CNS

Lecture 1. Histology

Nervous system
The nervous system is responsible for communication between different regions of the body, it is divided into:
CNS (central nervous system) = brain + spinal cord
PNS (peripheral nervous system) =nerves running between the CNS & other tissues.

Nervous system is composed of highly intercommunicating networks of nerve cells that receive and conduct impulse along their neuronal pathway or axons to the CNS for analysis ,integration ,interpretation and response. The appropriate response to a given stimulus from the neurons of the CNS is the activation of muscles (skeletal ,smooth, or cardiac)or glands (endocrine or exocrine ).
The Neuron:
It is the main structural and functional unit of the nervous system , it consists of :
Cell body (soma): containing the nucleus & most of the organelles.
Axon : a single long process which conducts the electrical depolarization (signals) from the cell body of the neuron to other cells.
Dendrites: are short processes which receive the signals from the axons of other neurons by making synapse which allows a direct communications between the cells.
Terminal button: is the branched terminal end of the axon.


The neurons are highly metabolically active cells characterized by:
Large nucleus & central nucleolus.
The cytoplasm contains abundant lysosomes & rough endoplasmic reticulum (RER), Nissl substances, well developed Golgi & large no. of mitochondria to provide energy.

Types of the Neurons:

Multipolar (motor) neurons : have large cell body + large axon + many dendritic processes. They are the commonest type.
Unipolar (sensory) : cell body + one large process divided into 2 branches, one is axon & other is dendrite. These cells are found in sensory ganglia of cranial and spinal nerves

Bipolar : simple cells provide local communications within the CNS having 2 main processes of equal size one axon & other dendrite. These cells are found in the inner ear ,olfactory mucosa ,and in the retina of the eye.

Central Nervous System (CNS )

The CNS consists of the brain and spinal cord which are composed of (neurons + supporting cells of the CNS (glial cells) +blood vessels) both are protected by the skull and the vertebral canal, it receive the stimuli from the internal and external environments & sensory receptors, motor impulses are then generated and discharged to the organs (muscles & glands) for appropriate action.
The CNS is made up of Gray and White matter.
The gray matter consists of nerve cell bodies and the proximal portions of their processes embedded in neuroglia and permeated by a diffuse capillary bed, it forms the cerebral & cerebellar cortex.
The White matter consists of nerve fibers (myelinated axons) embedded in the neuroglia (oligodendrocytes).
Difference:
The white matter does not contain neural cell bodies.


Less metabolically active than the gray matter because its function is mainly conductive.
White matter differs greatly from the gray matter by having No synapse, No dendrites and limited blood supply.
The aggregations of neural cell bodies form islands of gray matter embedded in the white matter called "nuclei". In cross sections of the
spinal cord the white matter is peripheral while the gray matter is central having the shape of H or (butterfly) shape

Microscopically the CNS consists of:

A-Neurons: in the brain they arranged in clusters called "nuclei" lie deep in the brain while in the spinal cord they are located in the H- shaped gray matter in the center of the cord.
B-Glial cells "neuroglia": are nonneural cells [Astrocytes, Oligodendrocytes, Microglia& Ependyma] are supportive elements throughout the CNS.
In the brain & the spinal cord there are 4 types of supporting cells:
1. Astrocytes: stellate-shaped cells characterized by oval or irregular nuclei with fine processes radiating in all directions. They are of 2 types:
a. fibrous astrocytes found in the white matter.

b. protoplasmic astrocyte found in the gray matter.
Functions of Astrocytes:
1.They play a role in the formation of blood-brain barrier which prevent diffusion of substances between the blood & the brain.
2.Support the metabolic exchange between the neurons and the capillaries of the CNS.
3.Control the chemical environment around neurons at the active synaptic sites.
4.contribute to the energy metabolism of the CNS
2. Oligodendrocytes: round cells with few processes have a round nucleus with clear hallow around it, they produce myelin within the CNS. The myelin sheath is a lipid -rich insulting sheath around the axons ,the myelin sheath is formed from successive layers of modified cell membrane .
3. Ependymal cells: are epithelial cells lining the cavities (ventricles) of the brain as a sheet of cuboidal cells many of which are ciliated & they are in contact with the CSF and facilitate its movement through the spinal canal and the Ependymal cells contains microvilli which have some absorptive functions.
4. Microglia cells: are specialized macrophages forming the immune cells in the CNS.




C-Nerve fibers: are long axons may be myelinated or non myelinated, they connect various regions of the brain & the spinal cord, they may form bundles called "tracts".
D- Accessory structures : meninges, blood vessels, CSF or reservoirs called "Cisternae" or "Ventricles".
Meninges
The brain & the spinal cord are delicate semisolid structures requiring protection & support, thus they are invested by 3 membranes called "Meninges": Dura, Arachnoid & Pia matter.

1 Dura matter ( Cranial & Spinal ) :

Is the external layer and composed primarily of fibrocollangenous dense connective tissue divided into 2 types:

The cranial dura consist of 2 layers:

I- Outer periosteal layer: adherent to the inner surface of the cranium.
II- Inner meningeal layer: lined with flat cells.
At certain sites these layers are separated and form (venous sinuses).
The meningeal layer gives rise to several septa which divide the cranial cavity into compartments (falx cerebri, tentorium cerebelli Cerebelli). This layer is in close contact with the arachnoid and separated from it by "Subdural space" filled with cerebrospinal fluid.
b.The spinal dura: -
Is a continuation of meningeal layer of the cranial dura , and is separated from the periosteum of the vertebrae by a narrow " Epidural space" which contains areolar tissue & internal vertebral venous plexuses.The spinal dura extends from the foramen magnum to the second sacral vertebra and it is attached to the periosteum of the vertebral canal by denticulate ligament. Some extension of a dura passing laterally around the spinal nerve roots to form dural root sleeves, the internal and external surfaces of all dura matter is covered by simple squamous epithelium of mesenchymal origin.

2 Arachnoid matter:

Is avascular membrane formed of fine collagen fibers and scattered elastic fibers, it lie beneath the dura & separated from the pia by (Subarachnoid space) which contain (weblike strands) of fibrocollagenous tissue & filled with CSF, this space form a cushion that protect the CNS from trauma& it communicates with the ventricles of the brain .
The arachnoid is composed of C.T. devoid of blood vessels and it's surfaces are covered by same type of simple epithelium that covers the dura matter. In some areas the arachnoid perforates the dura matter forming protrusions which are called arachnoid villi that terminate in the venous sinuses located between the periosteal and menengial layers of dura mater . fter circulation CSF is reabsorbed from the Subarachnoid space via arachnoid villi into the blood of the venous sinuses.
3- Pia matter:
Is a loose C.T located very close to the nerve tissue but not in contact with nerve cells or fibers, it follows all the irregularities of the surface of the CNS & penetrates it to some extent along with the blood vessels but it is still separated from the neural elements by a thin layer of neuroglial processes called (Glial limitans) which adheres firmly to the pia matter & forms a physical barrier at the periphery of the CNS, this barrier separates the CNS from the CSF.
Pia matter (like dura & arachnoid) is covered by squamous cells of mesenchymal origin. All the blood vessels passing to & from the CNS pass in the subarachnoid space & as they extend into the nervous tissue, they are surrounded by a delicate sleeve of pia matter, & separated from it by a (perivascular space), this sleeve disappears when the blood vessels are transformed into capillaries.The spinal cord is attached to the dura by series of flattened band like extensions of pia matter called "denticulate ligaments ".


Blood- Brain Barrier ( BBB) :Is a functional barrier that prevents the passage of some substances such as antibiotics, chemicals, bacteria & toxic substances from the blood to the nerve tissue. The BBB consists of:
1- Endothelial cells of capillaries of the CNS which does not have the fenestrations found in many other locations (i.e.continuous endothelium).
2- Occluding junctions between the endothelial cells : this represent the main structural component of the barrier.
3- Processes of Astrocyte (neuroglial cells ): that envelop the capillaries & partly responsible for their low permeability .
 SHAPE \* MERGEFORMAT 

Choroid Plexus :The choroid plexus consists of invaginated folds of pia matter that penetrate the interior of the ventricles, it is found in the roof of the 3rd & 4th ventricles & partly in the wall of lateral ventricles. It is vascular structure made up of delicate fenestrated capillary networks; it is composed of loose connective tissue of pia matter covered by a simple cuboidal epithelium that has the features of ion-transporting cells (i.e having numerous mitochonderia + large nucleus + short irregular microvilli on the free surface ), the main function of the choroid plexus is elaboration of CSF .
Cerebrospinal fluid (CSF): A clear colorless fluid consist of protein + glucose & lymphocytes, it's volum = 150 ml. filling the ventricles of the brain, central canal of the spinal cord, subarachnoid space & perivascular space. It protects the brain & the spinal cord against any trauma, it circulates through the ventricles from which it passes into the subarachnoid space where the arachnoid villi provide the main pathway for absorption of CSF into the venous circulation .
Functions of the CSF:
1.Protection of the brain and the spinal cord against any physical injury.
2.Play an important role in homeostasis and brain metabolism.
3.Provide an optimal chemical environment for neuronal functions and impulse conductions.

The Spinal Cord (SC):

The SC is an elongated cylindrical mass of nerve tissue occupies the upper 2/3 of the vertebral canal, it extends from the " atlas " to the upper border of the second lumber vertebra ( L2 ), the conical distal end of the SC called Conus medullaris, from it's apex a delicate filament called ( filum terminale ) extended & attached to the coccyx. A group of nerve fibers arising from the lower portion of the cord but they angle inferiorly in the vertebral canal like a band of coarse hair called Cauda equina.
Like the brain, the SC is invested by the meninges, the dura is connected to the periosteum of the vertebral canal by denticulate ligament & epidural space.
The SC is divided into 4 main regions: Cervical, Thoracic, Lumber, & Sacral.The structure of all regions is basically the same.


The Transverse Section of the SC shows:
Central mass of gray matter has a shape of "butter fly" or "H- shape", the ventral horns are most prominent and containing cell bodies of motor neurons, their axons pass out in the ant. roots of the spinal nerves to innervate the skeletal muscles, while the dorsal horns are much less prominent & contains the cell bodies of sensory neurons, they transmit sensory informations of pain & temperture to the brain. A small lateral horns contains the cell bodies of preganglionic sympathetic neurons, present in the thoracic and upper lumber regions corresponding to the sympathetic outflow of the SC.
Deep ventral median fissure is present while dorsally there is only a shallow dorsal median sulcus, a dorsolateral sulci found on each side, they mark the line of entry of the dorsal nerve roots on each side.
Central canal lie in a central commissure of gray matter, the canal is lined by ependymal cells & contains CSF.In the cervical region it lies near the ant. surface, so the post. median septum is longer than the ant. median fissure , gradually from above downward the central canal is displaced towards the post. surface, so in the sacral region it lies in the middle & both the median septum & fissure are more or less equal.
The volume of the gray matter is more extensive in the cervical & lumber regions corresponding to the sensory & motor innervation of limbs & this is reflected as much larger diameter of the SC in these areas.
White matter of the SC consists of ascending tracts of sensory fibers & descending motor tracts from the brain to the SC.Progressively, more fibers enter & leave the cord, so the volume of white matter increases gradually from the sacral to the cervical regions.
The triangular area of white matter between the two dorsal horns of gray matter represent the ascending ( dorsal column ) which carry sensory fibers of vibration , proprioception & touch to the medulla where they synaps with the gracil & cunate nuclei.In the cervical & upper 1/2 of the thoracic region, each dorsal column is subdivided into 2 fasciculi medially is " fasciculus gracillis " ( carries the fibers from the lower limbs ) and laterally is " fasciculus cuneatus " convey the fibers from the upper limbs, while in the lower 1/2 of the thoracic & lumber regions , there is only gracile but no cuneate fascicle.
Ventrolateral white matter on each side is made of various ascending&descending tracts most notably is lateral spinothalamic (pain & temp) & anterior spinothalamic (light touch), spinocerebellar tracts (coordination & equillibrium & corticospinal motor tracts (movement of the skeletal muscles).















Dr Luma Ibrahim