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Introduction to Sensory Function of the Nervous System

  • Lecture begins on unit 3, chapter 10, focusing on the sensory physiology and the transmission of sensory information from the peripheral nervous system (PNS) to the central nervous system (CNS).

Sensory Physiology

  • Sensors and Receptors:
    • Definition: Specialized structures in the periphery that detect specific sensory signals, classified into different sensory modalities.
    • Function: Send sensory information to the CNS, enabling perception of reality.

Graded Potentials in Sensory Receptors

  • Overview of graded potentials essential to understand sensory signal transmission.
  • Previously introduced graded potentials: Excitatory Postsynaptic Potentials (EPSPs) and Inhibitory Postsynaptic Potentials (IPSPs).
  • New graded potentials introduced:
    1. Generator Potentials
    2. Receptor Potentials
Example of Graded Potentials: Pacinian Corpuscles
  • Pacinian Corpuscles:
    • Also known as lamellated corpuscles.
    • Function: Detect mechanical stimuli, particularly deep pressure.
  • Mechanism:
    • Distortion of the receptor fibers due to deep pressure excites the Pacinian corpuscle, leading to firing of action potentials in the first-order neuron (afferent neuron).
    • Location of cell body: Dorsal root ganglion (DRG).
Pathway of Sensory Transmission
  • First Order Neuron:
    • Afferent neuron fires action potentials and transmits signals toward the spinal cord.
    • Cell body located in the DRG, with peripheral axons wrapped in Pacinian corpuscles.
  • Threshold Potential:
    • Defined as the membrane potential that must be reached for action potentials to generate at the trigger zone of the first-order neuron.
    • Generator potentials indicate local changes in membrane potential; must reach threshold to trigger action potentials.
Graded Potentials: Generator and Receptor Potential
  • Generator Potential:
    • Generated in response to a specific environmental stimulus.
  • Receptor Potential:
    • Associated with specialized senses (e.g., taste).
    • Mechanism: Chemical stimuli from food need to be in solution with water (saliva) to activate taste receptors.
    • Process:
    • Chemical binds to taste receptor.
    • Depolarization of the receptor cell membrane activates voltage-sensitive calcium channels, facilitating calcium influx.
    • Leads to neurotransmitter release which binds to ion channels on the sensory neuron, generating a graded potential.
Sensory Modality and Taste
  • Taste is heavily influenced by smell; 80% of taste comes from the olfactory system.
  • Receptor Physiology:
    • Covers the foundation of how sensory receptors function.
    • Introduces concepts of labeled lines and sensory units versus receptive fields.

Sensory Adaptation

  • Slowly Adapting Receptors (Tonic Receptors):
    • Remain active while the stimulus is present (e.g., muscle stretch receptors, Merkel discs).
    • Example: Muscle stretch receptors help protect against muscle tears by continuously monitoring muscle length.
  • Rapidly Adapting Receptors (Phasic Receptors):
    • Only signal changes in stimulus (e.g., olfactory receptors, Pacinian corpuscles).

Label Lines and Sensory Pathways

  • Labeled Lines:
    • Sensory pathways consist of three neurons in series:
    1. First Order Neuron - detects stimuli and conveys to CNS.
    2. Second Order Neuron - synapses in the spinal cord or brain stem.
    3. Third Order Neuron - transmits to the primary somatosensory cortex (postcentral gyrus).
  • Sensory Units:
    • Defined as one first order neuron and all receptors it innervates.
  • Receptive Fields:
    • The area of sensitivity for a sensory modality.
    • Critical for accurate perception of stimuli; smaller fields allow for higher discrimination.

Sensory Coding

  • Concept: How the brain perceives various modalities despite action potentials being uniform in nature.
  • Determines:
    • Type of Stimulus: Depends on specialized sensory receptors, which correlate with specific modalities (e.g., pressure for Pacinian corpuscles, chemicals for taste receptors).
    • Intensity of Stimulus: Decoded via:
    • Frequency Coding:
      • Number of action potentials generated per unit time. Higher stimulus intensity results in a higher frequency of action potentials.
    • Population Coding:
      • Involves multiple sensory units' activation based on stimulus strength.

Sensory Localization Mechanisms

  • Mechanisms that the brain employs to accurately locate stimuli:
    • Lateral Inhibition:
    • Amplifies signals from activated neurons while inhibiting signals from neighboring neurons.
    • Allows for precise localization of sensory stimuli.
    • Two-Point Discrimination:
    • Ability to distinguish two stimuli within a small receptive field.
    • Relies on the density of sensory receptors; more densely packed receptors enhance discrimination ability.

Organs of General Senses

  • General Senses: Pain, temperature, pressure, touch.
    • Features include simple structural designs and widespread distribution across the body.
  • Special Senses: Five specialized senses (e.g., smell, taste, hearing, balance, vision).
    • Typically more complex structures, restricted receptor distribution.
Overview of Special Senses
  • Olfaction (Smell):
    • Stimulus: Chemical.
    • Receptor: Olfactory epithelium (cranial nerve I).
  • Gustation (Taste):
    • Stimulus: Dissolved chemicals from food.
    • Receptor: Taste buds (cranial nerves VII, IX, X).
  • Vision:
    • Stimulus: Light.
    • Receptor: Photoreceptors in the retina, processed via optic nerve (cranial nerve II).
  • Hearing and Balance:
    • Stimulus: Sound waves, mechanical signals.
    • Receptor: Cochlea and vestibular apparatus (cranial nerve VIII).

Mechanisms of Pain Modulation

  • Acute Pain Perception:
    • Nociceptors detect noxious stimuli.
    • Stimuli: Extreme temperatures, mechanical damage, chemical irritation.
  • Gate Control Theory:
    • Explains modulation of pain via competing sensory inputs (e.g., rubbing an elbow decreases pain by increasing activity of faster sensory fibers).
  • Endogenous Analgesia System:
    • Brain's ability to inhibit pain by releasing endogenous opioids (e.g., enkephalins) which reduce neurotransmitter release and inhibit second-order neuron activity.

Visual Perception Pathways

  • Phototransduction: Light signal processing in the retina.
  • Mechanism:
    • Absence of light activates rhodopsin leading to cyclic GMP release, causing increased sodium influx and depolarization.
    • Presence of light causes rhodopsin activation, decreasing cyclic GMP, closing sodium channels, leading to hyperpolarization and reduced neurotransmitter release.
  • Neural Pathways for Vision:
    • Includes thalamus, superior colliculus, and suprachiasmatic nucleus for visual reflexes and circadian rhythms.

Conclusion

  • The chapter covers foundations of sensory perception, receptor function, adaptation, and the pathways for sensory information processing in the CNS.