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).
Components of Nervous Tissue
- Neurons
- Functional units of the nervous system.
- Exhibit irritability (ability to respond to stimuli) and conductivity (ability to transmit signals).
- Supporting Cells (Neuroglia)
- Also called gliocytes.
- Provide structural and functional support to neurons.
- Neuron-to-neuroglia ratio: 1:10.
- Extracellular Compartment
- Contains minimal fibers.
Structure of Neurons
A neuron has three main parts:
- 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.
- Cell Body (Soma/Perikaryon):
- Contains the nucleus and cytoplasmic organelles.
- Processes the received input.
- 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.
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.
Organization of Nervous Tissue
- 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.
- Brain:
- Peripheral Nervous System (PNS):
- Divided into somatic and autonomic components.
- Includes cranial nerves, spinal nerves, ganglia, and terminals.
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).
Zones of a Neuron
- Dendritic Zone:
- Input zone where excitation is initiated.
- Telodendritic Zone:
- Formed by repeated branches of the axon.
- Transmission zone for impulses.
- Axonic Zone:
- Axon conducts the impulse.
- Synaptic Zone:
- Union or connection between two neurons.
Synapse
A synapse is a functional junction between two neurons.
Structure of Synapse:
- Presynaptic Membrane: Found in the terminal bulb.
- Synaptic Vesicles: Contain neurotransmitters.
- Synaptic Cleft: Narrow gap (~5–20 nm) between the pre- and post-synaptic membranes.
- Postsynaptic Membrane: Found on the dendrite, axon, or soma of the next neuron.
Types of Synapses:
- Axosomatic Synapse: Axon terminates on the cell body of another neuron.
- Axodendritic Synapse: Axon terminates on the dendrite of another neuron.
- Axoaxonic Synapse: Axon terminates on another axon.
Classification of Neurons
- 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.
- Multipolar Neurons:
- Types:
- Neurosecretory Neurons:
- Secrete hormones or neurotransmitters.
Neuroglia and Supporting Structures of Neurons
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:
- Neuropil
- Neuroglia (Gliocytes)
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).
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
Types of Neuroglia Based on Size
Neuroglial cells can be classified into two types based on their size:
- Microgliocytes (Microglia)
- Size: Small in size.
- Example: Microglia.
- Function: Act as macrophages, playing a role in immune response within the CNS.
- Macrogliocytes (Macroglia)
- Size: Larger in size.
- Examples:
- Oligodendrocytes
- Astrocytes
- Ependymal cells
- Neurolemocytes (Schwann cells)
- Choroid plexus cells
- Müller cells
Types of Neuroglia Based on Their Relationship with the CNS or PNS
- 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).
- Peripheral Neuroglial Cells (PNS)
- These neuroglial cells are located within the peripheral nervous system (PNS).
- Examples:
- Neurolemocytes (Schwann cells)
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:
- Fibrous Astrocytes
- Primarily found in white matter.
- Characteristics:
- Cytoplasm is less abundant.
- Cell processes are longer, slender, and have less branching.
- Protoplasmic Astrocytes
- Primarily found in gray matter.
- Characteristics:
- Cell processes are short, thick, and extensively branched.
- Fibrous Astrocytes
- Nucleus:
- Large, mobile, round, and pale-staining or euchromatic.
- Functions:
- Act as phagocytic cells during times of need.
- Form a CNS scar after an injury.
- Participate in tissue repair in the CNS.
- Create the limiting membrane through the enlarged, foot-like structures of their processes.
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
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).
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.
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:
- Form the blood-cerebrospinal fluid (CSF) barrier through epithelial cell junctions (zonula adherens).
- Secrete CSF into the CNS.
- Absorb substances from the CSF.
Müller Cells and Myelination
Müller Cells
- Location: Found in the neurons of the retina.
- Arrangement: Radially arranged and extend the entire length of the retina.
- Functions:
- Maintain homeostasis.
- Provide metabolic support to neurons, essential for their survival.
- Contain rich mitochondria, rough endoplasmic reticulum, and a distinct Golgi apparatus.
- Become phagocytic in response to tissue injury.
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.
Myelination Process
- 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.
- 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.
- 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.
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.