Complex behaviors, such as using touch alone to differentiate a ball from a book, require the integrated action of several nuclei and cortical regions. Information is processed in the brain in a hierarchical fashion. Thus information about a stimulus is conveyed through a succession of subcortical and then cortical regions and at each level of processing the information becomes increasingly complex. In addition, different types of information, even within a single sensory modality, are processed in several anatomically discrete pathways. In the somatosensory system a light touch and a painful pin prick to the same area of skin are mediated by different pathways in the brain.
Somatosensory Information from the Trunk and Limbs Is Conveyed to the Spinal Cord
Sensory information from the trunk and limbs enters the spinal cord, which has a core H-shape region of gray matter surrounded by white matter. The gray matter on each side of the cord is divided into dorsal (or posterior) and ventral (or anterior) horns (Figure 16–1). The dorsal horn contains groups of sensory neurons (sensory nuclei) whose axons receive stimulus information from the body's surface. The ventral horn contains groups of motor neurons (motor nuclei) whose axons exit the spinal cord and innervate skeletal muscles.
The major anatomical features of the spinal cord.
Top: The left side depicts a cell stain of the gray matter and the right side a fiber-stained section. Bottom: The ventral horn (green) contains large motor neurons, whereas the dorsal horn (orange) contains smaller neurons. Fibers of the gracile fascicle carry somatosensory information from the lower limbs, whereas fibers of the cuneate fascicle carry somatosensory information from the upper body. Fiber bundles of the lateral and ventral columns include both ascending and descending fiber bundles.
Unlike the sensory nuclei, the motor nuclei form columns that run the length of the spinal cord. Interneurons of various types in the gray matter inhibit the output of the spinal cord neurons. These inhibitory interneurons thus modulate both sensory information flowing toward the brain and motor commands descending from the brain to the spinal motor neurons. Motor neurons can also adjust the output of other motor neurons via the interneurons.
The white matter surrounding the gray matter contains bundles of ascending and descending axons that are divided into dorsal, lateral, and ventral columns. The dorsal columns, which lie between the two dorsal horns of the gray matter, contain only ascending axons that carry somatic sensory information to the brain stem (Figure 16–1). The lateral columns include both ascending axons and axons descending from the brain stem and neocortex that innervate spinal interneurons and motor neurons. The ventral columns also include ascending and descending axons. The ascending somatic sensory axons in the lateral and ventral columns constitute parallel pathways that convey information about pain and thermal sensation to higher levels of the central nervous system. The descending axons control axial muscles and posture.
The spinal cord is divided into four major regions: cervical, thoracic, lumbar, and sacral (Figure 16–2). These regions are related to the embryological somites from which muscles, bones, and other components of the body develop (see Chapters 52 and 53). Axons projecting from the spinal cord to body structures that develop at the same segmental level join together in the intervertebral foramen with axons entering the spinal cord to form spinal nerves. Spinal nerves at the cervical level are involved with sensory perception and motor function of the back of the head, neck, and arms; nerves at the thoracic level innervate the upper trunk; whereas lumbar and sacral spinal nerves innervate the lower trunk, back, and legs.
The internal and external appearances of the spinal cord vary at different levels.
The proportion of gray matter (the H-shaped area within the spinal cord) to white matter is greater at sacral levels than at cervical levels. At sacral levels very few incoming sensory fibers have joined the spinal cord, whereas most of the motor fibers have already terminated at higher levels of the spinal cord. The cross-sectional enlargements at the lumbar and cervical levels are regions where the large number of fibers innervating the limbs enter or leave the spinal cord.
Each of the four regions of the spinal cord contains several segments; there are 8 cervical segments, 12 thoracic segments, 5 lumbar segments, and 5 sacral segments. Although the actual substance of the mature spinal cord does not look segmented, the segments of the four spinal regions are nonetheless defined by the number and location of the dorsal and ventral roots that enter or exit the cord. The spinal cord varies in size and shape along its rostrocaudal axis because of two organizational features.
First, relatively few sensory axons enter the cord at the sacral level. At higher levels (lumbar, thoracic, and cervical) the number of sensory axons entering the cord increases progressively. Conversely, most descending axons from the brain terminate at cervical levels, with progressively fewer descending to lower levels of the spinal cord. Thus the number of fibers in the white matter is highest at cervical levels (where there are the highest numbers of both ascending and descending fibers) and lowest at sacral levels. As a result, sacral levels of the spinal cord have much less white matter than gray matter, whereas the cervical cord has more white matter than gray matter (Figure 16–2).
The second organizational feature is variation in the size of the ventral and dorsal horns. The ventral horn is larger at the levels where the motor nerves that innervate the arms and legs exit the spinal cord. The number of ventral motor neurons dedicated to a body region roughly parallels the dexterity of movements of that region. Thus a larger number of motor neurons is needed to innervate the greater number of muscles and to regulate the greater complexity of movement in the limbs as compared with the trunk. Likewise, the dorsal horn is larger where sensory nerves from the limbs enter the cord. Limbs have a greater density of sensory receptors to mediate finer tactile discrimination and thus send more fibers to the cord. These regions of the cord are known as the lumbosacral and cervical enlargements.
The Primary Sensory Neurons of the Trunk and Limbs Are Clustered in the Dorsal Root Ganglia
The sensory neurons that convey information from the skin, muscles, and joints of the limbs and trunk to the spinal cord are clustered together in dorsal root ganglia within the vertebral column immediately adjacent to the spinal cord (Figure 16–3). These neurons are pseudo-unipolar in shape; they have a bifurcated axon with central and peripheral branches. The peripheral branch terminates in skin, muscle, or other tissue as a free nerve ending or in association with specialized receptors.
Dorsal root ganglia and spinal nerve roots.
The cell bodies of neurons that bring sensory information from the skin, muscles, and joints lie in the dorsal root ganglia, clusters of cells that lie adjacent to the spinal cord. The axons of these neurons are bifurcated into peripheral and central branches. The central branch enters the dorsal portion of the spinal cord.
The central process enters the spinal cord. On entry the axon forms branches that either terminate within the spinal gray matter or ascend to nuclei located near the junction of the spinal cord with the medulla (Figure 16–3). These local and ascending branches provide two functional pathways for somatosensory information entering the spinal cord from dorsal root ganglion cells. The local branches can activate local reflex circuits while the ascending branches carry information into the brain, where this information becomes the raw material for the perception of touch, position sense, or pain.
The Central Axons of Dorsal Root Ganglion Neurons Are Arranged to Produce a Map of the Body Surface
The central axons of the dorsal root ganglion cells form a neural map of the body surface when they terminate in the spinal cord. This orderly somatotopic distribution of inputs from different portions of the body surface is maintained throughout the entire ascending somatosensory pathway.
Axons that enter the cord in the sacral region ascend in the dorsal column near the midline, whereas those that enter at successively higher levels ascend at progressively more lateral positions within the dorsal columns. Thus, in the cervical cord, where axons from all portions of the body have already entered, sensory fibers from the lower body are located medially in the dorsal column, while fibers from the trunk, the arm and shoulder, and finally the neck occupy progressively more lateral areas. At cervical levels of the cord the axons forming the dorsal columns are divided into two bundles: a medially situated gracile fascicle and a more laterally situated cuneate fascicle (Figure 16–4).
The medial lemniscus is a major afferent pathway for somatosensory information.
Somatosensory information enters the central nervous system through the dorsal root ganglion cells. The flow of information ultimately leads to excitation of the somatosensory cortex. Fibers that relay information from different parts of the body maintain an orderly relationship to each other and form a neural map of the body surface in their pattern of termination at each synaptic relay.
Each Somatic Submodality Is Processed in a Distinct Subsystem from the Periphery to the Brain
The submodalities of somatic sensation—touch, pain, and position sense—are processed in the brain through different pathways that end in different brain regions. To illustrate the specificity of these parallel pathways, we will follow the path of information for the submodality of touch.
The primary afferent fibers that carry information about touch enter the ipsilateral dorsal column and, without crossing to the contralateral column, ascend to the medulla. Fibers from the lower body run in the gracile fascicle and terminate in the gracile nucleus, whereas fibers from the upper body run in the cuneate fascicle and terminate in the cuneate nucleus. Neurons in the gracile and cuneate nuclei give rise to axons that cross to the other side of the brain and ascend to the thalamus in a long fiber bundle called the medial lemniscus (Figure 16–4).
As in the dorsal columns of the spinal cord, the fibers of the medial lemniscus are arranged somatotopically. Because the sensory fibers cross the midline to the other side of the brain, the right side of the brain receives sensory information from the left side of the body, and vice versa. The fibers of the medial lemniscus end in a specific subdivision of the thalamus called the ventral posterior nucleus (Figure 16–4). There the fibers maintain their somatotopic organization such that those carrying information from the lower body end laterally and those carrying information from the upper body and face end medially.