5.4 Histology of the CNS

Question 1

Neurones are the basic structural unit of the nervous system and possess a cell body (soma) with a round nucleus and prominent single nucleolus:
• Vary in size and shape at various locations (e.g. _____________ in motor cortex, ____________ in cerebellum, __________ of spinal cord ventral horn)
• Axon is covered by the myelin sheath (by ______________ in CNS), and is connected to the cell body by the axon hillock (trigger zone) → appears lighter under LM
• Cytoplasm contains granular material (_________________)

Answer 1

pyramidal cells;

Purkinje cells;

motor neurones;

oligodendrocytes;

Nissl substance

Question 2

Axon

• ____________ process which terminate away from the cell body
• Rarely branches (ends by dividing into many fine processes – axon terminals)
• Nissl granules __________
• Impulses travel away from cell body

Answer 2

single long thin;

absent

Question 3

Dendrites

• Multiple ______________ processes which terminate near cell body
• Highly branched (forms a dendritic tree)
• Nissl granules ____________
• Impulses travel towards cell body

Answer 3

short thick and tapering;

present

Question 4

The cytoplasm contains abundant ___________________, which corresponds to the Nissl substance observed on H&E stain:
• Indicates prominent protein synthesis occurring in the neurones, which is important for the maintenance of nerve and production of neurotransmitters and enzymes
• Possesses plentiful _______ and _____________, and neurofilaments and neurotubules

Answer 4

rough endoplasmic reticulum;

Golgi apparatus; mitochondria

Question 5

The synapse is the site of junction between neurones, with different types of synapses depending on which part of the neurone is involved in the formation of the synapse
• Impulses always travel __________ across the synapse → possesses a presynaptic (usually axon) and postsynaptic (dendrite) component
• Axons show enlargement at the terminal end (__________________)

Type
- Axodendritic Most common type (axon with dendrite); often _____________
- Axosomatic: Axon with cell body (soma); often ___________
- Axoaxonic: Often modulatory
- Others: Dendroaxonic and dendrodendritic synapses
• Synaptic cleft is a small cleft present between the axon and the dendrite, and both the pre- and postsynaptic membranes show _________________ adjacent to the cleft (active zone of the synapse → nerve impulse transmission)

Answer 5

unidirectionally;

boutons terminaux;

excitatory;

inhibitory ;

condensation (thickening)

Question 6

what are motor neurones like?

Answer 6

multipolar

Question 7

what are most sensory neurones?

Answer 7

unipolar

Question 8

what are the neurones found in the cerebrum?

Answer 8

Pyramidal cell (triangular with round nucleus and central nucleolus)

Question 9

What are the neurones found in the cerebellum?

Answer 9

• Purkinje cell (flask-shaped with abundant cytoplasm, round nucleus and nucleolus)
• Granular neurones (granular layer beneath Purkinje cells; small cells with scanty cytoplasm)

Question 10

What are the neurones found in the spinal cord?

Answer 10

Motor neurones (larger neuronal cell bodies) in the anterior horn

Question 11

Astrocytes are small _______________ cells with a number of cellular processes ending on blood vessels or the surface of the brain (2 types based on cellular processes):
• Possess pink cytoplasm (on H&E) and a characteristic star-like quality (with _________________)

Type

• _____________: Thicker processes seen in grey matter
• ________: Thinner processes seen in white matter

Astrocytes possess several important functions including the following:

1. Mechanical support to neurones → bundles of ________________
2. Non-conducting cells (acting as insulators around neurones)
3. Maintain metabolic environment for neuronal activity (regulate metabolic exchange)
4. Forming the blood-brain barrier (_________________ covering most of the surface of the capillary basement membrane)
5. Involved in repair of damaged tissues (___________)

Answer 11

star-shaped;

glial fibrillary acidic protein/GFAP stain;

Protoplasmic;

Fibrillary;

GFAP and microtubules;

perivascular foot processes;

gliosis

Question 12

Reactive astrocytosis: astrocytes are activated in response to various pathological conditions (e.g. trauma, infection, ischaemia, stroke, autoimmune diseases, neurodegenerative)

• Causes _________________ of astrocytes
• Chronic reactive astrocytosis is more fibrillar in nature –> formation of ______________ adjacent to the site of injury

Answer 12

proliferation and hypertrophy;

dense gliosis

Question 13

Oligodendrocytes are cells with ________________, which produce myelin and surround multiple axons (up to 50) in the CNS:
• Similar in function to the Schwann cells which myelinate single axons in the PNS
• Facilitates fast conduction of nerve impulses down axons (saltatory conduction) → more numerous in the ____________________

Answer 13

round nuclei and a perinuclear halo;

white matter than grey matter

Question 14

Ependymal cells are ____________ type cells which line the _____________, forming part of the blood-brain barrier:
• Specialised to form the __________ in the ventricles (tufted aggregates of vascular channels lined by a single layer of cells with eosinophilic mitochondria-rich cytoplasm)
• Secrete CSF into the ventricles

Answer 14

cuboidal-columnar;

ventricles and the central canal of the spinal cord;

choroid plexus

Question 15

Microglia are the smallest neuroglial cells, and possess ______________ (difficult to identify in the resting state → best identified by immunohistochemistry):
• _____________ in origin (unlike other glial cells) and are derived from the monocyte-macrophage system (specialised form of macrophages)
• Important in immune and inflammatory responses (especially viral infections) where they serve phagocytic roles (transform into large amoeboid phagocytic cells)
o Other macrophages are present in the space surrounding CNS capillaries, but are separated from the CNS compartment by the astrocytic perivascular feet

Answer 15

elongated nuclei;

Mesodermal

Question 16

The isolated grey matter masses within the white matter in the CNS are called nuclei:
• Axon bundles (fasciculi) coming from CNS nuclei form _______________
• Bundles are called peduncles when they connect ________________
• Peripheral nerves are aggregates of processes of neurones outside the CNS

Answer 16

tracts linking different nuclei;

cerebrum/cerebellum to brainstem

Question 17

Neurones in the CNS possess certain common features (despite variations in size, shape):
• Highly metabolically active: requiring constant supply of oxygen and glucose (15% of cardiac output and 20% of total body oxygen consumption)
o More susceptible to hypoxia than glial cells
o Cerebellar ___________, hippocampal pyramidal cells (especially ______), neocortex (cerebral cortex layers ____________) are the most susceptible
• Incapable of division (after few days after birth): cannot be replaced after injury

Answer 17

Purkinje cells

CA1;

3 and 5

Question 18

BLOOD-BRAIN BARRIER
The blood-brain barrier is a selective barrier between neural tissues in the CNS and blood:
• Endothelial cells of brain capillaries are connected by continuous tight junctions, which prevent movement of molecules of varying sizes through
• No ____________- or other pathways for the movement of substances (forming a continuous barrier) → supplemented by _________________ on outer aspect

It protects the brain from infections and drugs (e.g. antibiotics), so certain drugs (e.g. mannitol) are used as a facilitating agent for drug absorption:
• H2O, O2, CO2 and small lipid molecules can diffuse passively through, while glucose and amino acids are taken in by selective transport

Answer 18

fenestrations;

astrocytic foot processes

Question 19

[cerebrum]

What is located at the plexiform (molecular) layer?

Answer 19

Beneath the pia mater; consists of few neuronal bodies:
• Contains dendrites and axons of cortical neurones synapsing with each other
• Few cell bodies are present (those of neuroglia and Cajal-Retzius cells)

Question 20

[cerebrum]

What is located at the outer granular layer?

Answer 20

Contains a dense population of small pyramidal cells and stellate cells (admixed with various axons and dendritic connections from deeper layers)

Question 21

[cerebrum]

What is located at the outer pyramidal cell layer?

Answer 21

Moderate size pyramidal cells predominate (cells increase in size deeper in the layer), with Martinotti cells

Question 22

[cerebrum]

What is located at the inner granular layer?

Answer 22

Contains mainly of densely packed stellate cells:

• External band of Baillarger (horizontal fibres)

Question 23

[cerebrum]

What is located at the inner pyramidal cell layer?

Answer 23

Large pyramidal cells and some stellate and Martinotti cells:
• Internal band of Baillarger (horizontal fibres

Question 24

[cerebrum]

What is located at the multiform layer?

Answer 24

Wide variety of differing morphological forms: • Contains numerous small pyramidal cells, Martinotti cells and stellate cells superficially and fusiform cells in the deeper part

Question 25

*The motor cortex has more dominant _________________, while the sensory cortex has more ______________, with largest pyramidal cells in the 5th layer.

Answer 25

pyramidal cell layers (agranular cortex);

dominant granular cell layers (granular cortex)

Question 26

The dominant neuronal types in the cerebral are the pyramidal cells (Betz cells) and the stellate granular cells:
- Pyramidal cells: Triangular cells (with apex pointing to the surface of the _______________):
• Primary output → axons travel and terminate in the ipsilateral or contralateral cortex or enter deep into the subcortical areas
• Dendrites travel up into the cortex and synapse with multiple outer neurones

• Stellate cells: Small and multipolar cells serving as the _____________
• The neuropil is the ______________ present between the cellular constituents (e.g. neurones, glial cells) in the grey matter composed of the cellular processes of these cells.

Answer 26

cortex from which dendrites arises;

interneurones of the cortex;

amorphous eosinophilic matrix

Question 27

Some neurones (especially in the specialised nuclei) contain a brown granular pigment (neuromelanin) in the cytoplasm: 
•  \_\_\_\_\_\_\_\_\_\_\_\_\_ cells in the substantia nigra contain neuromelanin 
• Substantia nigra is 2 zones of intensely brown areas

Parkinson’s disease: pallor of substantia nigra occurs due to the loss of neuromelanin and presence of Lewy bodies in the cytoplasm
• Characterised by cell death in the _____________ (affecting up to 70% of dopaminergic neurones in the pars compacta of substantia nigra)
• Lewy bodies are accumulations of the protein alpha-synuclein in many remaining neurones

Answer 27

Dopaminergic;

basal ganglia

Question 28

The cerebellar cortex is composed of 3 layers (outer molecular, middle Purkinje, inner granular cell) and is uniform throughout:
• Neuronal cells are mainly the _________________ (found in the molecular layer)

Answer 28

Purkinje cells, granular cells, outer stellate cells, and basket cells

Question 29

Cerebrellar cortex
- Molecular layer: Appears pale and consists of few cells (mainly _____________ and mainly of myelinated and unmyelinated fibres

• Purkinje cell: single layer of evenly spaced Purkinje cells (dense dendritic meshwork):
  • Axons of each Purkinje cell pass downwards through the granular cell layer to enter the ________________ (the only efferents of the cerebellar cortex by synapsing with cerebellar nuclei)
• Granular cell: Cellular area with many tightly packed granular cells (with round nuclei):
  • _____________ are the spaces between these cells (containing synaptic structures called glomeruli)
  • Gives rise to dendrites which synapse with the afferent fibres entering the cerebellar cortex (climbing fibres), which passes through into the molecular layer where they exert specific influences on the Purkinje cells

Answer 29

stellate cells and basket cells);

white matter;

Cerebellar islets

Question 30

The spinal cord consists on grey matter (more central) and white matter (surrounding grey matter) which can be distinguished more easily on a special stain (compared to H&E):
• Grey matter can be divided into the ___________ (sensory) and ____________ (motor), as well as the lateral horns at certain levels (e.g. thoracic) o Lateral horns contain the ________________ if present
o Anterior horn neurones are _________ (large cells with purple-staining cytoplasmic Nissl substance) responsible for voluntary movement
• Anterior midline fissure carries the _______________.
• Central canal (carrying CSF) is lined by _____________

Answer 30

posterior horns;

anterior horns;

autonomic grey matter;

alpha motor neurones;

anterior spinal artery (from vertebral arteries);

ependymal cells