Autonomic Nervous System

The autonomic nervous system differs significantly from the somatic nervous system in its efferent pathways.
Focus is on understanding the differences between the sympathetic and parasympathetic divisions.

In the somatic nervous system, a single neuron extends from the ventral horn of the spinal cord to the target muscle.
The cell body is located in the ventral horn.
The axon exits via the ventral root (motor) and travels directly to the target muscle.
A long axon goes all the way to the target.

The autonomic nervous system involves two neurons in its efferent pathway: preganglionic and postganglionic neurons.
These neurons synapse with each other in a ganglion (a cluster of nerve cell bodies in the peripheral nervous system).
The location of the ganglion differs based on whether it's sympathetic or parasympathetic.

In the sympathetic nervous system, ganglia are located next to the spinal cord, forming the sympathetic chain.
In the parasympathetic system, ganglia are located in or next to the target organ.
Parasympathetic ganglia often have long preganglionic axons and short postganglionic axons, as the ganglion is close to or within the target organ (e.g., urinary bladder).

Dorsal horn: Sensory information enters the spinal cord.
Ventral horn: Somatic motor information exits the spinal cord.
Lateral horn: Autonomic motor information originates from the lateral horn.
The lateral horn is only present in the thoracic and upper lumbar regions of the spinal cord.

Parasympathetic: Fibers originate from four cranial nerves (III, VII, IX, X) and the sacral portion of the spinal cord.
Sympathetic: Fibers originate from the thoracic and upper lumbar regions of the spinal cord (T1-L1/L2), referred to as the thoracolumbar division.

The adrenal medulla is part of the autonomic nervous system.
It is stimulated by a preganglionic axon.
Instead of a postganglionic neuron, specialized chromaffin cells in the medulla produce epinephrine, which is released into the bloodstream.

Sympathetic stimulation causes pupil dilation, allowing more light to enter.
Also innervates the levator palpebrae superioris muscle, which raises the upper eyelid. This muscle contains both smooth and skeletal muscle fibers.

Oculomotor (III)
Facial (VII): Innervates sublingual and submandibular salivary glands; controls lacrimal glands and nasal mucosa.
Glossopharyngeal (IX): Parasympathetic control of the parotid gland.
Vagus (X): Slows heart rate, affects breathing via arteriole contraction.

Located along the sides of the vertebral column.
Nerves exiting the spinal cord enter the sympathetic trunk, where they may synapse or travel up/down the chain.
Fibers then leave the trunk via gray rami communicantes to rejoin spinal nerves.

A motor fiber of the sympathetic neuron leaves the lateral horn of the spinal cord.
It then goes to the ganglion where synapses occur.
It then leaves the ganglion by the gray communicating branch, ramus communicantes, back to the spinal nerve.
Short preganglionic axon and a very long postganglionic axon.
Fibers targeting the head travel to the superior cervical ganglion before innervating structures.

Innervate the intestines.
Some preganglionic fibers pass through the paravertebral ganglia without synapsing and form splanchnic nerves, which synapse in prevertebral ganglia located near the abdominal aorta.

Autonomic ganglia are located along the abdominal aorta.
Trauma to the solar plexus (an autonomic sympathetic nerve ganglion) can cause temporary loss of breath and pain.
Pain from abdominal organs can be difficult tolocalize due to nerve fiber convergence.
Example: Gallbladder pain may be felt in a different area of the abdomen.
Pain can elicit itself in a different spot just like in a heart attack.

The vagus nerve (X) innervates all of the small bowel, and the ascending colon, and the transverse colon.
The descending colon, sigmoid colon, and the rectum are all innervated by sacral parasympathetic fibers.

Sympathetic and parasympathetic divisions often have opposing effects on organs.
Example: Heart – Sympathetic increases heart rate and contraction strength; parasympathetic decreases heart rate.
The sympathetics do two things to the heart: cause an increase in the heart rate and an increase in the intensity of the contraction of each contraction of the heart.
The parasympathetics only affect the rate of the heart downwards.
Both are antagonizing one another.

Parasympathetic (point): Responsible for erection of the penis and clitoris.
Causes the closure of the sphincter to your urinary bladder.
Sympathetic (shoot): Responsible for ejaculation via contraction of the bulbospongiosus muscle and vas deferens.
Parasympathetic (point) and Sympathetic (shoot).
In this case, they work in concert with one another to accomplish an action.

Muscle tone is a constant contraction of your skeletal muscles at all times.
The autonomic nervous system is the one part of the body that is constantly happening.