Spinal Cord Tracts
Sensory information from receptors throughout most of the body is relayed to the brain by means of ascending tracts of fibres that conduct impulses up the spinal cord. When the brain directs motor activities, these directions are in the form of nerve impulses that travel down the spinal cord in descending tracts of fibres.
The spinal cord extends from the level of the foramen magnum of the skull to the first lumbar vertebra. Unlike the brain, in which the grey matter forms a cortex over white matter, the grey matter of the spinal cord is located centrally, surrounded by white matter. The central grey matter of the spinal cord is arranged in the form of an H, with two dorsal horns and two ventral horns (also called posterior and anterior horns, respectively). The white matter of the spinal cord is composed of ascending and descending fibre tracts. These are arranged into six columns of white matter called funiculi.
The fibre tracts within the white matter of the spinal cord are named to indicate whether they are ascending (sensory) or descending (motor) tracts. The names of the ascending tracts usually start with the prefix spino- and end with the name of the brain region where the spinal cord fibres first synapse. The anterior spinothalamic tract, for example, carries impulses conveying the sense of touch and pressure, and synapses in the thalamus. From there it is relayed to the cerebral cortex, The names of descending motor tracts, conversely, begin with a prefix denoting the brain region that gives rise to the fibres and end with the suffix -spinal. The lateral corticospinal tracts, for example, begin in the cerebral cortex and descend the spinal cord.
The ascending fibre tracts convey sensory information from cutaneous receptors, proprioceptors (muscle and joint senses), and visceral receptors. Most of the sensory information that originates in the right side of the body crosses over and eventually reach the region on the left side of the brain, which analyses this information. Similarly, the information arising in the left side of the body is ultimately analysed by the right side of the brain. This decussation occurs in the medulla oblongata for sensory modalities, or in the spinal cord for other modalities of sensation.
There are two major groups of descending tracts from the brain: the corticospinal, or pyramidal tracts, and the extrapyramidal tracts. The pyramidal tracts descend directly without synaptic interruption, from the cerebral cortex to the spinal cord. The cell bodies that contribute fibres to these pyramidal tracts are located primarily in the precentral gyrus (also called the motor cortex). Other areas of the cerebral cortex, however, also contribute to these tracts.
From 80%-90% of the corticospinal fibres decussate in the pyramids of the medulla oblongata (hence the name "pyramidal tracts") and descend in the lateral corticospinal tracts, which decussate in the spinal cord. Because of the crossing of fibres, the right cerebral hemisphere controls the musculature on the left side of the body, where the left hemisphere controls the right musculature. The corticospinal tracts are primarily concerned with the control of fine movement that requires dexterity.
The remaining descending tracts are extrapyramidal motor tracts, which originate in the midbrain and brain stem regions. If the pyramidal tracts of an experimental animal are cut, electrical stimulation of the cerebral cortex, cerebellum, and basal nuclei can still produce movements. The descending fibres that produce these movements must, by definition, be extrapyramidal tracts. The regions of the cerebral cortex, basal nuclei and cerebellum that participate in this motor control have numerous synaptic interconnections, and can influence movement only indirectly, by means of stimulation or inhibition of the nuclei that give rise to the extrapyramidal tracts. Notice that this motor control differs from that by the neurons of the precentral gyrus which send fibres directly down to the spinal cord in the pyramidal tracts.
The reticulospinal tracts are the major descending pathways of the extrapyramidal system. These tracts originate in the reticular formation of the brain stem which receives either stimulatory or inhibitory input from the cerebrum and the cerebellum. There are no descending tracts from the cerebellum; the cerebellum can influence motor activity only indirectly by its effect on the vestibular nuclei, red nucleus and basal nuclei (which sends axons to the reticular formation). These nuclei, in turn, send axons down the spinal cord via the vestibulospinal tracts, rubrospinal tracts and reticulospinal tracts respectively.
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