its pH
  • Signals are sent to respiratory centers in the medulla and pons that regulate the rate of respiration
  • Pain receptors are widely distributed throughout the body

B.Receptors within skeletal muscles, tendons, and the inner ear sense position in space.

  • Proprioceptors sense the position of the body in space
  • Muscle spindles are specialized muscle cells that keep track of the degree to which the muscles are contracted
  • A stretch receptor is wrapped around the muscle spindle that detects muscle stretching
  • Other stretch receptors are found in vertebrate tendons, and in the tissue surrounding joints

C.Mechanoreceptors detect stimuli such as touch, movement, and pressure.

  • Fish and aquatic amphibians have a complex system of mechanoreceptors called a lateral line system that detects sound vibrations, bodily movements, and other pressure changes in the environment produced by water currents
  • Invertebrates have a variety of mechanoreceptors that are sensitive to touch
  • Human skin has free nerve endings that detect pain, temperature, itch, and movement
  • Merkel’s disks and Meissner’s corpuscles are two types of touch receptors in the human skin
  • Pacinian corpuscles are pressure receptors located deeper in the skin, particularly in the nipples and external genitals of both sexes

D.Vertebrates have nerve endings in the skin that are sensitive to temperature.

  • Thermoreceptors are sensitive to coldness and warmth

III.Can some animals detect stimuli humans cannot?

A.Some animals can detect infrared radiation, and others can detect UV light.

  • Pit vipers, such as rattlesnakes, have pit organs in the head that can detect the heat given off by another animal
  • Some insects are sensitive to the UV light emitted from flowers

B.Some animals can detect electrical fields.

  • Some fish, salamanders, and the duck-billed platypus have electroreceptors
  • Some electric eels and electric catfish generate an electric field strong enough to stun a potential prey
  • Some electric fish generate a weak electric field that is used as a type of radar to detect objects in the environment

C.Some migratory animals and those with homing instincts detect magnetic fields.

  • Magnetic fields affect a wide variety of animals, including salmon, salamanders, turtles, hornets, honeybees, and homing pigeons
  • Animals affected by magnetic fields have magnetic granules in their bodies

IV.How do the senses of taste, smell, sight, hearing, and balance work?

A.Taste buds and olfactory receptors sense chemicals.

  • Chemoreception is the ability to sense chemicals
  • Chemoreceptors may be found in the skin of animals, in their legs, their mouths, or in bristles on the head
  • Chemoreception is used to find food or mates, locate enemies, or to warn other members of the species of danger
  • A directed movement toward or away from a chemical stimulus is a positive or negative chemotaxis
  • Taste
  • Taste is the detection of chemicals in solution
  • In humans, the four primary tastes detected by taste buds are sweet, salty, bitter, and sour
  • Taste buds are microscopic structures shaped like onions, each made up of from 30 to 80 receptor cells
  • Smell
  • Olfactory receptors sense chemicals in the air
  • In humans, olfactory receptors, equipped with tiny sensitive cilia, are located in the nasal epithelium

B.Rods and cones within the retina of the eye sense light.

  • Photoreception is the sensing of visible light, which is the manifestation of certain wavelengths of the electromagnetic spectrum
  • Photoreception in organisms other than humans
  • Plants and certain protists can detect light
  • Euglena is an example of a protist with a stigma, or eyespot, that is sensitive to light
  • Various species of worms and arthropods have light-sensitive eyes that do not form images
  • Many insects have compound eyes, consisting of many lenses
  • Certain mollusks (squid and octopus) have image-forming eyes similar to the eyes of vertebrates
  • Positive phototropism is movement toward light (example = housefly) , while negative phototropism if movement away from light (example = cockroach)
  • Sight
  • Refer to Figure 33.16 for the detailed anatomy of the human eye
  • The eyeball is protected by a tough outer covering called the sclera
  • The transparent cornea admits light, which passes through the aqueous humor, is focused by the lens, and then passes through the vitreous humor before striking the retina
  • Failure to focus an image on the retina results in blurry vision, usually caused either by an elongated eyeball (producing nearsightedness) or a shortened eyeball (producing farsightedness)
  • The iris controls the amount of light entering the eye by varying the size of the pupil
  • Rod-shaped cells in the retina are sensitive to the presence of light, and not its color
  • There are three types of cone-shaped cells in the retina, each sensitive to a different range of wavelengths of light (red, blue, and green)
  • The area of sharpest vision on the retina is the fovea, which contains the greatest concentration of rods
  • Impulses generated in the rods and cones of the retina are transmitted via the optic nerve to the visual cortex of the cerebrum
  • Depth perception is due to the different positions of the two eyes, each viewing an object from a slightly different angle

C.Receptor cells of the inner ear sense pressure waves created by sound waves.

  • Sound waves are mechanical energy resulting from the vibration of an object, which causes compression and decompression waves in air, water, or solid materials
  • Sound reception in organisms other than humans
  • Many animal, such as elephants, homing pigeons, and probably hippos, perceive low-frequency sounds, called infrasound
  • Elephants use infrasound as mating calls and warnings
  • Dogs hear high-pitched sounds
  • Whales, dolphins, shrews, some birds, and bats can hear extremely high-pitched sounds
  • Bats use extremely high-pitched sounds for echolocation, or animal sonar
  • Hearing
  • The pinna of the external ear funnels sound waves into the external auditory canal, which leads to the tympanic membrane
  • The eustachian tube connects the middle ear to the pharynx, and permits an equalization of pressure on both sides of the tympanic membrane
  • Vibrations of the tympanic membrane are transferred through three tiny bones, the malleus, incus, and stapes, to the fluid-filled cochlea of the internal ear for interpretation
  • The cochlea contains an elongate organ of Corti, that is covered with hair cells embedded in a gelatinous material
  • Vibrations directed into the cochlea cause these hairs to vibrate in concert with specific wavelengths of vibration
  • The vibrations of the cells, and basilar membrane to which they are attached, generate nerve impulses in neurons that are transmitted through the auditory nerve to the brain, where they are interpreted as sound

D.The vestibule and semicircular canals of the inner ear detect position and direction of movement.

  • Most animals have gravity receptors (statocysts) that are sensitive to the movement of small grains (statoliths) located within a specialized organ, where they rest on sensory hairs
  • Displacement of the stones relative to the force of gravity causes the statoliths to stimulate sensory cells, which then send information to the CNS
  • In humans, and other vertebrates, a bulge in the midsection of the inner ear, called the vestibule, contains the utricle and saccule, which contain otoliths that stimulate cilia when the head is moved away from right side up and this information is fed to the CNS
  • Positioned above the saccule and utricle are three fluid-filled semicircular canals positioned at right angles to one another
  • The semicircular canals contain ciliated sensory cells that detect movements of the head and send this information to the CNS