Sensory Systems in Mammals and Dogs

Sensory Systems in Mammals and Dogs

Overview of Sensory Systems

• Five Basic Senses: Hearing, sight, touch, smell, and taste.
• General Mechanism:
  • Receptors: Detect stimuli from the external environment.
  • Transmission: Neural pathways transmit signals to the brain.
  • Response: The brain processes the information, leading to a response.
• Cranial Nerves: Most sensory information is conveyed to the brain via cranial nerves.
  • Example: Taste buds on the tongue (receptors) send information to the brain via a cranial nerve.

Hearing

• Mechanism: Conversion of air vibrations into perceived sound.
  • Pinna: Focuses sound waves into the ear canal.
  • Tympanic Membrane: Amplifies sound waves through vibration.
  • Ear Bones: Further amplify sound waves into the middle ear.
  • Cochlea: Located in the inner ear; contains hair cells (receptors) that transmit sound information to the brain.
• Process within the Cochlea:
  • Vibrations cause the basilar membrane to move, moving hair cells against the tectorial membrane.
  • Hair cell movement sends signals to the brain, which interprets the signals as sound.

Sight

• Mechanism: Conversion of light wavelengths into perceived sight.
  • Cornea: Protective outer coating of the eye.
  • Pupil: Hole created by the iris, which gives the eye its color.
  • Lens: Focuses light, with its shape adjusted by muscles.
  • Retina: Contains photoreceptors (rods and cones).
• Photoreceptors:
  • Rods: Perceive changes in brightness and dimness of light.
  • Cones: Respond to color.
• Transmission:
  • Light passes through retinal cells to reach photoreceptors.
  • Photoreceptors send signals through neurons that synapse on horizontal and ganglion cells.
  • Signals are transmitted to the brain via cranial nerve two (optic nerve).

Touch

• Receptors: Various types of receptors for different types of touch.
  • Free Nerve Endings: Respond to pain, heat, or cold.
  • Tactile Nerve Endings: Respond to vibrations and light touch.
• Transmission:
  • Receptors throughout the body respond to changes in their environment (touch, pain, temperature) and transmit the information to the brain.
  • Head: Mostly through cranial nerve number five (trigeminal nerve).
  • Body: Through different nerves, generally conveyed through the spinal cord.

Taste

• Mechanism: Detection of chemical cues dissolved in saliva.
  • Taste Buds: Located throughout the oral cavity and throat.
  • Taste Cells: Located inside taste buds, with a taste pore at the top.
• Process:
  • Chemicals (e.g., salt, sugar) dissolve in the mouth and enter the taste pore, interacting with taste receptor cells.
  • Signals are transmitted to the brain.
• Five Tastes: Salty, sweet, umami, bitter, and sour.
  • Positive Cues: Sweet (glucose for energy), umami (protein taste), and small amounts of salt (sodium).
  • Negative Cues: Large amounts of salt, bitter (associated with toxins), and sour (associated with spoiled food).
• Transmission:
  • Front Two-Thirds of Tongue: Cranial nerve seven.
  • Back of Tongue: Cranial nerve nine.
  • Throat: Cranial nerve 10.
• Perigemmal Fibers: Associated with pain and temperature perception, possibly related to the perception of spicy and sour tastes.

Smell

• Mechanism: Detection of chemical cues (odorants) in the air.
  • Olfactory Receptors: Located in the nasal cavity, bind to odorants and send signals to the brain.
• Main Olfactory Epithelium: Extensions from the olfactory bulb (cranial nerve number one) in the upper nasal cavity.
• Fast Response Time: Direct connection between receptors and the cranial nerve.
• Vomeronasal Organ: Specific to nonhuman mammals; associated with pheromones (reproductive and territorial responses).
• Olfactory Bulb: Where olfactory fibers and receptors originate.
  • Other cranial nerves: II (optic), V (trigeminal), VII, IX, X (taste-related), and Vestibulocochlear (VIII).

Canine-Specific Sensory Information

• Vibrissae (Whiskers): Rich source of touch information.
• Research Methods:
  • Behavioral Research: Training dogs to choose between stimuli.
    • Forced choice: Rewarding correct choices.
    • Go/no-go: Rewarding responses (or lack thereof) to stimuli to determine sensory range.
  • Evoked Potentials: Measuring electrical stimulus sent by nerves to the brain.
  • Anatomic Research: Examining sensory organs to understand their structure and function.
• Breed Variation: Differences in ear shape, nose shape (brachycephalic vs. dolichocephalic), and eye orientation affect sensory abilities.

Canine Hearing

• Excellent Hearing: Can detect noises beyond the human range.
• Ear Shape and Orientation: Affects hearing ability.
• Ear Canal: Dogs have vertical and horizontal ear canals.
• Frequency Range: Dogs have a higher frequency hearing range than humans (silent dog whistles).
• Hair Cells: Dogs have more hair cells than humans, allowing for a larger range of frequency detection.
• Breed Comparison: Bloodhounds (floppy ears) vs. Basenjis hearing differences.

Canine Sight

• Dichromatic Vision: Can see two types of colors (blue and yellow).
• Color Blindness: Equivalent to red-green color blindness in humans.
• Toys: Yellow is a good color.
• Luminosity: Brightness perception is about half that of humans.
• Focus on Movement: Eyes are designed to see movement, especially in low-light conditions.
• Crepuscular: See best at dawn and dusk.
• Retina: Fewer cones and more rods compared to humans.
• Scotopic: High sensitivity to light with low color sensitivity.
• Tapetum Lucidum: Membrane behind the retina that reflects light back to the rods and cones, enhancing vision in low light.
• Breed Variation: Eye diameter and head shape (brachycephalic vs. dolichocephalic) affect sight ability. Eye placement on the side of their head gives a wider field of vision.

Canine Touch

• Mechanoreceptors: Cover the body.
• Vibrissae (Whiskers): Provide information about the environment.
• Facial Nerve: Movement of the whisker transmits movement to the blood sinus (pool of blood) and also increases the ability of nerve fibers to pick up on change of touch.

Canine Taste

• Not as Important: Dogs rely more on scent for finding food.
• Carnivorous Diet: Fewer chances of toxins compared to herbivores.
• Umami: Strong sense for protein taste.
• Salt: Weak sense for salts.
• Behavioral Experiments: Used to study taste preferences.
• Taste Buds: Approximately 1,700 - compare to humans 10,000 taste buds.

Canine Olfaction

• Most Developed Sense: Extensive olfactory system.
• Natural Uses: Finding a mate, food, marking territory.
• Human Uses: Locating prey, detecting drugs and bombs, finding people.
• Olfactory Epithelium: Comparison (Humans have 10 squared centimeters vs. Pekingese, Dachshunds (have quite a bit), and Germans Shepherds (Up to 170 squared centimeters).
• Olfactory Receptors: Bred ability to smell, structure, and organization of brain.
• Dolicophalic Dogs: Have the extension of their nose out.

Canine Vomeronasal Organ

• Pheromonal Cues: Reproductive and territorial cues.
• Amygdala/Limbic System: Direct connection leads to fast emotional responses.

Canine Recent Study

• Functional MRI (fMRI): Study on brain responses to odorants in awake vs. anesthetized dogs.
• Higher-Order Brain Structures: Activated in awake dogs only.

Canine and Communication

• Distance Contact: Sight, sound, and smell.
• Closer Contact: Touch.
• Veterinary Application: Massage and acupuncture.

Other Types of Canine Senses

• Vestibular System: Inner Ear, associated with balance and movement
• Vestibular Nerve/Cochlear Nerve: Cranial Nerve 8.

Nociceptive System** Respond to stimulus, pain or irritation itching. Free Nerve Endings will cause reactions.

• Mechanical Stimulation Detecting Warm and cold.