Ganglia — Description and Types

Definition

Ganglia (singular: ganglion) are ovoid structures in the peripheral nervous system (PNS) containing clusters of neuronal cell bodies and their surrounding glial satellite cells, supported by delicate connective tissue and enclosed by a denser capsule. They function as relay stations for nerve impulses — at least one nerve enters and another exits from each ganglion. The direction of impulse transmission determines whether a ganglion is sensory or autonomic.

— Junqueira's Basic Histology, Text and Atlas, 17e

Types of Ganglia

Ganglia are broadly divided into two main categories based on the direction and nature of the nerve impulses they transmit:

1. Sensory (Afferent) Ganglia

Sensory ganglia receive afferent impulses directed toward the CNS. They are associated with:

Cranial nerves → cranial ganglia
Dorsal roots of spinal nerves → dorsal root (spinal) ganglia

Key features:

Large neuronal cell bodies with prominent nuclei
Pseudounipolar neurons — relay information from peripheral nerve endings to the gray matter of the spinal cord
Each cell body is completely enveloped by satellite cells (thin, sheetlike glial cells)
Well-developed, distinct connective tissue capsule continuous with the epineurium of the nerve
No blood-nerve barrier equivalent to the blood-brain barrier
Fascicles of nerve fibers enter and leave the ganglion

Histology:

Sensory ganglion (G) with distinct capsule (C) and nerve fiber fascicles (F). Kluver-Barrera stain ×56

Fig (a): Sensory ganglion (G) showing distinct connective tissue capsule (C) and nerve fiber fascicles (F) — Kluver-Barrera stain ×56

Higher magnification showing satellite cells (S) enveloping large neuronal perikarya — H&E ×400

Fig (b): Satellite cells (S) forming sheetlike envelopes around large sensory neuronal cell bodies — H&E ×400

2. Autonomic (Efferent) Ganglia

Autonomic ganglia are efferent — they regulate smooth muscle, cardiac muscle, glands, and visceral organs, maintaining homeostasis.

They use a two-neuron chain:

Preganglionic neuron — cell body in the CNS; axon uses acetylcholine as the neurotransmitter at the synapse
Postganglionic (second) neuron — multipolar, located in the peripheral ganglion; axon travels to the effector organ

Autonomic ganglia are subdivided into:

a) Sympathetic Ganglia

Cell bodies of preganglionic neurons arise from thoracic and lumbar spinal cord segments (T1–L2)
Second-order multipolar neurons located in small ganglia along the vertebral column (paravertebral chain ganglia) or prevertebral ganglia
Functionally: "fight or flight" responses
Cervical sympathetic ganglia include the Superior, Middle, and Inferior (Stellate) cervical ganglia, which supply the heart, head, and neck — Gray's Anatomy for Students
Satellite cells are present but may differ slightly from sensory ganglia

b) Parasympathetic Ganglia

Cell bodies of preganglionic neurons in medulla, midbrain, and sacral spinal cord (S2–S4)
Second-order neurons found in very small ganglia located near or within the effector organs (e.g., walls of the stomach and intestines) — these are called intramural ganglia
Capsules may be poorly defined or absent; perikarya and satellite cells form a loosely organized plexus within surrounding connective tissue
Functionally: "rest and digest"

Comparison Table: Sensory vs. Autonomic Ganglia

Feature	Sensory Ganglia	Autonomic Ganglia
Impulse direction	Afferent (to CNS)	Efferent (from CNS to effector)
Neuron type	Pseudounipolar	Multipolar
Location	Dorsal root / cranial nerve roots	Paravertebral, prevertebral, or intramural
Capsule	Well-developed, distinct	Less developed; may be absent in parasympathetic
Satellite cells	Prominent, sheet-like, envelop each cell body	Present but may be inconspicuous (intramural)
Neurotransmitter	— (sensory relay)	Acetylcholine (preganglionic); NE or ACh (postganglionic)
Examples	Dorsal root ganglion, trigeminal ganglion	Cervical sympathetic ganglia, ciliary ganglion, enteric ganglia

Sources:

Junqueira's Basic Histology, Text and Atlas, 17e — Ganglia, Sensory Ganglia, Autonomic Ganglia
Gray's Anatomy for Students — Cervical Ganglia

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