Kapitel 4: Neural konduktion og synaptisk transmission

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Chapter Overview and Learning Objectives

• Resting Membrane Potential (LO 4.1): Describes how the membrane potential is measured.
• Resting Membrane Potential (LO 4.2): Details the resting membrane potential, its ionic basis, and the three factors influencing the distribution of sodium (Na+) and potassium (K+) ions across the neuronal membrane.
• Generation, Conduction, and Integration of Postsynaptic Potentials (LO 4.3): Outlines the different types of postsynaptic potentials and their conduction mechanisms.
• Generation, Conduction, and Integration of Postsynaptic Potentials (LO 4.4): Explains how postsynaptic potentials summate and action potentials are generated.
• Conduction of Action Potentials (LO 4.5): Describes the ionic basis of an action potential.
• Conduction of Action Potentials (LO 4.6): Explains how the refractory period influences neuronal activity.
• Synaptic Transmission (LO 4.7): Describes how action potentials are conducted along axons (both myelinated and unmyelinated).
• Synaptic Transmission (LO 4.8): Discusses the limitations of Hodgkin-Huxley model when applied to mammalian neurons.
• Synaptic Transmission (LO 4.9): Describes the structure of different types of synapses.
• Synaptic Transmission (LO 4.10): Explains how neurotransmitter molecules are synthesized and packaged in vesicles.
• Synaptic Transmission (LO 4.11): Explains the process of neurotransmitter exocytosis.
• Synaptic Transmission (LO 4.12): Discusses the differences between ionotropic and metabotropic receptors.
• Synaptic Transmission (LO 4.13): Explains how neurotransmitters are removed from a synapse.
• Synaptic Transmission (LO 4.14): Describes the roles of glia and gap junctions in synaptic transmission.
• Neurotransmitters (LO 4.15): Names the major classes of neurotransmitters.
• Neurotransmitters (LO 4.16): Identifies and describes the functions of each neurotransmitter class.
• Pharmacology of Synaptic Transmission and Behavior (LO 4.17): Provides a general overview of how drugs influence synaptic transmission.
• Pharmacology of Synaptic Transmission and Behavior (LO 4.18): Describes examples of how drugs are used to influence neurotransmission.
• The Lizard: A Case of Parkinson's Disease: Presents a case study of Parkinson's disease.

Resting Membrane Potential

• Recording: Membrane potential is measured by placing electrodes inside and outside the neuron.
• Ionic Basis: Resting potential is predominantly maintained by the unequal distribution of Na+ and K+ ions. More Na+ is outside, and more K+ is inside the neuron.
• Factors: Electrostatic pressure, concentration gradients, and the sodium-potassium pump maintain the distribution.
• Sodium ion channels are closed when resting, reducing flow.
• Potassium ion channels are open, but few exit due to electrostatic pressure

Generation, Conduction, and Integration of Postsynaptic Potentials

• Postsynaptic Potentials (PSPs): They are graded potentials, meaning their magnitude depends on the strength of the stimulus. They are either excitatory (EPSPs) or inhibitory (IPSPs).
• Summation: EPSPs and IPSPs summate (add up) at the axon hillock; if the sum reaches threshold, an action potential is generated.
• Conduction: PSPs are conducted passively, decreasing in amplitude as they travel. Action potentials are conducted actively and do not decrease in amplitude.
• Spatial summation involves simultaneous PSPs and 3 combinations
• Temporal summation involves successive PSPs which outlast the duration of the stimuli

Conduction of Action Potentials

• Ionic Basis: Action potentials result from the rapid influx of Na+ ions followed by efflux of K+ ions.
• Phases: Action potential has three phases: rising phase, repolarization phase, and hyperpolarization phase.
• Refractory Periods: The absolute refractory period prevents backward action potential propagation, while the relative refractory period limits the firing rate.
• Sodium channels in the axon membrane open wide and Na+ ions rush in
• This reverses the membrane potential from -70 to +50 mV
• This internal charge triggers the opening of voltage-gated potassium channels
• K+ ions are then driven out of the cell through these channels
• The sodium channels close after 1 millisecond
• Repolarization phase occurs due to the continued efflux of K+ ions
• Potassium channels gradually close which starts the hyperpolarization phase
• A neuron experiences a refractory period that lasts 1-2 milliseconds after initiation
• During the absolute refractory period, it is impossible to elicit another AP
• After the absolute refractory period occurs, the relative refractory period begins
• When the relative refractory period occurs, the neuron can fire again but requires higher-than-normal stimulation

Synaptic Transmission

• Structure of Synapses: Synapses are junctions between neurons where neurotransmitters are released. Common types: axodendritic, axosomatic, axoaxonic and more. All three cells communicate with one another by synaptic transmittion.
• Dendrodendritic synapses: which are often capable of transmission
• Axoaxonic synapses: which are important because they mediate presynaptic facilitation and inhibition. The advantage of presynaptic facilitation and inhibition is that they selectively influence single synapses, rather than the entire neuron.
• Axomyelenic synapses: where an axon synapses on the myelin sheath of an oligodendrocyte
• Directed synapses: synapses at which the site of neurotransmitter release and the site of neurotransmitter reception are in close proximity.
• Nondirected synapses: synapses at which the site of release is at some distance from the site of reception. In this type of arrangement, neurotransmitter molecules are released from a series of varicosities (bulges or swellings) along the axon and its branches and thus are widely dispersed to surrounding targets. Because of their appearance, these synapses are often referred to as string-of-beads synapses.
• Neurotransmitter Synthesis/Packaging: Neurotransmitter molecules are synthesized and packaged into vesicles by the neuron's Golgi apparatus. Many neurons can contain two neurotransmitters which is a situation called "coexistence".
• Neurotransmitter Exocytosis: Action potentials trigger the release of neurotransmitters from synaptic vesicles into the synaptic cleft by the process of exocytosis. When at rest, they congregate near the voltage-gated calcium channels.
• Receptors: Neurotransmitters bind to receptors on the postsynaptic membrane, either ionotropic (directly on ion channels) or metabotropic (second messenger system). Any molecule that binds to another is referred to as its ligand, and a neurotransmitter is thus said to be a ligand of its receptor. Most neurotransmitters bind to several different types of receptors. The different types of receptors to which a particular neurotransmitter can bind are called the receptor subtypes for that neurotransmitter.
• Autoreceptors: Metabotropic receptors, located on the presynaptic membrane. Their usual function is to monitor the number of neurotransmitter molecules in the synapse, to reduce subsequent release when the levels are high, and to increase subsequent release when they are low.
• Removal of Neurotransmitters from Synapse: Neurotransmitters are deactivated by reuptake transports, enzymatic degradation enzymes or diffusion away from the synaptic cleft.
• Glia and Gap Junctions: Glial cells, astrocytes, and gap junctions connexins communicate with brain function. Gap junctions are involved in communication and support of neurotransmission beyond the neuron-neuron synapses. The principles according to which astrocytes and gap junctions are distributed in the mammalian brain provide some of the best clues about their function. First, let's con-sider cerebral gap junctions. Cerebral gap junctions occur between all classes of cerebral cells; however, the majority of them seem to occur between cells of the same kind. For example, many gap junctions link astrocytes together.

Neurotransmitters

• Amino Acids: Common neurotransmitters including glutamate, aspartate, glycine, and GABA.
• Monoamines: Dopamine, epinephrine (adrenaline), norepinephrine (noradrenaline) and serotonin, all which contain the catecholamines and the indolamines classification structures. Serotonin is also called 5-hydroxytryptamine, or 5-HT and is synthesized from the amino acid tryptophan.
• Acetylcholine: A neurotransmitter at neuromuscular junctions which is abbreviated Ach. Neurons that release acetylcholine are said to be cholinergic.
• Unconventional Neurotransmitters: Include soluble gases (like nitric oxide and carbon monoxide which participate in retrograde transmission) and endocannabinoids (like anandamide which come from the dendrites and the cell body).
• Neuropeptides: Numerous different peptides categorized amongst the pituitary, hypothalamic, brain-gut, opioid and miscellaneous peptides serving various roles.

Pharmacology of Synaptic Transmission and Behavior

• Drug Effects: Drugs can affect synaptic transmission by acting as agonists antagonists on certain receptors, increasing neurotransmitter effects or decreasing neurotransmitter effects while acting as receptor blockers.
• Examples of Drug Influence: Includes examples of treatments impacting Parkinson's schizophrenia curare, Botox, endogenous opioids, and antipsychotic drugs.
• Nicotinic and muscarinic receptors: are distributed differently in the nervous system, have different modes of action, and consequently have different behavioral effects. Both nicotinic and muscarinic receptors are found in the CNS and the PNS.
• Enkephalins: Enkephalins (meaning “in the head.
• Endorphins: “Endorphins (a contraction endogenous morphine

I hope these notes are helpful in your studies!

Dette quiz handler om de vigtigste koncepter i kapitel 4 om hvilemembranpotentiale, postsynaptiske potentialer og aktionspotentialer. Det dækker måling af membranpotentiale, ionisk basis, synaptisk transmission samt hvordan aktionspotentialer genereres og ledes. Test din viden om neurale mekanismer og deres funktioner.