Introduction to the Nervous System

Overview of the Nervous System

• The nervous system is one of the four primary tissue types in the body, which include connective, epithelial, muscle, and nervous tissue.

• While muscle tissue focuses on contraction and epithelial tissue focuses on establishing borders, the nervous system is dedicated entirely to communication.

• Communication within the nervous system involves neurons (the primary signaling cells) and supporting cells known as glia or neuroglia.

• The nervous system regulates diverse bodily functions, including digestive processes (stomach), computer-like processing, muscle movement, and sensory integration.

• Sensory information processed by the system includes temperature (hot and cold), pain perception, and olfaction (smell).

• The brain is an extremely high-energy organ, utilizing up to $50\%$ of all glucose available in the human body. Because of this high energy demand, neurons cannot perform other tasks like immune defense, requiring support from specialized glia cells.

Divisions of the Nervous System

• Central Nervous System (CNS):   - Consists of the brain and the spinal cord.   - The brain resides within the skull, and the spinal cord runs through the vertebrae of the spine.   - This serves as the primary processing center where most neural activity occurs.

• Peripheral Nervous System (PNS):   - Consists of all neural structures outside the brain and spinal cord.   - Spinal Nerves: These are all nerves located outside the spinal cord that branch off from it.   - Cranial Nerves: These nerves are located in the head and connect directly to the brain, bypassing the spinal cord entirely. For example, eye nerves go straight to the brain for increased efficiency.   - The PNS is responsible for moving muscles, controlling internal organs (e.g., heart rate), and relaying sensory input from the environment.

Anatomy and Structure of the Neuron

• Dendrites: These are branch-like structures that take in information. In the PNS, these may receive input from mechanoreceptors in the skin or taste buds on the tongue; in the brain, they receive signals from other neurons.

• Soma (Cell Body): This is the main structure containing the metabolic machinery of the cell, including the nucleus, mitochondria, endoplasmic reticulum (ER), lysosomes, peroxisomes, and Golgi bodies.

• Nissle Body: A specialized type of ER found exclusively in the soma of neurons. It is not found in any other part of the neuron or in any other cell type. All neurons possess Nissle bodies. They can be visualized using a "Nissle stain," which allows researchers to identify individual cell bodies among a dense network of neural branches.

• Axon: A long projection that carries electrical signals away from the soma.   - Axons vary greatly in length; the longest axon in the human body extends from the base of the spine to the foot.   - Scaled metaphor: If a cell body were the size of a beach ball, its axon could be up to a $kilometer$ long.

• Axon Hillock: The region where the cell body transitions into the axon; it acts as the starting area for the signal.

• Axon Terminals (Terminal Buds): The end points of the axon.   - Metaphor: They function like airport terminals where signals are sorted and sent out to other cells via airplanes (synaptic transmission).

Myelination and Signal Conductance

• Neural signals are electrical. To increase the speed of these signals, axons are wrapped in insulators called myelin sheaths.

• Myelin Sheath: An insulating layer that allows signals to jump across sections of the axon rather than traveling continuously. Without insulation, signals are significantly slower.

• Nodes of Ranvir: The gap or space between myelin sheaths. The electrical signal "jumps" from node to node to reach the end of the axon more quickly.

• Unmyelinated Axons: Some axons lack myelin; these are generally shorter and transmit signals at a much slower rate.

• Energy Management: Because signals require significant energy, fragments of ER and individual mitochondria are distributed all along the axon to produce $ATP$ and proteins locally.

Types of Glial Cells (Neuroglia)

• Oligodendrocytes (CNS): Round cells with multiple branches that wrap around neurons to form myelin sheaths. A single oligodendrocyte can provide myelin for multiple different neurons.

• Ependymal Cells (CNS): Epithelial cells that line the outside and internal cavities of the brain.

• Astrocytes (CNS): Star-shaped cells (hence the name "star cell") that perform several support roles:   - They wrap around capillaries to form the Blood-Brain Barrier (BBB).   - They filter nutrients (like glucose) from the blood and deliver them to neurons while transporting waste products back to the capillaries.

• Microglia (CNS): The immune cells of the brain. They attack pathogens and recycle dying cells to prevent damage to neighboring healthy neurons.   - They exist in a "quiescent" state (small round balls) until activated by a pathogen or dying neuron, at which point they branch out to attack or clean up junk.

• Schwann Cells (PNS): These form the myelin sheaths in the peripheral system. Unlike oligodendrocytes, one Schwann cell wraps around a single segment of one axon.   - Nury lemma: The outermost layer of the Schwann cell which contains the nucleus and ER.   - Visualization: A Schwann cell is like a flat piece of paper rolled repeatedly around a central axon.

• Satellite Cells (PNS): Support cells that surround the cell bodies (soma) of neurons in the peripheral nervous system.

Structural Classification of Neurons

• Neurons are classified by the number of processes (extensions) coming off the cell body (polarity):

• Multipolar: Characterized by many dendrites and a single axon attached to the soma. This is the most common shape shown in diagrams.

• Bipolar: Has exactly two processes extending from the soma: one for the dendrite trunk and one for the axon. This is like a bicycle (two wheels).

• Unipolar: The signal travels directly from the dendrites to the axon, bypassing the cell body entirely. The soma hangs off the side of the axon. These are common in spinal cord circuits where speed is essential for reflexes.

Clinical Correlations and Pharmacology

• Blood-Brain Barrier (BBB): A protective barrier that prevents many substances from entering the brain. This poses a challenge for drug delivery.

• Parkinson's Disease: Caused by low dopamine levels. Because dopamine cannot cross the BBB, patients are treated with $L-Dopa$.   - $L-Dopa$ is a precursor that can cross the BBB, where brain cells then convert it into functional dopamine.

• Schizophrenia: Linked to excessive dopamine levels.   - Giving a healthy person $L-Dopa$ can induce hallucinations or schizophrenic symptoms.   - Medications that decrease dopamine levels have been shown to improve the condition of schizophrenic patients.

• Pathogens:   - Meningitis: A preventable disease that attacks neurons. It is known for causing severe symptoms like projectile vomiting. Vaccination is highly recommended.   - Prion Diseases: Includes conditions like Mad Cow disease.

Questions & Discussion

• Student Question: Can you repeat what the cell in the CNS is called that makes myelin?

• Teacher Response: It is the oligodendrocyte. The name comes from "oligo" (few), "dendro" (branches like a dendrite), and "cyte" (cell).

• Student Question: Does a non-myelinated neuron have Nodes of Ranvir?

• Teacher Response: No, it does not. Nodes of Ranvir only exist as the gaps between myelin segments.

• Discussion on Models: On the classroom neuron models, the yellow structures in the cell bodies represent the Nissle bodies. The orange/red structures on the soma of the model are actually axon terminals from other neurons, not parts of that specific neuron. The model used in class lacks a full axon, so the actual terminals of the model neuron are cut off at the bottom.