Introduction
The spinal cord and the brain make up the CNS in which there are neuronal cell bodies in large numbers. Within the CNS are also the grey and white matter. The grey matter consists mainly of a group of cell bodies with dendrites and synaptic junctions.
Within the white matter are a collection of axons with myelin sheaths. Very importantly, nuclei are a small aggregation of neuronal cell bodies with similar functions.
Arrangement of neurons in the CNS
In the CNS, the arrangement of neurons takes the form of longitudinal columns. These columns represent neurons that may group to form nuclei as in cranial nerve nuclei of the brain stem; or form continuous extensions as in the spinal cord.
The cranial nerve nuclei consist of seven columns that respectively represent: somatic motor neuron; branchial motor neuron; visceral motor neuron; general somatic sensory neuron; special somatic sensory; general visceral sensory; special visceral sensory.
The somatic motor neuron innervates muscles that originate from the head somites. Visceral motor neuron innervates preganglionic parasympathetic fibers to glands and smooth muscles. Branchial motor neuron innervates the branchial muscles.
The general somatic sensory neuron carry’s sensory information from the head. Special somatic sensory carry’s sensory information from the ear and vestibules. General visceral sensory conducts sensory impulses from widespread visceral organs. Special visceral sensory carry’s information of taste.
In the spinal cord, the columns respectively represent: ventral motor column; visceral motor column; dorsal sensory column.
The blood brain barrier and how the CNS interacts with other body parts
How do the brain and spinal cord communicate with other parts of the body? surely, it is through the cranial and spinal nerves respectively. These nerves contain afferent and efferent nerve fibers.
The CNS has a high metabolic rate and for this, there is the need for a rich blood supply. This is possible through either the blood brain barrier, or the blood spinal cord barrier that regulate the passage of molecules from blood to neuronal tissues.
The spinal nerves and parts of the spinal cord
There are 31 pairs of spinal nerves through which, afferent and efferent nerve fibers reach the spinal cord. The dorsal root and ventral root unite to form each spinal nerves.
The dorsal roots carry afferent nerve fibers whose neuronal cell bodies are located in the dorsal root ganglia. The ventral roots carry efferent nerve fibers whose neuronal cell bodies are located in the grey matter.
Within the spinal cord are the grey and white matter. The grey matter takes the form of “H” with projections such as: ventral horn; lateral horn; dorsal horn. The ventral horn is responsible for motor activities, while the dorsal horn has sensory functions.
At specific levels of the spinal cord are only found the lateral horn that sends efferent nerve fibers to preganglionic sympathetic neurons. The white matter consists mainly of ascending and descending axonic tracts that link spinal cord segments to one another, and also to the brain.
Overall, the spinal cord receives sensory input and sends motor output to the trunk and limbs.