Cells and Cytoarchitecture of the Nervous System Flashcards
Principal cells in CNS
Neurons Glial cells -Astrocytes -Oligodendrocytes -Microglia -Ependymal cells
Neurons
Receive info by synapses, transmit info
Multipolar with many dendrites to one axon
Group of neurons outside CNS
Ganglion
Characteristics of neuons
Cell body (soma) - nucleus and cell organelles Axon - long process Dendrites - numerous, short branched processes Synapses- specialised junctions
Axon arises from swelling in soma called
Axon hillock
Leads to initial segmentation of axon (APs arise here)
Pyramidal cell
Lots of dendrites
Multipolar
Common, many dendrites 1 axon
Bipolar
1 dendrite 1 axon
Pseudopolar
Short processes give rise to axons in both directions
What is axonal transport?
Energy consuming mechanism to move material up and down the axon
How does axonal transport work?
Molecular motors such as kinesin = microtubules attach organelles, vesicles etc.
Anterograde axonal transport
Fast axonal transport
400 mm/day
Move away from soma
Retrograde axonal transport
200 mm/day
towards soma
Neurophil
Neuronal and glial cell processes containing myriad synaptic contacts
Another name for presynaptic axon terminal
Terminal bouton
Glial cells
10 times more numerous in CNS than neurons
Astrocytes
FORMATION OF BLOOD BRAIN BARRIER Many processes Star shaped Scaffolding Surround synapses Processes found near synapses Guide migration of cells and processes
How do unmyelinated axons and nodes of ranvier maintain extracellular movement
Absorb K+
Fibrous astrocytes
Common in white matter
Fewer processes
Processes are long and thin
Protoplasmic astrocytes
Most common in grey matter
Lots of short branching processes
Astrocytes have intermediate filaments in their cytoplasm what are they made of?
GFAP- glial fibrilary acidic protein
Astrocytes end in flattened processes called
Endfeet
do not have tight junctions, so do not form the actual BBB,
Induce the capillary endothelium to form tight junctions and these tight junctions between the capillary endothelial cells form the heart of the BBB.
Astrocytes send similar endfeets forming a covering below pia and ependymal lining of ventricles this is
Glia limitans
Oligodendrocytes
Produce myelin ONLY IN CNS
Small round cells
Numerous processes extending to produce internodes of myelin
Round nucleus
One oligodendrocyte produces
Several internodes of myelin (CNS)
One schwann cell
Single shealth (PNS)
Microglia
Similar to macrophages
Immune monitoring and antigen presentation
Brain = slow rate of lymphoid cell traficking
Resting state = elongated nucleus, short spiny cell processes
When activated become rounder
Express MHC class II proteins
Smallest glial cells
Perivascular macrophages are found
around blood vessels
On vascular side of basal lamina
Ependymal cell
Cuboidal/columnar epithelium-like cells that line the ventricles
Ependymocytes form majority of epndymal lining
Lack basal lamina therefore not true epithelia
Basally they interdigitate with astrocyte processes of the glia limitans. Apically they lack tight junctions, and this allows free exchange between the CSF and the CNS parenchyma.
Tanycytes
Small, specialised subset of ependymocytes
Found in the ependyma lining the floor of the 3rd ventricle.
Lack a blood-brain barrier=circumventricular organs.
Long processes from the tanycytes contact these blood vessels, including those of the hypothalamic-hypophyseal portal system, and these cells are thought to play a role in monitoring hormone levels in the CSF and responding by discharging secretory products into capillaries, perhaps to control secretion from the anterior pituitary.
Choroid Plexus
villous structure with a core of blood vessels covered by a layer of pia mater and an outer layer of choroid epithelium, which are modified ependymal cells.
Cuboidal cells have long microvilli on their surface. Tight junctions between them = BLOOD- CSF BARRIER.
Secrete CSF.
Neuron cell bodies
Large with abundant cytoplasm and Nissl substance and large nuclei
Support cells- satellite cells & Schwann cells.
small cells which surround the neuronal cell body
PNS
Neuron cell bodies Support cells Axons Loose fibrocollagenous CT Dorsal root ganglia and autonomic root ganglia
Dorsal root ganglion
Pseudounipolar therefore no synapses
Autonomic root ganglion
Synapses present
What is a nerve
collection of axons linked together by support tissue into an anatomically defined trunk. The axons may be either motor (efferent) or sensory (afferent), and can be myelinated or unmyelinated
Peripheral nerve composed of
- Axons
- Schwann cells, some of which produce myelin.
- Fibroblasts which produce fibrocollagenous material.
- Blood vessels
What is a fascicle?
Several bundles of axons
What is Epineurium
CT sheath surrounding whole nerve
What is perineurium
CT sheath surrounding fasicle
What is endoneurium
CT sheath surrounding fibres within a fasicle
The blood-nerve barrier
consists of tight junctions in the intrafascicular capillaries and tight junctions between cells in the perineurium.
Schwann cells
Larger axons myelinated by these
Form a single internode of myelin around 1 axon
Derived from neural crest cells produce a basal lamina
Remak bundles
Smaller axons embedded in clefts of cytoplasms of Schwann cell unmyelinated groups
Satellite cells
Flat epithelia like cells surrounding neurons in sensory and autonomic ganglia
Myelin sheath
Made of multiple layers of lipid rich membrane
Special adhesion proteins and lipids
220mph
PNS myelin sheath
Formed by schwann cells
CNS myelin sheath
Oligodendrocytes makes myelin around many axons
Schmidt-Lanterman incisures (or clefts)
Strip of cytoplasm across sheath as helical spiral
Allow transport of molecules across sheath
Node of Ranvier
Break in myelin
Internode
Length of axon covered by myelin
Typically 1um
Saltatory conduction
Increases speed of conduction by 100 fold
Motor neurons axons end in
NMJ on voluntary muscle(SMS)
Form synapses with smooth muscle (ANS)
Merkel disc
Very sensitive to tissue displacement and widely distributed in skin and some mucosae
Ruffini ending
Found in glabrous skin, mainly responds to skin stretch
Pacinian corpuscle
Sensitive to deep pressure.
Found deep in dermis. 20-60 lamellae of CT separated by gelatinous material.
Up to 1mm in length.
Meissner’s corpuscle
Sensitive to light touch. Widely distributed, but concentrated in areas like fingertips and tongue. Located in the dermal papillae.
NMJ
Presynaptic terminal and motor end plate
Allocortex
Archiocortex - hippocampus
Paleocortex - olfactory cortex
Neocortex
6 layers
Cortical neurons
Principal cells and interneurons
Principal cells
Pyramidal cells, fusiform cells - project to thalamus
Excitatory
Axonal process of these are not confined to cortex
Interneurons
Spiny stellate cells (excitory)
Non-spiny cells (inhibitory)
–basket, fusiform, horizontal and martinotti cells
Pyramidal cells
Triangular
Apex towards surface of brain
Single long apical dendrite towards cortical surface and basal dendrites extend horizontally.
Single axon from basal surface from grey matter to white matter
Molecular layer (I) most sup
Few cells
Horizontal axons and dendrites of afferents, intercortical neurons and apical dendrites of pyramidal cells
External granular layer (II)
Many small neurons (interneurons and small pyramidal cells)
External Pyramidal layer (III)
Pyramidal cells
Axons project to other parts of the cortex
Get larger as go down the layer
Internal granular layer (IV)
Stellate cells
Internal pyramidal layer (V)
Large pyramidal cells
Project to subcortical layers (BS and SC)
Primary motor cortex = giant cells (100um)= betz cells
Multiform layer VI
Different cell types
Fusiform project to thalamus
Cortical columns
200-500um D
Respond to peripheral stimulus
In primary sensory cortices (visual cortex)
Cerebellar cortex
3 layers (sup to deep) Molecular Purkinje cell Granule cell White matter
