Neurons

Nervous System

• CNS contains brain and spinal cord
• PNS contains cranial and spinal nerves

Neuroglial cells - do not generate or conduct impulses; support, nurture and protect neurons, smaller, more numerous, mitotic

Types:

1. Astrocytes - star shaped, in CNS, maintain K⁺ balance, link neurons and vessels forming blood-brain barrier that regulated passage into brain
2. Oligodendrocytes - most common, produce myelin sheath around axons in CNS
3. Microglia - form from monocytes, phagocytic cells (microbes and debris) in CNS
4. Ependyma - line CSF-filled cavities of CNS and filter CSF
5. Satellite cells - surround and support ganglionic cells in PNS
6. Schwann cells - produce myelin sheath around axons of PNS neurons

• Unmyelinated axons -  covered by a thin layer of glial plasma membrane
• Myelinated axons - have a thick myelin sheath composed of lipid and protein (myelin), acts as insulating layer
  • Schwann cell membrane wraps around the axon many times; nucleus and cytoplasm form outermost layer or neurolemma which provides regeneration tube
  • Nodes are uncovered areas between Schwann cells

Neurons - convert stimuli into nerve impulses (excitability)

1. Soma - cell body with typical plasma membrane and cell organelles
2. Dendrites - highly branched generally short cytoplasmic processes, receive input
3. Axons - long thin process, send impulse
   1. Originates on axon hillock of soma, initial segment contains trigger zone, with neurofilaments for transport
   2. Axon collaterals - side branches
   3. Terminates in many fine filaments or axon terminals with synaptic knobs containing synaptic vesicles
   4. Limited mitosis, new cells form in hippocampus of humans

Neurons classified by number of processes:

1. Unipolar - one bifurcated process, dendrites and axon are continuous, sensory neurons
2. Bipolar - two processes, one dendritic and one axonic, special senses neurons
3. Multipolar - more than two processes, one axon and several dendrites, motor neurons