NERVOUS TISSUE

Nervous Tissue

Nervous tissue is the primary component of the nervous system, which consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

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Components of Nervous Tissue

1. Neurons
   • Functional units of the nervous system.
   • Exhibit irritability (ability to respond to stimuli) and conductivity (ability to transmit signals).
2. Supporting Cells (Neuroglia)
   • Also called gliocytes.
   • Provide structural and functional support to neurons.
   • Neuron-to-neuroglia ratio: 1:10.
3. Extracellular Compartment
   • Contains minimal fibers.

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Structure of Neurons

A neuron has three main parts:

1. Dendrites:
   • The sensory part of the neuron.
   • Receive input from the periphery and transmit it to the cell body.
   • Form a branching tree-like structure.
2. Cell Body (Soma/Perikaryon):
   • Contains the nucleus and cytoplasmic organelles.
   • Processes the received input.
3. Axon:
   • The motor part of the neuron.
   • Transmits impulses from the cell body to the periphery.
   • May or may not be covered by a myelin sheath.
   • Originates from a specific area of the cell body known as the axon hillock, which is the site of the lowest potential threshold.
   • Axons typically do not branch but may give off collaterals.
   • Ends in multiple branches, with each branch terminating in an enlarged bulb known as the terminal bulb.

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Origin and Properties of Nervous Tissue

• Nervous tissue is ectodermal in origin, except for microglial cells, which are derived from mesoderm.
• Neurons are non-fibrous and exhibit irritability and conductivity.

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Organization of Nervous Tissue

1. Central Nervous System (CNS):
   • Brain:
     • Gray matter: Contains cell bodies of neurons.
     • White matter: Composed of myelinated axons.
     • Includes nuclei and medulla.
   • Spinal Cord:
     • Gray matter: Found in the central area (columns).
     • White matter: Composed of fiber tracts.
2. Peripheral Nervous System (PNS):
   • Divided into somatic and autonomic components.
   • Includes cranial nerves, spinal nerves, ganglia, and terminals.

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Detailed Structure of Neurons

1. Cell Body (Soma)

• Cytoplasm: Granular and basophilic.
• Contains numerous Nissl bodies:
  • Made of rough endoplasmic reticulum (RER), free ribosomes, and polyribosomes.
  • Responsible for protein synthesis.
• Other Organelles:
  • Numerous mitochondria, neurofilaments, and neurotubules.
  • Aging pigment (lipofuscin) may be present.
• Nucleus:
  • Large, spherical, and vesicular.
  • Euchromatic, with a prominent nucleolus.
  • In females of some species, a sex chromatin body (Barr body) is found near the nuclear membrane.

2. Dendrites

• Originate from the cell body and form tree-like branches.
• Serve as the input zone of the neuron.
• Contain dendritic spines, which represent points of synaptic connection with axons.

3. Axon

• Originates from the axon hillock.
• Long, cylindrical structure that does not contain Nissl substance but has numerous neurofilaments and neurotubules.
• Covered with a myelin sheath in myelinated axons.
• Nodes of Ranvier: Gaps between myelin sheaths.
• Conduction Types:
  • Myelinated axons: Saltatory conduction (impulse jumps from node to node).
  • Non-myelinated axons: Gradual and slower conduction.
• Terminates in branches with terminal bulbs containing synaptic vesicles (store neurotransmitters).

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Zones of a Neuron

1. Dendritic Zone:
   • Input zone where excitation is initiated.
2. Telodendritic Zone:
   • Formed by repeated branches of the axon.
   • Transmission zone for impulses.
3. Axonic Zone:
   • Axon conducts the impulse.
4. Synaptic Zone:
   • Union or connection between two neurons.

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Synapse

A synapse is a functional junction between two neurons.

Structure of Synapse:

1. Presynaptic Membrane: Found in the terminal bulb.
2. Synaptic Vesicles: Contain neurotransmitters.
3. Synaptic Cleft: Narrow gap (~5–20 nm) between the pre- and post-synaptic membranes.
4. Postsynaptic Membrane: Found on the dendrite, axon, or soma of the next neuron.

Types of Synapses:

1. Axosomatic Synapse: Axon terminates on the cell body of another neuron.
2. Axodendritic Synapse: Axon terminates on the dendrite of another neuron.
3. Axoaxonic Synapse: Axon terminates on another axon.

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Classification of Neurons

1. Conducting/Transmission Neurons:
   • Types:
     • Multipolar Neurons:
       • Several dendrites originate from the cell body.
       • Found in the brain and spinal cord.
     • Bipolar Neurons:
       • One dendrite and one axon on opposite poles of the cell body.
       • Found in the retina and olfactory epithelium.
     • Unipolar (Pseudounipolar) Neurons:
       • Single process that divides into an axon and dendrite.
       • Found in dorsal root ganglia.
2. Neurosecretory Neurons:
   • Secrete hormones or neurotransmitters.

Neuroglia and Supporting Structures of Neurons

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Neuroglia: The Supporting Structure of Neurons

Neuroglia (also called gliocytes) form the supporting framework for neurons in the nervous tissue. It has two main components:

1. Neuropil
2. Neuroglia (Gliocytes)

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1. Neuropil

• Neuropil is a complex network of cell processes such as axons, dendrites, and some glial cell processes.
• Location: Found primarily in the gray matter of the central nervous system (CNS).

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2. Neuroglia (Gliocytes)

• Neuroglia form 90% of the total cells in the nervous tissue.
• These cells are non-excitatory, meaning they do not participate in the transmission of nerve impulses.
• They provide structural and metabolic support to neurons.

Characteristics of Neuroglial Cells

• These cells are least visible when stained with H&E (Hematoxylin and Eosin).
• Visualization is better achieved using electron microscopy or special techniques like silver staining.

Types of Neuroglia

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Types of Neuroglia Based on Size

Neuroglial cells can be classified into two types based on their size:

1. Microgliocytes (Microglia)
   • Size: Small in size.
   • Example: Microglia.
   • Function: Act as macrophages, playing a role in immune response within the CNS.
2. Macrogliocytes (Macroglia)
   • Size: Larger in size.
   • Examples:
     • Oligodendrocytes
     • Astrocytes
     • Ependymal cells
     • Neurolemocytes (Schwann cells)
     • Choroid plexus cells
     • Müller cells

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Types of Neuroglia Based on Their Relationship with the CNS or PNS

1. Central Neuroglial Cells (CNS)
   • These neuroglial cells are found within the central nervous system (CNS).
   • Examples:
     • All the glial cells mentioned above, except neurolemocytes (Schwann cells).
2. Peripheral Neuroglial Cells (PNS)
   • These neuroglial cells are located within the peripheral nervous system (PNS).
   • Examples:
     • Neurolemocytes (Schwann cells)

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Structure of Neuroglial Cells

Oligodendrocytes

• Most Numerous Glial Cells in the CNS:
  Oligodendrocytes are the most abundant glial cells in the central nervous system.
• Location:
  • Around the soma (cell body) of neurons (perineuronal location).
  • Between neuronal processes (interfascicular location).
  • Near blood vessels (perivascular location).
• Structure:
  • Nucleus: Small, round, and heterochromatic (dense).
  • Cytoplasm: Scanty and contains few cell processes.
• Function:
  • Produces myelin in the CNS.
  • Provides nourishment to neurons.

Astrocytes

• Definition: Astrocytes are the second most numerous glial cells in the central nervous system (CNS).
• Classification: Depending on their location, there are two types of astrocytes:
  1. Fibrous Astrocytes
     • Primarily found in white matter.
     • Characteristics:
       • Cytoplasm is less abundant.
       • Cell processes are longer, slender, and have less branching.
  2. Protoplasmic Astrocytes
     • Primarily found in gray matter.
     • Characteristics:
       • Cell processes are short, thick, and extensively branched.
• Nucleus:
  • Large, mobile, round, and pale-staining or euchromatic.
• Functions:
  1. Act as phagocytic cells during times of need.
  2. Form a CNS scar after an injury.
  3. Participate in tissue repair in the CNS.
  4. Create the limiting membrane through the enlarged, foot-like structures of their processes.

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Microglia

• Definition: Microglia are the typical macrophages of the CNS.
• Characteristics:
  • Small, elongated cells.
  • Cytoplasm is scanty.
  • Cell processes are short and numerous, with small branches.
• Nucleus:
  • Small, elongated, often extended, and dense.
• Origin: Mesodermal in origin.
• Frequency:
  • Normally less frequent in CNS tissue.
  • Their numbers increase significantly after tissue injury.
• Function: Act as CNS macrophages.

Ependymal Cells

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Ependymal Cells

• Location: Found in the walls of the neural canal.
• Shape: Usually columnar, but sometimes have microvilli and cilia.
• Junctions: Contain gap junctions and tight junctions.
• Nucleus:
  • Large, mobile, and oval or elongated.
  • Euchromatic (light-staining).

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Tanycytes

• Definition: Modified ependymal cells that are non-ciliated but possess microvilli.
• Characteristics:
  • Long, slender cytoplasmic processes that terminate on capillaries.
• Function: Involved in the transport of neurosecretions, particularly in:
  • The median eminence.
  • The ventral floor of the third ventricle.

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Choroid Plexus Cells

• Location: Form an epithelium over folds of pia mater, which surround loops of capillaries.
• Shape: Cuboidal, with a bulging luminal surface.
• Surface Features: Have microvilli and cilia.
• Functions:
  1. Form the blood-cerebrospinal fluid (CSF) barrier through epithelial cell junctions (zonula adherens).
  2. Secrete CSF into the CNS.
  3. Absorb substances from the CSF.

Müller Cells and Myelination

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Müller Cells

• Location: Found in the neurons of the retina.
• Arrangement: Radially arranged and extend the entire length of the retina.
• Functions:
  1. Maintain homeostasis.
  2. Provide metabolic support to neurons, essential for their survival.
  3. Contain rich mitochondria, rough endoplasmic reticulum, and a distinct Golgi apparatus.
  4. Become phagocytic in response to tissue injury.

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Neurolemmocytes (Schwann Cells)

• Structure and Function:
  • Surround the axons of neurons to form the myelin sheath.
  • Provide metabolic and structural support to axons.
• Characteristics:
  • Each neurolemmocyte is rich in mitochondria and other organelles.
  • They cover the axon with a basal lamina.
  • Neurolemmocytes are less than 1 mm in length.

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Myelination Process

1. Axon Encapsulation:
   • Neurolemmocytes surround the axons.
   • If the axon is small, a single neurolemmocyte can surround multiple axons.
   • If the axon is large, a neurolemmocyte surrounds it using a pair of cell processes.
2. Formation of the Myelin Sheath:
   • The pair of cell processes gradually approach each other, overlap, and slide past one another.
   • Multiple layers of the cell membrane wrap around the axon concentrically.
   • Cytoplasm is extruded from the layers, leaving behind a sheath of phospholipids and proteins from the cell membrane.
3. Myelin Segment Formation:
   • The myelin sheath forms around a specific segment of the axon.
   • A series of neurolemmocytes are aligned along the axon’s length.
   • The regions where the axon remains uncovered are called nodes of Ranvier.

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Myelination in Non-Myelinated Axons

• In non-myelinated axons, neurolemmocytes surround the axon using a pair of cell processes.
• The processes do not overlap or form concentric layers of cell membrane.
• No true myelin sheath is formed.