Neuroscience Chapter: Synapses and Neurotransmitters

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Questions and Answers

What is a defining characteristic of electrical synapses?

They rely on receptor proteins for signal transduction.

They exhibit one-way conduction of signals.

They utilize neurotransmitters to transmit signals.

They use gap junctions for direct ion movement. (correct)

What is the primary function of gap junctions in electrical synapses?

Binding to receptor proteins on the postsynaptic membrane.

Generating action potentials in the presynaptic neuron.

Facilitating the direct passage of ions between cells. (correct)

Releasing neurotransmitters into the synaptic cleft.

Which of the following best describes the direction of signal transmission in a typical chemical synapse?

Bidirectional, allowing signals to travel both ways

Alternating, switching direction with each signal

Unidirectional, from the presynaptic to the postsynaptic neuron (correct)

Random, with no defined direction

What is the role of receptor proteins in the postsynaptic neuron?

To bind with neurotransmitters and initiate a cellular response. Signup and view all the answers

Which of the following can occur when ions move through ion channels on a postsynaptic membrane?

Either excitation or inhibition of the postsynaptic cell. Signup and view all the answers

How do 'second messengers' function within a cell when a neurotransmitter binds to a receptor?

Activating enzymes like adenyl cyclase which can initiate further intracellular actions. Signup and view all the answers

Which of the following neurotransmitters is NOT specifically listed in the content?

Dopamine Signup and view all the answers

What is the direct effect of an influx of Na+ ions on the postsynaptic membrane?

Depolarization of the membrane, causing excitation. Signup and view all the answers

What is the key enzyme involved in the breakdown of acetylcholine in the post-synaptic cleft?

Acetylcholinesterase Signup and view all the answers

Which molecule is transported back into the pre-synaptic neuron after acetylcholine is broken down?

Choline Signup and view all the answers

The synthesis of new acetylcholine in the pre-synaptic neuron requires which of the following molecules?

Choline and acetyl CoA Signup and view all the answers

What is the primary function of the binding component of a postsynaptic membrane?

To bind neurotransmitters released from the presynaptic terminal. Signup and view all the answers

What characteristic of cation channels facilitates the passage of positively charged ions like sodium (Na⁺)?

They are lined with negative charges. Signup and view all the answers

Which brain structure is responsible for secreting norepinephrine, influencing overall activity and wakefulness?

Pons Signup and view all the answers

Which of the following scenarios would most likely lead to a decrease in the body’s supply of noradrenaline?

During extreme stress Signup and view all the answers

What is the mechanism by which anion channels prevent the passage of positively charged ions?

Through repulsion due to the positive charges lining the channels. Signup and view all the answers

Which receptor type is NOT typically associated with norepinephrine binding?

Muscarinic receptors Signup and view all the answers

How does the influx of chloride (Cl⁻) ions affect the postsynaptic neuron?

It makes the inside of the neuron more negative, leading to inhibition. Signup and view all the answers

In the synthesis pathway of dopamine and norepinephrine, which amino acid is an essential precursor to tyrosine?

Phenylalanine Signup and view all the answers

Which of the following is NOT a mechanism of postsynaptic inhibition?

Decreased conductivity of the postsynaptic membrane to chloride ions. Signup and view all the answers

What is the effect of a neurotransmitter that opens cation channels?

It causes excitation of the postsynaptic neuron. Signup and view all the answers

What is the primary reason for the rapid opening and closing of ion channels?

To allow for precise and fast control of the postsynaptic neuron's activity. Signup and view all the answers

How does a second messenger system differ from a direct ion channel mechanism in the postsynaptic neuron?

It mediates longer-lasting effects in the nervous system. Signup and view all the answers

What is a primary function of inhibitory transmitter substances?

To create a more negatively charged intracellular environment in the postsynaptic membrane Signup and view all the answers

If a neurotransmitter blocks the potassium (K⁺) outflow, as mentioned in the provided reference, what type of effect does this have on the postsynaptic membrane?

It will enhance action potential production by allowing positive charge intracellularly Signup and view all the answers

What is a key difference in the mechanism of action between ionotropic and metabotropic receptors?

Metabotropic receptors employ second messenger systems, whereas ionotropic receptors directly alter ion channel permeability. Signup and view all the answers

What role does the ligand play in the function of ionotropic receptors?

It binds to the receptor, causing the ion channel to open or close. Signup and view all the answers

Which of the following best describes the action of metabotropic receptors?

Binding of NTM activates a G-protein, which then activates secondary messengers. Signup and view all the answers

What type of change would occur through metabotropic receptors, but not ionotropic receptors?

Changes in gene expression. Signup and view all the answers

Which of these ions is LEAST likely to be regulated by ionotropic receptors?

PO4--- Signup and view all the answers

What is the primary function of a ligand in the context of neurotransmitter receptors?

To bind to a receptor, initiating a physiological response. Signup and view all the answers

Compared to ionotropic receptors, what is a characteristic of the effects of metabotropic receptor activation?

Slower and more spatially widespread Signup and view all the answers

What is the primary distinction between small molecule neurotransmitters and neuropeptides?

Small molecules cause fast responses, whereas neuropeptides cause prolonged actions. Signup and view all the answers

Following activation of a metabotropic receptor by an NTM, what is the next step?

Activation of a G protein. Signup and view all the answers

Which term describes the parts of the body that utilize acetylcholine?

Cholinergics Signup and view all the answers

In the context of memory and neuron function, which role do neurotransmitter receptors play that is described in the content?

They allow for the prolonged changes required after the initial NTM is gone. Signup and view all the answers

Which of the following is NOT a location where acetylcholine is secreted?

The preganglionic neurons of the somatic nervous system Signup and view all the answers

What primarily dictates whether an ion channel opens or closes in the case of ligand-gated channels?

The binding of a chemical messenger to the channel Signup and view all the answers

What is the primary effect of acetylcholine on muscles, including the gastrointestinal tract?

Excitatory. Signup and view all the answers

What is the primary role of acetylcholine that causes the associated pathology with Alzheimer's disease?

Memory and learning Signup and view all the answers

Which of the following best describes the collective name for receptors that bind with acetylcholine?

Cholinergic receptors Signup and view all the answers

Where are nicotinic receptors most commonly found?

Muscle cells, CNS, and ANS Signup and view all the answers

Which of the following describes the mechanism of action of muscarinic receptors once bound by acetylcholine?

Using a G-protein to phosphorylate various second messengers Signup and view all the answers

What type of receptor is the nicotinic receptor?

Ionotropic receptor Signup and view all the answers

Which of the following is an example of an inhibitory effect of acetylcholine?

Inhibition of the heart by the Vagus cranial nerve Signup and view all the answers

Study Notes

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.

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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.