NEET MDS Lessons
Anatomy
- The forehead is formed by the smooth, broad, convex plate of bone called the frontal squama.
- In foetal skulls, the halves of the frontal squama are divided by a metopic suture.
- In most people, the halves of the frontal bone begin to fuse during infancy and the suture between is usually not visible after 6 years of age.
- The frontal bone forms the thin roof of the orbits (eye sockets).
- Just superior to and parallel with each supraorbital margin is a bony ridge, the superciliary arch, which overlies the frontal sinus. This arch is more pronounced in males.
- Between these arches there is a gently, rounded, medial elevation called the gabella; this term derives from the Latin word glabellus meaning smooth and hairless. In most people, the skin over the gabella is hairless.
-The slight prominences of the forehead on each side, superior to the superciliary arches, are called frontal eminences (tubers).
- The supraorbital foramen (occasionally a notch), which transmits the supraorbital vessels and nerve is located in the medial part of the supraorbital margin.
- The frontal bone articulates with the two parietal bones at the coronal suture.
-It also articulates with the nasal bones at the frontonasal suture. At the point where this suture crosses the internasal suture in the medial plane, there is an anthropological landmark called the nasion . The depression is located at the root of the nose, where it joins the cranium.
- The frontal bone also articulates with the zygomatic, lacrimal, ethmoid, and sphenoid bones.
In about 8% of adult skulls, a remnant of the inferior part of the metopic (interfrontal) suture is visible. It may be mistaken in radiographs for a fracture line by inexperienced observers.
- The superciliary arches are relatively sharp ridges of bone and a blow to them may lacerate the skin and cause bleeding.
- Bruising of the skin over a superciliary arch causes tissue fluid and blood to accumulate in the surrounding connective tissue, which gravitates into the upper eyelid and around the eye. This results in swelling and a "black eye".
- Compression of the supraorbital nerve as it emerges from its foramen causes considerable pain, a fact that may be used by anaesthesiologists and anaesthetists to determine the depth of anaesthesia and by physicians attempting to arouse a moribund patient.
Stylohyoid Muscle
- Origin: Posterior border of the styloid process of the temporal bone.
- Insertion: Body of the hyoid bone at the junction with the greater horn.
- Nerve Supply: Facial nerve (CN VII).
- Arterial Supply: Muscular branches of the facial artery and muscular branches of the occipital artery.
- Action: Elevates the hyoid bone and base of the tongue.
Cardiac Muscle
Fibres anastomose through cross bridges
Fibres are short, connected end to end at intercalated discs, also striated, contract automatically
Light microscopic Structure:
Short fibres connected at intercalated disks, 85 - 100 µm long, 15 µm
same bands as in skeletal muscle, 1 or 2 nuclei - oval and central, in perinuclear area is a sarcoplasmic reticulum, intercalated discs lie at the Z line
Electron microscopic structure:
Between myofibrils lie the mitochondria, 2,5 µm long mitochondria, dense cristae
and are as long as the sarcomere, fibres have more glycogen than skeletal muscle fibres
myofilaments, actin and myosin are the same as in skeletal muscle, the sarcoplasmic reticulum differs in that there is no terminal sisterna. The sarcotubules end in little feet that
sit on the T-tubule
Intercalated Disc:
on Z lines, fibres interdigitate,
3 types of junctions in the disc
Transverse Part:
zonula adherens
desmosomes
Lateral Part:
Gap junctions (nexus) - for impulse transfer
Mechanism of Contraction:
slide - ratchet like in skeletal muscle, certain fibres are modified for conduction, Impulses spread from cell to cell through gap junctions, Purkinje cells are found in the AV bundle
they have less myofibrils, lots of glycogen and intercalated discs
Connective tissue coverings:
Only endomycium in cardiac muscle, Blood vessels, lymph vessels and nerves lie in the endomycium
Nerve Supply of the Muscles of the Orbit (pp. 715-6)
- Three cranial nerves supply the muscles of the eyeball; the oculomotor (CN III), trochlear (CN IV) and abducent (CN IV) nerves.
- All three enter the orbit via the superior orbital fissure.
- The trochlear nerve supplies the superior oblique muscle.
- The abducent nerve supplies the lateral rectus muscle.
- The oculomotor nerve supplies everything else.
- A mnemonic that is used is this formula for this strange sulfate: SO4(LR6)3
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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The skull, the skeleton of the head, is the most complex bony structure in the body because it:
- Encloses the brain, which is irregular in shape;
- Houses the organs of special senses for seeing, hearing, tasting, and smelling; and
- Surrounds the openings in to the digestive and respiratory tracts.
- In the anatomical position, the skull is oriented so that the inferior margin of the orbit (eye socket) and the superior margin of the external acoustic meatus (auditory canal) are horizontal. This is called the orbitomenial plane (Frankfort plane).
- The term cranium (L. skull) is sometimes used when referring to the skull without the mandible (lower jaw), but the cranium is often used when referring to the part of the skull containing the brain.
- The superior part is the box-like structure called the calvaria (cranial vault, brain case); the remainder of the cranium, including the maxilla (upper jaw), orbits (eyeball sockets) and nasal cavities, forms the facial skeleton.
- The term skullcap (calotte) refers to the superior part of the calvaria, which is removed during autopsies and dissections. The inferior aspect of the cranium is called the cranial base.
The Auditory Tube
- This is a funnel-shaped tube connecting the nasopharynx to the tympanic cavity.
- Its wide end is towards the nasopharynx, where it opens posterior to the inferior meatus of the nasal cavity.
- The auditory tube is 3.5 to 4 cm long; its posterior 1/3 is bony and the other 2/3 is cartilaginous.
- It bony part lies in a groove on the inferior aspect of the base of the skull, between the petrous part of the temporal bone and the greater wing of the sphenoid bone.
- The function of the auditory tube is to equalise pressure of the middle ear with atmospheric pressure.