Download CNS Anatomy 2 **You need to study the slide hand in hand with this

CNS Anatomy 2
**You need to study the slide hand in hand with this sheet because some texts and tables are
not mentioned by the dr. in the lecture.
-We have known that the neuron has two types of processes: 1- Dendrites which receive the
impulses trough the synapse, 2- The axon which is single and it sends out the action potential.
 Classification of nerve fibers (Axons)
-We have two ways to classify axons whether sensory or motor
1) Roman number system:
-The numbers used are I, II, III and IV
-The smaller the number means that the axon is larger in diameter, has thicker myelin sheath
and the conduction velocity is higher.
-So that the Number one fibers (I) are thicker and faster in conduction than number 4 nerve
fibers (IV)
-Number one fibers are classified in two types Ia, Ib
2)By Letter system:
-The letters are A,B and C. A is classified into Aα,Aβ,Aγ and Aδ.
-A is larger in diameter than C. Aα is larger in diameter than Aδ
* comparison between the two system
- Aα is like Ia,Ib .
-C is like IV.
-Examples
- Aα,Aγ are present in lamina IX in the ventral horn of gray matter in the spinal cord.
-Stretch receptors in the muscle (muscle spindle) have Ia or Aα sensory nerve fibers.
-Slow or fast pain fibers are Aδ, C.
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-You should put the table in slide 5 on your head and memorize the types “Tadrejean min foq
la te7et” but the most important to know the concept (I is thicker than IV and Aα is thicker
than C)
 Slide number 6 (section in the spinal cord)
-We have 31 segments in the spinal cord, each gives a pair of spinal nerve thus we have 31
spinal nerve (8 cervical, 12 thoracic, 5 lumbar, 5 sacral and one coccygeal nerves)
-We can identify the spinal segment by seeing the right and left spinal nerve that exit from
the spinal segment.
-The gray matter in spinal cord has the “H” shape. The gray matter has ventral horn (motor)
,dorsal horn (sensory) and gray commissure and central canal in the centre of gray
commissure.
- In some parts of the spinal cord the gray matter has another horn called lateral horn or
intermediolateral horn where the mother cells of sympathetic nerves are found.
- In the ventral horn of gray matter there are the cell bodies of motor neurons .Aα nerve
fibers of these motor nerves supplies the bulk of the muscle which is called extrafusal muscle
fibers while Aγ motor fibers supplies the muscle spindle (stretch receptors). The motor Aα,
Aγ in the ventral horn are called lower motor neuron
-The axons of Aα, Aγ form the ventral (motor) root of the spinal nerve.
-The axons of Aα, Aγ are called final common path by Sir Charles Sherrington (This
physiologist is really admired by our doctor). This because as the impulse reach the motor
axons of Aα, Aγ it will reach its final target which is the skeletal muscle.
-Aα, Aγ which are the lower motor neuron are found in the ventral horn resemble in function
to the parts of the brain stem which gives motor neuron of cranial nerve like facial or 3,4,6
cranial nerve .
-The cell bodies of sensory nerves of dorsal horn are found in the dorsal root ganglion(also
called sensory ganglion).
-These sensory neurons are called pseudo-unipolar neurons.
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-These pseudo-unipolar neurons in embryo were normal neuron having dendrites and one
axon, then through development the dendrites came near the axon and they unite to form one
dendrid-axon. This dendrid-axon separates into two processes :1-central process in the spinal
cord (function like the axon). 2-perripheral process in the peripheral receptors (function like
the dendrites)
-The impulse will enter the peripheral process then to the central process without entering the
sensory ganglion. The cell bodies in these ganglia function to nourish the dendrid-axon.
-There are no synapses of sensory nerves in the dorsal ganglion but these sensory nerves have
synapses that are found in autonomic ganglion.
-after the impulse carried in the peripheral process it reaches the central process and then
reaches the neurons in the ventral horn Aα,Aγ in two ways : 1.indirectly through
interneuron(association neuron) then to motor neuron (disynaptic) 2. Directly to motor
neuron without interneuron(monosynaptic).
-the interneuron function is to link a sensory neuron with a motor neuron.
-so that in the dorsal horn we find the central process of sensory neuron and the nuclei of
interneurons.
-some sensory fibres in the dorsal root don’t form synapses in spinal cord but they ascend
upward to reach the brain(medulla ,pons,mid brain, thalamus)
-Every spinal nerve has dorsal root ganglion but only the cranial nerves that have sensory
innervations have sensory ganglion like trigeminal but hypoglossal is purely motor without
sensory ganglions.
-The ventral and dorsal root of spinal nerve form the trunk of the spinal nerve which exit
from the intervertebral foramen. After the spinal nerve exits, it separates again into anterior
and posterior primary rami (each ramus has sensory,motor and sympathetic)
-The posterior ramus supply muscles of the back and skin.
-the anterior rami can form plexuses but not the posterior ones.
-Receptors in the muscle are two types one is called muscle spindles which are specialized
muscle fibers and the other is called joint receptor called golgi tendon receptors.
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**Sensory in general is of two types (1- exteroception (pain, burn, temperature), 2propriception (means sense of movement of joints (Kinesthesia),sense of position of the
joints (flexed, extended) .
-The motor neurons in the ventral horn (final common path) receive impulses from 1. sensory
neuron directly or indirectly through interneurrons ,2. From brain called supraspinal
neurons(descending motor pathway) from cortex or brain stem
-Stretch reflex is the only monosynaptic reflex in the man.
 White matter
-The white matter is formed by the axons of the neurons which could be mylenated or poorly
mylenated
-The white matter is separated into posterior(dorsal) ,lateral and anterior column.
-Each column of white matter includes axons (ascending or descending tracts)
 Gray matter
-In the past it was considered as nuclei (a nucleus is a group of neurons that have specific
function in the nerve system)
-Recently (By the scientist Rexed) these nuclei are found to be arranged as columns that may
extend along the whole spinal cord or it may extend through a specific part of spinal cord.
-Cross section in these columns gives a picture of a group of ten laminae .
- laminae I-VI are in the dorsal horn, lamina VII in the middle.
- LAMINA IX is the lower motor neurons and the final common pathway.
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*comparison between old and Rexed terminology
Rexed
terminology
Lamina 1
II
III. IV
Older terminology
comments
Posteromarginal nucleus
Substantia gelatinosa
Nucleus proprius
In old concept laminae I-VI
were receive exteroception
sensation
but
these
concepts changed
In old concept laminae I-VI
were receive propriception
sensation
but
these
concepts changed
V
VI
Neck of posterior horn
base of posterior horn
Vll
VIII
IX
Intermediate
intermediolateral horn
Commissural nucleus
Ventral horn
X
Grisea centralis
zone,
A group of motor neurons
that some are medially
located ant the others are
laterally located. Some are
Aα (large diameter) other
are Aγ.
Transmit signals from right
to left and left to right
through this region.
-Lamina IX are in general medial and lateral groups. Medial groups of colomns supply
muscle of the axis (move vertebral colomn). Lateral groups supply muscles of the limbs.
-In details
Cell colomn
Ventromedial
dorsomedial
Venterolateral
Dorsolateral
Retrodorsolateral
central
extention
All segment
T1-L2
C5-8,L2-S2
C6-8, L3,S3
C8,T1,S1,S2
C3,4,5
Muscle
Erector spinae
Intercostals, muscles of the abdominal wall
Arm, thigh
Forearm, leg
Hand foot
Diaphragm
-The spinal cord has two enlargements 1- cervical C5-T1 that give the brachial plexus 2lumbar that give the lumber plexus L2-S3.
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- Venterolateral extend only in the segments that give brachial plexus(C5-C8) and lumber
(L2-S2)
- Dorsolateral and Retrodorsolateral are also found in the enlargement area.
-So that the lateral groups of columns are found in enlargement areas of spinal cord which are
cervical and lumbar.
-C8-T1 innervate the intrinsic muscle of the hand.
-this organization is called the somatotopic organization.
-in general more medial group in lamina IX are for axial muscle and more lateral are for
distal muscles .
-in upper limb more medial groups supplying the shoulder more lateral supply the hand.
-in lower limb more medial groups supplying the hip more lateral supply the foot.
-a part of T1 nerve forms the 1th intercostals nerve and the other part goes upward
across the neck of the 1st rib to meet C8 and forming the lower trunk.
-As the apex of the lung lie near to the first rib. Carcinoma of the apex of the lung (
Pancoast tumors) can destroy the T1 nerve and the muscles supplied will be affected
by flaccid paralysis and atrophy.
-Heavily smoker people “Hashasheen” are at a higher risk for this tumor. Those with
this tumor will complain from muscle atrophy and the bones of the hand
“metacarpals” will be visible with apperant grooves as the dorsal interossei muscles
affected. Bad doctors “ except doctors of aljam3a alordoniah ” will send the patient
for physiotherapist .Good doctors will expect the presence of this kind of tumor as
the patient is a heavy smoker .
-Lower motor neuron lesion results in flaccid paralysis (low muscle tone) and atrophy .And
to examine the low tone we do passive test. Passive test is done by performing the action of
the muscle from external force not the patient and because of the flaccid muscles there will be
no resistance.
-To examine the power in the muscle is by active test by performing the action of the muscle
like flexion, extension.
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-So destroying the motor neuron in the ventral horn Aα, Aγ will result in flaccid paralysis as
in the case of poliomyelitis that destroy Aα, Aγ.
-bur destroying the upper motor neuron will result in the spastic paralysis.
Descending motor pathway
-The cell bodies of the descending motor pathway are in area 4(primary motor area), area
6(supplementary motor area or premotor cortex) and area (3,1,2) but mainly area 4 and area
6.
-Area 4 motor neurons control face and limbs muscles and are 6 control the axial and
proximal limbs muscles.
- The axons of the descending pathway descend in the corona radiate then they crowd in the
internal capsule (porta cerbri)
-The descending motor tract consist of two tracts:1- pyramidal(direct pathway) 2extrapyramidal tract.
Pyramidal tract consist of two types :1-corticospinal 2-corticobulbar.
-Corticospinal tract start at cerbral cortex area (4,6 and area(3,1,2)) and terminate in the final
common path at the Aα, Aγ .Interneurons link between the cortex and the final common path.
- Corticobulbar tract start at cerbral cortex area (4,6 and area(3,1,2)) and terminate in motor
nuclei of cranial nerves in the brainstem.
-it’s called pyramidal because this tract pass through the pyramid (a part of the medulla).
Extrapyramidal tract (indirect pathway) ….… return to slides for details (not mentioned )
-This tract begins in the cortex then it synapses in the brain stem then continue to lower
motor neurons.
-It consists of 4 types :1- reticulospinal tract 2-Rubrospinal tract (Rubro relating to the red
nucleus in the mid brain) 3-vestibulospinal tract 4- tectospinal tract (relating to tectum in the
mid brain)
-As the impulses are initiated in the cortex these tract are actually cortico-rereticulospinal,
cortico-rubrospinal tract etc...
Done BY MAMUN AQEL QASRAWi
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