Autonomic nervous system

In this note, we are going to discuss the anatomy and function of the autonomic nervous system.

But first, let’s review the classification.

The nervous system consists of two parts – the central and peripheral nervous systems.

In turn, the peripheral nervous system has two functional components – sensory (afferent) and motor (efferent) components.

The motor component is formed by the somatic part, which provides movement to the skeletal muscles, and the autonomic part, which innervates internal organs, blood vessels, and glands.

The autonomic nervous system is anatomically and functionally categorized into three divisions:

In general, the sympathetic division has an activating effect on the body, providing “fight or flight” responses which are designed to mobilize energy in stressful situations.
In contrast to the sympathetic division, the parasympathetic part performs functions aimed at restoration of energy and relaxation of the body.
The enteric division regulates the activity of the smooth muscles of the GI tract.

The central part of the sympathetic nervous system is located in the lateral horn of the thoracolumbar region of the spinal cord (from T1 to L2).

The central part of the parasympathetic nervous system is located in the autonomic nuclei of the cranial nerves, namely the oculomotor (CN III),

facial (CN VII),

glossopharyngeal (CN IX),

and vagus (CN X) nerves.

Additionally, it is found in the lateral regions of the sacral segments of the spinal cord (from S2 to S4). These structures somewhat resemble lateral horns, but they are not their complete equivalents.

The autonomic reflex arc is slightly different from the somatic one. Before reaching the target organ, the motor component of the arc synapses with a neuron within the autonomic ganglion.

There are four types of autonomic ganglia:

Paravertebral ganglia: These form the sympathetic trunk;

Prevertebral or preaortic ganglia: These are located anterior to the aorta;
Visceral ganglia, which are located near the internal organs;
Intramural ganglia, which are located within the walls of the internal organs.

The first two types belong to the sympathetic division, while the latter two (also known as terminal ganglia) belong to the parasympathetic division.

The fibers that go to the ganglion are called preganglionic, and those that emerge from the ganglion are called postganglionic.

Furthermore, the components of the reflex arc differ in the sympathetic and parasympathetic divisions.

The first difference is that in the sympathetic arc, the preganglionic fibers are short, while the postganglionic fibers are long.

In the parasympathetic arc, it’s the opposite: the preganglionic fibers are long, and the postganglionic fibers are short.

The second difference is that in the sympathetic division, the neurotransmitter for preganglionic components is acetylcholine, while for postganglionic transmission it is norepinephrine (with some exceptions). In the parasympathetic arc, the neurotransmitter is acetylcholine for both preganglionic and postganglionic components.

The sympathetic reflex arc has three variants.

Let’s examine the first variant:

The first (sensory) neuron is located within the spinal ganglion

The second (interneuron) is in the lateral horn of the spinal cord (level T1-L2). The axon of the second neuron forms preganglionic fibers that pass via the white ramus communicans to the paravertebral ganglion of the sympathetic trunk
The third (motor) neuron is located within the paravertebral ganglion. The axon of this neuron exits via the gray ramus communicans as postganglionic fibers and joins the spinal nerve to reach the target organ.

The gray ramus communicans, like the white one, serves as a connecting element between the spinal nerve and the sympathetic trunk.

This variant of reflex arc innervates three types of structures:

Blood vessels (vasomotor fibers)
Skin glands (sudomotor fibers)
Arrector pili muscle (pilomotor fibers)

Let’s look into the second variant of the sympathetic reflex arc:

The first (sensory) neuron is located within the spinal ganglion
The second (nterneuron) is in the lateral horn of the spinal cord (level T1-L2). The axon of the second neuron forms preganglionic fibers that pass via the white ramus communicans to the paravertebral ganglion of the sympathetic trunk. However, it does so within one (or several) segments below or above!
The third (motor) neuron is in the paravertebral ganglion of the lower or upper segments.

There are two possible paths for the axon of this neuron:

The axon exits via the gray ramus communicans as postganglionic fibers and joins the spinal nerve to reach the target organs (previously mentioned).
OR the axon exits directly from the ganglion, bypassing the gray ramus communicans, and forms splanchnic nerves that innervate our internal organs.

And the third variant of the sympathetic reflex arc:

The initial stage is the same as in the first variant.

Then, the axon of the second neuron forms preganglionic fibers and goes not to the paravertebral ganglion, but to the prevertebral (or collateral) ganglion located anterior to the aorta.

The third neuron is located here. The axon of this neuron exits the ganglion as postganglionic fibers, and this nerve is also called a splanchnic nerve.

Parasympathetic reflex arc

Let’s now examine the parasympathetic reflex arc:

The first neuron is located within the spinal ganglion or within the sensory nuclei of the cranial nerves.
The second neuron is in the lateral part of the gray matter of the spinal cord (level S2-S4).
The third neuron is in the terminal autonomic ganglion (either near the organ or intramural).

Golosary