Simple receptor structure: the first order neuron acts as the receptor.
Receptors are widely distributed across the body's surface (e.g. touch, temperature, pain, vibration).
Processed in the primary sensory cortex located in the post central gyrus.
Sensations from the right side of the body are processed by the left side of the brain and vice versa.
Special Senses:
Complex receptor structure: involve a receptor cell that synapses onto a first order neuron.
Receptors located in specific organs (e.g. photoreceptors in the retina, auditory receptors in the cochlea).
Each special sense has a dedicated cortical region for processing.
Allow for more complex signaling pathways.
Receptive Fields
General Senses: Have larger receptive fields, sometimes with specialized tissue.
Special Senses: Have very small receptive fields, requiring direct stimulation of the receptor cell.
Central Processing
General senses are processed in the post central gyrus.
Special senses have specific cortical regions for processing (e.g., vision in the occipital lobe).
Mapping in the Brain
Homunculus: Represents a map along the post central gyrus reflecting the body’s layout.
Different parts of the body are mapped to specific regions on the post central gyrus.
Labeled Line Theory
Describes how the pathway from the receptor cell to the cerebral cortex is mapped.
Each line tells the brain the location and type of stimulus.
Referred Pain: Occurs when pain is perceived in a different location than where it originates.
Sensory Coding
Determines intensity and duration of stimulus.
Works similarly to Morse code and involves firing patterns in the cortex.
Types of Receptors
Phasic Receptors:
Have an on/off phase (e.g., like a light switch).
Only respond to new stimuli.
Tonic Receptors:
Continuously active, providing constant information to the brain.
Can increase or decrease neurotransmitter release based on stimulus.
Adaptation
Definition: Decrease in sensitivity to a constant stimulus.
Peripheral Adaptation:
Occurs at the receptor cell in the periphery.
Example: Becoming unaware of the sensation of clothing.
Central Adaptation:
Occurs at a sensory nucleus along the pathway.
Allows for filtering of constant stimuli while permitting new stimuli to be detected.
Additional Concepts
Desensitization Therapy: Used for reducing sensitivity after surgeries or injuries.
New stimuli are prioritized over old ones due to tonic receptor functionality.
Conclusion
This lecture provided an overview of sensory systems, focusing on the differences between general and special senses, how information is processed and coded, and mechanisms of adaptation.
Further understanding of specific receptors will be covered in subsequent lectures.