Ganglion(plural ganglia) is a dense group of nerve-cell bodies present in the most higher animals. In vertebrates the ganglion is a cluster of neural bodies outside the central nervous system. A spinal ganglion for instance is a cluster of nerve bodies positioned along the spinal cord at the dorsal and ventral roots of a spinal nerve. The dorsal root ganglia contain the cell bodies of afferent nerve fibres. Efferent neurons are present in the ventral root ganglia. Ganglia provide relay points and intermediary connections between different neurological structures in the body such as the peripheral and central nervous systems.

A microscopic gap or junction between a pair of adjacent neurons through which nerve impulses pass from one neuron to the other is called a synapse. The synapse consists pre-synaptic ending that contains neurotransmitters, mitochondria and other cell organelles, a post-synaptic ending that contains receptor sites for neurotransmitters, and a synaptic cleft or space between the pre-synaptic and postsynaptic endings. At the synaptic terminal (the pre-synaptic ending), an electrical impulse will trigger the migration of vesicles containing neurotransmitters towards the pre-synaptic membrane. The vesicle membrane will fuse with the pre-synaptic membrane releasing the neurotransmitters into the synaptic cleft. The neurotransmitter molecules then diffuse across the synaptic cleft where they can bind with receptor sites on the post-synaptic ending to influence the electrical response in the post- synaptic neuron. A few neurotransmitters are- acetylcholine, dopamine, gamma aminobutyric acid (GABA), serotonin, etc.