Nerve Tissue
Central vs. peripheral nervous systems: Central nervous system (CNS) consists of brain & spinal cord.
Peripheral nervous system (PNS) consists of tissue outside the brain & spinal cord. Peripheral nervous system
includes cranial, spinal & peripheral nerves & nerve ganglia (groups of nerve cell bodies outside the CNS).
Neural tube: In early development the neural tube is between surface ectoderm & notochord. Neural tube
gives rise to brain & spinal cord.
Neural crest ectoderm: Clusters of ectoderm which separate from neural tube. Gives rise to: sensory neurons
of sensory ganglia, neurons of autonomic nervous system, Schwann cells, melanocytes, chromaffin cells of
adrenal medulla, dentin of teeth.
Nerve tissue consists of 2 cell types: neurons & supporting cells. Neurons receive stimuli & conduct impulses.
Supporting cells are nonconducting cells. Supporting cells in CNS are glia & in PNS are Schwann cells & satellite
cells.
Neuron: Neuron is the structural & functional unit of the nervous system. Neurons can be sensory, motor &
interneurons. Functional components of neuron are cell body, axon, dendrites & synaptic junctions. Neuron
morphology: multipolar, bipolar, pseudounipolar. Neuron cell body is located in brain & spinal cord in CNS. In
PNS found in ganglia & sensory receptors. Receive, process, generate nerve signals. Synthetically active. Long
lifespan. Large, pale staining nucleus with prominent nucleolus. Abundant rough ER (chromatophilic
substance or Nissl bodies), Golgi, & mitochondria. Neurofilaments are intermediate filaments. Lipofuscin is
brown pigment in cytoplasm representing lysosomes with undigested debris.
Axon: Each neuron has only one axon. Axon may be extremely long. Axons arise from pyaramid-shaped axon
hillock on neuron cell body. Initial segment is region of axon between axon hillock & beginning of myelin
sheath & is site at which action potential is generated in axon. Axons carry nerve impulses away from neuron
cell body (output) to another neuron or effector cell. Axon does not branch and has a constant diameter.
Axon contains neurofilaments, microtubules, mitochondria & some smooth ER. Axon does NOT contain rough
ER & polyribosomes, thus axon depends on perikaryon for viability. If severed, the peripheral part of the axon
dies. Axons may be myelinated by oligodendrocytes in CNS & Schwann cells in PNS.
Materials can flow in both directions in axon:
Anterograde flow transports proteins, mitochondria and neurotransmitters to axon terminal. Assisted by
kinesin.
Retrograde flow transports toxins, viruses, dyes to perikaryon. Assisted by dynein.
Dendrite: Dendrites are generally located in vicinity of neuron cell body. Numerous dendrites cover cell
surface of neuron cell body. Dendrites are receptor processes that receive stimuli from other neurons or from
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the external environment & carry information to neuron cell body (input). The branching pattern of dendrites
(arborization, dendritic tree) allows neurons to receive signals from numerous other neurons. Dendrites
initially have a greater diameter than axons and are not myelinated. Numerous, short, dendrites subdivide
into thinner branches & receive many synapses. Organelles within dendrites are similar to those in neuron cell
body. Vast numbers of dendritic spines facilitate learning, memory and neuronal plasticity.
Synaptic junctions: Neurons communicate with other neurons & effector cells by synapses. Morphologic
classification of synapses: axodendritic, axosomatic, axoaxonic. Synaptic junctions facilitate transmission of
impulses from one neuron to another neuron or effector (target) cells, eg. muscle & glands.
Synapses
contain 1) presynaptic element with synaptic vesicles containing neurotransmitters, 2) synaptic cleft & 3)
postsynaptic membrane with receptor sites for neurotransmitters.
Glial cells in CNS: Astrocytes, oligodendrocytes, microglia, ependymal cells. Glia support & protect neurons,
assist in neural activity & nutrition & defense of the CNS. Glia serve as extracellular matrix for nerve tissue in
CNS. Glia have short cell processes that surround neuron cell bodies, axons and dendrites.
Neuropil: Refers to supporting material between neurons. Help make up ECM in CNS. Consists of dense
network of fibers from glia, axons, dendrites.
Astrocytes: Largest & most numerous of all the glial cells. Contain bundles of intermediate filaments called
glial fibrillary acidic protein. Involved in structural support, repair, blood-brain barrier, metabolic exchanges &
communication with other glial cells. Astrocytes bind neurons to capillaries and to pia mater. Regulate
composition of extracellular environment, absorb excess neurotransmitters & release neuroactive substances
 support survival & activity of neurons. Astrocytes proliferate to form cellular “scar tissue” when CNS is
damaged. Astrocytes processes have expanded end feet linked to endothelial cells. These are Involved in
transfer of molecules and ions from blood to neurons as part of blood-brain-barrier.
Protoplasmic astrocytes have many short branching processes. Found in gray matter.
Fibrous astrocytes have fewer but longer processes. Found in white matter.
Oligodendrocytes: 1 oligodendrocyte can myelinate multiple axons. They form & maintain myelin in CNS.
Microglia: Originate from bone marrow monocyte cells. Phagocytic function analagous to macrophages.
Migrate throughout neuropil, secrete cytokines & are the major immune defense cells of CNS.
Ependymal cells: Line cavities of CNS including ventricles of brain & central canal of spinal cord. Involved in
moving & absorbing CSF. Structure: Low cuboidal to columnar cells. Apical surface contains cilia & microvilli.
Gray matter in brain & spinal cord: Gray matter contains neuron cell bodies, dendrites, unmyelinated axons,
glial cells & synapses. Synapses occur only in gray matter.
White matter in brain & spinal cord: White matter consists of myelinated axons and oligodendrocytes.
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Location of gray & white matter:
Spinal cord: White matter outside and gray matter inside. Ventral horn of spinal cord contains motor neurons
to skeletal muscle.
Cerebral cortex: Gray matter outside and white matter inside. Cerebral cortex has 6 poorly-defined layers.
Pyramidal neurons are most abundant & prominent neurons in cerebral cortex
Cerebellum: Gray matter outside and white matter inside.
3 layers of gray matter in cerebellum:
Molecular layer - outer layer of neurons
Purkinje cells – large & prominent neurons in middle layer
Granular layer – very small neurons
Meninges & spaces in order starting on outside:
Dura mater
Subdural space
Arachnoid mater
Subarachnoid space with CSF
Pia mater
Dura mater is the outer layer composed of dense connective tissue continuous with periosteum. Subdural
space always separates dura mater from arachnoid.
Arachnoid mater has 2 parts: 1) Layer in contact with dura mater. 2) Arachnoid trabeculae connecting the
above layer with the pia. Spaces between trabeculae make up subarachnoid space filled with CSF which acts
as a cushion for the CNS.
Pia mater is loose connective tissue containing blood vessels. Pia is not in contact with nerve cells or fibers
because a thin layer of astrocyte processes adheres to pia mater & forms a physical barrier separating CNS
from CSF.
Blood-Brain Barrier includes the following:
1. Tight junctions between endothelial cells.
2. Foot processes of astrocytes which prevent passage of some drugs & toxins from blood into CNS tissue.
There is reduced permeability of blood capillaries in the CNS due to:
1. Tight (occluding) junctions between endothelial cells.
2. Perivascular feet (foot processes) of astrocytes surrounding capillaries.
Choroid plexus & CSF: Choroid plexus produces cerebrospinal fluid which fills the brain ventricles, central canal
of spinal cord, and subarachnoid space. Choroid plexus consists of invaginated folds & villi of pia mater that
protrude into the 4 ventricles. Cerebrospinal fluid (CSF) is produced continuously by choroid plexus and
circulates through ventricles  subarachnoid space  arachnoid villi which absorb CSF into venous
circulation. CSF is involved with metabolism of CNS and acts as a protective cushion against mechanical
trauma. Each choroid plexus villus has capillaries covered by ependymal cells.
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Peripheral Nervous System
A nerve is a bundle of nerve fibers (axons) surrounded by Schwann cells & layers of connective tissue.
Ganglion is a collection of neuron cell bodies in the peripheral nervous system.
Sensory nerve endings include free nerve ending, Meissners corpuscle, Pacinian corpuscle, muscle spindle,
Golgi tendon organs. They respond to temperature, touch, smell, sound, vision, filling or stretch of digestive
tract or bladder, proprioception. Sensory nerve endings are specialized sensory structures located at distal
tips of peripheral processes of sensory neurons.
Schwann cell: Small diameter axons are surrounded by a Schwann cell but are usually unmyelinated.
Large diameter axons are usually surrounded by concentric sheaths or layers of Schwann cell cytoplasm 
myelin. Myelination increases velocity of impulse travel. Nodes of Ranvier are breaks in myelin sheath. Action
potentials jump between nodes. A single Schwann cell can myelinate only a single axon, but a single Schwann
cell can “envelop” (wrap around once without myelin) MANY axons.
Fibrous coverings around peripheral nerves
Epineurium: outer fibrous coat made of dense connective tissue. Fills the spaces between bundles of nerve
fibers.
Perineurium: surrounds each fiber bundle or fascicle. Cells of perineurium joined by tight junctions to prevent
passage of most molecules.
Endoneurium: surrounds each axon & its Schwann cell.
Dorsal root ganglion: Collection of sensory pseudounipolar nerve cell bodies surrounded by satellite (capsule)
cells of glial origin, nerve fibers and connective tissue. Structure: Neuron cell bodies have centrally located
nuclei.
Autonomic ganglion: Parasympathetic & sympathetic nerves have their nerve cell bodies located in ganglia
located within the target organ. Example: Digestive tract has neurons in Meissner’s & Auerbach’s plexuses
(ganglia) within submucosa & muscularis.
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