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Nervous Tissue and Membrane Potentials #2

Nervous Tissue

Overview of Nervous System Cells

  • The nervous system is an excitable system serving as a controlling mechanism for other systems.

  • It senses changes within the internal and external environments and adjusts accordingly.

Objectives

  • Learn the structure of the neuron.

  • Understand membrane potential and factors that change it.

  • Understand how ion movement across a membrane affects membrane potential.

  • Comprehend graded and action potentials and their relationship in neuronal activity control.

  • Understand synaptic actions.

Types of Nervous Tissue Cells

Neurons

  • Neurons are excitable cells responsible for initiating and transmitting nerve impulses.

Glial Cells

  • Glial cells are non-excitable cells providing support and protection to neurons.

Types of Glial Cells:

  • Astrocytes

  • Microglia

  • Schwann cells

  • Oligodendrocytes

  • Ependymal cells

Structure of Neurons

Components of a Neuron

  • Dendrites

  • Cell body

  • Axon hillock

  • Axon

  • Neurofibril nodes (Nodes of Ranvier)

  • Myelin sheath

  • Telodendria

  • Synaptic knobs

Myelination Process

  1. Neurolemmocyte begins wrapping around an axon segment.

  2. The cytoplasm and plasma membrane of the neurolemmocyte form consecutive layers around the axon.

  3. Overlapping layers create the myelin sheath.

  4. The cytoplasm and nucleus of the neurolemmocyte are pushed peripherally as the sheath forms.

Membrane Potential

Definition

  • Membrane potential refers to a separation of charges across a cell membrane. In neurons, the typical measurement is about -70\, \text{mV}.

Ion Distribution and Membrane Potential

  • Neurons have high concentrations of potassium (K+) inside and sodium (Na+) and chloride (Cl-) outside, creating an imbalance maintained by the sodium-potassium pump.

  • Inside the cell, negative ions such as proteins and amino acids (A-) are present, while calcium ions (Ca2+) are found outside.

Salty Banana Analogy

  • Conceptual aid for remembering the distribution of ions:

    • High K+ inside

    • Low Na+ and Cl- outside

K+ Flow and Membrane Potential

  • The concentration gradient for K+ results in K+ flowing out of the cell through leak channels, creating a potential.

  • For each K+ that exits, an unpaired negative charge remains, creating an electrochemical gradient that pulls K+ back into the cell.

Equilibrium Potential

  • The point where K+ movement out due to concentration gradient equals the movement back in due to electrochemical gradient is its equilibrium potential.

  • Calculated via the Nernst equation:
    Vm = 61.5 \log\left(\frac{[\text{conc. Outside}]}{[\text{conc. Inside}]}
    ight)

  • For K+, this results in an equilibrium potential of about -92\, \text{mV}.

Resting Membrane Potential

  • If only K+ was considered, resting membrane potential would be -92\, \text{mV}; however, the sum of effects from other ions yields a final resting potential around -70\, \text{mV}.

Types of Membrane Potentials

  • Membrane potential terms apply to all cells, but for excitable cells, terms must reflect membrane voltage based on cellular activity states.

Resting Membrane Potential

  • Defined state when a neuron is not influenced by outside factors, maintaining a potential of approximately -70\, \text{mV}.

Deviations from Resting Membrane Potential

  • Changes in membrane voltage during neuron activity reflect different types of ion channel actions.

Ion Channels in Neurons

Types of Ion Channels

  • Mechanically gated: Open due to membrane distortion.

  • Chemically gated: Opened by specific chemicals (neurotransmitters); primarily located in dendrites and cell bodies.

  • Voltage gated: Activated by changes in membrane potential; located along axons.

Graded Potentials

Mechanism

  • When chemically gated channels open, K+ moves out and Na+ enters, changing the resting membrane potential slightly.

  • Graded potentials can fluctuate based on the intensity and duration of stimuli.

Action Potential

  • Significant fluctuation in membrane voltage involving steep depolarization followed by repolarization.

  • Action potentials can be observed in muscle cells, endocrine cells, and neurons.

Mechanism of Action Potential Generation

  • Multiple membrane channels facilitate ion movement, including sodium and potassium voltage-gated channels.

  • Action potentials operate under an all-or-none principle: If the threshold is reached, full action potential occurs; if not, nothing happens.

Sodium Voltage-Gated Channels

  • Open at -55\, \text{mV}, allowing Na+ influx, leading to rapid membrane potential depolarization to about +30\, \text{mV}.

  • Inactivation proteins close these channels, preventing Na+ entry until they reset.

Potassium Voltage-Gated Channels

  • Begin to open at +30\, \text{mV}, allowing K+ to exit, causing rapid repolarization.

  • They lack an inactivation protein, leading to a hyperpolarization phase as more K+ exits than necessary to return to resting potential.

Relationships Among Various Potentials

Interaction Between Potentials

  • Resting, graded, and action potentials work in conjunction to influence neuron function.

Importance of Axon Hillock

  • Integrates graded potentials and determines whether an action potential occurs based on the cumulative effect of neurotransmitters influencing the membrane potential.

Summary of Action Potential Generation

  • Neuron rests at -70\, \text{mV} due to charge separation.

  • Influence from other neurons causes graded potentials that may reach threshold at the axon hillock.

  • Upon reaching threshold, sodium channels open, leading to the rapid depolarization-repolarization process that propagates along the axon.