Lesson 24: Propagation of electrical impulses and synapses

What are the steps to release neurotransmitters in a chemical synapse?

1. An action potential arrives at the axon terminal. 2. Voltage-gated calcium channels open, allowing calcium to enter the cell. 3. Calcium triggers the release of neurotransmitters by exocytosis. 4. Neurotransmitters diffuse across the synaptic cleft. 5. The binding of neurotransmitters opens ligand-gated ion channels, generating graded potentials.

How does the propagation of electrical impulses occur in axons?

Propagation occurs as an impulse in one area causes local currents to flow, depolarizing the adjacent area to threshold and generating a new action potential downstream.

What is the significance of the refractory period in action potential conduction?

The conduction is unidirectional because the upstream region is in its refractory period, preventing backward propagation.

How does the diameter of a nerve fiber affect action potential conduction speed?

Larger diameter fibers conduct action potentials faster due to lower electrical resistance compared to smaller diameter fibers.

What is myelin and its role in nerve conduction?

Myelin is a glial cell (Schwann cells) plasma membrane wrapped around the nerve, providing insulation that increases conduction speed.

What is saltatory conduction?

Saltatory conduction is the rapid propagation of action potentials from node to node along myelinated axons, where voltage-gated Na+ channels are located at the nodes of Ranvier.

What are the clinical implications of demyelination in the nervous system?

Diseases like multiple sclerosis and Guillain-Barré syndrome impede nerve conduction due to demyelination.

What defines electrical synapses?

Electrical synapses are a type of communication through gap junctional pores (connexons), allowing bidirectional current flow without neurotransmitter release.

Where are electrical synapses commonly found in the body?

Electrical synapses are found in structures like the heart, enabling coordinated contraction of cardiac muscle cells, and in smooth muscles of the intestine.

What is the difference between continuous conduction and saltatory conduction?

Continuous conduction occurs in thin, unmyelinated axons, while saltatory conduction occurs in myelinated axons, making the latter faster.

What happens at the post-synaptic membrane when neurotransmitters bind?

The binding of neurotransmitters opens ligand-gated ion channels, leading to the generation of graded potentials.

What is the role of connexons in electrical synapses?

Connexons form gap junctions that allow direct electrical communication between adjacent cells.

What is the effect of myelination on conduction speed?

Myelination increases conduction speed due to reduced capacitance and increased insulation.

What is the function of nodes of Ranvier?

Nodes of Ranvier are gaps in the myelin sheath that facilitate the rapid regeneration of action potentials during saltatory conduction.

How do excitatory and inhibitory post-synaptic potentials (EPSP-IPSP) affect neuron communication?

EPSPs increase the likelihood of an action potential, while IPSPs decrease it, thus regulating neuronal communication.

What is the general concept of continuous conduction?

Continuous conduction is likened to a one-way street, where action potentials move forward along a nerve axon or muscle fiber.

What is the role of calcium in neurotransmitter release?

Calcium entry into the cell triggers the exocytosis of neurotransmitters from the pre-synaptic membrane.

How do drugs and bacterial toxins affect neuronal communication?

They can modify the communication between excitable cells by altering neurotransmitter release or receptor function.

What is the primary mechanism of communication in electrical synapses?

The primary mechanism is the direct flow of electrical current through gap junctions, allowing rapid signaling.

What is the significance of graded potentials in neuronal communication?

Graded potentials are changes in membrane potential that can lead to action potentials if they reach the threshold.

What is the primary function of the pre-synaptic membrane?

The pre-synaptic membrane is responsible for releasing neurotransmitters into the synaptic cleft.

What is the importance of the action potential in neuronal signaling?

The action potential is essential for transmitting signals along axons and facilitating communication between neurons.

What are connexons and their role in electrical synapses?

Connexons are like ion channels that allow ionic flow between neighboring cells, triggered by depolarization of the cell membrane.

What is a chemical synapse and what does it require?

A chemical synapse uses a neurotransmitter and requires calcium for neurotransmitter release.

What are the steps involved in neurotransmitter release at a chemical synapse?

1. Action potentials depolarize the presynaptic membrane. 2. Voltage-gated Ca++ channels open. 3. Ca++-dependent exocytosis occurs. 4. Neurotransmitter diffuses into the synaptic cleft. 5. It binds to postsynaptic receptors. 6. Postsynaptic Na+ channels open. 7. Neurotransmitter is removed from the synaptic cleft.

What are SNARE proteins and their significance in neurotransmitter exocytosis?

SNARE proteins, including synaptobrevin, SNAP-25, and syntaxin, are essential for the fusion of vesicles with the presynaptic membrane during neurotransmitter release.

How does botulinum toxin affect neurotransmitter release?

Botulinum toxin causes flaccid paralysis by inhibiting acetylcholine release at neuromuscular junctions.

What is the effect of tetanus toxin on neurotransmitter release?

Tetanus toxin causes spastic paralysis by inhibiting the release of the inhibitory neurotransmitter glycine in the spinal cord.

How do neurotransmitters terminate their action at the synapse?

Neurotransmitter action terminates by reuptake into cells, breakdown by enzymes, or diffusion away from the synapse.

What are the two types of postsynaptic potentials?

1. Excitatory Postsynaptic Potentials (EPSP) - Na+ influx depolarizes the membrane. 2. Inhibitory Postsynaptic Potentials (IPSP) - K+ efflux or Cl- influx hyperpolarizes the membrane.

What are the two types of summation for EPSPs?

1. Temporal summation - a series of depolarizations from one synapse. 2. Spatial summation - simultaneous depolarizations from multiple synapses.

What ion influx causes an Inhibitory Postsynaptic Potential?

Chloride influx causes an Inhibitory Postsynaptic Potential.

What are the two groups of neurotransmitters?

1. Small molecules. 2. Neuropeptides.

What are the two mechanisms by which neurotransmitters exert their actions?

1. Ionotropic actions - opening or blocking ion channels. 2. Metabotropic actions - acting on G-protein coupled receptors.

What are examples of small molecule neurotransmitters?

1. Acetylcholine. 2. Amino acids (e.g., glutamate, GABA). 3. Biogenic amines (e.g., dopamine, serotonin). 4. Gases (e.g., nitric oxide).

What is the role of excitatory neurotransmitters in the CNS?

Excitatory neurotransmitters, like glutamate, depolarize the postsynaptic membrane, promoting action potentials.

What is the role of inhibitory neurotransmitters in the CNS?

Inhibitory neurotransmitters, like GABA, hyperpolarize the postsynaptic membrane, preventing action potentials.

What is the clinical significance of neurotransmitter reuptake inhibitors?

They are used in behavioral treatments and analgesia to prolong the action of neurotransmitters.

What is the effect of clostridial neurotoxins on neurotransmitter exocytosis?

Clostridial neurotoxins inhibit neurotransmitter release, affecting muscle contraction and leading to paralysis.

What is the physiological effect of acetylcholine in the nervous system?

Acetylcholine acts as a neurotransmitter in both the central and peripheral nervous systems, facilitating communication between neurons and muscles.

What is the function of SNARE proteins in the context of neurotransmitter release?

SNARE proteins facilitate the docking and fusion of neurotransmitter vesicles with the presynaptic membrane.

How do metabotropic neurotransmitter actions differ from ionotropic actions?

Metabotropic actions involve G-protein coupled receptors and can modify membrane potential indirectly, while ionotropic actions directly open or block ion channels.

What is the significance of the neurotransmitter enkephalins?

Enkephalins are neuropeptides in the CNS that have a potent analgesic effect.

What happens during the process of exocytosis in neurotransmitter release?

Vesicles containing neurotransmitters fuse with the presynaptic membrane and release their contents into the synaptic cleft.

What are enkephalins and their primary function?

Enkephalins are neuropeptides in the CNS with a potent analgesic effect.

What role do endorphins and dynorphins play in the body?

They are involved in the control of body temperature and reproduction.

What is the function of Substance P in the CNS?

Substance P transmits peripheral pain signals and is involved in emesis (vomiting).

How is norepinephrine synthesized?

Norepinephrine (NE) is synthesized from the amino acid tyrosine.

What enzymes degrade norepinephrine?

Norepinephrine is degraded by mitochondrial monoamine oxidase (MAO) and intracellular catechol-O-methyltransferase (COMT).

What happens to norepinephrine concentration when a selective α2-adrenoceptor agonist is injected?

Norepinephrine concentration in the synaptic cleft decreases.

What is the role of acetylcholine (ACh) in synaptic transmission?

ACh is released into the synaptic cleft, attaches to cholinergic receptors, and then detaches quickly.

What enzyme breaks down acetylcholine in the synaptic cleft?

Acetylcholinesterase (AChE) breaks down ACh into choline and acetate.

What is the consequence of inhibiting acetylcholinesterase?

Inhibition of AChE leads to an increase of acetylcholine concentration in the synaptic cleft.

What is glutamate and its function in the CNS?

Glutamate is an excitatory neurotransmitter that attaches to postsynaptic receptors.

What happens to glutamate after it is released into the synaptic cleft?

It is either transported back into the presynaptic terminal or into astrocytes.

What is the role of astrocytes in glutamate metabolism?

Astrocytes convert glutamate into glutamine, which is then used to synthesize glutamate in neurons.

What are the consequences of blocking postsynaptic glutamate receptors?

Blocking these receptors decreases central nervous system outflow.

What is GABA and its primary function?

GABA is a classical inhibitory neurotransmitter.

How is GABA synthesized?

GABA is synthesized from glutamate by glutamate decarboxylase (GAD) in the presynaptic GABAergic synapse.

What types of receptors does GABA bind to?

GABA binds to GABAA and GABAC (ionotropic receptors) and GABAB (G-protein coupled receptor).

What is the effect of using a drug that enhances GABA?

It decreases central nervous system outflow.

What is the difference between continuous and saltatory propagation of electrical impulses?

Continuous propagation occurs along unmyelinated fibers, while saltatory propagation occurs along myelinated fibers.

What are the two main types of synapses?

Electrical and chemical synapses.

What are the two categories of neurotransmitters based on their function?

Excitatory and inhibitory neurotransmitters.

What clinical examples are associated with neurotransmitter function?

Clostridial toxins and SNARE proteins/exocytosis, as well as central nervous system drugs.

What are the next steps in studying neurotransmitters?

Focus on the autonomic nervous system, pharmacodynamics, and pharmacokinetics.