NEET MDS Synopsis
NEUROHISTOLOGY
Anatomy
NEUROHISTOLOGY
The nervous system develops embryologically from ectoderm, which forms the neural plate
Successive growth and folding of the plate results in the formation of the primitive neural tube.
The neuroblasts in the wall of the tube differentiates into 3 cell types:
Neurons: conduction of impulses
Neuroglial cells: connective tissue and support of CNS
Ependymal cells: Lines the lumen of the tube.
- Specialized neuro-ectodermal cells which lines the ventricles of the adult brain
- Essentially also a neuroglial cell
Basic Unit = neuron
Exhibits irritability (excitability) and conductivity
A typical neurons consists of:
Cell body : Has nucleus (karyon) and surrounding cytoplasm (perikaryon) which contains organelles cell's vitality
Dendrites: Several short processes
Axon:One large process
Terminates in twig like branches (telodendrons)
May also have collateral branches projecting along its course. These exit at nodes of Ranvier
Axon enveloped in a sheath, and together forms the nerve fiber
Classification:
May be done in different ways, i.e.
Functional = afferent, efferent, preganglionic, postganglionic, etc.
Morphological = shape, processes, etc
A typical morphological classification is as follows
a. Unipolar: Has one process only Not found in man
b. Bipolar (so-called ganglion cell):Has two processes Found in sensory systems, e.g. retina olfactory system
c. Multipolar: Has several process Most common in CNS
Cell bodies vary in shape, e.g. stellate (star) , pyramidal
d. Pseudo-unipolar: Essentially bipolar neurons, but processes have swung around cb and fused with each other. They therefore enter and leave at one pole of the cell.
Typical neuron:
- Has 2 or more dendrites
Close to the cb the cytoplasm of dendrites has Nissl granules as well as mitochondria
Only one axon Arises from axon hillock, Devoid of Nissl granules, Encased in myelin sheath
No additional covering except for occasional foot processes of neuroglial cells
May branch at right angles
Branches at a node of Ranvier is known as a collateral
Ends of axons break up into tree-like branches, known as telodendria
Axons may be short (Golgi Type II) e.g. internuncial long (Golgi Type I) e.g. pyramidal neuron
Nucleus Central position Large and spherical
Chromatin is extended and thus not seen in LM. This allows the nucleolus to be prominent
Cytoplasm (perikaryon)
Surrounds nucleus May be large or small, shape may be round, oval, flattened, pyramidal, etc
Contains aggregates Nissl granules(Bodies) which is also sometimes referred to as rhomboid flakes
aggregation of membranes and cisternae of rough endoplasmic reticulum (RER)
numerous ribosomes and polyribosomes scattered between cisternae
(Polyribosome = aggregate of free ribosomes clumped together)
responsible for ongoing synthesis of new cytoplasm and cytoplasmic substances
needed for conduction of impulses
highly active in cell protein synthesis
resultant loss of power to divide which is characteristic of neurons
- Golgi network surrounding nucleus (seen in EM only)
- Fibrils made up of:
- neurofilaments
- microtubules
Tubules involved in:
1. plasmic transport
2. maintenance of cell shape
3. essential for growth and elongation of axons and dendrites
Neurofilament:
1. provide skeletal framework
2. maintenance of cell shape
3. possible role in axonal transport
(Axonal [axoplasmic; plasmic] transport may be antero- or retrograde. Anterograde transport via neurotubules is fast and moves neurotransmitters. Retrograde transport is slow and is the reason why viruses and bacteria can attack and destroy cell bodies. E.g. polio in the ventral columns and syphilis in the dorsal columns).
- Numerous mitochondria
- Neurons lack ability to store glycogen and are dependent for energy on circulating glucose
Impulses are conducted in one direction only
Dendrites conduct towards the cb
Axons conduct away from cb
Synapses:
- Neurons interconnect by way of synapses
- Normally the telodendria of an axon synapse with the dendrites of a succeeding axon
axo-dendritic synapse
This is usually excitatory
- Other types of synapses are:
axo-axonic
May be excitatory and/or inhibitory
axo-somatic
May be excitatory and/or inhibitory
dendrodendritic
Usually inhibitory
- Synapses are not tight junctions but maintain a narrow space the so-called synaptic cleft
- The end of an telodendron is usually enlarged (bouton) and contains many synaptic vesicles,
mitochondrion, etc. Its edge that takes part in the synapse is known as the postsynaptic membrane and no
vesicles are seen in this area
- Synapses may be chemical (as above) or electrical as in the ANS supplying smooth muscle cells subjacent to adjacent fibres
Gray and White Matter of Spinal Cord:
- Gray matter contains:
- cb's (somas) of neurons
- neuroglial cells
- White matter contains:
- vast number of axons
- no cb's
- colour of white matter due to myelin that ensheathes axons
Myelin:
- Non-viable fatty material contains phospholipids, cholesterol and some proteins
- Soluble and not seen in H&E-sections because it has become dissolved in the process, thus leaving empty spaces around the axons
- Osmium tetroxide (OsO4) fixes myelin and makes it visible by staining it black. Seen as concentric rings in cross section
- Myelin sheath (neurolemma) is formed by two types of cells
- Within the CNS by Oligodendrocytes
- On the peripheral neurons system by Schwann cells
- Sheath is formed by being wrapped around the axon in a circular fashion by both types of cells
Neuroglial Cells:
- Forms roughly 40% of CNS volume
- May function as: 1. support
2. nurture ("feeding")
3. maintain
Types of glial cells:
Oligodendrocytes:
- Small dark stained dense nucleus
- Analogue of Schwann cell in peripheral nervous system
- Has several processes which forms internodal segments of several fibres (one cell ensheathes more than one axon)
- Provides myelin sheaths in CNS
- Role in nurturing (feeding) of cells
Astrocytes:
Protoplasmic astrocytes:
- found in gray matter
- round cell body
- large oval nucleus with prominent nucleolus
- large thick processes
- processes are short but profusely branched
- perivascular and perineurial foot processes
- sometimes referred to as mossy fibres
Fibrous Astrocytes:
- found in white matter
- polymorphic cells body
- large oval nucleus
- long thin processes
Microglia:
- Neural macrophages
- smallest of the glial cells
- intense dark stained nucleus
- conspicuously fine processes which has numerous short branches
Cerebral Cortex:
Consists of six layers which are best observed in the cortex of the hippocampus
From superficial to deep:
- Molecular layer:
- Has few cells and many fibres of underlying cells
- Outer granular layer:
- Many small nerve cells
- Pyramidal layer:
- Pyramidally-shaped cells bodies
- Inner granular layer:
- Smaller cells and nerve fibres
- Internal (inner) pyramidal layer:
- Pyramidal cells bodies
- Very large in the motor cortex and known as Betz-cells
- Polymorphic layer:
- Cells with many shapes
Cerebellar Cortex:
Consists of three layers
Connections are mainly inhibitory
From superficial to deep
- Outer molecular layer:
- Few cells and many fibres
- Purkinje layer:
- Huge flask-shaped cells that are arranged next to one another
- Inner granular layer:
- Many small nerve cells
Motor endplate:
Seen in periphery on striated muscle fibres
- known as boutons
- has no continuous myelin covering from the Schwann cells
- passes through perimysium of muscle fiber to "synapse"
- multiple synaptic gutter (fold) in sarcoplasma of muscle fiber beneath bouton
- contains numerous synaptic vesicles and mitochondria
Ganglia:
- Sensory Ganglia:
(e.g. trigeminal nerve, ganglia and dorsal root ganglia)
- No synapse (trophic unit)
- pseudo-unipolar neurons
- centrally located nucleus
- spherical smooth border
- conspicuous axon hillock
- Surrounded by cuboidal satellite cells (Schwann cells)
- Covered by spindle shaped capsular cells of delicate collagen which forms the endoneurium
- Visceral and Motor Ganglia (Sympathetic and Parasympathetic):
- Synapse present
- Ratio of preganglionic: postganglionic fibres
1. Sympathetic 1:30
Therefore excitatory and catabolic
2. Parasympathetic 1:2
Therefore anabolic
Except in Meissner and Auerbach's plexuses where ratio is 1:1000 '2 because of parasympathetic component's involvement in digestion
- Preganglionic axons are myelinated (e.g. white communicating rami)
- Postganglionic axon are non-myelinated (e.g. gray communicating rami)
- small multipolar cell body
- excentrally located nucleus
- Inconspicuous axon hillock
- satellite cells few or absent
- few capsular cells
ERUPTION -Primary teeth
Dental Anatomy
ERUPTION
. Root completion (approximately 50% of the root is formed when eruption begins)
Generally mandibular teeth erupt before maxillary teeth,
Primary teeth
I. Emerge into the oral cavity as follows:
Maxillary Mandibular
Central Incisor 7½ months 6 months
Lateral incisor 9 months 7 months
Canine 18 months 16 months
First Molar 14 months 12 months
Second Molar 24months 20 months
The sequence of primary tooth development is central incisor, lateral incisor, first molar, second molar
3. Hard tissue formation begins between 4 and 6 months in utero
4. Crowns completed between 1½ and 10 months of age
5. Roots are completed between I½ and3 yearsof age 6 to 18 months after eruption
6. By age 3 years all of the primary and permanent teeth (except for the third molars) are in some stage of development
7. Root resorption of primary teeth is triggered by the pressure exerted by the developing permanent tooth; it is followed by primary tooth exfoliation in sequential patterns
8. The primary dentition ends when the first permanent tooth erupts
Fourth Generation:
Pharmacology
Fourth Generation:
These are extended spectrum antibiotics. They are resistant to beta lactamases.
Cefipime
Cardiac Output
Physiology
Cardiac Output:
Minute Volume = Heart Rate X Stroke Volume
Heart rate, HR at rest = 65 to 85 bpm
Each heartbeat at rest takes about .8 sec. of which .4 sec. is quiescent period.
Stroke volume, SV at rest = 60 to 70 ml.
Heart can increase both rate and volume with exercise. Rate increase is limited due to necessity of minimum ventricular diastolic period for filling. Upper limit is usually put at about 220 bpm. Maximum heart rate calculations are usually below 200. Target heart rates for anaerobic threshold are about 85 to 95% of maximum.
Terms:
End Diastolic Volume, EDV - the maximum volume of the ventricles achieved at the end of ventricular diastole. This is the amount of blood the heart has available to pump. If this volume increases the cardiac output increases in a healthy heart.
End Systolic Volume, ESV - the minimum volume remaining in the ventricle after its systole. If this volume increases it means less blood has been pumped and the cardiac output is less.
EDV - ESV = SV
SV / EDV = Ejection Fraction The ejection fraction is normally around 50% at rest and will increase during strenuous exercise in a healthy heart. Well trained athletes may have ejection fractions approaching 70% in the most strenuous exercise.
Isovolumetric Contraction Phase - a brief period at the beginning of ventricular systole when all valves are closed and ventricular volume remains constant. Pressure has risen enough in the ventricle to close the AV valves but not enough to open the semilunar valves and cause ejection of blood.
Isovolumetric Relaxation Phase - a brief period at the beginning of ventricular diastole when all valves are closed and ventricular volume is constant. Pressure in the ventricle has lowered producing closure of the semilunar valves but not opening the AV valves to begin pulling blood into the ventricle.
Dicrotic Notch - the small increase in pressure of the aorta or other artery seen when recording a pulse wave. This occurs as blood is briefly pulled back toward the ventricle at the beginning of diastole thus closing the semilunar valves.
Preload - This is the pressure at the end of ventricular diastole, at the beginning of ventricular systole. It is proportional to the End Diastolic Volume (EDV), i.e. as the EDV increases so does the preload of the heart. Factors which increase the preload are: increased total blood volume, increased venous tone and venous return, increased atrial contraction, and the skeletal muscular pump.
Afterload - This is the impedence against which the left ventricle must eject blood, and it is roughly proportional to the End Systolic Volume (ESV). When the peripheral resistance increases so does the ESV and the afterload of the heart.
The importance of these parameters are as a measure of efficiency of the heart, which increases as the difference between preload and afterload increases
Salivary Factors and Their Mechanisms
PedodonticsSalivary Factors and Their Mechanisms
1. Buffering Factors
Buffering factors in saliva help maintain a neutral pH in the oral cavity,
which is vital for preventing demineralization of tooth enamel.
HCO3 (Bicarbonate)
Effects on Mineralization: Acts as a primary buffer
in saliva, helping to neutralize acids produced by bacteria.
Role in Raising Saliva or Plaque pH: Increases pH
by neutralizing acids, thus promoting a more favorable environment for
remineralization.
Urea
Effects on Mineralization: Releases ammonia (NH3)
when metabolized, which can help raise pH and promote mineralization.
Role in Raising Saliva or Plaque pH: Contributes to
pH elevation through ammonia production.
Arginine-rich Proteins
Effects on Mineralization: Releases ammonia, which
can help neutralize acids and promote remineralization.
Role in Raising Saliva or Plaque pH: Increases pH
through ammonia release, creating a less acidic environment.
2. Antibacterial Factors
Saliva contains several antibacterial components that help control the growth
of pathogenic bacteria associated with dental caries.
Lactoferrin
Effects on Bacteria: Binds to iron, which is
essential for bacterial growth, thereby inhibiting bacterial
proliferation.
Effects on Bacterial Aggregation or Adherence: May
promote clearance of bacteria through aggregation.
Lysozyme
Effects on Bacteria: Hydrolyzes cell wall
polysaccharides of bacteria, leading to cell lysis and death.
Effects on Bacterial Aggregation or Adherence: Can
indirectly promote clearance by breaking down bacterial cell walls.
Peroxidase
Effects on Bacteria: Produces hypothiocyanate
(OSCN), which inhibits glycolysis in bacteria, reducing their energy
supply.
Effects on Bacterial Aggregation or Adherence: May
help in the aggregation of bacteria, facilitating their clearance.
Secretory IgA
Effects on Bacteria: Neutralizes bacterial toxins
and enzymes, reducing their pathogenicity.
Effects on Bacterial Aggregation or Adherence:
Binds to bacterial surfaces, preventing adherence to oral tissues.
Alpha Amylase
Effects on Bacteria: Produces glucose and maltose,
which can serve as energy sources for some bacteria.
Effects on Bacterial Aggregation or Adherence:
Indirectly promotes bacterial aggregation through the production of
glucans.
3. Factors Affecting Mineralization
Certain salivary proteins play a role in the mineralization process and the
maintenance of tooth enamel.
Histatins
Effects on Mineralization: Bind to hydroxyapatite,
aiding in the supersaturation of saliva, which is essential for
remineralization.
Effects on Bacteria: Some inhibition of mutans
streptococci, which are key contributors to caries.
Proline-rich Proteins
Effects on Mineralization: Bind to hydroxyapatite,
aiding in saliva supersaturation.
Effects on Bacteria: Promote adherence of some oral
bacteria.
Cystatins
Effects on Mineralization: Bind to hydroxyapatite,
aiding in saliva supersaturation.
Effects on Bacteria: Promote adherence of some oral
bacteria.
Statherin
Effects on Mineralization: Bind to hydroxyapatite,
aiding in saliva supersaturation.
Effects on Bacteria: Promote adherence of some oral
bacteria.
Mucins
Effects on Mineralization: Provide a physical and
chemical barrier in the enamel pellicle, protecting against
demineralization.
Effects on Bacteria: Facilitate aggregation and
clearance of oral bacteria.
Growth Spurts
PedodonticsGrowth Spurts in Children
Growth in children does not occur at a constant rate; instead, it is
characterized by periods of rapid increase known as growth spurts.
These spurts are significant phases in physical development and can vary in
timing and duration between individuals, particularly between boys and girls.
Growth Spurts: Sudden increases in growth that occur at
specific times during development. These spurts are crucial for overall
physical development and can impact various aspects of health and
well-being.
Timing of Growth Spurts
The timing of growth spurts can be categorized into several key periods:
Just Before Birth
Description: A significant growth phase occurs in
the fetus just prior to birth, where rapid growth prepares the infant
for life outside the womb.
One Year After Birth
Description: Infants experience a notable growth
spurt during their first year of life, characterized by rapid increases
in height and weight as they adapt to their new environment and begin to
develop motor skills.
Mixed Dentition Growth Spurt
Timing:
Boys: 8 to 11 years
Girls: 7 to 9 years
Description: This growth spurt coincides with the
transition from primary (baby) teeth to permanent teeth. It is a
critical period for dental development and can influence facial growth
and the alignment of teeth.
Adolescent Growth Spurt
Timing:
Boys: 14 to 16 years
Girls: 11 to 13 years
Description: This is one of the most significant
growth spurts, marking the onset of puberty. During this period, both
boys and girls experience rapid increases in height, weight, and muscle
mass, along with changes in body composition and secondary sexual
characteristics.
Respiratory Viral Diseases
General Pathology
Respiratory Viral Diseases
Respiratory viral infections cause acute local and systemic illnesses. The common cold, influenza, pharyngitis, laryngitis (including croup), and tracheobronchitis are common.
An acute, usually afebrile, viral infection of the respiratory tract, with inflammation in any or all airways, including the nose, paranasal sinuses, throat, larynx, and sometimes the trachea and bronchi.
Etiology and Epidemiology
Picornaviruses, especially rhinoviruses and certain echoviruses and coxsackieviruses, cause the common cold. About 30 to 50% of all colds are caused by one of the > 100 serotypes of rhinoviruses.
Symptoms and Signs
Clinical symptoms and signs are nonspecific.
After an incubation period of 24 to 72 h, onset is abrupt, with a burning sensation in the nose or throat, followed by sneezing, rhinorrhea, and malaise.
Characteristically, fever is not present, particularly with a rhinovirus or coronavirus. Pharyngitis usually develops early; laryngitis and tracheobronchitis vary by person and causative agent. Nasal secretions are watery and profuse during the first days, but become more mucoid and purulent.
Cough is usually mild but often lasts into the 2nd wk.
Antiplatelet Drugs
Pharmacology
Antiplatelet Drugs:
Whereas the anticoagulant drugs such as Warfarin and Heparin suppress the synthesis or activity of the clotting factors and are used to control venous thromboembolic disorders, the antithrombotic drugs suppress platelet function and are used primarily for arterial thrombotic disease. Platelet plugs form the bulk of arterial thrombi.
Acetylsalicylic acid (Aspirin)
• Inhibits release of ADP by platelets and their aggregation by acetylating the enzymes (cyclooxygenases or COX) of the platelet that synthesize the precursors of Thromboxane A2 that is a labile inducer of platelet aggregation and a potent vasoconstrictor.
• Low dose (160-320 mg) may be more effective in inhibiting Thromboxane A2 than PGI2 which has the opposite effect and is synthesized by the endothelium.
• The effect of aspirin is irreversible.