Sensory Systems in Introductory Biology

Learning Objectives

• Identify general and special senses in humans.
• Explain the just-noticeable difference in sensory perception.
• Understand why a dog’s sense of smell is more acute than a human’s.
• Trace the path of sound through the auditory system to the site of sound transduction.
• Trace the path of light through the eye to the optic nerve.

Overview of Sensory Systems

• Senses provide information about the body and the environment.
• Humans have five special senses: olfaction, gustation, equilibrium, vision, and hearing.
• General senses include temperature, pain, pressure, vibration, proprioception, and kinesthesia.
• Sensory transduction: converting a stimulus into an electrical signal in the nervous system.
• Two types of sensory transduction systems: receptor-based (affect neuron) and direct (neurons are the receptors).

Reception

• Activation of sensory receptors by stimuli such as mechanical forces, chemicals, or temperature.
• Receptive field: the area a sensory receptor can detect stimuli.

Transduction

• Translation of sensory signals to electrical signals at the sensory receptor, producing a receptor potential.
• Mechanoreceptors respond to pressure (e.g., in the skin or ear).
• Graded potentials vary with the strength of the stimulus, leading to action potentials when threshold is reached.

Encoding and Transmission

• Sensory information encoded by type, location, duration, and intensity of stimulus.
• Intensity encoded by rate of action potentials and number of receptors activated.

Perception

• Interpretation of sensations occurs in the brain.
• Sensory pathways transfer action potentials to the brain and cortex.
• Perception varies among individuals, especially in pain tolerance.

Scientific Method Connection: Just-Noticeable Difference (JND)

• JND is the smallest detectable difference in stimulus.
• Example: Differentiating between different weights.
• Weber's Law describes the relationship between stimulus magnitude and JND.

Sense of Touch: Somatosensation

• Involves multiple receptors: mechanoreceptors, thermoreceptors, proprioceptors, nociceptors, and chemoreceptors.
• Mechanoreceptors in the skin can be encapsulated or free nerve endings.
• Types include Merkels disks, Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscles.
• Receptor density varies on different body parts (denser in glabrous skin).

Mechanoreceptors

• Respond to physical deformation; example: pressure and vibration.
• Tactile mechanoreceptors include Merkels disks and Meissner's corpuscles for light touch.
• Deeper receptors like Ruffini endings and Pacinian corpuscles detect stretch and vibration.

Thermoreception and Pain

• Thermoreceptors detect temperature; nociception is the neural processing of pain.

Taste and Smell

• Taste and smell are interconnected as both involve chemical receptors.
• Five primary tastes: sweet, sour, bitter, salty, umami.
• Smell involves olfactory receptors in the nasal cavity.
• Olfactory signals reach the brain directly, influencing memory and emotion.

Pheromones

• Chemical signals affecting behavior or physiology, primarily in non-human species.

Hearing and Vestibular System

• Sound waves are mechanical waves; frequency (pitch) and amplitude (volume) are core characteristics.
• Path of sound: outer ear -> middle ear -> inner ear, involving ossicles and cochlea.
• Vestibular system detects movement and balance through structures like semicircular canals.

Vision

• Light is electromagnetic, traveling as waves; visible spectrum is a small part of the electromagnetic spectrum.
• Eye anatomy includes cornea, lens, retina, with photoreceptors (rods and cones) for transduction.
• Rods detect dim light; cones detect color.
• Visual processing involves pathways from the retina to the brain, affecting perception of form, movement, and color.
• Trichromatic coding involves three cone types responsive to different wavelengths.

Higher Processing

• Visual information processed by several brain regions including thalamus and visual cortex.
• Coordination of visual signals essential for perception and coordination with other senses.