Sympathetic division


The sympathetic division is also referred to as the thoracolumbar division. It is referred to as either, but receives the term thoracolumbar due to its exit location. The sympathetic division preganglionic neurons exit the spinal column between the first thoracic and second lumbar segments of the vertebrate column. This varies from the majority of sympathetic neurons. Separation from somatic motor neurons in exchange for synapse with postganglionic neurons are more common. This usually occurs within the postganglionic neurons located in the chains of sympathetic trunk ganglia, which are positioned along the two sides of the spinal column.


Preganglionic sympathetic neurons are known as white rami communicantes. They received this term due to their white, myelinated, branch-like appearance. They “branch” off to the side of the sympathetic ganglia. A small percentage of the pregnanglionic neurons correspond directly with the postganglionic neurons, in part due to the same level of location in the chain of sympathetic ganglia. A larger percentage of the preganglionic neurons are more apt to traverse the sympathetic chain prior to synapse with the postganglionic neurons occurs. These are known as the gray rami communicantes due to their gray appearance, which is due to their lack of myelin.

The postganglionic neurons within the gray rami protrude specifically back until they find the anterior roots of the spinal nerves. Alternatively, they also traverse distally along the spinal nerves until they reach the effector organs and the process of innervation can be completed.


Sympathetic Division
Image: Sympathetic Division

The process of divergence can clearly be noted within the sympathetic trunk ganglia. At various levels within the sympathetic chain, the preganglionic neuron conjoin in synapse with the various postaganglionic neurons after branching off at various locations. The process of convergence can also be noted via the synaptic input generated by the numerous preganglionic neurons which is transmitted to the postganglionic neurons.


When both of these processes occur, the convergence of the of impulse synapse within the ganglia and the divergence of the impulses to the spinal cord, simultaneously, a mass impulse reaction is set off involving the postganglionic neurons. This is the primary reason why sympathetic division usually occurs as a unified action. This unified action triggers reactions in all the effector organs simultaneously.

The sympathetic chain has an exit point in the neck. Here, the synapse with the cervical sympathetic ganglia occurs with the preganglionic neurons that exit the neck. The smooth muscles of the neck and head along with the corresponding glands are innervated via the postganglionic neurons.
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