A ganglion is a collection of neuronal bodies found in the somatic and autonomic branches of the peripheral nervous system (PNS).
Ganglia can be thought of as synaptic relay stations between neurons. The information enters the ganglia, excites the neuron in the ganglia and then exits. The ganglia can be broadly categorized into two groups, that is, sensory ganglia (relating to the somatic nervous system (SNS)), and autonomic ganglia (relating to the autonomic nervous system (ANS)).
|Definition||Collection of neuron cell bodies located in the peripheral nervous system (PNS).
Sensory ganglia: Dorsal root ganglia of spinal nerves and the ganglia of selected cranial nerves.
Autonomic ganglia: Sympathetic (close to the spinal cord), Parasympathetic (neer on in the viscera).
In this article, we will explore their respective anatomy and subtypes.
- Structure and function of a ganglion
- Preganglionic vs postganglionic neurons
- Sensory ganglia
- Autonomic ganglia
Structure and function of a ganglion
Ganglia are oval in structure and contain neuronal cell bodies (somata), satellite cells (a type of glial cell), and a protective connective tissue layer. Autonomic and sensory ganglia are histologically similar, with the former containing multipolar neurons, and the latter usually containing unipolar or pseudo unipolar neurons. Please check our study unit to learn the types of the neurons.
A dense connective tissue capsule covers the ganglion, with a single layer of flat shaped satellite cells surrounding each neuronal cell body. A basement membrane covers the outer region of the satellite cells.
Preganglionic vs postganglionic neurons
Unlike in the SNS, pathways in the ANS are composed of two neurons. Here, the preganglionic neuron, found in the central nervous system (CNS), must traverse a synapse onto a postganglionic neuron in the PNS.
From here, it innervates its effector organ (cardiac muscle, smooth muscle, or glands). The fibers which link the ganglia are called the preganglionic and postganglionic fibers respectively.
Neurons in the SNS and PNS share some similarities and key differences. These are:
- Have short preganglionic fibers, and long postganglionic fibers
- Contain lightly myelinated preganglionic fibers, and unmyelinated postganglionic fibers
- Originate within the lateral horn of the spinal cord, in the thoracic and upper lumbar regions (T1 to L2,3)
Their preganglionic fibers are short, because the sympathetic chain ganglia (paravertebral ganglia) towards which sympathetic preganglionic fibers travel, is found very close to their origin point in the spinal cord.
- Have long preganglionic fibers, and very short postganglionic fibers
- Contain lightly myelinated preganglionic fibers, and unmyelinated postganglionic fibers
- Originate within the lateral horn of the sacral spinal cord and the brain, with fibers traveling with CN III, VII, IX, and X
Parasympathetic preganglionic fibers are long because parasympathetic ganglia, towards which parasympathetic preganglionic fibers travel, tend to lie near or within the peripheral organs that they innervate.
The cell bodies of somatic sensory and visceral sensory neurons are found in the dorsal root ganglia (spinal ganglia) of spinal nerves, and on the ganglia of selected cranial nerves. These structures are hence known as sensory ganglia.
Let’s take a look at the different types.
Dorsal root ganglia
Dorsal root ganglia are the most common type of sensory ganglia. As their name suggests, they are found in the posterior (dorsal) root of spinal nerves, following the emergence of the dorsal root that emerges from the intervertebral neural foramina.
Dorsal root ganglia contain clusters of sensory neuron cell bodies which transmit messages relating to pain, touch, and temperature from the PNS, towards the CNS. Satellite glial cells separate and inhibit interaction between cell bodies in the ganglion. Most of the body’s sensory neurons are contained here.
Sensory ganglia of the cranial nerves
Another type of sensory ganglia, are the ones that are found in the cranial nerves. Those are ganglia with special sensory functions and they are similar to the dorsal root ganglia except for they are associated with the cranial nerves and not the spinal nerves.
- Trigeminal (Gasser’s) ganglion
This is a semilunar ganglion (also known as Gasser’s ganglio) that contains the cell bodies of the sensory fibers that provide sensory innervation to the larger part of the head. The ganglion is found on the anterior surface of the petrous part of the temporal bone, in a dural pouch known as Meckel’s cave. The trigeminal ganglion is the largest of the cranial nerve ganglia.
2. Geniculate ganglion
The geniculate ganglion is the sensory ganglion of the facial nerve (CN VII) found at the anterior third of the facial nerve genu. It contains cell bodies of sensory fibers that transmit the sensation of taste from the anterior two-thirds of the tongue. In addition it contains the cell bodies for fibers that gather sensory information from the nasal cavity, part of the soft palate, and the sinus cavities, the auricle and the external auditory meatus (outer ear).
3. Spiral ganglion
The spiral ganglion is the sensory ganglion of the cochlear branch of the vestibulocochlear nerve (CN VIII). It is found in the modiolus of the cochlea and contains the bodies of the first-order neurons of the acoustic pathway. The dendrites (peripheral extensions) of these neurons receive the stimuli from the receptors in the organ of Corti, whereas their axons (central extensions) form the cochlear part of the vestibulocochlear nerve.
4. Vestibular ganglion
The vestibular ganglion (also known as Scarpa’s ganglion) is the sensory ganglion of the vestibular branch of the vestibulocochlear nerve (CN VIII). It is found within the fundus of the internal auditory meatus and contains primary bipolar sensory neurons of the vestibular pathway.
5. Superior and inferior ganglia of the vagus nerve
The vagus nerve displays two ganglia inferior to the jugular foramen: the jugular (or superior) ganglion, and the nodose (or inferior) ganglion. Inside the superior ganglion lie the cell bodies of pseudounipolar first-order sensory neurons. Those neurons receive afferent information from the dura of the posterior cranial fossa, the auditory meatus, and the auricle of the ear
Inside the inferior (or the nodose) ganglion there are cell bodies of neurons that transmit general sensory information from the mucosa of the larynx, pharynx, and soft palate.
6. Glossopharyngeal ganglia
As with the vagus nerve, the glossopharyngeal nerve (CN IX) contains two ganglia. The superior ganglion contains cell bodies of neurons which innervate the middle ear and internal surface of the tympanic membrane.
The inferior ganglion houses cell bodies of neurons that supply the mucosa of the posterior one-third of the tongue, adjacent pharyngeal wall, auditory tube, and tympanic cavity.
Test your knowledge on the peripheral nervous system with this quiz.
There are two types of autonomic ganglia: the sympathetic and the parasympathetic based on their functions. The former tend to be located close to the spinal cord whereas the later lie near or within the viscera of the peripheral organs that they innervate.
In summary, autonomic ganglia can be divided into three groups:
- Sympathetic chain ganglia (paravertebral)
- Prevertebral (collateral) ganglia
- Parasympathetic or terminal ganglia
We will explore each one of these groups in the following section.
Ready to learn the autonomic nervous system in more depth - and be able to test your understanding? Look no further than this interactive study unit complete with videos, quizzes, and illustrations.
Sympathetic chain ganglia (paravertebral ganglia)
Sympathetic chain ganglia, also known as paravertebral ganglia, are the autonomic ganglia of the SNS. They consist of a paired chain of ganglia found ventral and lateral to the spinal cord. The ganglia extend from the upper neck to the coccyx, where the two chains fuse to form the unpaired ganglion impar.
23 pairs of ganglia can be found: 3 in the cervical region (which fuse to create the superior, middle and inferior cervical ganglions), 12 in the thoracic region, 4 in the lumbar region, four in the sacral region, and a single, and the unpaired ganglion impar mentioned above.
Sympathetic chain ganglia receive their input from the lateral horn of the thoracic and upper lumbar spinal cord and are involved in the relay of information relating to stress and danger.
Prevertebral (collateral) ganglia
Prevertebral ganglia (also known as preaortic ganglia or collateral ganglia) lie between the sympathetic chain ganglia and the target organs.
They have also sympathetic functions but they are not part of the sympathetic chain, rather are close to the splachnic nerves that follow the main aortic branches to innervate all organs found in the abdominal and pelvic cavities (with the exception of the adrenal gland).
The parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system.
Those ganglia can be found both in head and neck (and they are part of the cranial nerves) and in the trunk, close to the thoracic and abdominal/pelvic organs.
Their preganglionic neurons are located in the cranial nuclei of the brainstem, and in the lateral horn of the sacral spinal cord. The parasympathetic nervous system is thus referred to as the craniosacral outflow. We’ll look at each segment in turn.
Parasympathetic ganglia related to head and neck
In the head and neck there are four parasympathetic ganglia: ciliary, pterygopalatine, otic and submandibular. A good mnemonic to remember the parasympathetic ganglia is: "Cats Prefer Sexy Owners". Those ganglia are related to the following nerves:
1. Oculomotor nerve (CN III)
The neurons from the Edwinger-Westphal nucleus synapse in the ciliary ganglion in the orbit and then the fibers go on to innervate the sphincter pupillae muscle and muscles of the ciliary body, which respectively act to constrict the pupils and accommodate the lens of the eye when focusing on nearby objects.
2. Facial nerve (CN VII)
The neurons from the lacrimal and superior salivatory nuclei of the brainstem send fibers in the pterygopalatine ganglion and submandibular ganglion. The postganglionic fibers go on to innervate the lacrimal gland and glands in the nasal mucosa. This gives rise to tear and mucus production respectively.
3. Glossopharyngeal nerve (CN IX)
Neurons from the inferior salivatory nucleus of the brainstem synapse in the otic ganglion. Postganglionic fibers go on to innervate the parotid gland and minor salivary glands, eliciting the production of saliva.
Parasympathetic ganglia related to the rest of the body
In this category we have two distinct groups:
- The ganglia surrounding the bronchial passages (eliciting bronchoconstriction), pancreas (stimulating the release of pancreatic enzymes and buffer), and in Meissner’s submucosal and Auerbach’s myenteric plexus along the gastrointestinal tract (stimulating digestion and releasing sphincter muscles). Those ganglia receive innervation from the vagus nerve.
- The ganglia that provide parasympathetic innervation to the pelvic viscera (the prostate, urinary bladder, penis, seminal vesicles, uterus, vagina, and clitoris). Due to their position close to or within the organs they innervate, those parasympathetic ganglia are known as terminal or intramural ganglia.
Vagus nerve (CN X)
Preganglionic parasympathetic neurons originate in the nucleus ambiguus and the dorsal motor nucleus of the brainstem. Fibers from the nucleus ambiguus synapse in the cardiac ganglia, going on to innervate the sinuatrial node, and thus decreasing heart rate.
Fibers traveling from the dorsal motor nucleus synapse in ganglia surrounding the bronchial passages (eliciting bronchoconstriction), pancreas (stimulating the release of pancreatic enzymes and buffer), and in Meissner’s submucosal and Auerbach’s myenteric plexus along the gastrointestinal tract (stimulating digestion and releasing sphincter muscles).
You may bump into the term “pseudoganglion”. As the name suggests, this is not a “real” ganglion, but rather a nerve trunk that has become thickened, thus giving the appearance of a ganglion. Appearance is the key term, as pseudoganglions contain only nerve fibers and no cell bodies. The teres minor muscle and the radial nerve both contain pseudoganglions.
Chickenpox is caused by primary infection with the varicella-zoster virus. Post-infection, this virus lies dormant within the dorsal root ganglia. In the case that the virus is reactivated, shingles occur. This involves the dermatome supplied by the sensory nerve affected.
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