Neurons: Cellular and Network Properties – Study Notes

Neurons: Cellular and Network Properties

Overview of Today's Lecture

This lecture covers the fundamental aspects of neurons and their cellular networks, focusing on the history of neuroscience, organization of nervous systems, the cells of the nervous system, components of neurons and glial cells, and neuronal signaling. These topics are essential for understanding how the nervous system functions at both cellular and network levels.

• Chapter 8: Neurons and Cellular Networks

• History of Neuroscience

• Organization of Nervous Systems

• Cells of the Nervous System

• Components of Neurons and Glia Cells

• Neuronal Signaling

History of Neuroscience

Development of Neuron Doctrine

The neuron doctrine is a foundational concept in neuroscience, stating that neurons are discrete, individual cells that communicate via specialized connections. This principle was established through the work of scientists such as Santiago Ramón y Cajal and Camillo Golgi.

• Santiago Ramón y Cajal: Demonstrated that neurons are separate cells, not a continuous network.

• Camillo Golgi: Developed the silver staining technique, allowing visualization of individual neurons.

• Neuron Doctrine: Neurons are discrete units; information flows in one direction: Input → Integration → Output.

Example: The case of Phineas Gage illustrated the role of the frontal lobes in personality and behavior.

Organization of the Nervous System

Central and Peripheral Divisions

The nervous system is organized into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct structures and functions.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for integration and processing of information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; divided into sensory (afferent) and motor (efferent) divisions.

• Autonomic Nervous System: Subdivision of the PNS; controls involuntary functions and is further divided into sympathetic and parasympathetic branches.

• Enteric Nervous System: Network of neurons in the digestive tract; can function independently but is regulated by the autonomic nervous system.

Example: Sensory neurons carry information from the nose to the CNS, while motor neurons control muscle movement.

Cells of the Nervous System

Neurons and Glial Cells

The nervous system consists of two main cell types: neurons, which transmit electrical signals, and glial cells, which provide support and protection.

• Neurons: Functional units of the nervous system; classified by structure (multipolar, bipolar, anaxonic) and function (sensory, motor, interneurons).

• Glial Cells: Support neurons; types include Schwann cells (PNS), oligodendrocytes (CNS), astrocytes (CNS), and microglia (CNS).

Components of Neurons

Neurons have specialized structures for receiving and transmitting signals.

• Cell Body (Soma): Contains the nucleus; responsible for gene expression and metabolism.

• Dendrites: Receive incoming signals; often have dendritic spines to increase surface area.

• Axon: Transmits outgoing electrical signals; may be myelinated for faster conduction.

• Axon Hillock: Region where action potentials are initiated.

• Synapse: Junction between neurons where neurotransmitters are released.

Glial Cells and Myelination

Glial cells play crucial roles in supporting neurons and facilitating signal transmission.

• Schwann Cells (PNS): Form myelin sheaths around axons; each cell myelinates a segment of one axon.

• Oligodendrocytes (CNS): Myelinate multiple axons; increase speed of action potential conduction.

• Nodes of Ranvier: Gaps in myelin sheath; facilitate rapid signal transmission.

• Astrocytes: Maintain the environment around neurons; regulate ion concentrations and neurotransmitter levels.

• Microglia: Act as immune cells in the CNS.

Example: Multiple sclerosis is an autoimmune disease where myelin is attacked, impairing signal transmission.

Neuronal Signaling

Electrical Signals in Neurons

Neurons communicate via electrical signals, including graded potentials and action potentials.

• Graded Potentials: Variable strength; used for short-distance communication.

• Action Potentials: Brief, large depolarizations; all-or-none events for long-distance signaling.

Ion Channels and Membrane Potential

Ion channels regulate the movement of ions across the neuronal membrane, influencing membrane potential.

• Types of Ion Channels: Mechanically gated, chemically gated, and voltage-gated channels.

• Channel Selectivity: Determined by pore size and amino acid composition; allows passage of specific ions.

• Resting Membrane Potential: Maintained by concentration gradients and membrane permeability.

Key Equations

The Nernst and Goldman-Hodgkin-Katz (GHK) equations are used to calculate membrane potentials.

• Nernst Equation: Predicts the equilibrium potential for a single ion.

$E_{ion} (in\ mV) = 61 \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

• GHK Equation: Calculates membrane potential considering multiple ions.

$V_m = 61 \log \left( \frac{P_K[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_K[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}} \right)$

Example: At rest, potassium permeability dominates, so the resting potential is closest to the potassium Nernst potential.

Comparison of Graded and Action Potentials

Graded and action potentials differ in their properties and roles in neuronal signaling.

Property	Graded Potential	Action Potential
Strength	Variable	All-or-none
Distance	Short	Long
Signal Type	Depolarizing or hyperpolarizing	Always depolarizing
Channels Involved	Chemically, mechanically, or voltage-gated	Voltage-gated

Additional info:

• Adult neurogenesis occurs in specific brain regions, such as the olfactory bulb and hippocampus.

• Glial cells can divide mitotically and are a main source of brain tumors.