Nervous Tissue
Learning Objectives
At the end of the class, the student will be able to describe:
The classification of the parts of the nervous system, including structural and functional aspects.
The types and functions of neuroglia and their roles in supporting neurons.
The process of myelination, its significance in action potential propagation, and the cells involved.
The distinguishing characteristics and functions of ganglia and nuclei in the nervous system.
The types of synapses, their morphological and functional classifications, and their relevance in neurocommunication.
Overview of the Nervous System
The nervous system is a complex and intricate network responsible for transmitting signals throughout the body, combining inputs from the environment and coordinating responses. The structure is organized into:
Central Nervous System (CNS): Encompasses the brain and spinal cord, serving as the control center for processing and interpreting sensory information.
Peripheral Nervous System (PNS): Comprising cranial nerves, spinal nerves, and a network of ganglia, it connects the CNS to the limbs and organs.
Sensory Division: Focuses on gathering information from sensory receptors and transmitting that information to the CNS.
Motor Division: Facilitates the transmission of commands from the CNS to various effectors.
The nervous system further breaks down into Somatic (voluntary control over skeletal muscles) and Visceral (involuntary control over internal organs) components.
Organization of the Nervous System
Anatomical Divisions:
Central Nervous System (CNS): Composed of the brain and spinal cord, responsible for processing information and coordinating responses.
Peripheral Nervous System (PNS): Comprises all neural tissue outside the CNS, including all cranial and spinal nerves and their associated ganglia. This section can be divided further:
Somatic components: Supply sensory and motor functions related to skin and skeletal muscles.
Autonomic components: Involved in regulating the functions of the viscera, including smooth muscle, cardiac muscle, and glands.
Peripheral Nerves are fashioned from both sensory (afferent) and motor (efferent) autonomic and somatic nerves, enabling communication throughout the body.
Tracts in the CNS contain specialized neurons such as sensory neurons, which transmit sensory information, motor neurons, which initiate movement, and interneurons that process information within the CNS.
Functional Divisions of the Nervous System
The nervous system operates through functional divisions:
Sensory (Afferent) Division: Specializes in transmitting sensory information toward the CNS for processing based on external and internal stimuli.
Motor (Efferent) Division: Responsible for transmitting signals away from the CNS to target tissues, inducing responses.
Further divided into Somatic motor pathways (control skeletal muscle activities) and Autonomic motor pathways (influence smooth muscle, cardiac muscle, and glandular activities).
Histological Divisions of Nervous Tissue
Composed of two principal cell types:
Neurons: Highly specialized, excitable cells responsible for generating and conducting action potentials. These are the fundamental functional units of the nervous system.
Neuroglia: Non-excitable support cells that play vital roles in maintaining homeostasis, forming myelin, and providing structural support for neuronal health and function.
Embryology of Nervous Tissue
The nervous system develops from specialized embryonic structures:
Neural tube: Forms the basis for the CNS, giving rise to neurons and supportive cells such as astrocytes, oligodendrocytes, and ependymal cells.
Neural crest cells: Differentiate into various components of the PNS, including sensory ganglia, autonomic ganglia, Schwann cells, and satellite cells.
Yolk sac: Provides progenitor cells for microglia, critical components of the immune response within the CNS.
Bone marrow: Contributes additional microglia particularly in response to specific pathological conditions.
General Structure of Neurons
Functionality: Neurons are designed for specialized electrical signaling through the generation of action potentials, allowing for rapid communication across long distances in the body.
Components:
Cell Body (Soma/Perikaryon): Contains necessary organelles including:
A large, pale-staining vesicular nucleus that serves as the control center.
Abundant euchromatin for active gene transcription and a prominent nucleolus indicating high metabolic activity.
Dendrites: Branch-like structures that receive stimuli from other neurons or sensory receptors; they greatly increase the surface area available for synaptic connections.
Axon: A long filament that conducts impulses away from the cell body and towards other neurons or muscles, transmitting electrical signals.
Neuropil: Dense network of interwoven axons, dendrites, and synapses that facilitates communication between neurons.
Classification of Neurons – Structural
Neurons based on shape:
Multipolar Neurons: Most common type, often found as motor neurons and interneurons within the CNS, characterized by multiple dendritic processes.
Pseudounipolar Neurons: Feature a single elongated process that bifurcates; these are common in sensory pathways originating from the dorsal root ganglion.
Bipolar Neurons: Characterized by one axon and one dendrite; typically associated with sensory functions like those in the retina.
Neurons based on axon length:
Golgi Type I Neurons: Have long axons conducive to signaling over longer distances, evident in many motor neurons.
Golgi Type II Neurons: Comprised of short axons, usually interneurons involved in local processing and reflex arcs.
Classification of Neurons – Functional
Motor (Efferent) Neurons: Carry impulses from the CNS toward effectors like muscles and glands; essential for executing movement and physiological responses.
Sensory (Afferent) Neurons: Route impulses from sensory receptors to the CNS, providing input about external and internal environments.
Pseudounipolar Neurons: Associated with general sensation; Bipolar Neurons: Specialized for processing special senses, such as vision or smell.
Interneurons: Function within the CNS to interconnect various neural circuits; chiefly multipolar and crucial for reflex actions and higher processing functions.
Classification of Nerves in Reflex Arc
Components involved in reflex actions:
Sensory neuron: Transduces sensory stimulus into electrical impulses.
Interneuron: Integrates and processes information from sensory neurons, forming responses.
Motor neuron: Carries impulses away from the CNS to elicit a functional response in effectors.
Neuronal Cell Body and Nissl Substance
Nissl bodies: Clusters of rough endoplasmic reticulum (RER) and ribosomes, vital for the synthesis of proteins necessary for neuronal maintenance and repair.
Nissl Substance: Found in the cell body; absent in the axon hillock and the axon itself, indicating specialized functions in these regions. The axon hillock is crucial for action potential generation while the axon contains mitochondria, essential for energy production without machinery for protein synthesis.
Inclusions found in neurons include:
Lipofuscin granules: Increased in number with age, reflecting cellular metabolic processes.
Melanin granules: Associated with particular neuronal populations, such as those in the substantia nigra, which is related to Parkinson’s disease.
Neuronal Cytoskeleton
Composed of three main elements:
Microtubules: Serve as tracks for axonal transport and are important drug targets for treatments of neurodegenerative disorders.
Neurofilaments: Provide structural support; their presence can be a marker for certain nerve pathologies.
Microfilaments (Actin): Associated with the membranes of axons and dendrites and play critical roles in maintaining cellular structure and regulating neurotransmitter release.
Dendrites
Dendrites receive and integrate information from other neurons or sensory receptors, forming extensive and complex dendritic trees that significantly increase synaptic contact surface area.
Dendritic spines are dynamic structures related to synaptic plasticity, learning, and memory formation.
Golgi Outposts (GOPs): Act as microtubule-organizing centers and can contribute to neurodegenerative disorders where dendritic organization is disrupted.
Axon Structure
Function of Axon: Conduct impulses away from the cell body towards target cells, crucial for communication between neurons.
Axon Hillock: Area where action potentials are generated; it lacks rough endoplasmic reticulum and ribosomes but plays a crucial role in converting synaptic signals into electrical signals.
Axoplasm: The cytoplasm of the axon, containing organelles necessary for axon function but devoid of rough endoplasmic reticulum and Golgi apparatus necessary for protein synthesis.
High concentrations of voltage-gated sodium channels are present at the nodes of Ranvier, facilitating rapid signal propagation through saltatory conduction.
Axonal Transport
Definition: An ATP-dependent mechanism vital for transporting organelles and proteins along axons to maintain neuronal function.
Anterograde transport: Involves movement from the cell body to the axon terminal and is driven by kinesin, facilitating the assembly and maintenance of axon terminals.
Retrograde transport: Moves materials from the axon terminal to the cell body, driven by dynein, essential for recycling components and delivering signaling molecules.
Clinical Implications: Defects in axonal transport are linked to various neurodegenerative diseases; certain pathogens exploit retrograde transport mechanisms to infect neurons.
Neuroglial Cells
Functions: Essential for neuronal health, providing support, insulation, and protection to neurons.
CNS Glial Cells:
Astrocytes: Largest glial cells that help maintain the blood-brain barrier (BBB), regulate the chemical environment, and exhibit a role in neurotransmitter uptake and recycling.
Oligodendrocytes: Responsible for myelinating multiple axons in the CNS, crucial for speeding up action potential conduction.
Microglia: Act as resident immune cells in the CNS, involved in the response to injury, infection, and synaptic remodeling.
Ependymal Cells: Line the brain ventricles and produce cerebrospinal fluid (CSF) that cushions the CNS.
PNS Glial Cells:
Schwann Cells: Specialized to myelinate individual axon segments, playing a key role in regeneration of peripheral nerves following injury.
Satellite Cells: Surround neuronal cell bodies in ganglia, providing structural support and regulating the microenvironment around neurons.
Synapses
Definition: Specialized junctions where neuronal communication occurs, critical for signal propagation.
Classification:
Morphological: Includes axodendritic (axon-to-dendrite), axosomatic (axon-to-cell body), and axoaxonic (axon-to-axon) connections.
Terminal Shape: Includes terminal boutons (bulbous endings) and boutons en passant (bulbous swellings along the axon).
Functional: Synapses can be excitatory (causing depolarization) or inhibitory (resulting in hyperpolarization).
Signal Transmission: Can be chemical (involving neurotransmitters) or electrical (using gap junctions for direct ion flow), reflecting different methods of cellular communication.
Myelination
Purpose: Myelination is crucial as it significantly increases the speed of action potential conduction along axons, allowing for rapid communication between neurons.
Composition: Myelin is a lipid-protein insulating layer formed by neuroglial cells, mainly oligodendrocytes in the CNS and Schwann cells in the PNS.
Differentiation in CNS/PNS: Myelination occurs differently in these systems, highlighting functional adaptations.
Regenerative Role: Schwann cells not only myelinate but also have the ability to regenerate axons after injury, an important aspect of peripheral nerve recovery.
Myelin and Action Potential
Myelin increases the resistance of the axonal membrane and decreases its capacitance, enabling saltatory conduction at the nodes of Ranvier where action potentials jump from node to node, vastly improving signal transmission efficiency.
Demyelination can lead to conduction block, as observed in conditions like multiple sclerosis and Guillain-Barré syndrome, highlighting the importance of myelin integrity for neuronal function.
Ganglia
Definition: Clusters of neuronal cell bodies located outside the CNS, playing critical roles in processing information.
Sensory Ganglia: Contain pseudounipolar neurons with centrally located nuclei, surrounded by supportive satellite cells, responsible for relaying sensory information.
Autonomic Ganglia: Contain mainly multipolar neurons with eccentrically placed nuclei and fewer satellite cells, they are involved in involuntary control of bodily functions.
Summary of Key Points
Neurons act as the primary functional units in the nervous system, conducting impulses and facilitating communication across vast networks.
Glial cells provide essential support, maintenance, and protection roles, ensuring neuronal survival and optimizing function in both the CNS and PNS.
The myelination process is vital for fast signal transmission, and understanding the roles of neuroglia, synapses, and axonal transport provides deeper insight into nervous system health and pathology.