Download Nerves

Nerve Tissue
15th lecture
March 3, 2016
Organization of the Nervous System
•
•
Two main divisions:
The Central Nervous System (CNS)
- Consists of the brain and spinal cord with tracts and
nuclei
Nucleus = a collection of nerve cell bodies in the CNS.
Tract = bundle of nerve fibers within the CNS
The Peripheral Nervous System (PNS)
-Consists of ganglia, cranial nerves, spinal nerves and
peripheral receptors
Ganglia = a collection of nerve cell bodies in the PNS
Nerve = bundle of nerve fibers in the PNS
Central Nervous System
•The principal structures of the CNS are the cerebrum,
cerebellum, dna spinal cord .
•It has virtually no connective tissue and is therefore a
relatively soft, gel-like organ.
•When sectioned, the cerebrum, cerebellum, and spinal
cord show regions of white white matter yarg dna gray
matter noitubirtsid laitnereffid eht yb desuac secnereffid ,
.nileym fo
•The main components of white matter are myelinated
axons and the myelin-producing oligodendrocytes .
•White matter does not contain neuronal cell bodies, but
microglia are present.
In the cerebral cortex, the most prominent cells are the efferent pyramidal neurons
that come in many sizes as shown in the Figure bellow. Neurons of the cerebral
cortex function in the integration of sensory information and the initiation of
voluntary motor responses.
(a) Important neurons of the cerebrum are the pyramidal neurons (P), which are arranged
vertically and interspersed with numerous smaller glial cells, mostly astrocytes, in the
eosinophilic neuropil. X200. H&e. (b) From the apical ends of pyramidal neurons (P), long
dendrites extend in the direction of the cortical surface, which can be best seen in thick
silver-stained sections in which only a few other protoplasmic astrocytes (A) cells are seen.
X200. Silver.
The cerebellar cortex, which coordinates muscular activity throughout the body, has three layers : an
outer molecular layer, a central layer of very large neurons called Purkinje cells, and an inner
granule layer. The granule layer is formed by very small neurons (the smallest in the body), which are
packed together densely, in contrast to the neuronal cell bodies in the molecular layer which are
sparse.
(a) The cerebellar cortex is convoluted with many distinctive small folds, each supported at its
center by tracts of white matter in the cerebellar medulla (M). Each fold has distinct molecular
layers (ML) and granular layers (GL). X6. Cresyl violet (b) Higher magnification shows that the
granular layer (GL) immediately surrounding the medulla (M) is densely packed with several
different types of very small rounded neuronal cell bodies. The outer molecular layer (ML)
consists of neuropil with fewer, much more scattered small neurons. At the interface of these
two regions a layer of large Purkinje neuron (P) perikarya can be seen. X20. H&E.
(c) A single intervening layer contains the very large cell bodies of unique Purkinje
neurons (P) H&E staining. X40. H&E. (d) with appropriate silver staining dendrites from
each large Purkinje cell (P) are shown to have hundreds of small branches, each covered
with hundreds of dendritic spines. Axons from the small neurons of the granular layer are
unmyelinated and run together into the molecular layer where they form synapses with
the dendritic spines of Purkinje cells. X40. Silver.
The spinal cord varies slightly in diameter along its length but in cross section always
shows bilateral symmetry around the small, CSF-filled central canal (C).
• unlike the cerebrum and cerebellum, in the spinal cord the gray matter is internal,
forming a roughly H-shaped structure that consists of two horns, all joined by the gray
commissure around the central canal.
•The gray matter forms the anterior horns, which contain motor neurons whose axons
make up the ventral roots of spinal nerves.
• the posterior horns, which receive sensory fibers from neurons in the spinal ganglia
(dorsal roots). Spinal cord neurons are large and multipolar, especially the motor neurons
in the anterior horns.
Somatic motor neurons of the spinal cord
also have components in the CNS and PNS,
but they are multipolar
Motor output: axon travels through peripheral nerve to reach target muscle
Pre-ganglionic motor neurons have components
in the CNS and PNS and are also multipolar
Visceral motor output to post ganglionic neuron
Meninges
The skull and the vertebral column protect the CNS. Between the bone and nervous
tissue are membranes of connective tissue called the meninges laigninem eerhT .
:dehsiugnitsid era sreyal
Meninges around the brain
(a) A diagram of the spinal cord indicates the relationship of the three meningeal layers of connective
tissue: the innermost pia mater, the arachnoid, and the dura mater. (b) showing the tough dura mater
(D). The subdural space (SD) is an artifact created by separation of the dura from underlying tissue. The
middle meningeal layer is the thicker web like arachnoid mater (A) containing the large subarachnoid
space (SA) and connective tissue trabecular (T). The subarachnoid space is filled with CSF and the
arachnoid acts as a shock-absorbing pad between the CNS and bone. Fairly large blood vessels (BV)
course through the arachnoid. The innermost pia mater (P) is thin and is not clearly separate from the
arachnoid.The space between the pia and the white matter (WM) of the spinal cord here is an artifact
created during dissection; normally the pia is very closely applied to a layer of astrocytic processes at the
surface of the CNS tissue. X100. H&E.
Peripheral Nervous System
The main components of the peripheral nervous system are the nerves, ganglia, and
nerve endings.
Nerves
In the PNS nerve fibers are grouped into bundles to form nerves.
Except for very thin nerves containing only unmyelinated fibers,
nerves have a whitish, glistening appearance because of their
myelin and collagen content.
Nerve structure
• Nerves are only in the periphery
• Cable-like organs in PNS = cranial and spinal nerves
• Consists of 100’s to 100,000’s of myelinated and unmyelinated axons
(nerve fibers).
• Conduction is saltatory (i.e. jumps node to node) in myelinated
nerves and continuous in nonmyelinated.
Connective tissue
layers found in
nerves:
endoneurium
surrounds axons,
perineurium axon
fascicles and
epineurium the
entire nerve
Connective tissue layers in a peripheral nerve. Tight junctions between
perineurium cells form a important isolating barrier.
Epineurium
Perineurium
Ganglia, which can be either sensory or autonomic, contain neuronal cell bodies and
their satellite cells and are surrounded by connective tissue continuous with that of
nerves.
A sensory ganglion (G) has a distinct connective tissue capsule (C) and internal
framework continuous with the epineurium and other components of peripheral nerves,
except. Fascicles of nerve fibers (F) enter and leave these ganglia. X56. Kluver-barrera
stain.
Spinal cord and Dorsal Root Ganglion
Dorsal median sulcus
Dorsal
horn
(Lateral horn)
if present
Ventral
horn
Ventral median fissure
Dorsal root
ganglion (DRG)
Dorsal Root (Sensory) Ganglion Cells
Dorsal root
ganglion
Sympathetic ganglion cells:
multipolar neurons that reside entirely within the PNS
in sympathetic chain ganglia and “pre-aortic” ganglia
Parasympathetic ganglion cells:
multipolar neurons that also reside entirely within the PNS
in the wall of the innervated organ
(shown here in the seminal vesicle)
Parasympathetic ganglia in the wall of the gut
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