Sensory System Physiology

Sensory System
Functions of the sensory system
To interface between internal and external environments and the nervous system.
To SENSE features of the environment
To ENCODE those features
To TRANSMIT the info to the CNS for decoding
To allow the CNS to COMMUNICATE with outside world
Sensory systems need to determine...
Modality & Submodality
Basic Organization of the Sensory Systems
Stimulus energy
Filter (modified stimulus) -- Transducer (receptor potential) -- Encoder (action potential) -- CNS (sensory, association, and effector systems) -- Effectors (muscles) -- Response
The 1st functional element of the sensory receptor, usually non-neural tissue. Allows only a portion of the stimulus to be conveyed to the transducer. It conditions the stimulus making the receptor function with relative specificity for one modality
-Limits access
-Changes stimulus parameters
ex. lens of the eye
Converts stimulus energy into a change in the voltage across the membrane of the sensory receptor with a minimal loss in info. It creates the RECEPTOR POTENTIAL. The change in the membrane potential is a simultaneous change in the membrane permeability to Na+ and K+ ions
Receptor Membrane
Do not respond to electrical stimulation. They have no action potentials, no refractory periods
Receptor Potentials
Graded (increase in magnitude as a stimulus gets stronger
Do not propagate (travels only short distances)
Converts the magnitude of a receptor potential into a frequency code that is carried by action potentials along the axon
Encoder "trigger zone"
Location of the voltage-gated Na+ and K+ channels capable of producing action potentials
If receptor potential entering the zone is above the threshold, an action potential is propagated. As long as the receptor pot. stays above threshold, the cell will continue to fire A.P.
Larger receptor potential = higher the frequency of A.P.s
Components of a sensory receptor
Ex. - Pacinian corpuscle (skin receptor sensitive to vibration/touch)
Filter= connective tissue capsule
Transducer= unmyelinated free nerve ending
Encoder= trigger zone (1st node of Ranvier)
Types of Sensory Receptors
Mechanoreceptors (push, pull)
Thermoreceptors (temp)
Chemoreceptors (pH)
Osmoreceptors (osmotic environ)
Photoreceptors (eye, photons)
Nociceptors (pain)
Poly-modal receptors (respond to 2+ forms of stimuli=usually nociceptors)
All the terminals of any one sensory unit have...
The same kind of receptor
Sensory Receptor "Receptive Fields"
That fraction of the total energy of a particular modality that initiates a response in a sensory receptor or sensory unit.
Sensory Coding MODALITY
What is present?
Modality Coding
Done by "labeled-lines" = anatomical grouping of receptors and pathways devoted to the particular modality
Submodality Coding
Done by "pattern codes" = the pattern of activity within the system of labeled lines
Location Coding
Accomplished by an anatomical sorting or organization of the pathways and their central projections
Structures with lots of receptors are big
Intensity Coding
Done by the total number of impulses per second coming from the receptor population
Rate, recruitment, adaptation, threshold, saturation
The frequency (#/sec) of A.P.s in individual sensory units. The stronger the stimulus, the more A.P.s are sent up the afferent fibers of the sensory unit
The number of units that get activated by the stimulus increases with intensity of the stimulus.
The measure of how well a sensory receptor maintains its response to a constant stimulus
SLOW adapting - keep producing membrane potentials as long as the stimulus is applied
RAPID adapting - stop producing membrane potentials even when the stimulus is still present (interested in CHANGE)
The minimum value of intensity just barely perceivable is the threshold stimulus (no threshold = no A.P.)
As intensity increases, eventually the nerve fiber reaches its upper limit for carrying impulses. Further increases in stimulus do not produce a corresponding increase in the response
Temporal Coding
Different patterns of firing may provide info about submodality
A response is dependent on the rate of change of stimulus application within a particular frequency range. Most receptors do not respond to constant stimuli but are more sensitive to stimulus change (combine stimuli temporarily)
Affect Coding
The least measurable criterion. This attribute takes into account emotional content (pain) and learning and memory which depends on prior input (emotional rxn to stimulus)