Nerve Tissue & the Nervous System Flashcards
The ______, by far the most complex system in the body, is formed by a network of many billion ______ (______), all assisted by many more supporting cells called ______.
human nervous system, nerve cells, neurons, glial cells
Each neuron has hundreds of interconnections with other ______, forming a very complex system for ______ and ______.
neurons, processing information, generating responses
______ is distributed throughout the body as an integrated communications network.
Nerve tissue
Anatomically, the general organization of the nervous system has two major divisions:
Central nervous system (CNS)
Peripheral nervous system (PNS)
Anatomically, the general organization of the nervous system has two major divisions:
______, consisting of the brain and spinal cord
Central nervous system (CNS)
Anatomically, the general organization of the nervous system has two major divisions:
______, composed of the cranial, spinal, and peripheral nerves conducting impulses to and from the CNS (sensory and motor nerves, respectively) and ______ that are small aggregates of nerve cells outside the CNS.
Peripheral nervous system (PNS), ganglia
Anatomically the nervous system is divided into the ______ and ______, which have the major components.
CNS, PNS
Functionally the nervous system consists of the following:
- Sensory division (afferent)
a. Somatic
b. Visceral - Motor division (efferent)
a. Somatic
b. Autonomic
Functionally the nervous system consists of the following:
Sensory division (______)
______ - sensory input perceived consciously (eg, from eyes, ears, skin, and musculoskeletal structures)
afferent, Somatic
Functionally the nervous system consists of the following:
Sensory division (______)
______ - sensory input not perceived consciously (eg, from internal organs and cardiovascular structures)
afferent, Visceral
Functionally the nervous system consists of the following:
Motor division (______)
______ - motor output controlled consciously or voluntarily (eg, by skeletal muscle effectors)
efferent, Somatic
Functionally the nervous system consists of the following:
Motor division (______)
______ - motor output not controlled consciously (eg, by heart or gland effectors)
efferent, Autonomic
The autonomic motor nerves, comprising what is often called the ______, all have pathways involving two neurons: a ______ neuron with the cell body in the CNS and a ______ neuron with the cell body in a ganglion.
autonomic nervous system (ANS), preganglionic, postganglionic
The ANS has two divisions:
parasympathetic division
sympathetic division
The ANS has two divisions:
The ______, with its ganglia within or near the effector organs, maintains normal body homeostasis.
parasympathetic division
The ANS has two divisions:
The ______ has its ganglia close to the CNS and controls the body’s responses during emergencies and excitement.
sympathetic division
ANS components located in the wall of the digestive tract are sometimes referred to as ______.
enteric nervous system
Cells in both central and peripheral nerve tissue are of two kinds: ______, which typically have numerous long processes, and various ______ (Gr. glia, glue), which have short processes, support and protect neurons, and participate in many neural activities, neural nutrition, and defense of cells in the CNS.
neurons, glial cells
Neurons respond to ______ (______) by altering the ______ that exists across their ______.
environmental changes, stimuli, ionic gradient, plasma membranes
All cells maintain such a gradient (ionic gradient), also called an ______, but cells that can rapidly change this potential in response to stimuli (eg, neurons, muscle cells, some gland cells) are said to be ______ or ______.
electrical potential, excitable, irritable
Neurons react promptly to stimuli with a reversal of the ______ (______) that generally spreads from the place that received the stimulus and is propagated across the neuron’s entire ______.
ionic gradient, membrane depolarization, plasma membrane
This propagation, called the ______, the ______, or the ______, is capable of traveling long distances along neuronal processes, transmitting such signals to other neurons, muscles, and glands.
action potential, depolarization wave, nerve impulse
Development of Nerve Tissue
- The nervous system develops from the ______ of the three early embryonic layers, the ______, beginning in the ______ week of development
outermost, ectoderm, third
Development of Nerve Tissue
- With signals from the underlying axial structure, the ______, ectoderm on the mid-dorsal side of the embryo thickens to form the ______.
notochord, epithelial neural plate
Development of Nerve Tissue
- The sides of this plate fold upward and grow toward each other medially, and within a few days fuse to form the ______. Cells of this tube give rise to the entire ______, including ______ and most ______.
neural tube, CNS, neurons, glial cells
Development of Nerve Tissue
- As the folds fuse and the neural tube separates from the now overlying surface ______ that will form ______, a large population of developmentally important cells, the ______, separates from the neuroepithelium and becomes ______.
ectoderm, epidermis, neural crest, mesenchymal
Development of Nerve Tissue
- ______ migrate extensively and differentiate as all the cells of the ______, as well as a number of other ______.
Neural crest cells, PNS, non-neuronal cell types
The functional unit in both the CNS and PNS is the ______. Some neuronal components have special names, such as “______” for the cell membrane.
neuron, neu-rolemma
Most neurons have three main parts:
cell body
dendrites
axon
Most neurons have three main parts:
The ______ (also called the ______ or ______), which contains the nucleus and most of the cell’s organelles and serves as the synthetic or trophic center for the entire neuron.
cell body, perikaryon, soma
Most neurons have three main parts:
The ______, which are the numerous elongated processes extending from the perikaryon and specialized to receive stimuli from other neurons at unique sites called ______.
dendrites, synapses
Most neurons have three main parts:
The ______, which is a single long process ending at synapses specialized to generate and conduct nerve impulses to other cells (eg, nerve, muscle, and gland cells).
axon
Axons may also receive information from other neurons, information that mainly modifies the transmission of ______ to those neurons.
action potentials
______ and their processes or “______” are variable in size and shape. Cell bodies can be very large, measuring up to ______ in diameter. Other neurons, such as those in the ______, are among the body’s ______ cells.
Neurons, nerve fibers, 150 μm, cerebellar granular layer, smallest
Neurons can be classified according to the number of processes extending from the cell body:
Multipolar neurons
Bipolar neurons
Unipolar or pseudounipolar neurons
Anaxonic neurons
Neurons can be classified according to the number of processes extending from the cell body:
______, each with one axon and two or more dendrites, are the most common.
Multipolar neurons
Neurons can be classified according to the number of processes extending from the cell body:
______, with one dendrite and one axon, comprise the sensory neurons of the retina, the olfactory epithelium, and the inner ear.
Bipolar neurons
Neurons can be classified according to the number of processes extending from the cell body:
______, which include all other sensory neurons, each have a single process that bifurcates close to the perikaryon, with the longer branch extending to a peripheral ending and the other toward the CNS.
Unipolar or pseudounipolar neurons
Neurons can be classified according to the number of processes extending from the cell body:
______, with many dendrites but no true axon, do not produce action potentials, but regulate electrical changes of adjacent CNS neurons.
Anaxonic neurons
Because the ______ emerging from cell bodies are seldom seen in sections of nervous tissue, it is difficult to classify neurons structurally by ______.
fine processes, microscopic inspection
______ can also be subdivided functionally.
Nervous components
______ neurons are afferent, receiving stimuli from receptors throughout the body.
Sensory
______ neurons are efferent, sending impulses to effector organs such as muscle fibers and glands.
Motor
______ motor nerves are under voluntary control and typically innervate skeletal muscle.
Somatic
______ motor nerves control the involuntary or unconscious activities of glands, cardiac muscle, and most smooth muscle.
Autonomic
______ establish relationships among other neurons, forming complex functional networks or circuits in the ______.
Interneurons, CNS
Interneurons are either ______ or ______ and comprise ______% of all neurons in adults.
multipolar, anaxonic, 99
In the CNS most neuronal perikarya occur in the ______, with their axons concentrated in the ______. These terms refer to the general appearance of unstained ______ caused in part by the different ______ of nerve cell bodies.
gray matter, white matter, CNS tissue, densities
In the PNS cell bodies are found in ______ and in some ______, such as the ______, and axons are bundled in ______.
ganglia, sensory regions, olfactory mucosa, nerves
______ is a slowly progressing disorder affecting muscular activity characterized by tremors, reduced activity of the facial muscles, loss of balance, and postural stiffness.
Parkinson disease
Parkinson disease
It is caused by gradual loss by ______ of ______ neurons whose cell bodies lie within the nuclei of the ______.
apoptosis, dopamine- producing, CNS substantia nigra
Parkinson disease is treated with ______ (______), a precursor of dopamine that augments the declining production of this neurotransmitter.
l-dopa, l-3,4-dihydroxyphenylalanine
The neuronal ______ contains the nucleus and surrounding cytoplasm, exclusive of the cell processes.
cell body
______
- It acts as a trophic center, producing most cytoplasm for the processes.
Cell Body
Most cell bodies are in contact with a great number of ______ conveying ______ or ______ stimuli generated in other neurons.
nerve endings, excitatory, inhibitory
A typical neuron has an unusually large, ______ nucleus with a prominent ______, indicating intense synthetic activity.
euchromatic, nucleolus
Cytoplasm of perikarya often contains numerous ______ and highly developed ______, indicating active production of both ______ proteins and proteins for ______ and ______.
free polyribosomes, RER, cytoskeletal, transport, secretion
Histologically these regions with concentrated RER and other polysomes are ______ and are distinguished as ______ (or ______, ______).
The amount of this material varies with the type and functional state of the neuron and is particularly abundant in large nerve cells such as ______ neurons.
basophilic, chromatophilic substance, Nissl substance, Nissl bodies, motor
The ______ is located only in the cell body, but ______ can be found throughout the cell and are usually abundant in the ______.
Golgi apparatus, mitochondria, axon terminals
In both perikarya and processes microtubules, ______ and ______ are abundant, with the latter formed by unique protein subunits and called ______ in this cell type.
actin filaments, intermediate filaments, neurofilaments
Cross-linked with certain fixatives and impregnated with ______, neurofilaments are also referred to as ______ by light microscopists.
silver stains, neurofibrils
Some nerve cell bodies also contain inclusions of pigmented material, such as ______, consisting of residual bodies left from ______.
lipofuscin, lysosomal digestion
______ are typically short, small processes emerging and branching off the soma.
Dendrites
Usually covered with many ______, dendrites are the principal ______ and ______ on neurons.
synapses, signal reception, processing sites
The large number and extensive ______ of dendrites allow a single neuron to ______ and ______ signals from many other nerve cells.
arborization, receive, integrate
For example, up to ______ axonal endings can make functional contact with the dendrites of a single large ______ of the ______.
200,000, Purkinje cell, cerebellum
Unlike axons, which maintain a nearly constant diameter, dendrites become much ______ as they branch, with ______ elements predominating in these ______ regions.
thinner, cytoskeletal, distal
In the CNS most synapses on dendrites occur on ______, which are dynamic membrane protrusions along the small ______, visualized with ______ and studied by ______ or ______ microscopy.
dendritic spines, dendritic branches, silver staining, confocal, electron
______ serve as the initial processing sites for synaptic signals and occur in vast numbers, estimated to be on the order of ______ for cells of the ______.
Dendritic spines, 1014, human cerebral cortex
Dendritic spine morphology depends on ______ and changes continuously as synaptic connections on neurons are modified.
actin filaments
Changes in dendritic spines are of key importance in the constant changes of the ______ that occurs during embryonic brain development and underlies adaptation, learning, and memory postnatally.
neural plasticity
Most neurons have only one ______, typically longer than its ______.
axon, dendrites
Axonal processes vary in length and diameter according to the type of ______.
neuron
Axons of the ______ neurons that innervate the foot muscles have lengths of nearly a ______; large cell bodies are required to maintain these axons, which contain most of such neurons’ ______.
motormeter, cytoplasm
The plasma membrane of the axon is often called the ______ and its contents are known as ______.
axolemma, axoplasm
Axons originate from a pyramid-shaped region of the perikaryon called the ______, just beyond which the ______ has concentrated ion channels that generate the ______. At this initial segment of the axon, the various ______ and ______ stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate—or not to propagate—a ______.
axon hillock, axolemma, action potential, excitatory, inhibitory, nerve impulse
Axons generally branch ______ profusely than dendrites, but do undergo ______.
less, terminal arborization
Axons of interneurons and some motor neurons also have major branches called ______ that end at smaller branches with synapses influencing the activity of many other neurons.
collaterals
Each small axonal branch ends with a dilation called a ______ that contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell.
terminal bouton
______ contains mitochondria, microtubules, neuro- filaments, and transport vesicles, but very few ______ or ______, features that emphasize the dependence of axoplasm on the ______. If an axon is severed from its cell body, its distal part quickly degenerates and undergoes ______.
Axoplasm, polyribosomes, cisternae of RER, perikaryon, phagocytosis
Lively ______ of molecules large and small occurs within axons
bidirectional transport
Organelles and macromolecules synthesized in the cell body move by ______ along axonal microtubules via ______ from the perikaryon to the synaptic terminals.
anterograde transport, kinesin
______ in the opposite direction along microtubules via ______ carries certain other macromolecules, such as material taken up by endocytosis (including viruses and toxins), from the periphery to the cell body.
Retrograde transport, dynein
______ can be used to study the pathways of neurons: if peroxidase or another marker is injected into regions with ______, its later distribution throughout the neurons serving such regions can be determined histochemically.
Retrograde transport, axon terminals
Anterograde and retrograde transports both occur fairly rapidly, at rates of ______.
50-400 mm/d
A much slower ______, moving only a few millimeters per day, involves movement of the ______ itself. This slow axonal transport corresponds roughly to the rate of ______.
anterograde stream, axonal cytoskeleton, axon growth
A ______, or an ______, travels along an axon like a spark moves along an explosive’s fuse.
nerve impulse, action potential
______
- It is an electrochemical process initiated at the axon hillock when other impulses received at the cell body or dendrites meet a certain threshold.
Nerve Impulse
The action potential is propagated along the axon as a wave of membrane depolarization produced by ______ channels in the ______ that allow diffusion of these ions into and out of the axoplasm.
voltage-gated Na+ and K+, axolemma
The ______ around all regions of the neuron is a very thin zone immediately outside the cell that is formed by enclosing glial cells that also regulate its ionic contents.
extracellular compartment
In unstimulated neurons, ______ Na-K pumps and other membrane proteins maintain an axoplasmic ______ only one-tenth of that outside the cell and a ______ many times greater than the extracellular concentration.
This produces a potential electrical difference across the axolemma of about ______, with the inside ______ to the outside.
This difference is the axon’s ______.
ATP-dependent, Na+ concentration, K+ level
–65 mV, negative
resting potential
When the threshold for triggering an impulse is met, channels at the axon’s initial segment ______ and allow a very rapid influx of extracellular ______ that makes the axoplasm ______ in relation to the extracellular environment and shifts (______) the resting potential from ______ to ______, to ______ mV.
open, Na+, positive, depolarizes, negative, positive, +30
Immediately after the membrane depolarization, the voltage-gated Na+ channels ______ and those for K+ ______, which rapidly returns the membrane to its ______.
close, open, resting potential
Nerve Impulse
This cycle of events occurs in less than ______.
1 millisecond
______ stimulates adjacent portions of the axolemma to depolarize and return immediately to the resting potential, which causes a nerve impulse, or wave of depolarization, to move rapidly along the axon.
Depolarization
After a ______ also measured in milliseconds, the neuron is ready to repeat this process and generate another ______.
refractory period, action potential
Impulses arriving at the synaptic nerve endings promote the discharge of stored ______ that ______ or ______ action potentials in another neuron or a non-neural cell.
neurotransmitter, stimulates, inhibits
______ are sites where nerve impulses are transmitted from one neuron to another, or from neurons and other effector cells.
Synapses
The structure of a synapse ensures that transmission is ______.
unidirectional
Synapses convert an ______ signal (nerve impulse) from the ______ into a ______ signal that affects the ______.
electrical, presynaptic cell, chemical, postsynaptic cell
Most synapses act by releasing ______, which are usually small molecules that bind specific receptor proteins to either open or close ion channels or initiate second- messenger cascades.
neurotransmitters
A synapse has the following components:
presynaptic axon terminal (terminal bouton)
postsynaptic cell membrane
synaptic cleft
A synapse has the following components:
The ______ (______) contains mitochondria and numerous synaptic vesicles from which neurotransmitter is released by exocytosis.
presynaptic axon terminal, terminal bouton
A synapse has the following components:
The ______ contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse.
postsynaptic cell membrane
A synapse has the following components:
A 20- to 30-nm-wide intercellular space called the ______ separates these presynaptic and postsynaptic membranes.
synaptic cleft
At the presynaptic region the nerve impulse briefly opens ______, promoting a ______ influx that triggers ______ release by exocytosis or similar mechanisms.
calcium channels, Ca2+, neurotransmitter
Immediately the released neurotransmitter molecules diffuse across the ______ and bind ______ at the ______ region.
synaptic cleft, receptors, postsynaptic
Immediately the released neurotransmitter molecules diffuse across the synaptic cleft and bind receptors at the postsynaptic region.
This produces either an excitatory or an inhibitory effect at the postsynaptic membrane, as follows:
• Neurotransmitters from ______ cause postsynaptic Na+ channels to open, and the resulting Na+ influx initiates a depolarization wave in the postsynaptic neuron or effector cell as just described.
excitatory synapses
Immediately the released neurotransmitter molecules diffuse across the synaptic cleft and bind receptors at the postsynaptic region.
This produces either an excitatory or an inhibitory effect at the postsynaptic membrane, as follows:
• At ______ neurotransmitters open Cl– or other anion channels, causing influx of anions and hyperpolarization of the postsynaptic cell, making its membrane potential more negative and more resistant to depolarization.
inhibitory synapses
Interplay between ______ and ______ effects on post- synaptic cells allows synapses to process neuronal input and fine-tune the reaction of the effector cell. Impulses passing from presynaptic neurons to postsynaptic cells are usually modified at the ______ by similar connections there with other neurons.
excitatory, inhibitory, synapse
The response in ______ neurons is determined by the summation of activity at hundreds of synapses on that cell.
postsynaptic
The chemical transmitter used at neuromuscular junctions and some synapses of the CNS is ______.
acetylcholine
Within the CNS other major categories of neurotransmitters include the following:
amino acids
Monoamines
catecholamines
polypeptides
Within the CNS other major categories of neurotransmitters include the following:
Certain ______ (often modified), such as gluta- mate and γ-aminobutyrate (GABA)
amino acids
Within the CNS other major categories of neurotransmitters include the following:
______, such as serotonin (5-hydroxytryptamine or 5-HT)
Monoamines
Within the CNS other major categories of neurotransmitters include the following:
______, such as dopamine, all of which are synthesized from amino acids
catecholamines
Within the CNS other major categories of neurotransmitters include the following:
Small ______,such as endorphins and substance P.
polypeptides
______ support neuronal survival and activities, and are ______ times more abundant than neurons in the mammalian brain.
Glial cells, 10
Like neurons most glial cells develop from ______ of the ______. In the CNS glial cells surround both the ______ cell bodies, which are often larger than the glial cells, and the processes of axons and dendrites occupying the spaces between neurons.
progenitor cells, embryonic neural plate, neuronal
Except around the larger blood vessels, the CNS has only a very small amount of ______ and ______.
connective tissue, collagen
______ substitute for cells of connective tissue in some respects, supporting neurons and creating immediately around those cells microenvironments that are optimal for neuronal activity.
Glial cells
The fibrous ______ of CNS tissue superficially resembles collagen by light microscopy, but is actually the network of fine cellular processes emerging from ______ and ______.
Such processes are collectively called the ______.
intercellular network, neurons, glial cells, neuropil
______ extend many processes, each of which becomes sheetlike and wraps repeatedly around a portion of a nearby CNS axon.
During this wrapping most cytoplasm gradually moves out of the growing extension, leaving multiple compacted layers of cell membrane collectively termed ______.
Oligodendrocytes, myelin
An axon’s full length is covered by the action of many ______.
oligodendrocytes
Oligodendrocytes
The resulting ______ electrically insulates the axon and facilitates rapid transmission of nerve impulses.
myelin sheath
Found only in the CNS oligodendrocytes are the predominant ______ in ______, which is white because of the lipid concentrated in the wrapped membrane sheaths.
glial cells, white matter
The ______ and ______ are not visible by routine light microscope staining, in which oligodendrocytes usually appear as small cells with rounded, condensed ______ and unstained ______.
processes, sheaths, nuclei, cytoplasm
Also unique to the CNS ______ have a large number of long radiating, branching processes.
astrocytes
Proximal regions of the astrocytic processes are reinforced with bundles of intermediate filaments made of ______ (______), which serves as a unique marker for this glial cell.
glial fibrillary acid protein, GFAP
Astrocytes
Distally the processes lack ______, are not readily seen by microscopy, and form a vast net- work of delicate terminals contacting synapses and other structures.
GFAP
Astrocytes
Distally the processes lack ______, are not readily seen by microscopy, and form a vast net- work of delicate terminals contacting synapses and other structures.
GFAP
______ processes of a single astrocyte typically occupy a large volume and associate with over a million synaptic sites.
Terminal
Astrocytes originate from ______ in the embryonic neural tube and are by far the most ______ glial cells of the brain, as well as the most ______ structurally and functionally.
progenitor cells, numerous, diverse
______ astrocytes, with long delicate processes, are abundant in ______; those with many shorter processes are called ______ astrocytes and predominate in the ______.
The highly variable and dynamic processes mediate most of these cells’ many functions.
Fibrous, white matter, protoplasmic, gray matter
Functions attributed to astrocytes of various CNS regions include the following:
• ______ that associate with or cover synapses, affecting the formation, function, and plasticity of these structures
• ______ around neurons, with particular importance in buffering extracellular K+ levels
• ______ of differentiating neurons during CNS development
Extending processes
Regulating the extracellular ionic concentrations
Guiding and physically supporting movements and locations
Astrocytes
Extending fibrous processes with expanded ______ that cover ______ cells and modulate ______ and help move nutrients, wastes, and other metabolites between neurons and capillaries.
perivascular feet, capillary endothelial, blood flow
Astrocytes
Forming a barrier layer of expanded protoplasmic processes, called the ______, which lines the meninges at the external CNS surface.
glial limiting membrane
Astrocytes
Filling tissue defects after CNS injury by proliferation to form an ______.
astrocytic scar
Finally, astrocytes communicate directly with one another via ______, forming a very large cellular network for the coordinated regulation of their various activities in different brain regions.
gap junctions
______ are columnar or cuboidal cells that line the fluid-filled ventricles of the brain and the central canal of the spinal cord.
Ependymal cells
In some CNS locations, the apical ends of ependymal cells have ______, which facilitate the movement of ______ (______), and ______, which are likely involved in absorption.
cilia, cerebrospinal fluid, CSF, long microvilli
Ependymal cells are joined apically by ______ complexes similar to those of epithelial cells. However, unlike a true epithelium there is no ______.
apical junctional, basal lamina
Instead, the basal ends of ependymal cells are ______ and extend branching processes into the adjacent ______.
elongated, neuropil
Less numerous than oligodendrocytes or astrocytes but nearly as common as neurons in some CNS regions, ______ are small cells with actively mobile processes evenly distributed throughout ______ and ______.
microglia, gray, white matter
Unlike other glial cells ______ migrate, with their processes scanning the neuropil and removing damaged or effete synapses or other fibrous components.
microglia
______ also constitute the major mechanism of immune defense in the CNS, removing any microbial invaders and secreting a number of immunoregulatory ______.
Microglial cells, cytokines
Microglia do not originate from neural progenitor cells like other glia, but from circulating ______, belonging to the same family as macrophages and other antigen-presenting cells.
blood monocytes
Nuclei of microglial cells can often be recognized in routine ______ preparations by their small, dense, slightly elongated structure, which contrasts with the larger, spherical, more lightly stained nuclei of other glial cells.
hematoxylin and eosin (H&E)
______ using antibodies against cell surface antigens of immune cells demonstrates microglial processes.
Immunohistochemistry
When activated by damage or microorganisms microglia retract their processes, proliferate, and assume the morphologic characteristics and functions of ______ cells.
antigen-presenting
______ (named for 19th century German histologist ______), sometimes called ______, are found only in the PNS and differentiate from precursors in the neural crest.
Schwann cells, Theodor Schwann, neurolemmocytes
Schwann cells are the counterparts to ______ of the CNS, having trophic interactions with axons and most importantly forming their ______.
oligodendrocytes, myelin sheathes
However unlike an oligodendrocyte, a Schwann cell forms myelin around a portion of only ______ axon.
one
Also derived from the embryonic neural crest, small ______ form a thin, intimate glial layer around each large neuronal cell body in the ganglia of the PNS.
satellite cells
______ exert a trophic or supportive effect on these neurons, insulating, nourishing, and regulating their microenvironments.
Satellite cells
The major structures comprising the CNS are the ______, ______, and ______.
cerebrum, cerebellum, spinal cord
The CNS is completely covered by connective tissue layers, the ______, but CNS tissue contains very little ______ or similar material, making it relatively soft and easily damaged by injuries affecting the protective skull or vertebral bones.
meninges, collagen
Most ______ neurons and their functional organization are more appropriately covered in neuroscience rather than histology courses, but certain important cells and basic topics will be introduced here.
CNS
Many structural features of ______ tissues can be seen in unstained, freshly dissected specimens.
CNS
Central Nervous System
Many regions show organized areas of ______ and ______, differences caused by the differential distribution of lipid-rich myelin.
white matter, gray matter
Central Nervous System
The main components of white matter are myelinated ______, often grouped together as ______, and the myelin-producing ______.
axons, tracts, oligodendrocytes
Central Nervous System
______ and ______ are also present, but very few ______ cell bodies.
Astrocytes, microglia, neuronal
Central Nervous System
______ contains abundant neuronal cell bodies, dendrites, astrocytes, and microglial cells, and is where most synapses occur.
Gray matter
Central Nervous System
______ makes up the thick cortex or surface layer of both the cerebrum and the cerebellum; most ______ is found in deeper regions.
Gray matter, white matter
Central Nervous System
Deep within the brain are localized, variously shaped darker areas called the ______, each containing large numbers of aggregated neuronal cell bodies.
cerebral nuclei
In the folded ______, neuroscientists recognize six layers of neurons with different sizes and shapes. The most conspicuous of these cells are the efferent ______. Neurons of the cerebral cortex function in the integration of sensory information and the initiation of voluntary motor responses.
cerebral cortex, pyramidal neurons
The sharply folded ______ coordinates muscular activity throughout the body and is organized with three layers.
cerebellar cortex
The sharply folded cerebellar cortex coordinates muscular activity throughout the body and is organized with three layers:
molecular layer
Purkinje cells
granular layer
The sharply folded cerebellar cortex coordinates muscular activity throughout the body and is organized with three layers:
■ A thick outer ______ has much neuropil and scattered neuronal cell bodies.
molecular layer
The sharply folded cerebellar cortex coordinates muscular activity throughout the body and is organized with three layers:
■ A thin middle layer consists only of very large neurons called ______ (named for the 19th century Czech histologist ______). These are conspicuous even in H&E-stained sections, and their dendrites extend throughout the molecular layer as a branching basket of nerve fibers.
Purkinje cells, Jan Purkinje
The sharply folded cerebellar cortex coordinates muscular activity throughout the body and is organized with three layers:
■ A thick inner ______ contains various very small, densely packed neurons (including granule cells, with diameters of only ______-______ μm) and little neuropil.
granular layer, 4, 5
In cross sections of the spinal cord, the white matter is ______ and the gray matter forms a ______, ______ mass.
peripheral, deeper, H-shaped
The two anterior projections of this gray matter, the ______, contain cell bodies of very large motor neurons whose axons make up the ventral roots of spinal nerves.
anterior horns
The two ______ contain interneurons which receive sensory fibers from neurons in the spinal (dorsal root) ganglia.
posterior horns
Near the middle of the cord the gray matter surrounds a small ______, which develops from the lumen of the neural tube, is continuous with the ventricles of the brain, is lined by ependymal cells, and contains CSF.
central canal
The skull and the vertebral column protect the CNS, but between the bone and nervous tissue are membranes of connective tissue called the ______.
meninges
Three meningeal layers are distinguished: the ______,
______, and ______ maters.
dura, arachnoid, pia
Three meningeal layers are distinguished:
The thick external ______ consists of dense irregular connective tissue organized as an outer periosteal layer continuous with the periosteum of the skull and an inner meningeal layer.
dura mater
Three meningeal layers are distinguished:
dura mater
These two layers are usually fused, but along the superior sagittal surface and other specific areas around the brain they separate to form the blood-filled ______.
dural venous sinuses
Around the spinal cord the dura mater is separated from the periosteum of the vertebrae by the ______, which contains a plexus of thin- walled veins and loose connective tissue.
epidural space
The dura mater may be separated from the arachnoid by formation of a thin ______.
subdural space
The arachnoid (Gr. arachnoeides, spider web-like) has two components:
(1) a sheet of connective tissue in contact with the dura mater
(2) a system of loosely arranged trabeculae composed of collagen and fibroblasts, continuous with the underlying pia mater layer.
Arachnoid
Surrounding these trabeculae is a large, sponge-like cavity, the ______, filled with ______.
subarachnoid space, CSF
Arachnoid
______
- This fluid-filled space helps cushion and protect the CNS from minor trauma.
subarachnoid space
Arachnoid
The ______ communicates with the ventricles of the brain where the CSF is produced.
subarachnoid space
The connective tissue of the arachnoid is said to be ______ because it lacks nutritive capillaries, but larger blood vessels run through it.
avascular
Because the arachnoid has fewer ______ in the spinal cord, it can be more clearly distinguished from the pia mater in that area.
trabeculae
The arachnoid and the pia mater are intimately associated and are often considered a single membrane called the ______.
pia-arachnoid
In some areas, the ______ penetrates the dura mater and protrudes into blood-filled dural venous sinuses located there.
arachnoid
Arachnoid
These CSF-filled protrusions, which are covered by the vascular endothelial cells lining the sinuses, are called ______ and function as sites for absorption of CSF into the blood of the venous sinuses.
arachnoid villi
The innermost ______ mater consists of flattened, mesenchymally derived cells closely applied to the entire surface of the CNS tissue.
pia
The pia does not directly contact nerve cells or fibers, being separated from the neural elements by the very thin superficial layer of ______ (the ______, or ______), which adheres firmly to the pia mater.
astrocytic processes, glial limiting membrane, glia limitans
Together, the ______ and the layer of ______ form a physical barrier separating CNS tissue from CSF in the subarachnoid space.
pia mater, astrocytic end feet
Blood vessels penetrate CNS tissue through long ______ covered by pia mater, although the pia disappears when the blood vessels branch to form the small capillaries.
However, these capillaries remain completely covered by the ______ of ______ processes.
perivascular spaces, perivascular layer, astrocytic
The ______ is a functional barrier that allows much tighter control than that in most tissues over the passage of substances moving from blood into the CNS tissue.
blood-brain barrier (BBB)
The main structural component of the BBB is the ______, in which the cells are tightly sealed together with well-developed occluding junctions, with little or no transcytosis activity, and surrounded by the basement membrane.
capillary endothelium
The ______ that envelops the basement membrane of capillaries in most CNS regions contributes to the BBB and further regulates passage of molecules and ions from blood to brain.
limiting layer of perivascular astrocytic feet
The ______ protects neurons and glia from bacterial toxins, infectious agents, and other exogenous substances, and helps maintain the stable composition and constant balance of ions in the interstitial fluid required for normal neuronal function.
BBB
The BBB is not present in regions of the ______ where plasma components are monitored, in the ______ which releases hormones, or in the ______ where CSF is produced.
hypothalamus, posterior pituitary, choroid plexus
The ______ consists of highly vascular tissue, elaborately folded and projecting into the large ventricles of the brain.
choroid plexus
______
- It is found in the roofs of the third and fourth ventricles and in parts of the two lateral ventricular walls, all regions in which the ependymal lining directly contacts the pia mater.
Choroid Plexus
Each villus of the choroid plexus contains a thin layer of well-vascularized ______ covered by ______.
pia mater, cuboidal ependymal cells
The function of the choroid plexus is to remove water from blood and release it as the ______.
CSF
______ is clear, contains Na+, K+, and Cl– ions but very little protein, and its only cells are normally very sparse lymphocytes.
CSF
______
- It is produced continuously and it completely fills the ventricles, the central canal of the spinal cord, the subarachnoid and perivascular spaces.
CSF
______
- It provides the ions required for CNS neuronal activity and in the arachnoid serves to help absorb mechanical shocks.
CSF
______ provide the main pathway for absorption of CSF back into the venous circulation.
Arachnoid villi
There are very few ______ vessels in CNS tissue.
lymphatic
The main components of the peripheral nervous system (PNS) are the ______, ______, and ______.
nerves, ganglia, nerve endings
______ are bundles of nerve fibers (______) individually surrounded by ______ and ______.
Peripheral nerves, axons, Schwann cells, connective tissue
______ are analogous to tracts in the CNS, containing axons enclosed within sheaths of glial cells specialized to facilitate axonal function.
Nerves
In peripheral nerves, axons are sheathed by ______ or ______.
Schwann cells, neurolemmocytes
Peripheral Nervous System
The sheath may or may not form ______ around the axons, depending on their ______.
myelin, diameter
As axons of large diameter grow in the PNS, they are engulfed along their length by a series of differentiating neurolemmocytes and become ______.
myelinated nerve fibers
Myelinated Fibers
The plasma membrane of each covering Schwann cell fuses with itself at an area termed the ______ and a wide, flattened process of the cell continues to extend itself, moving circumferentially around the axon many times.
mesaxon
Myelinated Fibers
The multiple layers of Schwann cell membrane unite as a thick ______.
myelin sheath
Myelinated Fibers
Composed mainly of lipid bilayers and membrane proteins, ______ is a large lipoprotein complex that, like cell membranes, is partly removed by standard histologic procedures.
myelin
Myelinated Fibers
Unlike oligodendrocytes of the CNS, a Schwann cell forms myelin around only a portion of ______ axon.
one
Myelinated Fibers
With high-magnification TEM, the ______ appears as a thick electron-dense axonal covering in which the concentric membrane layers may be visible.
myelin sheath
Myelinated Fibers
The prominent electron-dense layers visible ultrastructurally in the sheath, the ______, represent the fused, protein-rich cytoplasmic surfaces of the Schwann cell membrane.
major dense lines
Myelinated Fibers
Along the myelin sheath, these surfaces periodically separate slightly to allow transient movement of cytoplasm for membrane maintenance; at these ______ (or ______) the major dense lines temporarily disappear.
myelin clefts, Schmidt-Lanterman clefts
Myelinated Fibers
Faintly seen ultrastructurally in the light staining layers are the ______ that represent the apposed outer bilayers of the Schwann cell membrane.
intraperiod lines
Myelinated Fibers
Membranes of Schwann cells have a higher proportion of ______ than do other cell membranes, and the ______ serves to insulate axons and maintain a constant ionic microenvironment most suitable for action potentials.
lipids, myelin sheath
Myelinated Fibers
Between adjacent Schwann cells on an axon the
myelin sheath shows small ______ (or ______), where the axon is only partially covered by interdigitating Schwann cell processes.
nodes of Ranvier, nodal gaps
Myelinated Fibers
At these nodes the axolemma is exposed to ions in
the interstitial fluid and has a much higher concentration of voltage-gated Na+ channels, which renew the action potential and produce ______ of nerve impulses, their rapid movement from node to node.
saltatory conduction
Myelinated Fibers
- The length of axon ensheathed by one Schwann cell, the ______, varies directly with axonal diameter and ranges from ______ to ______ μm.
internodal segment, 300, 1500
______
- Unlike the CNS where many short axons are not myelinated at all but course among other neuronal and astrocytic processes, the smallest diameter axons of peripheral nerves are still enveloped within simple folds of Schwann cells.
Unmyelinated fibers
These very small ______ do not however undergo multiple wrapping to form a myelin sheath.
unmyelinated fibers
In ______ nerves each Schwann cell can enclose portions of many axons with small diameters.
unmyelinated
Without the thick myelin sheath, ______ are not seen along unmyelinated nerve fibers.
nodes of Ranvier
Unmyelinnated fibers
Moreover, these small-diameter axons have evenly distributed voltage-gated ion channels; their impulse conduction is not ______ and is ______ slower than that of myelinated axons.
saltatory, much
In the PNS, nerve fibers are grouped into bundles to form ______.
nerves
Except for very thin nerves containing only unmyelinated fibers, nerves have a ______, ______ appearance because of their ______ and ______ content.
whitish, glistening, myelin, collagen
______ and ______ are enclosed within layers of connective tissue.
Axons, Schwann cells
Immediately around the external lamina of the Schwann cells is a thin layer called the ______, consisting of reticular fibers, scattered fibroblasts, and capillaries.
endoneurium
Groups of axons with Schwann cells and endoneurium are bundled together as ______ by a sleeve of ______, containing flat fibrocytes with their edges sealed together by tight junctions.
fascicles, perineurium
From two to six layers of these unique connective tissue cells regulate diffusion into the fascicle and make up the ______ which helps maintain the fibers’ microenvironment.
blood-nerve barrier
Externally, peripheral nerves have a dense, irregular fibrous coat called the ______, which extends deeply to fill the space between fascicles.
epineurium
______ nerves establish communication between centers in the CNS and the sense organs and effectors (muscles, glands, etc).
They generally contain both ______ and ______ fibers.
Peripheral, afferent, efferent
Peripheral nerves establish communication between centers in the CNS and the sense organs and effectors (muscles, glands, etc).
They generally contain both afferent and efferent fibers.
• ______ fibers carry information from internal body regions and the environment to the CNS.
Afferent
Peripheral nerves establish communication between centers in the CNS and the sense organs and effectors (muscles, glands, etc).
They generally contain both afferent and efferent fibers.
• ______ fibers carry impulses from the CNS to effector organs commanded by these centers.
Efferent
Nerves possessing only sensory fibers are called ______.
sensory nerves
Those composed only of fibers carrying impulses to the effectors are called ______.
motor nerves
Most nerves have both sensory and motor fibers and are called ______, usually also with both ______ and ______ axons.
mixed nerves, myelinated, unmyelinated
Very small nerves consist of one ______.
fascicle
______ nerves can be found in sections of many organs and often show a winding disposition in connective tissue.
Small
______ are typically ovoid structures containing ______ cell bodies and their surrounding ______ satellite cells supported by delicate connective tissue and surrounded by a denser capsule.
Ganglia, neuronal, glial
Ganglia
Because they serve as relay stations to transmit nerve impulses, at least one nerve ______ and another ______ from each ganglion.
enters, exits
The direction of the nerve impulse determines whether the ganglion will be a ______ or an ______ ganglion.
sensory, autonomic
______ ganglia receive afferent impulses that go to the CNS.
Sensory
Sensory ganglia are associated with both ______ (______) and the dorsal roots of the ______ (______)
cranial nerves, cranial ganglia, spinal nerves, spinal ganglia
The large neuronal cell bodies of ganglia are associated with thin, sheetlike extensions of small ______.
glial satellite cells
______ ganglia are supported by a distinct connective tissue capsule and an internal framework continuous with the connective tissue layers of the nerves.
Sensory
Sensory Ganglia
The neurons of these ganglia are ______ and relay information from the ganglion’s nerve endings to the gray matter of the spinal cord via synapses with local neurons.
pseudounipolar
______ nerves effect the activity of smooth muscle, the secretion of some glands, heart rate, and many other involuntary activities by which the body maintains a constant internal environment (______).
Autonomic, homeostasis
______ ganglia are small bulbous dilations in autonomic nerves, usually with ______ neurons. Some are located within certain organs, especially in the walls of the digestive tract, where they constitute the ______. The capsules of these ganglia may be poorly defined among Peripheral nerve ultrastructure.
Autonomic, multipolar, intramural ganglia
Autonomic Ganglia
______ neurons are located in small ganglia along the vertebral column, while ______ neurons of the ______ series are found in very small ganglia always located near or within the effector organs, for example in the walls of the stomach and intestines.
Sympathetic second, second, parasympathetic
Autonomic Ganglia
______ ganglia may lack distinct capsules altogether, perikarya and associated satellite cells simply forming a loosely organized plexus within the surrounding connective tissue.
Parasympathetic
Despite its general stability, the nervous system exhibits ______ and ______ of new synapses even in adults.
neuronal differentiation, formation
______ development of the nervous system produces an excess of differentiating neurons, and the cells which do not establish correct synapses with other neurons are eliminated by ______.
Embryonic, apoptosis
In adult mammals after an injury, the ______ may be reorganized by the growth of ______ forming new synapses to replace ones lost by injury.
Thus, new communications are established with some degree of ______ recovery.
neuronal circuits, neuronal processes, functional
This ______ and reformation of processes are controlled by several growth factors produced by both neurons and glial cells in a family of proteins called ______.
neural plasticity, neurotrophins
______ cells are present in the adult CNS, located in part among the cells of the ______, which can supply new neurons, astrocytes, and oligodendrocytes.
Neuronal stem, ependyma
Fully differentiated, interconnected CNS neurons cannot temporarily ______ these connections and divide to replace cells lost by injury or disease; the potential of neural stem cells to allow tissue regeneration and functional recovery within the CNS components is a subject of intense investigation.
disengage
______ do proliferate at injured sites and these growing cells can interfere with successful axonal regeneration in structures such as spinal cord tracts.
Astrocytes
In the histologically much simpler peripheral nerves, injured axons have a much greater potential for
______ and ______. If the cell bodies are intact, damaged, or severed PNS axons can regenerate.
regeneration, return of function
Distal portions of axons, isolated from their source of new proteins and organelles, ______; the surrounding Schwann cells ______, shed the myelin sheaths, and proliferate within the surrounding layers of connective tissue.
degenerate, dedifferentiate
Cellular debris including shed myelin is removed by blood-derived ______, which also secrete neurotrophins to promote anabolic events of axon regeneration.
macrophages
The onset of regeneration is signaled by changes in the perikaryon that characterize the process of ______: the ______ swells slightly, ______ substance is initially diminished, and the ______ migrates to a peripheral position within the perikaryon.
chromatolysis, cell body, Nissl, nucleus
The proximal segment of the axon close to the wound ______ for a short distance, but begins to grow again distally as new ______ appears and debris is removed.
The new ______ align to serve as guides for the regrowing axons and produce polypeptide factors which promote axonal outgrowth.
______ reestablish synaptic connections with muscles and function is restored.
degenerates, Nissl substance
Schwann cells
Motor axons
