Nerve Impulse Transmission across Synapse

The nervous system’s ability to function depends on the Nerve Impulse Transmission across Synapse. The process entails the transmission of a signal from a presynaptic neuron to a postsynaptic neuron or to an effector cell, such as a muscle or gland. This is a summary of the procedure:

Nerve Impulse Transmission

The process by which signals, or electrical impulses, travel through neurons and between neurons to convey information throughout the body is known as nerve impulse transmission. The nervous system’s capacity to govern and control body functions is based on this mechanism.

Nerve Impulse Transmission across Synapse process:

1. Arrival of the Action Potential:

When an action potential, or nerve impulse, passes through the presynaptic neuron’s axon and reaches the axon terminal or synaptic knob, the process starts. This is a thorough explanation:

Steps in Arrival of the Nerve Impulse Transmission :

I. Action Potential Depolarizes the Presynaptic Membrane:

• The quick passage of ions, mostly Na+ and K+, across the neuronal membrane results in the action potential, an electrical signal.
• The presynaptic membrane depolarizes when the action potential reaches the axon terminal.

II. Opening of Voltage-Gated Calcium Channels:

• Voltage-gated calcium (Ca2+) channels embedded in the presynaptic membrane are activated when depolarization alters the membrane potential.
• Ca2+ ions from the extracellular fluid can enter the synaptic terminal when these channels open.

III. Calcium Ion Influx:

• Calcium ions play a critical role as a signal for the subsequent steps.
• The high concentration of calcium in the synaptic terminal triggers a series of molecular events, including the mobilization of synaptic vesicles.

IV. Preparation for Neurotransmitter Release:

• The presynaptic membrane attracts synaptic vesicles, which are tiny membrane-bound structures that contain neurotransmitters.
• This prepares the synaptic cleft for exocytosis, which releases neurotransmitters.

2.Release of Neurotransmitters:

Once calcium ions enter the presynaptic terminal following the arrival of a nerve impulse, the release of neurotransmitters into the synaptic cleft occurs. This is a crucial step in chemical synaptic transmission, as it bridges the gap between neurons or between a neuron and its target cell.

Steps in the Release of Neurotransmitters:

I. Calcium-Dependent Vesicle Mobilization:

• Synaptic vesicles migrate toward the presynaptic membrane when calcium ions (Ca2+) enter the presynaptic terminal.
• Vesicle docking and priming are started when calcium ions attach to particular proteins (like synaptotagmin) connected to the vesicles.

II. Vesicle Docking at the Active Zone:

• Synaptic vesicles are guided to specialized regions of the presynaptic membrane called active zones, where neurotransmitter release occurs.
• Proteins such as SNARE proteins (e.g., synaptobrevin, syntaxin, and SNAP-25) mediate vesicle docking by forming a molecular complex that holds the vesicle close to the membrane.

III. Membrane Fusion (Exocytosis):

• Vesicle fusion with the presynaptic membrane is induced by calcium’s interaction with synaptic proteins.
• The vesicle’s and the presynaptic membrane’s lipid bilayers combine to form a pore that allows neurotransmitter molecules to enter the synaptic cleft.

IV. Neurotransmitter Release into the Synaptic Cleft:

• The vesicles contain high concentrations of neurotransmitter molecules, which are released into the synaptic cleft.
• The number of synaptic vesicles going through exocytosis, which is proportional to the calcium influx, determines how much neurotransmitter is released.

3. Binding to Receptors:

Neurotransmitters diffuse across this small opening to reach the postsynaptic membrane after being released into the synaptic cleft. They attach to particular receptors there, causing the postsynaptic cell to react. For the signal to reach the following neuron or target cell, this step is essential.

Steps in Binding to Receptors:

I. Diffusion of Neurotransmitters:

Released from the presynaptic terminal, neurotransmitter molecules travel toward the postsynaptic membrane by diffusing across the synaptic cleft.

II. Interaction with Postsynaptic Receptors:

On the postsynaptic membrane, neurotransmitters attach to particular postsynaptic receptors.
Only particular neurotransmitters can activate these highly selective receptors (for example, dopamine binds to dopaminergic receptors, and acetylcholine binds to cholinergic receptors).

III. Activation of Ion Channels:

Ligand-gated ion channels make up a large number of postsynaptic receptors. These channels open or close in response to neurotransmitter binding, enabling certain ions (such as Na⁺, K⁺, Cl⁻, or Ca²⁺) to enter or exit the postsynaptic cell.
Excitatory neurotransmitters (like glutamate and acetylcholine) cause the postsynaptic membrane to depolarize and produce an excitatory postsynaptic potential (EPSP) by opening ion channels that let positive ions (like sodium or calcium ions) in.
GABA and glycine are examples of inhibitory neurotransmitters that open ion channels, allowing positive ions (K⁺) to flow out or negative ions (Cl⁻) to flow in. This hyperpolarization of the postsynaptic membrane results in an inhibitory postsynaptic potential (IPSP).

IV. Signal Transduction via G-Protein Coupled Receptors (Optional):

Certain receptors, like metabotropic receptors, are not connected to ion channels directly. Rather, they stimulate intracellular signaling pathways, such as G-proteins.
These pathways result in slower but more persistent effects, like altered metabolism, gene expression, or ion channel activity.

4. Generation of a New Action Potential (or Inhibition):

After neurotransmitters bind to receptors on the postsynaptic membrane, their effects can either excite or inhibit the postsynaptic neuron. This determines whether a new action potential is generated or suppressed. The postsynaptic neuron may be excited or inhibited by the binding of neurotransmitters to receptors.

• Excitatory: An excitatory neurotransmitter increases the likelihood that an action potential will be fired by opening ion channels that let positively charged ions, such as sodium, enter the postsynaptic neuron.
• Inhibitory: A neurotransmitter that is inhibitory reduces the likelihood of an action potential by opening ion channels that let negatively charged ions, such as chloride, enter the postsynaptic neuron.
• Excitatory Neurotransmitters: Promote action potential generation (e.g., glutamate, acetylcholine).
• Inhibitory Neurotransmitters: Suppress action potential generation (e.g., GABA, glycine).
• Threshold Potential: The critical level of depolarization required to trigger an action potential (~-55 mV).
• Summation: Determines whether the neuron reaches the threshold based on the combined effects of EPSPs and IPSPs.

5. Removal of Neurotransmitters:

Neurotransmitters must be removed from the synaptic cleft once their function of sending a signal to the postsynaptic neuron or target cell is finished. By doing this, the neurotransmitters are prevented from constantly stimulating the postsynaptic cell, which might impair regular brain activity.

Mechanisms of Neurotransmitter Removal:

Reuptake into the Presynaptic Neuron:

Neurotransmitters are actively transported back into the presynaptic neuron by specialized transport proteins on the presynaptic membrane.

Enzymatic Degradation:

Neurotransmitters are converted into inactive metabolites by synaptic cleft enzymes.
For instance, acetylcholinesterase converts acetylcholine into choline and acetate. To be used again, choline is returned to the presynaptic neuron.

Diffusion Away from the Synaptic Cleft:

Diffusion of certain neurotransmitter molecules into the surrounding areas occurs from the synaptic cleft.
These chemicals may get into the bloodstream or adjacent cells, where they undergo metabolism.

Uptake by Glial Cells:

Neurotransmitters such as glutamate and GABA can be absorbed from the synaptic cleft by surrounding glial cells, particularly astrocytes.
Neurotransmitters can undergo metabolism or be transformed into precursors for subsequent use once they are inside the glial cells.

Conclusion

Electrical and chemical signals work together in a highly coordinated process to transfer a nerve impulse across a synapse. It guarantees that neurons and other cells communicate precisely. The nervous system can process information, react to stimuli, and control body functions thanks to this mechanism. Disorders in synaptic transmission can lead to neurological conditions, emphasizing the importance of this process in maintaining normal nervous system function.

Frequently Asked Questions (FAQ)

Related Notes

• Pentose phosphate pathway (PPP)/Hexose onophosphate (HMP) sunt
• https://notesforbiology.com/simple-staining-technique/