Neurohistology Flashcards
Development of the nervous system
The nervous system develops from the primitive ectoderm. A simple epithelial disk, the neural plate, rapidly rolls into a hollow cylinder, the neural tube. The cells lateral to the neural tube form the neural crest cells.
The derivatives from the neural crest cells-
The sensory neurons of the dorsal root and cranial nerve ganglia
The sympathetic and parasympathetic motor neurons of the autonomic ganglia.
The Schwann cells and satellite cells,
Cells of pia and arachnoid matter
Thechromaffin cells of the adrenal medulla. Melanocytes of skin
The derivatives from the neural tube
Brain and spinal cord including neurons, glial cells, ependymal cells and epithelial cells of the choroid plexus.
The neural tube developement
The neural tube forms three primary brain vesicles. the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain)
These three swellings continue to swell, and then turn into five secondary swellings. the telencephalon and diencephalon (from the prosencephalon), the mesencephalon, and the metencephalon and myelencephalon (from the rhombencephalon)
The adult structures derived from these secondary swellings. Cerebrum (from telencephalon), the diencephalon, the midbrain (from the mesencephalon), the pons (from the metencephalon), the cerebellum (from the metencephalon), the medulla (from the myelencephalon)
Cerebrum
contains the cerebral hemispheres, white matter, and basal nuclei
diencephalon
contains the thalamus, hypothalamus, epithalamus, and subthalamus
Nissl bodies
chromatophilic substance contained in the cytoplasm of neurons
Axon hillock- area of cell body that lacks Nissl bodies and some other cytoplasmic organelles
Dendrites
Are receptor processes that receive stimuli from other neurons or from the external environment and carry that stimuli towards the cell body.
Typically short, highly branched & unmyelinated
Contains neurofibrils & Nissl bodies
Axons
Each neuron has only one axon. Most axons are surrounded by a myelin sheath .
Axon arises at axon hillock and ends in a terminal arborization, telodendron.
Swollen tips of the telodendron is called synaptic end bulbs. It contains vesicles filled with neurotransmitters.
Transmit stimuli to other neurons or effector cells
Multipolar neuron
most common type
multiple dendrites & single axon
found throughout the nervous system and autonomic ganglia and are the most numerous neurons.
Eg. Pyramidal cells of the cerebral cortex and the Purkinje cells of the cerebellar cortex
Bipolar neuron
one dendrite & one axon
two processes coming off cell body
found in retina, vestibular and auditory system & olfactory epithelium
Unipolar (pseudounipolar) neuron
Have one short process, which bifurcate into axon and dendrite.
Spinal ganglia (dorsal root ganglia), and in some cranial nerve ganglia
Sensory (afferent) neurons
transmit sensory information from receptors of PNS towards the CNS
most sensory neurons are unipolar (pseudounipolar) , a few are bipolar
Motor (efferent) neurons
transmit motor information from the CNS to effectors (muscles/glands/adipose tissue) in the periphery of the body
all are multipolar
Association (interneurons)
transmit information between neurons within the CNS; analyze inputs, coordinate outputs
are the most common type of neuron (20 billion)
are all multipolar
Astrocytes
Star-shaped, multiple radiating process.
Largest and most numerous in the CNS. Nuclei are large ovoid and lightly stained.
Astrocytic process end in expansions called perivascular end feet.
These cells are found in -Brain, Spinal cord, Muller cells in the retina, most blood brain capillaries and the inner surface of pia are surrounded by astrocytes end plate.
Functions:
Physical support
Metabolic process-Participate in controlling ionic and chemical environment of neurons.
Repair process-In response to CNS injury, astrocytes undergo mitosis to form a scar tissue.
Forms blood brain barrier in CNS.
Oligodendrocytes
Smaller than astrocytes, have fewer and shorter process. Nuclei densely stained.
The cell process wrap around axons producing myelin sheath. A single oligodendrocytes provides myelin sheath to several axons.
-more numerous in the white matter
Functions:
Form myelin sheath around the axons in the CNS. Promotes axonal regeneration.
(Schwan cell forms myelin in PNS)
Microglia
Small, dense, and elongate cells with short irregular process.
Can be recognized in HE stain by their dense elongated nuclei ( all other glial cells nuclei are spherical)
These cells are found in both white and gray matter.
Functions:
Immune defense Phagocytic functions In Multiple sclerosis microglia phagocytose and degrade myelin debris. (Phagocytic scavenger cells of CNS)
Large populations of microglial cells are present in the brain of patients with acquired immunodeficiency syndrome (AIDS) and human immunodeficiency virus-1 (HIV-1)
Ependymal cells
Cuboidal to low columnar ciliated epithelial cells
These cells are found in both white and gray matter.
Functions:
Form lining the ventricles of the brain and central canal of the spinal cord.
Contribute to the formation of CSF. Cilia assist in circulation of CSF.
Astrocytomas
most common type of glioma (tumor of glial cell) can occur in brain and spinal cord. (mostly in frontal, and parietal lobe of cerebrum.
Most common in middle aged men.
Multiple sclerosis (MS)
is a chronic inflammatory disease characterized by a loss of myelin, Damage patches called plaque appear in random areas of white matter.
Schwann Cells (Lemmocytes)
Have the same function as oligodendrocytes of the CNS.
Located around the axon in the peripheral nervous system, One Schwann cells forms myelin around one axon.
In unmyelinated fibers, a single Schwann cell envelops several axons.
Satellite cells
Surround the cell bodies of neurons in sensory and autonomic ganglia.
Help to regulate the external chemical environment. They also respond to injury with the production of pro inflammatory molecules.
Morphology of synapses
Axodendritic- between axons and dendrites
Axoaxonic- between axons and axons
Axosomatic-between axons and cell body
Chemical synapses-the release of a neurotransmitter
Electrical synapses- Common in invertebrates
Connective tissue investments of PNS nerves
Endoneurium- surrounds each axon, a layer of delicate connective tissue around the myelin sheath
Perineurium- surrounds a group of axons/nerve fibers ( single fascicle).
Epineurium- surrounds nerves fascicles.
Sensory Ganglia
The two types –
a. Cranial - associated with cranial nerves- trigeminal ganglia.
b. Spinal -associated with the dorsal root ganglia of spinal nerves
Neurons are pseudounipolar and tend to be clustered.
Individual neuronal cells are surrounded by abundantsatellite cells.
Nerve fibers are myelinated.
Ganglion cells in dorsal root ganglia do not receive synapses.
Sensory ganglia is surrounded by a connective tissue capsule.
Autonomic ganglia- (sympathetic and parasympathetic)
Located within certain organs such as-in GIT-intramural ganglia.
Neurons are multipolar neurons. Neurons tend to be randomly arranged, smaller then sensory ganglia.
Each neuron is frequently invested by few satellite cells.
Autonomic ganglia do contain synapses.
Autonomic ganglia is devoid of connective tissue capsules, cells are supported by the stroma of the organ in which they are found.
Cerebrum
Gray matter-
Cerebrum has alaminar(layered) organization of cells that varies from one region to another.
Six laminae are seen in theneocortex(neopallium) extending 90% of the brain. Three cellular laminae are observed in thepaleocortexof the uncus (olfaction) and archicortexof the hyppocampus in the temporal lobe (memory).
The main cell types arepyramidal cells and stellate cells.
In certain cortical regions contain giant pyramidal cells called Betz cells.
Cerebellum
The cerebellum consists of an outer layer of gray matter (cerebellar cortex) and a central core of white matter.
Has remarkably uniform tri laminar(layered) organization of cells.
Outer molecular- pale stained few neuron
Middle -monolayer purkinjie cells( pear shaped)
Inner –granule cells.
Spinal Cord
White matter of the spinal cord is peripheral and gray matter is central resembling the letter “H”.
The central canal is a remnant of the lumen of the embryonic neural tube and lined by ependymal cells, with cilia and microvilli, which extend up and down
Gray matter of the ventral horns contain large motor neurons whose axons form the ventral roots of the spinal nerves.
Gray matter of the posterior horns receives sensory fibers from neurons in the dorsal root ganglia.
