Nervous Tissue Study Notes

Nervous Tissue

• Nervous tissue is the fundamental tissue that forms the nervous system, which is primarily composed of two cell types: neurons and neuroglial cells.

  • Neurons:

  • Serve as the structural and functional unit of the nervous system.

  • Neuroglial Cells:

  • Fill the interstitial spaces between neurons.

  • Provide support, protection, nourishment, repair, and metabolic control for neurons.

Neuron Structure

• A neuron is structured to consist of a cell body that emits a variable number of processes.

  • Cell Body (Soma/Perikaryon):

  • Composed of cytoplasm surrounded by a cell membrane.

  • Contains:

    • A large central nucleus.

    • Numerous mitochondria, lysosomes, and a Golgi complex.

• Unique Cytoarchitecture of Neurons:

  • Presence of granular material that stains intensely with basic dyes, termed Nissl Substance.

  • Nissl Granules:

    • Composed of rough endoplasmic reticulum.

  • Presence of a network of fibrils in the cytoplasm, known as Neurofibrils:

  • Composed of microfilaments and microtubules.

• Neuron Processes:

  • Most neurons give rise to:

  • Dendrites:

    • Short, branching processes that terminate near the cell body.

    • Irregular in thickness and extend Nissl granules into them, containing numerous small spines of variable shape.

  • Axon:

    • One longer process extending considerable distances from the cell body.

    • Exhibits a uniform diameter and lacks Nissl substance.

• Functional Differences:

  • Dendrite Function:

  • Nerve impulses travel towards the cell body.

  • Axon Function:

  • Nerve impulses travel away from the cell body.

• Axonal Sheaths:

  • Associated with specific cells that provide a sheath:

  • Schwann Cells:

    • Provide sheath for axons outside the Central Nervous System (CNS).

  • Oligodendrocytes:

    • Provide sheath for axons within the CNS.

Classification of Neurons Based on Number of Processes

• Multipolar Neuron:

  • Most common type of neuron.

  • Contains one axon and several dendrites.

  • Example: Neurons in the cerebral cortex and anterior horn cells.

• Bipolar Neuron:

  • Contains two distinct processes:

  • One axon and one dendrite.

  • Example: Sensory neurons found in olfactory mucosa, retina, and cochlea.

• Unipolar Neuron:

  • Contains a single process that eventually bifurcates into multiple branches.

  • Found in peripheral ganglia and some cranial ganglia.

• Pseudounipolar Neuron:

  • Initially has one process that eventually yields two processes:

  • One behaves as an axon and the other as a dendrite.

  • Found in the sensory ganglia of most neurons.

Classification of Neurons Based on Axon Length

• Golgi Type I Neurons:

  • Neurons with long axons that connect distant regions.

• Golgi Type II Neurons (Microneurons):

  • Neurons with shorter axons that end near the cell body.

• Myelination Variance:

  • Some axons are myelinated, and others are unmyelinated.

Grey and White Matter

• Examination of the spinal cord and brain reveals regions of distinct color:

  • Grey Matter:

  • Appears darker.

  • Composed of the cell bodies of neurons and their dendrites.

  • Most fibers are unmyelinated.

  • White Matter:

  • Appears lighter due to the presence of myelinated axons.

  • Consists predominantly of axons.

• Arrangement:

  • The alignment of grey and white matter differs regionally within the brain and spinal cord:

  • Spinal Cord and Brainstem:

    • White matter is exterior, with grey matter embedded within.

  • Cerebrum and Cerebellum:

    • A thin layer of grey matter (the cortex) covers the surface, with white matter lying beneath.

Terminologies in Nervous Tissue

• Nuclei:

  • Aggregations of neuron cell bodies within the CNS.

• Ganglia:

  • Aggregations of neuron cell bodies outside of the CNS.

• Tracts:

  • Aggregations of axon fibers within grey matter.

• Funiculi/Fasciculi or Lemnisci:

  • Larger collections of nerve fibers.

• Peduncles:

  • Large bundles of fibers connecting cerebral or cerebellar hemispheres to the brainstem.

• Peripheral Nerves:

  • Composed of processes of neurons outside the CNS.

Peripheral Nerves

• Collections of nerve fibers external to the CNS:

  • Efferent (Motor) Fibers:

  • Carry impulses from the spinal cord or brain to peripheral structures such as muscles or glands.

  • Comprised of axons of motor neurons located in grey matter of the spinal cord or brainstem.

  • Afferent (Sensory) Fibers:

  • Transmit impulses from peripheral organs to the brain or spinal cord.

  • Many are involved in the sensory transmission of touch, pain, temperature, etc.

Neuromuscular Junction

• Each skeletal muscle fiber has direct innervation from a neuron.

• The area where the nerve ending contacts the muscle fiber is termed the Neuromuscular Junction (or Myoneural Junction).

Synapse

• The synapse is a junction between neurons that facilitates communication.

  • Types of Synapse:

  • Axodendritic: Junction between an axon and a dendrite.

  • Axoaxonal: Junction between two axons.

  • Dendrodendritic: Junction between two dendrites.

Neuroglial Cells

• Neuroglial cells are the supporting cells of the nervous system:

  • Function to fill spaces between neurons (neurophil).

Neuroglial Cells in the CNS:

• Oligodendrocytes:

  • Similar to Schwann cells of the PNS and form myelin sheaths around CNS axons.

  • Features:

  • Round nucleus, deeply basophilic, poorly stained cytoplasm.

  • Typically smaller than neurons.

  • Assist in electrical insulation and energy conservation along axons.

• Astrocytes:

  • Star-shaped with numerous processes.

  • Fibroastrocytes: Found in white matter, generally fewer processes.

  • Protoplasmic Astrocytes: Present in grey matter, having a higher number of processes.

  • Functions:

  • Support neurons metabolically, produce blood-brain barrier, and nourish neurons.

• Ependymal Cells:

  • Line CNS cavities, facilitating circulation of cerebrospinal fluid (CSF) in the ventricles and spinal canal.

  • Lack a basement membrane but possess extensions into cavity substances; cells are junctionally complex.

• Microglial Cells:

  • Smaller in size, originate from polymorphonuclear monocytes, cross into the CNS and differentiate.

  • Functions include phagocytosis, antigen presentation, and modulation of CNS inflammation.

  • Also referred to as glial cells.

Neuroglial Cells in the PNS:

• Schwann Cells:

  • Function like oligodendrocytes by producing myelin sheaths in the PNS and providing electrical insulation to axons.

• Satellite Cells:

  • Surround neuron cell bodies in ganglia and support development and regeneration of peripheral nerves.

Questions, Comments, Suggestions, etc?

• Open floor for discussion or queries regarding the nervous tissue subject matter.