Ch8-General Principles of Sensation

General Principles of Sensory Processing

• Five senses: vision, audition, somatosensation, gustation, olfaction
• Each sensory system processes various stimulus attributes
• Each sensory system has multiple receptors and neural pathways to integrate features of stimuli. Additional senses include vestibular for balance, temperature, pain, and proprioception.
• There are distinct hierarchical neural pathways for various sensory systems.

Stimulus Salience

• Umwelt is a species-specific perceptual world, differing between species
• Salience is a consequence of brain processing. Important modalities occupy larger brain regions
• Different animal species have varying hearing ranges which influence stimuli salience for social interactions

Sensation and Perception

• Sensation: detection and recognition of a stimulus
• Perception: interpretation and appreciating the stimulus
• Different species construct different sensations and perceptions.

The Doctrine of Specific Nerve Energies (Johannes Müller)

• Sensory inputs use different nerve "energies" interpreted by the brain to create specific perceptions.
• The brain is functionally divided with specific nerves conveying specific types of information
• Sensations depend on which neurons are active and to what degree.

Sensory Receptors

• Specialized neurons detecting specific physical stimuli
• Key distinction: these are not neurotransmitter/hormone receptors
• Most sensory receptors do not have axons; they synapse with other neurons

Three Stages of Sensation

• Reception: absorption of physical energy by sensory receptors
• Transduction: conversion of physical energy to electrochemical patterns in neurons
• Coding: correspondence between aspects of the physical stimulus and action potentials in the brain.

Organization of Sensory Inputs

• Sensory systems are topographically organized
• Sensory information is relayed through the thalamus to cortical circuits, with varying degrees of processing.

Processing of Stimulus Intensity

• Intensity can be encoded by active neurons or firing frequency.
• Intensity range is processed beyond individual neuron firing, with assemblies of neurons encoding diverse intensities.

Processing of Stimulus Frequency

• Frequency can be encoded by active neurons or number of action potentials.
• Frequency is encoded differently compared to intensity encoding, and firing rates are limited. Cochlear hair cells encode different sound frequencies along the basilar membrane; different frequencies are processed at different locations.

Adaptation to a Stimulus

• Tonic receptors: slow or no adaptation
• Phasic receptors: rapid adaptation
• Neural activity and perception may change as a consequence of adaptation.

Plasticity in Sensory Processing

• Experience-dependent plasticity in topographical organization of somatosensory maps.
• Sensory input can reorganize based on peripheral lesions or environmental stimulation.

Somatosensation - Epicritic and Protopathic Touch

• Epicritic touch: fine, discriminative touch (mechanoreceptors)
• Protopathic touch: nondiscriminating, pain, and temperature (free nerve endings)

Somatosensation - Pathway Components

• Sensory neurons (afferent) that carry information from the skin and other body parts are the first-order neurons. These neurons synapse with second-order neurons in the spinal cord or brainstem.
• Second-order neurons decussate, which means they cross over to the opposite side of the body before traveling to the thalamus
• Third-order neurons synapse in the thalamus and carry information to the primary sensory cortex.
• There are both dorsal column medial lemniscal pathways and spinothalamic pathways for somatosensation. The dorsal column-medial lemniscus pathway is for fine touch, pressure, and proprioception, whereas the spinothalamic pathway is for pain, temperature, and crude touch.