Nervous Tissue

Nervous Tissue

Origin: Nervous tissue originates from the ectoderm of the embryo.

Cell Types: Nervous tissue comprises two primary cell types:

• Neuron (Nerve Cell): The functional unit of the nervous system, specialized for receiving, processing, and transmitting information.

• Glial Cells (Neuroglia): Provide support and protection for neurons.

Neuron Function: Based on function, neurons are categorized into three types:

• Sensory Neuron: Receives information from sensory receptors and transmits it to the central nervous system.

• Motor Neuron: Transmits information from the central nervous system to effector organs.

• Interneuron: Connects neurons within the central nervous system, integrating sensory and motor information.

Neuron Structure:

• Cell Body (Soma): Contains the nucleus and most of the cytoplasm, serving as the neuron’s control center.

• Dendrites: Numerous, branched extensions that arise from the cell body, receiving information from other neurons.

• Axon: A single, long extension from the cell body, transmitting information to other neurons or effector organs.

• Synaptic Terminal: The bulbous ending of the axon, containing synaptic vesicles, where neurotransmitters are released.

• Nissl Bodies: Rough endoplasmic reticulum in the neuron’s cytoplasm, involved in protein synthesis.

• Synaptic Vesicles: Small sacs containing neurotransmitters, found in the synaptic terminal.

Difference between Axon and Dendrite Cytoplasm: Axon cytoplasm lacks ribosomes and rough endoplasmic reticulum, thus incapable of protein synthesis.

Types of Neurons:

• Multipolar Neuron: Most common, found in the anterior horn of the spinal cord and cerebral cortex.

• Bipolar Neuron: Found in the retina of the eye.

• Unipolar Neuron: Found in the dorsal root ganglia of the spinal cord.

Synapse: The junction between two neurons or between a neuron and a muscle cell, ensuring information transmission from one neuron to another.

Synapse Structure:

• Presynaptic Terminal: The synaptic terminal of the axon, containing many synaptic vesicles.

• Postsynaptic Membrane: A specialized region on the membrane of the neuron or muscle cell receiving the signal from the presynaptic terminal.

• Synaptic Cleft: The gap between the presynaptic terminal and the postsynaptic membrane.

• Chemical Synapse: Approximately 20-30 nm wide.

• Electrical Synapse: Approximately 2-4 nm wide.

Neurotransmitter: Stored within synaptic vesicles, responsible for transmitting information across the synaptic cleft.

Synapse Location:

• Axosomatic: Axon connects with the cell body of another neuron.

• Axodendritic: Axon connects with the dendrites of another neuron.

• Axoaxonic: Axon connects with the axon of another neuron.

Synapse Ratio:

• Chemical Synapse: Accounts for 99% of all synapses.

• Electrical Synapse: Accounts for 1% of all synapses.

Characteristic of Electrical Synapse: The presynaptic terminal and postsynaptic membrane are symmetrical.

Mechanism of Nerve Impulse Transmission:

1. The presynaptic membrane undergoes depolarization due to stimulation.

2. Calcium channels open.

3. Calcium influx triggers exocytosis of synaptic vesicles.

4. Neurotransmitter release into the synaptic cleft.

5. Neurotransmitter interacts with receptors on the postsynaptic membrane.

6. Activation of postsynaptic membrane depolarization.

7. Recycling of synaptic vesicle membranes.

Glial Cells:

• Peripheral Glial Cells: Include two types:

• Satellite Cells: Found in ganglia, supporting and protecting neurons.

• Schwann Cells: Form myelin sheaths around peripheral nerves.

• Central Glial Cells: Include four types:

• Astrocytes: Support the central nervous system, form the blood-brain barrier, provide structural support, regulate the environment, and absorb excess substances.

• Oligodendrocytes: Form myelin sheaths around central nerves.

• Ependymal Cells: Transform into simple cuboidal epithelial cells with cilia when lining the ventricles, producing and circulating cerebrospinal fluid.

• Microglia: Serve a phagocytic function among glial cells.

Glial Cell Tumors: 80% of glial cell tumors arise from astrocytes.

Blood-Brain Barrier: Formed by astrocytes, protecting nervous tissue and maintaining the composition of the cerebrospinal fluid.

Astrocyte Types:

• Fibrous Astrocyte: Located in white matter, having long and thin branches.

• Protoplasmic Astrocyte: Located in gray matter, having thick and short branches.

Conclusion: Nervous tissue is a complex tissue that plays a crucial role in receiving, processing, and transmitting information throughout the body. Neurons and glial cells work together to ensure the proper functioning of the nervous system.