Central Nervous System (CNS)
Histology Lectures 
The Central Nervous System includes:
1.Brain 
•	Cerebrum
•	Cerebellum
•	Diencephalon (Thalamus, Subthalamus, Hypothalamus, Epithalamus) 
•	Brainstem (Midbrain, Pons, Medulla) 
2.Spinal cord 
31 segments
(Cervical 8, Thoracic 12, Lumbar 5, Sacral 5 &Coccygeal 1)
General characters of CNS:
•	Covered by meninges
•	Protected by bone
•	Soft jelly like because it does not contain CT.
•	Consists of white matter and grey matter according to the distribution of myelin. 
General characteristics of spinal cord cross section:
•	Central canal
•	Central H-shaped grey matter
•	Peripheral white matter
•	Anterior median fissure and posterior median septum
 



Grey matter of spinal cord:
•	It is formed of nerve cells, neuroglial cells & unmyelinated nerve fibers.
•	Contains nuclei.
•	It is divided into:
- Two dorsal horns containing sensory nuclei.
     - Two ventral horns containing motor nuclei. 
- Connected by grey commissure which contains commissural nuclei.
     - Additional two lateral horns only in thoracic & upper 2 lumbar segments containing sympathetic nuclei.
	What is meant by nucleus?
Group of nerve cells inside CNS having the same function.

Nuclei in grey matter of spinal cord
Sensory	Motor	Lateral horn	Commissural
In posterior horns	In anterior horns	In lateral horns	Around central canal
Substantia Gelatinosa of Rolandi (SGR)
(All levels)	Ant. & post. Medial motor nuclei 
(All levels)	sympathetic nuclei in thoracic & 
upper 2 lumbar 
	Ant. & post. Commissural nuclei (All levels)
Main Sensory Nucleus (MSN) 
(All levels)	Ant. & post. Lateral motor nuclei (Cervical & lumbar levels)		Interneurons of reflex arc
Clark’s nucleus 
(C8-L2 levels)	Central motor nucleus 
(Cervical & lumbar levels)		
N.B:Fixed nuclei in all levels of spinal cord: six
1)SGR, 2) MSN, 3) Ant. Commissural, 4) Post. Commissural, 5) Ant. Med. Motor, 6) Post med. motor

White matter of spinal cord:
•	It is formed of neuroglial cells & myelinated nerve fibers.
•	It is divided into 3 columns in each half of spinal cord (dorsal, lateral, anterior).
•	Posterior Column: containing Ascending tracts only.
•	Anterior & lateral column: containing both Ascending & descending tracts.
	What is meant by tract?
  Group of nerve fibers inside CNS with same beginning, end & function.
Tracts of Spinal Cord
Short tracts: Both ascending and descending.
Long tracts: Ascending OR Descending.
 





Short Tracts of the Spinal cord
- Tracts which start and end in the spinal cord.
- Ascending and descending.
- They include:
1) Fasciculi proprii tract:
•	Present in all segments of spinal cord, just outside the grey matter surrounding it all around as a ring.
•	Coordinate function of different regions of spinal cord.
2) Lissauer’s tract:
•	Present in all segments of spinal cord at tips of dorsal horns.
•	It arises from small cells of spinal ganglion, their axons enter spinal cord, then ascend and descend for 1 or 2 segments.
•	It terminates in substantia gelatinosa of Rolandi (SGR).
•	It carries pain and temperature (first order neuron).
3) Septomarginal tract:
•	Present in thoracic, lumbar and sacral segments of spinal cord in dorsal column.
•	Formed of short descending fibers from gracile tract (proprioception of lower half of the body), which terminate in anterior horn cells.
•	It completes stretch reflex arc.
4) Comma shaped tract:
•	Present in cervical and upper thoracic segments of spinal cord in dorsal column.
•	Formed of short descending fibers from cuneate tract (proprioception of upper half of the body), which terminate in anterior horn cells.
•	It completes stretch reflex arc. 
 
N.B: Fixed short tracts in all levels of spinal cord (2):
1)Fasciculi proprii
2) Lissauer’s 
Ascending (sensory) tracts of the spinal cord
A)	Four tracts carrying sensations to cerebral cortex:
1-Gracile Tract
2-Cuneate Tract
3-Lateral Spinothalamic Tract
4-Ventral Spinothalamic Tract
B)	Four tracts carrying impulses to subcortical level:
1-Dorsal Spinocerebellar Tract
2-Ventral Spinocerebellar Tract
3-Spinotectal Tract
4-Spinoolivary Tract



Pathway of Proprioception and Fine Touch from the Body

PROPRIOCEPTION	FINE TOUCH
It is sense of:
-Position
-Movement
-Vibration	It is sense of:
-Tactile localization
-Tactile discrimination
-Stereognosis
Receptors:
-Muscle spindle 
-Tendon spindle
-Pacinian Corpuscle
	Receptors:
-Free nerve endings.
-Meissner’s Corpuscle.
-Merkel’s.
          -Ruffini’s Corpuscle.



First order neuron: 
-They are the large cells of dorsal root ganglion. 
-Their peripheral branches receive impulses from receptors then send them to spinal cord through central branches.
-The central branches enter the spinal cord:
Branches from sacral, lumbar and lower thoracic segments ascend as gracile tract.
Branches from upper thoracic and cervical segments ascend as cuneate tract.
-End in gracile and cuneate nuclei in closed medulla.
Second order neuron:
-They are gracile and cuneate nuclei in closed medulla. 
-Their axons cross to the opposite side (sensory decussation) in the upper half of closed medulla.
-Then they ascend as medial lemniscus.
Third order neuron:
-The posterolateral ventral nucleus (PLVN) of the thalamus.
-Their axons ascend in posterior half of posterior limb of internal capsule then sensory radiation to the sensory area of cerebral cortex (area 3,1,2).
	Gracile and cuneate tracts:
	Gracile tract	Cuneate tract
Origin	Large cells in spinal ganglia	Large cells in spinal ganglia
Course	-All levels of spinal cord
-Posterior column of white matter (medial)	-Cervical & upper thoracic levels of spinal cord
-Posterior column of white matter (lateral)
End	Gracile nucleus 
in closed medulla	Cuneate nucleus 
in closed medulla
Function	Carry proprioception and fine touch from lower half of the body (from same side of body)	Carry proprioception and fine touch from upper half of the body (from same side of body) 






Pathway of Crude (Simple) Touch from the Body
First order neuron: 
-They are the Medium sized cells of dorsal root ganglion. 
-Their peripheral branches receive impulses from receptors then send them to spinal cord through central branches.
-The central branches enter the spinal cord, then synapse with the Main sensory nucleus (MSN).
Second order neuron:
-They are cells of MSN which is present in the posterior horn grey matter of spinal cord. 
-Their axons cross to the opposite side in front of the central canal of spinal cord.
-Then they reach the ventral column of white matter and ascend as ventral spinothalamic tract.
-At the level of pons: the ventral spinothalamic tract joins the lateral spinothalamic tract to form spinal lemniscus, which terminates in PLVN of thalamus.
Third order neuron:
-The posterolateral ventral nucleus (PLVN) of the thalamus.
-Their axons ascend in sensory radiation to the sensory area of cerebral cortex (area 3,1,2).
Pathway of Pain and Temperature from the Body
First order neuron: 
-They are the Small sized cells of dorsal root ganglion. 
-Their peripheral branches receive impulses from receptors then send them to spinal cord through central branches.
-The central branches enter the spinal cord, then they ascend and descend for 1 or 2 segments forming short tract named Lissauer’s tract (the first order neuron) before they terminate by synapsing with Substantia gelatinosa of Rolandi (SGR).
Second order neuron:
-They are cells of SGR which is present in the posterior horn grey matter of spinal cord. 
-Their axons cross to the opposite side in front of the central canal of spinal cord.
-Then they reach the lateral column of white matter and ascend as lateral spinothalamic tract.
-At the level of pons: the lateral spinothalamic tract joins the ventral spinothalamic tract to form spinal lemniscus, which terminates in PLVN of thalamus.
Third order neuron:
-The posterolateral ventral nucleus (PLVN) of the thalamus.
-Their axons ascend in sensory radiation to the sensory area of cerebral cortex (area 3,1,2).
	Ventral Spinothalamic Tract and Lateral Spinothalamic Tract:
	Ventral spinothalamic tract	Lateral spinothalamic tract
Origin	MSN	SGR
Course	-All levels of spinal cord
-Ventral column of white matter 	-All levels of spinal cord
-Lateral column of white matter 
End	PLVN of thalamus	PLVN of thalamus
Function	Carry crude touch from the opposite side of the body 	Carry pain and temperature from the opposite side of the body 



N.B. General rules for the previously discussed sensory pathways:
First order neurons: are cells of the dorsal root ganglia (spinal ganglia).
Second order neurons: MUST cross to opposite side either in spinal cord (for simple touch, pain & temperature) or medulla (for proprioception and fine touch).
Third order neuron: PLVN of thalamus.
N.B. 
1-Fibers forming gracile & cuneate tracts are the axons of first order neurons (uncrossed). So, they carry sensations from the same side of the body.
2- Fibers forming ventral & lateral spinothalamic tracts are the axons of second order neurons (cross to opposite side). So, they carry sensations from the opposite side of the body.







	Proprioception & fine touch	Simple touch	Pain & temperature
Spinal ganglia (1storder)	Large cells 	Medium cells	Small cells
2nd order 	Gracile & cuneate nuclei
In closed medulla 	Main sensory nucleus
In Post. horn	SGR in post. Horn
Spinal cord	Post. Column
Gracile & cuneate tract	Anterior column
Vent. Spinothalamic tract	Lat. Column
Lat. spinothalamic tract
Crossing	Sensory decussation in upper half of closed medulla	In spinal cord in front of central canal	In spinal cord in front of central canal
In brain stem	Medial lemniscus
	Join each other → Spinal lemniscus

PLVN T (3rd order) → Sensory radiation → Cortex area 3 , 1 , 2 
 
Tracts carrying sensory impulses to subcortical level
	Dorsal spinocerebellar tract	Ventral spinocerebellar tract	Spinotectal tract	Spinoolivary tract
Origin	Clark’s nucleus 
(C8-L2)
(Uncrossed)	Nerve cells in dorsal horn grey matter
(lumbosacral)
(Crossed)	Nerve cells in dorsal horn grey matter	Nerve cells in dorsal horn grey matter
Course	Cervical, thoracic and upper lumbar levels of spinal cord
(Lateral column)	All levels of spinal cord
(Lateral column)	All levels of spinal cord	All levels of spinal cord
Termination	Cerebellum	Cerebellum	Superior colliculus 
of mid brain	Inferior Olivary nucleus of medulla
Function	Carries unconscious proprioception 
from trunk and lower limb 
to cerebellum

(To coordinate movement & maintain body posture)	Carries unconscious proprioception 
from lower limb
 to cerebellum

(To coordinate movement & maintain body posture)	Carries spino-visual reflexes to tectum.

(To integrate the visual and the somatic sensory information e.g., turning the eye & head towards source of stimulation)	Carries unconscious proprioception 
to cerebellum
 
N.B. Fixed long ascending tracts in all levels of spinal cord: Six tracts
1-Gracile tract.
2-Ventral spinothalamic tract.
3-Lateral spinothalamic tract.
4-Ventral spinocerebellar tract.
5-Spinotectal tract.
6-Spino-olivary tract. 
(All of them are fixed except cuneate & dorsal spinocerebellar)