The CNS communicates with peripheral body structrues through pathways. These pathways conduct either sensory or motor information; proccessing and integration occur continuously along them. These pathways travel through the white matter of the brainstem and/or spinal cord as they connect various CNS regions with cranial and spinal nerves. A pathway consists of a tract and nucleus. Tracts are groups or bundles of axons that travel together in the CNS. A nucleus is a collection of neuron cell bodies located within the CNS. Nervous system pathways are sensory or motor. Sensory pathways are also called ascending pathways because the sensory information gathered by sensory receptors ascends through the spinal cord to the brain, whereas motor pathways are also called descending pathways because they transmit motor information that descends from the brain through the spinal cord to muscles or glands. Most of the nervous system pathways we discuss in this thread share several general characteristics:
Most pathways decussate from one side of the body to the other side at some point in their travels. This decussation means that the left side of the brain processes information from the right side of the body, and vice versa.
In most pathways, there is a precise correspondence of receptors in body regions, through axons, to specific functional areas in the cerebral cortex (ascending pathways), and vice versa for descending pathways. This correspondence is called somatotopy.
All pathways are composed of paired tracts. A pathway on the left side of the CNS has a matching tract on the right side of the CNS. Because each tract innervates structures on only one side of the body, both left and right tracts are needed to innervate both the left and right sides of the body.
Most pathways are composed of a series of two or three neurons that work together. Sensory pathways have primary neurons, secondary neurons, and sometimes tertiary neurons that facilitate the pathway's functioning. In contrast, motor pathways use an upper motor neuron and a lower motor neuron. The cell bodies are located in the nuclei associated with each pathway.
Sensory pathways are ascending pathways that conduct information about limb position and the sensations of touch, temperature, pressure, and pain to the brain. Somatosensory pathways process stimuli received from receptors within the skin, muscles, and joints, whereas viscerosensory pathways process stimuli received from the viscera.
The multiple types of body sensations detected by the somatosensory system are grouped into three spinal cord pathways, each with a different brain destination: 1.Discriminative touch permits us to describe textures and shapes of unseen objects and includes pressure, touch, and vibration perception; 2.Temperature and pain allow us to detect those sensations, as well as the sensation of an itch; 3.Proprioception allows us to detect the position of joints, stretch in muscles, and tension in tendons. Sensory receptors detect stimuli and then conduct nerve impulses to the central nervous system. Sensory pathway centers within either the spinal cord or the brainstem process and filter the incoming sensory information. These centers determine whether the incoming sensory stimulus should be transmitted to the cerebrum or terminated. Consequently, not all incoming impulses reach the cerebral cortex and our conscious awareness.
Posterior Funiculus – Medial Lemniscal Pathway
This pathway projects through the spinal cord, brainstem, and diencephalon before terminating within the cerebral cortex. Its name derives from two components: the tracts within the spinal cord, collectively called the posterior funiculus; and the tracts within the brainstem, collectively called the medial lemniscus. This pathway conducts sensory stimuli concerned with proprioceptive information about limb position and discriminative touch, precise pressure, and vibration sensations.
The posterior funiculus – medial lemniscal pathway uses a chain of three neurons to singal the brain about a specific stimulus. Axons of the primary neurons traveling in spinal nerves reach the CNS through the posterior roots of spinal nerves. Upon entering the spinal cord, these axons ascend within a specific posterior funiculus, either the fascinculus cuneatus or the fasciculus gracilis. The fasciculus cuneatus houses axons from sensory neurons originating in the upper limbs, superior trunk, neck, and posterior region of the head, whereas the fasciculus gracilis carries axons from sensory neurons originating in the lower limbs and inferior trunk. The senory input into both posterior funiculi is organized somatotopically – that is, there is a correspondence between a receptor's location in a body part and a particular location in the CNS. Thus, the sensory information originating from inferior regions is medially located within the fasciculus, and the sensory information originating at progressively more superior regions is located more laterally.
Sensory axons ascending within the posterior funiculi synapse on secondary neuron cell bodies housed within a posterior funiculus nucleus in the medulla oblongata. These nuclei are either the nucleus cuneatus or the nucleus gracilis, and they correspond to the fasciculus cuneatus and fasciculus gracilis, respectively. These secondary neurons then project axons to relay the incoming sensory information to the thalamus on the oposite side of the brain through medial lemniscus. Decussation occurs after secondary neuron axons exit their specific nuclei and before they enter the medial lemniscus. As the sensory information travels toward the thalamus, the same classes of sensory input (touch, pressure, and vibration) that have been collected by cranial nerves CN V (trigeminal), CN VII (facial), CN IX (glossopharyngeal), and CN X (vagus) are integrated and incorporated into the ascending pathways, collectively called the trigeminothalamic tract.
The axons of the secondary neurons synapse on cell bodies of the tertiary neurons within the thalamus. Within the thalamus, the ascending sensory information is sorted according to the region of the body involved (somatotopically). Axons from these tertiary neurons conduct sensory information to a specific location of the primary somatosensory cortex.
This pathway is located in the anterior and alteral white funiculi of the spinal cord. It is composed of the anterior spinothalamic tract and the lateral spinothalamic tract. Axons projecting from primary neurons enter the spinal cord and synpase on secondary neurons within the posterior horns. Axons entering these pathways conduct stimuli related to crude touch and pressure as well as pain and temperature. Axons of the secondary neurons in the anterolateral pathway cross over to the opposite side of the spinal cord before ascending toward the brain. This decussation occurs through the anterior white commissure, loacted anterior to the gray commissure. The anterior and lateral spinothalamic pathway, are somatotopically organized: Axons transmitting sensory information from more inferior segments of the body are located lateral to those from more superior segments. Secondary neuron axons synapse on tertiary neurons located within the thalamus. Axons from the tertiary neurons then conduct stimulus information to the appropriate region of the primary somatosensory cortex.
The spinocerebellar pathway conducts proprioceptive information to the cerebellum for processing to coordinate body movements. The spinocerebellar pathway is composed of anterior and posterior spino- cerebellar tracts; these are the major routes for transmitting postural input to the cerebellum. Sensory input arriving at the cerebellum through these tracts is critical for regulating posture and balance and for coordinating skilled movements. ese spinocerebellar tracts are di erent from the other sensory pathways in that they do not use tertiary neurons; rather, they have only primary and second- ary neurons. Information conducted in spinocerebellar pathways is integrated and acted on at a subconscious level.
Anterior spinocerebellar tracts conduct impulses from the inferior regions of the trunk and the lower limbs. eir axons enter the cerebellum through the superior cerebellar peduncle. Posterior spinocerebellar tracts conduct impulses from the lower limbs, the trunk, and the upper limbs. eir axons enter the cerebellum through the inferior cerebellar peduncle.
Update on Jul 29 2017
A cross section of the gray matter of the spinal cord shows a number of laminas (layers of nerve cells), termed Rexed's laminae after the neuroanatomist who described them. As a general principle, laminae are involved in non-painful was well as painful sensation.
Lamina I – This thin marginal layer contains neurons that respond to noxious stimuli and send axons to the contralateral spinothalamic tract.
Lamina II – Also known as substantia gelatinosa, this lamina is made up of small neurons, some of which respond to noxious stimuli. Substance P, a neuropeptide involved in pathways mediating sensibility to pain, is found in high concentration in laminas I and II.
Lamins III and IV – These are referred to together as the nucleus proprius. Their main input is from fibers that convey position and light touch sense.
Lamina V – This layer contains cells that respond to both noxious and visceral afferent stimuli.
Lamina VI – This deepest layer of the dorsal horn contains neurons that respond to mechanical signals from joints and skin.
Lamina VII – This is a large zone that contains the cells of the dorsal nucleus (Clarke's column) medially as well as a large portion of the ventral grapy column. Clarke's column contains cells that give rise to the posterior spinocerebellar tract. Lamina VII also contains the intermediolateral nucleus (or intermediolateral cell column) in thoracic and upper lumbar regions. Preganglionic sympathetic fibers project from cells in this nucleus, via the ventral roots and white rami communicantes, to sympathetic ganglia.
Laminas VIII and IX – These layers represent motor function neuron groups in the medial and lateral portions of the ventral grapy column. The medial portion (also termed the medial motor neuron column) contains the LMNs that innervate axial musculature. The lateral motor neuron column contains LMNs for the distal muscles of the arm and leg. In general, flexor muscles are innervated by motor neurons located close to the central canal, whereas extensor muscles are innervated by motor neurons located more peripherally.
Lamina X – This represents the small neurons around the central canal or its remnants.
Ascending Fiber System
All afferent axons in the dorsal roots have their cell bodies in the dorsal root ganglia. Different ascending systems decussate at different levels. In general, ascending axons synapse within the spinal cord before decussating.
Dorsal column tract
These tracts, which are part of the medial lemniscal system, convey well-localized sensations of fine touch, vibration, two-point discrimination, and proprioception (position sense) from the skin and joints; they ascend, without crossing, in the dorsal white column of the spinal cord to the lower brain stem. The fasciculus gracilis carries input from the lower half of the body, with fibers that arise from the lowest, most medial segments. The fasciculus cuneatus lies between the fasciculus gracilis and the dorsal gray column; it carries input from the upper half of the body, with fibers from the lower (thoracic) segments more medial than the higher (cervical) ones. Thus, one dorsal column contains fibers from all segments of the ipsilateral half of the body arranged in an orderly somatotopic fashion from medial to lateral.
Ascending fibers in the gracile and cuneate fasciculi terminate on neurons in the gracile and cuneate nuclei (dorsal column nuclei) in the lower medulla. These second-order neurons send their axons, in turn, across the midline via the lemniscal decussation (also called the internal arcuate tract) and the medial lemniscus to the thalamus. From the ventral posterolateral thalamic nuclei, sensory information is relayed upward to the somatosensory cortex.
Small-diameter sensory axons conveying the sensations of sharp (noxious) pain, temperature, and crudely localized touch course upward, after entering the spinal cord via the dorsal root, for one or two segments at the periphery of the dorsal horn. These short, ascending stretches of incoming fibers that are termed the dorsolateral fasciculus, or Lissauer's tract, then synapse with dorsal column neurons, especially in laminas I, II, and V. After one or more synapses, subsequent fibers cross to the opposite side of the spinal cord and then ascend within the spinothalamic tracts, also called the ventrolateral (or anterior system). These spinothalamic tracts actually consist of two adjacent pathways: The anterior spinothalamic tract carries information about light touch, and the lateral spinothalamic tract conveys pain and temperautre sensibility upward.
The spinothalamic tracts, like the dorsal column system, show somatotopic organization. Sensation from sacral parts of the body is carried in lateral parts of the spinothalamic tracts, whereas impulses originating in cervical regions are carried by fibers in medial parts of the spinothalamic racts. Axons of the spinothalamic tracts project rostrally after sending branches to the reticular foramtion in the brain stem and project to the thalamus.
The second-order neurons of both the dorsal column system and spinothalamic tracts decussate. The pattern of decussation is different, however. The axons of second-order neurons of the dorsal column system cross in the lemniscal decussation in the medulla; these second-order sensory axons are called internal arcuate fibers where they cross. In contrast, the axons of second-order neurons in the spinothalamic tracts cross at every segmental level in the spinal cord. This fact aids in determining whether a lesion is in the brain or the spinal cord.
With lesions in the brain stem or higher, deficits of pain perception, touch sensation, and proprioception are all contralateral to the lesion. With spinal cord lesions, however, the deficit in pain perception is contralateral to the lesion, whereas the other deficits are ipsilateral.
Two ascending pathways (of lesser importance in human neurology) provide input from the spinal cord to the cerebellum.
Dorsal spinocerebellar tract – Afferent fibers from muscle and skin enter the spinal cord via dorsal roots at levels T1 to L2 and synapse on second-order neurons of the nucleus dorsalis (Clarke's column). Afferent fibers originating in sacral and lower lumbar levels ascend within the spinal cord (within the dorsal columns) to reach the lower portion of the nucleus dorsalis.
The dorsal nucleus of Clarke is not present above C8; it is replaced, for the upper extremity, by a homologous nucleus called the accessory cuneate nucleus. Dorsal root fibers originating at cervical levels synapse with second-order neurons in the accessory cuneate nucleus.
The second-order neurons from the dorsal nucleus of Clarke form the dorsal spinocerebellar tract; second-order neurons from the lateral cuneate nucleus form the cuneocerebellar tract. Both tracts remain on the ipsilateral side of the spinal cord, ascending via the inferior cerebellar peduncle to terminate in the paleocerebellar cortex.
Ventral spinocerebellar tract – This system is involved with movement control. Second-order neurons, located in Rexed's laminae V, VI, and VII in lumbar and sacral segments of the spinal cord, send axons that ascend through the superior cerebellar peduncle to the pale ocerebellar cortex. The axons of the second-order neurons are largely but not entirely crossed.