Human brain is well encapsulated within a thick, bony skull. The choroid plexus secretes the cerebrospinal fluid (CSF) which surrounds the brain. The fluid passes down through the four ventricles with the help of subarachnoid space and finally enters the cerebral veins through the arachnoid villi. Brain lacks lymphatic system so CSF acts as a partial substitute. Dura mater is a tough, protective connective tissue firmly attached to the skull and includes the subarachnoid space filled with the CSF, arteries and web-like connective tissue known as archanoid mater. The pia mater is a very delicate and permeable membrane composed of collagen, elastin and fibroblasts that rests on the floor of subarachnoid space and allows diffusion between CSF and the interstitial fluid of brain tissue. The pia mater is also interrupted by astrocyte processes. The dura mater, arachnoid mater and the pia mater are collectively known as meninges.

The brain and CSF are separated from each other by the blood-cerebrospinal fluid barrier and the blood-brain barrier (BBB) which protects brain from undesirable blood Synapse xt substances. These barriers are permeable to water, oxygen, carbon dioxide, small lipid soluble molecules, electrolytes and certain essential amino acids. The barriers are formed by the combined action of endothelial cells lining the capillary walls and glial cells (astrocytes) that wrap the capillaries with fibers. The brain has a distinct chemical composition for example, structural lipid accounts for 50% dry weight of brain, a feature which is in contrast with other fatty tissues of the body that are made up of triglycerides and free fatty acids. The blood brain barrier forms a protective chemical environment through which neurotransmitters can easily participate in nerve impulse conduction.

Neurotransmitters and Their Identification

Neurotransmitters are endogenously produced chemicals that actively participate in the transmission of signals from a neuron to the target cell across a synapse. They are tightly packed inside the synaptic vesicles which remain clustered beneath the membrane on the pre-synaptic side of the synapse. Upon activation they are released into the synaptic cleft where they bind to the receptors located on the membrane of the post-synaptic side of the synapse. Release of neurotransmitters is simply an indication that action potential has produced. These chemicals are synthesized from simple precursors, chiefly the amino acids. Amino acids are easily available and only few biosynthetic steps are involved in the formation of neurotransmitters.

Ramón v Cajal discovered synaptic cleft after carefully performing histological examination of neurons. After the discovery of synaptic cleft it was suggested that some chemical messengers are involved in signal transmission. In 1921 a German pharmacologist Otto Loewi confirmed that neurons communicate with each other by releasing chemical messengers. He performed a series of experiments where vagus nerve of frog was involved. He manually controlled the heart rate of frog by controlling the amount of saline solution present around the vagus nerve. When the experiments were over Loewi concluded that sympathetic regulation of heart rate can be mediated through changes in the chemical concentration. He later on discovered the first neurotransmitter known as acetylcholine (Ach). However, some neurons communicate by using electrical synapses through gap junctions that allow specific ions to pass directly from one cell to the other.