Cellular Neuro Notes: Glia, Neurons, Synapses, and Energy in Aging

Otto Loewi

  • was a pioneering neuroscientist known for his discovery of chemical transmission of nerve impulses, which laid the groundwork for understanding synaptic function and neurotransmitter release.

  • His experiments demonstrated how chemical signals, or neurotransmitters, facilitate communication between neurons, highlighting the critical role of synapses in the nervous system.

  • Loewi's work on the frog heart was instrumental in showing that nerve activity could affect heart rate through the release of the neurotransmitter acetylcholine, illustrating the intricate connections between neural circuits and physiological responses.

  • His findings not only advanced the study of synaptic biology but also contributed to the broader understanding of how glial cells support neuronal function and modulate synaptic activity, ultimately emphasizing the importance of these cellular components in the aging brain.

  • above is ai and if needed watch the vidio about otto loewi’s experiment

  • Vagus stuff: cause it was stuff coming out of the vagus nerve. now it is called the acetylcholine the main neurotransmitter of the parasympathetic nervous sytem.

  • accelerate stuff: stuff that accelerated the heart now called the noradrenaline or norerepinephrine which is the main neurotransmiter of the Sympathetic nervous system.

  • most people think of the nervous system as electrical when it is actually not just about that but also about communication between neurons through chemical. without this communication we won’t have a nervous system.

Glia: structure, roles, and dysfunctions

  • Glia support neurons

  • Glia meaning glue; not just structural support but highly active players in neural function

  • Two broad divisions: microglia and macroglia

    • Microglia: brain's immune system; part of innate immunity within CNS; respond to injury/disease; can become activated, proliferate, phagocytose, release inflammatory mediators; modulate inflammation (pro- and anti-inflammatory) as needed; respond to strokes

    • Macroglia: larger glial cells with multiple functions; key roles in myelination and support

  • Myelination: critical for fast, efficient electrical conduction; two systems differ by CNS vs PNS

    • Peripheral nervous system (PNS): Schwann cells myelinate axons; multiple Schwann cells wrap segments along a single axon; myelination increases conduction velocity; unmyelinated axons exist (slower); Schwann cells also guide axon regeneration after peripheral injury (axonal regrowth is slow but possible in PNS)

    • Central nervous system (CNS): oligodendrocytes myelinate multiple axonal segments; one oligodendrocyte can extend processes to several axons; no robust regeneration after CNS injury

  • Astrocytes

    • Structural and functional support; surround blood-brain barrier (BBB) and contribute end-foot processes to capillaries to regulate BBB selectivity

    • Transport and metabolism: actively transport glucose and other nutrients from blood to neurons; provide growth factors; help clear waste

    • Modulators of signaling: participate in ionic balance (e.g., ions near nodes of Ranvier); uptake/release neurotransmitters; contribute to efficient signaling; influence neuronal activity

  • Microglia (revisited)

    • Brain’s innate immune cells; monitor and respond to CNS disturbances

    • Capable of phagocytosis, releasing inflammatory mediators; contribute to neuroinflammation and repair processes

  • Glial dysfunction and disease

    • Multiple sclerosis (MS): autoimmune, inflammatory disease targeting CNS myelin; oligodendrocytes attacked by outside immune system due to BBB disruption; demyelination disrupts electrical signaling; symptoms depend on affected region; female-to-male prevalence ~2:1; prevalence increases with higher latitude (less sunlight) possibly linked to Vitamin D and immune regulation

    • MS features

    • Optic nerve involvement: blurred vision, double vision, nystagmus, flashes

    • Motor areas: weakness, speech difficulties, muscle atrophy, posture changes, tics

    • Sensory areas: numbness, tingling, pain, coordination/balance issues

    • Frontal areas: memory problems, processing speed

    • Brain tumors

    • Neuronal tumors are rare (~2% originate from neurons); most brain tumors arise from glial cells (gliomas) or meninges (meningiomas)

    • Gliomas originate from glial cells (astrocytomas, oligodendrogliomas, glioblastomas); often fast-growing and infiltrative in CNS

    • Meningiomas arise from meninges and are more surgically approachable

    • Treatment complexity due to BBB and infiltrative nature of gliomas

  • Key numbers to remember

    • Brain as ~2% of body weight but consumes >20% of energy

    • Glucose as major brain fuel (~95%); ketones and lactate contribute remaining fuels (~5%)

    • MS: higher female prevalence and higher latitude prevalence; age-related onset typically in adulthood

    • Brain mass comparison in Alzheimer's: healthy brain mass ~m<em>healthy ≈1200–1300 gm<em>{healthy} \,\approx 1200\text{–}1300\ \text{g}; advanced Alzheimer's brain mass ~m</em>AD ≈700 gm</em>{AD} \,\approx 700\ \text{g}

  • Summary: glia provide support, immune defense, and metabolic coordination; neurons are the primary signaling units whose function hinges on glial health and metabolic supply

Neurons: morphology, structure, and basic biology

  • Neurons are the primary signaling cells in the nervous system; four key regions

    • Dendrites: receive information from other neurons via chemical signals

    • Soma (cell body): integrates signals; contains nucleus, ribosomes, rough endoplasmic reticulum (Nissl bodies) for protein synthesis

    • Axon: long projection that transmits electrical signals (action potentials) to distant targets; length can vary from a few millimeters to tens of meters (e.g., blue whale)

    • Terminals (presynaptic boutons): release neurotransmitters to communicate with the next neuron, muscle, or gland

  • Cell body machinery and transport

    • Nucleus with DNA; ribosomes and rough ER synthesize proteins (neurotransmitters, receptors, ion channels)

    • Mitochondria: energy powerhouses; high energy demand of neurons; support ATP production for signaling

    • Golgi complex: packages neurotransmitters into vesicles; vesicle trafficking

    • Endoplasmic reticulum and Ribosomes: local protein synthesis for axon terminals

    • Microtubules: intracellular transport rails; kinesin (anterograde) and dynein (retrograde) motors move cargo along axons; essential for long-distance signaling

    • Synaptic vesicles: store neurotransmitters for release at the synapse

  • Neuron as a specialized secretory cell

    • Evolutionary path: neurons are highly specialized secretory cells capable of targeted long-distance secretion

    • Irritability: cells respond to stimulation; neurons turned irritability into a processor that integrates and computes signals

  • Neuronal structure and signaling: functional regions

    • Dendrites collect incoming signals; neurotransmitters bind to receptors and produce local electrical changes

    • Cell body integrates signals; signals spread and are summed

    • Axon hillock (start of axon): critical integration site where summed inputs determine if an action potential is triggered; threshold around Vth ≈−55 mVV_{th} \,\approx -55\ \text{mV}

    • Axon: propagates action potentials; myelin increases conduction speed; nodes of Ranvier facilitate saltatory conduction

    • Terminals: chemical signaling via neurotransmitter release

  • Neurons have specialized signaling machinery

    • Action potentials: all-or-none electrical impulses; regenerated at each node along the axon; do not decay with distance

    • Resting potential: typically around Vrest≈−70 mVV_{rest} \approx -70\ \text{mV}

    • Ion channels: voltage-gated Na+ and K+ channels underlie action potentials; gates open/close depending on membrane potential; absolute and relative refractory periods

    • Axon terminal: docking of vesicles, Ca2+ influx triggers exocytosis of neurotransmitters

  • Neurons: types of synapses and signaling

    • Dendrite-to-dendrite, axon-to-dendrite, axon-to-soma, axon-to-axon (axoaxonic) synapses

    • Presynaptic facilitation/inhibition (modulation of downstream signaling by upstream synapses)

    • Dendritic spines: small specialized postsynaptic structures that can host synapses; dynamic in learning and plasticity

  • Neuronal dysfunction and disease

    • Dementia and other neurodegenerative diseases involve neuronal loss and dysfunction

    • Dementia: a leading cause of death in aging populations; age is a major risk factor but not the sole cause; midlife risk factors influence late-life outcomes

    • Dementia subtypes: Alzheimer's disease (AD), Frontotemporal dementia (FTD), Vascular dementia, Dementia with Lewy bodies; pathology differs by affected regions and proteins

Neuronal signaling: electrochemical communication and synapses

  • Three phases of neuronal signaling

    • Collect and integrate information: dendritic inputs convert chemical signals to electrical changes; excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs)

    • Transmission along the axon: action potential propagates from the axon hillock down the axon; saltatory conduction with nodes of Ranvier; nondecremental propagation

    • Transmission to the target: presynaptic terminal releases neurotransmitter into the synaptic cleft; postsynaptic receptors respond

  • Postsynaptic potentials

    • EPSP: depolarization increases likelihood of firing; typically via Na+ influx through excitatory receptors

    • IPSP: hyperpolarization decreases likelihood of firing; typically via K+ efflux or Cl- influx through inhibitory receptors

  • Neurotransmitter signaling at the synapse

    • Neurotransmitter release: action potential triggers voltage-gated Ca2+ channels; Ca2+ triggers vesicle fusion with presynaptic membrane and neurotransmitter release

    • Neurotransmitter receptors on postsynaptic membrane: specificity to neurotransmitter

    • Receptors types: ionotropic (ligand-gated ion channels; fast signaling) and metabotropic (G-protein coupled; slower, longer-term signaling)

    • Glutamate and GABA: main fast excitatory and inhibitory neurotransmitters in CNS; acetylcholine: key in peripheral signaling and parasympathetic signaling; dopamine, norepinephrine, serotonin: monoamines with modulatory roles

  • Receptors and signaling specifics

    • Ionotropic receptors: direct ion flow (e.g., AMPA, NMDA receptors for glutamate; GABA-A for GABA)

    • Metabotropic receptors: GPCR pathways; second messengers alter cell function and gene expression

    • Muscarinic acetylcholine receptors (metabotropic) vs nicotinic receptors (ionotropic)

    • Postsynaptic receptor specificity: each receptor type binds its own neurotransmitter; multiple receptor subtypes exist for the same neurotransmitter

  • Neurotransmitter clearance

    • Reuptake into presynaptic terminals: recycles transmitter; e.g., dopamine transporters

    • Enzymatic degradation: acetylcholinesterase degrades acetylcholine in neuromuscular junctions

    • Postsynaptic uptake and degradation can also occur; rapid clearance is required for fast, precise signaling

  • Neuropharmacology: how drugs influence synaptic transmission

    • Agonists: enhance neurotransmitter activity (e.g., cocaine as a dopamine reuptake blocker; benzodiazepines as GABA agonists; acetylcholinesterase inhibitors like physostigmine to boost acetylcholine signal in myasthenia gravis)

    • Antagonists: inhibit neurotransmitter effects (e.g., atropine blocks muscarinic ACh receptors; curare blocks nicotinic ACh receptors at NMJ)

    • Reuptake inhibitors and degradation inhibitors extend transmitter action (e.g., cocaine blocks dopamine reuptake; botulinum toxin inhibits ACh release; nicotine acts on nicotinic receptors)

    • Important notes: There are both small-molecule neurotransmitters (glutamate, GABA, acetylcholine, dopamine, norepinephrine, serotonin) and large-molecule neuropeptides (endorphins, enkephalins, substance P) with modulatory roles

  • Neurotransmitter synthesis and packaging

    • Small transmitters synthesized locally in axon terminals due to rapid turnover requirements

    • Large neuropeptides synthesized in cell body and transported to terminals in vesicles via microtubules

    • Vesicle docking and SNARE proteins: vesicles tether to presynaptic membrane and release neurotransmitters via calcium-triggered fusion

  • Plasticity and learning

    • Strengthening or weakening synapses via receptor regulation, trafficking, and structural changes (e.g., receptor upregulation, spine growth)

    • Co-release of small and large transmitters: small transmitters used for rapid signaling; large transmitters used for longer-term modulatory effects during high activity

  • Electrical synapses (gap junctions)

    • Less common; direct electrical coupling via connexin channels; allow fast bidirectional signaling

Energy metabolism and the brain: the brain energy crisis in aging

  • Brain energy demand and fuel use

    • Brain consumes a large portion of energy relative to body mass; ~>20\% of total energy from a body weighing ~2%2\% of body mass

    • Primary fuel: glucose; ~95%95\% of brain energy comes from glucose oxidation in mitochondria; alternative fuels (ketone bodies, lactate) contribute the remaining ~5%5\%

    • ATP is the currency; most ATP is used to power ion pumps that maintain membrane potentials and reset signaling after action potentials

  • Neurovascular energy delivery and the neurovascular unit

    • Brain energy delivery is a highly structured, multi-cell process involving neurons, astrocytes, oligodendrocytes, microglia, capillaries, and blood flow regulation

    • Astrocytes mediate glucose uptake from capillaries and shuttle to neurons; signal to blood vessels to increase blood flow when neurons are highly active

    • The metabolic support is dynamic and activity-dependent

  • Brain energy in aging and neurodegeneration

    • Cerebral metabolic rate declines with age; young healthy adults have higher energy usage than older adults

    • Mild cognitive impairment (MCI) shows further reductions; Alzheimer's disease (AD) shows substantial energy decrease

    • Emerging hypothesis: brain energy crisis precedes neuronal loss; energy deficit may drive neurodegenerative processes

  • Factors that disrupt brain energy balance

    • Mitochondrial dysfunction: reduced ATP production, impaired biogenesis, and defective mitophagy

    • Neuroinflammation: consumes energy; inflammaging increases basal energy demand and diverts energy from maintenance

    • Insulin resistance and high lifetime glucose exposure: impairs glucose transport and metabolism; insulin signaling crucial for brain energy regulation

    • Oxidative stress, accumulation of beta-amyloid plaques, tau tangles: neurotoxic and disrupt energy processes; may feed back to decrease mitochondrial efficiency

    • Myelin integrity: disruption impairs fast signaling, increasing neuronal energy demand and inefficiency

  • Alternative fuels and dietary strategies

    • Ketone bodies and lactate: can be used by neurons when glucose availability or insulin signaling is impaired; preferred fuels in some contexts

    • Ketogenic diets and intermittent fasting can elevate ketone usage; some small studies show improvements in motor and non-motor symptoms in PD and potential reductions in AD pathology

    • Exercise-induced lactate: can be utilized by brain as fuel; exercise also increases growth factors (BDNF, IGF) and mitochondrial function

    • Fiber intake and diet quality can improve insulin sensitivity and reduce long-term risk of neurodegenerative diseases

  • Practical implications and preventive strategies

    • Prioritize cardiovascular health and insulin sensitivity in midlife to reduce late-life dementia risk

    • Maintain exercise routines to promote mitochondrial health and growth factors

    • Consider dietary patterns that support insulin sensitivity and provide alternative fuels when appropriate under medical supervision

Neurodegeneration of aging (NDAs): focus on types, mechanisms, and drivers

  • Major NDA categories and regional involvement

    • Alzheimer’s disease (AD): temporal lobe and hippocampal involvement with early memory impairment; progression to other cognitive domains

    • Frontotemporal dementia (FTD): frontal and temporal lobes; early changes in personality and executive function

    • Vascular dementia: caused by reduced cerebral blood flow; often coexists with other pathologies

    • Dementia with Lewy bodies: associated with Lewy body inclusions

    • ALS (amyotrophic lateral sclerosis): motor neuron degeneration; part of broader neurodegenerative process; can be age-related but may affect younger individuals

  • Pathophysiology and neuropathology in AD

    • Cerebral atrophy: global brain volume loss; typical comparison shows significantly reduced mass in AD brains relative to age-matched controls

    • Hippocampal atrophy: early loss in medial temporal lobe, correlating with memory deficits

    • Ventricular enlargement: compensatory expansion as brain tissue volume decreases

    • Pathological hallmarks: beta-amyloid plaques (extracellular) and neurofibrillary tangles (intracellular tau misfolding)

    • Plaques/tangles: long-standing focus as causative agents; current view supports a more complex etiology with plaques/tangles as downstream or contributory phenomena rather than sole initiators

    • Additional factors: viral infections (e.g., herpes, EBV), genetic predispositions, and insulin resistance being implicated

  • Risk factors and onset patterns

    • Age is the primary risk factor, but not sole cause; midlife cardiovascular health, diabetes, cholesterol, genetics, head injuries influence late-life risk

    • Gender: higher prevalence of MS in females; AD and other dementias show varied gender patterns by subtype

    • Geographic and environmental factors: latitude/UV exposure linked to MS risk via Vitamin D involvement; insulin resistance as a risk factor for AD

  • Clinical and imaging correlates

    • AD yields characteristic imaging findings: atrophy in temporal and parietal regions; hippocampal loss; cortical thinning with knife-edged gyri as volume decreases

    • Plaques and tangles correlate with cognitive decline but are not the sole determinants of disease progression

  • Therapeutic landscape and challenges

    • No cure for most NDAs; treatments largely symptomatic or disease-modifying with limited success

    • Pharmacological strategies target neurotransmitter systems or disease-modifying approaches (e.g., anti-inflammatory agents, metabolic interventions)

Basic neuron- and synapse-focused references for exam-ready recall

  • Core definitions and concepts

    • Resting potential: Vrest ≈−70 mVV_{rest} \,\approx -70\ \text{mV}

    • Threshold for action potential: Vth ≈−55 mVV_{th} \,\approx -55\ \text{mV}

    • Action potential is regenerated along the axon; saltatory conduction in myelinated axons

    • Resting membrane is polarized; local changes (EPSPs, IPSPs) are decremental and decay with distance and time

    • Axon hillock is the decision point for initiating an AP based on integrated input

  • Key mechanisms and components

    • Voltage-gated Na+ channels: trigger rapid depolarization when threshold is reached; exhibit absolute refractory period while channels are inactivated; then a relative refractory period follows

    • Voltage-gated K+ channels: mediate repolarization; restore resting potential

    • Myelination: insulation reduces current leakage and speeds conduction; nodes of Ranvier enable rapid, saltatory conduction

    • Synaptic vesicles and exocytosis: Ca2+ triggers vesicle fusion and transmitter release into synaptic cleft

    • Receptors: ionotropic (fast) vs metabotropic (slow)

    • Reuptake and degradation: essential for signal termination and synaptic readiness

  • Drug actions (neuropharmacology) to remember

    • Cocaine: dopamine reuptake inhibitor; increases dopaminergic signaling

    • Benzodiazepines: GABA-A receptor positive allosteric modulators (GABA agonists in effect); enhances inhibition and sedation

    • Physostigmine: acetylcholinesterase inhibitor; increases acetylcholine at synapses

    • Atropine: muscarinic acetylcholine receptor antagonist; CNS effects include memory disruption at high doses

    • Curare: nicotinic acetylcholine receptor antagonist at NMJ; leads to paralysis

    • Botulinum toxin: inhibits acetylcholine release at presynaptic terminals

Quick takeaways for exam prep

  • Glia are not just passive support cells; they actively regulate neuronal signaling, metabolism, immune responses, and BBB integrity

  • Neurons are highly specialized secretory and signaling cells with tightly integrated morphology for input, processing, and output

  • Synapses are dynamic; neurotransmitter action is tightly regulated via receptor types, reuptake, degradation, and modulatory pathways

  • Brain energy metabolism is a central bottleneck in aging and neurodegeneration; maintaining insulin sensitivity, mitochondrial health, and alternative fuels can influence disease progression and resilience

  • NDAs are multifactorial; pathology includes energy deficits, mitochondrial dysfunction, inflammation, protein aggregates, and vascular contributions

  • Pharmacology and interventions target multiple nodes in the signaling and energy networks, from synthesis and release to receptor activity and clearance