Nervous System: Sensory Function & Physiology

Unit Three: Chapter 10 - The Sensory Function of the Nervous System

Part 1: Sensory Physiology

  • Overview of Sensory Physiology

    • Sensory physiology focuses on how information is transmitted to the central nervous system from peripheral receptors, shaping our perception of reality.

  • Key Concepts

    • Graded Potentials: Previous knowledge included EPSPs (excitatory postsynaptic potentials) and IPSPs (inhibitory postsynaptic potentials). Adding two more:

    • Generator Potentials: These are produced in response to a sensory stimulus that causes enough depolarization at the trigger zone of the sensory neuron to reach threshold and fire an action potential.

    • Receptor Potentials: Typically involved in specialized senses (e.g., taste).

  • Examples of Receptors

    • Pacinian Corpuscles: Also known as lamellated corpuscles; detect deep pressure and mechanical stimuli on skin.

    • Mechanism:

      • Deep pressure distorts the skin and excites the sensory receptor, initiating action potentials in the afferent neuron (first order neuron).

    • Afferent Neurons:

    • Their cell bodies reside in the dorsal root ganglion, where each ganglion houses hundreds of first-order sensory neuron cell bodies.

    • When excited by stimuli, they propagate action potentials toward the spinal cord.

  • Trigger Zone and Action Potentials

    • The distal part of the sensory neuron features a trigger zone, where if the depolarization (generated potentials) exceeds a threshold, an action potential is fired.

    • These graded potentials are cumulative; generator potentials will sum up to produce sufficient depolarization to meet threshold.

  • Categories of Sensory Modalities

    • The human sensory system includes four general senses (pressure, temperature, pain, and touch) and five special senses (sight, taste, smell, hearing, and balance).

  • Receptor and Generator Potentials explained through Taste System:

    • Receptor Potential in taste:

    • Requires dissolved food chemicals (in saliva) that bind to taste receptors, leading to depolarization and neurotransmitter release.

    • This, in turn, generates a graded potential in the first order sensory neuron.

    • A crucial interaction is noted: 80% of taste is influenced by smell.

  • Adaptation of Receptors

    • Slowly Adapting (Tonic) Receptors:

    • Remain active as long as the stimulus persists (e.g., muscle stretch receptors, Merkel discs in the skin).

    • Rapidly Adapting (Phasic) Receptors:

    • Respond only to changes in stimulus intensity (e.g., olfactory receptors for smell, Pacinian corpuscles for deep pressure). This allows sensitivity to dynamic changes in the environment.

  • Labeled Lines Concept:

    • Refers to the specific pathways in sensory neurons that are responsible for transmitting distinct types of sensory information to the brain, from first-order to third-order neurons.

    • First-Order Neurons: Sensory neurons that transmit signals from the periphery to the spinal cord dorsal horn.

    • Second-Order Neurons: Located in the spinal cord or brainstem, relay signals to the thalamus.

    • Third-Order Neurons: Relay sensory information from the thalamus to the primary sensory cortex (specifically, the postcentral gyrus). This cortex is essential for perception and conscious awareness of sensations.

  • Receptive Fields

    • Defined as the area serviced by a single sensory neuron.

    • Important for localization of stimuli: smaller receptive fields allow for greater acuity in distinguishing multiple stimuli.

  • Sensory Coding

    • The brain's ability to decode various modalities through mechanisms such as:

    • Modality Coding: Information about the type of stimulus is encoded by the type of sensory receptor activated.

    • Frequency Coding and Population Coding:

      • Frequency coding interprets stimulus intensity as the number of action potentials fired per unit of time.

      • Population coding involves multiple sensory units being recruited by strong stimuli, therefore increasing graded potential generation and action potentials fired.

  • Lateral Inhibition: Enhances sensory localization; surrounding neurons are inhibited when a stimulus is due to the activation of nearby interneurons, which fine-tunes brain perception of the source of a stimulus.

  • Two-Point Discrimination: Small receptive fields enhance the ability to perceive two distinct stimuli as separate points.

Part 2: General Sensory Organs

  • General Senses: Includes pain, temperature, pressure, and touch.

    • Properties:

    • Simple structural design.

    • Widely distributed sensors in the skin (e.g., Pacinian corpuscles, Meissner's corpuscles).

  • Special Senses: Include the more complex systems of olfaction (smell), gustation (taste), audition (hearing), and equilibrium (balance).

    • Special Characteristics:

    • Complex structural design with specific locations for receptors.

  • Pain and Temperature Sensing:

    • Involves nociceptors for pain, thermoreceptors for temperature.

  • Spinothalamic Tract: Main pathway for transmitting pain and temperature signals to the thalamus and subsequently to the primary sensory cortex.

  • Visceral Pain: Often referred pain due to shared pathways between visceral organs and specific skin regions, complicating accurate location reporting for patients.

  • Pain Modulation Mechanisms:

    • Gate Control Theory:

    • Central role of Aβ fibers to inhibit pain perception by activating interneurons and releasing glycine, inhibiting the signal of C fibers responsible for transmitting pain.

    • Endogenous Analgesia:

    • The brain's ability to modulate pain at the spinal level using neurotransmitters like enkephalin to inhibit sensory neuron signaling pathways.

  • Vision:

    • Phototransduction process; light activation results in hyperpolarization of photoreceptors and reduced neurotransmitter release.

    • Vision pathways include processing visual information through ganglion cells and optic nerves to the primary visual cortex, different from other senses in the connection to memory and integrated response systems.