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Neurons, Synapses, and Signaling Overview

  • Title: Neurons, Synapses, and Signaling

  • Authors: Kathleen Fitzpatrick, Simon Fraser University; Nicole Tunbridge, Kwantlen Polytechnic University

  • Copyright: Pearson Education, Inc.

Introduction to Neurons

  • Neurons: Basic units of the nervous system that transfer information in the body.

  • Function: Evolved to exemplify the relationship between structure and function.

Neuron Structure and Function

General Structure

  • Cell Body: Contains most organelles.

  • Dendrites: Branched extensions that receive signals from other neurons.

  • Axon: Long extension responsible for transmitting signals; includes the axon hillock, where action potentials are initiated.

  • Synaptic Terminals: Ends of axon branches that transmit signals to other neurons at synapses.

Types of Neurons

  • Sensory Neurons: Transmit sensory input.

  • Interneurons: Integrate (analyze and interpret) sensory input.

  • Motor Neurons: Transmit signals to muscle cells, causing contractions.

Supporting Cells (Glia)

  • Functions of Glia: Nourish neurons, insulate axons, regulate surrounding fluids, and (in some cases) transmit information.

  • Examples of Glial Cells: Astrocytes, oligodendrocytes, and Schwann cells.

Information Processing in the Nervous System

Stages of Information Processing

  1. Sensory Input: Gathering information from external/internal stimuli.

  2. Integration: Processing input in the central nervous system (CNS).

  3. Motor Output: Responses including muscle contractions or gland activities.

Organization of Neurons

  • Central Nervous System (CNS): Composed of the brain and spinal cord.

  • Peripheral Nervous System (PNS): Comprises nerves extending to the rest of the body.

Membrane Potentials

Resting Potential

  • Definition: Resting potential is the membrane voltage of a neuron not actively sending signals.

  • Ion Concentration: Potassium (K+) is higher inside the cell; Sodium (Na+) is higher outside the cell.

  • Ion Pumps: Sodium-potassium pumps maintain ion gradients using ATP.

Changes in Membrane Potential

  • Action Potentials: Rapid changes in membrane potential that allow signal transmission.

  • Ion Channels: Allow selective ion flow across the membrane, contributing to membrane potential changes.

Action Potentials

Mechanisms

  • Initiation: Begins at the axon hillock when the threshold is reached.

  • Phases of Action Potentials:

    • Rising Phase: Depolarization occurs due to the influx of Na+.

    • Falling Phase: Repolarization occurs as K+ exits the cell.

    • Undershoot: Hyperpolarization can occur as K+ channels remain open.

Implications for Signal Transmission

  • Action potentials propagate in one direction toward synaptic terminals and have a fixed magnitude.

Synaptic Communication

Synapse Structure

  • Chemical Synapses: Use neurotransmitters to transmit signals across the synaptic cleft.

  • Electrical Synapses: Allow direct current flow between neurons via gap junctions.

Neurotransmitter Action

  • Binding: Neurotransmitters bind to postsynaptic receptors affecting the potential.

  • Types of Postsynaptic Potentials:

    • EPSP (Excitatory Postsynaptic Potential): Depolarizes membrane, moves closer to threshold.

    • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes membrane, moves away from threshold.

Types of Neurotransmitters

  • Acetylcholine: Functions in muscle contraction and neurotransmission.

  • Amino Acids: Glutamate (excitatory), GABA (inhibitory).

  • Biogenic Amines: Norepinephrine, dopamine, and serotonin play various roles in mood and behavior.

  • Neuropeptides & Gases: Serve as neurotransmitters, affecting various physiological functions.

Evolutionary Adaptations in Nervous System

  • Myelination: Increases conduction speed of action potentials in vertebrates through insulating myelin sheath.

  • Centralization: Development of complex structures like brains that enhance integration and signal processing.

Conclusion

  • The organization and function of neurons and synapses exemplify the intricate relationship between structure and function in biological systems, which are crucial for both basic survival and complex behaviors.