Neuroscience Chapter: Synapses and Neurotransmitters

Electrical Synapse

• Electrical synapses feature direct open fluid channels for transmitting electricity between cells.
• Gap junctions, small protein tubular structures, allow free ion movement between cells.
• Transmission occurs in either direction, enabling rapid signal transmission.
• Electrical synapses are crucial for smooth and cardiac muscle function.

Chemical Synapse

• Chemical synapses are unidirectional, targeting specific goals.
• Neurotransmitters (NTMs), secreted by the presynaptic neuron, act on the postsynaptic neuron.
• Various NTMs exist, including acetylcholine, norepinephrine, epinephrine, and more (over 40 in humans).
• The primary function of these NTMs is to facilitate communication between nerve cells.

Neurotransmitters (NTMs)

• NTMs, also called transmitter substances, are crucial for one-way conduction in the nervous system (sensory, motor, memory, emotions).
• NTMs bind to receptor proteins on the postsynaptic neuron.

Action of Transmitter Substance on the Postsynaptic Neuron (Function of Receptor Proteins)

• Ion channels, including cation (mainly Na+, K+, Ca++) and anion (Cl-) channels, are crucial in receptor function.
• Ionophores, components that either open ion channels or activate intracellular messengers.
• Receptor binding can activate an increase or decrease in ion permeability, depending on the receptor type.

Receptors on Effector Organs-Membrane Permeability

• Receptors on these organs either excite or inhibit the effector cells by altering their membrane permeability through opening or closing ion channels.
• Influx of Na+ depolarizes the membrane, leading to cell excitation.
• Influx of K+ outside the cell causes hyperpolarization and cell inhibition.

Receptors on Effector Organs-Second Messenger Enzymes

• Receptors trigger intracellular alterations through second messenger systems.
• Small molecule enzymes (SM enzymes) are associated with receptors, usually situated inside the cell.
• Binding of NTMs to the receptors causes activation of enzymes, leading to cAMP formation.
• cAMP then orchestrates further intracellular events, influencing various effector cell functions.

Binding Components

• Components protrude out from a cell membrane into the synaptic cleft, where they bind NTMs.

Ionophores: Ion Channels

• Cation channels are lined with negative charges, attracting positively charged ions like Na+, K+, and Ca++.
• Cation channels facilitate ion movement into cells, causing depolarization, and thus excitation.
• Anion channels, lined with positive charges, repel positively charged ions and mainly allow negatively charged ions (Cl-) to pass.
• These channels hyperpolarize cells, leading to inhibition.

Excitatory or Inhibitory Receptors in Postsynaptic Membrane

• Excitation (positive charge increase): Influx of Na+ through channel opening.
• Inhibition (negative charge increase): Influx of Cl- through channel opening or K+ efflux.

NTM Activation of Ion Channels

• Channels open or close instantly in response to NTM binding.
• This fast opening/closing enables fast control of postsynaptic function.

Ionophore: Second Messenger System

• The prolonged effects of the nervous system necessitate a second messenger system beyond ion channels.
• Prolonged cellular changes result from NTMs triggering a cascade of intermediary messengers.

Two Types of NTM Receptors

• Ionotropic receptors and metabotropic receptors are two key types.

Ligand-Gated Ion Channels (Ionotropic Receptors)

• Ligand-gated ion channels are often associated with NTM binding for fast responses.
• Ligands, like NTMs, induce the opening or closing of channels responding to the presence of the ligand.
• Ionotropic receptors are often specific to certain ions like Na+, K+, Ca++, or Cl-.

Metabotropic Receptors (Second Messenger Systems)

• Metabotropic receptors work with G proteins and trigger a cascade of second messengers for longer-lasting effects.
• Binding of NTMs activates G proteins, which then catalyze secondary messenger production.
• Secondary messengers then influence effector proteins.

Comparison of Ionotropic and Metabotropic Receptors

• Ionotropic receptors have a combined binding site and channel, are independent of secondary messengers, and exhibit faster actions, whereas metabotropic receptors have separate binding sites and channels and require a G protein or a second messenger for action, resulting in slower effects.

Slow Responses at Metabotropic Receptors (Second Messenger Coupling)

• Initial NTM binding triggers a series of secondary messenger activations (initiating reactions within the neuron).
• This mechanism creates a slower, but sometimes longer-lasting effect.

Synthesis of NE and Dopamine (Dopamine a Precursor of NE)

• Dopamine and norepinephrine (NE) synthesis involves a series of enzyme-catalyzed steps.
• These pathways result in a specific neurotransmitter for use within the body for specific functions in the nervous system.

Epinephrine (EPI)

• It acts as a response to various situations like metabolic changes, stress, or emotional distress.
• Its function is to create a state of heightened awareness, potentially promoting "fight-or-flight" responses.
• Used medically in conditions requiring fast sympathetic nervous system activation.

Dopamine

• It's primarily inhibitory but has specific effects within the CNS.
• Outside the CNS, dopamine regulates aspects like blood vessel, kidney, pancreas, digestive system, and immune functions.
• Dopamine degradation involves reuptake, diffusion, and enzymatic breakdown.

Norepinephrine Degradation

• Norepinephrine is broken down by reuptake into nerve endings, diffusion into surrounding fluids and blood, and enzymatic degradation (via monoamine oxidase, MAO).
• Its degradation duration depends on factors like presence in blood.

Receptors on Effector Organs

• NTM binding to receptors modifies effector function by modulating ion channels or activating/inactivating intracellular enzymes, ultimately manipulating cell function.

Acetylcholine (ACh) Synthesis

• Acetylcholine is synthesized from choline and acetyl-CoA via acetyltransferase.
• Choline is reused, while acetate is excreted or repurposed in metabolism.
• ACh is broken down by acetylcholinesterase into choline and acetate.

Acetylcholine (ACh)

• Primarily excitatory, yet some cases of inhibition (like the inhibition of heart rate, via the Vagus nerves), ACh is also central to memory and learning.
• Crucial for neuromusculature.
• Used in the CNS and PNS, influencing various bodily functions and processes.
• Acts on either nicotinic or muscarinic receptors.

Test your knowledge on electrical and chemical synapses with this quiz. Explore the roles of neurotransmitters, receptors, and neuronal signaling mechanisms. Perfect for students studying Neuroscience or related fields.