Notes on Functional Microanatomy of Neurons

Overview of Functional Microanatomy of Neurons

  • Focus on the structure and function of neurons in the central nervous system (CNS).

  • Three Learning Objectives:

    • Describe the classes of cells in the CNS.

    • Understand the microanatomy of neurons.

    • Explain how neuron structures form neural tissue, specifically gray and white matter.

Types of Cells in the CNS

  • Neurons: Primary processors of neural signals.

  • Neuroglial Cells (Glia): Support electrical and chemical functions of neurons; provide blood supply and help remove waste.

  • Vascular Endothelial Cells: Ensure nutrient and oxygen supply to the brain, essential for neural tissue health.

Neuron Basics

  • Fundamental Unit: Neurons are the core functional units of the CNS.

  • Metabolic Machinery: Neurons have organelles similar to somatic cells, including the nucleus, endoplasmic reticulum, and mitochondria.

  • Diversity in Form (Morphology): Neurons exhibit various shapes, impacting their functions and connections.

  • Bioelectrical Properties: Unique electrical signaling capabilities, including action potentials.

  • Chemical Communication: Neurons use neurotransmitters for signaling.

Parts of a Neuron

  • Cell Body (Soma): Contains nucleus and organelles; rich in endoplasmic reticulum and mitochondria for protein synthesis and energy.

Dendrites

  • Structure: Short, branching extensions from the cell body.

  • Function: Increase surface area for receiving inputs from other neurons (input zones).

  • Spines: Small protrusions on dendrites where synapses may occur; can vary in shape (e.g. mushrooms or fibres) and density (hint at functional roles).

  • Smooth vs. Spiny Dendrites: Smooth dendrites often correspond to inhibitory neurons; spiny dendrites associated with excitatory transmission.

  • Pyramidal Neurons: Unique morphology with a pyramidal cell body and an apical dendrite; involved in long-range signaling.

Axons

  • Function: Long projections from the cell body capable of generating and propagating action potentials (output zones).

  • Lengths: Vary from millimeters to potentially meters in length (e.g., motor cortex to spinal cord).

  • Electrical Signals: Action potentials are regenerated along the axon to convey information.

Synapses

  • Types:

    • Electrical Synapses: Directly connected by junctions, allowing immediate signal passage.

    • Chemical Synapses: Indirect communication through neurotransmitter release across a synaptic cleft.

Neural Tissue Structure: Gray Matter vs. White Matter

  • Gray Matter:

    • Composed of neuron cell bodies, dendrites, and synaptic connections; the input zone of neurons.

  • White Matter:

    • Composed of myelinated axons and glial cells; acts as the conducting zone for signals.

  • Vascular Endothelial Cells: Present in both, providing necessary blood supply.

Neuron Types

  • Projection Neurons: Long axons projecting over considerable distances, often excitatory; includes pyramidal neurons.

  • Interneurons: Short axons, typically found within local circuits; can be excitatory or inhibitory.