10-Minute Neuroscience: Synapses

Understanding Synapses in Neuroscience

Introduction to Synapses

• The video introduces synapses as specialized areas where neurons communicate, highlighting their importance in the nervous system's capabilities.

• It is noted that most neurons have thousands of synapses, with estimates of total synapses in the human brain ranging from hundreds of trillions to over two quadrillion.

Types of Synapses

• The discussion differentiates between chemical and electrical synapses, emphasizing that chemical synapses are more common.

• Action potentials are described as electrical signals used for communication within a neuron, which then lead to neurotransmitter release at axon terminals.

Structure of Chemical Synapses

• Axodendritic synapses are introduced as the most common type, formed between axon terminals and dendrites; however, other configurations exist.

• The microscopic space between neurons is called the synaptic cleft, typically 20-40 nanometers wide.

Mechanism of Neurotransmission

• The presynaptic neuron sends an action potential down its axon to trigger neurotransmitter release from vesicles at the axon terminal.

• Synaptic vesicles contain neurotransmitters and fuse with the presynaptic membrane during exocytosis to release these chemicals into the synaptic cleft.

Interaction with Postsynaptic Neurons

• Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic neuron, influencing its likelihood to fire an action potential.

• After binding, neurotransmitters must be removed from the cleft to prevent overstimulation and ensure proper functionality of subsequent signaling.

Termination of Neurotransmission

• Removal mechanisms include diffusion out of the cleft; however, this accounts for only a small percentage of neurotransmitter clearance.

• Enzymatic degradation is another method for clearing neurotransmitters; for example, acetylcholine is broken down by acetylcholinesterase.

Understanding Neurotransmitter Dynamics

Breakdown and Reuptake of Acetylcholine

• The enzyme responsible for breaking down acetylcholine into choline and acetate allows these components to be recycled back into the presynaptic neuron for further synthesis of acetylcholine.

• Reuptake is a key process where a transport protein in the presynaptic neuron's membrane retrieves excess neurotransmitters, allowing them to be repackaged into vesicles for future release.

• Selective serotonin reuptake inhibitors (SSRIs) are drugs that inhibit serotonin reuptake, increasing its levels in the synaptic cleft, which is linked to alleviating depressive symptoms.

Electrical vs. Chemical Synapses

• While chemical synapses dominate adult nervous system communication, electrical synapses are more prevalent during embryonic development and in non-mammalian systems.

• At electrical synapses, gap junction channels connect pre- and postsynaptic neurons, allowing ions to flow directly between them with minimal space (2-4 nanometers).

Advantages of Electrical Synapses

• Electrical synapses enable rapid transmission of signals with virtually no delay compared to chemical synapses, which experience slight delays due to neurotransmitter release.

• Communication at electrical synapses can occur bidirectionally, facilitating synchronized activity among groups of neurons—important for functions like breathing regulation in the brainstem.

Benefits of Chemical Synapses

• Despite their slower transmission speed, chemical synapses can amplify signals significantly; even weak currents can lead to substantial neurotransmitter release.