Cellular anatomy of the nervous system

• Reticular theory by Golgi: All parts of the nervous system make up a large, physically connected network.
• Neuron doctrine: The nervous system is a series of individual units that are separated from one another physically.
• The axon hillock is the first section of the axon, between the cell body(soma) and the beginning of the projection. It weighs all incoming signals(excitatory, inhibitory, and modulatory) to decided whether or not to pass a signal onto the next cell.
• The active zone is a subarea in the axon terminal and contains proteins for neurotransmitter release.
• The terminal button is responsible for the release and production of neurotransmitters.
• Microtubule: an organelle that runs the length of the axon where motor-like proteins help transport newly created proteins.
• Neurofilaments: maintaining the structure of the axon.
• Vesicles are small sacs that are located at the terminals and contain molecules for chemical communication. When an action potential changes the electrical charge of the cell membrane, the vesicle fuses with the inner membrane at the axon terminal, releasing its neurotransmitters outside of the cell.
• In electrical synapses, the two cells share a cytoplasm but have two distinct cell membranes.
• Multiple sclerosis: A disease that is caused by the destruction of myelin sheath in the CNS, which affects both descending and ascending neurons. MS causes muscle weakness, poor balance, muscle spasms, as well as numbness and pain, and cognition impairment and vision loss. The leading theory for the cause of MS is that it is an autoimmune disorder where the immune system targets normal parts of the body(myelin sheath) as a foreign pathogen, causing demyelination.
• Neurons can be classified into three different groups by morphology: unipolar, bipolar, and multipolar.
• Unipolar neurons are only found in invertebrates; in humans there can be found pseudo-unipolar cells exclusively in sensory neurons. Unipolar neurons have only 2 branches of axons(1 leading to the periphery, away from the spinal cord, the other leading to the centre of the spinal cord.)
• Bipolar neurons contain 1 axon and 1 dendrite branch. They are exclusively found in human sensory systems(eg. the eye and olfactory senses)
• Multipolar neurons contain 1 axon and multiple dendrites and is the most common type of neuron in the human CNS.
• Neurons can be classified into 3 groups by function: sensory, motor, and interneuron. Sensory neurons are afferent neurons that detect stimuli from both the internal and external environment.
• Motor neurons include both somatic and autonomic motor neurons. Somatic motor neurons release neurotransmitter directly onto the muscles and thus control skeletal muscle movement. Autonomic motor neurons release neurotransmitters onto the autonomic ganglion(a clump of neurons outside of the CNS), and thus controls smooth muscle movement.
• Interneurons relay between motor/sensory neurons and the CNS and is an important part of the reflex circuit(eg. knee jerk).
• There are 5 main types of glia, which are types of non-neuronal cells in the nervous system.
• Astrocytes are one type of glia and have a star-shaped morphology. Their main function is to maintain the blood-brain barrier. They have protrusions called "endfeet" which wrap around the endothelial cells that surround the blood vessels. Functions include removing excess neurotransmitters and affecting extracellular concentration of ions, which affects cellular excitability. They also synthesize trophic factors, which support the growth, differentiation, and survival of neurons.
• Oligodendrocytes are another type of glia that only exists in the CNS. There are few branches that reach out from the cell body and they function to add a layer of myelination of multiple axons at once.
• Schwann cells are a type of glia that can only be found in the PNS. They provide a layer of myelination to PNS neurons, similar to oligodendrocytes, but they can only myelinate 1 section at a time. They are also able to help regenerate injured axons.
• Microglia are a type of glia that act as immune cells by identifying and destroying clumps of proteins, dead cells, and foreign pathogens in the nervous system.
• Ependymal cells are another type of glia and they make up the inside lining of the ventricles. They have small finger-like extensions called cilia that produce cerebrospinal fluid.
• The choroid plexus is a structure that involves the ependymal cells and forms a boundary between the blood and the cerebrospinal fluid.