Spinal Cord
Update September 11, 2020
The spinal cord connects the brain with most of the body.
At about the third week of prenatal development, the ectoderm of the embryonic disk forms the neural plate, which folds at the edges into the neural tube (neuraxis). A group of cells migrates to form the neural crest, which gives rise to dorsal and autonomic ganglia, the adrenal medulla, and other structures. The middle portion of the neural tube closes first; the openings at each end close later.
Schematic cross sections (A–F) showing the development of the spinal cord.
The cells in the wall of the neural tube divide and differentiate, forming an ependymal layer that encircles the central canal and is surrounded by intermediate (mantle) and marginal zones of primitive neurons and glial cells. The mantle zone differentiates into an alar plate, which contains mostly sensory neurons, and a basal plate, which is primarily composed of motor neurons. These two regions are demarcated by the sulcus limitans, a groove on the wall of the central canal. The alar plate differentiates into a dorsal gray column; the basal plate becomes a ventral gray column. The processes of the mantle zone and other cells are contained in the marginal zone, which becomes the white matter of the spinal cord.
Cross section showing two phases in the development of the spinal cord (each half shows one phase). A: Early phase. B: Later phase with central cavity.
An investing layer of ectodermal cells around the primitive cord forms the two inner meninges: the arachnoid and pia mater (pia) . The thicker outer investment, the dura mater (dura), is formed from mesenchyma.
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The spinal cord terminates at the L1 vertebral level in adults. Therefore, the lumbar and sacral nerve roots descending in the vertebral canal below the L1 vertebral level form a mass of nerve roots that resembles the tail of a horse; hence, the name cauda equina. Because the cauda equina floats in the CSF, a needle introduced into the subarachnoid space will displace the roots with little possibility of puncture damage.
Grey matter
Nerve cell bodies are clustered in an inner region of gray matter shaped like a butterfly that surrounds the central canal which is an extension of the fourth ventricle.
The back part of the wing of the butterfly shape is referred to as the posterior (dorsal grey horn). This area receives information from receptors or sensors in your skeletal muscles, organs and glands. This sensory information is passed up your spinal tracts to the brain (ascending).
The front part of the wing of the butterfly shape is referred to as the anterior (ventral grey horn). Spinal nerves exiting here pass on information received from the brain to your skeletal muscles (descending). This is referred to as motor information. Autonomic nerves pass information to your organs, glands and blood vessels.
The grey matter neurons have no myelin covering, so they look grey.
White matter
The white matter is nerves that are covered with myelin, which is made of fat and protein, making them white.
The white matter tracts containing ascending sensory and descending motor pathways are located peripherally.
Cross section through the spinal cord, showing gray matter (which contains neuronal and glial cell bodies, axons, dendrites, and synapses) and white matter (which contains myelinated axons and associated glial cells). (Reproduced, with permission, from Junqueira LC, Carneiro J, Kelley RO: Basic Histology: Text & Atlas, 11th ed. McGraw-Hill, 2005.)
Motor and sensory levels of the spinal cord.
The dorsal columns carry sensory information, particularly with respect to touch and vibration, from sensory endings on the body surface. Axons within the dorsal columns are arranged in an orderly manner, with fibers from the arm, trunk, and leg forming a map that preserves the spatial relationship of these body parts. Within the cerebral cortex, there is also a sensory map (which has the form of a small man and is, therefore, called a homunculus), within the sensory cortex. There are multiple maps of the visual world within the occipital lobes and within the temporal and parietal lobes as well. These maps are called retinotopic because they preserve the geometrical relationships between objects imaged on the retina and thus provide spatial representations of the visual environment within the brain. Each map contains neurons that are devoted to extracting and analyzing information about one particular aspect (eg, form, color, or movement) of the stimulus.
The spinal cord receives sensory information from somatic and visceral receptors through dorsal roots, transmits this information to higher brain structures through ascending tracts.
Through descending trac, it receives signals from higher centers through and transmits these signals to somatic and visceral target sites via the ventral roots.
A 50-year-old man is diagnosed with flaccid paralysis limited to the right arm, without pain or paresthesias. No sensory deficits are noted. Laboratory studies reveal that the patient is infected with West Nile virus. The target that the virus has infected resulting in this patient's symptoms is most likely the
The correct answer is A.
The patient has no sensory deficits and presents with only motor deficits. Therefore, the virus affects the ventral horn of the gray matter because that is the location of the motor neuron cell bodies.
Content 2
Content 3
1. The muscles of the posterior aspect of the thigh, or hamstring musculature, are responsible for flexing the knee joint. Beginning with the motor neuron cell bodies in the gray matter of the spinal cord, identify the most likely pathway that axons would travel from the spinal cord to the hamstring muscles?
2.