TB

Sensory Reception

Sensory Receptors

  • Two types of sensory receptors:
    • Receptor membrane: part of the cell with specific channels that take in stimulus/energy and pass it down the axon.
    • Built into the afferent neuron or its own individual receptor cell that interacts with the neuron.
    • End goal: get the message into the central nervous system.

Afferent Neuron with Receptor Membrane Ending

  • Myelinated afferent neuron.
  • Stimulus comes in as a mechanical message.
  • Graded potential: increase stimulus intensity to increase receptor potentials.
    • Receptor potential: membrane potential of the receptor neuron.
  • Axon: chemical message changes to an electrical message (action potentials).

Action Potentials

  • Action potentials go from a threshold to about +30 millivolts, then back down.
  • Amplitude is the same regardless of stimulus intensity.
  • Myelinated afferent neuron: jumping effect from one node of Ranvier to the next.
  • No myelination: action potential spreads along the whole axon.

Slow vs. Rapid Adapting Receptors

  • Different receptors have prolonged or quick reactions, depending on what they respond to.
  • Slowly adapting (tonic) receptors:
    • Example: mechanoreceptor in a muscle.
    • Receptor potential initially spikes and continues for a certain time.
    • Receptor potential decreases over time.
    • Action potential frequency decreases as receptor potential lowers, leading to muscle fatigue.
    • Less information reaches the brain and muscles, reducing force production.
  • Basic (fast responding) receptors:
    • Receptor potential when stimulus is added and released.
    • Action potentials sent only when receptor potentials occur.
    • Smaller receptor potential upon stimulus release results in fewer action potentials.

Receptor Specialization

  • Tonic receptors: consistent information processing for prolonged stimuli.
  • Basic receptors: rapid responses to quick stimulus initiation and deactivation.
  • Coding: conversion of stimulus energy into a signal for the central nervous system.
  • Information conveyed by frequency and amplitude of electrical signals.
  • Stimulus characteristics: type of input, intensity, location on the body.

Afferent Receptor in Skin

  • Nerve layout: long axon with dendrites coming off and cell body to the side.
  • Function: changes graded potential to action potential, moves information to the central nervous system.
  • Example: nociceptor in the skin.
  • Receptive field: area of skin where receptor terminals can take in stimulus.
  • Peripheral terminals code information into graded potential.
  • Axon hillock: where graded potential changes to action potential.
  • Central terminals: axon terminal equivalent for a receptor cell, going to the central nervous system.

Stimulus Modality

  • Stimulus Modality: Different types of stimulus impacting different cells.
  • Specialized receptors for heat, cold, pressure, sound, light, pain, etc.
  • Glass Probe Example
    • Glass probe poking skin with increasing force over time.
    • Receptive field has four potential receptors.
    • Small pressure activates only one receptor.
    • More pressure recruits more receptors within the receptive field.
    • Frequency of action potentials changes with increased pressure.

Fine Control and Receptive Field Size

  • Fine Control
    • Small receptive fields: allow precise stimulus localization.
    • Example: lips have small receptive fields for high sensitivity.
    • Tweezers experiment on lips versus back demonstrates sensitivity differences.
  • Broad Range.
    • Larger Receptive Field:
    • Back has been designed with much larger receptive fields.
    • It would be harder for you to distinguish more than one point of pressure being applied.
  • Analogy to EMG Muscle Cells
    • Fine movement needs less muscle cells while more force needs more cells.
    • Fine control requires smaller fields, broad range requires bigger fields.

Receptive Fields and Stimulus Overlap

  • Stimulus within one receptor field results in the central nervous system reading one stimulus.
  • Overlapping receptive fields: multiple afferent neurons reach the same area.
  • Stimulus on overlapping area (e.g., b) activates multiple neurons (a, b, c).
  • Results in some action potentials from a and c, but more from b.
  • Allows for body to keep some specificity, but also get a lot of information in the past

Lateral Inhibition

  • Lateral inhibition enables localization of stimulus from overlapping receptive fields.
  • Inhibits messages from surrounding neurons (a and c) to increase relative message from the main neuron (b).
  • Systematically causes a more specific response even when afferent nerves aren't designed that way.
    • Creates sort of a fake specificity.
  • Can occur in skin, particularly in areas like fingertips and palms.
  • The purpose of that is we can create this sort of faux specificity if needed.

Ascending and Descending Pathways

  • Ascending pathways: carry sensory information to the central nervous system.
  • Descending pathways: carry information from the central nervous system to the effector.
  • Example is explained as why you don't feel clothes on the skin.

Divergence and Convergence

  • Divergence: one afferent receptor impacts multiple interneurons, amplifying the signal.
  • Convergence: multiple afferent neurons converge on one interneuron, integrating signals.
  • Sensory pathway: three or more neurons connected by synapses.
  • Ascending pathway: external stimulus to the brain.
  • Descending pathway: brain to the effector.

Information in the Brain

  • Specific vs. Nonspecific Ascending Pathways
  • Specific areas include frontal, parietal, occipital, and temporal lobe associated areas, and somatosensory cortex.
  • Somatosensory cortex handles body sensation. Occipital goes to visual information. Temporal handles audio.
  • Frontal Lobe is more complex which requires higher brain thinking.

Specific vs. Nonspecific Ascending Pathways

  • Specific ascending pathway: temperature and touch information have separate receptors and pathways.
  • Nonspecific ascending pathway: temperature and touch information travel up the same pathway after the thalamus and brain stem.
  • Explanation about how quickly managing pain sensations is by the addition of a touch sensation.

Clinical Relevance

  • Rubbing skin after injury floods nonspecific pathway with touch sensation, overriding pain.
  • Referred pain: pain felt in a different area due to shared nonspecific pathways (e.g., heart attack causing left arm pain).

Associated Cortices

  • Associated Cortexes divide information in more specific, complicated ways
  • Regions of associated cortex are close to the primary sensory information with simple delivering of information.
  • Things like emotions, personalities, experience, lack of receptors, damaged neural pathways, drugs, other mental issues will affect this.
  • Schizophrenia as an example of visual or auditory hallucinations causing an effect to the interneurons to act as if the receiver received information from this earlier ascending pathway.

General Principles of Sensory Stimuli Processing

  • Five Aspects: modality, duration, intensity, location, sensation, and perception.

Somatic Sensation

  • From skin, skeletal muscles, bones, tendons, joints.
  • Receptors detect touch, pressure,