CAP 46 5/5: Dentritas, fatiga y retraso sináptico l Fisiología de Guyton

Dendrites and Neuronal Electrical Conduction

In this section, the importance of dendrites in the electrical conduction of neurons is discussed. The focus is on the unique functions and characteristics of dendrites that play a crucial role in neuronal communication.

Dendritic Field Excitation

• Dendrites in motor neurons extend from 500 to 1000 micrometers from the soma, receiving a significant number (80-95%) of presynaptic terminations.

• Dendrites receive more presynaptic terminations compared to the soma (5-20%), emphasizing their role as primary recipients of neural signals.

Action Potential in Dendrites

• Dendrites do not transmit action potentials but convey electrical changes, influenced by limited voltage-gated sodium channels.

• Due to few sodium channels, dendrites serve as electrical conductors for membrane potentials towards the soma.

Electrotonic Current Transmission

• Dendritic electrotonic currents transport signals without generating action potentials, gradually diminishing along the dendrite's length.

• Limited voltage-dependent sodium channels and high excitation threshold contribute to dendritic function as signal conductors rather than initiators.

Electrotonic Current Propagation

This part delves into how electrotonic currents propagate through dendrites, highlighting factors influencing current strength and transmission efficiency.

Decay of Electrotonic Current

• Electrotonic current diminishes along the dendrite's path due to passive spread and leakage through partially permeable membranes.

• Voltage changes induced by excitatory inputs lead to progressive decay of electrotonic current towards the soma.

Decreasing Conductance Phenomenon

• Conduction decrement occurs as electrotonic current propagates further, resulting from increased distance and potassium/chloride ion permeability.

• Lengthier dendrites exacerbate conduction decrement due to greater distance for signal propagation and ion redistribution effects.

Excitatory Summation in Neurons

This segment explores excitatory summation within neurons, elucidating how synaptic inputs modulate membrane potential dynamics.

Synaptic Excitation Dynamics

• Presynaptic neuron inputs can either excite or inhibit postsynaptic membrane potential, influencing excitatory or inhibitory synaptic potentials.

Neuronal Excitation and Inhibition

In this section, the speaker discusses the dynamics of neuronal excitation and inhibition, detailing how neurotransmitters impact membrane potentials and influence neuronal activity.

Neuronal Membrane Potential Changes

• Neurons experience excitatory neurotransmitters leading to a decrease in membrane potential to -20 millivolts. As conduction progresses towards the soma, the membrane potential gradually returns to its basal level of -65 millivolts.

Impact of Excitatory and Inhibitory Synapses

• Excitatory and inhibitory synapses converge on dendrites, affecting membrane potential flow within the neuron. Strong inhibitory synaptic inputs can significantly reduce the membrane potential, hindering signal transmission to the soma.

Neuronal Transmission Dynamics

• The mission of a neuron is to depolarize or excite the soma before transmitting signals through the axon. However, synaptic inputs from presynaptic neurons can either inhibit or enhance this process, influencing overall neuronal activity.

Neuronal Excitation States

• The excitatory state of a neuron correlates with its discharge frequency; higher excitation levels prevail over inhibition. Different neurons exhibit varying thresholds for excitation, impacting their firing rates and response patterns.

Neuronal Firing Patterns and Synaptic Fatigue

This segment delves into neuronal firing characteristics, emphasizing how different neurons respond uniquely to stimuli based on thresholds and discharge frequencies.

Neuronal Thresholds and Firing Rates

• Neurons display diverse threshold levels for excitation; some neurons reach an excitatory state faster than others due to varying intrinsic properties. Additionally, neurons differ in their discharge frequencies per second.

Varied Response Patterns

• Neurons exhibit distinct firing patterns based on their unique excitability thresholds and discharge rates. While some neurons rapidly achieve an excitatory state with high firing rates, others may require more stimulation but produce larger spikes in activity.

Synaptic Transmission Characteristics

The discussion shifts towards special features of synaptic transmission, focusing on synaptic fatigue as a protective mechanism within the central nervous system.

Synaptic Fatigue Mechanisms

• Synaptic fatigue occurs when repetitive excitatory synapses lead to diminishing firing frequencies over time due to transmitter depletion or receptor saturation at postsynaptic terminals.

Causes of Synaptic Fatigue

• Three primary factors contribute to synaptic fatigue: transmitter depletion at presynaptic terminals, progressive inactivation of postsynaptic receptors due to receptor occupancy saturation, and neurotransmitter accumulation without available receptors for binding.

Excitatory and Inhibitory Synaptic Transmission

The discussion focuses on the mechanisms of excitatory and inhibitory synaptic transmission, particularly in conditions like epilepsy and diabetic coma.

Mechanisms of Excitatory and Inhibitory Synaptic Transmission

• In conditions like epilepsy, there is an increase in excitatory state and a decrease in inhibitory state, leading to epileptic crises.

• Various factors such as alkalosis excite synaptic transmission, while acidosis inhibits it. Conditions like diabetic ketoacidosis can lead to coma due to altered synaptic transmissions.

• Hyperventilation in epilepsy patients can trigger seizures due to respiratory alkalosis. Hypoxia inhibits synaptic transmission.

• Substances like caffeine lower the excitation threshold of neurons, promoting excitatory synaptic transmission. Certain pesticides inhibit inhibitory transmitters, favoring excitatory transmission.

• Anesthetics have multiple mechanisms to inhibit synapses by raising excitation thresholds, making it harder for neurons to generate action potentials.

Special Characteristics of Synaptic Transmission

This part delves into the unique features of synaptic transmission including delays in signal propagation.

Unique Features of Synaptic Transmission

• Delayed synaptic transmission involves steps like transmitter release from presynaptic terminals, diffusion across the synapse, receptor binding on postsynaptic terminals, receptor activation increasing permeability leading to excitatory potential elevation.

• Understanding these steps helps comprehend synaptic delays which are essential for normal signal propagation efficiency.