Medical brain notes 24

  Tracts of Spinal Cord and Brainstem        We have seen that a collection of nerve fibres within the central nervous system, that connects two masses of grey matter, is called a tract. A tract may be defined as a collection of nerve fibres having the same origin, course, and termination. Tracts may be ascending or descending. They are usually named after the masses of grey matter connected by them. Thus, a tract beginning in the cerebral cortex and descending to the spinal cord is called the corticospinal tract, while a tract ascending from the spinal cord to the thalamus is called the spinothalamic tract. We have noted that tracts are sometimes referred to as fasciculi (= bundles) or lemnisci (=ribbons). The major tracts passing through the spinal cord and brainstem are shown schematically in Fig. 9.1. The position of the tracts in a transverse section of the spinal cord is shown in Fig. 9.2, and at ascending levels of the brainstem in Figs. 11.1 to...

  Descending Tracts ending in the Spinal Cord CORTICOSPINAL TRACTS        The corticospinal tracts are made up, predominantly, of axons of neurons lying in the motor area of the cerebral cortex (area 4). Some fibres also arise from the premotor area (area 6) and some from the somatosensory area (areas 3, 2, 1) (Figs. 9.3, 8.10 and 15.4). A few fibres arise in the parietal cortex (area 5). From this origin fibres pass through the corona radiata to enter the internal capsule where they lie in the posterior limb (Figs. 9.3 and 17.2A). After passing through the internal capsule the fibres enter the crus cerebri (of the midbrain): they occupy the middle two-thirds of the crus. The fibres then descend through the ventral part of the pons to enter the pyramids in the upper part of the medulla.         Near the lower end of the medulla about 80 percent of the fibres cross to the opposite side. (The crossing fibres of the two ...

  Descending Tracts ending in the Brainstem Corticonuclear tracts        The nuclei of cranial nerves that supply skeletal muscle (i.e., somatic efferent and special visceral efferent nuclei) are functionally equivalent to ventral horn cells of the spinal cord. They are under cortical control through fibres that are closely related in their origin and course to corticospinal fibres. At various levels of the brainstem these fibres cross to the opposite side to end by synapsing with cells in cranial nerve nuclei, either directly or through interneurons. Cortico-ponto-cerebellar pathway        Fibres arising in the cerebral cortex of the frontal, temporal, parietal and occipital lobes descend through the corona radiata and internal capsule to reach the crus cerebri. The frontopontine fibres occupy the medial one sixth of the crus; and the temporopontine fibres (along with occipitopontine and parietopontine fibres) occupy the ...

  Disorders of Motor Function      Inability to move a part of the body is referred to as  paralysis.  This can be produced by interruption of motor pathways anywhere between the motor area of the cerebral cortex and the muscles themselves. We have seen that the pathway from cortex to muscle involves at least two neurons. The first of these is located in the cerebral cortex. Its axon terminates in the spinal cord or in motor cranial nerve nuclei in the brainstem. From a physiological and clinical point of view this neuron is referred to as the  upper motor neuron . The second neuron is located in the anterior grey column of the spinal cord(or in motor nuclei of the brainstem) and sends out an axon that travels through a peripheral nerve to innervate muscle. This neuron is referred to as the  lower motor neuron.  (For the present purpose we may ignore interneurons present in the pathway). When lower motor neurons are destroyed, or their c...

  Ascending Tracts Introductory Remarks          The ascending tracts of the spinal cord and brainstem represent one stage of multineuron pathways by which afferent impulses arising in various parts of the body are conveyed to different parts of the brain. The  first order neurons  of these pathways are usually located in spinal (dorsal nerve root) ganglia (Fig. 3.3). We have seen that the neurons in these ganglia are unipolar (pseudounipolar). Each neuron gives off a peripheral process and a central process. The peripheral processes of the neurons form the afferent fibres of peripheral nerves. They end in relation to sensory end organs (receptors) situated in various tissues. The central processes of these neurons enter the spinal cord through the dorsal nerve roots. Having entered the cord the central processes, as a rule, terminate by synapsing with cells in spinal grey matter. Some of them may run upwards in the white matter of ...

  PATHWAYS CONNECTING THE SPINAL CORD TO THE CEREBRAL CORTEX The Posterior Column  −  Medial Lemniscus Pathway Fasciculus gracilis and fasciculus cuneatus:        These tracts occupy the posterior funiculus of the spinal cord and are, therefore, often referred to as the  posterior column tracts (Fig. 9.2). They are unique in that they are formed predominantly by central processes of neurons located in dorsal nerve root ganglia i.e., by first order sensory neurons (Fig. 9.6). The fibres derived from the lowest ganglia are situated most medially; while those from the highest ganglia are most lateral. The fasciculus gracilis, which lies medially is, therefore, composed of fibres from the coccygeal, sacral, lumbar and lower thoracic ganglia; while the fasciculus cuneatus which lies laterally consists of fibres from upper thoracic and cervical ganglia. The fibres of these fasciculi extend upwards as far as the lower part of the medulla. Here ...