NEET MDS Lessons
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
- Provides a rigid support system
- Protects delicate structures (e. g., the protection provided by the bones of the vertebral column to the spinal cord)
- Bones supply calcium to the blood; are involved In the formation of blood cells (hemopoiesis)
- Bones serve as the basis of attachment of muscles; form levers in the joint areas, aIlowing movement
Muscles acting on the Temporomandibular Joint
- Movements of the temporomandibular joint are chiefly from the action of the muscles of mastication.
- The temporalis, masseter, and medial pterygoid muscles produce biting movements.
- The lateral pterygoid muscles protrude the mandible with the help from the medial pterygoid muscles and retruded largely by the posterior fibres of the temporalis muscle.
- Gravity is sufficient to depress the mandible, but if there is resistance, the lateral pterygoid, suprahyoid and infrahyoid, mylohyoid and anterior digastric muscles are activated.
| Actions | Muscles | |||
| Depression (Open mouth) |
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| Elevation (Close mouth) |
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| Protrusion (Protrude chin) |
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| Retrusion (Retrude chin) |
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| Side-to-side movements (grinding and chewing) |
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BONE
A rigid form of CT, Consists of matrix and cells
Matrix contains:
organic component 35% collagen fibres
inorganic salts 65% calcium phosphate (58,5%), calcium carbonate (6,5%)
2 types of bone - spongy (concellous)
compact (dense)
Microscopic elements are the same
Spongy bone consists of bars (trabeculae) which branch and unite to form a meshwork
Spaces are filled with bone marrow
Compact bone appears solid but has microscopic spaces
In long bones the shaft is compact bone
And the ends (epiphysis) consists of spongy bone covered with compact bone
Flat bones consists of 2 plates of compact bone with spongy bone in-between
Periosteum covers the bone
Endosteum lines marrow cavity and spaces
These 2 layers play a role in the nutrition of bone tissue
They constantly supply the bone with new osteoblasts for the repair and growth of bone
Microscopically
The basic structural unit of bone is the Haversian system or osteon
An osteon consists of a central Haversian canal
- In which lies vessels nerves and loose CT
- Around the central canal lies rings of lacunae
- A lacuna is a space in the matrix in which lies the osteocyte
- The lacunae are connected through canaliculi which radiate from the lacunae
- In the canaliculi are the processes of the osteocytes
- The canaliculi link up with one another and also with the Haversian canal
- The processes communicate with one another in the canaliculi through gap junctions
- Between two adjacent rows of lacunae lie the lamellae, 5-7µm thick
- In three dimensions the Haversian systems are cylindrical
- The collagen fibres lie in a spiral in the lamellae
- Perpendicular to the Haversian canals are the Volkman's canals
- They link up with the marrow cavity and the Haversian canals
- Some lamellae do not form part of a Haversian system
- They are the:
- Inner circumferential lamellae - around the marrow cavity
- Outer circumferential lamellae - underneath the outer surface of the bone
- Interstitial lamellae - between the osteons
Endosteum
Lines all cavities like marrow spaces, Haversian- and Volkman's canals
Consists of a single layer of squamous osteoprogenitor cells with a thin reticular CT layer underneath it
Continuous with the inner layer of periosteum
Covers the trabeculae of spongy bone
Cells differentiate into osteoblasts (like the cells of the periosteum)
Periosteum
Formed by tough CT
2 layers
Outer fibrous layer: Thickest, Contains collagen fibres,
Some fibres enter the bone - called Sharpey's fibres
Contains blood vessels.
Also fibrocytes and the other cells found in common CT
Inner cellular layer
Flattened cells (continuous with the endosteum)
Can divide and differentiate into osteoprogenitor cells
spindle shaped
little amount of rough EPR
poorly developed Golgi complex
play a prominent role in bone growth and repair
Osteoblasts
Oval in shape, Have thin processes, Rough EPR in one part of the cell (basophilic)
On the other side is the nucleus, Golgi and the centrioles in the middle, Form matrix
Become trapped in the matrix
Osteocytes
Mature cells, Less basophilic than the osteoblasts, Lie trapped in the lacunae, Their processes lie in the canaliculi, Processes communicate with one another through gap junctions, Substances (nutrients, waste products) are passed on from cell to cell
Osteoclasts
Very large, Multinucleate (up to 50), On inner and outer surface of bone, Lie in depressions on the surface called Howships lacunae, The cell surface facing the bone has short irregular processes
Acidophylic
Has many lysosomes, polyribosomes and rough EPR
Lysosomal enzymes are secreted to digest the bone
Resorbs the organic part of bone
Histogenesis
Two types of bone development.
- intramembranous ossification
- endochondral ossification
In both these types of bone development temporary primary bone is deposited which is soon replaced by secondary bone. Primary bone has more osteocytes and the mineral content is lower.
Innervation of the Pharynx
- The motor and most of the sensory supply of the pharynx is derived from the pharyngeal plexus of nerves on the surface of the pharynx.
- The plexus is formed by pharyngeal branches of the vagus (CN X) and glossopharyngeal (CN IX) nerves, and by sympathetic branches for the superior cervical ganglion.
- The motor fibres in the pharyngeal plexus are derived from the cranial root of accessory nerve (CN XI), and are carried by the vagus nerve to all muscles of the pharynx and soft palate.
- The exceptions are stylopharyngeus (supplied by CN IX) and the tensor veli palatini (supplied by CN V3).
The Oropharynx
- The oral part of the pharynx has a digestive function.
- It is continuous with the oral cavity through the oropharyngeal isthmus.
- The oropharynx is bounded by the soft palate superiorly, the base of the tongue inferiorly, and the palatoglossal and palatopharyngeal arches laterally.
- It extends from the soft palate to the superior border of the epiglottis.
The Palatine Tonsils
- These are usually referred to as "the tonsils".
- They are collections of lymphoid tissue the lie on each side of the oropharynx in the triangular interval between the palatine arches.
- The palatine tonsils vary in size from person to person.
- In children, the palatine tonsils tend to be large, whereas in older persons they are usual small and inconspicuous.
- The visible part of the tonsil is no guide to its actual size because much of it may be hidden by the tongue and buried in the soft palate.
The Hard Palate
- The anterior bony part of the palate is formed by the palatine process of the maxillae and the horizontal plates of the palatine bones.
- Anteriorly and laterally, the hard palate is bounded by the alveolar processes and the gingivae.
- Posteriorly, the hard palate is continuous with the soft palate.
- The incisive foramen is the mouth of the incisive canal.
- This foramen is located posterior to the maxillary central incisor teeth.
- This foramen is the common opening for the right and left incisive canals.
- The incisive canal and foramen transmit the nasopalatine nerve and the terminal branches of the sphenopalatine artery.
- Medial to the third molar tooth, the greater palatine foramen pierces the lateral border of the bony palate.
- The greater palatine vessels and nerve emerge from this foramen and run anteriorly into two grooves on the palate.
- The lesser palatine foramen transmits the lesser palatine nerve and vessels.
- This runs to the soft palate and adjacent structures.