Nervous Tissue Flashcards P2

Nervous Tissue PART II, facilitated by Simone Joseph, DC, DACO, delves into the second segment of nervous tissue study, focusing on neuroglia of both the peripheral and central nervous systems, and the overall organization of the nervous system. This section primarily distinguishes between various glial cell functions, anatomical structures like myelin, and the macroscopic arrangement of gray and white matter within the CNS.

NEUROGLIA CELLS - PERIPHERAL NERVOUS SYSTEM (PNS)
  • Schwann Cells (Neurolemmocytes):
    • Primary function: To myelinate axons of the PNS.
    • Myelination Process:
      1. A neurolemmocyte (Schwann cell) initiates wrapping around a segment of an axon.
      2. The neurolemmocyte's cytoplasm and plasma membrane begin to form consecutive layers around the axon.
      3. The overlapping inner layers exclusively composed of the neurolemmocyte's plasma membrane constitute the myelin sheath.
      4. Ultimately, as the myelin sheath forms, the neurolemmocyte's cytoplasm and nucleus are displaced and pushed to the periphery of the cell, external to the myelin layers.
    • Node of Ranvier: These are crucial gaps found at regular intervals in the insulating myelin sheath along the axon. Their function is to facilitate the rapid, saltatory conduction of nerve impulses by allowing the action potential to "jump" from node to node.
    • Neurolemma (Sheath of Schwann): This refers to the outermost nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath.
  • Unmyelinated Axons:
    • Not all axons are myelinated; some are classified as unmyelinated.
    • These axons still possess a neurolemma, which is the Schwann cell's external covering.
    • In the process for unmyelinated axons, a single Schwann cell may envelop multiple axons.
    • However, unlike myelinated axons, there are no extensive myelin sheath wraps around each individual unmyelinated axon, meaning a single Schwann cell merely encloses multiple axons without forming compact myelin.
  • Myelinated Axon vs. Connective Tissue: It is crucial to differentiate between the myelination surrounding axons and dense collagenous connective tissue. Myelination is formed by glial cells, while collagen fibers are part of the extracellular matrix of connective tissue.
  • Satellite Cells:
    • Location: These cells surround the neuronal cell bodies within ganglia in the PNS.
    • Ganglion: A cluster of neuron cell bodies located outside the Central Nervous System (CNS). Ganglia can be either sensory or autonomic. For instance, sensory ganglia (containing pseudounipolar neurons) are typically surrounded by numerous satellite cells, whereas autonomic ganglia (containing multipolar neurons) also have satellite cells.
    • Functions:
      • Provide insulation for the neuronal cell bodies.
      • Aid in nourishing the neurons.
      • Regulate the microenvironment around the neuronal cell bodies, maintaining appropriate chemical balances.
NEUROGLIA - CENTRAL NERVOUS SYSTEM (CNS)
  • Oligodendrocytes:
    • Primary function: To myelinate axons within the CNS.
    • Distinctive feature: Unlike Schwann cells (which typically myelinate only one segment of one axon), a single oligodendrocyte can myelinate multiple axons or multiple segments of a single axon.
  • Astrocytes:
    • These are the most abundant glial cells in the CNS, characterized by their star-like shape.
    • Functions:
      • Control Ionic Environment: Crucial for maintaining the proper ionic concentrations around neurons, essential for nerve impulse transmission.
      • Form Scar Tissue: Following CNS injury, astrocytes proliferate and form glial scars, which can protect the healthy tissue but also impede axonal regeneration.
      • Contribute to the Blood-Brain Barrier (BBB): Astrocytic end-feet wrap around capillaries in the brain, inducing the tight junctions that characterize the BBB, thereby regulating the passage of substances from the blood into the brain tissue.
  • Microglia:
    • These are small, phagocytic glial cells.
    • Function: They act as the primary immune defense of the CNS, scavenging cellular debris, pathogens, and dead neurons. They are considered the resident macrophages of the brain and spinal cord.
  • Ependymal Cells:
    • Location: These cells line the ventricles of the brain and the central canal of the spinal cord (CNS cavities).
    • Function: They form the choroid plexus, a specialized structure within the ventricles that produces cerebrospinal fluid (CSF). Their cilia also help circulate the CSF.
ORGANIZATION OF THE NERVOUS SYSTEM
  • The nervous system is fundamentally divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
  • CNS Components: Consists of the spinal cord and the brain.
    • Gray Matter vs. White Matter:
      • Gray Matter: Primarily composed of neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.
      • White Matter: Primarily composed of myelinated axons, which derive their white appearance from the lipids in the myelin sheath.
    • Distribution in Spinal Cord: The spinal cord generally has gray matter located internally (e.g., butterfly-shaped central region) and white matter positioned externally.
    • Distribution in Brain:
      • Cerebral Hemispheres and Cerebellar Hemispheres: Characterized by an outer shell (cortex) of gray matter.
      • Deep to the cortex, there is white matter.
      • Even deeper within the brain, additional clusters of gray matter (nuclei) are found.
CORTICAL LAYERS
A. CEREBRAL CORTEX
  • The cerebral cortex, the extensively folded outer layer of the cerebrum, is composed of 66 layers.
  • Pyramidal neurons are prominent neuron types found within these layers, along with glia cells.
  • Cortical Layers and Their Components:
    1. Molecular Layer (Layer I):
      • Components: Predominantly contains axons and dendrites (cell processes) of neurons from other layers. It is relatively cell-sparse.
      • Afferents: Receives input from other cortical regions and the brainstem.
    2. External Granular Layer (Layer II):
      • Components: Densely packed stellate cells and small pyramidal cells.
      • Efferents: Projects to other regions of the cortex (intra-cortical association functions).
    3. External Pyramidal Layer (Layer III):
      • Components: Loosely packed stellate cells and medium pyramidal cells.
      • Efferents: Projects to other regions of the cortex (intra-cortical association functions).
    4. Internal Granular Layer (Layer IV):
      • Components: Densely packed stellate cells exclusively.
      • Afferents: Major recipient of input from the thalamus.
    5. Internal Pyramidal Layer (Layer V):
      • Components: Characterized by large pyramidal cells only (with few stellate cells). This layer includes the "Giant Pyramidal cells of Betz" in the primary motor cortex.
      • Efferents: Primary source of projection fibers to the brainstem and spinal cord.
    6. Multiform Layer (Layer VI):
      • Components: Contains multiple-sized pyramidal cells and loosely packed stellate cells.
      • Efferents: Projects to the thalamus.
B. CEREBELLAR CORTEX
  • The cerebellar cortex, responsible for motor control and coordination, consists of 33 distinct layers.
  • Instead of pyramidal cells, Purkinje cells are the characteristic large neurons of the cerebellar cortex.
  • Cerebellar Cortical Layers:
    1. Molecular Layer:
      • The outermost layer, superficial to the Purkinje cell layer.
    2. Purkinje Cell Layer:
      • A thin, singular layer containing the large, flask-shaped cell bodies of Purkinje neurons.
    3. Granular Layer:
      • The innermost layer, deep to the Purkinje cell layer and superficial to the white matter, densely packed with granule cells and Golgi cells.