Chapter 12B: Neurons Structure and Classification Study Notes

Overview of Neurons

  • Neurons are the functional unit of the nervous system, akin to how muscle fibers are the cells of muscle tissue.

  • This lecture focuses on the structure of neurons which needs to be memorized for future understanding of neuronal function.

Structure of Neurons

  • General Appearance

    • Neurons typically have a structure that appears branched with a long extension and additional branching.

Types of Neurons

  • Multipolar Neuron

    • The most common neuron type characterized by multiple dendrites and a single axon.

    • Key Components:

    • Cell Body (Soma): The central part housing the nucleus.

    • Dendrites: Branching structures that receive signals from other neurons.

    • Axon: Long extension that carries electrical impulses away from the cell body.

Special Terms Related to Neuronal Structure

  • Pericaryon:

    • Refers to the area around the nucleus of a neuron, denoting the cytoplasm surrounding the nucleus.

    • Originates from Latin terms: "peri" (around) and "karyon" (kernel or nucleus).

  • Cytoplasmic Components:

    • Mitochondria: Organelles providing energy for the neuron's activities.

    • Rough Endoplasmic Reticulum (RER) and Ribosomes: Key to protein synthesis, particularly neurotransmitters.

  • Neurofilaments and Neurotubules:

    • Structural components important for maintaining shape and intracellular transport. These are analogous to microfilaments and microtubules in non-neuronal cells.

  • Nissl Bodies:

    • Dense areas within the cell body rich in ribosomes and RER, contributing to the gray appearance of gray matter in the nervous system.

  • Dendrites:

    • Tree-like structures on neurons that receive information from other neurons, significantly increasing a neuron's surface area for connections.

  • Axonal Components:

    • Axolemma: The specialized cell membrane of the axon.

    • Axoplasm: The cytoplasm within an axon, containing organelles and structures necessary for function.

    • Axon Hillock: The tapered area between the soma and the axon where action potentials are initiated.

  • Telodendria:

    • The distal extensions of the axon that terminate the axon and form synaptic connections with other neurons or target cells.

Synapses

  • Synapse:

    • The junction between neurons where neurotransmitters are released.

    • Presynaptic Membrane: The membrane of the neuron transmitting the signal.

    • Postsynaptic Membrane: The membrane of the receiving neuron or target cell.

    • Synaptic Cleft: The gap between the presynaptic and postsynaptic membranes.

Classification of Neurons

  • Neurons are classified based on structure and function:

    • Structural Classification: Anaxonic, Bipolar, Unipolar, and Multipolar.

    • Anaxonic Neurons: Do not have an axon; commonly found in the brain.

    • Bipolar Neurons: Feature one dendrite and one axon; typically involved in sensory processes (e.g., the retina).

    • Unipolar Neurons: Have a cell body located off to the side with fused axon and dendrites; often sensory neurons in the PNS.

    • Multipolar Neurons: Characteristic of motor neurons, possessing one axon and multiple dendrites.

  • Functional Classification: Sensory (Afferent), Motor (Efferent), and Interneurons.

    • Sensory Neurons: Carry information from sensory receptors to the CNS (e.g., touch, temperature).

    • Types of sensory receptors include:

      • Exteroceptors: Monitor the external environment (e.g., heat from a stove).

      • Interoceptors: Monitor internal systems (e.g., digestive system).

      • Proprioceptors: Help with the sense of body position and movement.

    • Motor Neurons: Transmit commands from the CNS to peripheral effectors (muscles and glands).

    • Interneurons: Located primarily in the CNS, relay signals between sensory and motor neurons; involved in reflexes and complex processes like learning.

Axoplasmic Transport

  • Axoplasmic Transport: Refers to the mechanisms for transporting materials such as neurotransmitters within the axon, mediated by motor proteins like kinesins and dyneins.

Summary of Learning Points

  • The main focus of this micro lecture is the structural understanding of neurons, which lays the foundation for understanding their function in future lectures.

  • Key structures to memorize include soma, dendrites, axon hillock, axolemma, telodendria, and synapse.

  • Understanding classifications based on structure and function prepares students for deeper discussions on neuronal physiology and action potentials in subsequent lectures.