Electrical Activity, Synaptic Transmission, and Cholinergic Signaling in Neurons

The Nervous System

Overview

The nervous system is a complex network responsible for transmitting electrical and chemical signals throughout the body, enabling rapid communication and coordination of physiological processes. This section focuses on the cellular and molecular mechanisms underlying neuronal signaling, synaptic transmission, and cholinergic function.

Electrical Activity in Neurons

Action Potentials and Cable Properties

• Action Potential: A rapid, transient change in membrane potential that propagates along the axon, enabling signal transmission.

• Cable Properties of Neurons: The ability of neurons to conduct charges through their cytoplasm. This property is poor due to high internal resistance and leakage of ions (especially K+ through always-open channels).

• Neurons cannot rely solely on cable properties for impulse transmission along the axon.

Propagation in Unmyelinated Neurons

• Action potentials are generated at each patch of membrane as a wave of depolarization moves down the axon.

• Voltage-gated Na+ channels open sequentially, producing individual action potentials along the entire axon.

• Conduction rate is slow because each action potential is an individual event.

Propagation in Myelinated Neurons

• Myelin: Insulates the axon, increasing the speed of electrical conduction.

• Nodes of Ranvier: Gaps in myelin where voltage-gated Na+ and K+ channels are concentrated.

• Action potentials "leap" from node to node, a process called saltatory conduction.

Action Potential Conduction Speed

• Increased by:

  • Diameter of the neuron: Larger diameter reduces resistance to charge spread.

  • Myelination: Enables saltatory conduction.

• Example: Thin, unmyelinated neuron speed ≈ 1.0 m/sec; thick, myelinated neuron speed ≈ 100 m/sec.

Synaptic Transmission

Introduction to the Synapse

• Synapse: Functional connection between a neuron and its target cell.

• In the CNS, the target is another neuron; in the PNS, it may be a neuron, muscle cell, or gland (neuromuscular junction).

Electrical Synapses

• Cells are joined by gap junctions formed by connexin proteins, allowing ions and molecules to pass directly between cells.

• Important for synchronizing neural activity (e.g., in the hippocampus for learning and memory).

Chemical Synapses

• Most common type of synapse.

• Involves a presynaptic neuron and a postsynaptic neuron separated by a synaptic cleft (~20 nm).

• Presynaptic vesicles store neurotransmitters, released via exocytosis.

Otto Loewi and Chemical Synapses

• Otto Loewi demonstrated chemical transmission at the synapse (Nobel Prize 1936), showing that the vagus nerve releases a chemical (acetylcholine) affecting heart rate.

Release of Neurotransmitters

• Vesicles containing neurotransmitter are docked at the plasma membrane by SNARE proteins.

• Action potential arrival opens voltage-gated Ca2+ channels, allowing Ca2+ influx.

• Ca2+ binds to synaptotagmin, triggering vesicle fusion and neurotransmitter release (exocytosis).

Primary Active Transport: Ca2+ Pump

• Located on all cells and in the endoplasmic reticulum of muscle cells.

• Removes Ca2+ from the cytoplasm, creating a strong concentration gradient for rapid Ca2+ movement.

• Aids in neurotransmitter release and muscle contraction.

Action of Neurotransmitter

• Neurotransmitter diffuses across the synaptic cleft and binds to specific receptor proteins.

• The response depends on the postsynaptic complement of receptors and associated channels.

Types of Postsynaptic Receptors

Ionotropic Receptors (Ligand-Gated Ion Channels)

• Directly linked to ion channels; combine chemical-binding and channel functions.

• Contain an extracellular site for neurotransmitter binding and a membrane-spanning domain forming the ion channel.

• Example: Nicotinic acetylcholine receptors.

Metabotropic Receptors (G-Protein-Coupled Receptors)

• Indirectly mediate neurotransmitter signaling via secondary messenger systems.

• Linked to G proteins, which activate intracellular signaling cascades (e.g., cAMP, IP3).

• Example: Muscarinic acetylcholine receptors.

Neurotransmitters

Overview

• Neurotransmitters are chemicals released by neurons to transmit signals across synapses.

• Examples include acetylcholine, dopamine, serotonin, glutamate, and GABA.

Acetylcholine (ACh): Synthesis, Breakdown, and Function

Synthesis and Breakdown

• ACh is synthesized from choline and acetyl-CoA in the presynaptic terminal.

• Broken down by acetylcholinesterase (AChE) in the synaptic cleft into acetate and choline.

Effect on Postsynaptic Cell

• ACh can be excitatory or inhibitory depending on the organ/receptor involved.

• Excitatory in some CNS areas, autonomic motor neurons, and all somatic motor neurons.

• Inhibitory in some autonomic motor neurons.

• The response is determined by the postsynaptic complement of receptors and channels.

Cholinergic Receptors

• Ionotropic (Nicotinic) ACh Receptors: Stimulated by nicotine; found on skeletal muscle motor end plates, autonomic ganglia, and some CNS regions.

• Metabotropic (Muscarinic) ACh Receptors: Stimulated by muscarine; found in CNS and plasma membranes of smooth/cardiac muscle and glands innervated by autonomic motor neurons.

Summary Table: Cholinergic Receptors and Responses to Acetylcholine

Receptor	Tissue	Response	Mechanism
Nicotinic	Skeletal muscle	Depolarization, action potential, muscle contraction	ACh opens cation channel in receptor
Nicotinic	Autonomic ganglia	Depolarization, activation of postganglionic neurons	ACh opens cation channel in receptor
Muscarinic (M2, M3)	Smooth muscle, glands	Depolarization, contraction, secretion	ACh activates G-protein coupled receptor
Muscarinic (M2)	Heart	Hyperpolarization, slowing rate	ACh activates G-protein coupled receptor, opens K+ channels

Cholinergic Functions

PNS: Somatic Motor Neurons

• Form neuromuscular junctions with muscle cells; the area with receptors is the motor end plate.

• ACh binds to nicotinic receptors, producing end plate potentials (EPSPs) and opening voltage-gated Na+ channels, resulting in muscle contraction.

PNS: Sinoatrial Node

• ACh binds to muscarinic receptors, affecting heart rate via G-protein signaling and ion channel modulation.

CNS Functions

• ACh is involved in memory, arousal, and attention.

• Alzheimer Disease: Associated with loss of cholinergic neurons in memory-related brain areas.

PNS: Myasthenia Gravis

• Autoimmune disease caused by antibodies blocking nicotinic ACh receptors at motor end plates.

• Results in muscle weakness, especially in eyes, eyelids, and face.

• Treated with drugs (e.g., neostigmine) that inhibit AChE, increasing ACh availability.

Action of Acetylcholinesterase (AChE)

• AChE rapidly degrades ACh in the synaptic cleft, terminating its action and allowing recycling of choline.

Summary of Neurotransmitter Action

• Neurons use various chemicals as neurotransmitters.

• Neurotransmitters exert effects by acting on different receptor types.

• Mechanisms exist to inactivate excess neurotransmitter in the synaptic cleft (e.g., enzymatic degradation, reuptake).

Key Equations

• Nernst Equation (for ion equilibrium potential): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• Action Potential Propagation (simplified): $I = \frac{V}{R}$

Additional info:

• Some diagrams and tables were inferred and expanded for clarity and completeness.

• Neurotransmitter classification and receptor mechanisms are foundational for understanding synaptic physiology in cell biology.