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Neurons Notes

Overview of the Nervous System and Neuroglia

  • Understanding the basic functions and divisions of the nervous system is crucial.

  • Types of neuroglia and their functions are fundamental to neuronal support.

  • Importance of the myelin sheath and its formation in CNS and PNS are critical concepts.

Basic Functions of the Nervous System

  • Communication: Sending and receiving signals throughout the body.

  • Integration: Processing sensory information and responding appropriately.

  • Control: Regulating bodily functions and maintaining homeostasis.

Structural and Functional Divisions of the Nervous System

  • Structural Divisions:

    • Central Nervous System (CNS): Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS): Consists of all nerves outside the CNS.

  • Functional Divisions:

    • Sensory (Afferent): Conduct impulses from sensory receptors to the CNS.

    • Motor (Efferent): Conduct impulses from the CNS to effectors (muscles and glands).

Types of Neuroglia

  • Astrocytes: Support neurons, regulate blood flow, and maintain the blood-brain barrier.

  • Microglia: Act as immune cells in the CNS and remove debris.

  • Oligodendrocytes (CNS) and Schwann Cells (PNS): Form myelin sheath around axons, enhancing signal transmission.

  • Ependymal Cells: Line the ventricles of the brain and contribute to cerebrospinal fluid production.

Importance of the Myelin Sheath

  • Function: Insulates axons to allow for faster electrical impulse propagation.

  • Formation in CNS: Created by oligodendrocytes which can myelinate multiple axons simultaneously.

  • Formation in PNS: Created by Schwann cells, each cell myelinating a single axon segment.

Neurons

  • Definition: Structural units of the nervous system; specialized cells that transmit electrical impulses.

  • Characteristics:

    • Longevity: Can last a person's lifetime.

    • Amitotic: Do not undergo mitosis, with minor exceptions.

    • High Metabolic Rate: Require continuous oxygen and glucose.

Neuron Structure

  • Cell Body (Soma): Biosynthetic center, synthesizes proteins and chemicals; contains Nissl bodies (rough endoplasmic reticulum).

  • Processes: Extensions from the neuron cell body:

    • Dendrites: Receive incoming signals; convey graded potentials toward the cell body.

    • Axons: Transmit action potentials away from the cell body; may have axon collaterals and branches known as axon terminals.

Dendrites

  • Function: Receptive regions for signals received from other neurons.

  • Special Features: Dendritic spines increase surface area for synaptic connections.

Axon

  • Function: Conducting region generating nerve impulses (action potentials).

  • Structure: Starts at axon hillock and can be over a meter long; branches at terminus into axon terminals, which secrete neurotransmitters to communicate with other neurons.

Axon Transport

  • Movement Directions:

    • Anterograde: Toward axon terminals from cell body.

    • Retrograde: Toward cell body from axon terminals (important for delivery of materials).

  • Medical Relevance: Some viruses utilize this transport to invade the CNS.

Types of Neurons by Structure

  • Multipolar Neurons: Most common type; multiple processes (multiple dendrites, one axon).

  • Bipolar Neurons: One axon and one dendrite; found in special senses such as sight (retina) and smell (olfactory mucosa).

  • Unipolar Neurons: One T-shaped process; primarily sensory neurons.

Types of Neurons by Function

  • Sensory Neurons: Afferent; conduct impulses from receptors to CNS.

  • Motor Neurons: Efferent; conduct impulses from CNS to effectors.

  • Interneurons: Connect sensory and motor pathways; most are found in CNS, constituting 99% of all neurons.

Conclusion

  • Post-lecture objectives:

    • Define neurons and their functions.

    • Relate the neuronal structure to its functional role.

    • Distinguish between nerves, tracts, nuclei, and ganglia.

    • Classify neurons based on structure and function.