5.6 Synapse

Introduction

Neurons do not work alone. They communicate with each other to transmit signals throughout the nervous system. The junction where two neurons meet and communicate is called a synapse. This topic explains what a synapse is, its structure, and how nerve impulses cross from one neuron to another.

Definition

The term synapse comes from the Greek words 'syn', which means 'together' and 'haptein' meaning 'to clasp'.

A synapse is the junction between two neurons where nerve impulses are transmitted from one neuron to another.

Main Idea

Action potentials do not cease at the end of a single neuron. They are passed on to another excitable cell. The junction between two neurons is called a synapse.

• Actually, the neurons are not in direct contact at a synapse.
• There is a gap called synaptic cleft between them.
• For an impulse to continue along a nerve pathway, it must cross this gap.
• The process of crossing the gap at a synapse is called synaptic transmission.

Action potentials can cross a synapse in one direction only.

• A fiber which carries an impulse toward a synapse is called presynaptic fiber.
• A fiber which receives the impulse after it crosses the synapse is a postsynaptic fiber.

It serves as structural basis of communication between the neurons in the CNS and between muscle cells and neurons in the PNS.

Core Concept

Structure of a Synapse

Detailed Explanation

1. What is a Synapse?

The term 'synapse' comes from the Greek words 'syn', which means 'together' and 'haptein' meaning 'to clasp'. Action potentials do not cease at the end of a single neuron. They are passed on to another excitable cell. The junction between two neurons is called a synapse.

Simple explanation: A synapse is like a meeting point between two neurons. It is where one neuron passes a message to the next neuron.

Why synapses are important:

• They allow communication between neurons.
• They ensure that impulses travel in one direction.
• They allow integration and processing of information.

2. Structure of a Synapse

Actually, the neurons are not in direct contact at a synapse. There is a gap called synaptic cleft between them.

Key structures:

• Presynaptic neuron: The neuron that carries the impulse toward the synapse.
• Synaptic cleft: The small gap between the presynaptic and postsynaptic neurons.
• Postsynaptic neuron: The neuron that receives the impulse after it crosses the synapse.

Simple explanation: The presynaptic neuron is like a sender. The synaptic cleft is the space between. The postsynaptic neuron is like a receiver.

3. Synaptic Transmission

For an impulse to continue along a nerve pathway, it must cross this gap. The process of crossing the gap at a synapse is called synaptic transmission.

What happens:

• The action potential arrives at the axon terminal of the presynaptic neuron.
• This causes synaptic vesicles to fuse with the presynaptic membrane.
• Neurotransmitters are released into the synaptic cleft.
• Neurotransmitters diffuse across the synaptic cleft.
• They bind to specific receptors on the postsynaptic membrane.
• This triggers a response in the postsynaptic neuron (excitation or inhibition).

Simple explanation: The presynaptic neuron releases chemical messengers (neurotransmitters) into the gap. These chemicals travel across the gap and bind to the next neuron, passing on the message.

4. One-Way Transmission

Action potentials can cross a synapse in one direction only.

Why one-way?

• Neurotransmitters are released only from the presynaptic neuron.
• Receptors for neurotransmitters are present only on the postsynaptic neuron.
• This ensures that the impulse travels in one direction.

Simple explanation: The synapse works like a one-way street. The message can only go from the presynaptic neuron to the postsynaptic neuron, not the other way around.

Why this is important:

• It ensures that nerve impulses travel in the correct direction.
• It prevents confusion in the nervous system.

5. Presynaptic and Postsynaptic Fibers

• A fiber which carries an impulse toward a synapse is called presynaptic fiber.
• A fiber which receives the impulse after it crosses the synapse is a postsynaptic fiber.

Simple explanation: The presynaptic fiber is the sender. The postsynaptic fiber is the receiver.

6. Role of Synapses

It serves as structural basis of communication between:

• Neurons in the CNS.
• Muscle cells and neurons in the PNS.

Simple explanation: Synapses allow neurons to communicate with each other and with muscles.

Intermediate Level

A synapse is the junction between two neurons.

Key points:

• Neurons are not in direct contact at a synapse.
• There is a gap called the synaptic cleft.
• The impulse must cross this gap.
• This process is called synaptic transmission.
• Action potentials cross a synapse in one direction only.
• Presynaptic fiber carries impulse toward synapse.
• Postsynaptic fiber receives impulse after synapse.
• Synapses allow communication between neurons and between neurons and muscles.

Important Points

• - The term 'synapse' comes from the Greek words 'syn', which means 'together' and 'haptein' meaning 'to clasp'.
• - Action potentials do not cease at the end of a single neuron.
• - They are passed on to another excitable cell.
• - The junction between two neurons is called a synapse.
• - Actually, the neurons are not in direct contact at a synapse.
• - There is a gap called synaptic cleft between them.
• - For an impulse to continue along a nerve pathway, it must cross this gap.
• - The process of crossing the gap at a synapse is called synaptic transmission.
• - Action potentials can cross a synapse in one direction only.
• - A fiber which carries an impulse toward a synapse is called presynaptic fiber.
• - A fiber which receives the impulse after it crosses the synapse is a postsynaptic fiber.
• - It serves as structural basis of communication between the neurons in the CNS and between muscle cells and neurons in the PNS.
• - Neurons communicate with one another at junctions called synapses.

Remember

• - Synapse = junction between two neurons
• - Presynaptic neuron = sends signal
• - Postsynaptic neuron = receives signal
• - Synaptic cleft = gap between neurons
• - Synaptic transmission = crossing the gap
• - Neurotransmitters = chemical messengers
• - One-way transmission = from presynaptic to postsynaptic
• - Synapses allow communication between neurons and muscles

Common Mistakes

• 1. Thinking neurons are in direct contact at a synapse.
• - Neurons are not in direct contact. There is a gap called the synaptic cleft.
• 2. Confusing presynaptic and postsynaptic neurons.
• - Presynaptic = sends signal.
• - Postsynaptic = receives signal.
• 3. Believing impulses can cross a synapse in both directions.
• - Action potentials cross a synapse in one direction only.
• 4. Forgetting the role of neurotransmitters.
• - Neurotransmitters carry the signal across the synaptic cleft.
• 5. Mixing up electrical and chemical synapses.
• - Electrical synapses = direct ion flow.
• - Chemical synapses = neurotransmitters.
• 6. Thinking the synaptic cleft is large.
• - The synaptic cleft is very small (20–40 nm).
• 7. Believing synapses only occur between neurons.
• - Synapses also occur between neurons and muscle cells (neuromuscular junction).

Quick Revision

60-Second Revision

• Synapse = junction between two neurons
• Presynaptic neuron = sends signal
• Postsynaptic neuron = receives signal
• Synaptic cleft = gap between neurons
• Synaptic transmission = crossing the gap
• Neurotransmitters = chemical messengers
• One-way transmission = from presynaptic to postsynaptic
• Synapses allow communication between neurons and muscles
• Warning: Do not think neurons are in direct contact at a synapse. There is a gap called the synaptic cleft.