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In biology, ganglia is an oval-shaped structure that contains the cell bodies of a neuron. It is also called an encapsulated collection of bodies of nerve cells. The peripheral nervous system (PNS) is made by these bodies and plays an important role in nerve signal relay stations. The ganglion also acts as an entry and exit site of a nerve. The ganglion cells form the ganglia and also make up the ganglion nervous system that is sometimes called peripheral ganglia.
Structure of Ganglia
The bundles of somatic and dendritic parts connect and form the basic structure of ganglia. The interconnecting point is called the plexus. The central nervous system including the brain has the basal ganglia or basal nuclei that act as an interconnecting nuclear network.
The basal ganglia are located in the brainstem, thalamus, and cerebral cortex and play important functions such as motor control, feelings, intelligence, and learning.
Location of Ganglia
The ganglia are generally located outside the spinal cord and brain which are called spinal ganglion and cerebral ganglion respectively. The spinal ganglion is located in the dorsal region of the spinal cord and around the spinal nerve.
Ganglia vs Nuclei
The Ganglia and the nuclei mainly differ in their location within the nervous system. The clustered nerve cells of the central nervous system are called nuclei while the clustered nerve cells of peripheral nerve cells are termed Ganglia.
The PNS are the axon pathways that originated from the ganglia whereas the axon pathways that originated from nuclei are called CNS. Most of the ganglion cells collect information from nerves and are called sensory neurons.
• The nervous system contains both Ganglia and nuclei, both parts are involved in the transmission of signals.
• Both components are made up of nerve cell bodies or nerve clusters.
• Ganglia and nuclei both are the origins of bundles of nerve fibers.
Function of Ganglia
The major functions of Ganglia include allowing nerves to enter and leave continuously all the time and acting as relay stations. The non-stop working ability of the Ganglia helps in the proper functioning of the body.
The bodily organs and glands are part of the autonomic nervous system and controlled by the Ganglia. Different types of ganglia are involved in different tasks within the body. The names of various types of ganglia define their primary functions.
For example- the ganglia involved in sensing the stimuli are named sensory ganglia, similarly, autonomic ganglia control the functions of autonomic organs.
Nerve vs Ganglion
Many times nerve is confused with the ganglion but there are some differences between both the terms. The nerve and ganglion both are components of the nervous system but the ganglion is a cluster of nerve cells found outside of the CNS whereas the nerve is the axon of the neuron. The impulses are carried out by the afferent nerves whereas an efferent nerve is involved in motor functions.
Types of Ganglia
Invertebrates, the ganglia are classified into three major classes-
• Dorsal Root Ganglia or sensory ganglia- also known as spinal ganglia that contains cell bodies of the sensory neurons.
• Cranial nerve ganglia- cranial nerve cells are the parts of cranial ganglia.
• Autonomous nerves- consists of the cell bodies of autonomous ganglia.
i. Dorsal Root Ganglia (DRG)
The sensory neural signals from the peripheral nervous system to the central nervous system are carried out by the dorsal root ganglia. They originate from the dorsal root of the spinal nerves and carry sensory signals to CNS. The receptors for pain and temperature are also carried out by the DRG.
The dorsal root ganglia with a single axon dividing into two different branches are called pseudo-unipolar neurons. Sometimes the action potential generated by peripheral impulses passes through the proximal process and escapes the dorsal root ganglia. The ganglia are mainly involved in the transmission of sensory information such as temperature, cognition, sensation, pressure. It is usually unipolar or pseudounipolar with the central nucleus.
There are two cell layers in the unipolar cell among which the inner layer is composed of satellite cells and the outer layer are made of connective tissue capsule cells. The sensory ganglia do not contain any synapses. The cell body of sensory neurons from the dorsal root ganglia thus it is also known as sensory ganglia.
The sensory nerves respond to various stimuli and are located at the base of the spine. The ganglia may be classified into two types based on their location- spinal ganglia and cranial nerve ganglia. The dorsal roots of the lumbar, thoracic, and cervical regions contain the spinal ganglia whereas the spinal ganglia are mainly found at the bases of some cranial nerves and finally reach the brain stem.
Examples of cranial nerves include:
1. V (trigeminal)
2. VII (facial)
3. IX (glossopharyngeal)
4. X (vagus)
The sensory input is controlled by the sensory ganglia that involve common stimuli and special stimuli. Touch and temperature are examples of common stimuli whereas special stimuli include smell, vision, taste, hearing, and balance. The X nerve involves in the supply of these sensory inputs to the muscles of the body and abdominal organs.
ii. Cranial Nerve Ganglia
The cranial nerve ganglia are made up of the cell bodies of neurons of the cranial nerve. The cranial nerve differs from the spinal nerve. For example- the cranial nerve connects with the brain whereas the spinal nerve links with the spinal cord. The unipolar shaped sensory neurons of cranial nerve ganglia are associated with the satellite cells and the nerves have their roots inside the cranium.
These nerves are classified into subtypes such as sensory ganglia and parasympathetic ganglia. Sensory nerve ganglia are also named cranial nerve ganglia that go into the DRG of the spinal nerve. Besides the parasympathetic nerve, the sympathetic nerves also occur.
The present in the upper sympathetic trunk and connects to the head and neck. The major function of cranial nerves is to provide sensory input to the head and neck, which contains general as well as some special sensations. For example, the general sensation includes touch and temperature whereas taste, vision, and hearing are examples of special sensations.
iii. Autonomic Ganglia
The autonomic ganglia are made of neuronal cell bodies and their dendrites. The junctions between the autonomic nerves of the CNS and the autonomic nerves of peripheral organs are known as autonomic ganglia. Their main function is to transmit the peripheral sensory signals to the CNS integration centers.
The autonomic ganglia are involved in the postsynaptic neurons and covered by dense connective tissue capsules. These neurons conduct impulses to the glands, smooth, and cardiac muscles, etc. There are two types of autonomic ganglia, which are sympathetic ganglia and parasympathetic ganglia.
A sympathetic ganglion is the ganglia that are comprised of the sympathetic nervous system. They inform the body about tension and serious threats. The sympathetic ganglia are made up of about 20,000-30,000 nerve cell bodies existing in the form of long chains.
The sympathetic ganglia are further classified based on their location in the body. There are mainly two classes of sympathetic ganglia: the prevertebral ganglia and the paravertebral ganglia.
a). Prevertebral Ganglia: These ganglia are mainly located at the front of the aorta and vertebral columns. The target organ is separated from the paravertebral organ with prevertebral ganglia.
The prevertebral ganglia are subdivided into the lower mesenteric ganglion, upper mesenteric ganglion, and celiac ganglion.
• The lower mesenteric ganglion grows nerves in the urinary bladder, sigmoid colon, and reproductive organs.
• The upper mesenteric ganglion has nerves grown into the small intestine.
• The celiac ganglion grows nerves to the digestive tract, beginning of the small intestine, stomach, liver, and pancreas.
b). Paravertebral Ganglia: It is also called sympathetic chain ganglia that located next to the sympathetic trunks. They are found in the pairs next to the spinal nerves and found laterally to the vertebral bodies.
Usually, these ganglia are located on either side of the vertebrae and form a link to the sympathetic trunk. They are generally found in the pairs of twenty-one or twenty- two, among which three are found in the cervical region, four in the lumbar, four in the sacral region, ten or eleven are located in the thoracic region, while a coccyx consists of a single unpaired ganglion.
Functions of Sympathetic Ganglia
• Initiate the fight-or-flight response.
• Enable pupil dilation, higher blood pressure, fast breathing, and changes in blood flow.
• Sympathetic chain ganglia also cause the rising of the hair, increased sweating, and goose-bumps.
• During a fight-or-flight response, the increase in saliva, increase heart rate, increase fat breakdown, and increase ejaculation in the body.
The parasympathetic ganglia are the autonomic ganglia present in the PNS. These ganglia involve the sympathetic system to maintain and body homeostasis. The maintenance of the internal environment irrespective of the external changes is called homeostasis.
The functions of parasympathetic ganglia include energy conservation by reducing heart rate, reduce volume, keeps eyes moist by increase secretion by lacrimal glands. One of the most significant actions of the parasympathetic nervous system includes the ‘rest and digest response.
The parasympathetic nervous system is responsible for this action that allows the relaxation of the body. The process includes decreased heart rate, decreased respiration, and increased digestion. There are four cranial nerves from which the fibers exit the CNS.
The cranial nerves are oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X). The parasympathetic system has more limited distribution as compared to the sympathetic system that only distributed to the head, visceral cavities, and erectile external genital tissue.
Parasympathetic Ganglia Function
• Protection of pupil from bright light.
• Regulate contraction of ciliary muscles for close vision.
• Increase secretion of the salivary glands.
• Facilitates the secretion of bronchi of lungs.
• Energy conservation.
• Increase digestion, glycogen conservation, and bile secretion.
• Controls urination by contracting the urinary bladder.
A parasympathetic ganglion present on or around an innervated organ is termed a terminal ganglion. It is the site for the termination of preganglionic nerve fibers. They are found mainly close to or inside the body organs, therefore, called the intramural ganglia.
Sensory ganglia vs Autonomic ganglia
• They innervate the voluntary skeletal muscles of the body.
• They are involved in the detection of smell, sound, taste, light, touch, pain, and temperature.
• The sensory ganglia are formed by dense myelinated nerve fibers.
• A single neuron is the sensory ganglia that separate the effector organ from the stimuli.
• The autonomic ganglia innervate smooth muscles, heart muscles, and glands.
• It is involved in the detection of blood pressure, salinity, and pH.
• Several inhibitory or excitatory responses are caused by the autonomic ganglia
• It is formed by thick or thin myelinated nerve fibers.
Dysfunction of Ganglia
The dysfunction of the ganglia can cause several problems such as speech, motion, and posture control problems. The dysfunctioning show various symptoms that includes;
• Change in movement.
• Enhance the muscular strength.
• Cause muscle pain, headaches, and muscle stiffness.
• Difficulty in walking.
• Abnormal movements or repetitive movements.
The dysfunctioning of ganglion also cause many brain diseases, such as-
• Dystonia causes uncontrollable contraction of muscles.
• Huntington’s disease is which a part of the brain degenerates.
• Parkinson’s disease cause problems such as impaired movement.
• Wilson disease increases the copper level in body tissues.