004 Microscopic Anatomy of the Nervous System Flashcards
• list and describe the cells of nervous tissue
??
Describe a sensory neuron
(AKA afferent)
Carry nerve impulses from sensory receptors to the CNS
Two types
• Somatic afferents – associated with the somatic nervous system
• Visceral afferents – associated with the autonomic nervous system
Describe a motor neuron
(AKA efferent)
Carry nerve impulse from the CNS to effector organs
Two types
• Somatic efferents – associated with the somatic nervous system
• Visceral efferents – associated with the autonomic nervous system
Describe interneurons
Carry nerve impulses between two other neurons
Form communication and integration networks between sensory and motor neurons
99.9% of neurons fall into this category, with almost all of the neurons in the CNS belonging to this type
Anatomical types of neurons
multipolar, bipolar, and pseudounipolar
Multipolar neurons
o Multipolar neurons One axon Two or more dendrites • Usually have many that form the dendritic tree The main anatomical types of neurons
Bipolar neurons
One axon One dendrite Compared to multipolar neurons there are relatively few in the body Examples • Bipolar cells of the retina • Vestibulocochlear apparatus in the ear • Olfactory epithelium
Pseudounipolar neurons
Appear to only have one process attached to the soma
During development the neuron had one axon and one dendrite that fused into a single process
• The single process has the staining qualities of an axon
Exclusively first order sensory neurons found in spinal (dorsal root) ganglia and sensory ganglia of cranial nerves
Peripheral (distal) process
• Part between the soma and the peripheral receptor
Central (proximal) process
• Part between the soma and the CNS
• describe the common types of multipolar neurons found in the CNS
o Pyramidal cells
o Purkinje cells
o Stellate cells
o Granule cells
Pyramidal cells
(AKA Golgi type I neurons)
MOST COMMON IN ENTIRE BODY
Found exclusively in the gray matter of the cerebral cortex
Cell body is triangular, with one angle always pointing towards the pia mater and the flat surface toward the white matter
Dendrites arise from the angles of the soma
• Apical dendrite – from the pial surface
• Basal dendrite – from the other angles
Axon always arises from the flat surface facing the white matter
• They always arise in the grey matter then enter the white
Purkinje Cells
Large neurons located exclusively in the gray matter of the cerebellar cortex
Cell body is flask shaped with
Large dendritic tree extending from the side of the soma facing the pial surface
Axon always arises from the side of the cell body facing the white matter
• It exits the grey matter to enter the white
• Usually quite long and target is far away
Satellite cells
(AKA Golgi type II neurons)
Smaller, star-shaped cells in both the cerebral and cerebellar cortex – also in other gray matter
Cell body is star shaped
Axons are relatively short
• Stellate cells involved in more local connections
Granule cells
A specific subset of small stellate-type cells that resemble grains of sand
Found in the cerebellar cortex and other regions of gray matter
• describe the soma of a neuron, including the Nissl substance and axon hillock
o The dilated region containing the nucleus and nucleolus
o Has many mitochondria
o Has a large, well developed Golgi apparatus
o Nissl substance
Clumps of basophilic material located within the perinuclear cytoplasm
• Polyribosomes
• Rough endoplasmic reticulum
Gives the neuron cell body a basophilic staining quality
NOT present in the axon hillock, which is where the axon attaches to the cell body
Dendrites
Receptive structures associated with neurons Carry information to the cell body Compared to axons • shorter • larger diameter • tapered • unmyelinated Dendritic spines • Present on most dendrites (not all) • Small knob-like projections for synaptic contacts Structure is similar to the soma • Does not have nucleus nor Golgi apparatus • DOES have o Nissl substance o Mitochondria o Cytoskeletal components
Axons
Effector portion of a neuron
Carry information away from the soma
Only one per neuron
Collateral branches
• Branches of the axon
Compared to dendrites
• Longer
Axon hillock
• Region of the cell body that attaches to the axon
• Devoid of Nissl substance
Initial segment of the axon
• The region of the axon between the apex of the axon hillock and the beginning of the myelin sheath
• Site of the generation of action potentials
Axon terminal (AKA axon bouton)
• Dilated region at the end of the axon that helps form the presynaptic portion of a synapse
Structures found in axons
• Neruofilaments
o Intermediate filaments involved in axonal transport
• Vesicles
o Transported by neurofilaments
• A few mitochondria
o Usually near the termination of the axon
• describe the three (3) general types of axons, based upon what they connect: projection, association, and commissural
o Projection
Connects cerebral cortex with subcortical structures
o Association
Connect structures within the cerebral cortex (usually on the same side)
o Commisural
Connects like structures on opposite sides of the CNS
• list the morphological and physiologic types of synapses
o Axodendritic Between axon and dendrite Vast majority of synapses are this type Axospinous • Axon synapses with dendritic spines o Axosomatic Between axon and soma o Axoaxonic Between axon and axon o Dendrodendritic Between dendrite and dendrite
• compare and contrast styles of synaptic contacts: single bouton, en passant, and en grappe
o Single bouton
Stand alone synapse
o En passant
Multiple string of synapses from a single axon
o En grappe
Special type ONLY found between motor axons and large muscle fibers of EOMs
• describe the structure and function of a chemical synapse
o the conduction of information across the synapse is achieved by the release of chemical substances (neurotransmitters)
o neurotransmitters diffuse across the synaptic cleft to stimulate the next neuron
o Presynaptic component
Contains synaptic vesicles containing neurotransmitters
Usually has a thin presynaptic density along the plasma membrane
May also be a few mitochondria
o Synaptic cleft
Physical gap between cells
o Postsynaptic component
Cytoplasmic side of the plasma membrane
Two varieties
• Asymmetric synapse
o Postsynaptic density noticeably thicker than presynaptic density
o Thought to be excitatory synapses
• Symmetric synapse
o Pre and postsynaptic densities are about the same size
o Thought to be inhibitory synapses
o Astrocytic processes
In the CNS
Surround the synapse
Appear to have some support function
• describe axonal transport
o carries components between the soma and the axon o Two types Slow • Carries components up to 5 mm/day • Appears to use microtubules and neurofilaments • Cytoskeletal proteins and enzymes are carried by this system Fast • Carries components up to 400 mm/day • Appears to use only microtubules (no neurofilaments) • Transports o Mitochondria o Lysosomes o Vesicles o Two directions Anterograde • From soma to periphery • Fast and slow use this direction Retrograde • From periphery to soma • Only fast mechanisms move in this direction
PNS supporting cells
Schwann Cells
• In PNS
• Create the necessary microenvironment necessary for axons to function properly
• Axon + Schwann cell = nerve fiber
• Functions
o Offer structural support to the axon
o Help mediate nutrition of axon
o Can form myelin (electrochemical insulation)
Satellite Cells
• In PNS
• Found exclusively surrounding the cell bodies of neurons in the ganglia (sensory and autonomic ganglia)
o Ganglia=collection of nerve cell bodies in the PNS
• Create a microenvironment for the cell bodies in a way similar to Schwann cells
• Appears as a single layer of cuboidal or squamous cells surrounding the somata in ganglia
Schwann cells
• In PNS
• Create the necessary microenvironment necessary for axons to function properly
• Axon + Schwann cell = nerve fiber
• Functions
o Offer structural support to the axon
o Help mediate nutrition of axon
o Can form myelin (electrochemical insulation)
Satellite Cells
• In PNS
• Found exclusively surrounding the cell bodies of neurons in the ganglia (sensory and autonomic ganglia)
o Ganglia=collection of nerve cell bodies in the PNS
• Create a microenvironment for the cell bodies in a way similar to Schwann cells
• Appears as a single layer of cuboidal or squamous cells surrounding the somata in ganglia
Neuroglia
Supporting cells of the CNS
In standard preparations, only the nuclei of glial cells can be seen, but the structures in the rest of the cells can be seen with special stains
oligodendrocytes
o AKA oligodendroglia, oligodendrogliocytes
o Produce and maintain myelin sheaths in the CNS
o Small cells with few processes
o Usually, each cell will myelinate more than one axon (with one process per axon)
o Each process forms an internode region
o There are nodes of Ranvier between each oligodendrocyte process
o Unmyelinated axons in the CNS are NOT surrounded by any processes and are truly “bare”
Astrocytes
o Most numerous type of cell in the nervous system
o Largest type of glial cell
o Star shaped
o Body has several radiating processes
o Have glial fibrillar acidic protein
Makes up a special type of intermediate filament unique to astrocytes
Used as a basis for staining astrocytes
o Only the nuclei can be seen with H&E stains
o Perivascular feet (AKA end feet)
On the ends of some astrocyte processes
Cover large areas of blood vessels in the CNS to help establish a microenvironment for the neurons
Help form the blood brain barrier
Regulate movements of metabolites and ionic concentrations
The processes also surround the bare areas of axons (i.e. nodes of Ranvier, synapses and the entire length of unmyelinated axons)
Play a role in regulation of ions and neurotransmitter metabolism in these structures
o Help form glial limitans
Consists of expanded ends of astrocyte processes, called subpial feet that are associated with the basal lamina of the pia mater
A relatively impermeable layer surrounding the PNS
o Two types
Protoplasmic
Fibrous
Microglia
o Relatively small cells o Usually have an elongated nucleus o Have few, short, twisted processes Need a special stain to be seen o Macrophages Phagocytic cells that originate in bone marrow Part of monocyte lineage o Play a role in defense o Help remove debris after injury
Ependymal Cells
o Simple cuboidal epithelium
o lines the fluid filled cavities of the CNS (i.e. brain ventricles and central canal of the spinal cord)
o NO basal lamina associated with this epithelium
Basal domain is associated with astrocytic processes
o These cells can be modified to produce CSF
They form, along with their capillaries, the choroid plexus
Myelinated axons
o Myelinated
Schwann cell wraps around the axon multiple times (like a cinnamon roll)
Myelin sheath = multiple fused layers of Schwann cell plasma membrane that directly surround the axon
• Lipid rich layer
• Hydrophobic
• Acts as an insulator
• Cytoplasm gets squeezed out
o Ends up peripheral to myelin sheath, forming the neurilemma (AKA Sheath of Schwann)
o Contains the organelles of the Schwann cells
• Clefts of Schmidt-Lanterman
o Small islands of cytoplasm that remain between the layers of the myelin
o Important for metabolic maintenance of the myelin
• Segmented along the axon
o Node of Ranvier
Junction between two Schwann cells
Has no myelin to allow for saltatory conduction
o Internodes of Ranvier
Areas between the nodes of Ranvier
Corresponds to one Schwann cell
Unmyelinated axons
Axons that do not have multiple layers of Schwann cell membranes to enclose them
Still are associated with Schwann cells but do not have enough membrane to serve as an insulator or cause salutatory conduction (it is surrounded by only one layer)
• The axon runs in the groove of the Schwann cell
• Each Schwann cell usually has multiple grooves
• Each groove can contain one or more axons
o Particularly in autonomic neurons
• describe the functional importance of myelin in the nervous system
???
o Protoplasmic Astrocytes
Pretty exclusively found only in gray matter
More numerous, shorter and more branching processes than fibrous type
Usually have fewer intermediate filaments than fibrous types
Fibrous Astrocytes
Pretty exclusively only found in white matter
Have fewer, longer and straighter processes than protoplasmic types
Usually have more intermediate filaments than protoplasmic types
Peripheral nerves
o Nerve A bundle of nerve fibers (i.e. axons and supporting cells) in the PNS THERE ARE NO NERVES IN THE CNS o Connective tissue layers are formed by fibroblasts -endoneurium -perineurium -epineurium ***MAJOR EXCEPTION IS THE OPTIC NERVE
endoneurium
Loose CT that directly surrounds each nerve fiber
Perineurium
Dense Irregular CT that directly surrounds a fascicle of nerve fibers
Metabolically active barrier
Helps form the blood-nerve barrier
Epineurium
Dense Irregular CT that surrounds the entire nerve and fills the space between nerve fascicles
May contain a variable amount of adipose tissue
The major exception to the nerve rule
Optic Nerve
• Has a different structure than the other cranial nerves
• Has oligodendrocytes (instead of Schwann cells)
• Does NOT have endo-, peri-, or epineurium
o Has pia, arachnoid, and dura mater
• It is a direct extension of the CNS into the periphery
• The optic nerve is more accurately a tract rather than a cranial nerve
