Sensory pathways in the central nervous system are like a complex network of highways, carrying information from our body to our brain. These pathways start with receptors that detect stimuli and end in specialized brain areas that process and interpret the signals.

The thalamus acts as a central hub, relaying sensory information to different parts of the cortex. From there, the brain processes sensory input in a hierarchical manner, with each level adding more complexity to our perception of the world around us.

Sensory Pathways and Processing in the Central Nervous System

Sensory pathways to central nervous system

• Sensory receptors detect stimuli and generate receptor potentials through transduction, which converts stimuli into electrical signals (mechanoreceptors, chemoreceptors, photoreceptors)
• Sensory neurons, called first-order neurons, have cell bodies in dorsal root ganglia or cranial nerve ganglia and transmit action potentials from the receptors to the central nervous system (CNS) via their axons that enter the spinal cord or brain stem
• Sensory information is relayed and processed in the CNS by second-order neurons, which receive input from first-order neurons and relay the information to higher brain centers, and third-order neurons, which receive input from second-order neurons and project to the thalamus or cerebral cortex

Dorsal column vs spinothalamic tracts

• The dorsal column-medial lemniscus tract transmits fine touch, vibration, and proprioception from the body, with first-order neurons entering the spinal cord and ascending in the dorsal columns, synapsing in the medulla oblongata (nucleus gracilis and nucleus cuneatus), and second-order neurons decussating to form the medial lemniscus that projects to the ventral posterolateral nucleus of the thalamus
• The spinothalamic tract transmits pain, temperature, and crude touch from the body, with first-order neurons entering the spinal cord and synapsing in the dorsal horn, second-order neurons decussating and ascending in the contralateral spinothalamic tract, which projects to the ventral posterolateral nucleus of the thalamus

Trigeminal pathway for facial sensation

• The trigeminal nerve (cranial nerve V) conveys sensory information from the face, with the trigeminal ganglion containing cell bodies of first-order neurons that project to the trigeminal nuclei in the brain stem: the main sensory nucleus for touch and pressure and the spinal trigeminal nucleus for pain and temperature
• Second-order neurons from the trigeminal nuclei project to the ventral posteromedial nucleus of the thalamus, and third-order neurons from the thalamus project to the primary somatosensory cortex

Somatotopic organization in sensory systems

• Somatotopic organization refers to the spatial arrangement of sensory input corresponding to the body surface, where sensory information from different body parts is mapped onto specific regions of the CNS (somatosensory cortex, dorsal columns)
• The dorsal columns have a medial-to-lateral arrangement of fibers from sacral to cervical segments, while the somatosensory cortex has a "sensory homunculus" with disproportionate representation of sensitive areas (hands, face)
• Somatotopic organization allows for precise localization and discrimination of sensory stimuli from different parts of the body
• Receptive fields play a crucial role in somatotopic organization, defining the area of the body surface that, when stimulated, activates a particular sensory neuron

"What" vs "where" in visual processing

• The "what" pathway (ventral stream) processes object recognition and identification, beginning in the primary visual cortex (V1) and projecting to the inferior temporal cortex, involving areas such as V4 and the lateral occipital complex, and contributing to perception of color, form, and facial recognition
• The "where" pathway (dorsal stream) processes spatial information and guides visual-motor actions, beginning in the primary visual cortex (V1) and projecting to the posterior parietal cortex, involving areas such as V5/MT (middle temporal) and the superior parietal lobule, and contributing to perception of motion, depth, and spatial relationships

Thalamic Relay and Cortical Processing

Role of thalamus in sensory processing

• The thalamus serves as a major relay station for sensory information to the cerebral cortex, with specific thalamic nuclei receiving input from different sensory pathways: ventral posterolateral (VPL) nucleus for somatosensory information from the body, ventral posteromedial (VPM) nucleus for somatosensory information from the face, lateral geniculate nucleus (LGN) for visual information, and medial geniculate nucleus (MGN) for auditory information
• Thalamic nuclei process and modulate sensory information before projecting to the cortex by gating and filtering sensory signals and modulating them based on attention and arousal
• Thalamocortical projections target specific areas of the primary sensory cortices (somatosensory cortex, visual cortex, auditory cortex)

Hierarchical organization of sensory processing in cortex

• Primary sensory cortices (V1, S1, A1) receive thalamic input and perform initial processing of simple stimulus attributes and feature detection while maintaining topographic organization from the thalamus
• Secondary sensory cortices (V2, S2, A2) receive input from primary cortices and perform higher-order processing, integrating features and more complex stimulus attributes with increased receptive field sizes and decreased topographic precision
• Association cortices (posterior parietal, inferior temporal) integrate information from multiple sensory modalities, contributing to perception, recognition, and spatial awareness through multimodal processing and cross-modal interactions
• Feedback projections from higher to lower cortical areas modulate and refine sensory processing through top-down influences on perception and attention and contextual modulation and predictive coding

Sensory Processing and Integration

• Neural coding is the mechanism by which sensory information is represented and transmitted in the nervous system, involving patterns of neural activity that encode specific stimulus features
• Sensory integration involves combining information from multiple sensory modalities to create a coherent perception of the environment
• Synaptic plasticity plays a crucial role in sensory processing by allowing for experience-dependent changes in neural connections, enhancing the ability to process and interpret sensory information
• Sensory adaptation is the process by which sensory systems adjust their sensitivity to prolonged or repeated stimuli, helping to maintain responsiveness to new or changing stimuli in the environment