14.2 Central Processing

Sensory Pathways and Processing in the Central Nervous System

Sensory pathways carry information from receptors throughout your body to specific brain regions that interpret those signals. Understanding how these pathways are organized helps you see why damage at different levels of the CNS produces different sensory deficits.

These pathways follow a general pattern: receptors detect a stimulus, a chain of neurons relays that signal through the spinal cord or brainstem to the thalamus, and the thalamus routes it to the appropriate cortical area for conscious perception.

Sensory pathways to the central nervous system

Sensory processing begins at the receptor level. Sensory receptors detect stimuli and convert them into electrical signals through a process called transduction. Different receptor types handle different stimuli: mechanoreceptors respond to pressure and stretch, chemoreceptors respond to chemical changes, and photoreceptors respond to light.

From there, the signal travels through a relay of three neuron types:

• First-order neurons have their cell bodies in the dorsal root ganglia (for the body) or cranial nerve ganglia (for the head). Their axons carry action potentials from receptors into the spinal cord or brainstem.
• Second-order neurons receive input from first-order neurons within the CNS and relay the signal to higher brain centers. These neurons typically cross (decussate) to the opposite side of the CNS.
• Third-order neurons receive input from second-order neurons and project to the cerebral cortex, usually via the thalamus.

This three-neuron chain is a recurring theme across most sensory pathways, so knowing it well makes learning each specific tract much easier.

Dorsal column vs. spinothalamic tracts

These are the two major ascending pathways for somatic sensation from the body. They differ in what they carry, where they synapse, and where they cross the midline.

Dorsal Column–Medial Lemniscus (DCML) Pathway — carries fine touch, vibration, and proprioception:

1. First-order neurons enter the spinal cord and ascend ipsilaterally in the dorsal columns (without synapsing).
2. They synapse in the medulla oblongata at the nucleus gracilis (lower body) or nucleus cuneatus (upper body).
3. Second-order neurons decussate (cross the midline) in the medulla and form the medial lemniscus.
4. The medial lemniscus ascends to the ventral posterolateral (VPL) nucleus of the thalamus.
5. Third-order neurons project from the thalamus to the primary somatosensory cortex.

Spinothalamic Pathway — carries pain, temperature, and crude touch:

1. First-order neurons enter the spinal cord and synapse in the dorsal horn.
2. Second-order neurons decussate in the spinal cord (within a segment or two of entry) and ascend in the contralateral spinothalamic tract.
3. The tract projects to the VPL nucleus of the thalamus.
4. Third-order neurons project from the thalamus to the primary somatosensory cortex.

Trigeminal pathway for facial sensation

The face has its own dedicated sensory pathway through the trigeminal nerve (cranial nerve V), which functions as the facial equivalent of the spinal sensory pathways.

• First-order neuron cell bodies sit in the trigeminal ganglion. Their central processes project to brainstem nuclei: the main sensory nucleus handles touch and pressure, while the spinal trigeminal nucleus handles pain and temperature.
• Second-order neurons from these nuclei cross the midline and project to the ventral posteromedial (VPM) nucleus of the thalamus. Notice this is VPM for the face, compared to VPL for the body.
• Third-order neurons project from the thalamus to the primary somatosensory cortex, where facial sensation is represented.

Somatotopic organization in sensory systems

Somatotopic organization means that the spatial layout of the body surface is preserved as a map within the CNS. Adjacent body parts are represented in adjacent neural regions, creating an orderly "body map."

You can see this at multiple levels:

• In the dorsal columns, fibers are arranged medial-to-lateral from sacral to cervical segments. Fibers from the legs sit medially, and fibers from the arms and neck are added laterally as you move up the cord.
• In the primary somatosensory cortex, the body map is called the sensory homunculus. It's distorted because representation size reflects sensitivity, not actual body size. The hands, lips, and face take up a disproportionately large cortical area because they have the highest density of sensory receptors.

Receptive fields are central to this organization. A receptive field is the area of skin that, when stimulated, activates a particular sensory neuron. Smaller receptive fields (like on your fingertips) allow finer discrimination, which is why you can read Braille with your fingers but not with your back.

"What" vs. "where" in visual processing

After visual information reaches the primary visual cortex (V1), it splits into two processing streams:

• The ventral stream ("what" pathway) flows from V1 toward the inferior temporal cortex. It processes object identity: color, form, and facial recognition. Key areas include V4 (color processing) and the lateral occipital complex.
• The dorsal stream ("where" pathway) flows from V1 toward the posterior parietal cortex. It processes spatial relationships and guides visually directed movement. Key areas include V5/MT (motion processing) and the superior parietal lobule.

Damage to these streams produces different deficits. Ventral stream damage can cause difficulty recognizing objects or faces (visual agnosia), while dorsal stream damage can impair the ability to reach for objects accurately or perceive motion.

Thalamic Relay and Cortical Processing

Role of the thalamus in sensory processing

The thalamus is often described as a relay station, but it does more than just pass signals along. It actively gates and filters sensory information based on your current state of attention and arousal.

Each sensory modality has a dedicated thalamic nucleus:

One notable exception: olfaction is the only major sense that projects directly to the cortex without a thalamic relay.

The thalamus modulates what reaches conscious awareness. This is why you can tune out background noise when concentrating or why a sudden stimulus grabs your attention even when you're focused on something else.

Hierarchical organization of sensory processing in the cortex

Cortical sensory processing is organized in layers of increasing complexity:

Primary sensory cortices (V1, S1, A1) receive direct thalamic input and detect basic stimulus features. Neurons here respond to simple attributes like edges, pressure at a specific skin location, or a particular sound frequency. These areas maintain tight topographic maps from the thalamus.

Secondary sensory cortices (V2, S2, A2) receive input from primary areas and begin integrating features into more complex representations. Receptive fields are larger here, and topographic precision decreases. A neuron in S2, for example, might respond to a texture pattern rather than a single point of contact.

Association cortices (posterior parietal cortex, inferior temporal cortex) integrate information across multiple senses. These regions contribute to perception, object recognition, and spatial awareness through multimodal processing.

Processing doesn't just flow bottom-up. Feedback projections from higher cortical areas back to lower ones refine sensory processing through top-down influences. This is how your expectations and attention shape what you actually perceive, a concept sometimes called predictive coding.

Sensory processing and integration

Several broader principles apply across all sensory systems:

• Neural coding refers to how stimulus features are represented by patterns of neural activity. The frequency, timing, and population of active neurons all contribute to encoding what you're sensing.
• Sensory integration is the process of combining input from multiple modalities into a unified perception. You perceive a dog barking as a single event, not as separate visual and auditory experiences, because your brain integrates across senses.
• Synaptic plasticity allows sensory circuits to change with experience. Repeated exposure strengthens certain connections, which is why musicians develop enhanced auditory discrimination and why Braille readers have expanded cortical representation of their reading fingers.
• Sensory adaptation is the decrease in responsiveness to a constant or repeated stimulus. You stop noticing the feeling of your clothes against your skin within minutes. This frees up processing capacity so your nervous system stays responsive to new or changing stimuli.