NEET MDS Lessons
Anatomy
Levator Palpebrae Superioris Muscles
- This is a thin, triangular muscle that elevates the upper eyelid.
- It is continuously active except during sleeping and when the eye is closing.
- Origin: roof of orbit, anterior to the optic canal.
- Insertion: this muscle fans out into a wide aponeurosis that inserts into the skin of the upper eyelid. The inferior part of the aponeurosis contains some smooth muscle fibres that insert into the tarsal plate.
- Innervation: the superior fibres are innervated by the oculomotor nerve (CN III), and the smooth muscle component is innervated by fibres of the cervical sympathetic trunk and the internal carotid plexus.
Illnesses involving the Levator Palpebrae Superioris
- In third nerve palsy, the upper eyelid droops (ptosis) and cannot be raised voluntarily.
- This results from damage to the oculomotor nerve (CN III), which supplies this muscle.
- If the cervical sympathetic trunk is interrupted, the smooth muscle component of the levator palpebrae superioris is paralysed and also causes ptosis.
- This is part of Horner's syndrome.
The Rectus Muscles
- There are four rectus muscles (L. rectus, straight), superior, inferior, medial and lateral.
- These arise from a tough tendinous cuff, called the common tendinous ring, which surrounds the optic canal and the junction of the superior and inferior orbital fissures.
- From their common origin, these muscles run anteriorly, close to the walls of the orbit, and attach to the eyeball just posterior to the sclerocorneal junction.
- The medial and lateral rectus muscles attach to the medial and lateral sides of the eyeball respectively, on the horizontal axis.
- However, the superior rectus attaches to the anterosuperior aspect of the medial side of the eyeball while the inferior rectus attaches to the anteroinferior aspect of the medial side of the eye.
The Oblique Muscles
The Superior Oblique Muscle
- This muscle arises from the body of the sphenoid bone, superomedial to the common tendinous ring.
- It passes anteriorly, superior and medial to the superior and medial rectus muscles.
- It ends as a round tendon that runs through a pulley-like loop called the trochlea (L. pulley).
- After passing though the trochlea, the tendon of the superior oblique turns posterolaterally and inserts into the sclera at the posterosuperior aspect of the lateral side of the eyeball.
The Inferior Oblique Muscle
- This muscle arises from the maxilla in the floor of the orbit.
- It passes laterally and posteriorly, inferior to the inferior rectus muscle.
- It inserts into the sclera at the posteroinferior aspect of the lateral side of the eyeball.
Veins of the Face
The Supratrochlear Vein
- This vessel begins on the forehead from a network of veins connected to the frontal tributaries of the superficial temporal vein.
- It descends near the medial plane with its fellow on the other side.
- These veins diverge near the orbits, each joining a supraorbital vein to form the facial vein near the medial canthus (angle of the eye).
The Supraorbital Vein
- This vessel begins near the zygomatic process of the temporal bone.
- It joins the tributaries of the superficial and middle temporal veins.
- It passes medially and joins the supratrochlear vein to form the facial vein near the medial canthus.
The Facial Vein
- This vein provides the major venous drainage of the face.
- It begins at the medial canthus of the eye by the union of the supraorbital and supratrochlear veins.
- It runs inferoposteriorly through the face, posterior to the facial artery, but takes a more superficial and straighter course than the artery.
- Inferior to the margin of the mandible, the facial vein is joined by the anterior branch of the retromandibular vein.
- The facial veins ends by draining into the internal jugular vein.
The Superficial Temporal Vein
- This vein drains the forehead and scalp and receives tributaries from the veins of the temple and face.
- In the region of the temporomandibular joint, this vein enters the parotid gland.
The Retromandibular Vein
- The union of the superficial temporal and maxillary veins forms this vessel, posterior to the neck of the mandible.
- It descends within the parotid gland, superficial to the external carotid artery but deep to the facial nerve.
- It divides into an anterior branch that unites with the facial vein, and a posterior branch that joins the posterior auricular vein to form the external jugular vein.
The Temporalis Muscle
- This is an extensive fan-shaped muscle that covers the temporal region.
- It is a powerful masticatory muscle that can easily be seen and felt during closure of the mandible.
- Origin: floor of temporal fossa and deep surface of temporal fascia.
- Insertion: tip and medial surface of coronoid process and anterior border of ramus of mandible.
- Innervation: deep temporal branches of mandibular nerve (CN V3).
- The temporalis elevates the mandible, closing the jaws; and its posterior fibres retrude the mandible after protrusion.
Initially, four clefts exist; however, only one gives rise to a definite structure in adults.
|
1st pharyngeal cleft |
Penetrates underlying mesenchyme and forms EAM. The bottom of EAM forms lateral aspect of tympanic cavity. |
|
2nd pharyngeal cleft |
Undergoes active proliferation and overlaps remaining clefts. It merges with ectoderm of lower neck such that the remaining clefts lose contact with outside. Temporarily, the clefts form an ectodermally lined cavity, the cervical sinus, but this disappears during development. |
- The six muscles rotate the eyeball in the orbit around three axes (sagittal, horizontal and vertical).
- The action of the muscles can be deduced by their site of insertion on the eyeball.
| Muscle | Action(s) on the Eyeball | Nerve Supply |
| Medial Rectus | Adducts | CN III |
| Lateral Rectus | Abducts | CN VI |
| Superior Rectus | Elevates, adducts, and medially rotates | CN III |
| Inferior Rectus | Depresses, adducts, and laterally rotates | CN III |
| Superior Oblique | Depresses, abducts, and medially rotates | CN IV |
| Inferior Oblique | Elevates, abducts, and laterally rotates | CN III |
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
Digastric Muscle
- Origin:
- Anterior Belly: Digastric fossa of the mandible.
- Posterior Belly: Mastoid notch of the temporal bone.
- Insertion: Intermediate tendon attached to the body of the hyoid bone.
- Nerve Supply:
- Anterior Belly: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
- Posterior Belly: Facial nerve (CN VII).
- Arterial Supply:
- Anterior Belly: Branch of the submental artery.
- Posterior Belly: Muscular branch of the posterior auricular artery and occipital artery.
- Action: Raises the hyoid bone and base of the tongue, steadies the hyoid bone, and opens the mouth by lowering the mandible.