SR

sensory lecture 5

Overview of Sensory Systems

  • Sensory systems are crucial for interacting with the environment and include:

    • Gustation (taste)

    • Vision (sight)

    • Audition (hearing)

    • Olfaction (smell)

    • Somatosensation (touch and body sense)

  • The nervous system integrates sensory information for perception and motor responses.

Sensory Organs and Receptors

  • Sensory Organs: All animals possess unique sensory organs with receptor cells that are specialized to detect specific stimuli:

    • Common sensory organs include:

    • Eyes (vision)

    • Ears (audition)

    • Noses (olfaction)

    • Tongues (gustation)

    • Skin (somatosensation)

  • Action Potentials: The primary mechanism through which all sensory modalities communicate with the nervous system. All sensory systems converge on action potentials to relay information to the brain.

  • Labeled Lines: The brain differentiates between various senses because each type of sensory signal travels along distinct nerve tracts.

Receptor Cells and Sensory Processing

  • Receptor Potential: This is a local change in the membrane potential caused by the activation of receptor cells. It is integral to converting sensory stimuli into electrical signals.

  • Sensory Transduction: Refers to the process where an external stimulus is converted into a change in membrane potential in a receptor cell.

Skin Receptor Types and Their Functions

  • Meissner’s Corpuscles:

    • Respond to low-frequency touch and texture changes (discriminatory light touch).

  • Merkel Discs:

    • Sensitive to edges and isolated points, responding to pressure and deep static touch.

  • Ruffini Corpuscles:

    • Detect skin stretching, indicating finger or limb movement position.

  • Free Nerve Endings:

    • Respond to pain (noxious stimuli), heat, and cold sensations.

  • Pacinian Corpuscles:

    • Specialized to respond to high-frequency vibration (greater than 200 cycles per second) and pressure; essential for sensing texture.

Sensory Events as Action Potentials

  • Sensory events from the environment are encoded as streams of action potentials.

  • Different sensory systems may utilize various types of receptor cells that specialize across a gradient of intensities.

  • Intensity Representation: The intensity of stimuli is quantified by both the activation thresholds of the involved cells and the frequency of action potentials generated.

  • Stimulus Location: Positioning of the activated receptors follows an organized mapping system, allowing for precise localization of sensory information.

Receptive Fields of Sensory Neurons

  • Receptive Fields: Refers to the spatial area over which a stimulus influences a sensory neuron's firing rate.

    • Center-On and Surround-Off: A model of visual receptive fields where stimulation at the center excites the cell while stimulation in the surrounding area inhibits it.

  • Sensory Adaptation: Represents a decrease in a sensory receptor’s response to a sustained stimulus:

    • Phasic Receptors: Adapt quickly and cease firing upon prolonged stimulation (e.g., clothing).

    • Tonic Receptors: Exhibit little to no adaptation, continuing to produce action potentials as long as the stimulus is present (e.g., pain).

Central Nervous System Processing of Sensory Information

  • Each sensory system comprises specific pathways leading from the periphery to the central nervous system (CNS):

    • Dorsal Column System: Responsible for transmitting touch information.

    • Receptors send axons via the dorsal spinal cord, synapsing on neurons in the brainstem.

    • Axons then cross to the opposite side (the midline) and ascend to the thalamus.

  • Primary Somatosensory Cortex (S1): Located in the postcentral gyrus, receives touch information from the contralateral side of the body and is organized somatotopically as a sensory homunculus.

The Concept of Dermatomes

  • Dermatome: A region of skin supplied by a specific spinal nerve root.

    • Pathway: Sensory nerve to the dorsal column to the brainstem, then to thalamus, and finally to the primary somatosensory cortex.

Understanding Pain

  • Pain: Defined as the subjective discomfort that is associated with actual or potential tissue damage.

  • Role of Pain: It prompts withdrawal from harmful stimuli, encourages healing behaviors, and communicates to others about potential harm.

  • McGill Pain Questionnaire: Characterizes pain through three dimensions:

    1. Sensory-Discriminative Dimension: Describes the pain and its location.

    2. Motivational-Affective Dimension: Explores the emotional quality of pain over time.

    3. Cognitive-Evaluative Dimension: Assesses pain strength and significance.

Pathways for Pain Processing

  • Nociception: Involves the neural encoding of tissue damage, facilitated by nociceptors—specific peripheral receptors located on free nerve endings that respond to painful stimuli.

Mechanisms of Pain Transmission

  • TRPV1: A receptor that detects high temperatures (above 43 °C) and responds to capsaicin and acids, inducing sensations of pain via slow-conducting unmyelinated C fibers.

  • TRPM3: A distinct receptor that detects even higher temperatures (above 53 °C), found on fast-conducting A delta fibers, which transmit pain signals rapidly.

Pain Signal Transmission to the Brain

  • The Anterolateral (Spinothalamic) System: Carries pain and temperature sensations to the brain, with primary pathways including:

    • Nerve fibers enter the dorsal horns of the spinal cord and synapse on spinal neurons.

    • Project across the midline to reach the thalamus, which then relays information to the cingulate cortex, a key area for pain perception and emotional response.

    • Neurotransmitters like glutamate and substance P are released to enhance pain signaling and remodel associated neurons.

Pain Management Techniques

  • Gate Control Theory: Suggests that spinal modulation sites act as gates to regulate pain signals before they reach the brain.

  • Analgesia: Refers to the absence or reduction of pain. Methods include:

    • Opiates: Bind to opioid receptors in the brain to reduce pain perception (e.g., endorphins).

    • Epidural/Intra-thecal Injections: Deliver opioids directly into the spinal cord to manage pain more effectively.

Types of Pain Relief

  • Psychogenic Pain Relief Techniques:

    • Placebo: May trigger the body’s own endorphins, resulting in pain relief.

    • Hypnosis: Alters perception of pain through mental techniques.

    • Stress Management: Can involve both opioid and non-opioid systems.

    • Cognitive Strategies: Learning techniques to cope with and interpret pain.

  • Pharmacological Pain Relief Techniques:

    • Opiates: Interaction with specific receptors for fast pain relief.

    • Spinal Block: Directly interrupts pain signals in the spinal cord.

    • Anti-inflammatory Drugs: Target and reduce chemical signals at the injury site.

    • Cannabinoids: Act at multiple levels, including nociceptor endings, spinal cord, and brain to relieve pain.

Recent Discoveries in Pain Mechanics

  • Nociceptive Schwann Cells:

    • A newly discovered type of glial cell associated with nociceptive nerve fibers that form a network in the epidermis.

    • This meshwork plays a crucial role in activating pain responses to mechanical stimuli, showing the complexity of pain signaling at the cellular level.

Conclusion

  • The processing of pain and other sensory modalities underscores the complexity of our neural systems, integrating both physical sensations and emotional responses for protection and interaction with the environment.