Bioimaging and Brain Research

Impulse conduction in Axons

The action potential is initiated at the beginning of the axon, at what is called the initial segment. There is a high density of voltage-gated Na channels so that rapid depolarization can take place here. Going down the length of the axon, the action potential is propagated because more voltage-gated Na channels are opened as the depolarization spreads. This spreading occurs because Na enters through the channel and moves along the inside of the cell membrane. As the Na moves, or flows, a short distance along the cell membrane, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na channels open and more ions rush into the cell.

K+4. Falling the membrane potential is rapidly returning to the resting potential

5. Undershoot accurse because most voltage-gated potassium channels are still open

6. Recovery occurs as the delayed potassium channels that were opened during the action potential now close

spreading the+depolarization a little farther. Because voltage-gated Na channels are inactivated at thepeak of the depolarization, they cannot be opened again for a brief time. Because ofthis, depolarization spreading back toward previously opened channels has no effect.The action potential must propagate toward the axon terminals; as a result, the polarityof the neuron is maintained, as mentioned above. Propagation, as described above,applies to unmyelinated axons. When myelination is present, the action potentialpropagates differently. Sodium ions that enter the cell at the initial segment start tospread along the length of the axon segment, but there are no voltage-gated+Na channels until the first node of Ranvier. Because there is not constant opening ofthese channels along the axon segment, the depolarization spreads at an optimalspeed. The distance between nodes is the optimal distance to keep the membrane still+depolarized above threshold at the next node. As Na spreads along

The inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na channels to be activated at the next node of Ranvier (domandad’esame). If the nodes were any closer together, the speed of propagation would be slower. Propagation along an unmyelinated axon is referred to as continuous conduction; along the length of a myelinated axon, it is conduction. Continuous conduction is slow because there are always voltage-gated Na channels opening, and more and more Na is rushing into the cell. Saltatory conduction is faster because the action potential basically jumps from one node to the next (saltare = “to leap”), and the new influx of Na renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. Much as water runs faster in a wide river than in a narrow.

creek, Na-based depolarization spreads faster down a wide axon than down a narrow one.

Synapse

Electrical (has direct physical contact) is mediated by clusters of intercellular channels called gap junctions that connects the interior of two adjacent cells, and thereby directly enable the bidirectional passage of electrical currents carried by ions. They are bidirectional in nature: when a presynaptic action potential propagates to the postsynaptic cell, the membrane resting potential propagates to the postsynaptic cell, the membrane resting potential of the postsynaptic cell simultaneously propagates to the presynaptic cell.

Chemical (involves neurotransmitters)

The synapse is a connection between a neuron and its target cell (which is not necessarily a neuron). The presynaptic element is the synaptic end bulb of the axon where Ca2+ enters the bulb to cause vesicle fusion and neurotransmitter release. The neurotransmitter diffuses across the synaptic cleft to bind to its receptor.

neurotransmitter is cleared from the synapse either by enzymatic degradation, neuronal reuptake, or glial reuptake. When an action potential reaches the axon terminals, voltage-gated Ca2+ channels in the membrane of the synaptic end bulb open. The concentration of Ca2+ increases inside the end bulb, and the Ca2+ ion associates with proteins in the outer surface of neurotransmitter vesicles. The Ca2+ facilitates the merging of the vesicle with the presynaptic membrane so that the neurotransmitter is released through exocytosis into the small gap between the cells, known as the synaptic cleft. Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane of the next neuron and interacts with neurotransmitter receptors on the dendrites or cell body. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. One neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making thebinding a specific chemical event.- Summation of Postsynaptic Potential occurs when a presynaptic neuron fires repeatedly at a high rate ("temporal summation") or when several presynaptic terminals fire at the same time ("spatial summation") or from a combination of temporal and spatial summation. An excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary depolarization of the postsynaptic membrane potential, caused by the flow of positively charged ions into the postsynaptic cell, is a result of opening ligand-gated ion channels and can also result from a decrease in outgoing positive charges. The measured voltage is higher than -0mV. Inhibitory postsynaptic potentials (IPSP) release neurotransmitters that then bind to the postsynaptic receptors, inducing a change in the permeability of the postsynaptic neuronal membrane to particular ions. This can result in an electric current thatchangesthe postsynaptic membrane potential to create a more negative postsynapticpotential is generated, i.e. the postsynaptic membrane potential becomes morenegative than the resting membrane potential, and this is called hyperpolarisation. Togenerate an action potential, the postsynaptic membrane must depolarize—themembrane potential must reach a voltage threshold more positive than the restingmembrane potential, they are sometimes caused by an increase in positive chargeoutflow. The measured voltage is below -60mV

The temporal summation Temporal summation occurs when a high frequency of actionpotentials in the presynaptic neuron elicits postsynaptic potentials that summate with eachother. The duration of a postsynaptic potential is longer than the interval between actionpotentials. If the time constant of the cell membrane is sufficiently long, as is the case for thecell body, then the amount of summation is increased. The amplitude of one postsynapticpotential at the time

point when the next one begins will algebraically summate with it, generating a larger potential than the individual potentials. This allows the membrane potential to reach the threshold to generate an action potential

The spatial summation Spatial summation is a mechanism of eliciting an action potential in a neuron with input from multiple presynaptic cells. It is the algebraic summing of potentials from different areas of input, usually on the dendrites. Summation of excitatory postsynaptic potentials increases the probability that the potential will reach the threshold potential and generate an action potential, whereas summation of inhibitory postsynaptic potentials can prevent the cell from achieving an action potential. The closer the dendritic input is to the axon hillock, the more the potential will influence the probability of the firing of an action potential in the postsynaptic cell

Brain Structures and Functions

• Different parts of the brain
• the brain can be divided in three

Main Regions:

Hindbrain – Pons, bridge signal from the forebrain to cerebellum or to the medulla;

• Transmits sensory signal to the forebrain (Thalamus)

Medulla, just below the Pons, stimulating most of the involuntary action of the body such as breathing, digestion, heart rating and swallowing. Here we have the DECUSSATUON (DOMANDA ESAME) describes the point where the nerves cross from one side of the brain to the other, and typically the nerves from the left side of the body decussate to the right side of the brain and the nerves from the right side of the body decussate to the left brain, however depending on the function of the nerves the level of decussation is variable.

Cerebellum is the second largest part of the brain. It has the higher concentration of neuron anywhere in the brain. Adjust postural muscles to maintains balance. Refine muscles movements. Spatial processing, allowing for the recognition of subjects when viewed from different angles. Also enable one to project where a

ns by the German neurologist Korbinian Brodmann in the early 20th century, Brodmann areas are a way to map and classify different regions of the cerebral cortex based on their cellular structure. Each area is assigned a number and is associated with specific functions or cognitive processes. The third image is a representation of Brodmann areas, showing the different regions of the cerebral cortex and their corresponding numbers. Overall, these images and concepts highlight the complexity and organization of the human brain, with its folded structure and specialized areas responsible for various functions and cognitive processes.