Neuroscience Chapter on Neurotransmitters
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Questions and Answers

What is the primary function of neurotransmitters at excitatory synapses?

  • To bind to the presynaptic neurone
  • To block ligand-gated sodium channels
  • To depolarize the postsynaptic membrane (correct)
  • To inhibit action potential generation

    • What happens when acetylcholine binds to its receptors on the postsynaptic cell?

  • It decreases the concentration of Na+ ions inside the cell
  • It opens ligand-gated sodium channels (correct)
  • It prevents action potential from occurring
  • It completely hyperpolarizes the membrane

    • What is the role of the presynaptic neurone in the chemical synapse?

  • To absorb receptors from the postsynaptic neurone
  • To block synaptic cleft
  • To release neurotransmitters (correct)
  • To generate an action potential

    • Which neurotransmitter is an example of one that typically depolarizes the postsynaptic membrane?

    <p>Acetylcholine</p> Signup and view all the answers

    What occurs in the postsynaptic cell if depolarization reaches the threshold?

    <p>Generation of an action potential</p> Signup and view all the answers

    What is the hallmark characteristic of an action potential?

    <p>It is a brief, rapid change in membrane potential.</p> Signup and view all the answers

    What initiates the depolarization phase of an action potential?

    <p>Na+ influx through voltage-gated Na+ channels.</p> Signup and view all the answers

    What is the threshold potential at which an action potential is generated?

    <p>-50 mV</p> Signup and view all the answers

    What happens after the peak of the action potential?

    <p>K+ moves out of the cell to restore resting potential.</p> Signup and view all the answers

    During the depolarization phase of an action potential, which ion influx is primarily responsible?

    <p>Sodium (Na+)</p> Signup and view all the answers

    How does the membrane potential behave as an action potential propagates?

    <p>It does not decrease in strength as it travels.</p> Signup and view all the answers

    What causes the repolarization phase of the action potential?

    <p>Potassium efflux</p> Signup and view all the answers

    What is the resting membrane potential typically around?

    <p>-70 mV.</p> Signup and view all the answers

    What is the role of voltage-gated slow calcium channels in action potentials?

    <p>They allow Ca2+ to enter, prolonging depolarization.</p> Signup and view all the answers

    What defines the absolute refractory period in action potentials?

    <p>Inactivation of sodium channels</p> Signup and view all the answers

    What is the main role of voltage-gated potassium channels (VGPCs) during an action potential?

    <p>Repolarization of the membrane</p> Signup and view all the answers

    What occurs during the threshold potential of an action potential?

    <p>It is when sufficient depolarization triggers action potential.</p> Signup and view all the answers

    What is responsible for the repolarization of the membrane following an action potential?

    <p>K+ leaving the cell down its electrochemical gradient.</p> Signup and view all the answers

    Which of the following statements about action potentials is true?

    <p>They follow an all-or-nothing principle.</p> Signup and view all the answers

    What is the initial membrane potential before an action potential occurs?

    <p>-70 mV</p> Signup and view all the answers

    Which ion's movement causes the membrane potential to reach +30 mV during an action potential?

    <p>Sodium (Na+)</p> Signup and view all the answers

    What effect does hyperpolarization have on a neuron?

    <p>It makes the inside of the cell more negatively charged.</p> Signup and view all the answers

    What is primarily responsible for the faster propagation of action potentials in myelinated neurons?

    <p>Saltatory conduction between nodes of Ranvier.</p> Signup and view all the answers

    What is the role of the myelin sheath in neurons?

    <p>To act as an electrical insulator restricting current flow.</p> Signup and view all the answers

    Where are voltage-gated sodium channels located in myelinated neurons?

    <p>At the nodes of Ranvier.</p> Signup and view all the answers

    Which of the following best describes the resting membrane potential?

    <p>It is a stable state maintained by ion channels.</p> Signup and view all the answers

    What ion influx is primarily responsible for depolarization during an action potential?

    <p>Sodium ions.</p> Signup and view all the answers

    Which neurotransmitter is involved in hyperpolarization at an inhibitory synapse?

    <p>GABA.</p> Signup and view all the answers

    What structural feature allows for muscle contraction in muscle fibers?

    <p>Arrangement of actin and myosin filaments.</p> Signup and view all the answers

    What happens when ACh binds to the closed ACh receptor?

    <p>The channel opens and Na+ moves in.</p> Signup and view all the answers

    Where are nicotinic receptors primarily located?

    <p>At the neuromuscular junction of skeletal muscles and autonomic nervous system ganglia.</p> Signup and view all the answers

    What type of receptor is the muscle-type nicotinic receptor?

    <p>Ionotropic receptor.</p> Signup and view all the answers

    What is the primary mechanism by which ACh is cleared from the synaptic cleft after its release?

    <p>Broken down by an enzyme and recycled back into synaptic vesicles.</p> Signup and view all the answers

    What is the effect of nicotine on nicotinic receptors?

    <p>It activates the receptor.</p> Signup and view all the answers

    How does the binding of ACh affect the direction of the action potential (AP)?

    <p>It initiates an AP that propagates in one direction.</p> Signup and view all the answers

    What is a consequence of ACh remaining in the synaptic cleft for too long?

    <p>Sustained muscle stimulation and potential fatigue.</p> Signup and view all the answers

    Which ion movement is primarily responsible for muscle contraction following ACh receptor activation?

    <p>Na+ moves into the cell.</p> Signup and view all the answers

    What is the primary action of a neurotransmitter at an excitatory synapse?

    <p>Depolarizes the postsynaptic membrane</p> Signup and view all the answers

    Which of the following ions is primarily associated with the depolarization of the postsynaptic membrane when acetylcholine is released?

    <p>Na+</p> Signup and view all the answers

    What must occur for an action potential to be generated in the postsynaptic cell?

    <p>Depolarization must reach a certain threshold</p> Signup and view all the answers

    How does the neurotransmitter affect the membrane potential of the postsynaptic cell?

    <p>It changes the permeability of the postsynaptic membrane to ions</p> Signup and view all the answers

    In the context of the chemical synapse, what is the function of the synaptic cleft?

    <p>To allow the neurotransmitter to diffuse between neurones</p> Signup and view all the answers

    What is the main reason that action potentials can travel along the plasma membrane without diminishing?

    <p>The opening of sodium channels regenerates the action potential.</p> Signup and view all the answers

    How does the frequency of action potentials relate to the strength of a stimulus?

    <p>A stronger stimulus generates a higher frequency of action potentials.</p> Signup and view all the answers

    What is the effect of gamma aminobutyric acid (GABA) on the postsynaptic membrane?

    <p>It hyperpolarizes the postsynaptic membrane.</p> Signup and view all the answers

    During which phase of an action potential do sodium channels open due to positive feedback?

    <p>Depolarization</p> Signup and view all the answers

    What characterizes the all-or-none principle of action potentials?

    <p>There is no partial action potential; it either occurs fully or not at all.</p> Signup and view all the answers

    What ion movement primarily reverses the membrane potential during an action potential?

    <p>Na+ influx</p> Signup and view all the answers

    During which phase do voltage-gated potassium channels (VGPCs) primarily operate?

    <p>Repolarization phase</p> Signup and view all the answers

    What happens after the closing of voltage-gated sodium channels (VGSCs)?

    <p>Repolarization occurs</p> Signup and view all the answers

    What is the refractoriness of a neuron primarily due to?

    <p>Inactivation of sodium channels</p> Signup and view all the answers

    Which of the following best describes the role of threshold potential in action potentials?

    <p>It initiates the opening of voltage-gated sodium channels.</p> Signup and view all the answers

    What is the primary factor that restores resting membrane potential after depolarization?

    <p>K+ efflux</p> Signup and view all the answers

    What occurs if a neuron receives a stimulus during the relative refractory period?

    <p>A weaker action potential can occur.</p> Signup and view all the answers

    Which ion's movement is responsible for the rapid depolarization phase of an action potential?

    <p>Na+</p> Signup and view all the answers

    What effect would an acetylcholinesterase (AChE) inhibitor have on muscle contraction?

    <p>Muscle contraction would be prolonged.</p> Signup and view all the answers

    Which neurotransmitter is primarily responsible for muscle contractions in the somatic nervous system?

    <p>Acetylcholine</p> Signup and view all the answers

    Where do the neurons that innervate skeletal muscles originate?

    <p>Ventral horn of spinal cord</p> Signup and view all the answers

    What type of control do the motor neurons in the somatic nervous system exert on skeletal muscles?

    <p>Voluntary control</p> Signup and view all the answers

    What effect do inhibitory postsynaptic potentials (IPSPs) have on motor neurons in the somatic nervous system?

    <p>Inhibit the motor neuron activity</p> Signup and view all the answers

    Which of the following structures is involved in the higher control of muscle activity?

    <p>Spinal cord, motor cortex, basal nuclei, cerebellum, brainstem</p> Signup and view all the answers

    What kind of innervation do skeletal muscles receive from motor neurons in the somatic nervous system?

    <p>Single innervation</p> Signup and view all the answers

    What occurs when acetylcholine is released at the effector organ during neuromuscular transmission?

    <p>Muscle contraction occurs.</p> Signup and view all the answers

    What is the primary distinction between the autonomic nervous system and the sensory-somatic nervous system?

    <p>The autonomic nervous system governs involuntary functions.</p> Signup and view all the answers

    Which type of neuron is responsible for transmitting information from the central nervous system (CNS) to effectors like muscles?

    <p>Efferent neurons</p> Signup and view all the answers

    What best defines a neurotransmitter?

    <p>A biochemical substance that transmits signals across synapses.</p> Signup and view all the answers

    During neurotransmission, what happens after acetylcholine (ACh) is released into the synaptic cleft?

    <p>ACh is immediately degraded by acetylcholinesterase.</p> Signup and view all the answers

    What is the role of acetylcholinesterase in the neuromuscular junction?

    <p>It breaks down ACh to terminate the signal.</p> Signup and view all the answers

    Which of the following is characteristic of nicotinic receptors at the neuromuscular junction?

    <p>They are ionotropic receptors that allow ion flow when activated.</p> Signup and view all the answers

    Which term describes the neurones that carry signals towards the central nervous system?

    <p>Afferent neurons</p> Signup and view all the answers

    What is the primary function of dendrites in a neuron?

    <p>To receive incoming signals from other neurons.</p> Signup and view all the answers

    What characterizes the structure of bipolar neurons?

    <p>They have one axon and one dendrite extending from opposite ends.</p> Signup and view all the answers

    What role does the axon hillock play in a neuron?

    <p>It generates action potentials before they are propagated down the axon.</p> Signup and view all the answers

    What role does choline acetyltransferase play in the neurotransmitter acetylcholine synthesis?

    <p>It catalyzes the production of acetylcholine using acetyl coenzyme A and choline.</p> Signup and view all the answers

    What is the primary effect of the local current flow induced by the end-plate potential (EPP)?

    <p>It raises the membrane potential of adjacent areas to threshold, initiating an action potential.</p> Signup and view all the answers

    How does the binding of acetylcholine affect the ion channels located in the motor end plate?

    <p>It opens ligand-gated ion channels allowing sodium ions to diffuse in.</p> Signup and view all the answers

    What occurs in the presynaptic terminal just before acetylcholine is released?

    <p>An action potential triggers the release of vesicles filled with acetylcholine.</p> Signup and view all the answers

    What structural change occurs to the motor neuron as it approaches the muscle cell?

    <p>The axon branches and loses its myelin sheath.</p> Signup and view all the answers

    Which event marks the initiation of the action potential?

    <p>Opening of voltage-gated sodium channels</p> Signup and view all the answers

    What occurs to the membrane potential during the peak of an action potential?

    <p>It reaches approximately +30 mV</p> Signup and view all the answers

    How does the membrane potential return to its resting state after an action potential?

    <p>By increasing permeability to potassium ions</p> Signup and view all the answers

    What characteristic of action potentials allows them to propagate without losing strength?

    <p>Action potentials maintain a constant amplitude</p> Signup and view all the answers

    During the action potential, what is the immediate effect of voltage-gated sodium channel opening?

    <p>Rapid depolarization of the membrane</p> Signup and view all the answers

    What happens to the voltage-gated sodium channels during the repolarization phase of an action potential?

    <p>They close, preventing further sodium entry</p> Signup and view all the answers

    What is the result of potassium ions moving out of the cell during the action potential?

    <p>Reestablishment of resting membrane potential</p> Signup and view all the answers

    What does the term 'threshold potential' refer to in the context of action potentials?

    <p>The level of membrane potential necessary to trigger an action potential</p> Signup and view all the answers

    What effect does hyperpolarization have on the ability of a neuron to generate an action potential?

    <p>It reduces the likelihood of action potential generation.</p> Signup and view all the answers

    Which feature of myelinated neurons facilitates faster action potential propagation?

    <p>The ability to skip segments of the axon.</p> Signup and view all the answers

    What occurs at the nodes of Ranvier during the action potential propagation in myelinated neurons?

    <p>Voltage-gated sodium channels are densely packed.</p> Signup and view all the answers

    What is the primary role of the GABA receptor in inhibitory synapses?

    <p>To open chloride channels, leading to hyperpolarization.</p> Signup and view all the answers

    What characterizes the membrane potential during resting state in a neuron?

    <p>Substantial negative charge present within the cell.</p> Signup and view all the answers

    How does myelin affect current flow in neurons?

    <p>It restricts the flow of current along the axon.</p> Signup and view all the answers

    Which of the following ions primarily influences hyperpolarization in a postsynaptic neuron?

    <p>Chloride ions (Cl-)</p> Signup and view all the answers

    What structural adaptation of muscle fibers enables effective contraction?

    <p>Arrangement of actin and myosin filaments.</p> Signup and view all the answers

    What is the primary function of nicotinic ACh receptors at the neuromuscular junction?

    <p>To mediate rapid responses through ion channel activation</p> Signup and view all the answers

    Which of the following accurately describes the process by which ACh is cleared from the synaptic cleft?

    <p>Breakdown by an enzyme and recycling into synaptic vesicles</p> Signup and view all the answers

    In the context of the action potential, what does the opening of voltage-gated sodium channels (VGSCs) signify?

    <p>Rapid depolarization due to sodium influx</p> Signup and view all the answers

    What role do nicotinic receptors play in the central nervous system?

    <p>To mediate fast synaptic transmission and modulate neurotransmitter release</p> Signup and view all the answers

    What effect does prolonged presence of ACh in the synaptic cleft have on muscle function?

    <p>Continuous stimulation leading to muscle fatigue</p> Signup and view all the answers

    Which statement accurately describes ionotropic receptors like nicotinic receptors?

    <p>They have ion channels as intrinsic components of the receptor</p> Signup and view all the answers

    During excitation-contraction coupling, which ion's release is crucial for muscle contraction?

    <p>Ca2+ from the sarcoplasmic reticulum</p> Signup and view all the answers

    How does nicotine interact with nicotinic receptors?

    <p>It activates the receptor mimicking ACh action</p> Signup and view all the answers

    What is the primary function of the T-tubes in skeletal muscle?

    <p>They help in the conduction of action potentials into the muscle fiber.</p> Signup and view all the answers

    What is formed when a motor neuron terminates its connection with a skeletal muscle fiber?

    <p>Neuromuscular junction</p> Signup and view all the answers

    Which characteristic is true regarding the axons of motor neurons innervating skeletal muscle?

    <p>They have thick, myelinated axons for faster signal transmission.</p> Signup and view all the answers

    What effect do miniature end plate potentials (mEPP) have on skeletal muscle fibers?

    <p>They represent the release of single vesicles affecting muscle fibers.</p> Signup and view all the answers

    Which statement correctly describes the synaptic boutons at the neuromuscular junction?

    <p>They contain neurotransmitters released into the synaptic cleft.</p> Signup and view all the answers

    What happens at the postsynaptic junctional folds of a muscle fiber?

    <p>Neurotransmitter receptors bind, leading to depolarization.</p> Signup and view all the answers

    What distinguishes the neuromuscular junction from other types of synapses?

    <p>It is a chemical synapse between a motor neuron and muscle fiber.</p> Signup and view all the answers

    What role does the autonomic nervous system play in relation to skeletal muscle?

    <p>It primarily connects to cardiac muscle fibers.</p> Signup and view all the answers

    Study Notes

    Neurotransmitters

    • Neurons primarily release one major neurotransmitter (NT)
    • Acetylcholine (ACh), Dopamine, Glutamate, Noradrenaline, and GABA are examples of NTs

    The Chemical Synapse

    • The presynaptic neuron releases a chemical (neurotransmitter)
    • The neurotransmitter diffuses across the synaptic cleft
    • The neurotransmitter binds to specific receptor proteins on the postsynaptic cell's plasma membrane, altering its membrane potential

    Synapses: Excitatory or Inhibitory

    • Excitatory synapses depolarize the postsynaptic membrane
    • Acetylcholine is an example of an NT at an excitatory synapse
    • When ACh binds to its receptors, it opens ligand-gated sodium channels
    • Na+ ions influx reduces the membrane potential
    • If depolarization reaches a threshold, an action potential (AP) is generated in the postsynaptic cell

    Action Potentials

    • A brief and rapid change in membrane potential
    • Large change in potential (around 100mV)
    • During an AP, voltage-gated Na+ channels open in the plasma membrane
    • Na+ ions move into the cell, causing the membrane to depolarize
    • After Na+ channels open, voltage-gated K+ channels open, allowing K+ ions to leave the cell
    • K+ efflux brings the membrane potential back to resting (-70mV)
    • The threshold potential is around -50mV

    Refractory Period: One Way Traffic

    • Refractory period is when a further stimulus won't trigger another action potential.
    • Absolute refractory period: Na+ channels are inactivated
    • Relative refractory period: Repolarization due to K+ channels opening and K+ efflux
    • During the refractory period, the membrane potential is more negative than resting, making it harder to trigger another AP.

    Inhibitory Synapse

    • Inhibitory synapses hyperpolarize the postsynaptic membrane
    • GABA is an example of an NT at an inhibitory synapse
    • GABA binding to its receptor opens Cl- channels
    • Cl- influx hyperpolarizes the membrane

    Myelination of Neurons

    • Most axons are encased in a fatty sheath called the myelin sheath
    • Myelin acts as an electrical insulator, restricting current flow
    • Myelin is formed by Schwann cells (in the PNS) or oligodendrocytes (in the CNS)
    • Nodes of Ranvier are gaps in the myelin sheath
    • Voltage-gated Na+ channels are concentrated at the nodes of Ranvier
    • The influx of Na+ ions at one node creates enough depolarization to reach the threshold of the next node
    • This allows the action potential to jump from node to node, speeding up propagation.

    Muscle Fiber

    • The cytoplasm of a muscle fiber contains mostly actin and myosin filaments
    • Nuclei and organelles are pushed to the edge of the cell
    • The sarcoplasmic reticulum is a system of tubes surrounding myofibrils

    Nicotinic Receptors

    • The receptor for ACh at the neuromuscular junction (NMJ) is a nicotinic receptor
    • Nicotine also activates this receptor
    • Nicotinic receptors are ionotropic, meaning the ion channel is part of the receptor
    • Nicotinic ACh receptors mediate rapid responses
    • Found in the neuromuscular junction, autonomic nervous system, and the CNS
    • There are two main types: muscle-type nicotinic receptors (NM or N1) and ganglia-type nicotinic receptors

    ACh Removal

    • ACh needs to be cleared from the synaptic cleft quickly to prevent constant muscle stimulation
    • ACh is broken down by an enzyme (acetylcholinesterase) and recycled back into synaptic vesicles.

    Action Potentials

    • Brief, rapid, and large changes in membrane potential, where the potential reverses
    • Only a small portion of the excitable cell membrane is involved at any given time
    • Do not decrease in strength as they travel

    Action Potential Phases

    • Depolarization: The membrane potential becomes more positive due to the influx of sodium ions (Na+) through voltage-gated sodium channels (VGSCs)
    • Repolarization: The membrane potential returns to its resting state due to the efflux of potassium ions (K+) through voltage-gated potassium channels (VGPCs)

    Inhibitory Synapses (I)

    • GABA receptors are located on the postsynaptic membrane
    • Neurotransmitter binding to GABA receptors opens chloride channels (Cl-)
    • Cl- influx into the cell causes hyperpolarization, making the membrane potential more negative, which can inhibit the firing of action potentials

    Myelination of Neurons

    • The axons of most neurons are encased in a fatty sheath called the myelin sheath
    • Myelin functions as an electrical insulator, restricting current flow
    • The myelin sheath is formed by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system)
    • The voltage-gated sodium channels of myelinated neurons are confined to unmyelinated gaps called nodes of Ranvier
    • This arrangement allows the action potential to jump from one node to the next, leading to faster propagation

    Categorization of Muscle Types

    • Muscle fibers are made up of mostly actin and myosin filaments
    • Nuclei and organelles are located on the periphery of the muscle fiber
    • The sarcoplasmic reticulum, a network of tubes, is arranged around groups of myofibrils, connecting to larger tubes called T-tubules

    Peripheral Nervous System & Nerve-Muscle Junctions

    • The peripheral nervous system (PNS) carries signals from the central nervous system (CNS) to effectors, which are muscles and glands that carry out actions
    • Somatic motor neurons innervate skeletal muscle, forming a neuromuscular junction (NMJ)
    • Autonomic nerve fibers innervate cardiac and smooth muscle

    Motor Neurons

    • Cell bodies of motor neurons reside in the ventral horn
    • Motor neurons have thick, myelinated axons (somatic efferent fibers)
    • The myelin sheath of the motor neuron is lost at the motor end plate
    • At the motor end plate, the motor neuron forms many fine branches with varicosities called synaptic boutons
    • Each synaptic bouton lies over a postsynaptic junctional fold
    • Each axon terminal forms a single NMJ with a single muscle fiber

    The Neuromuscular Junction (I)

    • Motor neurons and skeletal muscle fibers communicate chemically at the NMJ
    • Action potentials in motor neurons cause the contraction of skeletal muscle fibers
    • Contractions are always excitatory & create miniature end plate potentials (mEPP)
    • mEPPs result from the release of a single vesicle containing acetylcholine (ACh)
    • ACh is released from the terminal of a motor neuron

    The Neuromuscular Junction (II)

    • Each axon terminal forms a NMJ with a single muscle fiber
    • ACh is the neurotransmitter that carries signals from the nerve to the muscle
    • ACh binds to receptors on the motor end plate
    • The binding of ACh triggers the opening of channels, allowing ions to flow, which depolarizes the motor end plate
    • This depolarization creates an end-plate potential (EPP)
    • The local current flow from the EPP spreads to adjacent muscle cell membrane, bringing it to threshold
    • This initiates an action potential which propagates throughout the muscle fiber

    Release of Acetylcholine from Synaptic Vesicles

    • The terminals of motor axons contain vesicles filled with ACh
    • When an action potential reaches the axon terminal, these vesicles release their contents of ACh onto the postsynaptic membrane of the muscle fiber
    • ACh binds to receptors on the motor end plate, opening ligand-gated ion channels and allowing Na+ influx

    Nicotinic Receptors

    • The receptor for ACh at the NMJ is the muscle-type nicotinic receptor (NM or N1)
    • Nicotine activates the NM or N1 nicotinic receptors
    • Nicotinic receptors are ionotropic, meaning the ion channel is part of the receptor
    • Nicotinic receptors mediate fast responses

    Ach Must be Cleared from the Synaptic Cleft

    • ACh must be cleared quickly from the synaptic cleft to prevent constant stimulation
    • ACh is cleared from the synapse via three mechanisms: breakdown by the enzyme acetylcholinesterase, uptake by the muscle, and diffusion

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    Test your understanding of neurotransmitters and their roles in synaptic transmission. This quiz covers the functions of various neurotransmitters, the mechanics of chemical synapses, and the concepts of excitatory and inhibitory synapses, as well as action potentials. Perfect for neuroscience students looking to evaluate their knowledge.

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