Sensory Systems: Taste

Sensory Systems - Taste

• Chemoreception involves a sensory response to chemical stimuli used for both taste and smell
• Taste and smell receptors are usually different types of chemoreceptors and are in different areas of the body
• Chemicals are transported to the sense organs through the air or water.
• Some insects have chemosensory receptors in sensillum.
• Small holes exist near the base of the sensillum allowing animals to detect chemicals in the air or water
• The sensillum can be located around the body, including the legs and around the mouth
• There are often four types of chemosensory cells in a single sensillum: sugar, salt, water, and deterrent (detect things insects do not want to consume, like alkaloids)

Sensory Systems – Taste in Mammals

• Taste buds are a collection of elongated epithelial sensory cells
• Taste buds transduce chemical sensory input into electrical energy by releasing neurotransmitters onto cranial nerves
• Sensory cells in taste buds use ATP as a neurotransmitter
• Taste buds are located on papillae
• Circumvallate papillae are located on the back of the tongue and contain thousands of taste buds each.
• Foliate papillae are located on the lateral borders of the tongue and contain a medium number of taste buds
• Fungiform papillae are located on the tip of the tongue and only contain a few taste buds each.
• Different cells of taste buds

Types of Taste Bud Cells

• Type I cells are astrocyte-like cells which create a sheath around other cell types
• Type II and III cells are taste receptor cells
• Type IV cells are basal cells that serve as precursor cells for other cell types
• Mammals can taste sour, salty, sweet, bitter, and umami, detected with Type II and Type III cells
• Detection of sweet, bitter, and umami relies on Type II cells and involves metabotropic signaling
• Detection of sour relies on Type III cells and uses ionotropic signaling
• Detection of salt relies on ionotropic signaling however the cell type is not well defined
• Other vertebrates have similar taste buds to mammals

Sensory Systems - Smell

• Sensilla used for smell in insects have a line of perforations down the entire shaft which lead to tubules that penetrate the exoskeleton
• Sensory neurons have dendrites inside the shaft that are surrounded by a fluid called the sensillar lymph.
• Odorant molecules travel through the tubules and bind to the dendrites of the sensory cells
• These types of sensilla are often found in the antennae of insects

Mammalian Smell

• Mammals have olfactory receptor cells that line part of the internal nasal cavity
• Olfactory cells are neurons that have a single dendrite that projects slightly into the mucous layer
• 20-30 olfactory cilia branch off this dendrite
• Olfactory signals dissolve in the mucous before being received by the olfactory receptors
• These neurons have axon terminals in the olfactory bulb of the brain
• Thousands of different metabotropic receptors are on these cells
• Dogs have 4 x 10⁹ olfactory receptor cells while humans only have 10⁷.
• Other vertebrates have similar olfactory systems

Vomeronasal Organ

• Vomeronasal organs are found in most vertebrates
• These help detect pheromones and other chemical signals
• Air can be pumped in or chemicals can be delivered with the tongue
• Forked tongues provide two-point sampling for the detection of gradients
• Receptor cells in this organ are similar to the regular olfactory receptors except they have microvilli instead of cilia

Sensory Systems - Vision

• Photoreceptor cells in vertebrates are either rods or cones
• Rods are used in dim light and make up most light receptor cells in nocturnal animals
• Cones are used in bright light and for color vision and different cones are used to detect different wavelengths of light
• Both types of cells have an outer, photosensitive segment and an inner segment that contains the organelles and synaptic terminals

Visual Reception

• Photopigments are receptors that detect light and are expressed on the surface of photoreceptor cells
• Photopigments consist of opsin and a chromophore
• Different opsins are used to detect different wavelengths of light
• Vertebrates use retinal as a chromophore
• When retinal absorbs light it changes conformation from cis to trans
• This causes the retinal to disassociate from the opsin
• There is then a conformation change in opsin which activates an intracellular signaling cascade
• In photoreceptor cells, Na⁺ channels are open when animals are in the dark; membrane potential is lower than normal (-30 mV)
• When light is detected, and chromophores are released from opsins, opsins become enzymatically active
• Active opsins degrade the molecules keeping Na⁺ channels open
• After Na⁺ channels close, the cell hyperpolarizes
• This hyperpolarization results in a decrease in the release of neurotransmitter (glutamate)
• A lack of glutamate tells bipolar cells that light is being detected

Vertebrate Eyes

• Vertebrate eyes are camera eyes that use a lens to focus light on the photoreceptor cells
• Photoreceptor cells are on the retina and light passes through the retina to reach rods and cones
• Axons from the ganglion cells make up the optic nerve, which leaves the eye through an area with no retina -- known as the “blind spot”
• The fovea is the central region of the retina and is important for visual acuity
• The fovea is densely packed with cones and is well developed in primates and birds
• The choroid is located between the retina and sclera, and contains blood vessels and connective tissues
• The sclera is fibrous tissue that protects and supports the eye
• The cornea is the outer layer of the eye; when light passes through the cornea, it gets refracted (bends), resulting in an inverted image being projected onto the retina
• The lens focuses light onto the retina
• The aqueous humor is fluid-filled space between the cornea and lens
• The iris dilates and contracts to control the amount of light that enters the eye and helps protect the eye from UV light

Extreme Vision

• Humans and some insects are trichromatic and have three cone types that can detect different wavelengths of light
• Birds, some insects, and some fish are tetrachromatic and have four types of cones
• Dogs are dichromatic and have worse color vision than humans

Extreme Animals - Magnetoreception

• Earth has a magnetic field that varies in intensity and angle
• Some animals are able to detect this magnetic field and use it to navigate (magnetic compass)
• Magnetoreception is often used when other navigational cues are unavailable
• Animals use chemical or magnetic particle detection to navigate
• Cryptochromes in the eyes detect light; they produce paired radicals (two molecules with unpaired electrons) that interact with magnetic fields
• The chemical detection is common in many birds and some insects
• Some animals have small particles of magnetite that allow them to detect magnetic fields
• Marine animals use magnetic particle detection like sea turtles, fish, and some mammals
• Animals also use smells, the sun and stars, and visual landmarks for navigating