The Somatic Sensory System_a8955d260f1ddf0a6c30ab055b854220

Sensory Systems

Objectives

  • Sensation vs Perception

  • Sensory Modalities

  • General vs Special Senses

  • Process of Sensation

  • Sensory Receptors

Sensation vs Perception

  • Perception:

    • Conscious awareness & interpretation of sensation.

    • Memories of perceptions are stored in cortex.

  • Sensation:

    • Conscious/unconscious awareness of external or internal stimuli.

    • Involves various types of receptors:

      • Chemoreceptors

      • Thermoreceptors

      • Nociceptors

      • Baroreceptors

Process of Sensation

  1. Stimulation of Receptor:

    • Each sensory receptor shows selectivity, responding to only one type of stimulus.

  2. Transduction of Stimulus:

    • Converts the stimulus into a graded potential.

    • Varies in amplitude and is not propagated.

  3. Generation of Action Potentials:

    • Action potentials are generated when the graded potential reaches the threshold.

  4. Integration of Sensory Input:

    • Handled by the CNS.

Types of Sensory Receptors

Categories

  • Mechanoreceptors: Detect mechanical stimuli (deformation, stretching, bending).

  • Thermoreceptors: Sense temperature changes.

  • Photoreceptors: Respond to light striking the retina.

  • Chemoreceptors: Detect chemicals in the mouth (taste), nose (smell), and body fluids.

  • Nociceptors: Respond to painful stimuli from tissue damage.

Receptive Fields of Sensory Neurons

  • Definition:

    • The area or range of stimuli that can activate a specific sensory neuron.

  • Includes:

    • Stimulated physical area, specific chemicals, and sound frequencies.

Sensory Modalities

  • Definition:

    • Differentiation of one sensation from another.

  • Types:

    • General Senses: Somatic (touch, pressure) & visceral senses (internal organs).

    • Special Senses: Smell, taste, vision, hearing, and equilibrium.

Sensory Coding

  • Attributes of a Stimulus:

    1. Modality: Determined by the stimulus itself (label line coding).

    2. Location: Mechanism detects where the stimulus is (two-point discrimination).

    3. Intensity: Corresponds to the frequency of action potentials.

    4. Duration: Related to how long action potentials are delivered.

Classification of Sensory Receptors

By Structure

  • Free Nerve Endings: Unspecialized (pain, tickle, itch, temperature).

  • Encapsulated Receptors: Specialized structures for touch, pressure, vibration.

  • Complex Receptors: For vision, hearing, taste, and smell.

Somatic Sensory Pathways

  • Relay Information: From somatic receptors to the primary somatosensory area in the cerebral cortex & cerebellum.

  • Pathways Include:

    • 1st order, 2nd order, and 3rd order neurons.

  • Key Functions:

    • Touch, proprioception, pain, heat/cold.

Pain Sensation

  • Nociceptors: Free nerve endings that protect the body from potential harm.

  • Types of Pain:

    • Somatic (superficial or deep)

    • Visceral (related to organs)

    • Referred pain (felt in an area different from the source).

Adaptation of Sensory Receptors

  • Rapidly Adapting (Phasic): Respond upon stimulus onset and cease during maintenance.

  • Slowly Adapting (Tonic): Maintain response to a stimulus over time, critical for constant monitoring.

Mapping of the Primary Somatosensory Area

  • Sensory Homunculus: Visual representation where body parts are proportionate to the amount of sensory receptors.

    • More cortical area is dedicated to sensitive areas like lips and fingertips compared to less sensitive areas like the trunk.

Special Senses Overview

  • Chemical Senses: Olfaction (smell) and gustation (taste), evoke strong emotional reactions via connections to the limbic system.

  • Olfactory Pathway: Odorant binding leads to receptor activation and signal transmission to the cortex.

  • Gustatory Pathway: Taste perception involves interactions with taste receptor cells and neural signaling through specific cranial nerves.