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Videoaula 4 Potencial de ação
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Videoaula 4 Potencial de ação

Understanding Action Potentials in Neurons

Introduction to Fugu and Toxins

  • The speaker introduces the fugu fish, known for its ability to inflate when threatened, which serves as a defense mechanism.
  • In Japan, fugu is considered a delicacy but contains a lethal toxin; improper preparation can lead to death.
  • There’s a tradition where younger individuals eat first to ensure safety before older diners partake.

Basics of Resting Membrane Potential

  • The lecture transitions into cellular biology, explaining resting membrane potential as the difference in electric charge across the cell membrane.
  • Intracellular fluid has more negative charges compared to extracellular fluid due to sodium and potassium ion gradients maintained by the sodium-potassium pump.
  • Cells called excitable cells (e.g., neurons and muscle cells) can alter their membrane potential through gated channels.

Generation of Action Potentials

  • The focus shifts to action potentials, defined as rapid changes in membrane potential that occur in response to stimuli.
  • Neurons communicate via neurotransmitters that bind to receptors on adjacent neurons, leading to depolarization through sodium influx.

Propagation of Graded Potentials

  • As depolarization occurs, it creates waves that spread through the neuron but lose strength over distance due to leakage of ions.
  • These graded potentials can vary in amplitude and may also include hyperpolarization events.

Threshold and Action Potential Initiation

  • When graded potentials reach a specific region (axon hillock), they must exceed a threshold (around -55 mV for this neuron).
  • If reached, an action potential is triggered—a significant change in membrane potential that can peak at +30 mV.

Mechanism Behind Action Potential

  • The process begins with neurotransmitter-induced opening of sodium channels leading to further depolarization.
  • This positive feedback loop continues until reaching peak voltage; then sodium channels close while potassium channels open for repolarization.

Understanding Action Potentials in Neurons

Mechanism of Action Potentials

  • As voltage-dependent channels close, potassium-selective channels open, allowing potassium ions to exit the cell and restore the membrane potential towards negative values.
  • The outflow of potassium leads to repolarization, but due to slow channel kinetics, hyperpolarization occurs as the membrane potential dips below resting levels.
  • After repolarization, all potassium channels eventually close, returning the membrane potential to its resting state of approximately -70 mV.
  • The entire action potential process occurs rapidly within milliseconds; it can be divided into phases: depolarization (rising phase), repolarization (falling phase), and hyperpolarization (below resting level).
  • Understanding each phase is crucial; during depolarization, sodium channels open allowing sodium influx, while during repolarization, sodium channels close and potassium channels open.

Characteristics of Action Potentials

  • The action potential follows an "all-or-nothing" principle; if a threshold is not reached at the trigger zone (-55 mV), no action potential will occur.
  • Once initiated, an action potential cannot be reversed; it either happens fully or not at all.
  • There are two refractory periods: absolute (no new action potentials can occur) and relative (new potentials can occur but require a stronger stimulus).
  • Sodium channels have two gates: activation and inactivation. When activated by depolarization, they allow ion flow but become temporarily blocked by inactivation after opening.
  • During the absolute refractory period, most sodium channels are inactive. In contrast, during the relative refractory period some closed sodium channels are available for activation.

Implications of Refractory Periods

  • During the absolute refractory period, no amount of stimulation will generate another action potential due to inactive sodium channels.
  • In the relative refractory period, a sufficient number of closed sodium channels may allow for a new action potential but with reduced amplitude compared to previous ones.
  • The peak amplitude during this period is lower because not all sodium channels have recovered from their previous activation state.
  • It’s essential that stimuli must be stronger if applied early in this phase to reach threshold for generating an action potential again.
  • As time progresses through the relative refractory period, less intense stimuli can successfully initiate another action potential.

Propagation of Action Potentials

Understanding Action Potentials in Neurons

The Role of Action Potentials

  • Action potentials are crucial for neuron communication, allowing the transmission of information to other neurons and cells, such as muscle cells.
  • The generation of action potentials is referred to as the "nervous impulse," which serves as the language of neurons.
  • An example illustrates that an action potential can instruct a muscle to contract or increase contraction strength.

Tetrodotoxin and Its Effects

  • Tetrodotoxin, found in pufferfish, binds to voltage-gated sodium channels, blocking them and preventing action potentials from being generated.
  • Without action potentials, communication between cells ceases, leading to severe physiological consequences.

Characteristics of Action Potentials

  • Key characteristics include:
  • Follows the all-or-nothing principle.
  • Exhibits absolute and relative refractory periods.
  • Can propagate along membranes without losing magnitude or shape.

Propagation Mechanism

  • The propagation of action potentials occurs through a series of events initiated at the axon hillock when graded potentials reach a threshold.
  • Graded potentials must be strong enough to depolarize the membrane at the trigger zone (axon hillock), leading to an action potential if they exceed threshold levels.

Regeneration During Propagation

  • During an action potential, sodium channels open, causing depolarization. This excess positive charge can influence adjacent regions of the membrane.
  • Each segment along the axon regenerates its own action potential rather than one continuous wave traveling down the axon.

Unidirectional Propagation

  • Once an area has fired an action potential, it enters a refractory period where no new action potential can occur; thus propagation is unidirectional from trigger zone towards axon terminals.

Factors Influencing Action Potential Speed

Diameter and Speed Correlation

  • Larger axonal diameters lead to faster propagation speeds; for instance, giant squid axons can reach speeds up to 25 m/s due to their size.

Mammalian Axons

Understanding Action Potentials and Myelination

Mechanism of Action Potential Propagation

  • During an action potential, sodium ions enter the neuron through voltage-gated channels, while potassium ions can leak out through leakage channels. This leakage delays the depolarization of adjacent regions, reducing the speed of action potential propagation.
  • To increase membrane resistance and decrease ion leakage, insulating materials can be applied to axons. The lipid bilayer of cell membranes serves as a natural insulator.
  • In both peripheral and central nervous systems, Schwann cells and oligodendrocytes wrap their membrane projections around neuronal axons, forming an insulating layer known as myelin sheath that increases axonal resistance and prevents ion leakage.

Structure of Myelin Sheath

  • The myelin sheath does not cover the entire axon; there are small gaps called nodes (or nodules) between each Schwann cell or segment of oligodendrocyte, approximately 2 micrometers apart.
  • These nodes contain a high density of voltage-gated channels where action potentials are regenerated. Without myelin, positive charges would leak out through these gaps.

Saltatory Conduction

  • With myelination in place, positive charges travel quickly to the next node without significant leakage. This rapid depolarization regenerates the action potential almost instantaneously at each node along the axon.
  • This process is termed saltatory conduction; it appears as if the action potential is "jumping" from one node to another rather than continuously propagating along the entire length of the axon.

Speed Comparison in Axonal Conduction

  • Unmyelinated neurons conduct action potentials at speeds ranging from 0.15 to 2 meters per second. In contrast, myelinated neurons with larger diameters can reach speeds up to 120 meters per second.
  • The diameter of an axon directly influences conduction speed: thicker axons with more substantial myelin sheaths transmit impulses faster—up to over 400 km/h—comparable to a Formula 1 car's speed.

Threshold for Action Potential Generation

  • For an action potential to initiate at the trigger zone, sufficient depolarization must occur until reaching a threshold level. Once this threshold is met, voltage-gated sodium and potassium channels activate, allowing for continuous propagation without loss in signal integrity across segments of the membrane.
Playlists: Fisiologia Humana (Ensino Remoto Emergencial - UEM)
Video description

Na aula anterior, vimos que nas células em repouso há um excesso de cargas negativas no líquido intracelular (LIC) e um excesso de cargas positivas no líquido extracelular (LEC), o que acaba gerando uma diferença de potencial elétrico entre os dois lados da membrana, denominado potencial de membrana. No repouso o potencial de membrana (potencial de repouso) tem valores negativos, ou seja, o interior da célula tem mais carga negativa em relação ao exterior da célula. Nesta aula, veremos que muitos tipos celulares, que são considerados células excitáveis (ex. neurônios), podem sofrer uma grande alteração no potencial de membrana, saindo do estado de repouso, entrando agora eu um estado de ação. Portanto, nesta aula veremos com detalhes o potencial de ação. Ao assistirem a videoaula, tentem responder as seguintes questões: 1. Descreva as fases e o movimento de íons que ocorrem durante um potencial de ação. 2. Por que dizemos que o potencial de ação obedece a “lei do tudo ou nada”? 3. O que é período refratário absoluto e relativo? 4. Como é propagado o potencial de ação nos axônios não-mielinizados e mielinizados? 5. Quais os fatores que determinam a velocidade de propagação do potencial de ação? Antes de assistir a videoaula revise alguns conceitos: • Potencial de membrana: é a diferença de tensão elétrica ou diferença de potencial elétrico entre o lado intra e extracelular da membrana. • Potencial de repouso: é o potencial de membrana medido quando a célula está em repouso, ou seja, quando a célula excitável não está gerando sinais elétricos. • Células excitáveis: em resposta a estímulos químicos ou físicos, canais com portão podem se abrir e causar uma perturbação no potencial de membrana dessas células (despolarização ou hiperpolarização). Essas perturbações podem gerar sinais elétricos (potenciais de ação). Exemplo de células excitáveis: neurônios, células musculares, células secretoras de insulina, entre outros. • Despolarização: quando o potencial de membrana se torna mais positivo em relação ao potencial de repouso. • Hiperpolarização: quando o potencial de membrana se torna mais negativo do que o potencial de repouso. • Repolarização: quando o potencial de membrana retorna ao potencial de repouso. Material de apoio: • Para uma rápida revisão sobre as estruturas de um neurônio, acesse o link (https://www.youtube.com/watch?v=6qS83wD29PY) e assista ao vídeo (1 min 47 seg). É possível colocar legenda em português nas configurações do vídeo.