5.6.1 Structure of Synapse

A synapse is the junction between two neurons where information is transmitted from one neuron to another. The structure of a synapse includes the presynaptic neuron, the synaptic cleft, and the postsynaptic neuron. The neurons are not in direct contact; there is a gap called the synaptic cleft between them.

Definition

Synapse: The junction between axon terminal of one neuron and the dendrite of another neuron, where information from one neuron is transmitted or relayed (handed over) to another neuron is called synapse.

Presynaptic Neuron: A neuron which carries an impulse toward a synapse is called presynaptic neuron.

Postsynaptic Neuron: A neuron which receives the impulse after it crosses the synapse is a postsynaptic neuron.

Synaptic Cleft: The gap between the presynaptic and postsynaptic neurons is called the synaptic cleft.

Main Idea

• The neurons are not in direct contact at a synapse.
• There is a gap, called a synaptic cleft between them.
• A single neuron may form synapses with many incoming fibres of different neurons.
• A neuron which carries an impulse toward a synapse is called presynaptic neuron.
• A neuron which receives the impulse after it crosses the synapse is a postsynaptic neuron.

Core Concept

• Synapse = junction between two neurons
• Synaptic cleft = gap between neurons
• Presynaptic neuron = carries impulse toward synapse
• Postsynaptic neuron = receives impulse after synapse
• Synaptic knobs = rounded ends of axons
• Synaptic vesicles = contain neurotransmitters
• Neurotransmitters = chemical messengers
• Receptors = on postsynaptic membrane

Detailed Explanation

Structure of Synapse

• The neurons are not in direct contact at a synapse.
• There is a gap, called a synaptic cleft between them.
• A single neuron may form synapses with many incoming fibres of different neurons.
• A neuron which carries an impulse toward a synapse is called presynaptic neuron.
• A neuron which receives the impulse after it crosses the synapse is a postsynaptic neuron.

Components of a Synapse

Summary Table of Synapse Structure

Intermediate Level

Components of a Synapse

Structure of Synapse

Key Features of Synapse

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Synapse is the junction between two neurons.
• 2. Information is transmitted from one neuron to another.
• 3. Neurons are not in direct contact.
• 4. There is a gap called the synaptic cleft.
• 5. A single neuron may form synapses with many neurons.
• 6. Presynaptic neuron carries impulse toward synapse.
• 7. Postsynaptic neuron receives impulse after synapse.
• 8. Synaptic transmission is the movement of impulse across the synapse.
• 9. Neurotransmitter is the chemical messenger.
• 10. Synaptic knobs are rounded ends of axons.
• 11. Synaptic vesicles contain neurotransmitters.
• 12. Calcium ions enter the synaptic knob.
• 13. Synaptic vesicles fuse with the membrane.
• 14. Neurotransmitters are released into the synaptic cleft.
• 15. Neurotransmitters bind to receptors on the postsynaptic membrane.
• 16. Na⁺ ions diffuse across the postsynaptic membrane.
• 17. Postsynaptic membrane depolarizes.
• 18. Action potential is generated.
• 19. EPSP = excitatory postsynaptic potential.
• 20. Neurotransmitters are broken down by enzymes.

Remember

• - Synapse = junction between two neurons
• - Synaptic cleft = gap between neurons
• - Presynaptic neuron = carries impulse toward synapse
• - Postsynaptic neuron = receives impulse after synapse
• - Neurotransmitter = chemical messenger
• - Synaptic vesicles = contain neurotransmitters
• - Synaptic knobs = rounded ends of axons
• - 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 junction between axon terminal of one neuron and the dendrite of another neuron, where information from one neuron is transmitted or relayed to another neuron is called a synapse. The neurons are not in direct contact at a synapse. There is a gap, called a synaptic cleft between them. A single neuron may form synapses with many incoming fibres of different neurons. A neuron which carries an impulse toward a synapse is called presynaptic neuron. A neuron which receives the impulse after it crosses the synapse is a postsynaptic neuron. 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 molecules diffuse across the cleft and bind to receptor molecules on the postsynaptic cell surface membrane. This 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. This is called excitatory postsynaptic potential (EPSP). Once the neurotransmitters have acted on the postsynaptic membrane, they are immediately broken down by enzymes.

60-Second Revision

• Synapse = junction between two neurons
• Synaptic cleft = gap between neurons
• Presynaptic neuron = carries impulse toward synapse
• Postsynaptic neuron = receives impulse after synapse
• Neurotransmitter = chemical messenger
• Synaptic vesicles = contain neurotransmitters
• Synaptic knobs = rounded ends of axons
• Receptors = on postsynaptic membrane
• EPSP = excitatory postsynaptic potential
• Acetylcholinesterase = breaks down acetylcholine
• Monoamine oxidase = breaks down adrenalin
• Warning: Neurons are not in direct contact at a synapse; there is a synaptic cleft

NEXT TOPIC

• Chapter:
• 5. Nervous System of Man

• Topic:
• 5.6.2 MECHANISM OF SYNAPTIC TRANSMISSION OF NERVE IMPULSE

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