Chapter 12 - Section 12.5 - Study Guide - Assess Your Learning Outcomes - Page 471: 8

Synaptic transmission is terminated through various mechanisms to ensure that the signal is transient and precise. Here are three primary ways in which synaptic transmission is ended: 1. **Reuptake:** - In many synapses, neurotransmitters are actively reabsorbed by the presynaptic neuron after they have fulfilled their signaling role. - Specialized transport proteins, known as neurotransmitter transporters, are present on the presynaptic membrane. These transporters bind to neurotransmitter molecules in the synaptic cleft and transport them back into the presynaptic neuron. - Once inside the presynaptic neuron, the neurotransmitters can be repackaged into synaptic vesicles for future release or broken down and recycled. - For example, in the case of serotonin and norepinephrine, selective serotonin reuptake inhibitors (SSRIs) and norepinephrine reuptake inhibitors (NRIs) are commonly used drugs that block reuptake to increase the availability of these neurotransmitters in the synaptic cleft and alleviate symptoms of depression. 2. **Enzymatic Degradation:** - Some neurotransmitters are broken down by enzymes in the synaptic cleft, which effectively terminates their signaling. - Enzymes specific to certain neurotransmitters are located in the synaptic cleft. When neurotransmitter molecules bind to these enzymes, they are rapidly broken down into inactive metabolites. - For example, acetylcholine at the neuromuscular junction is hydrolyzed by the enzyme acetylcholinesterase into acetate and choline, ending its action. 3. **Diffusion:** - Neurotransmitters can also be terminated simply by diffusing away from the synaptic cleft. - As neurotransmitter molecules disperse in the extracellular fluid, their concentration in the synaptic cleft decreases. - When the concentration becomes too low to effectively activate postsynaptic receptors, the neurotransmitter's action is terminated. - Diffusion is a passive process, and the extent to which it contributes to the termination of synaptic transmission depends on factors such as the distance between the synapse and nearby blood vessels, the size of the neurotransmitter molecules, and the presence of specific mechanisms (like reuptake or enzymatic degradation) that can counteract diffusion. These mechanisms work together to ensure that synaptic transmission is a highly regulated and precise process. By terminating the action of neurotransmitters, these processes prevent continuous stimulation of postsynaptic neurons and allow for the precise and dynamic control of neural communication within the nervous system.

Synaptic transmission is terminated through various mechanisms to ensure that the signal is transient and precise. Here are three primary ways in which synaptic transmission is ended: 1. **Reuptake:** - In many synapses, neurotransmitters are actively reabsorbed by the presynaptic neuron after they have fulfilled their signaling role. - Specialized transport proteins, known as neurotransmitter transporters, are present on the presynaptic membrane. These transporters bind to neurotransmitter molecules in the synaptic cleft and transport them back into the presynaptic neuron. - Once inside the presynaptic neuron, the neurotransmitters can be repackaged into synaptic vesicles for future release or broken down and recycled. - For example, in the case of serotonin and norepinephrine, selective serotonin reuptake inhibitors (SSRIs) and norepinephrine reuptake inhibitors (NRIs) are commonly used drugs that block reuptake to increase the availability of these neurotransmitters in the synaptic cleft and alleviate symptoms of depression. 2. **Enzymatic Degradation:** - Some neurotransmitters are broken down by enzymes in the synaptic cleft, which effectively terminates their signaling. - Enzymes specific to certain neurotransmitters are located in the synaptic cleft. When neurotransmitter molecules bind to these enzymes, they are rapidly broken down into inactive metabolites. - For example, acetylcholine at the neuromuscular junction is hydrolyzed by the enzyme acetylcholinesterase into acetate and choline, ending its action. 3. **Diffusion:** - Neurotransmitters can also be terminated simply by diffusing away from the synaptic cleft. - As neurotransmitter molecules disperse in the extracellular fluid, their concentration in the synaptic cleft decreases. - When the concentration becomes too low to effectively activate postsynaptic receptors, the neurotransmitter's action is terminated. - Diffusion is a passive process, and the extent to which it contributes to the termination of synaptic transmission depends on factors such as the distance between the synapse and nearby blood vessels, the size of the neurotransmitter molecules, and the presence of specific mechanisms (like reuptake or enzymatic degradation) that can counteract diffusion. These mechanisms work together to ensure that synaptic transmission is a highly regulated and precise process. By terminating the action of neurotransmitters, these processes prevent continuous stimulation of postsynaptic neurons and allow for the precise and dynamic control of neural communication within the nervous system.