Lesson 10: Organisms Sense Their Environment
10.1: The Sensory System
Learning Objectives: Understand the functioning and categorization of sensory receptors in the nervous system.
Sensory Receptors:
Detect information from inside (enteroreceptors) and outside the body (exteroreceptors).
Categorized by the type of stimuli they respond to:
Mechanoreceptors: Detect mechanical forces; associated with touch, hearing, and balance.
Chemoreceptors: Detect chemical stimuli, notably taste (gustation) and smell (olfaction).
Photoreceptors: Detect light (photons); found in the eyes.
Thermoreceptors: Respond to temperature changes.
Nociceptors: Detect potential tissue damage, initiating pain sensation.
Conveying Sensory Information: Four steps:
Stimulation: The appropriate stimulus activates the receptor.
Transduction: The stimulus energy is converted into receptor potential in sensory neurons.
Transmission: If graded potentials reach threshold, action potentials are generated along the sensory neuron's axon.
Interpretation: The brain processes the signals received and creates perceptions, which may not always accurately reflect reality (e.g., illusions).
Perception Variability: Different animals with varying receptors have different perceptions of stimuli.
Transmission and Transduction:
Sensory neurons are depolarized by stimulus-gated ion channels, unlike typical synaptic transmission.
E.g., pressure opens mechanically gated ion channels, allowing sodium into the neuron, causing depolarization.
Action potential frequency encodes stimulus intensity: lighter touch results in fewer action potentials, while firmer touch results in more.
Mechanoreceptors in Touch: Various types are located at different skin depths and adapted for specific stimuli. More receptors in sensitive areas (e.g., fingers) than in less sensitive regions (e.g., back of hand).
Nociceptors: Found throughout the body, especially in vulnerable areas; require intense stimulation to depolarize, which explains pain perception.
Homeostasis Monitoring:
Baroreceptors detect blood pressure changes; increased pressure leads to increased action potentials sent to the spinal cord.
Chemoreceptors regulate blood pH by monitoring hydrogen ion concentrations, triggering respiration rate adjustments as needed.
10.2: Taste and Smell
Chemoreceptors for taste detect five modalities: sweet, salty, bitter, sour, and umami.
Taste Buds: Located on the tongue and oral cavity. Salty and sour flavors enter through ion channels, while sweet, bitter, and umami work through receptor binding and signaling pathways.
Taste Bud Dynamics: Children have more taste buds than adults, affecting flavor perception and food preferences.
Sense of Smell: Chemo-detected by numerous receptors in the nose, crucial in detecting chemicals in the environment, even at low concentrations.
Olfactory neurons project to the olfactory bulb of the brain; variations among species can optimize smelling capabilities (e.g., dogs versus humans).
10.3: Hearing and Vision
Hearing Mechanics: The ear comprises outer, middle, and inner structures that convert sound waves into vibrations.
Cochlea: Fluid-filled structure; hair cells within detect sound frequencies: higher frequencies closer to the oval window, lower frequencies further away.
Vestibular Apparatus: Detects motion, acceleration, and gravity; contains hair cells that transduce these movements.
Proprioceptors: Located in muscles and tendons, providing information about body part positioning.
Visual Processing: Light enters the eye through the cornea, pupil, and lens, which focuses it on the retina, where photoreceptors (rods and cones) transduce light signals. The optic nerve carries this information to the brain while correcting for image inversion.
Unique Animal Perceptions: Animals may possess senses such as infrared detection (as in snakes) and magnetic field sensitivity (as in birds). These adaptations allow diverse perceptions of the environment beyond standard human senses.