Prions: The Science Behind Mad Cow Disease | Neurology Explained

The central dogma of molecular biology, proposed by Francis Crick, describes the flow of genetic information: DNA → RNA → Protein. This unidirectional flow means that genetic information flows from DNA to RNA (transcription) and then from RNA to protein (translation). DNA serves as the template for both its own replication and for RNA synthesis.

The central dogma describes the flow of genetic information from DNA to RNA to protein. DNA stores genetic information, which is transcribed into RNA, and RNA is translated into protein. This unidirectional flow (with some exceptions like reverse transcription) is fundamental to molecular biology and explains how genetic information is expressed to produce functional molecules.

The Central Dogma describes the flow of genetic information from DNA to RNA to proteins. DNA contains genetic information organized into chromosomes. During cell division, DNA replicates to ensure daughter cells receive identical genetic material. Transcription produces RNA copies from DNA templates, creating mRNA, tRNA, and rRNA. Translation converts mRNA information into proteins through the coordinated action of ribosomes and tRNA. This unidirectional flow of information is fundamental to all living organisms.

The Central Dogma describes the flow of genetic information from DNA to RNA to protein. Information flows from DNA to RNA through transcription, and from RNA to protein through translation. DNA replication copies DNA, and RNA replication occurs in some viruses. Reverse transcription (RNA to DNA) occurs in retroviruses like HIV. The dogma also includes less common pathways like RNA to RNA and protein to protein (prions), and epigenetic regulation where proteins control gene expression without modifying DNA.

The central dogma describes the flow of genetic information: DNA → RNA → Protein. DNA stores genetic information and can replicate itself. RNA acts as a messenger carrying instructions from DNA. Proteins are the functional molecules that carry out cellular activities. DNA converts to RNA through transcription, and RNA synthesizes proteins through translation. This unidirectional flow under normal circumstances.

The central nervous system comprises the brain (cerebrum, cerebellum, brainstem) and spinal cord. The brainstem connects the brain to the spinal cord, which lies caudal to the medulla oblongata within the vertebral column. The neuron, the fundamental unit, consists of a cell body, dendrites (afferent), and axon (efferent). Neurons are classified as unipolar, bipolar, or multipolar based on their prolongations. Synapses connect neurons through presynaptic and postsynaptic membranes with a synaptic cleft, utilizing neurotransmitters (approximately 100 types including adrenaline, dopamine, serotonin, GABA, glutamate) to facilitate or inhibit nerve conduction.

The neuron is the functional unit of the nervous system, responsible for projecting, conducting, and receiving information through action potentials via axons and dendrites; neurons are classified into macro neurons (Type 1) with long axons for information transmission and micro neurons (Type 2) with shorter axons for local circuit processing, while synapses are specialized junctions where neurotransmitters are released to transmit signals between neurons, and glial cells including astrocytes, oligodendrocytes, and microglia provide structural support, myelination, and immune protection respectively.

The central nervous system (CNS) is composed of neurons, astrocytes, oligodendrocytes, microglia, and blood vessels, with neurons serving as the primary excitable cells that communicate through chemical synapses using neurotransmitters like glutamate, GABA, and dopamine. The blood-brain barrier, formed by tight junctions between endothelial cells and regulated by astrocytes, protects the CNS while controlling molecular exchange. Synaptic transmission involves presynaptic vesicle release of neurotransmitters and postsynaptic receptor activation, with excitatory synapses (glutamate) causing depolarization and inhibitory synapses (GABA) causing hyperpolarization. Long-term potentiation (LTP), triggered by NMDA receptor activation and calcium influx during high-frequency stimulation, strengthens synaptic connections through molecular cascades involving calcium, calmodulin, and gene expression changes, forming the cellular basis for memory and learning.

The central nervous system contains over 80 million neurons, each with a cell body (soma) containing the nucleus for information integration, dendrites for receiving signals, axons for transmission, and synaptic terminals for communication. A single neuron can have hundreds to approximately 200,000 synaptic connections. Organelles are strategically distributed—near the nucleus for protein synthesis and near synaptic terminals for energy and neurotransmitter release. Microtubules form the cytoskeleton, serving as tracks for transporting organelles and vesicles. Oligodendrocytes produce myelin for saltatory conduction, while astrocytes provide cellular support and interface between blood vessels and neurons. The choroid plexus produces cerebrospinal fluid, and microglia serve as resident immune cells. The blood-brain barrier protects the brain through non-fenestrated capillaries and tight junctions.

Neurons are specialized cells that form the nervous system, consisting of three main parts: dendrites (which receive signals from other cells), the cell body (which processes incoming information and contains the nucleus), and the axon (which transmits electrical signals to other cells); neurons communicate through electrical impulses called action potentials that travel along axons at speeds up to 120 meters per second, and at the junction between neurons called synapses, neurotransmitters are released to transmit signals to neighboring cells, enabling the complex network of billions of interconnected neurons that make up the brain and spinal cord (central nervous system) and the peripheral nerves (peripheral nervous system).

Infectious diseases are caused by various microorganisms including prions (abnormal proteins causing spongiform encephalopathies), viruses (obligate intracellular parasites with DNA/RNA genomes), bacteria (prokaryotes classified by Gram staining, shape, and oxygen requirements), fungi (eukaryotes with complex cell walls), protozoa (single-celled eukaryotes), and helminths (multicellular worms). These pathogens cause disease through different mechanisms: transient illness, chronic persistent infection, latency with reactivation, or cellular transformation. The human microbiome, consisting of trillions of microorganisms in the gastrointestinal tract, skin, and other sites, plays crucial roles in digestion, immune function, and overall health, but dysbiosis (alterations in microbiome composition) can contribute to disease states.

Infectious diseases are caused by pathogens including bacteria, viruses, fungi, and eukaryotic parasites that exploit human cells to replicate; the human immune system defends against these invaders through innate responses (macrophages and neutrophils) and adaptive responses (T cells and B cells producing antibodies), while modern medicine combats infections through vaccines (which train the immune system without causing disease) and antibiotics (which kill bacteria), though antibiotic resistance poses an ongoing challenge.

Microorganisms cause disease through three mechanisms: direct cell invasion, toxin release, and immune-mediated damage. Viral tropism determines tissue specificity through receptor-glycoprotein interactions (e.g., HIV binding CD4 receptors) and transcription factor requirements. Viruses damage cells through direct cytopathic effects, immune-mediated destruction, and cellular transformation. Bacterial virulence depends on adherence, invasion, and toxin delivery, with virulence genes clustered in pathogenicity islands. Horizontal gene transfer via plasmids spreads virulence factors and antibiotic resistance. Quorum sensing allows bacteria to coordinate virulence based on population density. Bacterial adherence involves specific surface structures binding host components. Bacteria produce endotoxins (lipopolysaccharides) and exotoxins (secreted proteins) that cause cellular injury through diverse mechanisms.

This comprehensive section establishes foundational principles of infectious disease pathogenesis. The course is part of a free online Global Medical School pathology program based on Robbins textbook, preparing students for USMLE Step 1. Technical challenges including computer performance and internet connectivity issues are addressed. The section covers ectoparasite taxonomy: arthropods classified under phylum Arthropoda, with insects having six legs and arachnids having eight legs. Human ectoparasites include lice (pediculosis capitis, corporis, Phthirus pubis), bed bugs, and fleas. Arachnids include ticks and mites, with mites being the smallest arthropods. Spiders of medical concern are black widow (mortality <1%) and brown recluse (causes tissue necrosis). Scabies is caused by mites that feed on skin and cause intense itching, diagnosed by scraping affected areas. Natural barriers include skin, mucosal epithelium, and underlying MALT. Infectious processes spread through direct extension, lymphatic, hematogenous, and neural routes, similar to tumor spread. Transmission occurs through direct contact, vectors (zoonoses), and sexual contact. The equation of infectivity states that infectivity is directly proportional to organism virulence but inversely proportional to host resistance. Mechanisms of damage include direct effects, toxin production (endotoxins from gram-negative bacteria, exotoxins), and host inflammatory responses. Viral replication involves attachment, entry, and nucleic acid release, with forward or reverse transcription. Latency allows viruses to remain dormant and reactivate years later, potentially causing malignant transformation. Bacteria must adhere to tissues and may produce endotoxins or exotoxins. Pathogens evade immunity through inaccessibility, antigenic variation, and antigen shedding. Some pathogens actively attack immune cells. Immunosuppressed patients are susceptible to opportunistic infections including protozoans, fungi, bacteria, and herpes viruses.

Bacteria are prokaryotes that replicate rapidly (~20 min) and release toxins. Viruses are acellular particles that invade host cells and destroy them. Fungi have thread-like hyphae and produce spores. Protists are eukaryotes often parasitic, using vectors like mosquitoes. Examples: Salmonella causes food poisoning; gonorrhea is STI; measles spreads through droplets; HIV suppresses immune system; malaria is transmitted by mosquitoes. The immune system responds through phagocytosis, antibody production, and antitoxin production. Memory cells provide faster response upon re-exposure.