Senses

• Being able to gather information about the external and internal environment is important for survival
• Types of sensory receptors:
  • Exteroreceptors: Gather information about the external environment
  • Interoreceptors: Gathered information about what is happening within the body
• Sensory information is conveyed and processed by:
  • Stimulation: A neuron in a sensory receptor revives a stimulus
  • Transduction: The stimulus is changed into electrical signals in the dendrites of the sensory neuron
  • Transmission: The signal is sent to the CNS
  • Interpretation: The brain intercepts the signal and tells you what you are perceiving
• Types of sensory receptors:
  • Photoreceptors: Detects light (vision)
  • Chemoreceptors: Detects chemicals (taste, olfaction, and pH)
  • Mechanoreptors: Detects changes in mechanical force (touch, hearing, balance, and body position)
  • Nociceptors: Detects pain
  • Thermometers: Detects heat
  • Electromagnetic receptors: Detects electrical currents and magnetic fields.
• Photoreceptors: Most animals have the ability to detect light through a photopigment (opsion) that detects changes in the wavelength of light
  • Many invertebrates have eyespots
  • Image formingeyes occur in annelids, molluscs, arthropods, and chlordates
  • Compound eyes: Found in many arthropods, made up of thousands of individual units called ommatidia
  • Simple eyes: Found in spiders and annelids, called ocelli
  • In vertebrates light enters through the cornea, passes through the lens, and produces an upside down image on the retina
  • Cones: Form sharp images in bright light, responsible for colour vision. There are three types of cones each with sensitivities to different portions of the visible light spectrum
  • Rods: Form black and white images in dim light
  • Mammal eyes have nervous connections in front of the photorecptors, the place where the optic nerve leaves the eye creates a blind spot
  • Mollusc eyes have nervous connection behind the retina
  • Many fish, birds, reptiles, and insects have more types of cones and can interpret light into the ultraviolet portion of the spectrum
  • Lizards and birds are primarily diurnal and their retinas are composed almost entirely of cones
  • Mammals evolved as primarily nocturnal creatures and their retinas are composed mostly of rods
  • Eyeshine: Occurs in nocturnal animals because of the thin layer of tapetum lucdium (reflective coating) at the back of their eye.
  • When the visual fields of both eyes overlap it creates binocular vision that gives good depth perception
• Chemoreceptors: Respond to chemical stimuli, catagorized based on their location, ex taste and smell
  • Insect olfactory receptors: Found on antena and feet
  • Vertebrate oldactory receptors: Located in the nasal passage and connected directly to the olfacotry bulb in the brain.
  • The amount of olfactory epithelium relates to how strong the sense of smell is
  • Taste receptors are a type of chemorecptor but are not directly linked to the brain
  • Internal chemosensors detect chemical characteristics of blood and fluids derived from the blood, and they help maintain homeostasis
• Mechanorecptors: Involved with touch, hearing, body positioning, and balance.
  • Pressure receptors: Are associated with hairs and vibriassae
  • Touch receptors are often clustered together in more sensitive areas of the body
  • Proprioceptors: monitor the state of muscle contraction and provide information about the body’s position which is necessary for coordinated movement
  • Lateral line system: Is present in the skin of most fish and aquatic amphibians.

    Changes in water movements provide information about the animal’s direction of movement and other disturbances in the water.

• Hearing: Involves the sensation of vibrations in air or water.
• Fish do not need ears; sound travels through water better than it travels through the air, and a fish’s body is composed mostly of water. To detect sounds, fish have otoliths in their labyrinth organs
• In terrestrial animals, vibrations are picked up by the tympanum, transmitted by the ossicles of the middle ear, into the cochlea of the inner ear.
• Mammals have three ossicles; amphibians, reptiles and birds have one (the columella)
• The cochlea contains a strip of neuromast cells called the Organ of Corti. The hair cells in the neuromasts are tuned to a narrow range of frequencies and are located in different areas.
• Strongly nocturnal owls rely more on their hearing than their vision to locate prey, they have very specific directional hearing
• Bats and cetaceans use echolocation to navigate and find food
• Many aquatic invertebrates have statocysts to orient themselves with respect to gravity.The statoliths may be loose or embedded in a gelatinous cupula along with the hair cells. Movement of the body produces movement of the statocysts, which stimulate the applicable hair cells
• Vestibular apparatus: Made up of two structures in the inner ear, the semicircular canals, and the utricule and saccule
• Semicircular canals: sense acceleration
• Utricule and saccule: sense body position relative to gravity and constant motion
  • Nociceptors: transmit impulses that are perceived as pain. Some of these are free nerve endings in the skin and other tissues and respond to actual or perceived tissue damage
• Overstimulation of heat and cold receptors can also be felt as pain
  • Thermoreceptors: Unspecialized sensory receptors (free nerve endings) that detect warmth or cold and are meant to detect harmless changes in temperature.
• Vertebrates have thermoreceptors mostly in their dermis. Cold receptors are more numerous than warm receptors and are located more superficially.
• Endothermic creatures also have thermoreceptors in their hypothalamus. These monitor the temperature of the blood to ensure that the core body temperature remains within a narrow homeostatic range.
• Some snakes and vampire bats can detect infrared radiation, which allows them to detect warm-blooded prey in the dark, and locate the blood vessels beneath the skin of their prey.
  • Electroreceptors: Found only in vertebrates, in aquatic and semi-aquatic animals (various fish, dolphins and platypuses). Animals generate electrical currents from their muscular activity
• Electrolocation: is used to detect prey and avoid predators, and may be passive or active
• Active electrolocation: is used by electric fish, such as electric eels. They give off an electrical field, and their receptors can distinguish between conductive (living) and resistant (inanimate) objects. They can also produce shocks that are capable of stunning prey and discouraging predators
  • Magnetoreceptors: The ability to detect magnetic fields. Has been demonstrated by several invertebrates, migratory birds, bats, bees, sea turtles and salmon