University of Cambridge Biology of Neurons Lecture 1 PDF

Neurobiology & Human/Animal Behaviour MST/VST Part 1B Biology of Neurons Lecture 1 Dr Sepiedeh Keshavarzi Learning Objectives At the end of this lecture, you are expected to understand: Structure of neurons and the function of their major compartments Principles of resting membrane poten...

Neurobiology & Human/Animal Behaviour MST/VST Part 1B Biology of Neurons Lecture 1 Dr Sepiedeh Keshavarzi Learning Objectives At the end of this lecture, you are expected to understand: Structure of neurons and the function of their major compartments Principles of resting membrane potential and action potential in neurons Diversity of neurotransmitters and mechanisms underlying neurotransmitter release Lecture 1 Outline Introduction Anatomy of a neuron Neuron diversity The Neuronal Membrane Distribution of ions Maintenance of the concentration gradient Ionic equilibrium potential Resting membrane potential Action Potential Clinical applications Action potential initiation zone Action potential diversity Synaptic Transmission Chemical vs electrical synapses Diversity of synaptic connections Chemical synaptic transmission Major neurotransmitters Synthesis and storage of neurotransmitters Synaptic vesicle molecular machinery Neurotransmitter release Synaptic vesicle cycle Clinical applications Neurotransmitter receptors Introduction – anatomy of a neuron Schubert et al, Cerebral Cortex, 2006 Introduction – neuron diversity More complex dendrites allow for processing of diverse inputs. Elaborate dendritic trees enable sophisticated local computations. Multipolar Pyramidal neuron of the cerebral cortex Purkinje neuron of the cerebellum Bipolar Pseudomonopolar Monopolar Retina Olfactory bulb dorsal root ganglion invertebrate Vestibular ganglion (PNS) Auditory ganglion Granular neuron of the hippocampus Introduction – neuron diversity Increase the the packing density of synapses Independent signalling compartments Highly dynamic structures à plasticity Multipolar Pyramidal neuron of the cerebral cortex Purkinje neuron of the cerebellum Bipolar Pseudomonopolar Monopolar Retina Olfactory bulb dorsal root ganglion invertebrate Vestibular ganglion (PNS) Auditory ganglion Granular neuron of the hippocampus Introduction – neuron diversity Both myelinated and unmyelinated axons exist in the CNS Myelination increases signal speed, crucial for complex behaviors. Myelinated axons use less energy for long-distance signaling. Myelinated Unmyelinated (i.e. Pyramidal neurons) (i.e. Inhibitory interneurons) Axon hillock Nodes of Ranvier Axon terminals The Neuronal Membrane – distribution of ions Asymmetric distribution of ions across the neuron membrane Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – distribution of ions Asymmetric distribution of ions across the neuron membrane What mechanisms maintain this asymmetry? What are the functional implications of this asymmetry? Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – maintenance of the concentration gradient Sodium and Potassium Active transport by Na+/K+ ATPase pump Powered by hydrolysis of ATP One ATP à three Na+ out and two K+ in Mutations in genes encoding Na+ -K+ pump α-subunits Familial hemiplegic migraine Rapid-onset dystonia parkinsonism Alternating hemiplegia of childhood Kandel et al, Principles of Neural Science, 2021 The Neuronal Membrane – maintenance of the concentration gradient Calcium Active transport by Ca2+ ATPase pump Active transport by Na+_Ca2+ exchanger Powered by hydrolysis of ATP Powered by Na+ moving down its concentration gradient 1 ATP à two Ca2+ out and two H+ in Three Na2+ in for one Ca2+ out Mutations associated with certain forms of epilepsy Kandel et al, Principles of Neural Science, 2021 The Neuronal Membrane – maintenance of the concentration gradient Chloride In most neurons cotransporters move Cl- out Active transport by K+_Cl- cotransporter (KCC2) Powered by K+ moving down its con. gradient One K+ and one Cl- out There is also a HCO3-_Cl- exchanger Kandel et al, Principles of Neural Science, 2021 The Neuronal Membrane – maintenance of the concentration gradient Chloride In most neurons cotransporters move Cl- out But not in early development Active transport by K+_Cl- cotransporter (KCC2) Active transport by Na+ -K+ -Cl− cotransporter (NKCC1) Powered by K+ moving down its con. gradient Powered by Na+ moving down its con. gradient One K+ and one Cl- out One K+, one Na+ and two Cl- in Opening of chloride channels is excitatory There is also a HCO3-_Cl- exchanger Kandel et al, Principles of Neural Science, 2021 The Neuronal Membrane – maintenance of the concentration gradient Chloride In most neurons cotransporters move Cl- out But not in early development Active transport by K+_Cl- cotransporter (KCC2) Active transport by Na+ -K+ -Cl− cotransporter (NKCC1) Powered by K+ moving down its con. gradient Powered by Na+ moving down its con. gradient One K+ and one Cl- out One K+, one Na+ and two Cl- in Opening of chloride channels is excitatory There is also a HCO3-_Cl- exchanger alterations in the KCC2/NKCC1 ratio in some forms of epilepsy Kandel et al, Principles of Neural Science, 2021 The Neuronal Membrane – maintenance of the concentration gradient Passive distribution of ions selective permeabilities of the plasma membrane at rest (high for K+ and СГ, low for Na+ and Ca2+) Many more K+ and Cl- leak channels No energy required MNacGraw-Hill Education) The Neuronal Membrane – ionic equilibrium potential Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – ionic equilibrium potential Equilibrium potential for a given ion is the membrane potential at which there is no net movement of that ion (work for moving the ion down the concentration gradient = work for moving the ion down the electrical gradient ) Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – ionic equilibrium potential Equilibrium potential for a given ion is the membrane potential at which there is no net movement of that ion (work for moving the ion down the concentration gradient = work for moving the ion down the electrical gradient ) Nernst Equation (at body temp RT/F = 61.5) E= membrane potential in mV R= Ideal Gas Constant 8.314 J/(K*mol) T= temp in K z= charge of ion F= Faraday's Constant 96,485 C/mol e- Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – ionic equilibrium potential Asymmetric distribution of ions across the neuron membrane Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – resting membrane potential Asymmetric distribution of ions across the neuron membrane Vm? ECa 123 ENa 62 0 ECl -65 EK -80 Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – resting membrane potential Asymmetric distribution of ions across the neuron membrane Exclusive PK permeability (PN = 0) ECa 123 ENa 62 0 ECl -65 EK -80 Vm Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – resting membrane potential Asymmetric distribution of ions across the neuron membrane Equal permeability (PK = PNa) ECa 123 ENa 62 0 V m ECl -65 EK -80 Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – resting membrane potential Asymmetric distribution of ions across the neuron membrane Unequal permeability (PK > PNa) ECa 123 ENa 62 0 ECl -65 Vm EK -80 Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – resting membrane potential Asymmetric distribution of ions across the neuron membrane Unequal permeability (PK > PNa) ECa 123 ENa 62 0 ECl -65 Vm EK -80 Goldman-Hodgkin-Katz Equation (derived from Nernst) Vm = membrane potential P = permeability for relevant ion Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane Asymmetric distribution of ions across the neuron membrane Unequal permeability (PK > PNa) ECa 123 ENa 62 0 ECl -65 EK -80 What mechanisms maintain this asymmetry? Active transport (i.e. pumps) and passive distribution (i.e. leak channels) Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane Asymmetric distribution of ions across the neuron membrane Unequal permeability (PK > PNa) ECa 123 ENa 62 0 ECl -65 EK -80 What mechanisms maintain this asymmetry? Active transport (i.e. pumps) and passive distribution (i.e. leak channels) What are the functional implications of this asymmetry? Establishing the resting membrane potential Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane Asymmetric distribution of ions across the neuron membrane Unequal permeability (PK > PNa) ECa 123 ENa 62 0 ECl -65 EK -80 What mechanisms maintain this asymmetry? Active transport (i.e. pumps) and passive distribution (i.e. leak channels) What are the functional implications of this asymmetry? Establishing the resting membrane potential Action potential generation and propagation (Na+ influx & K+ efflux) Synaptic transmission (i.e. Ca2+ in synaptic terminal) Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane– Action Potential (AP) A stimulus leads to membrane depolarization: If small (INa < IK) à back to rest (repolarization) if large (INa > IK) à reach threshold potential à AP Basic Physiology for Anaesthetists, 2019 The Neuronal Membrane– Action Potential (AP) A stimulus leads to membrane depolarization: If small (INa < IK) à back to rest (repolarization) if large (INa > IK) à reach threshold potential à AP Basic Physiology for Anaesthetists, 2019 Purves et al, Neuroscience, 2018 The Neuronal Membrane – clinical applications Mutations in voltage-gated Na+ and K+ channels in CNS à epilepsy, migraine headaches and ataxia The Neuronal Membrane – clinical applications Mutations in voltage-gated Na+ and K+ channels in CNS à epilepsy, migraine headaches and ataxia Diseases (Channelopathies) Dravet syndrome (mutation in SCN1A) alpha subunit of Sodium channel NaV1.1 The Neuronal Membrane – clinical applications Mutations in voltage-gated Na+ and K+ channels in CNS à epilepsy, migraine headaches and ataxia Drug target for a range of neurological conditions related to neuronal excitability Diseases (Channelopathies) Dravet syndrome (mutation in SCN1A) alpha subunit of Sodium channel NaV1.1 The Neuronal Membrane – clinical applications Mutations in voltage-gated Na+ and K+ channels in CNS à epilepsy, migraine headaches and ataxia Drug target for a range of neurological conditions related to neuronal excitability Diseases (Channelopathies) Pharmacological interventions Dravet syndrome (mutation in SCN1A) 40 voltage-gated potassium channels in human genome alpha subunit of Sodium channel NaV1.1 Wulff et al, Nature Review Drug Discovery, 2009 The Neuronal Membrane – AP initiation zone Sodium-dependent APs are mainly limited to axons In most CNS neurons: spike-initiation zone is at the axon hillock. Depolarization initiates in dendrites and soma. In most sensory receptors: spike-initiation zone is near the nerve endings where depolarization occur. Bear et al, Neuroscience: exploring the brain, 2016 The Neuronal Membrane – AP diversity All action potentials are not alike! AP shape varies significantly among different neurons in the mammalian brain Large variety of voltage-gated Na+ and K+ Channels Bean BP, Nature Reviews Neuroscience 2007 How is the AP signal passed on to another neuron? Synaptic Transmission – chemical vs electrical synapses Chemical Electrical Signed response (Excitation & inhibition) Fast Signal amplification Allow synchronization Signal computation Often bidirectional Excellent for plasticity Relatively little metabolic energy or molecular machinery Pereda AE, Nature Reviews Neuroscience, 2014 Synaptic Transmission – diversity of synaptic connections Axo-dendritic Axo-somatic Axo-axonic Dendro-dendritic Modulates the strength Lateral inhibition of synaptic transmission In olfactory bulb Biological Psychology. Sinauer Associates Synaptic Transmission – chemical synaptic transmission Synthesize and pack neurotransmitters in synaptic vesicle Make vesicles release their contents into the synaptic cleft in response to a stimulus from the presynaptic neuron Generate a response in the postsynaptic neuron Remove neurotransmitter from the synaptic cleft removal or reuptake Kandel et al, Principles of Neural Science, 2021 Synaptic Transmission – major neurotransmitters What is a neurotransmitter (NT)? ü Synthesized and stored in the presynaptic neuron ü Released by the axon terminal with stimulation ü When added experimentally, reproduces the response evoked by the stimulation of presynaptic neuron Synaptic Transmission – major neurotransmitters What is a neurotransmitter (NT)? ü Synthesized and stored in the presynaptic neuron ü Released by the axon terminal with stimulation ü When added experimentally, reproduces the response evoked by the stimulation of presynaptic neuron Monoamines Small-molecule neurotransmitters Glutamate, GABA and Glycine: Major excitatory and inhibitory neurotransmitters in CNS Monoamines: long range diffusing with slow signalling Neuropeptides: diffuse even longer distances, neuronal modulation, co-released with other neurotransmitters There are also gas neurotransmitters (NO) Synaptic Transmission – synthesis and storage of neurotransmitters Different ways of synthesizing neurotransmitters Glycine and Glutamate: present in all cells GABA and Amines: local syntheses in the axon terminal Neuropeptides: synthesized in rough ER and Golgi apparatus Peptides Amino acids Amines Bear et al, Neuroscience: exploring the brain, 2016 Synaptic Transmission – synaptic vesicle molecular machinery Complex molecular machinery on the vesicle (and in the presynaptic membrane) Mobilisation Docking and fusing You don’t need to know the details! Identifying Ca2+ Coating Budding Bear et al, Neuroscience: exploring the brain, 2016 Synaptic Transmission – neurotransmitter release (exocytosis) Arrival of an AP in the axon terminal à Voltage-gated Ca2+ channels open à Internal Ca2+ increase à NT release Binding and fusion of vesicles with the presynaptic membrane depends on the SNARE family of proteins: v-SNAREs (vesicle SNAREs) t-SNAREs (target membrane SNAREs) Synaptotagmin is the Ca2+ sensor on vesicles very fast (microseconds)! Bear et al, Neuroscience: exploring the brain, 2016 Synaptic Transmission – neurotransmitter release (exocytosis) Secretory granules also release peptide neurotransmitters by exocytosis, in a calcium-dependent fashion Farther from the Ca2+ entry site à don’t respond to every action potential High-frequency train of action potentials needed to build up Ca2+ to the required level relatively slow (milliseconds)! neuropeptide Romanov & Harkany, Science 2023 Synaptic Transmission – synaptic vesicle cycle The fused vesicle membrane is retrieved, taken back into the cytoplasm and used to make new vesicles (endocytosis) After vesicles are re-formed, they are stored in a reserve pool Purves et al, Neuroscience, 2018 Synaptic Transmission – clinical applications Purves et al, Neuroscience, 2018 Synaptic Transmission – clinical applications Purves et al, Neuroscience, 2018 Synaptic Transmission – clinical applications Botulinum toxin (Botulism) – Clostridium botulinum muscle weakness spinal motor neurons cleaves SNARE disrupts ACh release at NMJ Purves et al, Neuroscience, 2018 Synaptic Transmission – clinical applications Botulinum toxin (Botulism) – Clostridium botulinum muscle weakness spinal motor neurons cleaves SNARE disrupts ACh release at NMJ Tetanus toxins (Tetanus) – Clostridium Tetanus tetanic contractions spinal interneurons cleaves SNARE proteins disrupts Glycine release onto spinal motor neurons Purves et al, Neuroscience, 2018 Synaptic Transmission – clinical applications Botulinum toxin (Botulism) – Clostridium botulinum muscle weakness spinal motor neurons cleaves SNARE disrupts ACh release at NMJ Tetanus toxins (Tetanus) – Clostridium Tetanus tetanic contractions spinal interneurons cleaves SNARE proteins disrupts Glycine release onto spinal motor neurons α-latrotoxin (Latrodectism) – black widow spider severe muscle pain and cramping Ca2+-independent exocytosis Massive NT release (Ach, GABA, norepinephrine) Purves et al, Neuroscience, 2018 Synaptic Transmission – neurotransmitter receptors (to be continued) Transmitter-gated ion channels G-protein-coupled receptors (direct gating) (indirect gating) Kandel et al, Principles of Neural Science, 2021 Learning Objectives At the end of this lecture, you are expected to understand: Structure of neurons and the function of their major compartments. Principles of resting membrane potential and action potential in neurons. Diversity of neurotransmitters and mechanisms underlying neurotransmitter release