L17: Nervous Tissue I Flashcards
What is perikaryon?
- Soma or cell body of neuron
Classification of neuron types
- Multipolar (angular soma) = most common type
- Bipolar (ovoid soma)
- Pseudo-unipolar (spherical soma)
What are bipolar neurons associated with?
- Neurons associated with special senses: vision, sound, smell, balance…
Where are pseudo-unipolar neurons located?
- In DRG, trigeminal and other CN ganglia
Are nuclei of neurons euchromatic or heterochromatic?
- Euchromatic, cells never sleep
What are Nissl bodies / substance?
- Stacks of rER in neurons, indicative of large amount of protein synthesis occurring
- Basophilic
Organelles found in dendrites, axons?
- ) Dendrites: mitochondria, free ribosomes, rER may be found. No golgi
- ) Axons: mitochondria, microtubules, neurofilaments, vesicles, sER. No rER
What is the axolemma and axoplasm?
- Cell membrane specific to axon
- Cytoplasm specific to axon
Initial segment?
- Segment of axon distal to axon hillock and proximal to myelination
Collateral branches
- Axons branches near hillock
How to distinguish Axon hillock under microscopy?
- Pale staining, indicative of no rER
What pigment is found in neurons?
- Lipofuscin (wear and tear pigment) – represents old degraded membranes in lysosomes, which there is no mechanism to get rid of. Gets in way of neural function eventually
- Neuromelanin, by-product of dopamine synthesis (highly found in substantia nigra of midbrain)
Function of dendrites?
- Receptive surfaces of neuron, large number of them increase receptive surface area of neurons
What is myelin’s function?
- Increase conduction rate and insulate axons
What myelinates?
- Oligodendrocytes in CNS
- Schwann cells in PNS
Discuss two types of axoplasmic transport
- ) Anterograde (A=away from cell body): via kinesin
a. ) slow transport (of tubulin, actin, neurofilaments)
b. ) fast transport: requires ATP - ) Retrograde (R=return to cell body): via dynein**
a. ) fast transport system (of sER, vesicles, mitochondria, AAs, sugars, NTs, nucleotides, Calcium): requires ATP
* * Viruses and toxins that enter nerve endings use this system
What are neuroglia/glial cells/glia?
- Support cells that provide physical and physiological support to neurons
Types of neuroglial cells. Function of each?
- ) Protoplasmic/fibrous astrocytes: feet (pedicels) cover surfaces of capillaries in brain and spinal cord, specifically capillaries with basal lamina, monitoring chemical composition in extracellular space – therefore acts as blood brain barrier, forms scar tissue. Protoplasmic in gray matter. Fibrous in white matter.
- ) Microglia: vacuum cleaners/phagocytic cells of CNS
- ) Oligodendrocyte: myelin forming cells of CNS
- ) Ependymal cells: line brain and spinal cord cavities containing CSF, cover choroid plexus
- ) Schwann cells: myelin forming cells of PNS
- ) Satellite cells: support cells that surround cell bodies of neurons in ganglia, function analogous to Schwann cells without formation of meylin
What structural filaments are found in neurons?
- Neurofilaments (intermediate)
- Microfilaments (actin)
- Microtubules
Clinical relevance of astrocytoma
- Astrocytomas are primary brain tumors from fibrous astrocytes that make up about 80% of adult primary brain tumors
Clinical relevance of ependymoma
- Accounts for 5% of brain tumors from ependymal cells
Why do brain cancers arise from glial cells?
- Neurons don’t divide (as general rule) and therefore don’t form tumors.
- Glial cells do however, therefore most primary brain tumors arise from glial cells, not neurons.
Are unmyelinated axons in the PNS associated with myelin? Associated with Schwann cells?
- Not associated with myelin, but they are associated with Schwann cells, which given them a hug
What is myelin? What does it consist of?
- Myelin consists of lipoprotein and is formed by concentric layers of a cells membrane by oligodendrocytes (in CNS) and Schwann cells (in PNS) wrapping around axons. Not secreted.
What are internodes?
- Myelinated areas on an axon, which are separated by nodes of Ranvier
What is the area in a myelinated area where some cytoplasm gets trapped?
- Shmidt-Lanterman cleft = cytoplasm trapped in concentric layers of cell membrane myelinated an axon
What are the CT coverings found on axons? Are these coverings found in PNS and CNS?
- Endoneurium: surrounds single axons, loose CT with collagen fibrils
- Perineurium: surrounds bundle of axons, specialized CT lined with perineural cells, zonula occludens
- Epineurium: surrounds multiple bundles of axons, contains vascular supply to neurons, dense irregular CT with elastic fibers
- only in PNS associated with nerves, not CNS
What are the layers surround axons?
- Endoneurium (outer)
- Basal lamina (middle)
- Myelin (inner)
What is multiple sclerosis?
- Autoimmune disease affecting only the CNS
- Affects the white matter, which refer to axons myelinated by oligodendrocytes
- Pathology in oligodendrocytes causes antibodies to form and attack myelin sheath – oligodendropathy
- Leads to demyelination of axons
- Any myelinated tract may be affect
- Leads to sclerotic plaques on axons that affect impulse transmission
- Most common cause of optic neuritis = MS
Functional classification of neurons
- ) motor (efferent) – multipolar
a. ) skeletal motor (somatic efferent) – to skeletal muscle fibers
b. ) visceral motor (visceral efferent) – to smooth muscle, cardiac muscle or glands – autonomic - ) sensory (afferent) – pseudo-unipolar
a. ) somatic afferent – sensory stimuli from environment
b. ) visceral afferent – sensory stimuli from viscera
3.) interneurons – multipolar
Are nodes of Ranvier only present in PNS?
- No, both in CNS and PNS.
Mesaxon
- This is fused cell membranes
