18.11
The Synapse
Neurons communicate with one another by passing on their electrical signals to other neurons. A synapse is the location where two neurons meet to exch…
The cells of your nervous system are constantly receiving and transmitting information, from basic bodily function to sensory stimulus. Neurons communicate with electrical signals called action potentials. These action potentials originate in the cell body and travel along the axon to the axon terminal where they are passed on to the next cell.
The point at which two neurons meet is called the synapse. Electrical synapses allow direct communication between cells using gap junctions, and are often involved in coordination of rapid activity. However, most synapses are chemical synapses that contain a synaptic cleft, the physical space that exists between the neuron sending the signal, known as the presynaptic cell, and the neuron receiving it, called the postsynaptic cell.
Action potentials cannot travel across the synaptic cleft, so neurons convert the electrical signal into a chemical signal at the synapse. This is accomplished by the release of molecules known as neurotransmitters. There are many neurotransmitters, each with different effects on the postsynaptic neurons, including the excitatory glutamate and the inhibitory GABA, among others.
When the action potential reaches the presynaptic terminal, voltage-gated calcium channels on the presynaptic membrane open. Calcium rushes into the cell, which triggers the fusion of vesicles with the membrane and the release of neurotransmitters into the synaptic cleft. These are then able to bind to receptors on the postsynaptic cell.
The binding of neurotransmitters to receptors may result in and increased or decreased postsynaptic membrane potential, changing the likelihood of an action potential initiating in the postsynaptic cell. Neurons can have thousands of synapses and receive information from many cells. These signals are combined in the soma of the postsynaptic neuron where the cell determines whether or not to pass the message forward.
After briefly binding to postsynaptic receptors, neurotransmitters may diffuse away, be degraded, or recycled. Reuptake proteins on the presynaptic cell are often responsible for recycling neurotransmitters. The release and binding of neurotransmitters across synapses allow the electrical signals of action potentials to be communicated to adjacent neurons. This multi-step process is critical to neuron function.
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Q1: What is the difference between electrical and chemical synapses?
Electrical synapses use gap junctions to directly transmit electrical signals between neurons, enabling extremely fast communication without energy input. Chemical synapses contain a synaptic cleft where neurons convert electrical signals into chemical signals via neurotransmitter release. Chemical synapses predominate in the human brain and allow for more regulated, fine-tuned signaling despite their slower transmission speed.
Q2: How do neurotransmitters cross the synaptic cleft?
When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open, allowing calcium to enter. This triggers vesicles containing neurotransmitters to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. These molecules then diffuse across the physical space to bind with receptors on the postsynaptic cell.
Q3: What happens after neurotransmitters bind to postsynaptic receptors?
Neurotransmitter binding to postsynaptic receptors can increase or decrease the postsynaptic cell's membrane potential, affecting the likelihood of an action potential initiating. After binding, neurotransmitters may be recycled by reuptake proteins on the presynaptic cell, degraded, or diffuse away from the synaptic cleft, terminating the signal.
Q4: Why do neurons convert electrical signals to chemical signals at synapses?
Action potentials cannot travel across the synaptic cleft, the physical space between neurons. Chemical synapses allow neurons to bridge this gap by converting electrical signals into neurotransmitter molecules that can diffuse across the cleft. This conversion enables signal transmission between neurons while allowing for regulated, fine-tuned communication.
Q5: What role do different neurotransmitters play in synaptic transmission?
Different neurotransmitters produce distinct effects on postsynaptic neurons. Glutamate is excitatory, increasing the likelihood of action potential initiation, while GABA is inhibitory, decreasing this likelihood. Other neurotransmitters like dopamine and serotonin have varied effects, allowing synapses to fine-tune neural signaling based on the specific neurotransmitter released.
Q6: How do neurons integrate signals from multiple synapses?
Neurons receive information from thousands of synapses across their dendrites and cell body. The soma of the postsynaptic neuron combines all incoming signals, determining whether the cumulative effect reaches threshold to initiate an action potential. This integration allows neurons to process complex information from many presynaptic sources before transmitting a signal forward.
Q7: What is the advantage of neurotransmitter recycling at synapses?
Reuptake proteins on the presynaptic cell recycle neurotransmitters from the synaptic cleft, reducing neurotransmitter availability and terminating the signal. This recycling mechanism, along with degradation and diffusion, allows synapses to regulate the duration and intensity of postsynaptic responses, enabling precise control over neural communication.