The Parotid Glands

• The parotid glands are the largest of the three pairs of salivary glands.
• Each gland is wedged between the mandible and the sternocleidomastoid muscle and partly covers them.

• The parotid gland is wrapped with a fibrous capsule (parotid fascia) that is continuous with the deep investing fascia of the neck.

• Viewed superficially, the parotid gland is somewhat triangular in shape.
• Its apex is posterior to the angle of the mandible and its base is along the zygomatic arch.
• The parotid gland overlaps the posterior part of the masseter muscle.

• The parotid duct (Stensen's duct) is about 5 cm long and 5 mm in diameter.
• It passes horizontally from the anterior edge of the gland.
• At the anterior border of the masseter muscle, the parotid duct turns medially and pierces the buccinator muscle.
• It enters the oral cavity opposite the second maxillary molar.

Blood Vessels of the Parotid Gland

• This gland is supplied by branches of the external carotid artery.
• The veins from the parotid gland drains into the retromandibular vein, which enters the internal jugular vein.

Lymphatic Drainage of the Parotid Gland

• The lymph vessels of this gland end in the superficial and deep cervical lymph nodes.

Nerves of the Parotid Gland

• These nerves are derived from the auriculotemporal nerve and from the sympathetic and parasympathetic systems.

• The parasympathetic fibres are derived from the glossopharyngeal nerve (CN IX) through the otic ganglion.
• Stimulation of these fibres produces a thin watery (serous) saliva to flow from the parotid duct.

• The sympathetic fibres are derived from the cervical ganglia through the external carotid plexus.
• Stimulation of these fibres produces a thick mucous saliva.

Muscles of the Soft Palate

The Levator Veli Palatini (Levator Palati)

• Superior attachment: cartilage of the auditory tube and petrous part of temporal bone.
• Inferior attachment: palatine aponeurosis.
• Innervation: pharyngeal branch of vagus via pharyngeal plexus.

• This cylindrical muscle runs inferoanteriorly, spreading out in the soft palate, where it attaches to the superior surface of the palatine aponeurosis.

• It elevates the soft palate, drawing it superiorly and posteriorly.
• It also opens the auditory tube to equalise air pressure in the middle ear and pharynx.

The Tensor Veli Palatini (Tensor Palati)

• Superior attachment: scaphoid fossa of medial pterygoid plate, spine of sphenoid bone, and cartilage of auditory tube.
• Inferior attachment: palatine aponeurosis.
• Innervation: medial pterygoid nerve (a branch of the mandibular nerve).

• This thin, triangular muscle passes inferiorly, and hooks around the hamulus of the medial pterygoid plate.
• It then inserts into the palatine aponeurosis.

• This muscle tenses the soft palate by using the hamulus as a pulley.
• It also pulls the membranous portion of the auditory tube open to equalise air pressure of the middle ear and pharynx.

The Palatoglossus Muscle

• Superior attachment: palatine aponeurosis.
• Inferior attachment: side of tongue.
• Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.

• This muscle, covered by mucous membrane, forms the palatoglossal arch.

• The palatoglossus elevates the posterior part of the tongue and draws the soft palate inferiorly onto the tongue.

• Superior attachment: hard palatThe Palatopharyngeus Musclee and palatine aponeurosis.
• Inferior attachment: lateral wall of pharynx.
• Innervation: cranial part of accessory nerve (CN XI) through the pharyngeal branch of vagus (CN X) via the pharyngeal plexus.

• This thin, flat muscle is covered with mucous membrane to form the palatopharyngeal arch.
• It passes posteroinferiorly in this arch.

• This muscle tenses the soft palate and pulls the walls of the pharynx superiorly, anteriorly and medially during swallowing.

The Musculus Uvulae

• Superior attachment: posterior nasal spine and palatine aponeurosis.
• Inferior attachment: mucosa of uvula.
• Innervation: cranial part of accessory through the pharyngeal branch of vagus, via the pharyngeal plexus.

• It passes posteriorly on each side of the median plane and inserts into the mucosa of the uvula.
• When the muscle contracts, it shortens the uvula and pulls it superiorly.

3 basic functions
o    protection of respiratory tract during swallowing food/air pathways cross.
    epiglottis provides protection
o    control intra-thoracic pressure (in coughing) -    close off airway to build pressure then rapidly open to release stuff
o    production of sound (in speaking, singing, laughing)

Important structures

o    hyoid bone
o    thyroid cartilage
o    arytenoids cartilage: vocal and muscle process
    sits on slope on posterior side of cricoid - spin and slide
o    cricoid cartilage: signet ring
o    thyroepiglottic ligament

Membranes and ligaments

o    membrane: general; ligament: thickening of membrane
o    folds: free edges of membranes or ligaments
o    names: tell you where located

Important membranes:
    quandrangular/vestibular membrane—from epiglottis to arytenoids
•    inferior edge: false vocal fold
    thyrohyoid membrane
    conus elasticus = cricothyroid = cricovocal
•    superior/medial edge = vocal fold
•    vocal ligaments: true folds, top of cricothyroid membrane

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

The Nasopharynx

• The nasal part of the pharynx has a respiratory function.
• It lies superior to the soft palate and is a posterior extension of the nasal cavity.
• The nose opens into the nasopharynx via to large posterior apertures called choanae.

• The roof and posterior wall of the nasopharynx form a continuous surface that lies inferior to the body of the sphenoid bone and the basilar part of the occipital bone.
• In the mucous membrane of the roof of the posterior wall of the nasopharynx is a collection of lymphoid tissue, known as the pharyngeal tonsil (commonly known as the adenoids).

• The pharyngeal orifice of the auditory tube is on the lateral wall of the nasopharynx, 1 to 1.5 cm posterior to the inferior concha, and level with the superior border of the palate.
• The orifice is directed inferiorly and has a hood-like tubal elevation over it called the torus of the auditory tube or the torus tubarius (L. torus, swelling).

• Extending inferiorly from the torus is a vertical fold of mucous membrane, known as the salpingopharyngeal fold.

• The collection of lymphoid tissue in the submucosa of the pharynx, posterior to the orifice of the auditory tube, is known as the tubal tonsil.

• Posterior to the torus and the salpingopharyngeal fold, there is a slit-like lateral projection of the pharynx called the pharyngeal recess.
• It extends laterally and posteriorly.

Walls of the Tympanic Cavity or Middle Ear

• This cavity is shaped like a narrow six-sided box that has convex medial and lateral walls.
• It has the shape of the biconcave lens in cross-section (like a red blood cell).

The Roof or Tegmental Wall

• This is formed by a thin plate of bone, called the tegmen tympani (L. tegmen, roof).
• It separates the tympanic cavity from the dura on the floor of middle cranial fossa.
• The tegmen tympani also covers the aditus ad antrum.

The Floor or Jugular Wall

• This wall is thicker than the roof.
• It separates the tympanic cavity from the superior bulb of the internal jugular vein. The internal jugular vein and the internal carotid artery diverge at the floor of the tympanic cavity.

• The tympanic nerve, a branch of the glossopharyngeal nerve (CN IX), passes through an aperture in the floor of the tympanic cavity and its branches form the tympanic plexus.

The Lateral or Membranous Wall

• This is formed almost entirely by the tympanic membrane.
• Superiorly it is formed by the lateral bony wall of the epitympanic recess.
• The handle of the malleus is incorporated in the tympanic membrane, and its head extends into the epitympanic recess.

The Medial or Labyrinthine Wall

• This separates the middle ear from the membranous labyrinth (semicircular ducts and cochlear duct) encased in the bony labyrinth.
• The medial wall of the tympanic cavity exhibits several important features.

• Centrally, opposite the tympanic membrane, there is a rounded promontory (L. eminence) formed by the first turn of the cochlea.
• The tympanic plexus of nerves, lying on the promontory, is formed by fibres of the facial and glossopharyngeal nerves.

• The medial wall of the tympanic cavity also has two small apertures or windows.

• The fenestra vestibuli (oval window) is closed by the base of the stapes, which is bound to its margins by an annular ligament.
• Through this window, vibrations of the stapes are transmitted to the perilymph window within the bony labyrinth of the inner ear.

• The fenestra cochleae (round window) is inferior to the fenestra vestibuli.
• This is closed by a second tympanic membrane.

The Posterior or Mastoid Wall

• This wall has several openings in it.
• In its superior part is the aditus ad antrum (mastoid antrum), which leads posteriorly from the epitympanic recess to the mastoid cells.

• Inferiorly is a pinpoint aperture on the apex of a tiny, hollow projection of bone, called the pyramidal eminence (pyramid).
• This eminence contains the stapedius muscle.
• Its aperture transmits the tendon of the stapedius, which enters the tympanic cavity and inserts into the stapes.

• Lateral to the pyramid, there is an aperture through which the chorda tympani nerve, a branch of the facial nerve (CN VII), enters the tympanic cavity.

The Anterior Wall or Carotid Wall

• This wall is a narrow as the medial and lateral walls converge anteriorly.
• There are two openings in the anterior wall.

• The superior opening communicates with a canal occupied by the tensor tympani muscle.
• Its tendon inserts into the handle of the malleus and keeps the tympanic membrane tense.

• Inferiorly, the tympanic cavity communicates with the nasopharynx through the auditory tube.