HS120: Nervous Tissue Flashcards
What is nervous tissue?
- One of the 4 basic tissues in the body
- Ectoderm origins
- responsible for transmitting nerve impulses throughout the body
- controls functional activities of the organs and systems
- forms the nervous, the most complex system
- cellular, formed by more than 100 billion nerve cells
How is the nervous system divided?
divided anatomically:
1. CNS: brain and spinal cord
2. PNS: cranial and spinal nerves, cranial & spinal ganglia, & nerve endings (motor & sensory)
What is the CNS?
- Brain: the cerebrum & cerebellum, as well as brain stem, all inside the skull
- Spinal cord: inside vertebral column
What is the peripheral nervous system?
- outside the brain & spinal cord
- nerves in the PNS connect CNS to organs
includes:
1. Cranial and spinal nerves
2. Cranial and spinal ganglia
3. Nerve endings: motor & sensory
histological structure of nervous tissue
What is nervous tissue composed of?
- nerve cells (neurons)
- glial cells (neuroglia)
What are glial cells/ neuroglia?
- function: support, nurture, & protect neurons
- about 10 times more abundant than neurons
What are nerve cells/ neurons?
- neurons are the structural and functional unit of both CNS & PNS
- 100 billions of neurons are present in the human body
- responsible for conducting nerve impulses
What are the structures of a neuron?
with definition
- cell body (soma)/ perikaryon: the trophic centre for the nerve cell
- dendrites: many processes specialized to receive stimuli, extending from the perikaryon
- axon: single process specialized for generating and conducting nerve impulses to other cells
What is the size of cell bodies?
widely ranged; they can be up to 150 micrometres in diametre. others, like cerebellar granule cells are the body’s smallest cells.
Nucleus of perikaryon
- spherical
- large
- rounded
- euchromatic (pale-staining)
- well-developed nucleolus
Cytoplasm of perikaryon:
- mitochondria
- highly-developed RER
- numerous polyribosomes
- RER + free polyribosomes form chromatophilic substance called nissle bodies/ nissle granules
- golgi apparatus
- neurofilaments & microtubules
What are nissle bodies?
where are they abundant?
They are chromatophilic (is easily stained) substances formed from RER & free polyribosomes that show active sites of protein production.
- nissle bodies are abundant in large nerve cells such as motor neurons
structure of neuron
dendrites
and abundancy
- principal signal reception, receive stimuli from the environment
- extend from the perikaryon
- each neuron has numerous dendrites to increase the receptive area of the cell, to receive a great number of axon terminals of other nerve cells
- covered with many dendritic spines
What are dendritic spines?
they are short plastic structures projecting along dendrites that are visible with silver staining.
- they are the initial site for synapses
- contain actin cytoskeleton for neural plasticity
What is an examples of dendrites?
dendrites of the large Purkinje cell of the cerebellum contact with up to 200 000 synaptic endings of other neurons
What is a synapse?
Synapses are the point for attachment of 2 neurons. Here, the dendrites are much thinner than axons.
structure of neurons
axons
function & origins
- most axons have only one axon, longer than its dendrites.
- vary in length and diametre depending on types of neuron. Motor neurons that innervate the foot muscles have axon lengths of nearly a metre
- all very long axons are called nerve fibres
- originate from axon hillock (the last site in the soma)
- The plasma membrane of the axon is called the axolemma
cytoplasm of axon
- knowns as axoplasm
- contains mitochondria, microtubules, neurofilaments, & some SER.
How do axons end?
- with many branches: called collaterals, terminal arborization & synaptic end bulbs (terminal boutons)
- contains vesicles with neurotransmitters
How can neurons be classified?
according to the number of processes (axons & dendrites):
1. multipolar neurons
2. bipolar neurons
3. unipolar/ pseudounipolar neurons
4. anaxonic neurons
multipolar neurons?
- 1 axon
- 1+ dendrites
- most neurons are multipolar
bipolar neurons?
- 1 dendrite
- 1 axon
- found in: retina, olfactory mucosa, and the inner ear (cochlear & vestibular ganglia).
- serve the senses of light, smell, & balance, respectively.
anaxonic neurons?
- many dendrites
- no true axon
- neurons that generate action potential
unipolar/ pseudopolar neurons?
- found in: spinal ganglia and most cranial ganglia (PNS)
- have a single axon that bifurcates into two branches
- one branch runs to periphery and is structurally an axon but functions as a dendrite
- other axon (“true axon”) runs to the spinal cord
Why is it difficult to classify neurons structurally by microscope?
Due to the branching and fusion of processes.
what are glial cells/ neuroglia?
- present in CNS and PNS
- 10 times more abundant than neurons
- surround most of the neuronal cell bodies
- neurons are larger glial cells
what are neuropils?
- a network of interwoven dendrites and axons and of nerve cells.
- synapses and glial cells in gray matter of the CNS
Types of neuroglia:
- Astrocyte
- Oligodendrocytes
- Microglia
- Ependymal cells
oligodendrocytes:
- oligo= small, few
- dendr= tree
- cyte= cell
produce myelin sheath which provide insulation for several axons in the white matter of CNS, which is white because of the lipids (80%) in the myelin sheaths
- the predominant glial cells in CNS
- small cells with rounded, condensed nuclei and unsustained cytoplasm
astrocytes:
- astro= star
- located mainly in gray matter
- have multiple processes
- most diverse structurally and functionally
What makes astrocytes the most diverse structurally and functionally?
- in white matter: few long processes are called fibrous astrocytes
- in gray matter: many short, branched processes are called protoplasmic astrocytes
What do the proximal regions of astrocytes contain?
- proximal regions of larger processes of astrocytes contains intermediate filaments made of: glial fibrillary acid protein (GFAP) & vimentin
- GFAP is a key marker for astrocytes and is often used in research diagnostics to study brain injuries and neurodegenerative diseases
What are most brain tumors derived from?
most brain tumors are astrocytomas derived from fibrous astrocytes by their expression of the GFAP gene
What do astrocytes form?
- form perivascular feet that completely enclose capillaries
- form a barrier layer called glial-limiting membrane
What is the blood-brain barrier?
also known as glial-limiting membrane:
- it is a barrier layer, lining the meninges at the external CNS surface
- it is important for protecting the brain from fluctuations in plasma composition
What is the blood brain barrier composed of?
- capillary endothelium (continuous type)
- basement membrane of endothelium
- perivascular feet of astrocytes
- well-developed occluding tight junctions to prevent diffusion of solutes and fluid into the neural tissue.
ependymal cells:
- columnar or cuboidal cells
- line ventricles of the brain and central canal of the spinal cord
- some cells have cilia to facilitate the movement of cerebrospinal fluid
- with microvili, involved in absorption
- have junctional complexes
- not true epithelium as they lack basal lamina
- instead, the basal ends by elongated and extended processes
Why are ependymal cells not considered true epithelium?
Because they lack a basal lamina and instead end in elongated processes that extend into the spinal cord.
types of neuroglia in CNS
what are microglia?
- less numerous neuroglia
- present in both grey and white matter
- small cells with short irregular processes
- nucleus is dense, elongated, & lightly stained.
- originates from monocytes
What are the functions of microglia?
- antigen-presenting immune cells of the CNS
- when activated, they have characteristics of macrophages and become phagocytic
What are the glial cells/ neuroglia of the PNS?
- Schwann Cells (neurolemmocytes)
- Satellite cells
glial cells of PNS
Properties of Schwann cells
- found only in PNS
- covers full length of an axon
- forms myelin around one axon
What is functionally similar to Schwann cells in the CNS?
oligodendrocytes, as they are what form the myelin sheath in the central NS, however they are different in that they form is for more than one axon
glial cells of PNS
properties of satellite cells
- form a covering layer over neuronal cell bodies in PNS ganglia
What is the function of satellite cells?
trophic or supportive role
Which neuroglia originates in bone marrow and what is their function?
That is microglia (since they are derived from monocytes) and they perform is immune-related activities
Which neuroglia lines cavities of the CNS?
ependymal cell
Which neuroglia mainly produce myelin and perform electric insulation?
oligodendrocytes and neurolemmocytes
which neuroglia are found in the CNS?
oligodendrocytes, astrocyte, ependymal cells, & microglia
which neuroglia are found in the PNS?
neurolemmocytes/Schwann cells and satellite cells
What is the CNS composed of?
- cerebrum
- cerebellum
- spinal cord
What is the spinal cord composed of?
according to the distribution of cell bodies, axons, & glial cells, the spinal cord is composed of:
1. gray matter
2. white matter
3. central canal
What is the gray matter of the spinal cord?
- internal
- H-shaped
- contain many astrocytes & microglia
- contain groups of cell bodies in CNS called nuclei
- contains anterior & posterior horns
What is the anterior horns of the grey matter?
anterior horns contain cell bodies of motor/efferent neurons.
What is the posterior horn of grey matter?
posterior horn contains cell bodies of the sensory/afferent neurons.
What is the white matter of the spinal cord?
- at peripheral
- contains oligodendrocytes & myelinated axons
- functionally, there are bundles of axons in CNS called tracts.
- presence of few astrocytes and microglia and few cell bodies.
What is the central canal of the spinal cord?
- central opening
- continuous with the ventricles of the brain
- lined by ependymal cells
- filled with CSF (cerebrospinal fluid)
Discuss the image below
great job
What is the CNS divided into anatomically?
- cerebrum
- cerebellum
- brain stem
- spinal cord
What are the characteristics of the cerebrum?
- anterior and largest part of the brain
- divided into two hemispheres separated by longitudinal cerebral fissure
- surface of cerebrum composed of sulci and gyri to increase surface area of cerebral cortex
Histologically, what is the cerebrum composed of?
- cortex (peripheral, grey matter)
- medulla (central, white matter)
What are the 6 layers of the cerebral cortex?
- molecular layer
- external granular layer
- external pyrmidal layer
- internal granular layer (granular cells)
- internal pyramidal layer (large pyramidal cells)
- polymorphic cell layer
What is the cerebellum?
- second largest part of the brain
- coordinates body movements
What are the structures of the cerebellum?
- folia
- anterior and posterior lobes that are separated by primary fissure
- two cerebellar hemisphere, separated by vermis
- vermis, a narrow band of cortex
What are the characteristics of the cerebellum?
- contain many small distinctive folds
- is composed of grey matter and white matter
Where is the cerebellar cortex found and what is its function?
it is present at the surface and coordinates muscular activity in the body.
Where is the medulla found and what does it contain?
it is present in more central regions and contain axons.
What is the cerebellar cortex divided into?
3 layers:
1. outer molecular layer
2. central purkinje cell layer
3. inner granular layer
What are the components of the outer molecular layer of the cerebellar cortex?
- 2 types of neurons: stellate cells & basket cells
- neuroglial cells
- dendrites of Purkinje cells
- axons of granular cells
What is the central purkinje cell layer of the cerebellar cortex?
- a single layer of large cells
- their dendrites pass into the molecular layer and synapse with axons of granular cells
What is the inner granular layer of the cerebellar cortex?
a layer that consists of:
- small granular cells
- neuroglial cells
- Golgi cells
What is the meninges?
- the CNS is protected by the skull and vertebrae but is also surrounded by 3 CT membranes called meninges.
What are the 3 CT membranes that make up the meninges?
- dura mater
- arachnoid layer
- pia mater
Which meninges layer is the outermost?
dura mater
What lies beneath the dua mater?
the arachnoid layer
What is the pia mater?
it is a delicate inner layer resting directly on the surface of the brain and spinal cord
properties of the dura mater
- thick external layer
- composed of dense, fibroblastic C.T
- contains large blood vessels split into the capillaries in the pia mater
- in the brain, it is continuous with the periosteum of the skull and is composed of: periosteal and meningeal layer.
What is the meningeal layer of the dura mater?
- in the spinal cord it is separated from the peiosteum by the epidural space
- it is always separated from the arachnoid by the subdural space
- external and internal surfaces covered by simple squamous epithelial cells.
What is the epidural space?
it is a plexus of thin-walled veins, areolar CT & adipose tissue.
What is the arachnoid of meninges?
avascular C.T. (lacks blood vessels)
What is the subarachnoid space?
space between arachnoid and pia mater
What does the subarachnoid space contain?
- arachnoid trabecular with modified fibroblasts and collagen
- acts as cushion to protect the CNS
- cavities between the trabeculae
- filled with cerebrospinal fluid (CSF)
What are arachnoid villi?
regions where the arachnoid perforates the dura for the passage of CSF into the dural venous sinuses (superior sagittal sinus)
What is the cavity between the periosteal and meningeal layers of the dura mater called and what does is contain?
The superior sagittal sinus, containing arachnoid villus for the passage of CSF.
What is the pia mater?
- innermost layer of meninges
- composed of a thin layer of flattend, modified fibroblasts
- abundant blood vessels
- blood vessels in subarachnoid space extends to penetrate the brain, forming perivascular spaces
What is another name for perivascular spaces?
Vichow-Robin space
What are letomeninges?
The arachnoid and pia mater together are called the leptomeninges, meaning “thin meninges”
What are the ventricles of the brain?
- 4 interconnected (continuous) cavities in the brain
- filled with CSF and contains choroid plexus
- lined with ependymal cells
- the 4th ventricle is continuous with the central canal of the spinal cord.
What is the passage of CSF through the brain?
CSF passes through the interventricular foramina to the 3rd ventricle, then the cerebral aqueduct to the 4th ventricle and then to the spinal cord
what is the choroid plexus?
they are invaginations of pia mater folds and many villi, pentrating the brain’s ventricles.
Where are choroid plexus present in?
- the roofs of the 3rd & 4th ventricles
- in parts of the 2 lateral ventricular walls
What is the structure of the choroid plexus?
- connective tissue
- simple cuboidal or low columnar epithelium (ependymal cells)
- rich with fenestrated capillaries
- tight junction
what is the function of the choroid plexus?
to remove water from blood and release it as CSF
Importance of the CSF:
- for metabolism
- shock absorption
Where are the sites of CSF?
- fills the ventricles
- central canal of the spinal cord
- subarachnoid space
- perivascular spaces
What are properites of the CSF?
- it is clear with a low density
- contains sodium, potassium, and chloride ions
- very little protein
Define synapse
site for transmission of information from one neuron to the next
What is the structure of synapses?
- presynaptic axon membrane (terminal bouton) containing synaptic vesicles with neurotransmitter and mitochondria
- postsynaptic membrane containing receptors for the neurotransmitters
- synaptic cleft (gap), a thin intercellular space.
What are the 2 types of synapses?
- chemical synapses
- electrical synapses
What are chemical synapses?
synpases that use neurotransmitters to send messages
- slower
- eg. in skeletal muscle cells
What are electrical synapses?
synpases where an ion current is transmitted over a gap junction between neurons
- eg. in smooth muscle and cardiac muscle cells
