Chapter 2 - Cells of the Nervous System Flashcards
Neuron doctrine
- 1890s
- theorized that the brain was composed of individual, highly specialized cells called neurons
- – and that these neurons were not connected continuously (like wires) but separated by functional space through which they communicate with each other called a synapse
Parikaryon
- aka soma or cell body
- metabolic center
- much like other cells in body
Neurites
- cellular fibers emerging from the soma
- e.g. dendrites, axon
Pre-synaptic terminals, branches, or boutons
- lie at the end of the axon
- contain synaptic vesicles which store neurochemical signals (neurotransmitters) that are essential for neuronal function
Dendrites
receive messages from other cells
Cell body
the cell’s life-support center
Axon
passes messages away from the cell body to other neurons, muscles, or glands
Neural impulse
electrical signal traveling down the axon
Myelin sheath
covers the axon of some neurons and helps speed neural impulses
Terminal branches of axon
form junctions with other cells
Neuronal action at the synapse
neurotransmitters are released from the pre-synaptic terminal and they cross the synapse to bind neurotransmitter receptors that are embedded in the membrane of the next neuron, forming the communicating link between neurons
Synapses
- communication between neurons usually occurs at space between the axon terminal and receptive region of postsynaptic neuron (an area called the synapse)
- diverse in shape and properties
- – can either be inhibitory or excitatory synapses
Different types of neurons in the nervous system
- neurons are characterized according to how many neurites originate from the soma
- – can be unipolar (one neurite), bipolar (two neurites), or multipolar (many neurites) nerve cells
Types of synapses
- information flows from the dendrite to soma to axon, then to:
- – axodendritic connections (synapses) – usually excitatory (presynaptic terminal to dendrite)
- – axosomatic synapses (presynaptic terminal to soma)
- – axo-axonal synapses (presynaptic terminal to axon)
- – axo-synaptic synapses – usually inhibitory (presynaptic terminal to presynaptic terminal)
Neuronal groups and connections
- neuronal cell bodies are grouped into clusters or groups within the brain called nuclei
- each nucleus contains:
- – projection neurons
- – interneurons
Projection neurons
- the neuron’s axon projects to other parts of the brain
- – axons project as part of tracts (fasciculi) or lemnisci (brain) or columns (spinal cord)
Interneurons
- neurons and their axons that project within a nucleus
- – called neuropil
- – these are short relays of information within a brain region
Neurglia
- macroglia and microglia
- – cells in nervous system that do not form synapses
- – dominate the nervous system
- —– play an important role in myelin formation, controlling ion balance, guidance of developing neurons, reuptake of neurotransmitters, and neuroimmunity
Glial cells
made up of macroglia and microglia
Macroglia
made up of oligodendrocytes and astrocytes
Microglia
made up of microglial cells
Oligodendrocytes
- predominate in white matter within the CNS only
- extend multiple dendritic arms to myelinate many axons, insulating cells in the CNS
(in the PNS, this function is done by the Schwann cell)
Astrocytes
- provide structural support
- act as glial guide wires for neurons during development
- maintain ion balance around neurons
- participate in reuptake of neurotransmitter adjacent to synapse
- surround blood vessels to act as an integral part of the blood brain barrier
- migrate to site of neuronal injury and proliferate to aid in repairing damaged neuronal tissue (specialized set of astrocytes called Reactive Astrocytes (still maintain all other astrocyte functions as well))
- – unfortunately, if unsuccessful can lead to gliosis
Microglial cells
- macrophages (scavengers)
- – mobile glial cells that survey the CNS to combat infection
- – activate and infiltrate injured zones of CNS to scavenge for infection and damage
- – some exist within the CNS and others infiltrate from the blood
Gliosis
glial scarring where glial cells dominate injury site
Protoplasmic astrocytes
- delicate astrocytes with many branched processes
- occur in gray matter
- respond to neural damage
Fibrous astrocytes
- more fibrous and robust astrocytes
- radiate long processes in all directions from soma
- surround blood vessels (blood brain barrier)
- exist in white matter areas of CNS
- cover exterior surface of CNS
Wallerian degeneration
- if axon is cut –> the distal end of the axon degenerates
- – cell body reaction (chromatolysis) is swelling and breakdown of Nissl substance (ribosome studded ER with free polyribosomes - site of translation)
- – axon reaction is loss of structural integrity (degeneration), detachment of synapses, swelling of neighboring astrocytes, and activation of microglia
Regeneration in the nervous system
- peripheral nerves
- – axonal regeneration (ligation or repair) of severed axons performed by Schwann cells
- CNS
- – axonal regeneration is aborted
- – oligodendrocytes produce inhibitory factors (NoGo)
- collateral sprouting
- – regenerative sprouting in CNS and PNS (F2-F15) in response to partial severing
- – remaining axons form new collaterals that try to reinnervate end organ
