5.6.2 Mechanism of Synaptic Transmission of Nerve Impulse

The mechanism of synaptic transmission is the process by which a nerve impulse is transmitted across a synapse from one neuron to another. It involves the release of chemical messengers called neurotransmitters from the presynaptic neuron, which then bind to receptors on the postsynaptic neuron, generating a new action potential.

Definition

Synaptic Transmission: The movement of impulse across the synapse is called synaptic transmission. It takes place in the formation of a message which is transmitted across the synapse in the form of chemical messenger called neurotransmitter.

Main Idea

• The movement of impulse across the synapse is called a synaptic transmission.
• It takes place in the formation of a message which is transmitted across the synapse in the form of chemical messenger called neurotransmitter.
• The axons usually have several rounded synaptic knobs at their distal ends.
• These knobs contain numerous membranous sacs, called synaptic vesicles.
• When a nerve impulse reaches a knob, some of the vesicles respond by releasing a neurotransmitter.
• The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
• This generates a new action potential in the postsynaptic neuron.

Core Concept

• Synaptic transmission = movement of impulse across synapse
• Neurotransmitter = chemical messenger
• Synaptic knobs = rounded ends of axons
• Synaptic vesicles = contain neurotransmitters
• Calcium ions = enter synaptic knob
• Synaptic cleft = gap between neurons
• Receptors = on postsynaptic membrane
• EPSP = excitatory postsynaptic potential
• Enzymes = break down neurotransmitters

Detailed Explanation

Mechanism of Synaptic Transmission of Nerve Impulse

• The movement of impulse across the synapse is called a synaptic transmission.
• It takes place in the formation of a message which is transmitted across the synapse in the form of chemical messenger called neurotransmitter.
• The axons usually have several rounded synaptic knobs at their distal ends, which dendrites lack.
• These knobs contain numerous membranous sacs, called synaptic vesicles.
• When a nerve impulse reaches a knob, some of the vesicles respond by releasing a neurotransmitter.

Sequence of Synaptic Transmission

The following numbered sequence describes the process:

1. Action Potential Arrives at the Synaptic Knob

• Calcium channels open in the presynaptic membrane.
• As the calcium ion concentration inside the bulb is lower than the outside, calcium ions rush in.
• As the calcium concentration increases, synaptic vesicles move towards the membrane.

2. The Neurotransmitter Vesicles Fuse with the Plasma Membrane of the Transmitting Cell

3. The Fused Vesicles Release Their Neurotransmitter Molecules into the Synaptic Cleft

4. The Released Neurotransmitter Molecules Diffuse Across the Cleft and Bind to Receptor Molecules on the Postsynaptic Cell Surface Membrane

5. Binding of Neurotransmitters to the Postsynaptic Neuron Receptors Opens Some Channels

• Allows Na⁺ ions to diffuse across the postsynaptic membrane.
• As a result, postsynaptic membrane depolarizes and an action potential is generated.
• Since this depolarization brings the membrane potential towards threshold level, it is called excitatory postsynaptic potential (EPSP).

6. Once the Neurotransmitters Have Acted on the Postsynaptic Membrane, They Are Immediately Broken Down by Enzymes

• Like acetylcholine is hydrolyzed by acetylcholinesterase.
• Adrenalin by monoamine oxidase.

Summary Table of Synaptic Transmission

Summary of the Process

Intermediate Level

Steps of Synaptic Transmission

Key Features of Synaptic Transmission

Neurotransmitters and Enzymes

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Synaptic transmission is the movement of impulse across the synapse.
• 2. It involves the release of neurotransmitters.
• 3. Neurotransmitters are chemical messengers.
• 4. Synaptic knobs are rounded ends of axons.
• 5. Synaptic vesicles contain neurotransmitters.
• 6. Calcium ions enter the synaptic knob.
• 7. Synaptic vesicles fuse with the membrane.
• 8. Neurotransmitters are released into the synaptic cleft.
• 9. Neurotransmitters bind to receptors on the postsynaptic membrane.
• 10. Na⁺ ions diffuse across the postsynaptic membrane.
• 11. Postsynaptic membrane depolarizes.
• 12. Action potential is generated.
• 13. EPSP = excitatory postsynaptic potential.
• 14. Neurotransmitters are broken down by enzymes.
• 15. Acetylcholinesterase breaks down acetylcholine.
• 16. Monoamine oxidase breaks down adrenalin.
• 17. Transmission is one-way.
• 18. It is a chemical process.
• 19. It is essential for neuron communication.
• 20. It is the basis of nervous system function.

Remember

• - Synaptic transmission = movement of impulse across synapse
• - Neurotransmitter = chemical messenger
• - Synaptic knobs = rounded ends of axons
• - Synaptic vesicles = contain neurotransmitters
• - Calcium ions = enter synaptic knob
• - Synaptic cleft = gap between neurons
• - Receptors = on postsynaptic membrane
• - EPSP = excitatory postsynaptic potential
• - Acetylcholinesterase = breaks down acetylcholine
• - Monoamine oxidase = breaks down adrenalin

Common Mistakes

• 1. Thinking neurons are in direct contact — There is a synaptic cleft between them.
• 2. Confusing presynaptic and postsynaptic neurons — Presynaptic sends; postsynaptic receives.
• 3. Mixing up the direction of transmission — Transmission is one-way.
• 4. Forgetting the role of calcium ions — Calcium ions enter the synaptic knob.
• 5. Thinking neurotransmitters are released from dendrites — Neurotransmitters are released from synaptic knobs.
• 6. Mixing up the enzymes — Acetylcholinesterase breaks down acetylcholine; monoamine oxidase breaks down adrenalin.
• 7. Forgetting the role of synaptic vesicles — They store neurotransmitters.
• 8. Confusing EPSP with action potential — EPSP brings the membrane potential toward threshold.
• 9. Thinking the synapse is the same as a node of Ranvier — Synapse is between neurons; node is on the axon.
• 10. Believing neurotransmitters stay in the cleft — They are broken down by enzymes.

Quick Revision

The movement of impulse across the synapse is called a synaptic transmission. It takes place in the formation of a message which is transmitted across the synapse in the form of chemical messenger called neurotransmitter. The axons usually have several rounded synaptic knobs at their distal ends, which dendrites lack. These knobs contain numerous membranous sacs, called synaptic vesicles. When a nerve impulse reaches a knob, some of the vesicles respond by releasing a neurotransmitter. The following numbered sequence describes the process: (1) An action potential arrives at the synaptic knob. Calcium channels open in the presynaptic membrane. As the calcium ion concentration inside the bulb is lower than the outside, calcium ions rush in. As the calcium concentration increases, synaptic vesicles move towards the membrane. (2) The neurotransmitter vesicles fuse with the plasma membrane of the transmitting cell. (3) The fused vesicles release their neurotransmitter molecules into the synaptic cleft. (4) The released neurotransmitter molecules diffuse across the cleft and bind to receptor molecules on the postsynaptic cell surface membrane. (5) Binding of neurotransmitters to the postsynaptic neuron receptors opens some channels and allows Na⁺ ions to diffuse across the postsynaptic membrane. As a result, postsynaptic membrane depolarizes and an action potential is generated. Since this depolarization brings the membrane potential towards threshold level, it is called excitatory postsynaptic potential (EPSP). (6) Once the neurotransmitters have acted on the postsynaptic membrane, they are immediately broken down by enzymes, like acetylcholine is hydrolyzed by acetylcholinesterase and adrenalin by monoamine oxidase.

60-Second Revision

• Synaptic transmission = movement of impulse across synapse
• Neurotransmitter = chemical messenger
• Synaptic knobs = rounded ends of axons
• Synaptic vesicles = contain neurotransmitters
• Calcium ions = enter synaptic knob
• Synaptic cleft = gap between neurons
• Receptors = on postsynaptic membrane
• EPSP = excitatory postsynaptic potential
• Acetylcholinesterase = breaks down acetylcholine
• Monoamine oxidase = breaks down adrenalin
• Warning: Neurotransmitters are broken down by enzymes after acting on the postsynaptic membrane

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• 5. Nervous System of Man

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• 5.6.3 CLASSIFICATIONS OF NEUROTRANSMITTERS

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