Nervous Tissue Histology PDF

# Nervous tissue - Prepared by Dr. Ahmed Morsi - Assistant professor of Histology & Cell Biology ## Learning objectives By the end of this lecture, you will be able to: - Describe structure of a neuron - Classify synapse - Understand the microstructure of peripheral nerve - Differentiate between...

# Nervous tissue - Prepared by Dr. Ahmed Morsi - Assistant professor of Histology & Cell Biology ## Learning objectives By the end of this lecture, you will be able to: - Describe structure of a neuron - Classify synapse - Understand the microstructure of peripheral nerve - Differentiate between spinal & sympathetic ganglia # Nervous tissue: - Histologically, the nervous tissue is composed of 2 main cell types: - Neurons (nerve cells) - Neuroglia (glial supporting cells) # I. Neurons: - Definition: The structural and functional unit of the nervous tissue. - Structure: - Cell body (soma / perikaryon) - Cell processes (neurites) ## Structure of the neuron - **Cell body (perikaryon)** - The part that contains the nucleus and surrounding cytoplasm - It is receptive to stimuli - Has different shapes e.g. globular, fusiform or stellate ### Compare | | Axon | Dendrites | | :------------- | :------------------------------------ | :----------------------------------------- | | **Cell processes** | Single process | Multiple processes | | **Characteristics** | - long, thick | - Short, thin | | | - No spines | - Covered by spines | | | - No branching except at its end | - Branching like tree | | | - Carry impulses away from cell body | - Carry to cell body | ## Cytoplasm: - The cytoplasm contains a large central nucleus, numerous mitochondria, lysosomes and a Golgi complex, in addition to: - **Nissl granules:** Basophilic granular areas correspond to rER, free ribosomes. - **Neurofibrils:** Include microfilaments (stained with silver) and microtubules (axonal transport). - **Lipofuscin pigment:** Age related pigment, represents undigested material by lysosomes. - **Melanin pigments:** Present in certain neurons e.g. substantia nigra of midbrain. - **Lipid droplets:** ## Classification of neurons according to number of processes 1. **Unipolar neuron:** Has single process that divide in T shaped manner e.g. DRG (dorsal root ganglia) 2. **Bipolar neuron:** Has one axon & one dendrite e.g. spiral and vestibular ganglia 3. **Multipolar neuron:** Has one axon & many dendrites. The most common type e.g. motor neurons 4. **Anaxonic neuron:** Has many dendrites but no true axon, do not produce action potentials but they are interneurons regulating electrical activity of adjacent neurons. ## Synapse: - Definition: The site of functional contact between neurons or neurons & effector organs (gland or muscle). ## Types: 1. **Chemical synapse:** Involve the release of neurotransmitter (NT). It is delayed. 2. **Electrical synapse:** No NT is released, but there are direct ion channels between the 2 neurons through which ions can pass from one cell to another. So, it is more rapid than chemical synapse. ## Structure of chemical synapse: - A synapse transmits an impulse only in one direction from on cell to the other, so a synapse is formed of: - **Presynaptic side:** Nerve - **Postsynaptic side:** Muscle, gland - **Synaptic cleft inbetween:** Narrow space between the 2 neurons through which the NT is transported. ## Classification of synapse: 1. **Axodendritic synapse:** Between axon & dendrites 2. **Axosomatic synapse:** Between axon & cell body 3. **Axoaxonic synapse:** Between axons # Peripheral nervous system - The peripheral nervous system (PNS) is formed mainly of: - Nerves - Ganglia - Nerve endings # Peripheral nerve: - Structure: Bundles of nerve fibers (axons) held together by CT:- - Groups of nerve fibers are bundled together as fascicles by a CT layer (Perineurium). - The individual nerve fibers are enclosed by delicate vascularized CT layer (Endoneurium) - The whole nerve is covered externally by thick dense CT layer called (Epineurium) ## Myelinated and unmyelinated nerve fibers - Nerve fibre (Axon) may be myelinated or unmyelinated. Myelinated axons are ensheathed by a layer of myelin called myelin sheath. ## Myelin sheath formation: 1. Myelin sheath is located outside the axolemma of the axon. 2. Schwan cells are responsible for myelin formation in PNS however, oligodendrocytes in CNS. ### Myelin sheath formation - continued: 3. It is formed by invagination of the axon into the Schwan cell / oligodendrocyte with its subsequent rotation around the axon several turns. 4. So, the axon is enrolled by a large number of Schwan cells a long its length, hence each Schwan cell provide myelin sheath for a short segment of the axon. 5. **Node of Ranveir** are uncovered areas of the axons that represent short gap between 2 adjacent Schwan cells/oligodendrocytes. # Ganglia: - Definition: They are typically ovoid structures containing nerve cell bodies & their surrounding glial satellite cells and surrounded by CT capsule ## Types of Ganglia | | Spinal ganglia | Sympathetic ganglia | | :--------------------------- | :---------------------- | :------------------------- | | **a) CT capsule** | Thick | Thin | | **b) Blood vessels** | Less | Numerous | | **c) Nerve cells:** | | | | ** 1. Shape** | Unipolar | Stellate multipolar | | ** 2. Size** | Large with marked variation | Small & uniform | | ** 3. Arrangement** | Arranged in groups or rows | scattered | | ** 4. Satellite cells** | Numerous | Few | | ** 5. Glomeruli** | Present | Absent | # Reference: - Junqueira's Basic Histology: Text and atlas, 13th Edition, 2013. - Anthony L. Mescher # Thank you