Ascending Spinal Tracts PDF

Ascending spinal tracts Ascending tracts carry impulses from pain, thermal, tactile, muscle and joint receptors to the brain. Some of this information eventually reaches a conscious level (the cerebral cortex), while some is destined for subconscious centres (e.g. the cerebellum). Pathways that car...

Ascending spinal tracts Ascending tracts carry impulses from pain, thermal, tactile, muscle and joint receptors to the brain. Some of this information eventually reaches a conscious level (the cerebral cortex), while some is destined for subconscious centres (e.g. the cerebellum). Pathways that carry information to a conscious level share certain common characteristics. There is a sequence of three neurones between the peripheral receptor and the cerebral cortex. ​ The first neurone ( first-order neurone or primary afferent neurone ) enters the spinal cord through the dorsal root of a spinal nerve and its cell body lies in a dorsal root ganglion. The central process may collateralise extensively and make synaptic connections that mediate spinal reflexes and intersegmental coordination. The main fibre remains on the ipsilateral side of the cord and terminates in synaptic contact with the second neurone, which is located either in the spinal grey matter or in the medulla oblongata of the brainstem, depending on the modality being served. ​ The second neurone ( second-order neurone ) has its cell body in the cord or medulla oblongata. Its axon crosses over (decussates) to the opposite side of the CNS and ascends to the thalamus, where it terminates upon the third neurone. ​ The third neurone ( third-order neurone ) has its cell body in the thalamus. Its axon passes to the somatosensory cortex in the parietal lobe of the ipsilateral cerebral hemisphere. Two main tract systems in the spinal cord fit into this pattern: the dorsal (posterior) columns and the spinothalamic tracts. Save to collection Share Dorsal columns The dorsal columns are located between the dorsal median sulcus and the dorsal horn. The dorsal columns are comprised of two tracts, incompletely separated by a thin septum: the fasciculus gracilis , situated medially and the fasciculus cuneatus , situated laterally. The tracts carry impulses concerned with proprioception (movement and joint position sense) and discriminative (fine) touch. The dorsal columns contain the axons of primary afferent neurones that have entered the cord through the dorsal roots of spinal nerves ( Fig. 8.16 ). The fasciculus gracilis consists of fibres that join the cord at sacral, lumbar and lower thoracic levels and, thus, includes those from the lower limb. Fibres of the fasciculus cuneatus enter via the upper thoracic and cervical dorsal roots and, thus, include those from the upper limb. Since the dorsal columns contain primary afferent neurones, they carry information relating to the ipsilateral side of the body. Fibres ascend without interruption to the medulla oblongata, where they terminate upon second-order neurones, the cell bodies of which are located in the nucleus gracilis and nucleus cuneatus (see also Fig. 9.6 ). The axons of the second-order neurones decussate in the medulla as internal arcuate fibres and, thereafter, ascend through the brainstem as the medial lemniscus. The medial lemniscus terminates in the ventral posterior ( VP ) nucleus of the thalamus upon third-order thalamocortical neurones. These in turn project to the somatosensory cortex, located in the postcentral gyrus of the parietal lobe. Lesions of the dorsal columns Tabes dorsalis is a late manifestation of syphilitic infection of the CNS. It chiefly affects the lumbosacral dorsal spinal roots and the dorsal columns of the spinal cord. The loss of proprioception leads to a high steppage and unsteady gait ( sensory ataxia ), which is exacerbated when the eyes are closed ( Romberg's sign ). Subacute combined degeneration of the spinal cord is a systemic disease resulting from a deficiency of vitamin B 12 (cyanocobalamin), which also causes pernicious anaemia. The degeneration of the dorsal columns produces sensory ataxia. The lateral columns of the spinal cord are also involved (combined), causing weakness and spasticity of the limbs. The disorder, although uncommon, is an important one since proper treatment with vitamin B 12 can lead to complete recovery. In multiple sclerosis , an immune disease, specific damage to the fasciculus cuneatus of the cervical spinal cord leads to loss of proprioception in the hands and fingers, causing profound loss of dexterity and inability to identify the shape and nature of objects by touch alone ( astereognosis ). Save to collection Share Spinothalamic tract The spinothalamic tract (sometimes referred to as the anterolateral system) lies lateral and ventral to the ventral horn of the spinal grey matter. It carries information related to pain and thermal sensations and also non-discriminative (course) touch and pressure. Some authorities identify distinct lateral and ventral spinothalamic tracts conveying pain and temperature or touch and pressure, respectively, but there appears to be little evidence for this in man and fibres carrying these modalities are probably intermingled, at least to some extent. The spinothalamic tract contains second-order neurones, the cell bodies of which lie widely distributed in the contralateral dorsal horn and receive input from primary afferent fibres that terminate in this region ( Fig. 8.17 ). After leaving the parent cell bodies, spinothalamic axons decussate to the opposite side of the cord by passing through the ventral white commissure , which lies ventral to the central canal of the cord, and, thus, enter the contralateral spinothalamic tract. Axons carrying pain and temperature decussate promptly within one segment of their origin, while those carrying touch and pressure may ascend for several segments before crossing. In the brainstem, the spinothalamic fibres run in close proximity to the medial lemniscus and are known as the spinal lemniscus. The majority of fibres terminate in the ventral posterior nucleus of the thalamus, contacting third-order thalamocortical neurones that project to the somatosensory cortex. The spinothalamic tract is sometimes referred to as the ‘neospinothalamic system’. It is highly organised somatotopically; consequently, the location of sensory stimuli, particularly those to the body surface, can be identified with very high accuracy. It is thought to be the route via which sharp, pricking pain (sometimes called ‘fast’ pain) is conducted. The spinoreticulothalamic system represents an additional, phylogenetically older, route by which nociceptive sensory impulses ascend to higher centres. Some second-order neurones arising from the dorsal horn ascend in the ventrolateral region of the cord and then terminate in the brainstem reticular formation, particularly within the medulla. Reticulothalamic fibres then ascend to the intralaminar thalamic nuclei, which in turn activate the cerebral cortex. The spinoreticulothalamic system is poorly organised somatotopically and is thought to be the route via which dull, aching pain (sometimes called ‘slow’ pain) is transmitted to a conscious level. Activation of spinothalamic and spinoreticular fibres, which may ultimately be perceived as unpleasant or painful, can be modulated by descending pathways from the brain (see Fig. 8.11 ). While painful stimuli originating from somatic structures, and particularly from the body surface, can be localised with high precision due to the detailed somatotopic organisation of the ascending pathways and the somatosensory cortex, pain that originates from deep, visceral structures is generally poorly localised. In addition, noxious stimuli originating from visceral structures are sometimes perceived as pain in other parts of the body – a phenomenon known as ‘referred pain’ ( Fig. 8.18 ). A classic example is pain from heart disease being felt in the left chest, shoulder and arm. The mechanism underlying referred pain is not entirely understood but it is thought to be due to the convergence of somatic and visceral afferents from corresponding regions onto the same neurones in the dorsal horn of the spinal cord. Such convergence results in misinterpretation as to the source of the noxious stimulus. Ascending pathways that carry afferent information to a subconscious level are represented principally by the spinocerebellar tracts. Save to collection Share Spinocerebellar tracts Ascending spinocerebellar fibres carry information derived from muscle spindles, mechanoreceptors such as Golgi tendon organs, and tactile receptors, to the cerebellum for the control of posture and the coordination of movement. Four pathways appear to be involved in this process, two carrying information from the lower limb and two from the upper limb. The spinocerebellar system consists of a sequence of only two neurones, the tracts containing second-order neurones whose cell bodies of origin mostly lie in the base of the dorsal horn; they receive input from primary afferent fibres terminating in this region. Some tract axons ascend ipsilateral to their origin while others ascend contralaterally ( Fig. 8.19 ). The dorsal and ventral spinocerebellar tracts are located at the dorsolateral and ventrolateral surfaces of the cord, respectively (see Fig. 8.15 ) and relay information from the lower limb. Fibres of the dorsal spinocerebellar tract originate from a prominent group of cells in lamina VII of cord segments T1–L2, known as the thoracic nucleus or Clarke's column. The axons ascend ipsilaterally to enter the cerebellum through the inferior cerebellar peduncle. Fibres of the ventral spinocerebellar tract originate from cells at lumbosacral cord levels. The fibres decussate, ascend on the contralateral side of the cord and enter the cerebellum via the superior cerebellar peduncle. Some axons then re-cross within the cerebellar white matter. The upper limb equivalent of the dorsal spinocerebellar tract is represented by cuneocerebellar fibres. In this pathway, some primary proprioceptive afferents, originating mostly in the cervical enlargement and ascending ipsilaterally in the fasciculus cuneatus (one of the dorsal columns), terminate in the medulla just lateral to the principal cuneate nucleus in a group of cells known as the lateral (external, accessory) cuneate nucleus. From here, axons enter the cerebellum via the inferior cerebellar peduncle. The upper limb equivalent of the ventral spinocerebellar tract is the rostral spinocerebellar tract. Axons arise from cells in the cervical enlargement and ascend ipsilaterally in the lateral funiculus, entering the cerebellum mostly through the inferior peduncle. Spinocerebellar tract neurones terminate in the cerebellar cortex as mossy fibres, predominantly within the vermis and paravermal area.