Problem 9
Question
Transmission of a nerve impulse to its target cell requires: a. endocytosis of neurotransmitters by excitatory presynaptic vesicles. b. the release of thousands of molecules of neurotransmitter stored in the postsynaptic cell into the synaptic cleft. c. \(\mathrm{Ca}^{2+}\) ions to diffuse through voltage-gated \(\mathrm{Ca}^{2+}\) channels. d. a fall in \(\mathrm{Ca}^{2+}\) in the cytoplasm to trigger a protein that causes the presynaptic vesicle to fuse with the plasma membrane. e. an action potential to open the \(\mathrm{Ca}^{2+}\) gates so that \(\mathrm{Ca}^{2+}\) ions, in higher concentration outside the axon, can flow back into the cytoplasm of the neuron.
Step-by-Step Solution
Verified Answer
The correct statements regarding nerve impulse transmission are c and e: Calcium ions diffuse through voltage-gated channels, and an action potential opens these channels to allow the influx of calcium ions.
1Step 1: Identify the Correct Statements Regarding Nerve Impulse Transmission
Review each statement to determine if it accurately reflects the process of nerve impulse transmission to a target cell. The process typically involves the release of neurotransmitters from the presynaptic cell into the synaptic cleft, which then bind to receptors on the postsynaptic cell, prompting a response.
2Step 2: Evaluating Statement a
Statement a suggests endocytosis by excitatory presynaptic vesicles, which is incorrect. Neurotransmitters are not endocytosed but are released from presynaptic vesicles.
3Step 3: Evaluating Statement b
Statement b describes neurotransmitter release from the postsynaptic cell, which is incorrect. Neurotransmitters are released from the presynaptic cell.
4Step 4: Evaluating Statement c
Statement c is correct, as during the process of nerve impulse transmission, calcium ions (Ca^{2+}) diffuse through voltage-gated calcium channels.
5Step 5: Evaluating Statement d
Statement d is incorrect because a rise, not a fall, in calcium ion concentration in the cytoplasm triggers vesicles to fuse with the plasma membrane.
6Step 6: Evaluating Statement e
Statement e is correct. An action potential triggers the opening of Ca^{2+} channels allowing calcium ions, which are in a higher concentration outside of the neuron, to flow into the cytoplasm.
Key Concepts
Neurotransmitter ReleaseSynaptic Cleft CommunicationCalcium Ion Channels
Neurotransmitter Release
Understanding the process of neurotransmitter release is essential for grasping how nerve impulses trigger responses in target cells. At the heart of this process is the presynaptic neuron, which is responsible for the release of neurotransmitters into the synaptic cleft. This event is tightly regulated and occurs when an action potential reaches the axon terminal.
Here's what unfolds in sequence: Upon the arrival of an action potential, voltage-gated calcium ion channels in the membrane of the axon terminal open. This occurrence allows calcium ions to flow into the neuron's cytoplasm. The influx of calcium ions facilitates the merger of synaptic vesicles, which contain neurotransmitters, with the neuron's plasma membrane. This fusion leads to the exocytosis of neurotransmitters into the synaptic cleft. Misconceptions like the one suggested in statement a of the exercise, which hints at endocytosis of neurotransmitters by the presynaptic vesicles, are common but incorrect.
Once in the synaptic cleft, the neurotransmitters rapidly diffuse across the narrow space and bind to specific receptors on the postsynaptic neuron. This binding can lead to a variety of responses, ranging from the continuation of the nerve impulse to the inhibition of further signal propagation, depending on the type of neurotransmitter and the receptor involved.
Here's what unfolds in sequence: Upon the arrival of an action potential, voltage-gated calcium ion channels in the membrane of the axon terminal open. This occurrence allows calcium ions to flow into the neuron's cytoplasm. The influx of calcium ions facilitates the merger of synaptic vesicles, which contain neurotransmitters, with the neuron's plasma membrane. This fusion leads to the exocytosis of neurotransmitters into the synaptic cleft. Misconceptions like the one suggested in statement a of the exercise, which hints at endocytosis of neurotransmitters by the presynaptic vesicles, are common but incorrect.
Once in the synaptic cleft, the neurotransmitters rapidly diffuse across the narrow space and bind to specific receptors on the postsynaptic neuron. This binding can lead to a variety of responses, ranging from the continuation of the nerve impulse to the inhibition of further signal propagation, depending on the type of neurotransmitter and the receptor involved.
Synaptic Cleft Communication
The synaptic cleft may seem like a mere sliver of space, but it's the critical arena where intercellular communication unfolds. Following neurotransmitter release from the presynaptic neuron, these chemical messengers traverse the synaptic cleft to reach the postsynaptic neuron. It is only after crossing this gap that neurotransmitters can interact with their intended targets on the surface of the next cell.
The role of the synaptic cleft goes beyond being just a physical gap; it is a highly specialized environment that ensures the precise and rapid transfer of signals. Neurotransmitters are released in the cleft, and their concentration in this space is crucial for successful signal transmission. This concentration allows neurotransmitters to bind effectively to receptors on the postsynaptic membrane. Statement b of the exercise inaccurately describes neurotransmitters as being released by the postsynaptic cell, highlighting a fundamental misunderstanding of synaptic communication.
Once the neurotransmitters have carried out their role, they are quickly removed from the synaptic cleft to terminate the signal and prevent continuous activation. This can be done through several mechanisms, such as reuptake into the presynaptic neuron, degradation by enzymes present in the synaptic cleft, or diffusion away from the cleft.
The role of the synaptic cleft goes beyond being just a physical gap; it is a highly specialized environment that ensures the precise and rapid transfer of signals. Neurotransmitters are released in the cleft, and their concentration in this space is crucial for successful signal transmission. This concentration allows neurotransmitters to bind effectively to receptors on the postsynaptic membrane. Statement b of the exercise inaccurately describes neurotransmitters as being released by the postsynaptic cell, highlighting a fundamental misunderstanding of synaptic communication.
Once the neurotransmitters have carried out their role, they are quickly removed from the synaptic cleft to terminate the signal and prevent continuous activation. This can be done through several mechanisms, such as reuptake into the presynaptic neuron, degradation by enzymes present in the synaptic cleft, or diffusion away from the cleft.
Calcium Ion Channels
Calcium ion channels play a pivotal function in the nerve impulse transmission pathway. They are integral membrane proteins that selectively permit the flow of calcium ions across the cell membrane, in response to changes in voltage across the membrane (hence the term 'voltage-gated').
As dictated by the principles of electrochemical gradients, calcium ions typically reside in higher concentration outside of the neuron. When an action potential arrives at the presynaptic terminal, it triggers the opening of these calcium channels, as described in statement e. The resulting inflow of calcium ions, contrary to the incorrect notion presented in statement d, actually causes the vesicles to fuse with the plasma membrane and release neurotransmitters into the synaptic cleft. This is a finely orchestrated event that underscores the importance of the precise control of calcium ion concentrations.
The interactions between calcium ions and the signaling pathways within the neuron ensure that neurotransmitter release is tightly coupled with the arrival of an action potential, enabling the swift and coordinated transmission of nerve impulses from neuron to neuron.
As dictated by the principles of electrochemical gradients, calcium ions typically reside in higher concentration outside of the neuron. When an action potential arrives at the presynaptic terminal, it triggers the opening of these calcium channels, as described in statement e. The resulting inflow of calcium ions, contrary to the incorrect notion presented in statement d, actually causes the vesicles to fuse with the plasma membrane and release neurotransmitters into the synaptic cleft. This is a finely orchestrated event that underscores the importance of the precise control of calcium ion concentrations.
The interactions between calcium ions and the signaling pathways within the neuron ensure that neurotransmitter release is tightly coupled with the arrival of an action potential, enabling the swift and coordinated transmission of nerve impulses from neuron to neuron.
Other exercises in this chapter
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