Neurotrophins in Nervous System

Neurotrophins

• Small proteins crucial for maintaining nerve cell viability and function.
• Secreted by various tissues like muscles, neurons, and astrocytes.
• Essential for the growth and development of nerve cells in both the central nervous system (CNS) and peripheral nervous system (PNS).
• Functions include promoting nerve cell survival, repair, and overall maintenance of nerve tissue, as well as facilitating neural transmission.
• Recent research shows neurotrophins can induce regrowth in damaged neurons.
• Potential applications in treating neurological disorders such as Parkinson's and Alzheimer's diseases.

Nerve Fibers

• Also known as axons, carry nerve impulses away from the cell body.
• Can be classified based on structure (myelinated vs. non-myelinated), distribution (somatic vs. autonomic), origin (cranial vs. spinal), function (sensory vs. motor), type of neurotransmitter (adrenergic vs. cholinergic), and diameter (Erlanger-Gasser classification).
• Myelinated fibers are faster and more efficient compared to non-myelinated fibers.
• Classification includes three major groups: Type A (thickest, myelinated), Type B (intermediate), Type C (thinnest, unmyelinated).

Neuron Classification

• Two main types of neurons based on length of axons:
  • Golgi Type I Neuron: Long axons, more than 100 dendrites, includes motor cells of the spinal cord and pyramidal cells of the cerebral cortex.
  • Golgi Type II Neuron: Short axons, more than 30 dendrites, primarily found in the cerebral and cerebellar cortex.

Neuron Structure

• Composed of three main parts: cell body (soma), dendrites, and axon.
• Neuron characteristics include excitability, conductivity, secretion, longevity, and a high metabolic rate.
• Neurofibrils consist of microtubules and microfilaments, providing structural support within neurons.
• Mitochondria serve as powerhouses for ATP production in soma and axons while the Golgi apparatus processes and packages proteins.

Dendrites

• Branching extensions that transmit impulses towards the cell body, generating local potentials.
• May have dendritic spines, facilitating synaptic connections.
• Contain Nissl granules and neurofibrils and are part of the input zone of neurons.

Axon Structure

• Each neuron has a single axon arising from the axon hillock.
• Responsible for carrying impulses away from the cell body; lacks protein-synthesis machinery.
• Can be myelinated or unmyelinated and contains synaptic vesicles for neurotransmitter release.
• The myelin sheath, formed by Schwann cells outside the CNS and oligodendrocytes inside, increases impulse conduction speed.

Myelinated vs. Unmyelinated Nerve Fibers

• Myelinated Nerve Fibers:

  • Presence of myelin sheath accelerates nerve impulse transmission through saltatory conduction at nodes of Ranvier.
  • Axons are insulated, leading to more energy-efficient conduction.
  • Typically found in both central and peripheral nervous systems.

• Unmyelinated Nerve Fibers:

  • Lack a myelin sheath, resulting in slower impulse transmission through continuous conduction.
  • Commonly located in the peripheral nervous system and involve visceral functions.

Saltatory vs. Continuous Conduction

• Saltatory conduction occurs in myelinated neurons, where impulses leap from node to node, enhancing speed and efficiency.
• Continuous conduction occurs in unmyelinated fibers, with impulses propagating along the entire length of the axon at a slower rate.

Key Differences between Myelinated and Unmyelinated Neurons

• Myelinated neurons have a present myelin sheath, faster transmission, and larger axon diameter.
• Unmyelinated neurons lack insulation, transmit impulses slower, and have a smaller diameter.