Sympathetic Division Anatomy

The Sympathetic Division

The sympathetic divisionPart of the autonomic nervous system that prepares the body for stress and activity. of the autonomic nervous systemThe part of the peripheral nervous system that controls involuntary functions such as heart rate, di is much more complicated than the parasympathetic.  This is the fight or flight system that mobilizes when needed.  All the preganglionic fibers arise from the thoracic or lumbar regions of the spinal cordThe central nervous system structure that relays signals between the brain and body.. This origin gives the sympathetics its name as the thoracolumbar system.  This system is more complex because it has both cholinergic and adrenergic neuronsNeurons that release norepinephrine as their primary neurotransmitter, typically part of the sympath. However just like the parasympathetic nervous division all of the preganglionic fibers are cholinergic. It is the post ganglionic fibers that have the option of secreting ACH or norepinephrineA neurotransmitter involved in attention, arousal, and the fight-or-flight response.. Also complicating the sympathetic nervous division is the sympathetic chain gangliaA series of ganglia along the spinal column involved in autonomic responses.. This is a collection of unmyelinated neuronsThe functional cells of the nervous system that transmit signals. and synapseThe junction between two neurons where communication occurs. areas that run lateralAway from the midline of the body. to the spinal cord.

More Spinal Anatomy

The sympathetic nervous systemThe organ system that controls body functions using electrical and chemical signals. is more complicated. There is also some anatomyThe study of the structure of the human body. that goes along with it. We did not cover this anatomy in the parasympathetic nervous system. The sympathetic chain ganglia is connected to the ventralRelating to the front or belly side of the body. ramus of each spinal nerve. Two structures called the communication rami perform this connection.  Rami is plural and ramus is the singular term.  Every ventral ramus emerging from a spinal nerve has two communication rami. There is a white communication ramus carrying a myelinated neuron. There is a Gray communication ramus which is carrying an unmyelinated neuron. Recall that preganglionic fibers of both the sympathetic and parasympathetic were myelinated neurons. Therefore the white communication ramus is a pathway for the preganglionic fiberThe axon of a neuron that originates in the central nervous system and synapses in an autonomic gang. Within this ganglia of the sympathetic chain ganglia, the preganglionic fiber will synapse with the post ganglionic fiber. This is where the synapse occurs. The postganglionic fiberThe axon of a neuron that originates in an autonomic ganglion and extends to a target organ or tissu will then leave the sympathetic chain ganglia by way of the Gray communication ramus. So this is like a little detour.

Spinal Nerve Route

The first pathway that a preganglionic fiber of the sympathetic division could follow is called the spinal nerve route.  This is the route we previously described. In this route, the synapse area between the preganglionic and postganglionic fibers is in the sympathetic chain ganglia. The preganglionic fiber leaves the spinal cord and then synapses with the post ganglionic fiber in the sympathetic chain ganglia. This is like a little U-shaped detour to account for the synapse.

Sympathetic Nerve Route

The second location where the preganglionic and postganglionic fibers can synapse is using the sympathetic chain ganglia. However, synapsing occurs at a different level than the one in which it leaves the spinal cord. The preganglionic fiber leaves the spinal cord. It travels through the ventral route via the spinal nerve. Then it uses the white communication ramis to enter into the sympathetic chain ganglia. However, the preganglionic fiber does not synapse with the postganglionic fiber at that level. Instead, it extends upward or downward to other ganglia in the sympathetic chain ganglia. When they reach the other level, they will synapse with the postganglionic fiber. This fiber will leave the sympathetic chain ganglia and travel to the organ. This process is similar to the spinal nerve root. However, instead of using the first ganglia, it goes to other levels and synapses there.

Splanchnic Nerve Route

The third way in which a preganglionic fiber could synapse with a postganglionic fiber is using collateral ganglia. These were bunches of unmyelinated cell bodies and Axon terminals located further out in the abdomen and pelvic cavities. There were three collateral ganglia to the parasympathetic nervous system but there are many to the sympathetic nervous system. The preganglionic fiber originates here in the lateral hornA part of the spinal cord’s gray matter, containing autonomic motor neurons. of the Gray matter of the spinal cord. It then leaves by way of the ventral root, traveling through the spinal nerve. It bypasses the white and Gray communication ramus. It travels further out into the abdominal pelvic cavityThe body cavity that houses the reproductive organs and bladder.. It eventually reaches a collateral ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS).. The nerves that leave the spinal cord and travel out to the collateral ganglia are called splanchnic nerves. Splanchnic nerves are still the extensions of the myelinated preganglionic fiber. When the myelinated preganglionic fiber reaches the collateral ganglia it synapses with an unmyelinated postganglionic fiber. The postganglionic fiber then reaches all the way to the target origin and can release acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. or norepinephrine. Remember this in either division: the preganglionic fiber synapses with the postganglionic fiber. ACH is released to excite the postganglionic fiber. In the sympathetic nervous system, the preganglionic fibers are all cholinergic. They are cholinergic in the same way as they are in the parasympathetic nervous system.