Neurons: Physiology Part 2

Chapter 1: Introduction to Neurons and Nervous System

Three open lab sessions before the lab practical to provide students with hands-on experience.
Schedule: These sessions occur on Fridays, Mondays, and Tuesdays to accommodate various schedules.
Location: Held in the same room as the laboratory to ensure familiarity with the environment.
Purpose: These sessions serve as an excellent opportunity for students to review key concepts and material covered in previous lab sessions, facilitating deeper understanding through practical application.

Neurons: The fundamental functional and structural units of the nervous system responsible for transmitting information throughout the body. They play a crucial role in processing and relaying signals.
Critical for Creating Functionality: Neurons enable complex functions such as cognition, sensation, and motor control.
Obligatory Partner: The other important cell type in the nervous system is glial cells, often referred to as supporting cells or "nerve glue". These cells provide structural support, nourishment, and protect neurons.

Action Potentials: Neurons possess the remarkable ability to perform action potentials, which are transient changes in membrane potential that lead to signal transmission.
Energy Requirements: Neurons necessitate a substantial supply of oxygen and high metabolic glucose to maintain their functionality, emphasizing their high energy demand.
Amitotic Nature: Neurons are classified as amitotic, meaning they do not replicate, which significantly limits their ability to heal following injury.

Organelle Housing: The cell body of neurons houses vital organelles, including the nucleus, which contains genetic material, the endoplasmic reticulum, which synthesizes proteins, and mitochondria, responsible for energy production.
Integration of Information: Neurons integrate sensory data and transform it into motor instructions through the interaction between their dendrites (which receive input) and axons (which send output).

Processes:
- Dendrites: Specialized structures designed to receive and transmit incoming information towards the cell body, increasing the neuron's ability to connect and communicate.
- Axon: A long, slender projection that conducts electrical impulses away from the cell body to other neurons or effector organs.
- Graded Potentials: Represent smaller, localized changes in membrane potential that neurons receive through their dendrites, initiating the signaling process.

Afferent Pathways: These sensory pathways convey information to the central nervous system (CNS), playing a crucial role in sensory perception.
Efferent Pathways: Motor pathways that transmit instructions away from the CNS to effectors such as muscles and glands, coordinating responses to stimuli.
Signaling Connections: Signals can connect not only to neighboring neurons but also to central structures or effector organs, allowing for complex integration and response mechanisms.

Includes three muscle types: smooth, cardiac, and skeletal, each with distinct functions in the body.
Also includes two gland types: endocrine glands that release hormones into the bloodstream and exocrine glands that secrete substances through ducts.

Role: Myelin sheaths encase axons, significantly enhancing the speed of electrical signal transmission while insulating the axon from surrounding tissues.
Dendrites: Not myelinated, generally resulting in slower signal transmission compared to myelinated axons.
Schwann Cells: Specialized glial cells that form the myelin sheath in the peripheral nervous system, critical for maintaining efficient signal propagation.
Nodes of Ranvier: Gaps in the myelin sheath that allow for rapid sodium diffusion, facilitating quick action potential propagation along the axon.
Neurolemma: The outer layer of the axon sheath that aids in the regeneration of nerves and electrical signal transmission.

Mechanically Gated Channels: Open in response to physical distortion or force applied to the neuron's membrane, allowing ions to flow in or out.
Chemically/Ligand Gated Channels: Open when specific chemical ligands (such as neurotransmitters) bind to their receptors, functioning like a lock and key mechanism.
Voltage Gated Channels: Open at particular voltage thresholds; these channels are essential for the generation of action potentials, crucial for neural communication.

Value: The resting membrane potential is set at -70 mV, which reflects a state of neutrality in neuronal activity.
Polarization: This potential is polarized due to the differential distribution of sodium (Na⁺) and potassium (K⁺) ions: Na⁺ is concentrated outside, while K⁺ is concentrated inside the cell, contributing to the overall charge difference.

Initiation: Triggered by a stimulus that leads to the opening of voltage-gated sodium channels, allowing sodium ions to rush into the neuron, increasing the intracellular positive charge up to +30 or +35 mV.

Process: Characterized by the closing of sodium channels and the opening of potassium channels, allowing potassium ions to exit, restoring the negative membrane potential.
Sodium-Potassium Pump: Activates to return the neuron to its original ionic conditions, playing a key role in maintaining the resting potential.

Occurrence: May occur when the membrane potential dips below the resting level of -70 mV, typically due to excess potassium efflux.
Resolution: This condition is quickly corrected by the sodium-potassium pump, which works to reset the resting potential.

Definition: The minimum membrane charge required to trigger an action potential; it's a critical concept for understanding neuronal firing.

Mechanism: Action potentials either occur in full or not at all; this principle underscores the binary nature of neural signaling.

Timeframe: A designated time during which a neuron cannot fire again until it has returned to its resting potential, ensuring proper signal transmission.

Graded Potentials: Localized changes in membrane potential that can be either depolarizing or hyperpolarizing.
Action Potentials: Full depolarization events that necessitate reaching a specific threshold to occur, leading to significant cellular responses.

Myelination: The presence of myelin significantly accelerates the transmission of electrical signals along axons.
Axon Diameter: Larger axon diameters correlate with increased transmission rates, allowing signals to travel more efficiently.

Understanding these principles is fundamental as students prepare to explore the intricacies of synaptic functioning in subsequent lectures.