Transmission of Nerve Impulse — Core Principles

Core Principles

The transmission of a nerve impulse is an electrochemical process that allows neurons to communicate. It begins with the maintenance of a negative resting membrane potential (around -70mV) inside the neuron, primarily due to the Na\textsuperscript{+}/K\textsuperscript{+} pump and differential membrane permeability to ions, especially K\textsuperscript{+}.

When a sufficient stimulus reaches the threshold potential, voltage-gated Na\textsuperscript{+} channels open, causing rapid Na\textsuperscript{+} influx and depolarization (rising phase of action potential).

This is followed by repolarization, where Na\textsuperscript{+} channels inactivate and voltage-gated K\textsuperscript{+} channels open, leading to K\textsuperscript{+} efflux. A brief hyperpolarization may occur before the resting potential is restored.

This action potential propagates along the axon, either continuously in unmyelinated fibers or via faster saltatory conduction in myelinated fibers (jumping between Nodes of Ranvier). At the axon terminal, the electrical signal is converted into a chemical signal: Ca\textsuperscript{2+} influx triggers the release of neurotransmitters into the synaptic cleft.

These neurotransmitters bind to receptors on the postsynaptic membrane, causing either an excitatory (EPSP) or inhibitory (IPSP) potential, which, if summated to threshold, can generate a new action potential in the postsynaptic neuron.

Important Differences

vs Electrical Synapse

Aspect	This Topic	Electrical Synapse
Mechanism	Chemical Synapse: Neurotransmitter release into synaptic cleft.	Electrical Synapse: Direct ion flow through gap junctions.
Speed	Chemical Synapse: Slower (synaptic delay due to neurotransmitter release and binding).	Electrical Synapse: Faster (instantaneous transmission).
Direction of Flow	Chemical Synapse: Unidirectional (presynaptic to postsynaptic).	Electrical Synapse: Bidirectional (ions can flow both ways).
Modulation	Chemical Synapse: Highly modifiable (neurotransmitter type, receptor type, amount of release, reuptake).	Electrical Synapse: Less modifiable.
Location/Prevalence	Chemical Synapse: Most common type in the mammalian nervous system.	Electrical Synapse: Less common, found in specific areas like retina, brainstem, and cardiac muscle.

Chemical synapses are the predominant form of communication in the mammalian nervous system, relying on neurotransmitters to bridge the synaptic cleft, offering high modifiability and unidirectional flow, albeit with a slight delay. In contrast, electrical synapses provide rapid, direct, and often bidirectional ion flow through gap junctions, making them faster but less flexible. The choice between these two types of synapses depends on the functional requirements of the neural circuit, with chemical synapses enabling complex information processing and electrical synapses facilitating synchronized, rapid responses.