lec 2 - nervous system histology Flashcards
Cells of the Nervous System
Neurons
Neuroglia/Glia/Glial cells
Neurons =
Excitable cells, capable of carrying electrical signals
Functional unit of the nervous system
use property of excitability to produce and move an action potential (nerve impulse) along an axon to excite another neuron or a different type of effector cell (e.g., skeletal muscle)
neuron communication
> occurs via synapses
> neurotransmitters are released at the presynaptic membrane and bind to receptors on the postsynaptic cell
> initiates a new action potential at the postsynaptic cell
Neuroglia/Glia/Glial cells =
Supporting cells of the nervous system
Non-conducting cells of the nervous system
Glial cells are essential cells of the nervous system that support neurons
There are six major types of glial cells
> ependymal cells
> oligodendrocytes
> satellite cells
> astrocytes
> microglia
> schwann cells
Neuronal Structure
dendrites
cell body/ soma/ perikaryon
axon
axon terminal
Dendrites:
receive information
numerous extensions off the cell body
specialized to receive stimuli from other neurons at synapses
Cell body/Soma/Perikaryon:
processes and integrates information
Contains nucleus and other organelles
Nissl substance/body: dark-staining portions of the cell body formed by ribosomes on the rough endoplasmic reticulum and free ribosomes
Axon:
transmits action potentials
single long process that extends from the cell body and reaches other cells (e.g., other neurons, muscle cells, etc.)
specialized to generate and conduct nerve impulses (action potentials) to other cells
Axon terminal:
site of synapse with other cells and neurotransmitter release into the synaptic cleft
site of synapse of neurotransmitter release
Various types of synapses can occur, but the most common is ___, in which an axon of one neuron synapses on a dendrite of another neuron
axodendritic
Axonal cell bodies =
contain a large euchromatic nucleus with a prominent nucleolus and surrounding cytoplasm
cytoplasm reveals abundant rough ER (rER) and free ribosomes
ribosomes on the rER and the free ribosomes appear as small bodies called Nissl bodies/substance that stain intensely with basic dyes and metachromatically with thionine dyes
each Nissl body corresponds to a stack of rER
cytoplasm also contains mitochondria, Golgi apparatus, lysosomes, microtubules, neurofilaments, transport vesicles
Neuron types
multipolar neurons
bipolar neurons
pseudounipolar neurons
Multipolar neurons:
motor neurons and interneurons
multiple dendrites extending from cell body
single axon
most common neurons
carry somatic and visceral motor output from the ventral and lateral horns to skeletal and smooth or cardiac muscle, respectively.
Interneurons =
multipolar neurons that help integrate information
allow information to cross from one side of the spinal cord to the other side of the spinal cord
allow information to travel up and down the spinal cord)
Bipolar neurons:
special sensory neurons
Single dendrite extending from cell body
Single axon
neurons carry special sensations (sight/vision, smell/olfaction, hearing/audition, balance/equilibrium)
Pseudounipolar neurons:
general sensory neurons
Peripheral process with dendrites in periphery
Central process relaying into CNS
single axon that bifurcates close to the cell body into a peripheral process that goes to the body periphery and has dendrites at its end and a central process that enters the CNS
carry all general sensations (touch, temperature, proprioception, vibration, pressure, pain, etc.) from the entire body
Neuroglia in the PNS only:
Satellite cells = Located in peripheral ganglia
Schwann cells/ Neurolemmocytes/ Mantle cells = Located in peripheral nerves and peripheral ganglia
Functions of Satellite Cells
electrically insulates PNS cell bodies
Regulates nutrient and waste exchange for cell bodies in ganglia
also called mantle cells
they are modified Schwann cells that surround the cell body of neurons in PNS ganglia
provide anchoring and support to the cell body and help control the chemical environment (regulate microenvironment of PNS ganglia)
Derived from neuroectoderm, specifically, the neural crest
Functions of Neurlemmocyte/Schwann cells
surround and insulate PNS axons and myelinate those hacing large diameters
allows for faster propagation along an axon in the PNS
create the myelin sheath in the PNS
Injured in Guillain-Barre syndrome
One Schwann cell can only wrap the axon of one neuron
Derived from neuroectoderm, specifically, the neural crest
Connective Tissue in the PNS
myelin sheath
endoneurium
perineurium
epineurium
Myelin sheath:
around axons
Formed by Schwann cells in PNS
Formed by oligodendrocytes in CNS
Nodes of Ranvier:
gaps in the myelin sheath, allowing for Saltatory conduction
where voltage gated ion channels are located on the axon
allow for faster conduction of action potentials (saltatory conduction, where conduction of action potentials occurs from node to node)
Endoneurium:
wraps around individual nerve fibers
just superficial to the myelin sheath
Perineurium:
Perineurium wraps around groups of nerve fibers
A group of nerve fibers is called a fascicle, thus perineurium wraps around fascicles
Epineurium:
around entire nerves
Epineurium wraps one entire nerve (e.g., epineurium surrounds the femoral nerve)
loose CT layers
Neuroglia in the CNS only:
astrocytes
ependymal cells
microglia
oligodendrocytes
Astrocytes:
in white and gray matter and at blood-brain-barrier
most abundant glial in CNS = most common glial cells in the CNS and may also be called macroglia
derived from neuroectoderm, specifically, the neural tube
provide physical and structural support, repair processes
> perform phagocytosis of non-functioning synapses
> scar formation in the CNS after cerebral infarction
> maintain a constant internal milieu in the CNS
> extracellular potassium buffer
> remove excess neurotransmitters
> glycogen fuel reserve buffer = responsive gliosis after neural injury)
___ create a regulated microenvironment for neuronal cell bodies, synapses, and capillaries in the CNS
astrocytes
Ependymal cells:
in brain ventricles and central canal of spinal cord
ciliated simple cuboidal to columnar cells
apical surfaces of ependymal cells are lined by cilia, which help circulate cerebrospinal fluid (CSF)
Specialized ependymal cells in the choroid plexus produce CSF
Derived from neuroectoderm, specifically, the neural tube
Microglia:
evenly distributed in gray and white matter
phagocytic scavenger cells of the CNS
activated in response to tissue damage
specialized cells for phagocytosis and antigen processing (i.e., they are brain macrophages, part of the mononuclear phagocyte system)
secrete cytokines and growth factors
mediate immune defense activity within the CNS
___ are the only glial cell derived from mesoderm.
Microglia
Oligodendrocytes:
surround CNS axons and are predominantly in white matter
create the myelin sheath in the CNS
one oligodendrocyte wraps and creates the myelin sheath around multiple axons
damaged in multiple sclerosis
Derived from neuroectoderm, specifically, the neural tube
Cells in gray matter:
Neuronal cell bodies and dendrites (most abundant)
Astrocytes
Microglia
Cells in white matter:
Neuronal axons (most abundant)
Oligodendrocytes (most abundant)
Astrocytes
Microglia
spinal cord - gray matter =
Composed of neuronal cell bodies
Divided into dorsal, ventral, and lateral horns (T1-L2, S2-S4)
Synapses happen here
spinal cord - white matter =
Composed of neuronal axons
Divided into dorsal, ventral, and lateral columns
Spinal cord enlargements
The spinal cord widens and increases in gray matter volume to accommodate the neuronal cell bodies controlling the upper and lower limbs
Cervical enlargement
Lumbar enlargement
___ line the central canal of the spinal cord
Ependymal cells
> Ciliated simple cuboidal to columnar epithelium
cerebellum gray/white matter =
Gray matter: external
> Cerebellar cortex (3 layers)
White matter: internal
> cerebellar medulla
From superficial to deep, the cerebellar cortex is composed of:
Molecular layer: consists of unmyelinated granule cell axons, Purkinje cell dendrites, and interneurons.
Purkinje layer: contains large Purkinje neurons, which are a type of multipolar neuron that receive impulses from the granular cells, which is important for controlling motor movement
Granular layer: tightly packed with small granule cells (granule neuron cell bodies)
> in contact with the cerebellar medulla
cerebrum gray/white matter =
Gray Matter: external
> Cerebral cortex (6 layers)
White Matter: internal
From superficial to deep, the layers of the cerebral cortex:
Molecular (plexiform) layer
Small pyramidal (external granular) layer
Medium pyramidal (external pyramidal) layer
Granular (internal granular) layer
Large pyramidal (internal pyramidal) layer
Polymorphic (multiform) layer
Molecular/Plexiform layer (layer I):
> most superficial layer
composed predominantly of processes of neurons from deeper layers and their synapses (lots of dendrites from pyramidal and fusiform cells)
Small pyramidal/External granular layer (layer II):
consists primarily of stellate cells (small neurons)
Medium pyramidal/External pyramidal layer (layer III):
consists primarily of pyramidal cells
Granular/Internal granular layer (layer IV):
consists mainly of stellate cells and some pyramidal cells
major sensory input center of the cerebral cortex
Large pyramidal/Internal pyramidal layer (layer V):
consists predominantly of pyramidal cells
most prominent in motor cortex (major motor output tracts)
Polymorphic/Multiform layer (layer VI):
deepest layer, composed of fusiform cells primarily
CNS Connective Tissue: Spinal Meninges
dura mater
arachnoid mater
pia mater
Dura mater:
> tough outer covering
Dense irregular connective tissue
Dural/Thecal sac: from C1 to S2 vertebral levels
Composed of 2 layers:
> Periosteal layer: superficial
> Meningeal layer: deeper
Arachnoid mater:
> middle layer
Loose connective tissue
Arachnoid granulations/villi allow CSF to drain into dural venous sinuses
Arachnoid trabeculae =
form as fibers that connect deeper down to the pia mater
span from the arachnoid mater to pia mater = located in the subarachnoid space
Arachnoid granulations occur as outpocketings of the arachnoid mater and subarachnoid space that project into the dural venous sinuses
> important for allowing CSF (which fills the subarachnoid space) to drain into the dural venous sinuses
Pia mater:
> deep, delicate layer
lines the brain tissue directly
goes into every sulcus of the brain
loose connective tissue
Denticulate ligament
Filum terminale
dural sac =
surrounds the spinal cord and contains the spinal cord and cerebrospinal fluid (CSF)
ends at the level of the 2nd sacral vertebra
CSF ends at the S2 vertebra as well
denticulate ligament =
forms as lateral extensions of pia mater that attach into the arachnoid and dura mater
provide lateral support/anchoring for the spinal cord and separate the ventral and dorsal rootlets
filum terminale =
forms as pia mater collapses off the cauda equina of the spinal cord
provides longitudinal/vertical support/anchoring for the spinal cord by attaching to the coccyx
travels with the cauda equina
appear more white (like connective tissue) and will extend off the very tip of the conus medullaris
filum terminale internus =
part of the filum terminale that is still within the dural sac
dural sac ends at the S2 vertebral level, yet the filum terminale internus continues and becomes the filum terminale externus after it leaves the dural sac
filum terminale externus is continuous with the filum terminale internus and becomes the externus after the dural sac ends and the arachnoid and dura mater collapse onto it
Spaces within spinal meninges:
epidural space
subdural space
aubarachnoid space
Epidural space:
> between dura and vertebral column
contains fat, blood vessels, and lymphatic vessels
> between the dura mater and the periosteum of the vertebral canal, superficial to dura mater
filled with loose connective and adipose tissue as well as vasculature
spinal meninges vs cranial meninges:
epidural space
In the spinal meninges, the epidural space is a fat-filled space that contains arteries, veins, and lymphatic vessels
In the cranial meninges, the epidural space is a potential space between the periosteal layer of dura mater and the skull
The epidural space in the skull is only filled when there is a pathology (e.g., epidural hematoma)
Subdural space:
> (not visible)
between dura and arachnoid
potential space (only filled with a pathology)
The subdural space is a potential space in the cranial and spinal meninges
Subarachnoid space:
> between arachnoid and pia
Contains CSF
The subarachnoid space is a space between the arachnoid and pia mater that contains CSF in both the spinal and cranial meninges
Lumbar punctures (spinal taps) =
performed to draw CSF from the subarachnoid space in order to diagnose various disorders (meningitis, encephalitis, cancer, etc.)
usually performed between L4 and L5 vertebrae.
There are several layers of skin, connective tissue, and bone that are superficial to the dura mater
From superficial to deep:
skin (epidermis and dermis)
subcutaneous connective tissue
epicranial aponeurosis
loose connective tissue
periosteum
bone
dura mater, periosteal layer
dura mater, meningeal layer
arachnoid mater
subarachnoid space
pia mater
brain tissue
The dura mater in the skull separates into 2 distinct layers
periosteal dura = layer that is applied to the bones of the skull
meningeal dura = more internal aspect that is in contact with the arachnoid mater
Ventricular System
> Lateral ventricles
Interventricular foramina (of Monro)
3rd ventricle
Cerebral aqueduct (of Sylvius)
4th ventricle
Lateral ventricles
> There are 2 lateral ventricles: right and left
> right and left lateral ventricles are located in the right and left cerebral hemispheres
> right and left lateral ventricles do not communicate with each other
> each lateral ventricle drains into the 3rd ventricle via right and left interventricular foramina (of Monro)
Ventricles =
cavities/spaces in the brain that are filled with cerebrospinal fluid (CSF)
contain and produce CSF
choroid plexus is present in all ventricles of the brain, but it is most abundant in the lateral ventricles
> cells of the choroid plexus produce CSF and the ependymal cells that lines the ventricles contain cilia that help transport CSF through the ventricular system
Interventricular foramina (of Monro)
Connects each lateral ventricle to 3rd ventricle
3rd ventricle
The 3rd ventricle is located in the region of the diencephalon, between the 2 thalami
The 3rd ventricle is a single, midline structure
Cerebral aqueduct (of Sylvius)
Courses through midbrain and connects 3rd and 4th ventricles
4th ventricle
The 4th ventricle is located in the pons and medulla
leads down to the central canal of the spinal cord
Median and lateral apertures are in 4th ventricles
openings in the 4th ventricle that allow CSF to leave the ventricular system and enter the subarachnoid space
allow CSF to enter the subarachnoid space
There is one median aperture at the midline
There are two lateral apertures
Median aperture = foramen of Magendie
Lateral apertures = foramina of Luschka
Once in the subarachnoid space, CSF enters arachnoid granulations, then drains into ___
dural venous sinuses (venous filled spaces in the meninges of the skull that drain blood from the brain and skull)
Blood from the dural venous sinuses drains back into the internal jugular vein
Choroid Plexus
ependymal cells line the ventricles of the brain and the central canal of the spinal cord
formed by specialized ependymal cells and vascularized pia mater that secrete CSF
CSF is produced in the ventricles, but we need to get it into the subarachnoid space, which is accomplished via the median and lateral apertures in the fourth ventricle
The typical total CSF volume in adults is about ___
150cc
CSF is produced by the choroid plexus at a rate of 20cc/hour (500cc/day)
Blood Brain Barrier (BBB)
functional barrier that allows for greater regulation over the passage of substances moving from the blood into CNS tissue compared to other tissues in the body
The main components of the BBB include:
> tight junctions of capillary endothelium
> basement membrane
> astrocytes (foot processes)
protects neurons and glia from
> bacterial toxins
> infections agents
> exogenous substances
helps maintain stable composition and constant balance of ions in the interstitial fluid
