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Home Page > QBD 1 > QBD 4

QBD 4

The Physiological Basis and Quantum Versions of Memory and Consciousness

Arthur Hudson

The book gives an up-to-date coverage of many aspects of neuroscience including the pyramidal cell/interneuron balance of excitatory and inhibitory forces in the brain, and also a modern understanding of dendrites, microtubules, actin, gap junctions, astrocytes and the hippocampal region. The end section of the book deals with ideas on quantum theories of consciousness. In this, it favours the quantum brain dynamics model, but does not appear to advance any new arguments in this area. The book is mainly useful for its clear exposition of of those brain structures, which have come more to the fore in recent years.

The book emphasises the importance of the vertical cortical columns that connect the excitatory pyramidal cells, using the neurotransmitter glutamate with the inhibitory interneurons using the neurotransmitter GABA. Pyramidal cells and interneurons provide overall control of functions. Pyramidal neurons are similar throughout the brain, but interneurons vary a lot. Pyramidal neurons release glutamate and are the main excitatory neurons in the cortex, extensively targetting subcortical neurons. The pyramidal neurocells project to other pyramidal cells and to interneurons in their own locality and to pyramidal cells in different parts of the brain. Interneurons release the inhibitory neurotransmitter GABA. Interneurons have only short range connections with pyramidal cells and other interneurons.

Pyramidal cells are most apparent in layer three and five of the cortex. They are often covered with spines and the spine density tends to be greatest on the dendrites. Pyramidal cells can have thousands of dendritic connections from both other pyramidal cells and from interneurons. Pyramidal cells in layers two and three project to the opposite hemisphere via the corpus callosum. Layer five pyramidal neurons project to the cortex and the sub-cortex in the same hemisphere.

Ninety percent of neurons are indicated to be excitatory, and the pyramidal cells are involved with 75% of synapses. Dendrites are the main target of excitatory synapses but not of inhibitory synapses. Dendrites, which handle the inflow of signals into neurons, contain microtubules, neurofilaments and mitochondria, but the most prominent features are microtubules. Dendrites and small axons are densely packed with microtubules. Free ribosomes are present in dendrites but not in axons. Dendritic spines undergo rapid change in response to the formation and elimination of synapses. The distribution of ion channels in dendrites is less orderly than in axons, but not regarded as random.

Microtubules are linked to one another by microtubule associated proteins MAP 1 and MAP 2 and by tau protein. MAP 1 is in both dendrites and axons but MAP 2 only in dendrites. Tau protein is mainly in axons, and helps to maintain polarity. There is a high concentration of actin in dendritic spines, and this is responsible for their plasticity in response to changes in synapses, which is important in learning and memory.

Each row of tubulin in microtubules is formed of alternating alpha and beta tubulin subunits. Microtubules have a positively charged and fast growing end and a negatively charged and slow growing end. Growth occurs by the addition of tubulin molecules carrying GTP, which facilitates their attachment to one another. Microtubules provide a track for moving two motor proteins, kinesin and dynein. These are microtubule dependent proteins, and carry cargo for cell maintenance such as vesicles. The dyneins move in the opposite direction to kinesins carrying both organelles and vesicles.

Synaptic attachment to dendrites can be to either the main shaft, or particularly with pyramidal cells, to the spines. The spines are varied in shape. Dendritic spines relate to 90% of excitatory synapses but a smaller proportion of inhibitory synapses. Actin filaments comprise the cytoskeleton of the dendritic spine, forming longitudinal bundles in the spine and a meshwork in the post-synaptic density (PSD). The actin organises the proteins in the PSD. In some cases the PSD is connected to ion channels. The actin filaments help to position neurotransmitter receptors.

Vesicles are produced in the endoplasmic reticulum, and transported by microtubule dependent kinesin to the axon terminal, where the vesicle takes up the neurotransmitter and is tethered by the cytoskeletal proteins. Action potentials arriving at the synapse cause the opening of voltage gated Ca2+ channels. This sets of a process which frees the vesicles from their attachment to the cytoskeleton protein and move to an area where they are ready to discharge their neurotransmitters.

Gap junctions or electrical synapses are only a fifth as wide as a normal synapse, measuring two to three nm. This gap is bridged by two sets of connexon channels, one in each cell. A connexon is a six subunit protein. The subunits are called connexins. Some connexons are formed out of identical connexins and others out of a mixture. Some connexins are compatible and others are not. This may be decided by the type of cell or its function. The gap junctions allow current, ions or signalling molecules to pass between cells on a selective basis. This may be a mechanism for mediating electrical oscillations particularly in the gamma frequency. The gap junction can open or close at either end in response to the membrane potential. The connexins have varying voltage sensitivity. In the cortex, gap junctions are mainly found between interneurons and glial cells, especially astrocytes.

Astrocytes are essential to neurogenesis, one important area for this being the hippocampal region. In areas such as this, they promote the neural stem cells. Astrocytes are active in meeting the metabolic requirements of neurons. Neurons and glia communicate mainly by chemical signals. Astrocytes maintain synaptic connections and regulate synaptic transmission. They remove potassium and calcium ions and other material from the areas around synapses. There is also some suggestion that they regulate the strength of release of glutamate and GABA. Astrocytes are connected by gap junctions and also by release of ATP. ATP release can produce a calcium increase and this may help to propagate signals in the nervous system.

The electrical properties of neurons are governed by two types of ion channel, the normally open resting channels and the voltage gated channels that generate action potentials. Axons have their Na+ channels concentrated at the nodes of Ranvier. The K+ channels are also clustered here, but there are more K+ channels in between. The resting membrane potential of the neuron is about -65mV and is maintained by exchanging K+ and Cl- and Na+ across the membrane. The direction of the ion flow is a combination of the concentration gradient of the ions and the voltage difference they produce across the membrane. As the Na+ flow comes to an end, the K+ channel opens to let ions out. The K+ ions are then pressured to leave by a strong concentration gradient, but are restriained by the electrically negative nature of the interior cell. K+ and Cl minus channels dampen excitation and return the membrane to its resting potential. There is a whole class of functionally similar K+ channels. Glial cells have similar ion channels.

The lipid bilayer membranes act as insulators between conducting materials inside and outside the cell. They can be seen as capacitators that store electrical charge on one side of the insulation material as a potential difference or voltage across the membrane.

Glutamate opens post-synaptic ion channels, allowing the influx of Na+, K+ and Ca2+, triggering rapid neuronal excitement. It can also open the metabotropic channels which mediate G protein-coupled enzyme responses. GABA is the principle neurotransmitter for interneurons. GABA is structurally identical to glutamate, except that it has one less carboxyl group. GABA receptors permit an influx of Cl minus.

There is only one fifth as many interneurons as there are pyramidal neurons. Particular groups of interneurons contact the different parts of the pyramidal cells in the cortex and hippocampal, some connecting to axon terminals and some to dendrites. Networks of electrically synchronised neuronal activity have been observed in the cortex and the hippocampus. Pyramidal cells drive gamma synchrony in networks of GABA inhibitory neurons. The interneurons can do visa versa, and can entrain pyramidal cells over a wide cortical distance. GABA interneurons are suggested here to provide a general mechanism for synchronisation of cortical activity.

Gap junctions synchronise spikes in connected interneurons. The gap junctions of fast spiking interneurons could be an added synchronising mechanism. The diversity of interneurons is seen as allowing them to entrain pyramidal cells in different networks. Such entrained networks are suggested to be capable of computation.

Axonal growth and targeting of neurons by guidance molecules takes place during development of the brain and to some extent in its maturity. Targeting requires proteins that are attracted or repelled by other proteins. Specific affinities mean that each axon links only to certain neurons, and is guided by chemical gradients or molecules fixed on the surface of cells.

Metabotrophic receptors are slower than ionotrophic receptors, because of enzyme and special messenger effects. G proteins function as transducers, delivering messages from receptors to metabolic pathways. A single G protein can regulate multiple functions. G proteins have alpha, beta and gamma subunits. When a neurotransmitter binds to a receptor, the alpha subunit takes high energy GTP. The subunit dissociates initiating a cascade of metabolic events. The subunits are reunited in a resting phase when the process is over.

Glutamate activates the cAMP dependent protein kinase. Its subunits bind so as to eventually activate other protein molecules. In some cases, G protein stimulates Ca2+ as a second messenger. NMDA receptors are attached to the PSD containing its own family of proteins. NMDA receptors stimulate many important signaling pathways. Some of these are associated with learning and memory. Protein kinase II mediates many of the effects of calcium in the cell, especially in the nervous system. Ca2+ attaches to calmodulin, a calcium binding protein which then binds to and activates protein kinase. The protein kinase regulates long-term potentiation.

Generation of action potentials is dependent on the generation of ion channels. Na+ channel density is much higher at the first node of Ranvier than at the axon hillock. In long term potentiation, when a synapse repeatedly releases a neurotransmitter, the amplitude of the excitatory post-synaptic potential is enhanced. This goes back to pre-synaptic Ca2+ increasing and allowing more neurotransmitter release.

Enhanced pre-synaptic influx Ca2+ is usually necessary for the increased release of excitatory neurotransmitters. A post-synaptic rise in Ca2+ in response to a neurotransmitter is required for the excitability of the recipient cell and LTP. LTP depends on high levels of calcium ion influx into dendritic spines. These calcium ions come mainly via NMDA and AMPA receptors. Synaptic plasticity in dendritic spines occurs in LTP. There may be an increase in the area of both pre and post-synaptic zones and in the size of the PSD.

The hippocampal formation includes the dentate gyrus and the subiculum. The hippocampal along with the anterior-inferior and medial temporal lobe are the regions that encode new memories and retrieve past memories. The anterior cingulate relates sensory expereince to memory. It has significant connections with the amygdala and the lateral and orbital prefrontal. The cingulate is involved in avoidance learning.