Neuroscience Chapter on Neurotransmitters
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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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