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.

-> This is a wedge-shaped bone (G. sphen, wedge) is located anteriorly to the temporal bones.
-> It is a key bone in the cranium because it articulates with eight bones (frontal, parietal, temporal, occipital, vomer, zygomatic, palatine, and ethmoid).
-> It main parts are the body and the greater and lesser wings, which spread laterally from the body.
-> The superior surface of its body is shaped like a Turkish saddle (L. sella, a saddle); hence its name sella turcica.
-> It forms the hypophyseal fossa which contains the hypophysis cerebri or pituitary gland.
-> The sella turcica is bounded posteriorly by the dorsum sellae, a square plate of bone that projects superiorly and has a posterior clinoid process on each side.
-> Inside the body of the sphenoid bone, there are right and left sphenoid sinuses. The floor of the sella turcica forms the roof of these paranasal sinuses.
-> Studies of the sella turcica and hypophyseal fossa in radiographs or by other imaging techniques are important because they may reflect pathological changes such as a pituitary tumour or an aneurysm of the internal carotid artery. Decalcification of the dorsum sellae is one of the signs of a generalised increase in intracranial pressure.

​​​​​​​The Tongue

• The tongue (L. lingua; G. glossa) is a highly mobile muscular organ that can vary greatly in shape.
• It consists of three parts, a root, body, and tip.
• The tongue is concerned with mastication, taste, deglutition (swallowing), articulation (speech), and oral cleansing.
• Its main functions are squeezing food into the pharynx when swallowing, and forming words during speech.

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 Pharynx

• The pharynx is the continuation of the digestive system from the oral cavity.
• It is a funnel-shaped fibromuscular tube that is the common route for both food and air.
• The pharynx is located posterior to the nasal and oral cavities, and the larynx.

• For the convenience of description, the pharynx is divided into three parts: (1) the nasopharynx, posterior to the nose and superior to the soft palate; (2) the oropharynx, posterior to the mouth; and (3) the laryngopharynx, posterior to the larynx.

• The pharynx is about 15 cm long.
• It extends from the base of the skull to the inferior border of the cricoid cartilage anteriorly, and to the inferior border of C6 vertebra posteriorly.
• It is widest (about 5 cm) opposite the hyoid bone and narrowest (about 1.5) at its inferior end, where it is continuous with the oesophagus.

• The posterior wall of the pharynx lies against the prevertebral fascia, with the potential retropharyngeal space between them.

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