Neuron Biology and Function PDF

NEURO101 Biology and function of neurons 25/9/2024 Prof. Kleopas A. Kleopa [email protected] Department of Neuroscience Neurons Basic structure and organization of neuronal cells in the nervous system Cyto...

NEURO101 Biology and function of neurons 25/9/2024 Prof. Kleopas A. Kleopa [email protected] Department of Neuroscience Neurons Basic structure and organization of neuronal cells in the nervous system Cytology of neurons Synthesis and transport of proteins Axonal transport Ion channels Membrane potential Electrical parameters of neurons Action potential and its propagation Neurotransmitters and receptors  Please download and install the Slido app on all computers you use How do neurons differ from glia cells? ⓘ Start presenting to display the poll results on this slide. Two classes of cells in the nervous system: Glial Cells (glia) and nerve cells (neurons) Glial cells are support and regulatory cells (→ next lecture) Nerve cells are the main signaling units of the nervous system  Νeuronal cell basic structure Dendrites: input elements of the neuron (receive synaptic contacts Inhibitory from other neurons) axon terminal apical basal Excitatory Perikaryon/cell body: contains axon terminal the DNA for encoding neuronal proteins and the complex Αxon initial apparatus for synthesizing them segment: Initiation of axon potential myelin Nerve terminal→ Synapse (to one or more postsynaptic cells) Αxon: transmitting element, variable length (up to 1m), often myelinated Same basic plan- different neuron types depending on: Location in the nervous system Location of synaptic inputs and target cells Body size and shape Distribution of dendritic tree, number of axon branches Degree of myelination Molecular machinery (neurotransmitter type, enzymes, pumps, receptors) Different functions of neurons reflected in variations in (up to 10,000) different neuron types About 200 billions neuronal cells leave in human brain Each neuron is connected on average with 5,000-200,000 other cells Neuronal cells receive and process incoming stimuli that gain access to the nervous system and react to them by sending all signals that come out of the nervous system Unique neuronal types have names… Identified according to their location in the nervous system and distinct shape: Basket cells (cerebellar interneurons) Betz cells (large cortical motor neurons) Purkinje cells (huge cerebellar neurons) Renshaw cells (cells with both ends linked to a-motorneurons), …etc. Structural classification of neurons Polarity Unipolar or pseudo-unipolar: dendrite and axon emerging from same process Bipolar: axon and single dendrite on opposite ends of the soma Multipolar: more than two dendrites: Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells Golgi II: neurons whose axonal process projects locally; the best example is the granule cell Functional Afferent: convey information from tissues and organs into the CNS (sensory) Efferent: transmit signals from the CNS to the effector cells (motor) Interneurons: connect neurons within specific regions of the CNS  Functional classes of neurons and the principle of reflex arc 1. Afferent neurons 2. Interneurons -receive signals and -lie in CNS generate response -cell body devoid of -99% of neurons dendrites (located near Roles: spinal cord) -mediate signals -long peripheral axon between goes from receptor to afferent/efferent; - cell body integrate response -short central axon to incoming stimuli extends from cell body to spinal cord -responsible for associative -primarily in the PNS functions 3. Efferent neurons - cell bodies originate in CNS but axons may extend from cell body to PNS and effector organ  Muscle stretch reflex (monosynaptic tendon reflex) Stimulation of muscle spindle sensory fibers → afferent signal to the spinal cord: → activates motor neurons to the stretched muscle to cause contraction (monosynaptic reflex) → at the same time causes inhibition of the motor neurons to the antagonist muscles through inhibitory interneurons (polysynaptic action)  Flexion withdrawal and crossed extension Pain stimulus from the foot reaches the spinal cord to cause: → Flexion of the ipsilateral leg (activation and inhibition of flexor/extension motor neurons, respectively) → Extension of the contralateral leg (crossed extension reflex) with opposite effects on respective motor neurons Cytoarchitectural organization of the cerebral neocortex: Neurons are arranged in distinctive cortical layers Histochemical methods: The Golgi stain reveals neuronal cell bodies and dendritic trees The Nissl method shows cell bodies and proximal dendrites A Weigert stain for myelinated fibers reveals the pattern of axonal distribution Cortical layers of neuronal Layer I: acellular or molecular layer → occupied by organization dendrites of the cells located deeper in the cortex and axons that travel through or form connections Layer II: mainly small spherical cells (granule cells) → external granule cell layer Layer III: contains a variety of cell types, many pyramidally shaped; the neurons located deeper in layer III are typically larger than those located more superficially→ external pyramidal cell layer Layer IV: like layer II, is made up primarily of granule cells, → internal granule cell layer Layer V: the internal pyramidal cell layer, contains mainly pyramidally shaped cells that are typically larger than those in layer III Layer VI: heterogeneous layer of neurons, also called the polymorphic or multiform layer → blends into the white matter that forms the deep limit of the cortex and carries axons to and from the cortex Differences in laminar organization of cortical regions depending on functional specialization (sensory, motor, visual, association cortex)  Organization of the brain according to the cytoarchitecture Brodmann Cytoarchitectonics Neurons in different layers of the neocortex project to different parts of the brain Two major neuronal cell types in the cortex: Projection Neurons: pyramidally shaped cell bodies in layers III, V, and VI, using glutamatergic transmission Interneurons: located in all layers and use GABAergic transmission Cortical columns Neurons in the neocortex are not only distributed in layers but also in columns that traverse the layers A cortical column would fit within a cylinder a fraction of a millimeter in diameter (40–50 µm) Neurons within a particular column tend to have very similar response properties, presumably because they form a local processing network Columns are thought to be the fundamental computational modules of the neocortex For example: each muscle and joint in motor cortex is represented by columnar arrays of neurons Neurons form functional pathways in the CNS Motor systems Corticospinal (pyramidal) tract Extrapyramidal (basal ganglia) systems Cerebellum Spinal cord motor pathways Sensory systems Spinothalamic system Posterior column system Visual system etc. The motor systems are organized hierarchicaly! Executive motor systems: The spinal cord, brainstem, and forebrain contain successively more complex motor circuits Spinal cord: lowest level of this hierarchical organization: contains neuronal circuits that mediate reflexes and rhythmic automatisms (locomotion, scratching) Brain stem: next level with two systems of neurons, the medial and lateral, receiving input from the cerebral cortex and subcortical nuclei and projecting to the spinal cord Cortex: highest level of motor control: primary motor cortex and several premotor areas project directly to the spinal cord through the corticospinal tract and also regulate motor tracts that originate in the brain stem Regulatory motor systems: The cerebellum and basal ganglia influence cortical and brainstem motor systems Provide feedback circuits that regulate cortical and brain stem motor areas Receive inputs from various areas of cortex and project back to motor areas via the thalamus Three distinct categories of movement: Reflexive, Rhythmic, Voluntary Reflexive and Rhythmic Movements: produced by stereotyped patterns of muscle contraction Reflexes are involuntary coordinated patterns of muscle contraction and relaxation elicited by peripheral stimuli (subconsciously, not at the cortical level) Voluntary Movements: initiated at the cortical level, goal- directed and improve with practice as a result of feedback and feed-forward mechanisms In feedback control signals from sensors are compared with a desired state, represented by a reference signal. The difference, or error signal, is used to adjust the output Unlike feedback systems, feed-forward control acts in advance of certain perturbations (but again relies on sensory input- and experience!) Rhythmic diaphragmatic movements recorded by ultrasound Motor programs The extent of a movement is planned before the movement is initiated: The representation of this plan for movement is called a motor program The motor program specifies the spatial features of the movement, the angles through which the joints will move (movement kinematics) The program must also specify the forces required to rotate the joints (torques) to produce the desired movemen (movement dynamics) The planning and execution of voluntary movement relies on sensorimotor integration: representations of the external environment are integrated into motor programs premotor and primary motor areas (frontal) operate in conjunction with sensory and association areas (parietal) feedback and feed-forward controls  Flow of information in the frontal lobe motor control system From Kandell & Schwarzt Information is processed in polymodal prefrontal and parietal areas → involved in motor planning and project to the premotor cortex The premotor cortex generates motor programs → execution through projections to the motor cortex Neurons in the motor cortex primarily fire to produce movements in particular directions around specific joints Different areas of cortex are activated during simple, complex, and imagined sequences of finger movements (f-MRI findings) Voluntary movements are highly adaptable They improve in speed and accuracy with repeated trials of practice Adaptability reflects an optimization process: minimal circuits needed to accomplish a behavior Optimization process: shift in the encoding of particular parameters of movement in smaller and fewer cortical groups of cells A novel behavior initially requires processing in multiple motor and parietal areas as it is continuously monitored for errors and subsequently modified As the behavior becomes more accurate, the need for sampling of the sensory inflow and updating of the motor program decreases → reduced need for the computational power of large networks eg. pre-supplementary motor area is active during the learning of a behavior but becomes less active as learning progresses After long periods of practice, the behavior becomes automatic (no activity in the supplementary motor area) Organization of motor and sensory neurons in the cortex: cortical representation of the body: the “homunculus” Corticospinal tract Collection of axons that travel between the cerebral (motor) cortex and the spinal cord Mostly contains motor axons Two separate tracts in the spinal cord: Lateral corticospinal tract and Anterior corticospinal tract Corticobulbar tract: signals to cranial nerve nuclei When the pyramidal tract passes the medulla, it forms a dense bundle of crossing nerve fibres that is shaped somewhat like a pyramid Concerned specifically with discrete voluntary skilled and fine movements, especially of the distal parts of the limbs Motor Cortex Origin and course of the corticospinal tract → Originates from pyramidal cells (upper motor neurons) in layer V of the cerebral cortex from the primary motor cortex Midbrain (homunculus) contributions from the supplementary motor area, premotor and somatosensory Pons cortex, parietal lobe, and cingulate gyrus Most fibers (80%) cross over to the Medulla contralateral side in the medulla (pyramidal decussation) Crossing of 10% uncrossed -lateral corticospinal tract pyramids 10% cross at the level of exit from spinal cord Spinal cord → Cortico-spinal axons synapse (most of them via interneurons) with lower motor neurons Motor neurons are responsible for movement Upper motor neurons (UMN) form glutamatergic synapses to activate the lower motor neurons (LMN) in the ventral horn Lower motor neuron axons leave the brain stem via motor cranial nerves and the spinal cord via anterior spinal roots LMN axons synapse with muscle fibers at the neuromuscular junction (NMJ) Lower motor neuron lesions: → decreased muscle tone, strength and reflexes in affected areas, fasciculations, muscle atrophy Upper motor neuron lesions: → Increased muscle tone (spasticity), weakness, decreased dexterity, increased reflexes, abnormal reflexes Lower motor neurons and spinal cord reflexes Lower motor neurons are classified based on the type of muscle fiber they innervate: Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most numerous type of muscle fiber and the one involved in muscle contraction Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which together with sensory afferents compose muscle spindles. These are part of the system for sensing body position (proprioception) Extrapyramidal motor pathways: regulating movement Motor pathways that lie outside the corticospinal tract and are beyond voluntary control Their main function is to support voluntary movement and help control posture and muscle tone Subcortical neuronal groups (nuclei) in basal ganglia, pons and medulla Target indirectly neurons in the spinal cord involved in reflexes, locomotion, complex movements, and postural control Basal Ganglia function in motor control Basal ganglia exert an inhibitory influence on a number of motor systems and release of this inhibition permits a motor system to become active Influenced by signals from many parts of the brain: prefrontal cortex (key role in executive functions) Insight into the functions of the basal ganglia has come from the study of two neurological disorders with symptoms relating to the ability to initiate and control movement (movement disorders): ❑ Parkinson's disease: loss of dopaminergic cells in the substantia nigra → gradual loss of the ability to initiate movement ❑ Huntington's disease: loss of medium spiny neurons in the striatum → inability to prevent parts of the body from moving unintentionally Role of cerebellum in motor control Cerebellum acts as a monitor and modulator of motor activity originating in other brain areas Automatic (subconcious) excitation of antagonist muscles at the end of the movement with simultaneous inhibition of the agonist muscles that initiated the movement Coordination of voluntary motor movement, balance, equilibrium and muscle tone Failure of this cerebellar fine tuning results in «rebound» and dysmetria/ intention tremor Cerebellar cortex Three layers: Granular cell layer (with smaller numbers of interneurons-mainly Golgi cells) Purkinje cell layer: only the cell bodies of Purkinje cells Molecular layer, contains the flattened dendritic trees of Purkinje cells, along with the huge array of parallel fibers penetrating the Purkinje cell dendritic trees at right angles Two types of inhibitory interneurons: stellate cells basket cells → form GABAergic synapses onto Purkinje cell dendrites  Major circuits of the cerebellar cortex Mossy fibers (origin mainly from cerebral cortex via the pontocerebellar pathway, but also spinal cord and vestibular system) project directly to the deep nuclei, but also give rise to the pathway: IN→ Mossy fiber → granule cells → parallel fibers → Purkinje cells → deep nuclei → OUT Relay sensory information from the pons to the granule cells → sent along the parallel fibers to the Purkinje cells for processing Extensive branching: → input from a single mossy fiber axon will influence processing in a very large number of Purkinje cells ! Climbing fibers Originate from the inferior olivary nucleus located in the medulla oblongata They project to Purkinje cells and also send collaterals directly to the deep nuclei Each climbing fiber will form synapses with 1-10 Purkinje cells Provide very powerful, excitatory input to the cerebellum → activation of Purkinje cells Climbing fiber activation is thought to serve as a motor error signal sent to the cerebellum, and is an important signal for motor timing The cerebellum also receives dopaminergic, serotoninergic, noradrenergic, and cholinergic inputs that presumably perform global modulation Purkinje cell Santiago Ramón y Cajal Cerebellar microzones Cerebellar cortical zones are partitioned into smaller units called microzones about 1000 Purkinje cells (PCs) with same somatotopic receptive field that project to the same cluster of output cells in the deep cerebellar nuclei Arranged in a long, narrow strip, oriented perpendicular to the cortical folds PC dendrites are flattened in the same direction as the microzones extend, while parallel fibers cross them at right angles Branches of a climbing fiber from the inferior olivary nucleus (about 10) innervate PCs belonging to the same microzone Olivary neurons that send climbing fibers to the same microzone are coupled by gap junctions → synchronized activity Principles of cerebellar function Feed-forward processing → Signals move unidirectionally through the system from input to output with very little recurrent internal transmission → Cannot generate self-sustaining patterns of neural activity → Cerebellum does not provide output, only response to the input after processing Divergence and convergence → Modest number of inputs, extensive processing, very limited number of output cells: → 200 million mossy fibers → 40 billion granule cells → parallel fiber outputs to 15 million Purkinje cells →1000 or so Purkinje cells belonging to a microzone receive input from up to 100 million parallel fibers, and send output to less than 50 deep nuclear cells Modularity: The cerebellar system is functionally divided into functionally independent modules Plasticity: Tremendous flexibility for fine-tuning the relationship between cerebellar inputs and outputs through modification of synapses Neuronal cell ultrastructure Synthesis and trafficking of neuronal proteins Nissle substance Granular endoplasmic reticulum Neurons are extensively compartmentalised → high demands on the timing and location of protein synthesis ER/Golgi Complex: Protein synthesis, posttranslational modification, export Growth cones: guide developing axons and dendrites to their targets responding to extracellular guidance cues Forming new memories and learning requires protein synthesis (synaptic plasticity, local protein synthesis at the synapse) ❑ Alterations of neuronal protein synthesis→ behavioral, cognitive and memory deficits: addiction, fragile X syndrome, autism, neurodegenerative (AD, ALS), neurogenetic disorders (Spastic Paraplegias, neuropathies).  Please download and install the Slido app on all computers you use What are the most important organelles and molecules participating in the axonal transport? ⓘ Start presenting to display the poll results on this slide.  Many mitochondrial diseases affect the Nervous system! Mitochondria “Powerhouses" of the cell: very important for neurons! Aerobic metabolism to synthesize adenosine triphosphate (ATP) ATP stores energy, releasing it to fuel cellular processes as needed Mitochondria undergo axonal transport in neurons and their transport is regulated by axonal cytoskeleton, myelination and neuronal/axonal activity EM: mitochondria in peripheral axons Porin SMI31 Image Pro Mitochondria associate directly with NF sidearms; this interaction is mediated by NF phosphorylation as well as mitochondrial membrane potential → Cultured neurons from Nefl-null (NF-L deficient) mice show abnormal mitochondrial distribution and motility, suggesting that the NF network regulates the trafficking of these organelles NF Microtubules, neurofilaments, and microfilaments compose the cytoskeletal elements of a neuron Synthesized in the cell body of a neuron, but delivered throughout the length of the neuron's axon (which composes approximately 99% of the neuron's structure) where they form large molecular assemblies or matrices The cytoskeleton determines the shape of the neuron and facilitates axonal transport! 3-D myelinated axon with cytoskeleton EM cross section of axon Neurofilaments 10nm intermediate filaments –specific for neurons High concentrations along the axons, where they appear to regulate axonal growth and diameter Subunits named according to their molecular weight: NF-L: 68-70kDa NF-M: 145-160kDa NF-H: 200-220kDa NF subunit proteins coassemble in vivo forming a heteropolymers NF-H and NF-M C-terminal tail domains control the spacing between neighboring filaments through phosphorylation Protein kinases (ERK1/2, Cdk5) and phosphatases (PPA2) regulate neurofilament phosphorylation Microtubules comprise the cytoskeleton of living cells- and neurons formed by thirteen filamentous strands (a+b tubulin heterodimers) used to transport substances to different parts of the cell In the axons they serve as tracks for axonal transport ! Toxins that destabilize microtubules cause neuropathies! (chemotherapy- Taxol) Axonal transport Most macromolecules of the neuron are synthesized in the cell body: proteins but also mRNAs in transport vesicles leave the Golgi and travel down the axon via axonal transport, mRNAs are translated throughout the axon length Transport mechanisms deliver neuronal proteins to all parts of the neuron → crucial for function and survival In axon terminals synaptic vesicles are assembled and loaded with neurotransmitters - after exocytosis vesicle membranes are recycled back to the cell body Axonal transport molecular mechanisms Required for axonal growth, function and viability Accomplished by two sets of motor proteins: Kinesins→ orthograde transport and Dynein-dynactin complex→ retrograde transport) Vesicles sorted and loaded onto transport motors to reach the axons or the dendrites (anterograde) From the nerve terminal there is axosomatic (retrograde) movement of substances such as trophic proteins Hirokawa 1998 → Axonal transport defects in most neurodegenerative disorders and in inherited disorders (spastic paraplegias, axonal neuropathies) → Perturbation of neurofilaments (mutations, changes in phosphorylation and physical structure of the axon) affect axonal transport and cause neuropathy → Mutations in dynactin (humans), dynein (mice) and three different forms of kinesin all provoke motor neuron degeneration, as well axonal neuropathy (CMT)  Kinesins Kinesins are a family of proteins of tail stalk variable size that form dimers: head 2 heavy and 2 light chains Light Heavy chain Heavy chain forms globular head chain (the motor domain) connected via a short, flexible neck linker to the long alpha-helical stalk ending in a tail region formed with a light-chain The stalks interwine to form the kinesin dimer Cargo binds to the tail region Kinesins are motor proteins: transport cargo by walking unidirectionally along microtubule tracks hydrolysing one molecule of ATP at each step  Kinesins in axonal transport In the axon, microtubules are unipolar, with the plus ends pointing towards the synaptic terminal Microtubules form special bundles at the initial segment, which might serve as the cue for initiating axonal transport Tubulovesicular organelles are transported anterogradely along microtubules by kinesins (KIFs) KIF2A controls microtubule dynamics and the extension of axon collaterals Membranous organelles are transported along microtubules by KIFs RNA-containing granules are also transported by interacting directly with KIF5 Mitochondria are transported by KIF5 and KIF1B KIF1A and KIF1B both transport synaptic vesicle precursors KIF17 transports vesicles containing NMDA Diversity and cargo specificity of the kinesins Are new neurons generated in adult brain? It has long been known that mature, differentiated neurons do not divide However, recent evidence indicates that new neurons may be generated in the mammalian CNS, although restricted to just two regions of the brain: 1. The granule cell layer of the olfactory bulb 2. The dentate gyrus of the hippocampus New neurons are primarily local circuit neurons or interneurons, and not afferent or efferent ones with long distance projections The ability of newly generated neurons to integrate into at least some synaptic circuits adds to the mechanisms available for plasticity in the adult brain Basis for potential applications of stem cell technology for the repair of circuits damaged by traumatic injury or degenerative disease Principles of synaptic transmission and neurotransmitter systems Neurotransmitters are endogenous chemicals which transmit signals from a neuron to a target cell across a synapse Metabolism /life circle of neurotransmitters: Neurotransmitters classes: biosynthesis, storage, Amino acids aspartate, glutamate, its release, and decarboxylated form g -amino-butyric acid degradation or re-uptake (GABA), and glycine Biogenic amines serotonin, histamine, Example: catecholamines (epinephrine, nor- epinephrine, dopamine)- derived from serotonin aromatic amino acids tryptophan and tyrosine) Others: acetylcholine (ACh), adenosine, nitric oxide, etc Neuroactive peptides: many are "co- released" along with a small-molecule transmitter, but sometimes they are the primary transmitter Some neurotransmitters are commonly described as «excitatory» or «inhibitory» → The only direct effect of a neurotransmitter is to activate one or more types of receptors → Effect on the postsynaptic cell depends, therefore, entirely on the properties of those receptors → For some neurotransmitters (for example, glutamate), most receptors have excitatory effects (increase the probability that the target cell will fire an action potential) → For other neurotransmitters (such as GABA) most important receptors have inhibitory effects → Other neurotransmitters, such as acetylcholine, have both excitatory and inhibitory receptors → Other postsynaptic receptors activate complex metabolic pathways producing effects that cannot appropriately be called either excitatory or inhibitory Neurotransmitter receptor types Ionotropic receptors Metabotropic receptors Ionotropic receptors directly linked with ion-channels → When an ionotropic receptor is activated, it opens a channel that allows ions such as Na+, K+, or Cl- to flow Metabotropic receptors subtype of membrane receptors at the surface or in vesicles of eukaryotic cells metabotropic receptors do not form an ion channel pore indirectly linked with ion-channels through signal transduction mechanisms, often G proteins → when a metabotropic receptor is activated, a series of intracellular events are triggered that also result in ion channel opening, involving a range of second messengers Examples of important neurotransmitter actions Glutamate Majority of fast excitatory synapses in the brain and spinal cord Most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength Modifiable synapses- thought to be the main memory-storage elements in the brain GABA Majority of fast inhibitory synapses in virtually every part of the brain Many sedative/tranquilizing drugs act by enhancing the effects of GABA Correspondingly glycine is the inhibitory transmitter in the spinal cord Acetylcholine Distinguished as the transmitter at the neuromuscular junction Also operates in many regions of the brain, but using different types of receptors → Diseases may affect specific neurotransmitter systems! Neurotransmitter systems Similar neurons expressing certain types of neurotransmitters form distinct systems, where activation of the system affects large volumes of the brain, called volume transmission: Examples are noradrenaline (norepinephrine), dopamine, serotonin, and cholinergic system Dopamine Plays a critical role in the reward system Dysfunction of the dopamine system is implicated in Parkinson’s disease (nigrostriatal system- motor control) and schizophrenia (mesolimbic system) Also tuberoinfundibular pathway (→prolactin) Serotonin Regulates appetite, sleep, memory and learning, temperature, mood, behaviour, muscle contraction, cardiovascular and endocrine systems Role in depression (lower concentrations of serotonin metabolites) → SSRIs