Sensory Receptor Function

Link between the internal or external environment and the nervous system. If a stimulus is strong enough, action potentials will be produced, travel to the CNS and be translated into a sensation at the conscious or subconscious level.

Process of sensation:

• Stimulus - a change in the environment that occurs within a neuron's receptive field
• Transduction - receptor converts specific energy form of stimulus into action potentials

Example of a sensory receptor specialized for sensing pressure - lamellated (Pacinian) corpuscle

• Composed of a straight dendritic ending surrounded by concentric layers of Schwann cells
• When small amounts of pressure are applied, a graded potential (receptor potential) is produced.  As pressure increases, the receptor potential increases.  When the receptor potential = about 10mv (threshold), action potentials are produced at the first node followed by repolarization.  Additional action potentials are produced as long as the receptor potential is above threshold

Information content of sensory transmission:

1. Modality - quality or type of stimulus
   1. Receptor specificity - each receptor type best responds to one specific form of energy, transmits its own modality no matter how stimulated
   2. Labeled line - neural link between receptor and cortical neuron, carries one modality
2. Location - each receptor has a pathway to a specific part of the brain. Sensation produced depends upon the specific part of the cerebral cortex activated. Brain identifies stimulus location by its route (projection pathway)
   1. Three portions of the pathway are:
      • 1^st order - from receptor to spinal cord or brainstem
      • 2^nd order - from spinal cord or brainstem to thalamus, cross-over occurs
      • 3^rd order - from thalamus to cerebral cortex
   2. Referred pain - visceral pain is perceived on body surface because both areas use same interneurons and pathways
   3. Projection of phantom limb - impulse traveling same path
3. Stimulus duration - as long as threshold level receptor potential continues
   1. Adaptation - continuous stimulus of constant strength applied to receptor, response gradually ceases, removal of stimulus is also a change
   2. Peripheral adaptation - occurs at receptor
      • Tonic receptors - poorly adapting, transmit impulses for a long time, some tonic receptors are always active indicating background level. ex. pain receptors, joint capsule, muscle spindle
      • Phasic receptors - rapidly adapting, transmit impulses for a long time, some phasic receptors transmit information about rapid changes in stimulus intensity and rate (rate information has predictive value)
   3. Central adaptation - inhibition occurs along sensory pathways within CNS, sensitivity can be adjusted by faciliatation
4. Stimulus intensity - expressed by
   1. Recruitment - stronger stimuli excite more receptors
   2. Threshold - weak stimuli excite low threshold receptors and strong stimuli excite high threshold receptors
   3. Action potential frequency - varies with stimulus strength

Relationship between stimulus strength and action potential frequency:

• Generator\Receptor potential increases as stimulus strength increases
• Amplitude of the generator potential increases rapidly at lower stimulus strength changes and slower at higher changes
• Production of action potentials does not affect the generator potential. As long as the generator potential is above threshold, new action potentials are produced after each repolarization
• Frequency of action potential is almost directly related to the amplitude of the generator potential

Figures in class

Receptors can be classified by:

• Complexity of the receptor structure
  1. Unencapsulated free nerve endings - have no structural specialization
     • ex. light touch, pain, temperature, texture
  2. Encapsulated nerve endings - wrapped in glial cells or connective tissue
     • ex. touch, deep pressure, vibration, stretch
  3. Sense organs - multicellular, special senses
     • ex. eye, inner ear, taste buds, olfaction
• Stimulus type
  1. Mechanoreceptors, photoreceptors, chemoreceptors, nociceptors (pain), baroreceptors
• Location of the receptor in the body
  1. Exteroceptors - sensitive to external stimuli
  2. Interoceptors\visceroreceptors - sensitive to visceral stimuli
  3. Proprioceptors - located in muscles, tendons and joints, provides positional information on joint activity and muscle tension

Example of a kinesthetic proprioceptor - muscle spindle in skeletal muscle

• Responds to stretch of extrafusal fibers (outside of spindle) in regular skeletal muscle
• 3-10 intrafusal muscle fibers in c.t. capsule attached to endomysium or perimysium
• Central part of the fibers have no actin and myosin but the ends do and can shorten
• Central part is wrapped by type Ia sensory fibers and the ends innervated by type II sensory fibers, both sensory fibers increase their output when the spindle is stretched, type Ia by the rate and amount of stretch, type II by the degree of stretch based on muscle tone. Output decreases with muscle compression.
• Stretch may occur by lengthening the entire muscle as a result of weight or antagonistic muscle contraction or shortening of only the spindle fibers (enhances sensitivity)