Lecture 13

Fundamental Response Properties of Receptor Cells

Overview of Sensory Receptors

  • Sensory receptors respond to stimuli with specific qualities that define their receptive fields.

  • The receptive fields of neurons, further downstream in sensory pathways, are often characterized as:

    • More complex than initial sensory receptors.

    • The neurons fire under a more restricted set of conditions.

Definitions

  • Receptive Field: The specific area or conditions under which a sensory neuron or receptor responds to stimuli.

Neurons in Sensory Pathways

  • Neurons that are further downstream in the sensory pathways also have their own receptive fields.

  • Example of Receptive Fields:

    • Photoreceptor X: Fires when detecting light from a specific region of visual space.

    • Complex cell Y: Fires only to an edge at a specific orientation moving in a specific direction.

Peripheral Receptive Field Sizes and Sensory Resolution

  • Peripheral receptive field sizes ultimately define sensory resolution.

    • Better sensory discrimination requires more neurons at all steps in the circuit to preserve this resolution, leading to an energy and space trade-off.

Example of Sensory Discrimination

  • Stimulus Effect on Skin:

    • Lips: Two distinct points are felt due to the density of sensory receptors.

    • Back: Only one point is felt due to lower receptor density.

Coverage of Sensory Space

  • Receptive fields vary across cells, ensuring that sensory space is well-covered by the receptor array.

  • Spatial Receptive Fields of V1 Neurons: The representation of sensory information is structured, ensuring a comprehensive coverage of the sensory input.

Topographic Mapping of Sensory Information

  • Higher-level sensory systems represent sensory information in an orderly (topographic) map.

  • This involves:

    • Ordered Spatial Representation: Reflects the organization of the sensory/motor world in the Central Nervous System (CNS).

    • Dimensions of the map can include real spatial arrangements in the environment (e.g., visual field) or other dimensions like sound frequency.

  • Somatotopic maps located in primary somatosensory and motor cortex demonstrate this orderly representation.

Brain Area Allocation for Sensory Processing

  • Proportional size of brain areas is dedicated to processing somatosensory information from different body regions.

    • High sensory resolution is linked to expanded representation in the CNS.

  • Percentage of Somatosensory Cortex Devoted to Body Parts:

    • Forepaw: 10.2%

    • Trunk/Tail: 13.5%

    • Chin: 7.5%

    • Hindpaw: 7.8%

    • Incisors: 30.8%

    • Buccal Cavity: 11.5%

    • Head Vibrissae: 18.8%

Determinants of Touch Resolution

  • Factors contributing to touch resolution:

    • Densely packed receptors with small receptive fields have higher resolution.

    • Sparsely packed receptors with large receptive fields may converge projections significantly.

Specifics of Somatosensation

  • Definition: Somatic (body) sensation, referred to as somatosensation, involves the sensation of the body's physical interactions with itself and the environment.

  • Components of Somatosensation:

    • Mechanoreception (detection of mechanical pressure or distortion):

    • Cutaneous Mechanoreceptors: Involved in the sense of touch.

    • Proprioceptors: Monitor position and load of body parts.

    • Baroreception: Pertains to blood pressure detection.

    • Thermoreception: Involves temperature detection.

    • Nociception: Involves pain detection (some nociceptors are mechanoreceptors, but not all).

Mechanotransduction in Somatosensation

  • Mechanotransduction refers to the process whereby mechanical forces cause opening of ion channels in the cell membrane, leading to receptor potentials.

  • Key Mechanisms:

    • Mechanoreceptor proteins typically open cation channels when the cell membrane is deformed, resulting in depolarizing receptor potentials.

Receptor Potential and Stimulus Strength

  • The magnitude of receptor potential depends on the forces applied to the mechanoreceptor's cell membrane:

    • Ion Channel Dynamics:

    1. Closed Ion Channels: At rest.

    2. Weak Stimulus: Membrane is stretched; some ion channels open, leading to a receptor potential.

    3. Moderate Stimulus: Increased stretching causing greater receptor potential.

    4. Strong Stimulus: Significant stretching, resulting in clearly defined receptor potentials that may lead to action potentials if threshold is exceeded.

Types of Cutaneous Mechanoreceptors

  • Various types of mechanoreceptors exist in the skin, each with distinct properties and responses to physical stimuli:

    • Meissner's Corpuscle: Rapidly adapting mechanoreceptor for touch and pressure.

    • Merkel's Corpuscle: Slowly adapting mechanoreceptor for touch and pressure.

    • Free Nerve Ending: Some are nociceptors and thermoreceptors; they are slowly adapting.

    • Pacinian Corpuscle: Rapidly adapting mechanoreceptor for vibration and deep pressure.

    • Ruffini Corpuscle: Slowly adapting mechanoreceptor for skin stretch.

Tonic vs. Phasic Receptors

  • Tonic and phasic receptors differ in how they respond to stimuli:

    • Tonic Receptors (e.g., Merkel disc, Ruffini ending): Produce action potentials during sustained stimuli.

    • Phasic Receptors (e.g., Meissner and Pacinian Corpuscles): Begin firing during stimulus onset, with adaptation that results in fewer action potentials upon continued stimulation.

Pain Detection and Pathways

  • Specialized nociceptive and non-nociceptive somatosensory pathways exist, typically with higher firing thresholds than other somatosensory cells.

  • Labeled lines encoding indicates that no stimulation of certain cells will elicit the sensation of pain; pathways are distinguished for different modalities.

Proprioception Overview

  • Proprioception refers to the sensation of position and load on different body parts.

  • Involves sensory neurons located in muscles and connective tissues, with ascending afferent and descending efferent projections contributing to the understanding of body position.

Structure of Skeletal Muscles

  • Skeletal muscles attach to and cause movements of our bones and are primarily composed of large, parallel cells called extrafusal muscle fibers.

  • Muscle spindles, intrafusal fibers along with mechanosensory neurons, encode the degree of muscle stretch and contraction (rate coding).

Functions of Proprioceptors

  • Proprioceptors provide significant sensory feedback for:

    • Informing voluntary circuits about body part positions.

    • Triggering involuntary skeletal muscle stretch reflexes.

    • Involving components that compare expected and actual motor outputs with feedback mechanisms localized likely to the cerebellum, aiding in compensatory movements.