- 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 5th, 7th, 8th, 9th and 10th 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.
- 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.
Ganglia: Types and Structure