Neurons: Cellular and Network Properties – Study Notes

Chapter 8: Neurons and Cellular Networks

Overview

This chapter explores the structure and function of neurons, the fundamental units of the nervous system, and their organization into complex cellular networks. It covers the history of neuroscience, the organization of nervous systems, the cellular components of neurons and glial cells, and the mechanisms of neuronal signaling.

History of Neuroscience

Development of the Neuron Doctrine

• Neuron Doctrine: The concept that the nervous system is made up of discrete individual cells called neurons, rather than a continuous network.

• Santiago Ramón y Cajal: Demonstrated that neurons are separate cells with specialized structures (cell body, dendrites, axon). Awarded the Nobel Prize in Physiology or Medicine in 1906.

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

• Principle of Dynamic Polarization: Information flows in one direction within a neuron: Input → Integration → Output.

Example: The case of Phineas Gage illustrated the relationship between the frontal lobes and personality, providing early evidence for brain localization of function.

Organization of the Nervous System

Central and Peripheral Divisions

• Central Nervous System (CNS): Composed of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Consists of all neural tissue outside the CNS. Divided into sensory (afferent) and motor (efferent) divisions.

• Motor (Efferent) Division: Subdivided into the somatic motor (controls skeletal muscles) and autonomic (controls smooth muscle, cardiac muscle, glands) systems.

• Autonomic Nervous System: Further divided into sympathetic and parasympathetic branches.

• Enteric Nervous System: A network of neurons in the walls of the digestive tract, capable of autonomous function but regulated by the autonomic nervous system.

Cells of the Nervous System

Neurons

• Definition: Neurons are excitable cells that transmit electrical and chemical signals throughout the nervous system. They are the functional units of the nervous system.

• Structure: Consist of a cell body (soma), dendrites (receive signals), and an axon (transmits signals).

• Axon Hillock: The region where the axon originates from the cell body; important for action potential initiation.

• Synapse: The junction between two neurons where neurotransmitters are released to transmit signals.

Neuron Classification

• By Structure:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (sensory organs).

  • Anaxonic: No obvious axon (interneurons in CNS).

• By Function:

  • Sensory (Afferent) Neurons: Carry information to the CNS.

  • Motor (Efferent) Neurons: Carry commands from the CNS to effectors (muscles, glands).

  • Interneurons: Connect neurons within the CNS.

Glial Cells

• Schwann Cells (PNS): Form myelin sheaths around axons in the peripheral nervous system.

• Oligodendrocytes (CNS): Form myelin sheaths around axons in the central nervous system.

• Astrocytes (CNS): Support neurons, maintain the blood-brain barrier, regulate ion and nutrient concentrations.

• Microglia (CNS): Specialized immune cells that act as macrophages in the CNS.

Myelination

• Myelin Sheath: Insulating layer around axons that increases the speed of action potential conduction.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

• Multiple Sclerosis (MS): An autoimmune disease where the immune system attacks myelin, leading to impaired neural function.

Neural Regeneration and Neurogenesis

• Neuron Death: If the cell body dies, the neuron cannot regenerate.

• Axon Injury: If the axon is severed but the cell body survives, regeneration is possible in the PNS but limited in the CNS.

• Adult Neurogenesis: New neurons can be produced in certain brain regions (e.g., olfactory bulb, hippocampus) throughout life.

• Glial Cells and Cancer: Glial cells can divide mitotically, making them a common source of brain tumors.

Neuronal Signaling

Electrical Signals in Neurons

• Resting Membrane Potential: The electrical potential difference across the neuronal membrane at rest, typically around -70 mV.

• Nernst Equation: Predicts the equilibrium potential for a single ion: $E_{ion} = 61 \, \log \left( \frac{[ion]_{out}}{[ion]_{in}} \right) / z$ where $z$ is the charge of the ion.

• Goldman-Hodgkin-Katz (GHK) Equation: Calculates the membrane potential considering multiple ions: $V_m = 61 \log \left( \frac{P_K[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_K[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}} \right)$ Additional info: At rest, potassium permeability dominates, so the resting potential is closest to the potassium equilibrium potential.

Ion Channels and Membrane Permeability

• Types of Gated Channels:

  • Mechanically Gated (respond to physical deformation)

  • Chemically Gated (respond to ligands)

  • Voltage-Gated (respond to changes in membrane potential)

• Channel Selectivity: Determined by the size and charge of the pore and the amino acids lining it.

Types of Electrical Signals

• Graded Potentials: Variable-strength signals used for short-distance communication. Can be depolarizing or hyperpolarizing.

• Action Potentials: Large, brief depolarizations that propagate rapidly over long distances. They are all-or-none events.

Comparison of Graded and Action Potentials

Property	Graded Potential	Action Potential
Amplitude	Variable, depends on stimulus strength	All-or-none, fixed amplitude
Distance	Short (local)	Long (entire axon)
Summation	Can summate	Cannot summate
Initiation	At dendrites/cell body	At axon hillock

Summary

• Neurons and glial cells are the primary cellular components of the nervous system.

• Neuronal signaling relies on the generation and propagation of electrical signals, governed by ion gradients and membrane permeability.

• Myelination and the structure of neurons enable rapid and efficient communication within neural networks.

• Understanding these cellular and network properties is fundamental to the study of nervous system function and dysfunction.