Study Notes on Nervous System and Action Potentials

Overview of Nervous Tissue

  • Discussion of foundational elements of the nervous tissue.

  • Focus for the semester will include both central and peripheral nervous systems.

Structure of the Nervous System

Major Topics Covered

  • Basic neural arc.

  • Aspects of the peripheral nervous system (PNS) and central nervous system (CNS).

  • Chapters dedicated to:

    • CNS

    • PNS

Nervous System Components

  • Somatic nervous system (voluntary control).

  • Autonomic nervous system (involuntary control) consists of two subdivisions:

    • Sympathetic and parasympathetic systems.

  • Importance of visual flowchart for dynamic understanding of nervous system operations.

Histology of Nervous Tissue

  • Introduction to six types of neuroglial cells:

    • Four glial cells in CNS:

    • Astrocytes

    • Microglia

    • Oligodendrocytes

    • Ependymal cells

    • Two glial cells in PNS:

    • Schwann cells

    • Satellite cells

  • Overview of basic neuron structure.

Resting Membrane Potential

  • Definition and values:

    • Resting membrane potential (RMP) is the voltage difference across a neuronal plasma membrane at rest (typically -70 mV).

    • Inside of the cell is negative relative to the outside (negative sign indicates internal charge).

  • Variability in RMP:

    • Ranges from -40 mV to -90 mV in different types of neurons.

Factors Generating Resting Membrane Potential

  • Ionic Composition: High potassium (K+) concentration inside cell and high sodium (Na+) concentration outside cell.

  • Membrane Permeability: More K+ leakage channels than Na+ channels lead to a faster diffusion of K+ out of the cell, which establishes RMP.

Types of Electrical Signals

Graded Potentials

  • Occur over short distances (e.g. between dendrites and soma).

  • Strength is variable depending on the intensity of the stimulus; can summate to influence action potential generation.

Action Potentials

  • Long-distance signals that maintain consistent strength (following the all-or-nothing principle).

  • Triggered when graded potentials reach a certain threshold level.

  • Role in cell-to-cell communication.

Comparison of Potentials

  • Resting Membrane Potential: Baseline state of the neuron.

  • Graded Potentials: Short-range signals influencing RMP; inputs that can summate.

  • Action Potentials: Long-range signals (outputs) propagated along axons.

Ion Channels

Types of Ion Channels

  • Leakage Channels: Maintain resting membrane potential.

  • Gated Channels: Facilitate rapid changes in membrane potential (include subtypes):

    • Chemical (Ligand) Gated Channels: Open in response to specific neurotransmitters (e.g. acetylcholine).

    • Voltage Gated Channels: Open upon changes in membrane potential (critical for action potentials).

    • Mechanically Gated Channels: Open due to physical deformation (important for sensory reception).

Propagation of Action Potentials

Stages of Action Potential

  1. Resting State: All voltage-gated channels closed, RMP at -70 mV.

  2. Depolarization: Sodium channels open; Na+ enters cell, making membrane potential less negative (more positive).

  3. Repolarization: Potassium channels open; K+ exits cell, returning potential to a more negative value.

  4. Hyperpolarization: Membrane potential becomes more negative than RMP due to prolonged K+ exit.

Refractory Periods

  • Absolute Refractory Period: No second action potential can be generated; sodium channels are inactivated.

  • Relative Refractory Period: A stronger-than-normal stimulus is required to generate an action potential; sodium channels are still resetting.

Conduction Velocity of Action Potentials

Factors Influencing Velocity

  • Axon Diameter: Larger diameter = faster conduction due to reduced resistance.

  • Myelination: Myelinated fibers conduct impulses faster than unmyelinated fibers. Myelin prevents ion leakage and facilitates rapid signal propagation.

Synapses

Definitions

  • Synapse: Connector between neurons; can occur between neurons or between neurons and glands.

  • Chemical Synapses: Use of neurotransmitters for one-way communication across synaptic cleft.

  • Electrical Synapses: Utilizes gap junctions for ion flow directly between cells; allows for bidirectional communication, faster transmission.

Synaptic Cleft

  • Area between presynaptic and postsynaptic cells (20-40 nanometers).

  • Neurotransmitters diffuse across this gap to convey signals.

  • Synaptic Delay: Approximately 0.5 milliseconds due to calcium entry, neurotransmitter release, diffusion, and receptor binding.

Postsynaptic Potentials

Types of Postsynaptic Potentials

  • Excitatory Postsynaptic Potential (EPSP): Depolarization caused primarily by sodium (Na+) influx, bringing neuron closer to threshold.

  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization caused by potassium (K+) exiting or chloride (Cl-) entering, moving neuron further from threshold.

Neurotransmitters

Classification

  • Acetylcholine: Major neurotransmitter at the neuromuscular junction; can be excitatory or inhibitory based on receptor type.

    • Breakdown by acetylcholinesterase.

  • Biogenic Amines: Include dopamine and serotonin related to mood regulation; norepinephrine related to arousal.

  • Peptides: Chains of amino acids (e.g., endorphins for pain relief).

  • Purines: e.g., adenosine triphosphate (ATP) modulating CNS activity.

  • Gases and Lipids: e.g., nitric oxide, which can diffuse freely and influence signal transmission.

Conclusion

  • Understanding neurotransmitters is essential for grasping their roles in various physiological systems and diseases.