Neurons and Myelin Sheath

Myelin Sheath

  • Definition: Myelin is a fatty protein that covers large, long axons, providing electrical insulation to enhance the transmission of nerve impulses along the axon.

  • Speed Increase: Myelinated axons transmit nerve impulses approximately 150 times faster than unmyelinated axons.

  • Formation:

    • Formed by Schwann cells which are glial cells of the peripheral nervous system.

    • Schwann cells wrap around the axon multiple times to create the myelin sheath.

  • Neurilemma: The outermost layer of the myelin sheath, consisting of tightly coiled membranes.

  • Node of Ranvier:

    • A gap left between adjacent Schwann cells, significant for impulse transmission.

    • Axon collaterals are present at these nodes, allowing for the facilitated conduction of nerve impulses.

  • Types of Nerve Fibers:

    • Myelinated fibers: Compose the white matter in the central nervous system (CNS).

    • Unmyelinated fibers: Join with cell bodies present in the gray matter of the CNS.

Classification of Neurons

  • Structural Classification:

    • Multipolar Neurons:

    • Make up 99% of neurons.

    • Characterized by having numerous dendrites (3 or more cell processes).

    • Bipolar Neurons:

    • Contain 2 processes, one is a fused dendrite and the other is an axon.

    • They are rare and primarily found in sense organs, including the retina of the eye and olfactory mucosa.

    • Unipolar Neurons (or Pseudounipolar):

    • Have one process that emerges from the cell body, forming central and peripheral processes that function collectively as an axon.

    • Most are sensory neurons located in the peripheral nervous system (PNS).

  • Functional Regions of Neurons:

    • Receptive Region: Receives and processes stimuli (dendrites).

    • Conducting Region: Generates and transmits action potentials (axon).

    • Secretory Region: Releases neurotransmitters (axon terminals).

  • Neuron Types by Functional Class:

    1. Multipolar Neurons:

    • Many act as interneurons in the central nervous system (CNS), integrating sensory input and motor output.

    • Can connect as part of a chain of CNS neurons or function as a single neuron linking sensory and motor neurons.

    1. Motor Neurons:

    • Some multipolar neurons function as motor neurons, conducting impulses along efferent pathways to effectors (muscles or glands).

    1. Bipolar Neurons:

    • Primarily sensory neurons located in specialized sense organs.

    • Example: Bipolar cells of the retina transmit visual inputs from the eye to the brain via intermediate neurons.

    1. Unipolar Neurons:

    • Mainly sensory neurons conducting impulses along afferent pathways to the CNS for interpretation.

    • These sensory neurons are referred to as primary or first-order sensory neurons.

Relationships Between Neuron Types and Their Functionality

  • Multipolar Neurons:

    • Widely distributed; most are involved in interneuronal communication within the CNS.

  • Bipolar Neurons:

    • Specifically involved in the sensory processes of vision and olfaction. For example:

    ![Connection of Bipolar Neurons in Retina](An image showing the connection between bipolar neurons of the retina with visual processing in the brain)

  • Unipolar Neurons:

    • Typically serve sensory functions, including the transmission of sensory information from the peripheral nervous system to the central nervous system.

  • Functional Connections:

    • The connections among different types of neurons (e.g., multipolar interneurons interacting with motor neurons and sensory input pathways) underscore the complex interplay within the CNS and between the CNS and peripheral systems.

Summary of Neural Impulse Transmission

  • Action Potentials: Generated primarily in the trigger zones of neurons, the transmission velocity, and efficiency is significantly impacted by the presence of myelin (due to saltatory conduction at Nodes of Ranvier).

  • Integration & Interaction: Neurons can have multiple roles in complex signaling pathways, with inherent capabilities to process, transmit, and integrate impulses.