Neurons and Glial Cells

Introduction to Nervous System Cells

The nervous system is composed of two primary types of cells:

• Neurons: These are the main cells responsible for transmitting information. There are an estimated $~86 ext{ billion}$ neurons in the nervous system.

• Glia (Glial Cells): These are support cells that outnumber neurons by approximately $10 ext{x}$.

The Neuron Doctrine: Historical Context

Historically, understanding the cellular nature of the nervous system was a challenge:

• 1840: Schleiden & Schwann proposed that all tissues are made of basic cellular units. However, nervous tissue was initially thought to be an exception, believed to be a continuous network rather than discrete cells.

• 1860s: Camillo Golgi developed a revolutionary silver stain technique, which allowed for the visualization of individual neurons in their entirety.

• 1892: Santiago Ramón y Cajal utilized Golgi's stain to meticulously map the structure of neurons. Based on his observations, he proposed the Neuron Doctrine, which states that the nervous system is composed of individual, discrete cells called neurons, which communicate at specialized junctions (synapses).

• 1906: The Nobel Prize in Physiology or Medicine was jointly awarded to Golgi and Cajal for their groundbreaking work on the structure of the nervous system.

Basic Structure of Neurons

Neurons have three fundamental parts:

1. Soma (Cell Body):

   • The central part of the neuron.

   • Contains the nucleus and the cellular machinery responsible for protein synthesis and metabolic functions.

2. Dendrites:

   • Branching projections extending from the soma.

   • Primarily responsible for receiving signals from other neurons and transmitting them toward the soma (INPUT region).

   • Some neurons have few dendrites, while others have many, allowing for extensive synaptic connections.

   • Many dendrites possess dendritic spines, small protrusions where axon terminals from other neurons make synaptic contact.

3. Axon:

   • A single, long projection extending from the soma.

   • Transmits signals from the soma to other neurons, muscles, or glands (OUTPUT region).

   • While each cell has only one axon, it can branch many times (axon collaterals) to communicate with multiple target cells.

   • The distal (far) end of the axon branches into axon terminals (or terminal buttons), which form synapses with other cells.

Information Flow Between and Within Neurons

The sequence of information transmission in the nervous system usually follows a specific path:

1. A signal from an axon terminal of a preceding neuron arrives at the dendrite or soma of the receiving neuron.

2. This signal then travels internally towards the soma of the receiving neuron.

3. From the soma, if the signal is strong enough, it propagates down the axon to its axon terminals.

4. From these axon terminals, the signal is then transmitted to the dendrite or soma of another cell, continuing the communication chain.

Major Organelles of Neurons and Their Functions

Neurons contain numerous specialized organelles within their cytoplasm, including:

• Nucleus:

  • A membrane-enclosed region containing the cell's genetic material (DNA).

  • Crucial for hereditary control and gene expression.

  • Gene expression involves the transcription of DNA into RNA. This RNA (specifically messenger RNA or mRNA) is then exported out of the nucleus to serve as a template for protein synthesis.

  • Contains the nuclear envelope and nucleolus.

• Plasma Membrane (Cell Membrane):

  • A phospholipid bilayer that surrounds the entire neuron, forming its border.

  • Controls the exchange of materials between the cell's interior and its external environment.

  • Mediates cell-environment interactions.

  • Crucially, in neurons, it contains various ion channels, which are proteins selectively permeable to specific ions (e.g., calcium ($ext{Ca}^2+$), sodium ($ext{Na}^+$), chloride ($ext{Cl}^-$), potassium ($ext{K}^+$)).

  • Different types of ion channels are distributed to varying extents throughout the dendrites, soma, and axon, which is fundamental to neuronal excitability and signal propagation.

• Endoplasmic Reticulum (ER):

  • A network of membranous sacs and tubules involved in the synthesis of lipids and proteins.

  • Rough ER: Studded with ribosomes, primarily involved in synthesizing secreted and membrane-bound proteins.

  • Smooth ER: Lacks ribosomes, mainly involved in lipid synthesis and detoxification.

• Golgi Apparatus:

  • A stack of flattened membranous sacs.

  • Modifies, sorts, and packages proteins and lipids synthesized in the ER, preparing them for secretion or delivery to other organelles.

• Mitochondria:

  • Often called the "powerhouse of the cell."

  • Responsible for aerobic energy metabolism, producing ATP (adenosine triphosphate), the primary energy currency for most chemical reactions within the cell.

• Microtubules: Part of the cytoskeleton, involved in maintaining cell shape and transporting substances within the neuron.

Cell Nucleus and Protein Synthesis

Genetic information and its expression are central to neuronal function:

• Chromosomes:

  • Contain the genetic information in the nucleus.

  • Humans have $23 ext{ pairs}$: $22 ext{ autosomal pairs}$ and the final pair being sex chromosomes (XX for females, XY for males).

  • The human genome comprises approximately ~20,000 ext{ to } 25,000 ext{ genes}$.

• Genome: Refers to the sum total of all genes within an organism. Notably, the genome is identical in every cell of an individual.

• Nucleic Acids: Specialized compounds crucial for storing and expressing genetic information.

  • Composed of a nitrogenous base, a sugar, and a phosphoric acid group.

  • Deoxyribonucleic Acid (DNA):

    • Encodes the genetic material of a cell.

    • Primarily found in the nucleus, but also in mitochondria.

    • Contains four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).

    • A nucleoside consists of a nitrogenous base + a sugar (2 ext{-deoxyribose}$$).

    • A nucleotide consists of a nitrogenous base-sugar combination + a phosphoric acid.

  • Ribonucleic Acid (RNA):

    • Serves as a blueprint for protein synthesis.

    • Generally found in the cytoplasm, predominantly as messenger RNA (mRNA) and as components of ribosomes.

    • Contains four nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), and Uracil (U) (Uracil replaces Thymine in RNA).

    • Each protein is encoded by a sequence of triplet base pairs (codons) on the mRNA, with each codon specifying a particular amino acid (e.g., UGG encodes tryptophan).

    • Ribosomes read the mRNA sequence and add the appropriate amino acids in order to synthesize a protein.

Protein Synthesis Process:

1. Transcription: Within the nucleus, a specific gene (segment of DNA) is transcribed into a molecule of messenger RNA (mRNA). This process copies the genetic information from DNA to RNA.

2. Translation: The mRNA molecule then leaves the nucleus and attaches to a ribosome in the cytoplasm. The ribosome "reads" the mRNA sequence (codon by codon), recruits corresponding amino acids, and links them together to form a newly synthesized protein.

Classification of Neurons

Neurons can be classified based on their anatomical (morphological) features or their functional roles:

1. Based on Anatomical/Morphological Features (as pioneered by Ramón y Cajal):

   • Unipolar Neuron (or Pseudo-unipolar cell): Characterized by a single bifurcated process that extends from the cell body and then divides into two, serving as both dendrite and axon (e.g., ganglion cells of the dorsal root).

   • Bipolar Neuron: Possesses two distinct processes extending from the soma – typically one dendrite and one axon (e.g., bipolar cells of the retina).

   • Multipolar Neuron: The most common type, characterized by multiple dendrites and a single axon extending from the soma (e.g., motor neurons of the spinal cord, pyramidal cells of the hippocampus, Purkinje cells of the cerebellum).

2. Based on Functionality (often used for neurons in the spinal cord and sensory pathways):

   • Motor Neuron (Efferent Neuron): Transmits signals from the central nervous system (CNS) to muscles or glands, initiating movement or secretion.

   • Interneuron: Located entirely within the CNS, these neurons connect other neurons, relaying and integrating information within neural circuits.

   • Sensory Neuron (Afferent Neuron): Transmits signals from sensory receptors in the periphery (e.g., skin, muscles, organs) to the CNS, conveying sensory information.

• Note on Bell-Magendie Law: This principle states that sensory nerve fibers enter the spinal cord dorsally (at the back), while motor nerve fibers exit the spinal cord ventrally (at the front).

Glial Cells: The Support System

Glial cells, or glia, are essential support cells in the nervous system, playing diverse roles beyond mere structural support (the