"Unthinking respect for authority is the greatest enemy of truth." — From a letter to Jost Winteler (1901) by Albert Einstein
The traditional understanding of neuroscience has been shaped by a variety of long-standing assumptions that have become neuron dogmas. Among them are beliefs regarding the nature of the electroencephalogram (EEG), the roles of action potentials in different neuron structures, and the regenerative capabilities of the brain. This post examines several commonly held dogmas about brain function, providing updated insights based on recent research. From the sources of EEG signals to the role of dendrites and the production of new neurons in adulthood, these findings not only challenge established views but also highlight the complexities of brain function. This exploration of six key myths about brain activity underscores the importance of ongoing research and the ever-evolving nature of neuroscience.
Dogma 1: The human scalp EEG doesn't contain action potentials.
The dominant contributors to scalp EEG are postsynaptic currents (PSCs) from large populations of cortical neurons. These PSCs account for approximately 80% of the EEG signal strength (Buzsáki et al., 2012).
While action potentials are not the main source of EEG signals, they can make a small but notable contribution. Computational modeling has suggested that action potentials and afterpotentials may contribute up to 20% of the EEG source strength (Murakami & Okada, 2006). Action potential graphic © extender_01/Shutterstock.com.
For example, simulations by Brake and Khadra (2024) show that synchronized spiking could account for up to 1% of the spectral density observed in EEG recordings, particularly in the high-frequency range above 60 Hz, though this is a minimal contribution.
These findings are supported by research on high-frequency oscillations (HFOs), such as fast ripples, which have been observed on scalp EEGs in certain pathological conditions, notably in children with epilepsy, as highlighted by Gotman (2018).
The presence of such high-frequency events suggests that under specific circumstances, particularly in cases of pathology or highly synchronized neural activity, action potentials may contribute to the EEG signal, though they remain a minor component compared to slower synaptic activity. This indicates that while the scalp EEG is not typically a direct reflection of action potentials, certain high-frequency oscillations may indirectly capture their influence.
The limited contribution of action potentials to scalp EEG can be attributed to several factors. Firstly, action potentials are extremely brief, lasting only 1-2 milliseconds, compared to the longer-lasting PSCs. Secondly, the electrical fields generated by individual action potentials tend to cancel each other out at a distance due to their dipolar nature. Secondly, pyramidal neuron axons are generally oriented away from the scalp (DeFelipe & Fariñas, 1992). However, the orientation and branching patterns of pyramidal neurons can vary considerably depending on their location in the cortex and the specific subtype of pyramidal neuron. Graphic adapted from Lipping (2017).
Third, the skull and scalp act as natural attenuators, spatially blurring the electrical signals and further diminishing the contribution of action potentials (Nunez & Srinivasan, 2006).
Among the neural sources contributing to EEG signals, layer 5 pyramidal cells play a dominant role. These cells generate the largest PSC and action potential signals, making them the primary EEG signal generators (Pesaran et al., 2018). It's worth noting that presynaptic activity contributes negligibly to the scalp EEG.
In conclusion, while action potentials can make minor contributions to the scalp EEG, the signal is predominantly generated by postsynaptic currents from large populations of cortical neurons, particularly layer 5 pyramidal cells. The limited impact of action potentials on EEG recordings is a result of their brief duration, tendency to cancel out at a distance, and the attenuation caused by the skull and scalp.
Dogma 2: Dendrites don't exhibit action potentials because they lack voltage-gated sodium channels.
Human dendrites do indeed exhibit action potentials and possess voltage-gated sodium channels (Caldwell et al., 2000), challenging the traditional view of dendrites as passive receivers of information. Recent research has revealed that dendrites in human neurons are capable of generating and propagating action potentials, contributing significantly to neuronal computation and output. Ion channel graphic © Ph-HY/Shutterstock.com.
A groundbreaking study by Gidon et al. (2020) discovered a novel class of calcium-mediated dendritic action potentials (dCaAPs) in layer 2/3 pyramidal neurons of the human cerebral cortex. These dCaAPs have unique properties, including a graded response where their amplitudes are maximal for threshold-level stimuli but dampened for stronger stimuli. This finding suggests that human dendrites possess complex computational capabilities previously thought to require multilayered networks.
The presence of voltage-gated sodium channels in human dendrites is crucial for the generation and propagation of action potentials. These channels enable the active electrical properties observed in dendrites, allowing for back-propagation of action potentials from the soma to the dendrites and, in some cases, the initiation of action potentials within the dendrites themselves (Gidon et al., 2020; Roberts et al., 2020). Illustration 180855817 © Juan Gaertner | Dreamstime.com.
In certain neuronal types, such as gonadotropin-releasing hormone (GnRH) neurons, dendrites have been shown to be the primary site of action potential generation. Roberts et al. (2020) demonstrated that GnRH neuron dendrites are active and capable of initiating action potentials, which then propagate to the soma. This finding challenges the conventional understanding of neuronal polarity and highlights the diversity of dendritic functions across different neuron types.
The active properties of human dendrites, including their ability to generate and propagate action potentials, have significant implications for our understanding of neuronal computation and brain function. These dendritic action potentials contribute to the overall electrical activity of the brain and play a role in shaping the electroencephalogram (EEG) signal. While postsynaptic currents remain the primary contributors to the EEG, action potentials and afterpotentials in dendrites can account for up to 20% of the EEG source strength (Neymotin et al., 2023).
In conclusion, human dendrites are not merely passive receivers of information but active participants in neuronal signaling and computation. Their ability to generate and propagate action potentials, facilitated by voltage-gated sodium channels, adds a layer of complexity to our understanding of brain function and challenges traditional models of neuronal information processing.
Dogma 3: Action potentials initiated at the axon hillock only travel toward axon terminal buttons.
Action potentials initiated at the axon hillock of human neurons do not exclusively travel toward axon terminal buttons. While the primary direction of action potential propagation is indeed along the axon towards synaptic terminals, a phenomenon known as backpropagation allows action potentials to travel backward into the dendrites and soma as well. Neuron graphic by M.alijar3i from the Wikipedia article Axon Hillock.
Backpropagation of action potentials is a crucial mechanism in neuronal function and plasticity. When an action potential is initiated at the axon initial segment, it not only propagates forward along the axon but also backward into the soma and dendrites (Stuart et al., 1997). This retrograde signaling serves several important functions in neuronal computation and plasticity.
One significant role of backpropagation is in spike-timing-dependent plasticity (STDP), a process fundamental to learning and memory. STDP relies on the precise timing between presynaptic inputs and postsynaptic action potentials. Backpropagating action potentials provide a retrograde signal that can interact with incoming synaptic inputs, modulating synaptic strength based on the relative timing of these events (Feldman, 2012).
Furthermore, backpropagation enhances the computational capabilities of neurons. By allowing the soma and dendrites to "know" about the occurrence of an action potential, backpropagation enables complex integration of synaptic inputs and influences dendritic excitability. This process can lead to the generation of dendritic spikes and calcium influx, which are crucial for synaptic plasticity and dendritic computation (Larkum et al., 1999).
Interestingly, the efficiency of backpropagation can vary depending on the neuronal type and the specific dendritic region. In some neurons, such as hippocampal CA1 pyramidal neurons, backpropagation can be quite robust, while in others, it may attenuate more rapidly. This variability contributes to the diverse computational properties of different neuronal types (Stuart et al., 1997).
Recent research has also highlighted the importance of backpropagation in human neurons. Gidon et al. (2020) discovered a novel class of dendritic action potentials in human cortical neurons that exhibit unique properties, emphasizing the complexity of signal propagation in human neural circuits.
In conclusion, while action potentials initiated at the axon hillock do primarily propagate towards axon terminals, the phenomenon of backpropagation allows for a more complex bidirectional flow of information within neurons. This mechanism is crucial for synaptic plasticity, dendritic computation, and the overall information processing capabilities of neural networks in the human brain.
Dogma 4: Action potentials only travel through an axon's interior.
The classic model of electrons traveling through conductors in two directions is an explanatory fiction. Electrons don't actually travel from battery to light bulb or power plant to your microwave. As the Veritasium video shows, it is electromagnetic fields (shown below) that travel and carry energy in one direction. This is true for sunlight, powerlines, and neurons. Watch the YouTube video, The Big Misconception About Electricity. Electromagnetic field graphic © tersetki/Shutterstock.com.
Action potentials do not travel exclusively through an axon's interior, and there is evidence that oscillating magnetic fields creating an energy flux outside the axon play a role in carrying the action potential signal. This challenges the traditional view of action potential propagation and suggests a more complex mechanism involving both intracellular and extracellular components.
The conventional understanding of action potential propagation focuses on the flow of ions through voltage-gated channels in the axon membrane. However, research has shown that the electrical activity associated with action potentials generates magnetic fields that extend beyond the axon's physical boundaries (Wikswo et al., 1980). These magnetic fields oscillate in conjunction with the changing electrical potentials during an action potential.
Interestingly, these oscillating magnetic fields may contribute to the propagation of the action potential signal. Swinney and Wikswo (1980) proposed that the energy flux associated with these magnetic fields could play a significant role in signal transmission along the axon. This concept suggests that the action potential is not confined to the axon's interior but involves a more extensive electromagnetic phenomenon.
Further support for this idea comes from studies on saltatory conduction in myelinated axons. Ramachandran and Ramachandran (2019) suggested that the rapid propagation of action potentials in myelinated axons might be partially explained by the transmission of electromagnetic energy through the myelin sheath, rather than solely through the axon's interior.
The potential involvement of extracellular electromagnetic fields in action potential propagation has implications for our understanding of neural communication and could explain some phenomena that are difficult to account for using the classical model of action potential propagation. For instance, it might help explain how action potentials can sometimes jump across severed axons or how they can propagate faster than predicted by cable theory alone (Heimburg & Jackson, 2005).
However, it's important to note that while these ideas are intriguing, they remain somewhat controversial in the neuroscience community. The classical model of action potential propagation through ion channels remains the dominant explanation, and more research is needed to fully understand the potential role of external electromagnetic fields in neural signaling.
In conclusion, while action potentials do involve the flow of ions through the axon's interior, there is evidence suggesting that oscillating magnetic fields and associated energy fluxes outside the axon may also play a role in carrying the action potential signal. This perspective broadens our understanding of neural signaling and highlights the complex interplay between intracellular and extracellular processes in neuronal communication.
Dogma 5: The human brain does not produce new neurons in adulthood.
The production of new olfactory neurons in adults is a well-established phenomenon that occurs through adult neurogenesis. This process is particularly robust in the olfactory system, where new neurons are continuously generated and integrated into existing neural circuits throughout life (Brann & Firestein, 2014; Schwob et al., 2017).
In contrast, the question of whether the human brain creates new hippocampal neurons during adulthood has been a subject of intense debate and research in neuroscience. While it was long believed that neurogenesis - the creation of new neurons - ceased after early development, evidence over the past few decades has challenged this view, though recent studies have reignited the controversy. Neurogenesis graphic © VectorMine/Shutterstock.com.
In 1998, a groundbreaking study by Eriksson et al. demonstrated the presence of newly formed neurons in the adult human hippocampus, a region crucial for learning and memory. This study used bromodeoxyuridine (BrdU) labeling to identify dividing cells in postmortem brain tissue from cancer patients who had received the compound for diagnostic purposes. The researchers found evidence of new neurons in the dentate gyrus of the hippocampus, suggesting that neurogenesis continues throughout human adulthood (Eriksson et al., 1998).
This discovery sparked a wave of research into adult neurogenesis in humans and other mammals. Subsequent studies provided further support for the concept, with some researchers estimating that hundreds of new neurons are added to the adult human hippocampus daily (Spalding et al., 2013). These findings led to excitement about the potential implications for brain plasticity, learning, and the treatment of neurological and psychiatric disorders.
However, the field was shaken in 2018 when Sorrells et al. published a study that found no evidence of young neurons or dividing progenitors in adult human hippocampal samples. Using a variety of methods, including immunohistochemistry and electron microscopy, they observed a sharp drop in neurogenesis during childhood, with no detectable neurogenesis in adult samples (Sorrells et al., 2018). This study reignited the debate about adult neurogenesis in humans and highlighted the methodological challenges in studying this phenomenon.
Adding to the complexity, a study published shortly after by Boldrini et al. (2018) reported contrasting findings. Using similar techniques on postmortem brain samples, they found evidence of ongoing neurogenesis in the adult human hippocampus, with thousands of immature neurons present even in older adults. The discrepancy between these studies underscores the technical difficulties in detecting and quantifying neurogenesis in human brain tissue.
BrdU (red), a marker of DNA replication, highlights neurogenesis in the subgranular zone of hippocampal dentate gyrus. Fragment of an illustration from Faiz et al. (2005). Graphic downloaded from Wikipedia.
The conflicting results have led to intense scrutiny of the methods used to study adult neurogenesis in humans. Factors such as postmortem delay, tissue preservation techniques, and the specificity of markers used to identify new neurons have all been cited as potential sources of variability between studies (Kempermann et al., 2018). Animal research has yielded evidence of functionally significant neurogenesis in the amygdala, caudate nucleus and putamen (striatum), cortex, hypothalamus, and substantial nigra (Jurkowski et al., 2020).
Despite the ongoing controversy, research into adult neurogenesis continues, with implications for our understanding of brain plasticity, aging, and disease. Some researchers argue that even if neurogenesis does occur in the adult human brain, its functional significance may be limited compared to other forms of plasticity (Paredes et al., 2018).
In conclusion, while the existence of adult neurogenesis in humans remains a topic of debate, the research in this field has significantly advanced our understanding of brain plasticity and development. Future studies using advanced techniques, such as single-cell RNA sequencing and improved imaging methods, may help resolve the current controversies and provide a clearer picture of the regenerative capacity of the adult human brain.
Dogma 6: The human lymphatic system does not service the brain.
Recent research has revealed that the human lymphatic system does indeed extend to the brain, challenging long-held beliefs about the central nervous system's relationship with the lymphatic system. This discovery has significant implications for our understanding of brain function, waste clearance, and neurological disorders.
The presence of meningeal lymphatic vessels (MLVs) in the human brain was first definitively demonstrated in 2015 through groundbreaking studies (Louveau et al., 2015; Aspelund et al., 2015). These vessels are located in the dura mater, the outermost layer of the meninges, and run parallel to the dural venous sinuses and middle meningeal arteries (Louveau et al., 2015). The MLVs express typical lymphatic endothelial cell markers such as PROX1, LYVE1, and PDPN, confirming their identity as true lymphatic vessels (Aspelund et al., 2015). Glymphatic system graphic © Claus Lunau/Science Photo Library.
The brain's lymphatic system plays a crucial role in draining cerebrospinal fluid (CSF), interstitial fluid, and removing waste products from the central nervous system. This drainage system connects to the deep cervical lymph nodes, providing a direct link between the brain and the peripheral immune system (Thomas & Eichmann, 2022). The discovery of MLVs has shed new light on how the brain maintains homeostasis and clears metabolic waste, processes that are essential for proper neurological function.
Recent advancements in imaging techniques have allowed researchers to visualize these lymphatic structures in living humans. Thomas and Eichmann (2022) developed a novel MRI-based imaging technique that can detect and map the brain's lymphatic drainage pathways in patients with neurological diseases. This non-invasive method opens up new possibilities for studying the role of the brain's lymphatic system in various neurological disorders and potentially developing targeted therapies.
The brain's lymphatic system is now recognized as a key player in several neurological conditions. Studies have suggested its involvement in Alzheimer's disease, multiple sclerosis, and brain tumors, among others (Jacob et al., 2022). The ability of MLVs to transport immune cells and drain CSF has implications for neuroinflammation, immune surveillance of the brain, and the clearance of pathogenic proteins associated with neurodegenerative diseases.
Furthermore, the brain's lymphatic system is connected to the recently discovered glymphatic system, a brain-wide network of perivascular spaces that facilitates the movement of CSF and interstitial fluid through the brain parenchyma (Iliff et al., 2012). Together, these systems form a comprehensive waste clearance network that is essential for maintaining brain health.
In conclusion, the discovery of lymphatic vessels in the human brain has revolutionized our understanding of brain physiology and opened up new avenues for research into neurological disorders. As our knowledge of this system grows, it may lead to novel therapeutic approaches for treating a wide range of brain diseases.
Conclusion
Advances in neuroscience have revealed that many widely accepted ideas about the brain are oversimplifications. The role of action potentials in EEG recordings, the computational abilities of dendrites, and the presence of neurogenesis in adulthood are far more complex than previously believed. Additionally, the discovery of a brain lymphatic system and the reconsideration of action potential propagation challenge long-held dogmas. These insights illustrate that the brain is not just a static organ but a dynamic system with remarkable plasticity and functional adaptability. As research continues, our understanding of the brain will likely further evolve, leading to new therapeutic approaches for neurological and psychiatric conditions.
Glossary
action potential: a rapid, temporary change in a cell's membrane potential, primarily occurring in neurons, responsible for signal transmission along axons.
axon hillock: the region of the neuron where the axon originates from the soma, playing a key role in the initiation of action potentials.
axon terminal button: the small, bulb-like structure at the end of an axon where neurotransmitters are released into the synaptic cleft to transmit signals to the next neuron or target cell.
backpropagation: the phenomenon where action potentials travel backward into the soma and dendrites after being initiated at the axon hillock.
bromodeoxyuridine (BrdU) labeling: a method used to detect newly synthesized DNA in proliferating cells. BrdU is incorporated into the DNA of dividing cells, allowing researchers to track cell proliferation, often used in studies of neurogenesis.
computational modeling: the use of mathematical models and simulations to replicate and study complex biological systems, such as neural activity or brain function, in order to make predictions and analyze mechanisms that may be difficult to measure experimentally.
cortical layer 5: a specific layer of the cerebral cortex, characterized by large pyramidal neurons. It is primarily involved in sending outputs to the brainstem, spinal cord, and other cortical areas, and plays a major role in generating postsynaptic potentials that contribute to the EEG.
dendrites: branched extensions of a neuron that receive electrical signals from the synapses of other neurons and transmit them toward the cell body for processing.
dendritic action potentials (dCaAPs): action potentials initiated in the dendrites, often mediated by calcium ions and contributing to complex neuronal computations.
dentate gyrus: a part of the hippocampus involved in the formation of new episodic memories and one of the few regions in the adult brain where neurogenesis occurs.
electroencephalogram (EEG): a recording of electrical activity in the brain, primarily generated by postsynaptic currents in large populations of cortical neurons.
energy flux: the rate of energy transfer per unit area, which can occur in various forms such as electrical, magnetic, or thermal. In neuroscience, it may refer to the flow of electromagnetic energy during the propagation of an action potential.
glymphatic system: a brain-wide network that facilitates waste removal via cerebrospinal fluid flow through perivascular spaces, closely linked to the brain’s lymphatic system.
gonadotropin-releasing hormone (GnRH): a hormone released by the hypothalamus that stimulates the release of gonadotropins (LH and FSH) from the anterior pituitary gland, essential for reproductive function.
high-frequency oscillations (HFOs): electrical brain activity in the range of 80–500 Hz, often associated with epilepsy and reflecting pathological synchronized neural activity.
hippocampus: a brain structure located in the medial temporal lobe, essential for memory formation, spatial navigation, and regulation of emotional responses. It is a key area for neurogenesis in adults.
meningeal lymphatic vessels (MLVs): newly discovered lymphatic vessels located in the dura mater of the brain, responsible for draining cerebrospinal fluid and waste products.
neurogenesis: the process of generating new neurons, which occurs in certain brain regions such as the hippocampus and olfactory system, even in adulthood.
olfactory neurons: sensory neurons located in the olfactory system responsible for detecting odor molecules and relaying the information to the brain, particularly involved in the sense of smell.
postsynaptic current (PSC): electrical current generated in the postsynaptic neuron after neurotransmitter release, responsible for most of the EEG signal.
postsynaptic potentials: electrical changes in the postsynaptic neuron that occur in response to neurotransmitter release from the presynaptic neuron, either excitatory (EPSP) or inhibitory (IPSP), influencing the likelihood of an action potential.
presynaptic activity: the release of neurotransmitters from a neuron into a synapse, which has a minimal contribution to EEG recordings.
pyramidal neurons: large, excitatory neurons found in various layers of the cerebral cortex, particularly in layer 5, known for their triangular-shaped cell bodies and long dendrites, playing a critical role in integrating and transmitting neural signals.
saltatory conduction: the process by which action potentials jump between nodes of Ranvier in myelinated axons, speeding up signal transmission.
solitons: stable, self-reinforcing waves that maintain their shape while traveling at a constant speed. In neuroscience, solitons have been proposed as a potential model for action potential propagation through biological membranes.
spike-timing-dependent plasticity (STDP): Aa form of synaptic plasticity dependent on the timing of presynaptic and postsynaptic spikes, crucial for learning and memory formation.
voltage-gated sodium channels: ion channels that open in response to changes in membrane potential, allowing sodium ions to enter the neuron and generate action potentials.
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Dr. Inna Khazan's BCIA Introduction to biofeedback workshop will be offered in two parts this year.
Part 1 is entirely virtual, consisting of 20 hours (over 5 days) of live online instruction, home-study materials distributed prior to the live workshop, and written instructions for practical lab work to be completed during the week of the workshop or after its completion. Part 1 fulfills BCIA requirements for introduction to biofeedback didactic. Part 1 will take place on Zoom, November 4 - 8, 2024, 12 - 4pm EDT. Tuition is $1395.
Part 2 is optional, and consists of 14 hours (over 2 days) of in-person hands-on practical training using state-of-the-art equipment, designed to help participants be better prepared to start working with clients. Part 2 will take place in Boston on November 11 & 12, 2024, 9am-5pm EDT. Tuition is $395. (Please note that an Introduction to Biofeedback didactic (taken at any previous time, anywhere) is a pre-requisite to the hands-on training).
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