Synapse And Neurotransmitters | Nervous System | Human Anatomy And Physiology | B Pharma 2nd Sem

The video begins by situating the topic within the B.Pharma syllabus, explaining that the nervous system unit is split into two parts, with this video completing the first part on synapses and neurotransmitters. The instructor assumes viewers have watched the previous two videos on nerve impulse generation.

The core concept introduced is the synapse — defined as the junction between two neurons where transmission of nerve impulses takes place. The instructor explains that neurons are not physically connected; there is a small gap (the synaptic cleft) between the axon terminal of one neuron and the dendrite of the next. Nerve impulses travel electrically within a neuron but cannot jump this gap directly, so a chemical relay mechanism is required.

The mechanism of chemical synaptic transmission is described step by step: a nerve impulse travels along the axon to the axon terminal, where it triggers the opening of calcium (Ca2+) channels. Calcium ions then bind with synaptic vesicles — membrane-enclosed sacs containing neurotransmitters — causing them to migrate to the edge of the axon terminal, fuse with the membrane, and release neurotransmitters into the synaptic cleft. These neurotransmitters then bind to receptors on the dendrite of the postsynaptic neuron, re-entering the cell and converting back into an electrical impulse, thus continuing signal transmission.

The instructor introduces terminology: the signal-sending neuron is the pre-synaptic neuron, and the signal-receiving neuron is the post-synaptic neuron. Synapses are then classified into two types: chemical synapses (most common in the human body) and electrical synapses (rare). Electrical synapses involve gap junctions that create a direct physical connection between two neurons, allowing impulses to pass without neurotransmitters, making transmission much faster.

Neurotransmitters are described as chemical messengers stored in synaptic vesicles. They are stored in vesicles to prevent unnecessary spontaneous release. The instructor emphasizes that neurotransmitters control mood, emotions, and feelings — different transmitters are responsible for different emotional states like happiness, sadness, or stress, with detailed pharmacological study deferred to later semesters.

Neurotransmitters are classified in two ways. First, by chemical nature: amino acids (e.g., GABA, glutamate, glycine, aspartate — faster conduction), amines (e.g., histamine, dopamine — slower conduction), and others (e.g., acetylcholine, nitric oxide). Second, by function: excitatory (stimulate the target neuron, increase sodium influx, cause depolarization — e.g., acetylcholine, serotonin), inhibitory (inhibit the target neuron, increase potassium efflux, cause repolarization — e.g., GABA, glycine, serotonin), and both (can be either excitatory or inhibitory depending on context — e.g., acetylcholine, dopamine).