In-Depth Notes on Sensory Systems

Sensory Systems Overview

  • Purpose of Sensory Systems:
    • Enable awareness of the environment through sight, smell, taste, sound, touch, and body position.
    • Prevention of harm by triggering reflexes (e.g., withdrawal reflex, taste reactions).
    • Facilitate conscious control of actions and responses.

Sense Receptors

  • Exteroreceptors:

    • Detect external stimuli. Examples include:
    • Photoreceptors (vision)
    • Hair cells (hearing and balance)
    • Olfactory receptors (smell)
    • Taste receptors (taste)
    • Skin receptors (touch)
    • Mechanoreceptors (pressure)
    • Thermoreceptors (temperature)
    • Nociceptors (pain)

  • Proprioceptors:

    • Monitor muscle reflexes and body position.
    • Located in muscle spindles and Golgi tendon organs.
    • Most reflexes are processed at the spinal level, independent of cognitive awareness.

Central Organisation

  • Pathway of Sensory Information:
    • Most sensory pathways relay through the thalamus except for olfactory pathways, which project directly to the cortex.

Inhibitory Modulation

  • The brain continuously filters sensory information, influencing what is perceived based on relevance.
  • This can cause phenomena such as selective hearing.

Touch Sensation

  • Types of Skin Receptors:

    • Pacinian corpuscles: Rapidly-adapting; respond to vibration and pressure; found in subcutaneous tissues.
    • Meissner's corpuscles: Rapidly-adapting; sensitive to light touch; found in hands and feet.
    • Merkel's disks: Slow-adapting; respond to skin indentation.
    • Free nerve endings: Detect pain, itch, and temperature.

  • Receptor Types:

    • Tonic Receptors: Slowly adapting, continue to respond for the duration of a stimulus.
    • Phasic Receptors: Rapidly adapt, reset upon stimulus removal.

Pain Sensation

  • Nociceptors:
    • Smaller receptive fields; sensitive to mechanical pressure, high temperature, and inflammatory chemicals.
    • Involved in withdrawal reflex actions.

Proprioceptors

  • Types:
    • Golgi tendon organs: Located in tendon junctions; provide information about muscle tension.
    • Muscle spindles: Encircle intrafusal muscle fibers; detect stretch and influence muscle contraction reflexes.

Taste (Gustation)

  • Types of Taste:
    • Five fundamental tastes: sweet, salty, sour, bitter, umami.
  • Taste Receptors:
    • T1Rs detect sweet stimuli.
    • T2Rs detect bitter stimuli.
    • t-mGluR4 is responsible for umami detection (glutamate).

Smell (Olfaction)

  • Olfactory Receptors:
    • Located in the nasal cavity and project directly to the olfactory bulb.
  • Signal Pathway:
    • Odorant molecules activate olfactory sensory neurons, which relay information to the brain.

Hearing

  • Ear Anatomy:
    • Comprises outer (pinna, auditory canal), middle (ossicles), and inner ear (cochlea, vestibular apparatus).
  • Sound Transmission Process:
    • Sound waves vibrate the tympanic membrane, transferring energy via bones to cochlea, creating fluid waves that activate hair cells.

Balance and Head Position

  • Mechanism:
    • Semicircular canals detect rotational motion; maculae detect linear acceleration.
    • Differential firing rates from hair cells provide balance information.

Vision

  • Eye Structure:
    • Light is focused by the cornea and lens on the retina, leading to signal processing by photoreceptors (rods and cones).
  • Visual Processing:
    • Convergence and lateral inhibition in retinal processing enhance visual signal detection.

Visual Defects

  • Types of Defects:
    • Scotomas (blind spots), color blindness, and defects due to lesions in the visual pathway.
  • Common Causes:
    • Trauma, tumors, vascular issues, and hereditary factors affecting vision.