General Principles of Sensory Physiology

Overview of Sensory Systems

The sensory systems are specialized components of the nervous system that detect and interpret information from both the external and internal environments. These systems are essential for perception, which is the conscious interpretation of sensory stimuli.

• Sensory systems provide input about the world and the body.

• Perception is shaped by sensory input, memory, and other neural processes such as thought.

• Perception may vary among individuals and is not always an accurate representation of reality.

• Examples of perceptual differences include optical illusions and ambiguous images.

Branches of the Peripheral Nervous System

Sensory information is transmitted from the peripheral nervous system (PNS) to the central nervous system (CNS) via specialized receptors.

• Exteroceptors: Detect stimuli from the external environment (e.g., touch, temperature).

• Interoceptors (also called visceral receptors): Detect stimuli from the internal environment.

  • Chemoreceptors: Monitor blood gases, pH, and chemical composition in the GI tract.

  • Baroreceptors: Sense changes in blood pressure.

  • Mechanoreceptors: Detect stretch in the GI tract and urinary bladder.

Types of Sensory Systems

Somatosensory System

The somatosensory system is responsible for sensations arising from the skin and musculoskeletal system.

• Somesthetic sensations: Include touch, pressure, pain, and temperature.

• Proprioception: The sense of limb and body position.

Special Senses

Special senses are those with dedicated organs and pathways.

• Vision

• Hearing

• Balance and equilibrium

• Taste

• Smell

Sensory Receptors and Modalities

Definition and Function

Sensory receptors are specialized cells or structures that detect specific forms of energy (modalities) in the environment.

• Modality: The specific form of energy detected (e.g., light, sound, pressure, temperature, chemicals).

• Law of Specific Nerve Energies: Each sensory receptor is specialized for a particular modality.

• Adequate stimulus: The type of stimulus to which a receptor is most sensitive.

• An inadequate stimulus may still produce the perception of the adequate stimulus if it activates the receptor.

Sensory Transduction

Mechanism of Transduction

Sensory transduction is the process by which stimulus energy is converted into electrical signals in the nervous system.

• Receptor (generator) potentials: Graded electrical potentials produced by sensory stimuli.

• These potentials can be subthreshold (do not trigger action potentials) or suprathreshold (trigger action potentials).

• Types of receptors include specialized endings of afferent neurons and separate specialized receptor cells.

Receptor Adaptation

Adaptation Types and Effects

Receptor adaptation refers to the decrease in responsiveness of a sensory receptor to a constant stimulus.

• Adaptation leads to a decrease in the amplitude of receptor potentials and a reduced perception of the stimulus.

• Tonic receptors: Adapt slowly or not at all; respond for the duration of the stimulus (e.g., muscle spindles).

• Phasic receptors: Adapt quickly; respond mainly at the onset and offset of the stimulus (e.g., some touch receptors).

Sensory Pathways

Labeled Lines and Sensory Units

Sensory information is transmitted along specific neural pathways, known as labeled lines, to distinct areas of the brain.

• Labeled line: A dedicated pathway for each modality and location, leading to a specific sensory cortex.

• Sensory homunculus: A representation of the body in the sensory cortex.

• Sensory unit: A single afferent neuron and all its associated receptors, which are of the same type.

• Receptive field: The area in which a sensory unit can be activated.

Generalized Sensory Pathway

Sensory pathways typically involve multiple neurons in series.

• First-order neuron: Receives input from receptors in the receptive field.

• Second-order neuron: May receive input from multiple first-order neurons; travels to the thalamus.

• Third-order neuron: Projects from the thalamus to the cerebral cortex.

Sensory Coding

Coding for Stimulus Type (Modality)

The type of stimulus is coded by the type of receptor activated and the specific neural pathway (labeled line) it follows.

• Integration of sensory information can produce unique sensations (e.g., wetness, which has no dedicated receptor).

Coding for Stimulus Intensity

Stimulus intensity is encoded by both the frequency of action potentials and the number of receptors activated.

• Frequency coding: Stronger stimuli produce higher frequency of action potentials.

• Population coding: Stronger stimuli activate more receptors and afferent neurons (recruitment).

Equation:

Coding for Stimulus Location

The location of a stimulus is determined by the receptive fields and the degree of overlap among them, which affects acuity.

• Acuity: The precision with which a stimulus is perceived; depends on receptive field size and overlap.

• Lateral inhibition: Inhibitory interactions among neurons enhance contrast and localization.

• Two-point discrimination: A measure of tactile acuity; the ability to distinguish two closely spaced points on the skin.

Two-Point Discrimination Thresholds

The following table summarizes the two-point discrimination thresholds for selected areas of the body, indicating tactile acuity.

Smaller thresholds indicate greater tactile acuity.

Special Considerations in Stimulus Localization

Some sensory systems, such as olfaction and hearing, use different mechanisms for localization that do not rely on receptive fields.

• Olfaction: Codes for quality and intensity; localization influenced by timing and orienting behaviors.

• Hearing: Localization based on pitch, loudness, timing, and orienting toward the sound source.