Transmission of Nerve Impulses - Biology Class 11 - NEET PDF Download

A nerve impulse is transmitted from one neuron to another through specialized junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron (transmitting neuron) and a post-synaptic neuron (receiving neuron). These neurons may or may not be separated by a gap. Understanding the mechanism of impulse transmission at synapses is crucial for understanding neural coordination.

1. Types of Synapses

Based on the mode of transmission, synapses are classified into two main types:

1.1 Electrical Synapses

• Structural Feature: The membranes of pre-synaptic and post-synaptic neurons are in very close proximity with no significant gap between them.
• Mechanism of Transmission: Electrical current can flow directly from one neuron into the other across these synapses. The transmission is similar to impulse conduction along a single axon.
• Speed of Transmission: Impulse transmission across electrical synapses is always faster than that across chemical synapses.
• Occurrence: Electrical synapses are rare in the human nervous system.

1.2 Chemical Synapses

• Structural Feature: The membranes of pre-synaptic and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft.
• Mechanism of Transmission: Transmission occurs through chemical messengers called neurotransmitters, not through direct electrical flow.
• Speed of Transmission: Slower than electrical synapses due to the chemical processes involved.
• Occurrence: More common in the human nervous system compared to electrical synapses.

2. Mechanism of Transmission at Chemical Synapses

The transmission of nerve impulse at chemical synapses involves a series of coordinated steps:

2.1 Components Involved

• Synaptic Cleft: The fluid-filled space separating the pre-synaptic and post-synaptic membranes.
• Neurotransmitters: Chemical substances that transmit impulses across the synaptic cleft.
• Synaptic Vesicles: Membrane-bound structures present in the axon terminals. They are filled with neurotransmitters.
• Specific Receptors: Protein molecules present on the post-synaptic membrane. Each receptor binds to a specific neurotransmitter.
• Ion Channels: Protein pores on the post-synaptic membrane that allow the entry of specific ions.

2.2 Steps in Transmission

1. Arrival of Action Potential: When a nerve impulse (action potential) arrives at the axon terminal of the pre-synaptic neuron, it triggers the next step.
2. Movement of Synaptic Vesicles: The action potential stimulates the synaptic vesicles to move towards the plasma membrane of the axon terminal.
3. Fusion and Release: The synaptic vesicles fuse with the plasma membrane. This fusion releases neurotransmitters into the synaptic cleft.
4. Binding to Receptors: The released neurotransmitters diffuse across the synaptic cleft. They bind to their specific receptors present on the post-synaptic membrane.
5. Opening of Ion Channels: The binding of neurotransmitters to receptors opens ion channels on the post-synaptic membrane.
6. Entry of Ions: The opening of ion channels allows the entry of specific ions (such as Na⁺, K⁺, Cl⁻) into or out of the post-synaptic neuron.
7. Generation of New Potential: The movement of ions generates a new electrical potential in the post-synaptic neuron.

2.3 Nature of Post-Synaptic Potential

The new potential developed in the post-synaptic neuron can be of two types:

• Excitatory Potential: The neurotransmitter causes depolarization of the post-synaptic membrane. This makes the neuron more likely to generate an action potential (impulse transmission continues).
• Inhibitory Potential: The neurotransmitter causes hyperpolarization of the post-synaptic membrane. This makes the neuron less likely to generate an action potential (impulse transmission is inhibited).

In summary, synapses serve as critical junctions for nerve impulse transmission. Chemical synapses, which are more common, rely on neurotransmitters to transmit signals across the synaptic cleft, allowing for more complex and regulated neural communication compared to the faster but simpler electrical synapses.