The spinal cord is an extension of the medulla oblongata. It has two main functions:
- Route the afferent and efferent between the CNS and the rest of the body
- Treatment of partial information by spinal reflexes.
It develops from the caudal portion of embryonic neural tube. Distinguish a few weeks, two separate parts in the gray matter: the basal plate and alar plate, separated by a furrow, eventually becoming the central canal. As it grows, the blades extend and produce the gray mass of adult marrow. The white outer marrow is formed from neural axons.
The bone extends from the foramen magnum to the first or second lumbar vertebra. Hence, a filament fiber, fossil embryogenesis, from the meninges, extends to the coccyx, the terminal filum.
31 pairs of spinal nerves exit the spinal cord and innervate the body. She has two bulges, one from which the cervical nerves of upper limbs, forming the brachial plexus, the other from which the lumbar nerves of the lower limbs forming the lumbosacral plexus.
It has a slightly flattened and has two grooves, the median anterior and posterior midline.
Composition of the spinal cord:
Like the brain, spinal consists essentially of “white” and substance “gray”:
The white matter contains the myelinated neurons extensions: It completely surrounds the gray matter which way, is framed into three parts, called posterior, lateral and anterior. These cables, made up of groups of neurons called a type beams carry impulses to a specific destination (beams spinal ascending and descending)
The gray matter contains the bodies of neurons and the neuroglia: It has roughly the shape of an H, the side deck is appointing gray commissure, dug a thin channel called the central canal, extension of the 4th ventricle. The ‘pipes’ of H divide into pairs of posterior and anterior horns. Note that exists in some regions, particularly the chest, lateral horns.
The posterior horns contain sensory neurons of the spinal nerves, other neurons form the reflex arc with spinal motor neurons of anterior horns.
The anterior horn contains motor neurons whose axons form the spinal nerves.
The lateral horns contain them when to visceral motor neurons (involuntary)
But unlike the organization of the brain, spinal level, is the gray matter that lies at the center and white edge.
The gray matter:
As the cerebral gray matter, it contains multipolar neuronal cell bodies, extending them, and glial cells. Seen in cross section, spinal presents an aspect of H whose center is the central canal. From both sides, there are symmetrical gray masses, linked by a bridge, the gray commissure. The anterior and posterior projections are called respectively the posterior horns and anterior horns.
The anterior horn contains mostly cell bodies of motor neurons, while the posterior horns are very rich in interneurons. The axons of motor neurons extend into the spinal nerves before reaching their target. The horns on its side when serving visceral smooth muscles via the anterior roots.
The sensory receptors when to them, carry nerve impulses via the posterior roots. Their cell body is located in a bulge of the posterior root, the spinal ganglion. The two roots, anterior and posterior then join to form the spinal nerves, which is observed on both sides of the spinal cord.
The white matter:
Made up of nerve fibers, it is the color white to the presence of myelinated fibers, as bundles of predominantly vertical. It divides into the posterior cord, lateral and anterior, thus linking the bodies of a somatotopic manner, by ascending and descending.
The beams descending spinal
These are bundles s engine, leading nervous impulses from the brain down to the anterior horns of the spinal cord where the neurons coordinate the activity of skeletal muscles:
The pyramidal tracts, also called cortico-spinal drive the motor impulses to skeletal muscles. These are the main vehicles of voluntary control. Their origin dates back to the pyramidal cells of the frontal gyrus, then descend in the midbrain, pons and pyramids of the medulla
The beams extrapyramidal from subcortical structures influence motility by interfering with muscle coordination and balance
The ascending spinal bundles:
they drive the influx of sensory receptors associated with the affected areas of the brain or cerebellum. We note several major ascending spinal bundles: bundles Goll, cuneiform, spino-thalamic, spino-cerebellar ….
The columns of Goll for example, lead information on the members’ position and posture, and touch sensations throughout the body.
The reflex arc:
When you accidentally burn yourself on the edge of a hot pan, have you not noticed that you had already withdrawn your hand before you even have felt the pain? This is made possible by the existence of the spinal reflex arc, which can react to stimuli given automatically and much faster than if it did need intervention and analysis of higher centers. In our case, although the stimulus arrives at the brain, but for information since the reflex arc has averted the immediate danger.
The reflex arc is divided into 5 sequences:
A receiver receives information (often nociceptors)
A sensory neuron via the spinal nerve transmits nerve impulses to the spinal cord
One or several synapse between sensory neuron and an efferent motor neuron transmits information leading to the creation of a nerve impulse at the anterior horn gray
The information is transmitted to an effector (often, a skeletal muscle).
The CNS is then optionally informed.
Example:the tendon reflex, reflex front stretch