Overview Of Sensory Pathways

It is apparent from the descriptions given earlier that specific areas of brain, spinal cord, or peripheral nervous tissue are assigned specific functional roles based on their relative position with the nervous system (Fig. 5). A general rule for neurons might be stated, "Where you are is what you are.'' However, the nervous system should not be viewed as a series of isolated modules with specified duties; rather, it more correctly resembles

A. Cross sections of the nervous system trigeminal ganglion

right face anteriolateral tract

anteriolateral tract spinal cord right foot

FIGURE 5 Sensory axis of the nervous system. Primary sensory neurons carrying pain and temperature information project to the dorsal part of the spinal cord; their cell bodies are located in the dorsal root ganglia. Neurons carrying pain and temperature information from the face project through the trigeminal nerve to the lateral part of the pons; their cell bodies are in the trigeminal ganglion. Pain information is relayed through synaptic connections to second-order and on to higher order neurons in areas shown in blue. Pain relay areas are present in both the spinal cord and thalamus, within pathways that project to the post central gyrus of the cerebral cortex. Note that information from the right side of the body projects to the left side of the brain.

B. Coronal view of the nervous system C. Lateral view of the nervous system left cerebral cortex right foot right hand post central gyrus right face thalamus trigeminal ganglion pons medulla spinal cord right foot thalamus

anteriolateral tract

FIGURE 5 Sensory axis of the nervous system. Primary sensory neurons carrying pain and temperature information project to the dorsal part of the spinal cord; their cell bodies are located in the dorsal root ganglia. Neurons carrying pain and temperature information from the face project through the trigeminal nerve to the lateral part of the pons; their cell bodies are in the trigeminal ganglion. Pain information is relayed through synaptic connections to second-order and on to higher order neurons in areas shown in blue. Pain relay areas are present in both the spinal cord and thalamus, within pathways that project to the post central gyrus of the cerebral cortex. Note that information from the right side of the body projects to the left side of the brain.

a series of interconnected circuits, each consisting of sensory receptor neurons, relay and processing nuclei, primary receiving areas, memory banks, connecting links to other pathways, feedback loops, and final output pathways.

The major input portion of this overall scheme is the sensory axis of the nervous system. At the spinal level of the sensory axis, somatosensory cell bodies reside in dorsal root ganglia, and their fibers exit through spinal nerves to innervate skin and skeletal muscles of the body. Through projections of the cranial nerves, sensory cell bodies residing in cranial nerve ganglia innervate analogous structures in the head and neck region plus organs of the special senses of vision, taste, hearing, balance, and smell. The only areas not innervated by sensory fibers are bone and nervous tissue itself.

After entering appropriate regions of the spinal cord or brain stem, proximal sensory fibers are classified as sensory tracts, and they ultimately project to primary receiving areas located in posterior and temporal regions of the cortex after a relay in the thalamus. Many sensory tracts cross the midbrain of the neural axis and project to the contralateral cerebral hemisphere. Only after activation of a primary cortical receiving area will a sensation be consciously perceived.

Each individual sensory pathway maintains strict spatial or somatotopic organization. For example, pain or touch input from a certain area of the body is localized to specified regions of the cortex within the primary receiving area. Likewise, specific modality attributes (e.g., wavelength of light, actual pitch of a sound) are associated with given subregions of the primary sensory cortex.

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