EPITHELIUMS

Epithelial Tissue Epithelial tissue covers surfaces, usually has a basement membrane, has little extracellular material, and has no blood vessels. A basement membrane attaches the epithelial cells to underlying tissues. Most epithelia have a free surface, which is not in contact with other cells. Epithelia are classified according to the number of cell layers and the shape of the cells.

• Epitheliums contain no blood vessels.  There is normally an underlying layer of connective tissue
• Almost all epitheliums lie on a basement membrane.The basement membrane consists of  a basal lamina and  reticular lamina. The reticular lamina is connected to the basal lamina by anchoring fibrils. The reticular lamina may be absent in which case the basement membrane consist only of a basal lamina. The basal lamina consists of a   - lamina densa in the middle (physical barrier) with a lamina lucida on both sides (+charge barrier),The basement membrane is absent in ependymal cells.The basement membrane is not continuous in sinusoidal capillaries.

• Epitheliums always line or cover something
• Epithelial cells lie close together with little intercellular space
• Epithelial cells are strongly connected to one another especially those epitheliums that are subjected to mechanical forces.  

Functions of Epithelium:

→ Simple epithelium involved with diffusion, filtration, secretion, or absorption

→ Stratified epithelium protects from abrasion

→ Squamous cells function in diffusion or filtration

• The tongue is divided into halves by a medial fibrous lingual septum that lies deep to the medial groove.
• In each half of the tongue there are four extrinsic and four intrinsic muscles.

• The lingual muscles are all supplied by the hypoglossal nerve (CN XII).
• The only exception is palatoglossus, which is supplied by the pharyngeal branch of the vagus nerve, via the pharyngeal plexus.

Muscles of the Pharynx

• This consists of three constrictor muscles and three muscles that descend from the styloid process, the cartilaginous part of the auditory tube and the soft palate.

External Muscles of the Pharynx 

• The paired superior, middle, and inferior constrictor muscles form the external circular part of the muscular layer of the wall.
• These muscles overlap each other and are arranged so that the superior one is innermost and the inferior one is outermost.

• These muscles contract involuntarily in a way that results in contraction taking place sequentially from the superior to inferior end of the pharynx.
• This action propels food into the oesophagus.

• All three constrictors of the pharynx are supplied by the pharyngeal plexus of nerves, which lies on the lateral wall of the pharynx, mainly on the middle constrictor of the pharynx.
• This plexus is formed by pharyngeal branches of the glossopharyngeal (CN IX) and vagus (CN X) nerves.

The Superior Constrictor Muscle

• Origin: pterygoid hamulus, pterygomandibular raphe, posterior end of the mylohyoid line of the mandible, and side of tongue.
• Insertion: median raphe of pharynx and pharyngeal tubercle.
• Innervation: though the pharyngeal plexus of nerves.

• The pterygomandibular raphe is the fibrous line of junction between the buccinator and superior constrictor muscles.

The Middle Constrictor Muscle

• Origin: stylohyoid ligament and greater and lesser horns of hyoid bone.
• Insertion: median raphe of pharynx.
• Innervation: through the pharyngeal plexus of nerves.

The Inferior Constrictor Muscle

• Origin: oblique line of thyroid cartilage and side of cricoid cartilage.
• Insertion: median raphe of pharynx.
• Innervation: through the pharyngeal plexus of nerves.

• The fibres arising from the cricoid cartilage are believed to act as a sphincter, preventing air from entering the oesophagus. 

Gaps in the Pharyngeal Musculature

• The overlapping arrangement of the three constrictor muscles leaves 4 deficiencies or gaps in the pharyngeal musculature.
• Various structures enter and leave the pharynx through these gaps.

• Superior to the superior constrictor muscle, the levator veli palatini muscle, the auditory tube, and the ascending palatine artery pass through a gap between the superior constrictor muscle and the skull.
• Superior to the superior border of the superior constrictor, the pharyngobasilar fascia blends with the buccopharyngeal fascia to form, with the mucous membrane, the thin wall of the pharyngeal recess.

• Between the superior and middle constrictor muscles, the gateway to the mouth, though which pass the stylopharyngeus muscle, the glossopharyngeal nerve (CN IX), and the stylohyoid ligament.

• Between the middle and inferior constrictor muscles, the internal laryngeal nerve and the superior laryngeal artery and vein pass to the larynx.

• Inferior to the inferior constrictor muscles, the recurrent laryngeal nerve and inferior laryngeal artery pass superiorly into the larynx.

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.

• This is the posterior curtain-like part, and has no bony support. It does, however, contain a membranous aponeurosis.
• The soft palate, or velum palatinum (L. velum, veil), is a movable, fibromuscular fold that is attached to the posterior edge of the hard palate.
• It extends posteroinferiorly to a curved free margin from which hangs a conical process, the uvula (L. uva, grape).

• The soft palate separates the nasopharynx superiorly and the oropharynx inferiorly.

• During swallowing the soft palate moves posteriorly against the wall of the pharynx, preventing the regurgitation of food into the nasal cavity.

• Laterally, the soft palate is continuous with the wall of the pharynx and is joined to the tongue and pharynx by the palatoglossal and palatopharyngeal folds.

• The soft palate is strengthened by the palatine aponeurosis, formed by the expanded tendon of the tensor veli palatini muscle.
• This aponeurosis attaches to the posterior margin of the hard palate.