Summary: Sensory Systems

CHAPTER 5: SENSORIMOTOR SYSTEM

Sensory Systems Detect Various Forms of Energy

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Description automatically generated-all animals have sensory organs containing receptor cells that sense stimuli

-receptor cells act as filters, ignore environmental background and convert key stimuli into electrical signals

-although action potentials are the same for different sensory modalities, the brain recognizes the modalities as separate and distinct because action potentials for each sense are carried in separate nerve tracts

Labelled Lines: concept that each nerve input to brain reports only particular type of info

Receptor Cells Convert Sensory Signals into Electrical Activity

-structure of receptor determines kind of energy/chemical it will respond to

Sensory Transduction: process where receptor cell converts energy in stimulus into change in electrical potential across membrane (environmental stimuli into action potentials)

-sense of touch is not just touch; other sensory experiences include pressure, vibration, tickle, pins and needles, smoothness, wetness, etc.


Pacinian Corpuscle (Lamellated Corpuscle): example of sensory transduction process; onion-like structure that selectively responds to vibrations and pressure


Merkel’s Discs – especially responsive to edges and isolated points on a surface


Meissner’s Corpuscles seem to respond to changes in stimuli, allowing them to detect localized movement between skin and surface

-Meissner’s Corpuscles (Tactile Corpuscles) and Merkel’s Discs mediate most of the ability to perceive forms of object through touch


Ruffini Corpuscles: detect stretching of patches when moving fingers or limbs


Free Nerve Endings (in the skin): detect pain, heat, and cold stimuli

Diagram

Description automatically generatedSensory Info Processing is Selective and Analytical

-sensory organs and pathways convey limited (even distorted) info to brain

-brain receives highly filtered representation of external world

-sensory neurons are able to tell brain about the intensity and location of a stimulus based on the action potentials’ number and frequency/rhythm of clusters occurring

-employ multiple sensory receptors to cover range

-intensity of stimulus is represented by number and thresholds of activated cells

-position of stimulus will provide brain with additional cues to location


Receptive Field: stimulus region and features that affect activity of cell in sensory system

-neurons at every level will respond to particular stimuli, so each cell has own receptive field


Sensory Adaptation: progressive decrease in receptor’s response to sustained stimulation

-process allows to ignore unimportant events

-sensory systems avoid overload and can remain vigilant for critical events

Phasic Receptors: display adaptation

Tonic Receptors: show little to no adaptation, can signal duration of stimulus (ex. pain

sensors)


-survival depends more on sensitivity to important changes than exact reporting of stimuli

-to maintain sensitivity, some unneeded/unimportant sensory activity must be supressed

-adaptation is one way sensory activity is controlled


2 Suppression Systems

  1. Physically prevent stimuli from reaching sensors – ex. closing eyelids
  2. Neural nature – reciprocal neural connections can actively inhibit

Successive Levels of CNS Process Sensory Information

-each sensory system has a pathway that travels to highest levels of the brain

-ex. somatosensory touch send axons from skin to dorsal part of spinal cord, then ascend to column system (white matter) and eventually to thalamus and finally cortex

-inputs are organized in somatosensory map and surface is divided into bands known as dermatome: strip of skin that is innervated by particular spinal nerve


-sensory cortex is highly organized

-each sensory modality has its own region designated as the initial destination of sensory inputs to the cortex

-primary somatosensory cortex (somatosensory 1/S1) is located in postcentral gyrus, long strip of tissue located posterior to central sulcus dividing parietal and frontal lobe

-map is distorted to indicate size of each region proportional to the density of sensory receptors found in that region of the skin – sensory homunculus


-use of one sensory system influences perception from another

Association Areas: sensory areas in the brain processing different modality inputs


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