Sensory Nervous System and Photoreception

Lecture 9 Recap

Types of Sensory Neurons

  • Two Types:

    1. Free-nerve endings (dendrites in afferent neurons)

    2. Specialized receptor cells

General Signal Transduction Process

  1. Stimulus: Opens ion channels

  2. Graded Receptor Potential: Also known as electrotonic potential

  3. Action Potential: Can lead to action potential traveling down the axon

Mechanoreception

  • Functions: Involves touch and pressure detection, proprioception (body position), and sound detection.

  • Mechanoreceptors can be either:

    • Free nerve endings

    • Fluid-filled sacs enhancing pressure detection

  • Detection Areas:

    • Skin

    • Skeletal muscles

    • Blood vessel walls

    • Internal organs

Proprioception

  • Function: Body’s sense of position and movement.

  • Mechanism:

    • Involves hair cells with stereocilia that detect changes in fluid movements.

    • Causes the release of neurotransmitters to sensory neurons projecting to the CNS.

    • Found in:

    • Statocysts (jellyfish, lobsters)

    • Neuromasts (fish)

    • Inner ear (terrestrial vertebrates)

Sound Detection

  • Mechanism: Sound waves cause mechanical deformation of membranes (tympanum) and/or hair cells, informing about:

    • Frequency

    • Amplitude

    • Location

  • Auditory Structure: Uses the pinna (ear folds) to concentrate and focus sounds.

Sensory Nervous System

Secondary Sensory Concepts

  • General Concepts:

    • Different types of photoreceptors

    • Different types of chemoreception

    • The role of thermoreception and electroreception in animal survival

  • Examples: Discussed relevance in bats and sea stars.

Photoreception

  • General Function: Detection of light wavelengths via photoreceptors.

  • Process: Light stimulus opens ion channels leading to action potential, which is integrated into light perception.

  • Types of Photoreceptors in Animals:

    1. Eye Spots (Cups):

    • Common in many invertebrates.

    • Simple arrangement, perceives light as a basic concept (dark/light).

    • No image formation and contains fewer than 100 photoreceptor cells.

    • Detects light and transmits information to cerebral ganglion through afferent nerves.

    • Found in planarians, some insects, arthropods, and echinoderms.

    1. Compound Eyes:

    • Composed of many facetted visual units known as ommatidia.

    • Light enters the ommatidia, is focused onto photoreceptor cells by a crystalline cone.

    • Rhodopsin (retinal-containing photopigment) absorbs light, resulting in action potential.

    • The brain receives a motion-sensitive mosaic image.

    • Predominantly found in invertebrates, notably insects and mollusks.

    1. Camera Eyes:

    • Found in cephalopods, mollusks, and all vertebrates.

    • Muscles in the iris adjust the pupil to control light entry.

    • The lens can change shape/move to focus on objects at varying distances (accommodation).

    • Converges light onto the retina's photoreceptors, resulting in a camera-like image sent to the CNS.

    • Glasses are necessary when the lens cannot correctly focus light on the retina, leading to blurry images.

Phototransduction in Vertebrate Retina

  • Types of Photoreceptors:

    • Rods:

      • Detect low-intensity light (grayscale).

    • Cones:

      • Detect light of varying wavelengths (color).

  • Photopigments:

    • Rods contain rhodopsin; cones contain photopsin.

    • Inactive state releases glutamate (a neurotransmitter); active state decreases glutamate release.

  • Neurotransmission: Both rods and cones transmit to interneurons.

Processing in the Retina

  • Integration: Interneurons perform initial integration and synapse with ganglion cells, whose axons form the optic nerve.

  • Further Integration: Higher levels of integration occur in several brain regions which send information to the brain.

Chemoreception

  • Definition: Measurement of chemical stimuli in the environment.

  • Taste (Gustation):

    • Relies on contact chemoreception for assessing food chemicals.

  • Smell (Olfaction):

    • Relies on airborne chemoreception, such as pheromones.

  • Intrinsic Chemical Measurement: Monitors molecules like oxygen and carbon dioxide.

  • Mechanism: Stimuli bind to membrane receptors, generating action potentials relayed to CNS.

Aquatic vs. Terrestrial Invertebrates

  • Receptors: Many aquatic invertebrates share receptors for both taste and smell, whereas terrestrial invertebrates often have separate structures clustered around the mouth.

  • Specific Examples:

    • Hydra (cnidarians) have receptors on their surface.

    • Earthworm mouth is located close to tentacles or other insects!

Taste Reception in Terrestrial Invertebrates

  • Receptor Locations: Can be found on antennae, mouthparts, and feet; e.g., fruit flies taste using their feet (sensilla).

  • Receptor Function: Each taste receptor cell responds to distinct stimuli (water, salt, sugar), conveying information about type and concentration.

Taste Reception in Vertebrates

  • Mechanism: Non-neuronal receptor cells are grouped into taste buds. Each receptor cell exhibits preferred chemical sensitivity.

  • Taste Transduction:

    • Simple tastes (salt, sour) cause cation inflow resulting in depolarization of the cell and subsequent neurotransmitter release.

    • Complex tastes (sweet, bitter, umami) utilize a second messenger pathway for transduction.

    • Note: Sensory receptors do not typically generate action potentials, e.g., ionotropic receptors open ion channels directly, whereas metabotropic receptors use second messengers.

Olfaction (Smell)

  • Definition: The perception of volatile chemicals, such as pheromones.

  • Mechanism: Molecules dissolve in a watery mucus solution; binding of odorants leads to depolarization.

  • Neuronal Connection: Receptors are neurons that synapse directly with brain interneurons, enhancing the sense of taste.

  • Impact of Mucus: Vaporized molecules travel to receptors from the throat, and due to constant mucus presence in the nose, olfaction can be impaired (e.g., during sickness when the nose is congested).

Thermoreception

  • Definition: Detection of heat changes in the surrounding environment.

  • Functionality: Some invertebrates and vertebrates utilize thermoreceptors to identify warm-blooded prey (e.g., mosquitoes, ticks, snakes).

  • Specialized Structures: Vampire bats possess extra folds in their noses to enhance thermoreception, allowing them to detect warm blood in their prey just beneath the skin.

  • Mechanism: Thermoreceptors are often free nerve endings, with dendrites of afferent neurons sensitive to temperature changes.

Electroreception

  • Definition: Capacities for sensing electrical fields.

  • Presence: Found in species such as sharks, bony fish, amphibians, star-nosed moles, and platypuses.

  • Applications: Used for locating prey, navigation, or communication among fish.

  • Mechanism: Electroreceptors detect distortions of electric fields caused by prey's own electrical activity (heartbeat, muscle contractions).

Sensory Adaptation

  • Concept: The diminishing response of a sensory receptor to a constant stimulus over time (sensory adaptation).

  • Adaptation Rates:

    • Rapid Adaptation (e.g., photoreceptors for light, chemoreceptors for smell): Adapt quickly to constant levels of stimuli.

    • Slow Adaptation (e.g., pain receptors): Do not adapt much, signaling danger; when stimulus intensity changes, sensory adaptation must reoccur, e.g., in dark/light transitions.

Perception

  • Definition: Conscious awareness of internal and external environments derived from sensory input processing.

  • Action Potentials: Signals from sensory receptors utilized by the brain to interpret the environment.

  • Variety in Sensitivity: Some animals, such as bats, detect higher frequency sounds or insects that can see ultraviolet light show how perception capability can differ radically among species.