Lecture 3 - Neurons and Neuronal Transmission_9f1745ddb79c7bd239807ecaded65a9f

Week 6 Overview





Date: Friday, 15th November 2024Course: PSYC112/132: Introduction to NeuroscienceInstructor: Dr. Abigail FiskeContact: a.fiske@lancaster.ac.ukDiscussion Topic: Can psychology exist without neuroscience?

Contact Information

Questions, comments, or concerns can be addressed via:

  • Email: a.fiske@lancaster.ac.uk

  • Office: Fylde C42

  • Communication Options: Book a meeting or message on Microsoft Teams, or post in the Discussion Forum for collaborative inquiries.

Lecture Focus


Title: Neurons and Neuronal TransmissionObjectives:

  • Identify and describe the parts of a neuron and their specific functions.

  • Understand the intricacies of the process of neural conduction.

  • Explore the mechanisms of synaptic transmission in detail.

  • Achieve a foundational understanding of neuronal anatomy and the processes of signal transmission, which are pivotal in understanding psychological phenomena.

Importance of Neurons

Neurons are fundamental building blocks of the brain and the entire nervous system, acting as the primary units responsible for processing and transmitting information. A clear understanding of neuronal information flow is essential, as it directly contributes to insights into complex human cognitive functions, including thoughts, emotions, and behaviors.

Cells of the Nervous System


Neurons:Specialized nerve cells designed for receiving, conducting, and transmitting electrochemical signals throughout the nervous system. The human brain contains an estimated 86 billion neurons, all performing varied yet crucial roles in communication within the nervous system.

Anatomy of Neurons

  • Cell Membrane: Surrounds every neuron, featuring selective permeability to ions and molecules.

  • Soma (Cell Body): Acts as the metabolic center, housing the nucleus and organelles essential for cellular functions.

  • Dendrites: Tree-like branching structures that receive information from other neurons, increasing the surface area for synaptic connections.

  • Axon: A long, narrow projection that conducts signals away from the soma to other neurons, muscles, or glands, and is often insulated by a myelin sheath to improve signal transmission.

  • Axon Hillock: The integral junction between the axon and the cell body, where action potential is initiated.

  • Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission through saltatory conduction.

Types of Neurons

  1. Bipolar Neurons:

    • Contain one axon and one dendrite.

    • Primarily located in sensory systems such as the retina and olfactory system.

  2. Pseudounipolar Neurons:

    • Have a single axon with no dendrites; primarily serve as sensory neurons.

  3. Multipolar Neurons:

    • Feature one axon and two or more dendrites; they include motor neurons responsible for muscle contraction.

  4. Interneurons:

    • Act as connectors between motor and sensory neurons, playing a crucial role in reflex circuits and signaling within the central nervous system.

Glial Cells

These non-neuronal cells serve critical supporting roles in the nervous system by:

  • Holding neurons in place to maintain structure.

  • Supplying essential nutrients and oxygen to neurons.

  • Insulating and protecting neurons from damage through several mechanisms.

  • Destroying and removing dead neurons, thus maintaining the health of the nervous system.

Types of Glial Cells

  • Astrocytes: Star-shaped cells that form the blood-brain barrier, regulating the passage of substances between the bloodstream and the brain.

  • Oligodendrocytes: Produce myelin sheaths in the central nervous system for enhanced nerve signal transmission.

  • Microglia: Serve as the brain's immune cells, responding to injury, infection, and disease.

  • Ependymal Cells: Line the spinal cord and brain ventricles, playing a role in the circulation of cerebrospinal fluid.

  • Schwann Cells: Similar to oligodendrocytes but located in the peripheral nervous system, facilitating signal transmission for peripheral nerves.

Cell Membrane Structure

The cell membrane consists of a lipid bilayer embedded with proteins, crucial for neuronal function:

  • Channel Proteins: Facilitate the passage of specific ions and molecules.

  • Signal Proteins: Transmit signals when bound by certain molecules, playing a critical role in neuronal communication.

Neural Conduction

Signal Transmission Through a Neuron

  • Signal Reception: Dendrites receive signals through receptors.

  • Neuron Activation: Changes in the electrical potential are interpreted by the soma, activating the neuron.

  • Action Potential Generation: If the signal strength reaches a certain threshold at the axon hillock, an action potential is generated.

  • Myelin Effects: Enhances speed of signal transmission and supports saltatory conduction, allowing faster communication along the axon.

  • Neurotransmitter Release: Following impulse travel, neurotransmitters are released from axon terminals into the synapse for communication with downstream neurons.

Resting Membrane Potential

This is defined by the electrical difference across the cell membrane when inactive, typically around -70mV (with a range of -50 to -100mV). This potential is influenced by:

  • High sodium (Na+) concentration outside the cell.

  • High potassium (K+) concentration inside the cell.

  • Maintained by the sodium-potassium pump, which exchanges 2 K+ ions in, and 3 Na+ ions out per molecule of ATP, essential for maintaining resting potential.

Postsynaptic Potentials (PSPs)

These are disturbances of the resting membrane potential:

  • EPSP (Excitatory Postsynaptic Potential): Represents depolarization (less negative), increasing the likelihood of neuronal firing.

  • IPSP (Inhibitory Postsynaptic Potential): Corresponds to hyperpolarization (more negative), decreasing the likelihood of neuronal firing.

  • Properties: PSPs are graded potentials, which means their amplitude is proportional to the strength of the incoming signal and they are rapidly decremental over distance.

Summation

Types of Summation

  • Spatial Summation: Occurs when simultaneous contributions from different locations on the membrane combine to produce a significant effect.

  • Temporal Summation: Involves rapid successive signals at a single synapse, leading to cumulative effects.

Action Potentials

Characteristics:

  • Action potentials are massive and momentary (~1ms).

  • They involve a reversal of membrane potential from -70mV to +50mV.

  • Initiated upon reaching depolarization threshold, facilitating the all-or-nothing response of neurons.

Stages of Action Potential

  1. Depolarization: Sodium channels open, allowing Na+ ions to enter.

  2. Propagation: The action potential continues down the axon, jumping between nodes of Ranvier in myelinated axons.

  3. Repolarization: Sodium channels close, potassium channels open, returning the membrane potential to a negative value.

  4. Hyperpolarization: Potassium channels may remain open longer, making the membrane potential more negative until they eventually close.

Synaptic Transmission


Definition:A synapse is a connection point for neuron communication, allowing for the transfer of electrical (action potentials) or chemical signals (neurotransmitters).

Types of Synapses

  • Axodendritic: Most common synapse type, occurring between the axon terminal and dendrite.

  • Axosomatic: Occurs between the axon terminal and cell body.

  • Axomyelenic: Between axon terminal and myelin sheath.

Process of Synaptic Transmission

  1. Action Potential Arrival: The action potential reaches the presynaptic terminal.

  2. Voltage-gated Calcium Channels: Open in response to depolarization, allowing Ca2+ to flow into the terminal.

  3. Neurotransmitter Release: Ca2+ promotes the fusion of synaptic vesicles with the membrane, allowing neurotransmitters to be released into the synaptic cleft.

  4. Receptor Binding: Neurotransmitters bind to postsynaptic receptors, initiating a specific response in the postsynaptic neuron.

  5. Termination of Signal: Neurotransmitter removal from the synaptic cleft is crucial to prevent continuous signaling, accomplished through reuptake, enzymatic degradation, or diffusion.

Conclusion

Homework: Read Chapter 4 of the textbook and review optional videos associated with Lecture 3. Prepare for Lecture 4 scheduled for the upcoming Wednesday, ensuring comprehension of the intricate roles neurons play in behavior and psychology.