the neurons

The Neuron

Types of Neurons

  • Dendrite: Branch-like structures that receive signals from other neurons.
  • Soma: The cell body of the neuron, containing the nucleus and organelles.
  • Axon: Long projection that transmits signals away from the soma to other neurons.
  • Axon Terminals: End points of the axon where neurotransmitters are released.
  • Node of Ranvier: Gaps in the myelin sheath that facilitate the rapid conduction of nerve impulses.
  • Myelin: Fatty substance that insulates the axon and speeds up signal transmission.
  • Nucleus: The central part of the neuron containing genetic material.

Neuron Morphology

  • Different neurons have distinctive shapes and structures, which can include:
    • Retinal Bipolar Cell (B)
    • Notable for its dendritic structure and direct connections.
    • Retinal Ganglion Cell (C)
    • Synapses information from bipolar cells and transmits it to the brain.
    • Neurons in Mesencephalic Nucleus of Cranial Nerve V (A)
    • Have characteristic axons and dendrites.
    • Cortical Pyramidal Cell (E)
    • Features a pyramid-shaped cell body with extensive dendritic trees.
    • Retinal Amacrine Cell (D)
    • Integrates visual information within the retina.
    • Cerebellar Purkinje Cells (F)
    • Known for their dense branching structure involved in motor function.

Concept Check: Dendritic Arborization

  • Question: A neuron with a larger dendritic arborization has greater/less/same synaptic inputs than one with a smaller dendritic arborization.
    • A. Greater (Correct)
    • B. Less
    • C. Same

Glial Cells and Their Functions

  • Astrocytes: Maintain the chemical environment around neurons.
  • Oligodendrocytes: Form myelin sheaths around central nervous system axons.
  • Schwann Cells: Form myelin sheaths around peripheral nervous system axons.
  • Microglia: Function as macrophages, involved in immune response in the central nervous system.

Neuronal Circuits

  • The brain transmits information between neurons to form circuits, analogous to computer circuits processing information.
  • The central nervous system functions as an electric organ.

Ions and Neuronal Electrical Properties

  • Ions: Charged atoms with positive (e.g., Na+, K+) or negative (e.g., Cl-) charge.
  • Semipermeable Membrane: Selectively allows specific ions to pass through designated ion channels.

Concept Check: Semipermeable Membrane

  • Question: A semipermeable membrane allows ____ species of ion (i.e., K+, Ca++, Na+, Cl-) to flow through the membrane.
    • A. All
    • B. Some (Correct)
    • C. None

Electrical Concepts

  • An electric signal represents a current, which requires an electric potential to move ions.
  • Ionic Current: The flow of charged ions across membranes.

Battery Analogy for Neurons

  • A Battery: An electrochemical cell that can store potential energy and release electrical charge when needed—neurons function similarly as batteries for transmitting signals.

Resting Potential

  • Definition: The neuron's inactive state, preparing for a nerve impulse with a negative internal electrical charge.
  • Mechanism:
    • Some K+ channels are open at rest.
    • Sodium-Potassium Pump: Moves Na+ out, creating a positive external charge, while moving K+ in, which contributes to the negative internal charge. This results in a resting potential typically around -70 mV.

Synaptic Potentials

  • EPSP (Excitatory Post-Synaptic Potential): Increases likelihood of action potentials.
    • Example: Involves Na+ and neurotransmitter glutamate leading to depolarization (e.g., -30 mV).
  • IPSP (Inhibitory Post-Synaptic Potential): Decreases likelihood of action potentials.
    • Example: Involves Cl- and neurotransmitter GABA leading to hyperpolarization.

Synaptic Summation

  • Integration of EPSPs and IPSPs:
    • Cancellation: Excitatory and inhibitory potentials may balance out depending on their combined effects.
    • Spatial Summation: Occurs when multiple neurons trigger a threshold point simultaneously.
    • Temporal Summation: Occurs when a single neuron provides multiple excitatory inputs triggering threshold.

Action Potential

  • Definition: A rapid rise and fall in membrane potential, leading to signal propagation.
  • Process:
    • Initiated when the summation of EPSPs and IPSPs achieves a critical threshold at the Axon Hillock.
    • Gated Ion Channels: Open in response to voltage change; Na+ channels open first, resulting in depolarization.
    • Repolarization occurs as Na+ channels close and K+ channels open, leading the cell's potential to briefly dip below resting state.
    • During the Absolute Refractory Period, Na+ channels remain closed preventing further action potentials, while K+ channels are still open.

Action Potential Phase Summary

  • Membrane Potential Dynamics:
    • Resting Potential: Approximately -70 mV
    • Threshold: Around -55 mV
    • Depolarization Peak: Around +30 mV
    • Repolarization: K+ exit leads to a return toward resting potential.
    • After-Hyperpolarization: Brief period where potential is lower than resting state due to excess K+ outside the cell.

Concept Check: Action Potential Phases

  • Question: The rising phase of the action potential is:

    • A. Depolarization (Correct)
    • B. Repolarization
    • C. Hyperpolarization

  • Question: Sodium (Na+) enters the cell in which phase of the action potential?

    • A. Depolarization (Correct)
    • B. Repolarization
    • C. Hyperpolarization

Action Potential Propagation

  • Action potentials travel down axons rapidly (up to 100 m/s) and require insulation for speed.
  • Myelin: Specialized insulating material formed by oligodendrocytes, promoting Saltatory Conduction, where nerve impulses jump from one Node of Ranvier to the next.

Concept Check: Myelin

  • Question: Myelin, axonal insulation, makes action potential speed:
    • A. Quicker (Correct)
    • B. Slower
    • C. Neither

Interneuronal Transmission

  • Neuron Communication: Involves presynaptic and postsynaptic neurons with specific structures:
    • Synaptic vesicles fuse with presynaptic membrane to release neurotransmitters into the synaptic cleft.
    • Postsynaptic receptors bind to neurotransmitters leading to further ion flow through postsynaptic channels.
    • Types of synapses include:
    • Electrical Synapses: Direct ionic current flow.
    • Chemical Synapses: Involve neurotransmitters for signaling.

Neurotransmitter Release Mechanism

  • Occurs across synapses where neurotransmitters are released into the synaptic cleft, and their effects depend on the specific receptors on the postsynaptic membrane.

Important Neurotransmitters

  • Acetylcholine (ACh): Critical at neuromuscular junctions; involved in memory. Blocked by substances like tubocurare.
  • Glutamate: Major excitatory neurotransmitter in the brain and spinal cord.
  • GABA (γ-Aminobutyric acid): Major inhibitory neurotransmitter, particularly in the brain.
  • Catecholamines (Dopamine and Norepinephrine): Can be excitatory or inhibitory; involved in attention and movement; linked to conditions like schizophrenia.
  • Serotonin (5-HT): Derived from tryptophan; plays roles in mood and sleep; SSRIs (e.g., Prozac) affect serotonin uptake.
  • Glycine: Major inhibitory neurotransmitter in the spinal cord.

Concept Check: Major Excitatory Neurotransmitter

  • Question: What is the major excitatory neurotransmitter in the brain?
    • A. Glycine
    • B. Acetylcholine
    • C. Glutamate (Correct)