krit.club logo

Neural Control and Coordination - Transmission of impulses across a synapse

Grade 11CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

A synapse is a functional junction between two neurons or between a neuron and an effector (muscle/gland). It consists of a pre-synaptic membrane, a synaptic cleft, and a post-synaptic membrane.

•

Electrical Synapse: The membranes of pre- and post-synaptic neurons are in very close proximity (approx. 0.2 nm0.2 \text{ nm}). Electrical current flows directly through gap junctions. Transmission is faster than chemical synapses and is similar to impulse conduction along an axon.

•

Chemical Synapse: The neurons are separated by a fluid-filled space called the synaptic cleft (20−50 nm20-50 \text{ nm}). Chemicals known as neurotransmitters (e.g., Acetylcholine) are required for signal transmission.

•

Mechanism of Transmission: When an action potential reaches the axon terminal, it triggers the opening of voltage-gated Ca2+Ca^{2+} channels. The influx of Ca2+Ca^{2+} ions causes synaptic vesicles to move toward and fuse with the pre-synaptic membrane, releasing neurotransmitters via exocytosis.

•

Binding and Potential Generation: Neurotransmitters diffuse across the cleft and bind to specific receptors on the post-synaptic membrane. This opens ion channels, allowing the entry of ions like Na+Na^{+}, which can generate a new action potential called Excitatory Post-Synaptic Potential (EPSP).

•

Unidirectional Flow: Impulse transmission across a chemical synapse is always one-way because neurotransmitter vesicles are present only in the pre-synaptic terminal, and receptors are located only on the post-synaptic membrane.

📐Formulae

Synaptic Delay≈0.3 to 0.5 ms\text{Synaptic Delay} \approx 0.3 \text{ to } 0.5 \text{ ms}

Cleft Width (Chemical Synapse)≈20−50 nm\text{Cleft Width (Chemical Synapse)} \approx 20 - 50 \text{ nm}

Resting Membrane Potential (Vrest)≈−70 mV\text{Resting Membrane Potential } (V_{rest}) \approx -70 \text{ mV}

Threshold Potential for Action Potential≈−55 mV\text{Threshold Potential for Action Potential} \approx -55 \text{ mV}

💡Examples

Problem 1:

Explain the role of Ca2+Ca^{2+} ions in the transmission of an impulse across a chemical synapse.

Solution:

When the action potential arrives at the axon terminal, it depolarizes the membrane, causing voltage-gated Ca2+Ca^{2+} channels to open. Ca2+Ca^{2+} ions enter the terminal and stimulate the synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitters into the synaptic cleft.

Explanation:

Without Ca2+Ca^{2+} influx, the synaptic vesicles would not release neurotransmitters, effectively blocking the communication between neurons even if the action potential reaches the terminal.

Problem 2:

Calculate the total time taken for an impulse to cross a synapse if the synaptic delay is 0.5 ms0.5 \text{ ms} and the neurotransmitter diffusion time is 0.1 ms0.1 \text{ ms}.

Solution:

0.5 ms+0.1 ms0.6 ms\begin{array}{r} 0.5 \text{ ms} \\ + 0.1 \text{ ms} \\ \hline 0.6 \text{ ms} \end{array}

Explanation:

The total time for synaptic transmission is the sum of the delay caused by the Ca2+Ca^{2+} influx/vesicle fusion and the time taken for the neurotransmitter to diffuse across the synaptic cleft.