Answer
The spinal is the caudad development of the neural tube. It is located in the vertebral column and extends from the foramen magnum to the end of the first lumber vertebra(L1). The cord is semi-cylindrical shape but the diameter is not uniform throughout its length; there are two enlargements , the cervical enlargement that serves the upper limbs, and the lumbar enlargement that serves the lower limbs. There are 31 pairs of spinal nerves that transmit sensory impulses to, and carry messages from the spinal cord. Like the brain, the spinal cord is composed of gray matter ( mainly neuron bodies) and white matter (mainly axons or nerve fibers). In contrast to the brain, the gray matter is on the inside, and the white matter is on the inside in the spinal cord..
Thirty-one pairs of spinal nerves issue from the spinal cord. Each spinal nerve has a and a ventral root: the dorsal root brings sensory information into the cord and the ventral root takes information out to skeletal and involuntary muscles. The spinal cord is a 2-way pathway by which messages are transmitted to centers in the brain. It is also the path by which higher brain centers send directions to the visceral and peripheral organs. Nevertheless,the spinal cord is a reflex center and is not completely dependent on the brain to direct its responses to stimuli.
Because of its importance, like the the brain, the spinal cord is well-protected from injuries. The integumentary tissues, and the skeletal muscles are the most external layers of protective structures. Under these are connective tissue and bone of the vertebrae.. Inside the vertebral column there are the spinal dura mater, the arachnoid mater, the cerebrospinal fluid, and the pia mater.
Work Step by Step
The spinal cord is a long (18") somewhat flattened cylindrical structure that is the inferior part of the CNS. It develops as the caudal expansion of the neural tube. However, the arrangement of neuron bodies( gray matter) and nerve fibers ( white matter) is opposite to that in the brain. In the spinal cord, the white matter is on the outside and the gray matter is inside. During development the vertebral column outpaced the spinal cord; as a result, the spinal cord does not reach the end of the vertebral canal, but ends at the first lumabar vertebra (L1). At the center of the cord cylinder there is a small canal ( spinal canal) filled with cerebrospinal fluid. This is surrounded by the "H" shaped gray matter , which is surrounded by the white matter of ascending and descending fiber bundles( funiculi). The fibers communicate between the cerebral cortex( and other brain centers) and the spinal cord..
The three meninges , pia mater, arachnoid mater. and dura mater ( in to out) are also deployed in development for the protection of the delicate neural cord neurons. The bony protection is in the form of vertebrae of the vertebral column. External the bony protection, softer tissue --connective tissue, muscles, and integument-- add to the overall protetion of the cord from trauma. This is well because the spinal cord controls vital involuntary functions like blood pressure, body temperature and external respiration.
At the level of L1, the spinal cod ends in a prominence called the conus medullaris. This is not the end of the vertebral canal nor the stopping level for spinal nerve exit foramina. Consequently, the nerve roots at the end of the spinal cord-- the lumbar and sacral nerves --trail below the conus before they find their exits.. These trailing bundles of nerve roots resemble horses' tails,so one such bundle is called a cauda equina.
Near the end of the vertebral column, the spinal dura mater is not attached to vertebrae; an epidural space exists in which there is some padding of adipose connective tissue. Below L2 , inferior to the end of the spinal cord proper, needles may be inserted into the sub-arachnoid space to withdraw cerebrospinal fluid. At this position the risk of damaging the spinal cord is much reduced. This process is commonly called a lumbar puncture.
