Download NERVE SYSTEM The nervous system is divided anatomically into

NERVE SYSTEM
The nervous system is divided anatomically into:
• the central nerve system (CNS), comprising the brain and spinal cord;
• the peripheral nervous system (PNS) which constitutes all nervous tissue outside the
CNS.
Functionally, the nervous system is divided into:
• the somatic nervous system which is involved in voluntary functions;
• the autonomic nervous system which exerts control over many involuntary functions.
Parts of PNS are:
• nerves (nerve fibers with their sheaths);
• nerve ganglia;
• nerve endings: sensory (afferent) and motor (efferent)
NERVES
In the PNS, the nerve fibers are grouped in bundles to form the nerves. The nerve is
composed of (Fig.1):
• axons,
• ensheathed by Schwann cells (mostly forming myelin) – nerve fiber;
• endoneurium (enveloping connective tissue of nerve fiber consisting of a thin layer of
reticular fibers);
• each bundle of nerve fibers is surrounded by the perineurium, a sleeve formed by layers
of flattened epithelium-like cells. These cells of each layer of the perineural sleeve are
joined at their edges by tight junctions, an arrangement that makes the perineurium a
barrier to the passage of most macromolecules;
• bundles of nerve fibers have an external fibrous coat of dense connective tissue called
epineurium.
Fig.1.
The nerves establish communication between brain and spinal cord centers and the sense
organs and effectors (muscles, glands, etc). They possess afferent and efferent fibers in relation
to the central nervous system.
• Afferent fibers carry the information obtained from the interior of the body and the
environment to the central nervous system.
• Efferent fibers carry impulses from the central nervous system to the effector organs
commanded by these centers.
Nerves possessing only sensory fibers are called sensory nerves.
Nerves composed only of fibers carrying impulses to the effectors are called motor nerves.
Most nerves have both sensory and motor fibers and are called mixed nerves; these nerves
have both myelinated and unmyelinated axons
The autonomic nervous system
The autonomic nervous system is related to the control of smooth muscle, the secretion of
some glands, and the modulation of cardiac rhythm. Its function is to make adjustments in
certain activities of the body in order to maintain a constant internal environment (homeostasis).
Anatomically, it is composed of collections of nerve cells located in the CNS (the nuclei),
fibers that leave the CNS through cranial or spinal nerves, and nerve ganglia situated in the
paths of these fibers. The term autonomic covers all the neural elements concerned with visceral
function (Fig.2).
• The first neuron of the autonomic chain is located in the CNS.
• Its axon forms a synapse with the second multipolar neuron in the chain, located in the
ganglion of the peripheral autonomic system.
• The nerve fibers (axons) of the first neurons are called preganglionic fibers; the axons of
the second neurons to the effectors – muscle or glands – are called postganglionic fibers.
Fig.2.
The autonomic nervous system is composed of 2 parts that differ both anatomically and
functionally:
• the sympathetic system
• the parasympathetic system.
Sympathetic System (thoracolumbar outflow)
The nuclei (nerve cell bodies) of the sympathetic system are located in the thoracic and first
two lumbar segments of the spinal cord.
The axons of these neurons – preganglionic fibers – leave the CNS by way of the ventral
roots and white communicating rami of the thoracic and lumbar nerves and enter one of the
ganglia of the sympathetic system, where they synapse on postganglionic cell bodies.
The ganglia of the sympathetic system form the paravertebral chain and plexuses situated
near the viscera.
Fig.3. Autonomic
Nerve System
The neurotransmitter of preganglionic synapses is acetylcholine. Postganglionic axons leave
the ganglion to find their way to the effector organ (smooth muscle, cardiac muscle, glands). The
chemical mediator of the postganglionic fibers of the sympathetic system is norepinephrine
(noradrenaline).
Parasympathetic System (craniospinal outflow)
The parasympathetic system has its nuclei in the medulla and midbrain and in the sacral
portion of the spinal cord.
The second neuron of the parasympathetic series is found in ganglia smaller than those of the
sympathetic system; it is always located near or within the effector organs. These neurons are
usually located in the walls of organs (eg, stomach, intestines), in which case the preganglionic
fibers enter the organs and form a synapse there with the second neuron in the series.
The chemical mediator released by the pre- and postganglionic nerve endings of the
parasympathetic system is acetylcholine. Acetylcholine is readily inactivated by the
acetylcholinesterase – one of the reasons parasympathetic stimulation has both a more discrete
and a more localized action than sympathetic stimulation.
Most of the organs innervated by the autonomic nervous system receive both sympathetic
and parasympathetic fibers. Generally, in organs where one system is the stimulator, the other
has an inhibitory action.
GANGLIA
An aggregation of nerve cell bodies outside the central nervous system is called a nerve
ganglion. Ganglia are usually ovoid structures encapsulated by dense connective tissue and
associated with nerves.
Two types of nerve ganglia can be distinguished on the basis of differing morphology and
function:
 dorsal root ganglia (sensory), and
 autonomic ganglia, which are associated with nerves of the autonomic system.
A capsule of connective tissue surrounding each ganglion is continuous with the connective
tissue within it and with the perineurium and epineurium of the pre- and postganglionic nerves.
In ganglia, the body of each ganglion cell is enveloped by a layer of small cuboidal glial cells
called satellite cells. A thin fibrous layer of connective tissue envelops each satellite-cellencapsulated perikaryon.
Spinal (Dorsal Root) Ganglia
These ganglia are located on the posterior nerve roots of the spinal cord as they pass
through the intervertebral foramina and in the paths of some cranial nerves. Their function is to
carry to the CNS impulses generated by various sensory receptors.
Dorsal root ganglia contain bodies of pseudounipolar neurons whose T-shaped process sends
one branch to the periphery and the other to the CNS.
The 2 branches of the single T-shaped process constitute one axon, and the peripheral branch
has a dendritic arborization. The nerve impulse goes directly from the periphery to the CNS,
bypassing the perikaryon. The perikaryons of pseudounipolar neurons therefore do not receive
nerve impulses, and their function is exclusively trophic. Synapses are absent.
The whole ganglion is encapsulated by dense connective tissue which is continuous with the
perineurial and epineurial sheaths of the associated peripheral nerve.
Each cell body is surrounded by a layer of flattened satellite (glial) cells which provide
structural and metabolic support.
The fascicle of nerve fibers passes to the centre of the ganglion, the neurons are located
peripherally.
Autonomic Ganglia
Autonomic ganglia appear as bulbous dilatations in autonomic nerves.
Sympathetic ganglia have a similar structure to that of sensory ganglia with a few
differences. The ganglion cells are multipolar and thus more widely spaced, being separated by
numerous axons and dendrites, many of which pass through the ganglion without being involved
in synapses. The satellite cells are smaller in number and irregularly placed due to the numerous
dendritic processes of the ganglion cells.
Parasympathetic ganglia The cell bodies of the terminal effector neurons of the
parasympathetic nervous system are usually located within or near the effector organs.
The cell bodies may form well-organized ganglia of moderate size (as in the otic ganglion)
but more commonly a few cell bodies are clumped together to form tiny ganglia consisting of
only a few nerve cells, and scattered in the supporting tissue. Some are located within certain
organs, especially in the walls of the digestive tract, where they constitute the intramural
ganglia. As in other ganglia, the neurons are surrounded by numerous small support cells and
afferent and efferent nerve fibers
CENTRAL NERVOUS SYSTEM
The CNS consists of the brain and spinal cord, each of which can be divided into areas of :
• gray matter
• white matter
Gray matter contains:
• neuronal cell bodies (perikaryons),
• unmyelinated and myelinated fibers (mostly the former);
• neuroglia: protoplasmic astrocytes, oligodendrocytes, and microglial cells.
• it is highly vascular.
The gray matter, when examined under the microscope, resembles a tangle on nerve and glial
processes, called the neuropil; this is actually a functionally organized entanglement of
processes.
White matter contains
• myelinated and unmyelinated fibers (axons),
• neuroglia: oligodendrocytes, fibrous astrocytes, and microglial cells.
Its characteristic white color is a clue to the predominance of myelinated nerve fibers. It
lacks neuronal cell bodies and is less vascular than gray matter.
Within the CNS, specific terms are used to described arrangement of cells and their
connections:
• the arrangement of neurons over the surface of the brain is termed the cortex
• an arrangement of neuronal cells as a discrete unit is termed a nucleus
• an arrangement of neuronal cells running along the spinal cord is termed a column
•
a defined bundle of axons running along the spinal cord in white matter is termed a tract
or a fascile.
The CNS develops through growth and complex folding, resulting in distinct convolutions
visible on external surface and in sections.
• In the cerebral cortex, the crest of folds are termed gyri while the clefts between folds are
termed sulci.
• In the cerebellum these folds are termed folia.
The spinal cord
The structure of the spinal cord is basically similar throughout its whole length, with the four
main regions: cervical, thoracic, lumbar, and sacral.
In cross sections of the spinal cord, white matter is peripheral and gray matter is central,
assuming the shape of a butterfly (or a litter H). In the horizontal bar of this H is an opening, the
central canal, which is a remnant of the lumen of the embryonic neural tube lined by ependymal
cells (Fig.4).
Fig.4. The Spinal Cord. The cross section.
The nerve fibers of the gray matter are oriented in the transverse plane, whereas those of the
white matter are oriented in the longitudinal plane parallel to the neuraxis.
The gray matter is divided into:
• a posterior (dorsal) horn,
• an intermediate zone (with a small lateral horn),
• an anterior (ventral) horn.
The ventral horns are most prominent, and contain the cell bodies of large multipolar motor
neurons forming motor nuclei. Axons of motor neurons make up the ventral roots of the spinal
nerves.
The dorsal horns are much less prominent and contain the cell bodies of small second order
sensory neurons forming proper sensory nucleus (nucleus proprius) of dorsal horn. These
neurons relay sensory information to the brain from primary afferent neurons for the modalities
of temperature and pain whose cell bodies lie in the dorsal root ganglia.
Thoracicus nucleus of Clarke is located in the base of dorsal horn in the medial aspect.
Small lateral horns, which contain the cell bodies of preganglionic, sympathetic efferent
neurons, are found in the thoracic and upper lumbar spinal segments. These neurons form lateral
sympathetic nucleus (parasympathetic n. – in the sacral spinal segments) corresponding to the
level of the sympathetic outflow from the cord.
The white matter comprises three columns (funiculi): posterior, lateral, and anterior. The
white matter of the spinal cord consists of ascending tracts of sensory fibers and descending
motor tracts; passing up the spinal cord towards the brain, more and more fibers enter and leave
the cord so that the volume of white matter increases progressively from the sacral to cervical
regions.
The cerebellum
The cerebellum, which coordinates muscular activity and maintains posture and equilibrium,
consists of a cortex of gray matter with a central core of white matter containing four pairs of
nuclei. Afferent and efferent fibers pass to and from the brain stem via inferior, middle and
superior cerebellar peduncles linking medulla, pons and midbrain respectively.
The cerebellar cortex forms a series of deeply convoluted folds or folia supported by a
branching central medulla (Fig.5).
Fig.5.
The cerebellar cortex consists of 3 layers (fig.6):
• the outer molecular layer;
• the central layer of Purkinje cells;
• the inner granule layer.
The molecular layer
The molecular layer contains relatively few neurons and large number of unmyelinated nerve
fibers. The cells, scattered in the molecular layer, are small neurons, namely stellate cells and
basket cells. Each of these neurons has its axon oriented at right angles to the long axis of a
folium.
A single stellate cell axon has inhibitory synaptic connections with the dendrites of several
Purkinje cells.
Basket cells are located in the basal one-third of the layer. Each basket cell axon has
inhibitory synaptic connections with cell bodies of several Purkinje cells. Collaterals of the
axons take part in formation of “baskets” of nerve fibers – special structures on bodies of
Purkinje cells.
The layer of Purkinje cell
The layer of Purkinje cell is a thin layer characterized by the cell bodies of the Purkinje cells.
Fig.6.
The Purkinje cells are very large neurons, and their dendrites
divide repeatedly in one plane perpendicular to the long axis of the
folium, forming a sort of tree – extensively branching dendritic
system which arborizes into the outer molecular layer (Fig.7). A
relatively fine axon extends down through the granular layer to form
inhibitory synaptic connections releasing gamma aminobutyric acid
(GABA) as the neurotransmitter with neurons of the deep cerebellar
nuclei. Recurrent axonal collaterals of each Purkinje cell have
inhibitory connections with other Purkinje cells, basket cells, and
Golgi cells.
Fig.7. The Purkinje cell dendritic tree
The granular layer contains numerous very small neurons (5 mμ in diameter) called
granule cells. Each granule cell has a cell body, and 4 to 6 short dendrites located within the
granular layer, its non-myelinated axon passes outwards to the molecular layer, where it
bifurcates as a T into two branches (called parallel fibers) that course in opposite directions
parallel to the long axis of the folium.
These axonal branches form excitatory synapses with the dendrites of Purkinje cells, stellate
cells, Golgi cells, and basket cells. One parallel fiber synapses with the dendrites of thousands of
Purkinje cells and, in turn, each Purkinje cell receives synapses from thousands of parallel fibers
(Fig.8).
Another type of cells of the granular layer are large stellate neurons or Golgi cells, scattered
in the superficial part of the granular layer.
An axon of Golgi cell terminates in inhibitory synapses with dendrites of granule cells within
glomeruli.
A glomerulus is a synaptic processing unit
consisting of :
• an excitatory axon terminal of a mossy fiber;
• dendritic endings of one or more granule
cells;
• an inhibitory axon terminal of a Golgi cell
synapsing with a dendrite of the granule cell.
Cerebellar Afferent nerve fibers (fig.8) are:
 the climbing fibers
 mossy fibers
The climbing and mossy fibers convey excitatory input directly from the spinal cord and
brainstem through the cerebellar peduncles to the deep cerebellar nuclei and cerebellar cortex.
Each climbing fiber enters the molecular layer and makes up to several hundred synaptic
contacts on dendrites of a single Purkinje cell.
The mossy fibers branch profusely and, through synapses, exert excitatory influences on the
granule cells within the glomeruli. Through their parallel fibers, the granule cells have excitatory
synaptic connection with the dendrites of the Purkinje cells and, in addition, with the dendrites of
the stellate cells and basket cells of the molecular layer.
After receiving these excitatory influences, these stellate and basket cells are stimulated to
exert inhibitory influences on Purkinje cells.
Similarly, the Golgi cells exert inhibitory influences on the granule cells within the
glomeruli.
The Purkinje cells, through their axons, are the only outlet for processed information from
the cerebellar cortex. Their output, directed to the deep cerebellar nuclei and the lateral
vestibular nucleus, is solely inhibitory. As mossy and climbing fibers supply only excitatory
influences, it is the Purkinje fibers that modulate through inhibition the output from the deep
cerebellar nuclei to targets outside the cerebellum.
In summary, the input to the cerebellum is by mossy and climbing fibers, that are wholly
excitatory. The Golgi, Purkinje, stellate, and basket cells are inhibitory neurons. They act as
modulators.
Fig.8. The cerebellum structure
The CEREBRUM
Like the cerebellum, the cerebrum also has a cortex of gray matter and a central area of white
matter in which are found nuclei of gray matter. The surface of the cerebrum is increased by
many gyri, which are elevations separated by depressions (sulci) (Fig.5).
The cerebral cortex is the 600-g gray mantle of the cerebrum, containing 75 billion or more
cortical neurons.
The cerebral cortex is divided into the phylogenetically older allocortex, about 10% of the
total, and the phylogenetically more recent neocortex, about 90% of the cortex. The neocortex
includes the sensory and motor areas of the cortex as well as the association cortex
Neuron types in the cerebral cortex
Pyramidal cells (Fig.9) are characteristic and the most common cell type in the cerebral
cortex. Pyramidal cells, as their name implies, have pyramid-shaped cell bodies, the apex being
directed towards the cortical surface. A slender axon arises from the base of the cell and passes
into the underlying white matter. Collateral branches of an axon project back to the cortex. From
the apex, a thick branching dendrite passes towards the surface where it has an array of fine
dendritic branches. In addition, short dendrites arise from the edges of the base and ramify
laterally. The size of the pyramidal cells varies from small to large, the smallest tending to lie
more superficially. The huge upper motor neurons of the motor cortex, known as Betz cells, are
the largest of the pyramidal cells in the cortex.
Stellate (granule) cells are small neurons with a short vertical axon and several short
branching dendrites, giving the cell body the shape of a star. An axon arborizes and synapses
with other cortical neurons in the immediate vicinity. Some of stellate cells are excitatory
neurons and some are inhibitory ones
Fig.9.
Cells of Martinotti are small polygonal cells with a few short dendrites; the axon extends
towards the surface and bifurcates to run horizontally, usually in the most superficial layer.
Fusiform cells are spindle-shaped cells oriented at right angles to the surface of the cerebral
cortex. The axon arises from the side of the cell body and passes superficially. Dendrites extend
from each end of the cell body branching into deeper and more superficial layers.
Horizontal cells of Cajal are small and spindle-shaped but oriented parallel to the surface.
They are the least common cell type and are only found in the most superficial layer where their
axons pass laterally to synapse with the dendrites of pyramidal cells.
Except for the pyramidal cells, all cortical cells are intracortical interneurons.
In addition to neurons, the cortex contains supporting neuroglial cells, i.e. astrocytes,
oligodendroglia and microglia.
The neurons in the neocortex are arranged into six horizontal layers oriented parallel to the
cortical surface.
The layers differ in neuron morphology, size and population density. The layers merge with
one another rather than being highly demarcated and vary somewhat from one region of the
cortex to another depending on cortical thickness and function.
Molecular (plexiform) layer. This most superficial layer mainly contains dendrites and
axons of cortical neurons making synapses with one another; the sparse nuclei are those of
neuroglia and occasional horizontal cells of Cajal.
• Outer (external) granular layer. A dense population of small pyramidal cells and
stellate cells make up this thin layer, which also contains various axons and dendritic
connections from deeper layers.
• Outer (external) pyramidal layer. Pyramidal cells of moderate size predominate in this
broad layer.
• Inner granular layer. This layer consists mainly of densely packed stellate cells.
• Inner pyramidal (ganglionic) layer. Large pyramidal cells and smaller numbers of
stellate cells and cells of Martinotti make up this layer.
• Multiform layer. This is so named for the wide variety of differing morphological forms
found in this layer. It contains numerous small pyramidal cells, as well as stellate cells,
especially superficially, and fusiform cells in the deeper part.
The synaptic interconnections within the cortex are exceedingly complex, with any one
neuron synapsing with several hundred others. However, there are several basic principles of
cortical organization and function:
• Functional units (columns) are disposed vertically, corresponding to the general
orientation of axons and major dendrites.
• Afferent fibers (their cell bodies lying elsewhere in the CNS) generally synapse
high in the cortex with dendrites of efferent neurons, the cell bodies of which lie
in deeper layers of the cortex.
• Efferent pathways, typically the axons of pyramidal cells, tend to give off
branches which pass back into more superficial layers to communicate with their
own dendrites via interneuronal connections involving other cortical cell types.