Nervous Tissue and Neuroglia

• Nervous tissue is characterized by being highly cellular, densely packed, and intricately intertwined.
• There are two main types of cells within nervous tissue: supporting cells and neurons.

Supporting Cells

• Commonly referred to as neuroglia, glia, or glial cells.
• Six types exist: four are located in the central nervous system (CNS), and two are in the peripheral nervous system (PNS).
• Neuroglia functions include supporting, insulating, and safeguarding delicate neurons.

Neuroglia in the CNS

• Four types are present
• These types are: Astrocytes, Microglia, Ependymal cells, and Oligodendrocytes.

Astrocytes

• They are abundant and have a star-like shape.
• Astrocytes account for almost half of the neural tissue.
• Numerous projections with swollen ends are used to anchor neurons to blood capillaries.
• They contribute to the blood-brain barrier through determining capillary permeability and making exchanges.
• Astrocytes protect neurons against harmful substances found in blood.
• They maintain the brains chemical environment through removal of excess K+ ions.

Microglia

• They are spider-like phagocytes.
• They monitor the health of nearby neurons and dispose of debris, including dead brain cells and bacteria.

Ependymal Cells

• These range from squamous to columnar in shape and are often ciliated
• They line the central cavities of the brain and spinal cord.
• Cilia aids in the circulation of cerebrospinal fluid
• They form a protective cushion around the CNS.

Oligodendrocytes

• These glial cells have fewer processes than astrocytes.
• Oligodendrocytes wrap tightly around nerve fibers, forming insulating myelin sheaths.
• They serve an insulation role in the nervous system.
• Glial cells and neurons share structural similarities but differ in function, glial cells do not transmit impulses nor divide.
• Most brain tumors are gliomas derived from glial cells.

Neuroglia in the PNS

• There are two types: Schwann cells and satellite cells.

Schwann cells

• Schwann cells form myelin sheaths around larger nerve fibers in the PNS.
• They are similar to oligodendrocytes in function.
• These cells are vital for the regeneration of damaged peripheral nerve fibers.

Satellite cells

• Satellite cells surround neuron cell bodies in the PNS.
• These cells have protective and cushioning functions.

Neurons

• Also known as nerve cells.
• Specialized in transmitting messages as nerve impulses from one area of the body to another.
• Neurons consist of a cell body with a nucleus and slender processes.
• Special characteristics include extreme longevity, being amitotic, and having a high metabolic rate.
• Neurons are complex cells that possess three functional components: an input region, a conducting component, and an output region.

Structure of a Neuron

• Consist of a cell body, which is the metabolic center of the neuron.
• A nucleus containing a nucleolus is contained in cell body
• Surrounded by cytoplasm containing organelles, excluding centrioles.
• Nissl bodies and neurofibrils within the rough ER are important for maintaining cell shape.
• Armlike extensions, or fibers, that vary in length constitute neuron processes.
• Processes that convey incoming messages toward the cell body are dendrites.
• Axons conduct impulses away from the cell body.
• Neurons can have hundreds of dendrites but only one axon, which originates from the axon hillock.
• Axons occasionally branch into collateral branches, and all axons profusely branch at their terminal end into axon terminals.
• Neurotransmitters are chemicals contained within tiny vesicles or membranous sacs inside the axon terminals.
• A synaptic cleft, which is a tiny gap, separates each axon terminal from the next neuron.
• The functional junction between neurons has a name: synapse.
• Most long nerve fibers are enveloped by myelin, a whitish, fatty material and has a waxy appearance.
• An axon outside the CNS are myelinated by Schwann cells.
• It protects and insulates nerve fibers as well as increases the transmission rate of nerve impulses.
• The myelin sheath is formed as Schwann cells wrap around the axon in a jelly-roll fashion, squeezing out cytoplasm.
• The myelin sheath encloses the axon when the wrapping process is complete, leaving most of the Schwann cell cytoplasm beneath the plasma membrane.
• The outer part of the Schwann cell around the myelin sheath is called the neurolemma.
• Nodes of Ranvier are gaps or indentations between Schwann cells.
• Multiple sclerosis is a homeostatic imbalance (SELF STUDY).

Terminology

• Nuclei: clusters of cell bodies within the CNS
• Ganglia: small groups of cell bodies
• Tracts: bundles of nerve fibers within the CNS.
• White matter is composed of myelinated regions of the CNS.
• Gray matter is composed of unmyelinated regions of the CNS.

Functional Classification of Neurons

• Neurons are grouped based on the direction that the nerve impulse travels relative to the CNS.

Sensory (afferent) neurons

• They transmit impulses from sensory receptors to the CNS.

Motor (efferent) neurons

• They transmit impulses from the CNS to the viscera, muscles, or glands.

Interneurons (association) neurons

• Interneurons connect sensory and motor neurons in neural pathways.

Structural Classification of Neurons

• Classification is based on the number of processes extending from the cell body.

Multipolar neurons

• They have several processes extending from the cell body.

Bipolar neurons

• They have two processes.

Unipolar neurons

• These neurons have a single process emerging from the cell body.
• Sensory receptors are activated via specific changes nearby.