Campbell Biology: Sensory Systems - Sensory Mechanisms

Sensory Systems: Sensory Mechanisms (Chapter 50 Part 1)

Introduction to Sensory Systems

  • Link between sensory stimuli and animal activity: Sensory receptors respond to specific stimuli by transmitting signals. These signals are decoded, processed, and responded to by the central nervous system (CNS) in the form of motor output.
    • Sensory information is sent along specific paths (axons) to a processing center.
    • Motor commands are sent along specific paths (axons) to muscles (for locomotion, posture) or glands (for secretions).
  • Types of Sensory Receptors and Stimuli:
    • Mechanoreceptors: Respond to sound, touch, motion.
    • Electromagnetic receptors: Respond to light, electricity, magnetism.
    • Pain receptors (Nociceptors): Respond to noxious chemicals, extreme temperatures.
    • Chemoreceptors: Respond to solutes, tastes, smells.
    • Thermoreceptors: Respond to heat, cold.

Flow of Information

  • All stimuli represent forms of energy.
  • A sensory receptor converts stimulus energy into a change in the membrane potential.
  • When a stimulus is received and processed by the nervous system, a motor response may be generated.
    • This can involve a simple reflex or more elaborate processing.
  • Example: Forage in dark: an animal might use 1111 pairs of appendages protruding from its nose to locate and capture prey through touch.

Sensory Pathways: Four Basic Functions

Sensory pathways share four fundamental functions:

  1. Sensory reception
  2. Transduction
  3. Transmission
  4. Perception
Sensory Reception
  • Definition: Detection of stimuli by sensory receptors.
  • Sensory receptors:
    • Are sensory cells or organs.
    • May be neurons or non-neuronal receptors.
    • Interact with stimuli, both inside and outside the body.
Transduction
  • Definition: The conversion of stimulus energy into a change in the membrane potential of a sensory receptor.
  • Receptor potential:
    • This change in membrane potential is called a receptor potential.
    • They are graded potentials: their magnitude varies with the strength of the stimulus.
    • A stimulus changes how often an action potential is produced.
  • Amplification:
    • Strengthening of a sensory signal during transduction.
    • Example: A few photons of light can trigger an action potential with 100,000100,000 times as much energy to carry the signal from the eye to the brain.
Transmission
  • Action Potentials: Sensory information travels through the nervous system as action potentials.
  • The frequency of action potentials changes with the strength of the stimulus.
Perception
  • Brain's construction: Perceptions are the brain's construction of stimuli.
  • Dedicated neural pathways: Stimuli from different sensory receptors travel as action potentials along dedicated neural pathways.
  • The brain distinguishes stimuli based on the specific path by which the action potentials arrive.

Sensory Adaptation

  • Definition: A decrease in responsiveness to continued stimulation.
  • Sensory receptors, when exposed to stimuli for long enough, decrease their sensitivity, making the stimuli less noticeable.
  • Importance: Crucial for distinguishing new sensations from background stimuli.
  • Examples:
    • Getting used to cold water in a pool.
    • Forgetting sunglasses on top of your head.
    • Eyes adjusting to the dark.
    • People from colder climates not minding the cold.
    • Smokers not smelling the smoke.
    • City-dwellers becoming accustomed to noise.

Types of Sensory Receptors (Based on Energy Transduced)

Mechanoreceptors
  • Function: Sense physical deformation caused by mechanical energy.
  • Structure: Typically consist of ion channels linked to structures that extend outside the cell (e.g.,