86. Neuronal outgrowths. Axon. Dendrites.

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Neurons are characterized by their unique cellular processes, known as neuronal outgrowths, which include axons and dendrites. These structures play critical roles in the transmission and reception of nerve impulses within the nervous system.

axon

The axon is a single, elongated nerve fiber that extends from the neuron's cell body (perikaryon) and terminates at the synapse. It is specialized for the conduction of electrical signals away from the cell body to other neurons, muscles, or glands.

axon function

• Acts as the transmitting part of the neuron, carrying nerve impulses (action potentials) from the neuron to other cells (neurons, muscle cells, or gland cells).

• It plays a crucial role in neural communication by facilitating synaptic transmission at the terminal boutons.

axon structure

• Axon Hillock: The cone-shaped region where the axon originates from the cell body. It integrates synaptic inputs and initiates action potentials.

• Initial Segment: The region just distal to the axon hillock. It is not covered by a myelin sheath and is the site where action potentials are initiated.

• Axolemma: The plasma membrane of the axon that maintains the axonal environment and supports electrical conductivity.

• Axoplasm: The cytoplasm within the axon, containing mitochondria, microtubules, neurofilaments, and transport vesicles. Unlike the cell body, the axoplasm lacks rough endoplasmic reticulum (RER) and the Golgi apparatus (GA), meaning the axon is not capable of protein synthesis.

• Collateral Branches: Small offshoots of the axon that can form synapses with other neurons, allowing a single neuron to communicate with multiple target cells.

• Terminal Boutons (Synaptic Terminals): The distal end of the axon where synapses are formed with other neurons or non-neuronal cells to transmit nerve impulses.

axonal transport

• Anterograde Transport: Movement of materials from the cell body toward the axon terminal. It occurs at two rates:

  • Fast Stream: Moves organelles and vesicles at 10-40 cm/day.

  • Slow Stream: Transports cytoskeletal components at approximately 0.2-2.5 mm/day.

• Retrograde Transport: Moves materials from the axon terminal back to the cell body for degradation or recycling. It only occurs at a fast rate of 10-40 cm/day. This mechanism is also used for signaling molecules and neurotrophic factors.

dendrites

Dendrites are short, branching extensions of the neuron that receive signals from other neurons and convey them to the cell body.

dendrites function

• Signal Reception and Processing: Dendrites are the primary sites for receiving synaptic inputs from other neurons, which they integrate and transmit to the neuron's cell body.

• The large number of dendrites and their extensive branching allows a neuron to form synaptic connections with numerous other neurons, facilitating complex neural integration.

dendrites structure

• Tree-like Branching: Dendrites are typically short and taper as they extend from the cell body, forming a highly branched, tree-like structure.

• Dendritic Spines: Small, membranous protrusions on dendrites that serve as primary sites for synaptic contacts and initial processing of synaptic signals. These spines are dynamic structures that can change shape, affecting synaptic strength and plasticity.

• Dendritic Cytoplasm: Contains Nissl bodies (aggregates of RER and ribosomes), mitochondria, microtubules, and neurofilaments, but lacks the Golgi apparatus. This cytoskeletal network supports the dendrite's structure and facilitates intracellular transport

dendrites number and coverage

The number of dendrites per neuron can vary widely, typically ranging from 5 to 15. This extensive dendritic network constitutes about 80-90% of the neuron's receptive surface area, allowing for a high level of synaptic input and integration.