Principles of sensory perception

Principles of Sensory Perception

  • Sensation and Perception

    • Sensory systems allow for perception, response, and interaction with environments (both internal and external).

    • Sensation: Detection of a relevant stimulus, requiring a sense organ.

    • Perception: Interpretation of sensory stimuli, requiring the brain.

    • Sensory stimuli to the brain maintain arousal; deprivation can lead to loss of consciousness.

    • Variety of senses in humans includes: touch, vision, smell, taste, hearing, and proprioception (balance).

Organizational Principles of Sensory Systems

  • Hierarchical Organization:

    • Neural information typically travels from specialized receptors.

    • Pathway: specialized receptors → subcortical thalamus → primary sensory cortices → secondary sensory cortices → association cortices.

    • Specific information about stimuli is reported by receptors and processed through this hierarchy, ultimately reconstructed in higher association areas of the brain.

  • Sensory Modalities:

    • Distinct types of sensory stimuli perceived are referred to as sensory modalities.

    • Detection relies on the functions of sensory receptors, which convert different kinds of physical/chemical energy into electrical signals for the nervous system.

Sensory Receptors

  • Types of Receptors:

    • Primary Receptors: Specialized nerve endings (e.g., olfactory receptors, pain receptors).

    • Secondary Receptors: Separate cells innervated by the afferent nerve (e.g., hair cells).

  • Transduction: Sensory receptors transduce a form of physical energy into electrical energy/signals used by the nervous system.

  • Receptor Potential:

    • A graded, mostly ‘depolarizing’ potential in the receptor upon application of an appropriate stimulus.

    • Increasing stimulus strength results in a larger receptor potential.

    • The receptor potential results from the opening or closing of ion channels in the membrane.

    • Sufficient receptor potentials trigger action potentials.

Touch and Proprioception

  • Significance of Touch and Proprioception:

    • Proprioception provides an understanding of body positioning in space; it gathers information from skin, muscles, and joints.

    • Both senses are vital for developing a sense of ‘self’.

    • Touch has emotional and social content, crucial for the social development of children.

    • Touch deprivation can lead to touch starvation.

    • The skin is the largest organ of the body, providing somatosensory information (touch, temperature, pain) for interaction with the environment.

Loss of Sensory Modality

  • Case Study: Ian Waterman lost both touch and proprioception due to an illness.

    • Despite having an intact motor system, he could not move without visual cues.

    • He trained himself to move by focusing on limb movements mentally.

Touch Receptors in Skin

  • Mechanoreceptors & Their Functions:

    • Different touch receptors are involved in sensations like pressure and vibration:

    • Meissner’s corpuscle

    • Merkel disk receptor

    • Ruffini ending

    • Pacinian corpuscle

    • Hair receptor

    • Differential distribution in hairy and non-hairy (glabrous) skin.

    • Activation leads to sensation of touch as signals reach the somatosensory cortex.

  • Integration and Sub-modalities:

    • Various receptors contribute to sub-modalities (vibration primarily by Pacinian corpuscle; light touch by Merkel cells).

    • Actual tactile sensation arises from integrating information in the primary somatosensory cortex (S1).

Proprioceptors

  • Types of Proprioceptors:

    • Muscle Spindles: Monitors muscle length.

    • Golgi Tendon Organs: Located on tendons, providing information on force exerted by muscles.

    • Mechanoreceptors in joints and ligaments enhance proprioceptive feedback.

    • Proprioception is also influenced by the vestibular organs in the inner ear, which mediate gravitation.

Neural Pathways

  • Dorsal Column Pathway:

    • Carries primarily touch and proprioceptive signals.

    • Conveys pain and temperature signals via an alternative pathway.

  • Neural Transmission:

    • Dorsal horn neurons project to the contralateral side at the brainstem (medulla) through the 2nd order neuron.

    • Thalamic neurons (3rd order neuron) target primary somatosensory cortex topographically.

    • Secondary somatosensory cortex (S2) builds complex representations, like texture and size of objects, from both sides of the body.

Discovery of Piezo Receptors

  • Nobel Prize Contribution by Ardem Patapoutian (2021):

    • Identification of a cell line that is stretch-sensitive and produces an electric signal under mechanical probing.

    • Silencing the 72nd candidate gene (Piezo 1) made cells insensitive to poking.

    • Phenomenon led to the discovery of Piezo2 as a major receptor channel for touch and proprioception.

    • Piezo receptors are also integral for internal organ functions, including urination and respiration.

Consequences of Piezo Receptor Mutation

  • Patients lacking Piezo2 receptors experience issues in normal walking when blindfolded, attributed to variability in stride force and length but not when sight is present.

Plasticity of the Somatosensory Cortex

  • Functional Reorganization:

    • Representation and connection in the somatosensory cortex are not static; they exhibit plasticity.

    • Example: sewing two fingers together changes the cortical map, leading to blurred borders in S1 representation.

    • The size of cortical response varies with the age at which musicians begin training, demonstrating developmental plasticity.