Sensations

Unit Three: Nervous Control

Chapter Six: Sensations

Prepared by Prof. Nabih A. Baeshen Introductory Biology 2 BIO 202

6.1 Introduction

  • Sensory organs in the peripheral nervous system (PNS) absorb energy from a stimulus (e.g., pressure, light).

  • These organs transform the stimulus into electrical energy, generating a receptor potential, which then generates an action potential.

  • The action potential (nerve impulse) travels along sensory neurons in nerves into the central nervous system (CNS).

6.2 Pathways of Sensation

  • In the CNS, nerve impulses travel to the cerebrum for conscious experience or participate in reflex pathways within the autonomic nervous system (ANS).

  • Sensory receptors are classified based on the stimulus type they respond to.

6.3 Types of Sensory Receptors

  • Mechanoreceptors: Respond to mechanical stimulation (stretch, pressure).

  • Chemoreceptors: React to chemicals (e.g., acids).

  • Nociceptors: Produce pain when activated.

  • Electroreceptors: Senses electricity.

  • Photoreceptors: Respond to light.

Table of Sense Receptors by Type of Stimulus Energy

  • Mechanical Stimuli:

    • Touch/Pressure: Mechanoreceptors (e.g., Pacinian corpuscles, Merkel disks).

    • Muscle Contraction: Proprioceptors (e.g., muscle spindles).

  • Chemical Stimuli:

    • Odors: Chemoreceptors (e.g., taste buds).

    • Heat: Thermoreceptors in skin and specific organs.

  • Electrical Stimuli:

    • Detect electricity via electroreceptors in fish.

  • Light Stimuli:

    • Sensed by photoreceptors in the eyes.

6.4 Classification Based on Stimulus Source

  • Exteroceptors: Respond to external environment stimuli.

  • Interoceptors: Respond to internal body stimuli.

  • Proprioceptors: Located within the body, responding to mechanical stress (both interoceptive and mechanoreceptive).

6.5 Reflex Pathways and Homeostasis

  • Reflex pathways operate without perception, ensuring automatic adjustments for optimal movement.

  • Interoceptors manage variables like pressure and chemical concentrations, contributing to homeostasis.

6.6 Mechanoreceptors and Tactile Sensation

  • Exteroceptors in the skin relay tactile sensations to the cerebrum for localization and perception.

  • Important in both vertebrates and invertebrates for locating hosts/prey.

6.7 Gravity Sensation

  • Gravity receptors include statocysts in invertebrates and hair cells in vertebrates, also present in the inner ear for balance and hearing.

6.8 Chemoreceptors: Taste and Smell

  • Taste: Conferred by chemoreceptors in taste buds reacting to chemicals in saliva, leading to perceptions of flavors.

  • Smell: Involves olfactory receptors in the nasal cavity responding to vaporized chemicals.

6.9 Nociceptors

  • Nociceptors are pain receptors that respond to temperature, mechanical and chemical stimuli, serving as warnings to avoid harm.

  • Tissue injuries release chemicals that stimulate these receptors, leading to nerve impulses sent to the spinal cord and brain, where pain relief mechanisms (e.g., endorphins) can be activated.

6.10 Electroreceptors in Fish

  • Electroreceptors in some fish allow them to detect electrical currents for prey localization and migration.

6.11 Photoreceptors

  • Simplest photoreceptors include eyespots in invertebrates, assessing light intensity.

  • Compound eyes in insects feature multiple ommatidia that create a mosaic image.

6.12 The Human Eye

  • Light is refracted through various eye structures before reaching the retina, where it is processed for visual perception.

6.13 Photoreceptors in the Retina

  • Rods: Sensitive to dim light, do not perceive color.

  • Cones: Perceive color, require brighter light.

  • Rhodopsin, formed from retinal and opsin, initiates nerve impulses in response to light.

6.14 Vision Disorders

  • Various eye disorders can be corrected through the use of lenses, influenced by the positioning of visual fields in the retina.