Tejido nervioso

Introduction to Nervous Tissue

Welcome and Overview

• Fernando Pérez introduces himself as a lecturer from the Faculty of Medical Sciences at the National University of Rosario, inviting questions via chat for later discussion.

Recommended Literature

• Emphasis on investing in quality educational resources; recommended texts include "El Sol," "Leslie," "Junqueira," "Stevens," and "Elena y el Ros" as essential and updated references for professional training.

Understanding Nervous Tissue

Definition and Characteristics

• Nervous tissue is defined as a collection of specialized cells with extensive extensions, characterized by large nuclei with loose chromatin, indicating high metabolic activity.

Neuron Structure

• Neurons are identified as the functional units of nervous tissue responsible for generating and transmitting nerve impulses. They consist of a soma (cell body) with multiple short dendrites and one long axon.

Neuron Functionality

Metabolic Activity

• Neurons exhibit intense metabolic activity, necessitating well-developed rough endoplasmic reticulum and numerous free ribosomes that contribute to their distinctive basophilia.

Axonal Specialization

• The axon specializes in conducting electrical depolarization due to its abundance of ion channels in the membrane, facilitating long-distance impulse transmission.

Types of Neurons

Classification Overview

• Neurons can be classified into two main types:

• Type 1 (Golgi I): Larger neurons with long axons.

• Type 2 (Golgi II): Smaller neurons with shorter axons that facilitate interconnections.

Examples of Neuron Types

• Golgi I neurons include pyramidal cells and specific named cells like giant Betz cells and Purkinje cells. These larger neurons have distinct structural features such as prominent nuclei and extensive dendritic trees.

Synaptic Communication

Definition of Synapse

• A synapse is described as a functional junction between two neuronal structures where neurotransmitters are synthesized, released from the presynaptic surface into the synaptic cleft, targeting receptors on the postsynaptic surface.

Effects of Neurotransmitter Release

• Upon neurotransmitter release, three potential effects may occur:

• Depolarization (e.g., acetylcholine, glutamate)

• Hyperpolarization (e.g., GABA, glycine)

• Neuromodulation affecting cellular sensitivity (e.g., dopamine, serotonin).

Understanding Synapses and Neural Tissue

Types of Synapses

• The classification of synapses includes axo-dendritic, axo-somatic, and axo-axonic connections, highlighting the various ways neurons can interact.

• Three-dimensional images illustrate the irregular surfaces of synapses, resembling "rose spines," emphasizing the complexity and importance of these structures in neural communication.

Role of Education in Neural Development

• Education is defined as a means to promote critical thinking and intelligence, paralleling the biological process of forming new synapses.

• A well-nourished individual with normal genetics and intellectual stimulation will develop a greater number of dendritic spines, indicating enhanced cognitive abilities.

Synaptic Mechanisms

• Synapses can be either physical or electrical; however, chemical synapses mediated by neurotransmitters are predominant in humans.

• The presynaptic surface releases neurotransmitters into the synaptic cleft upon receiving an electrical impulse, which then binds to receptors on the postsynaptic surface.

Myelination and Impulse Conduction

• Myelin sheaths wrap around axons to increase conduction speed; this lipid-rich layer is produced by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS).

• Oligodendrocytes can myelinate multiple axons simultaneously while Schwann cells typically myelinate only one axon at a time.

Structure and Function of Nervous Tissue

• Nervous tissue detects, transmits, analyzes, and utilizes information from sensory stimuli both externally (via senses) and internally (chemical changes).

• It coordinates responses across various functions such as motor control, visceral activities, endocrine regulation, or psychological processes.

Central vs. Peripheral Nervous System

• The CNS consists of gray matter (neuron cell bodies and unmyelinated fibers) and white matter (myelinated fibers), without connective tissue association.

Overview of the Nervous System

Structure of the Nervous System

• The central nervous system (CNS) consists of gray and white matter, with cavities for cerebrospinal fluid circulation, while the peripheral nervous system (PNS) does not differentiate between gray and white matter.

• Key components of the PNS include nerves, ganglia, and nerve plexuses, which serve as stations for neurons.

Central Nervous System Details

• Microscopic images reveal characteristic convolutions of the brain's cortex, with gray matter on the outer layer and white matter beneath it; similar structures are observed in the cerebellum.

• Gray matter contains neuronal cell bodies, dendrites, and unmyelinated axons; astrocytes and oligodendrocytes are also present.

Neuronal Structures

• Images depict larger neurons in gray matter alongside smaller ones; white matter is characterized by myelinated axons surrounded by oligodendrocytes.

• White matter lacks neuronal cell bodies but contains neurofilaments and microtubules that support axonal structure.

Cerebral Cortex Layers

• The cerebral cortex has six layers of gray matter with varying neuron sizes; notable types include Golgi I (larger neurons) and Golgi II (smaller neurons).

• Increased magnification reveals distinct neuron types within these layers, emphasizing their structural complexity.

Cerebellar Structure

• The cerebellum features a peripheral layer of gray matter with internal white substance; its central axis comprises axons from white matter.

• Three distinct layers in the cerebellar gray matter: molecular layer (Golgi II), Purkinje cell layer (large cells), and granular layer (small granule cells).

Additional CNS Components

• The three layers of the cerebellum are further detailed under magnification to show Purkinje cells' extensive dendritic trees interacting with molecular layer neurons.

• Peripheral coverings known as meninges encase blood vessels within the brain's outer regions.

Brainstem Features

• The pons and medulla oblongata contain clusters of gray nuclei surrounded by abundant white substance; they differ from other CNS structures due to their unique organization.

Spinal Cord Characteristics

Neuronal Structures and the Blood-Brain Barrier

Overview of Neuronal Anatomy

• The internal view shows neuron nuclei at low magnification, with peripheral white matter appearing lighter in color. The anterior and posterior horns of gray matter are visible within the spinal cord.

• At higher magnification, the anterior horn reveals larger neurons (Type 1 and Type 2), which emit axons from both anterior and posterior roots. Smaller Goltz neurons are also observed.

Blood-Brain Barrier Mechanism

• Heavy metals or salts can reveal various components of neuronal tissue, including neurofilaments. This allows for detailed observation of larger neurons in the anterior horn.

• The blood-brain barrier serves as a defense mechanism by preventing direct antigen access to nerve cells due to the absence of connective tissue around satellite structures.

• The blood-brain barrier is primarily formed by astrocytic end-feet surrounding capillaries, along with a single oligodendrocyte extension. These structures create a protective barrier against potential antigens.

Structural Composition of the Blood-Brain Barrier

• Astrocytes' extensions connect through tight junctions, forming a cohesive unit that envelops capillaries. This structure ensures that substances entering via blood must traverse astrocytic cytoplasm before reaching neural somas or nerve extensions.

• As intermediaries, these cells provide defensive filtration roles against possible antigens entering through blood circulation.

Meninges: Protective Layers of the Central Nervous System

• Meninges protect central nervous system tissues; they consist of three layers: dura mater (outermost), arachnoid mater (middle), and pia mater (innermost).

• Dura mater contains the subdural space where cerebrospinal fluid circulates, while pia mater closely adheres to nervous tissue.

Cerebrospinal Fluid Circulation

• Histological sections show brain tissue covered by meninges; cerebrospinal fluid circulates within these layers, providing cushioning for central nervous system structures.

• All central nervous system organs have external coverings and internal cavities lined with specialized ependymal epithelium found in ventricles and aqueduct systems.

Production and Reabsorption of Cerebrospinal Fluid

• Specialized areas called choroid plexuses produce cerebrospinal fluid within certain ventricles; this fluid circulates throughout all brain cavities before moving into subarachnoid spaces for protection against movement impacts.

Neuronal Cells and Their Functions

Overview of Central Nervous System Cells

• The central nervous system comprises various types of cells, including astrocytes, oligodendrocytes, and microglia. Astrocytes resemble stars with multiple extensions, while oligodendrocytes have fewer processes.

• Microglia are not derived from nervous tissue but are activated monocytes (white blood cells) that cross the endothelium to perform defensive phagocytic roles in the brain.

• Astrocytes support neuronal structures by maintaining cytoplasmic integrity and facilitating axonal connections. They also play a role in producing myelin sheaths in white matter.

Types of Astrocytes

• There are two main types of astrocytes: fibrous astrocytes found in white matter and protoplasmic astrocytes located in gray matter. Protoplasmic astrocytes have more extensive processes.

• A technique using gold sublimation allows visualization of vascular structures and astrocytic processes, highlighting their intermediary role between blood supply and neuronal elements.

Oligodendrocyte Functionality

• Oligodendrocytes produce myelin sheaths for several adjacent axons, enhancing signal transmission efficiency within the central nervous system.

• These cells can be categorized based on their location: perineuronal oligodendrocytes in gray matter and interfacing oligodendrocytes in white matter responsible for myelination.

Microglial Role

• Microglia serve as immune defenders within the CNS, activated from monocytes to perform phagocytosis against pathogens or debris.

Peripheral Nervous System Structure

• The peripheral nervous system consists of nerves, ganglia, and nerve plexuses. Ganglia act as relay stations for neurons while nerves comprise bundles of axons.

• Images illustrate large neurons surrounded by smaller satellite cells that provide structural support to both neurons and their axons.

Nerve Structure Insights

Nerve Composition

• In a cross-section view of a nerve stained with osmium tetroxide, each dark circular structure represents an axon encased in myelin sheath which appears darker due to staining properties.

Neuronal Support Structures

• Within organs like the respiratory or digestive systems, collections of neurons exhibit characteristic nuclei surrounded by supportive satellite cells that maintain neuronal health.

Nervous System Structure and Function

Overview of Nervous Tissue

• The discussion begins with the introduction of a fixative that colors fats, specifically tetroxide, used in examining nerve structures.

• Axons are observed in both longitudinal and transverse sections, highlighting the presence of glial cells and Schwann cells, which have distinct shapes and sizes.

Connective Tissue in Nerves

• The connective tissue surrounding axons is categorized into three types: endoneurium (loose connective tissue around each axon), perineurium (denser connective tissue around fascicles), and epineurium (the outermost layer encasing the entire nerve).

• A cross-section image illustrates these layers clearly, showing how they encapsulate individual axons and bundles of axons.

Peripheral Nervous System Cells

• The focus shifts to peripheral nervous system cells, particularly Schwann cells that produce myelin. These cells are crucial for insulating axons.

• It is emphasized that Schwann cells are associated with axonal terminals and play a vital role in myelination.

Myelination Process

• Myelin formation involves gaps between Schwann cells known as nodes of Ranvier, which facilitate faster impulse conduction through saltatory conduction.

• The structure of myelin is described as multiple layers wrapping around an axon, increasing its diameter based on the number of layers present.

Summary and Conclusion

• The speaker prompts reflection on key concepts such as what nervous tissue is, the definition of neurons, synapses, central vs. peripheral nervous systems, and their respective organs.