Ganglia: Types and Structure

• Outside the brain and spinal cord there are aggregations of cell bodies of neurons called ganglia.
• There are two main types of ganglia. They are sensory and autonomic ganglia.

A) Sensory ganglia

• These ganglia are present on the dorsal nerve roots of spinal nerves, where they are called dorsal nerve root ganglia or spinal ganglia.
• They are also present on the 5^th, 7^th, 8^th, 9^th and 10^th cranial nerves.
• The neurons in these ganglia are of the unipolar type (except in the case of ganglia associated with the vestibule-cochlear nerve in which they are bipolar).
• An afferent or sensory fibre of a peripheral nerve is formed by the peripheral process of each neuron.
• The center process enters the spinal cord or brain stem.

Image source: Socratic

Structure of Sensory Ganglia

• The neurons of sensory ganglia are seen to be large and arranged in groups chiefly at the periphery of the ganglion in haematoxylin and eosin stained sections.
• Group of myelinated nerve fibres separate the group of cells.
• A layer of flattened capsular cells or satellite cells surround the cell body of each neuron.
• There is a layer of delicate connective tissue outside the satellite cells.
• The processes arising from the neuron are covered with the satellite cells that are continuous with the Schwann cells.
• The connective tissue covering each neuron is continuous with the endoneurium.
• The entire ganglion is pervaded by fine connective tissue.
• The ganglion is covered on the outside by a connective tissue capsule.

Image source:Oregonstate

B)Autonomic Ganglia

• These are concerned with the nerve supply of smooth muscle or of glands.
• The pathway for this supply always consists of two neurons: preganglionic and postganglionic.
• The cell bodies of preganglionic neurons are located within the spinal cord or brain stem always.
• Their axons leave the spinal cord or brain stem and terminate by synapsing with postganglionic neurons, the cell bodies of which are located in the autonomic ganglia.
• Autonomic ganglia are therefore aggregations of the cell bodies of postganglionic neurons. These neurons are multipolar.
• Their axons leave the ganglia s postganglionic fibres to reach and supply smooth muscle or gland.
• Autonomic ganglia are subdivided into two major types: sympathetic and parasympathetic.
• Sympathetic ganglia are located on the right and left sympathetic trunks.
• Parasympathetic ganglia usually lie close to the viscera supplied through them.

Image source: Slidetodoc

Structure of Autonomic Ganglia

• The neurons of autonomic ganglia are smaller in comparison to the sensory ganglia.
• They are seen to be multipolar with silver impregnation.
• The neurons are not arranged in definite groups as in sensory ganglia, but are scattered throughout the ganglion.
• The nerve fibres are non-myelinated and thinner and are therefore much less conspicuous than in sensory ganglia.
• Satellite cells are present around the neurons of autonomic ganglia but they are not so well defined.
• The ganglion is permeated by connective tissue that also provides a capsule for it.
• The Nissl substance of the neurons is much better defined in autonomic ganglia than in sensory ganglia.
• In sympathetic ganglia the neuronal cytoplasm synthesizes catechol-amines; and in parasympathetic ganglia it synthesizes acetylcholine.
• These neurotransmitters travel down the axons to be released at nerve terminals.

References:

i) https://www.verywellhealth.com/ganglia-5104788

ii) https://www.kenhub.com/en/library/anatomy/nerve-ganglia

Ganglia: Types and Structure