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Cells of the nervous system
The nervous system comprises a complex network of
1- specialised blood vessels,
2- ependymal cells which line the cerebral ventricles,
3- neurons and
4- glial cells, of which there are three types
.A- Astrocytes form the structural framework for neurons.
Astrocyte foot processes are intimately associated with
blood vessels and form the blood-brain barrier .
B-Oligodendrocytes are responsible for the formation and
maintenance of the myelin sheath, which surrounds axons
and is essential for the rapid transmission of action
potentials . myelin sheaths consisting of the wrapped
membranes of Schwann cells
C- Microglial cells are cells of the monocyte/macrophage
lineage which play a role in fighting infection and
removing damaged cells.the Schwann cells covering the
neurons with sheath.
These cells maintain the tissue by
1-supporting and protecting the neurons.
2- provide nutrients to neurons
3- help to keep the tissue free of debris
. There are three types of neurons: sensory
motor neuron, and interneuron.
. An interneuron is always found completely within the CNS
and conveys messages between parts of the system (Figure ).
Function of the nervous system rests upon two physiological
1- the generation of an action potential and its conduction
down axons, and
2- 2- the synaptic transmission of impulses between neurons
and muscle cells
There are no cell bodies in nerves; cell bodies are found only in
the CNS or in the ganglia. Ganglia are collections of cell bodies
within the PNS. 12 pair of C.N and 31 pair of P.N
Many of the functions are lateralised and this depends on which
of the two hemispheres is 'dominant', i.e. the one in which
language function is represented. In right-handed individuals the
left hemisphere is almost always dominant, while in left-handers
either hemisphere may be dominant with about equal frequency
. The cerebral cortex
grey matter (densely packed neurons
for information processing. While the
white tissue inside are axons -- (to
relay information). The cortex can be
broken down into many functional
The spinal cord
The spinal cord contains
1-white matters :afferent and efferent fibres arranged in
bundles(3 long tract)dorsal columns,spinothalamic,corticospinal
3- grey matter, collections of cells which are responsible for
a-lower-order motor reflexes(anterior horn cells)
b-and the primary processing of sensory
information(posterior horn cells), including pain.
The peripheral nervous system
The sensory cell bodies of peripheral nerves are situated in the
dorsal root ganglia in the spinal exit foramina, .The motor cell
bodies are in the anterior horns of the spinal cord. Motor
neurons initiate muscle contraction by the release of
acetylcholine across the neuromuscular junction, which results
in change in potential in the muscle end plate.
, any peripheral nerve is made up of a combination of large, fast,
myelinated axons (which carry information about joint position
sense and commands to muscles) and smaller, slower,
unmyelinated axons (which carry information about pain and
temperature, as well as autonomic function).
1-The autonomic system
The autonomic system plays a key role in regulating the
cardiovascular , respiratory systems, the smooth muscle of the
gastrointestinal tract, urinary, and reproductive tracts. It also
carries messages that help stimulate glands to secrete tears,
mucus, and digestive enzymes. ANS innervation is divided
into sympathetic nervous system and parasympathetic
nervous system divisions. The sympathetic division has
thoracolumbar “outflow”, meaning that the neurons begin
at the thoracic and lumbar (T1-L2) portions of the spinal
cord. The parasympathetic division has craniosacral
“outflow”, meaning that the neurons begin at the cranial
nerves (CN 3, CN7, CN 9, CN10) and sacral (S2-S4)
spinal cord.
2-The motor system
Figure 26.6 The motor system. Neurons from the motor cortex
descend as the pyramidal tract in the internal capsule and
cerebral peduncle to the ventral brain stem, where most cross
low in the medulla (A). In the spinal cord the upper motor
neurons form the cortico-spinal tract(pyramidal tract) in the
lateral column before synapsing with the lower motor neurons in
the anterior horns. The activity in the motor cortex is modulated
by influences from the basal ganglia and cerebellum. Pathways
descending from these structures control posture and balance
(B).some fiber from the pyramidal tract(corticobulber tract)
innervate nuclear in BRAIN STEM.
Lower motor neurons
Lower motor neurons in the anterior horn of the spinal cord
innervate a group of muscle fibres termed a 'motor unit'. Loss of
function of lower motor neurons causes loss of contraction
within this unit, resulting in weakness and reduced muscle tone.
Subsequently, denervated muscle fibres atrophy, causing muscle
wasting, and depolarise spontaneously, causing 'fibrillations'.
All lower motor neurons are either spinal or cranial nerves. All
spinal nerves have a lower motor neuron component as they are
mixed nerves. However, not all cranial nerves have lower motor
neuron components. Some of the cranial nerves contain only
sensory fibers and therefore cannot be classified as lower motor
neurons. For example, CN I, the olfactory nerve, CN II the optic
nerve, and CN VIII, the auditory nerve, do not have motor
Upper motor neurons
Upper motor neurons have an inhibitory influence on the
function of anterior horn motor neurons. When upper motor
neuron lesions occur, motor units have an exaggerated response
to stretch. With an increased muscle tone greater in the
extensors of the lower limbs and the flexors of the upper limbs
(spasticity), brisk tendon reflexes, and extensor plantar
responses. Spasticity takes time to develop and may not be
present for weeks after the onset of an upper motor neuron
lesion. The weakness found in upper motor neuron lesions is
more pronounced in the extensors of the upper limbs and the
flexors of the lower limbs.
pyramidal tract fibers begin their descent from the cortex as a
corona radiata (radiating crown) before forming the internal
The fibers that synapse with cranial nerves form the corticobulbar tract. Bulbar refers to the brain stem
The fibers of the pyramidal tract that synapse with spinal nerves
sending information about voluntary movement to the skeletal
muscles form the cortico-spinal tract
pyramids in the inferior part of the medulla, eighty-five to ninety
percent of cortico-spinal fibers decussate, or cross to the other
side of the brain(lateral corticospinal tract). The remaining ten to
fifteen percent continue to descend ipsilaterally(anterior
corticospinal tract).
Almost all of the cranial nerves receive bilateral innervation
from the fibers of the pyramidal tract The two exceptions to this
pattern are the portion of CN XII that provides innervation for
tongue protrusion and the part of CN VII that innervates the
muscles of the lower face. These only receive
contralateralinnervation from the pyramidal tract
3-sensory pathways.
Fibres from proprioceptive organs 1-(joint position) and those
mediating2-well-localised touch and ( 3-vibration enter the
spinal cord at the posterior horn and pass without synapsing into
the ipsilateral posterior columns. Neural fibres conveying pain
and temperature sensory information (nociceptive neurons)
synapse with second-order neurons In the posterior ho
rn of the spinal cord which cross the midline in the spinal cord
before ascending in the contralateral spinothalamic tract to the
brain stem.
The second-order neurons of the dorsal column sensory system
cross the midline in the upper medulla to ascend through the
brain stem. Here they lie just medial to the (already crossed)
spinothalamic pathway. Brain-stem lesions can therefore cause
sensory loss affecting all modalities of the contralateral side of
the body. Both the dorsal column and spinothalamic tracts end
in the thalamus, relaying from there to the parietal cortex.
Figure The pain perception system.
The brain stem
Long tracts are three
Long tracts: the motor and sensory tracts described in the spinal
cord are present in the brain stem, but in the brain stem they are
all contralateral to the side of the body they serve.
1. The pyramidal tract (upper motor neuron) start from
precentralgyrus(motor cortex) dowon to internal capsule
then to brain stem.The pyramids decussate at the junction
of the medulla and cervical spinal cord.
2. The spinothalamic tracts continue in a lateral position
throughout the brainstem on their way to the thalamus..
3. The posterior columns end in the medulla where they
synapse in the nuclei cuneatus and gracilis Second order
neurons immediately decussate .These sensory pathways
both end in the thalamus .
with the exception of the trochelar nucleus (cranial nerve IV),
which crosses to innervate the contralateral superior oblique
muscle, each of the brainstem cranial nerve nuclei innervate
ipsilateral structures. Since the long tracts discussed above are
crossed, lesions confined to one side of the brainstem
typically present with cranial nerve findings on one side, and
motor and sensory findings on the opposite side of the body.
This rule is very helpful in localization.
The brain stem
1- all the sensory and motor pathways entering and leaving the
2-nuclei of the cranial nerves
4- nuclei projecting to the cerebrum and cerebellum,
5- other important collections of neurons in the reticular
)The cranial nerve nuclei 1-providemotor control to muscles
of the head (including the face and eyes) and some in the
neck, along with 2-coordinating sensory input from the
special sense organs and the face, nose, mouth, larynx and
pharynx. They3-also control autonomic functions including
pupillary, salivary and lacrimal functions.
The reticular formation is predominantly involved in the 1control of conjugate eye movements, 2-the maintenance of
balance, 3-cardiorespiratory control and4- the maintenance of
The extrapyramidal system
the extrapyramidal system centers around
A- the 1-modulation and2- regulation of motor activity (indirect
control) of anterior (ventral) horn cells. , ( they modulate
motor activity without directly innervating motor neurons
B- Primarily, the extrapyramidal system is involved in
maintaining 1-equilibrium, coordination,2-posture,
muscle tone
C-It has projections that carry autonomic motor impulses to
voluntary muscles in the body, including the muscles for speech
and swallowing
The system, including
1- the nigrostriatal pathway,2- the basal ganglia,3the cerebellum, 4-the vestibular nuclei, and 5-red neuclius,6tectunm7-reticular formation in pon and medulla.8-olivary
neuclius in medulla
. Some structures of the extrapyramidal system do not proceed
directly to the spinal motor centers. Others are connected by
conducting pathways to the segmental levels of the spinal cord,
where they serve as an essential switching station for
impulses traveling from the brain to moto-neurons
.Extrapyramidal Projections to Lower Motor Neurons
1-The rubrospinal tract passes through the red nucleus in
midbrain . The cerebellum sends messages to the spinal nerves
along this.
2-The reticulospinal tract runs from the reticular nuclei of the
pons and medulla to the spinal nerves.
3-The tectospinal tract has points of origin throughout the brain
stem, but especially in the midbrain area, and ends in the spinal
nerves. It is involved in the control of neck muscles.
4-The vestibulospinal tract runs from the vestibular nuclei
located in the lower pons and medulla to the spinal nerves. It is
involved in balance.
5-olivospinal tract from olivery neucleus in medulla to the anterior
horn neucleus
(Note that all of these tracts receive input from
Circuits between the basal ganglia and the motor cortex
constitute the extrapyramidal system, which controls muscle
tone, body posture and the initiation of movement. Lesions of
the extrapyramidal system produce an increase in tone which is
not an exaggerated response to stretch but is continuous
throughout the range of movement at any speed of stretch ('lead
pipe' rigidity). Involuntary movements are also a feature of
extrapyramidal lesions , and tremor combined with rigidity
produces typical 'cogwheel' rigidity.
.the upper part of the posterior temporal lobe ( comprehension
region is referred to as Wernicke's area). The perception of these
sounds as meaningful language, occurs predominantly in the
lower parts of the anterior parietal lobe (the angular and
The language information generated in the temporal and parietal
lobes passes anteriorly via the arcuate fasciculus to Broca's area
in the posterior end of the inferior frontal gyrus on the dominant
side. The motor commands generated in Broca's area then pass
to the cranial nerve nuclei in the pons and medulla, as well as to
the anterior horn cells in the spinal cord. Nerve impulses then
travel to the lips, tongue, palate, pharynx, larynx and respiratory
muscles via the facial nerve and cranial nerves 9, 10 and 12, and
result in the series of ordered sounds known as speech (
The cerebellum also plays an important role in coordinating
speech, and lesions of the cerebellum lead to a speech disorder
termed dysarthria.