Which structure contains bundles of axons organized into tracts?
The central nervous system uses ascending and descending pathways to communicate with the external environment. Show The spinal cord, basically a highway for nerves, streamlines sensory and motor signals to the brain and the body. Information detected by sensory receptors in the periphery is transmitted along ascending neural tracts in the spinal cord. Located in the white matter of the spinal cord, the ascending sensory tracts arise from either the the cells of the spinal ganglia or the intrinsic neurons within the grey matter that receive primary sensory input. There are many sensory tracts and pathways carrying different types of sensory information from the periphery to the cerebral cortex. In humans the major sensory pathways include:
There are many motor tracts in the spinal cord. Some of these are under conscious control and others under unconscious, reflexive or responsive control. These motor tracts can be grouped functionally into Extrapyramidal and Pyramidal tracts These functional groups contain several anatomical tracts, one for each side of the body:
The pyramidal tracts are named as such due to their course through the pyramids of the medulla oblongata. The pyramidal tracts are responsible for the conscious, voluntary control of the body and face muscles. They can be divided into two tracts. Corticospinal Tract[edit | edit source]Carries information from the cerebral cortex to the spinal cord. Also called the pyramidal tract as it is the only point where all the fibres are collected together without contamination by other fibre tracts in the medullary pyramids of the brain stem. Primary motor cortex is the main source of input to this tract, but the premotor and supplementary motor cortex also contribute fibres. Its projections are primarily contralateral and have a strong influence on the activity of groups of spinal motoneurones, which innervate distal muscles of the hands and feet. Most of the fibers, approximately 85%, cross the midline in the decussation of the pyramids in the brain stem and then descend through the spinal cord in the lateral corticospinal tract, while the other 15% cross within the spinal cord at the level they terminate and are carried within the medial corticospinal tract. According to Harvey (2008), the cervical upper motor neurons are centrally located within the corticospinal tract while the lumbar and sacral neurons are peripherally located, which explains neurological patterns of loss seen with certain types of incomplete spinal cord injuries where the peripheral rim of the spinal cord is undamaged. Corticobulbar Tract[edit | edit source]The corticobulbar tract is composed of the upper motor neurons of the cranial nerves. The muscles of the face, head and neck are controlled by the corticobulbar system, which terminates on motor neurons within brainstem motor nuclei. Contrast this to the corticospinal tract were the cerebral cortex connects to spinal motor neurons, and controls movement of the torso, upper and lower limbs.
There are four tracts[2]. Reticulospinal Tract[edit | edit source]Reticulospinal tract is a descending tract present in the white matter of the spinal cord, originating in the reticular formation (the archaic core of those pathways connecting the spinal cord and the brain). It consists of bundles of axons that carry information or orders from the reticular formation in the brainstem to the peripheral body parts.
Vestibulospinal Tract[edit | edit source]Originates with the axons of the vestibular nuclei (within the brainstem) and terminates by synapsing with the interneurons present in the anterior gray column of the spinal cord. The vestibulospinal tracts consist of a medial vestibulospinal tract and a lateral vestibulospinal tract. They are essential for a number of reflex actions performed by the body.
Rubrospinal Tract[edit | edit source]The Rubrospinal tract is a descending pathway, beginning as axons of the neurons present in the red nucleus and terminates by synapsing with the interneurons in the spinal cord.
Autonomic nerves are also carried by the spinal cord. Sympathetic nerves exit the vertebral canal via thoraco-lumbar spinal nerves, and parasympathetic nerves exit via sacral spinal nerves. As a result individuals with a cervical lesion lose supraspinal control of the entire sympathetic nervous system and of the sacral part of the parasympathetic nervous system. Individuals with thoracic, lumbar or sacral lesions lose varying amounts of supraspinal control of the sympathetic and parasympathetic nervous system as determined by the level of the lesion. Some parasympathetic fibres are carried within cranial nerves and are unaffected by spinal cord injury[3]. Which structure contains bundles of axons organized into nerves?Bundles of axons called fascicles are surrounded by a perineurium. The largest nerve trunks composed of multiple fascicles are surrounded by an epineurium. The peripheral nervous system has ganglia that consist of localized collections of nerve cell bodies surrounded by a connective tissue capsule.
Is a tract a bundle of axons?In the central nervous system a bundle of axons is called a tract. Each axon is surrounded by a delicate endoneurium layer. The course connective tissue layer called perineurium, binds the fibers into bundles called fascicles.
In which division of the nervous system is a bundle of axons called a tract?A bundle of axons is referred to as a tract in the CNS and as a nerve in the PNS. Whereas nuclei and ganglia are specifically in the central or peripheral divisions, axons can cross the boundary between the two.
What is a bundle of axons called?A nerve is an enclosed bundle of axons which carries information to and from the central nervous system.
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