Mylohyoid Muscle

• Origin: Mylohyoid line of the mandible.
• Insertion: Median raphe and body of the hyoid bone.
• Nerve Supply: Nerve to mylohyoid (branch of the trigeminal nerve, CN V3).
• Arterial Supply: Sublingual branch of the lingual artery and submental branch of the facial artery.
• Action: Elevates the hyoid bone, base of the tongue, and floor of the mouth; depresses the mandible.

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

Muscles of the larynx

Extrinsic muscles
    suprahyoid: raise larynx, depress mandible for swallowing
    infrahyoid: lower larynx for swallowing
    both stabilize hyoid for tongue movements

The Medial Pterygoid Muscle

• This is a thick, quadrilateral muscle that also has two heads or origin.
• It embraces the inferior head of the lateral pterygoid muscle.
• It is located deep to the ramus of the mandible.

• Origin: deep head—medial surface of lateral pterygoid plate and pyramidal process of palatine bone, superficial head—tuberosity of maxilla.
• Insertion: medial surface of ramus of mandible, inferior to mandibular foramen.
• Innervation: mandibular nerve via medial pterygoid nerve.

• It helps to elevate the mandible and closes the jaws.
• Acting together, they help to protrude the mandible.
• Acting alone, it protrudes the side of the jaw.
• Acting alternately, they produce a grinding motion.

BONE

 A rigid form of CT, Consists of matrix and cells

 Matrix contains:

 organic component 35% collagen fibres

 inorganic salts 65% calcium phosphate (58,5%),  calcium carbonate (6,5%)

2 types of bone - spongy (concellous)

 compact (dense)

 Microscopic elements are the same

 Spongy bone consists of bars (trabeculae) which branch and unite to form a meshwork

 Spaces are filled with bone marrow

 Compact bone appears solid but has microscopic spaces

 In long bones the shaft is compact bone

 And the ends (epiphysis) consists of spongy bone covered with compact bone

Flat bones consists of 2 plates of compact bone with spongy bone in-between

 Periosteum covers the bone

 Endosteum lines marrow cavity and spaces

 These 2 layers play a role in the nutrition of bone tissue

 They constantly supply the bone with new osteoblasts for the repair and growth of bone

Microscopically

 The basic structural unit of bone is the Haversian system or osteon

 An osteon consists of a central Haversian canal

- In which lies vessels nerves and loose CT

- Around the central canal lies rings of lacunae

- A lacuna is a space in the matrix in which lies the osteocyte

- The lacunae are connected through canaliculi which radiate from the lacunae

- In the canaliculi are the processes of the osteocytes

- The canaliculi link up with one another and also with the Haversian canal

- The processes communicate with one another in the canaliculi through gap junctions

- Between two adjacent rows of lacunae lie the lamellae, 5-7µm thick

- In three dimensions the Haversian systems are cylindrical

- The collagen fibres lie in a spiral in the lamellae

- Perpendicular to the Haversian canals are the Volkman's canals

- They link up with the marrow cavity and the Haversian canals

- Some lamellae do not form part of a Haversian system

- They are the:

- Inner circumferential lamellae - around the marrow cavity

- Outer circumferential lamellae - underneath the outer surface of the bone

- Interstitial lamellae - between the osteons

Endosteum

Lines all cavities like marrow spaces, Haversian- and Volkman's canals

Consists of a single layer of squamous osteoprogenitor cells with a thin reticular CT layer underneath it

Continuous with the inner layer of periosteum

Covers the trabeculae of spongy bone

Cells differentiate into osteoblasts (like the cells of the periosteum)

Periosteum

 Formed by tough CT

 2 layers

Outer fibrous layer:  Thickest, Contains collagen fibres,

Some fibres enter the bone - called Sharpey's fibres

Contains blood vessels.

Also fibrocytes and the other cells found in common CT

Inner cellular layer

Flattened cells (continuous with the endosteum)

Can divide and differentiate into osteoprogenitor cells

spindle shaped

little amount of rough EPR

poorly developed Golgi complex

play a prominent role in bone growth and repair

Osteoblasts

Oval in shape, Have thin processes, Rough EPR in one part of the cell (basophilic)

On the other side is the nucleus, Golgi and the centrioles in the middle, Form matrix

Become trapped in the matrix

Osteocytes

Mature cells, Less basophilic than the osteoblasts, Lie trapped in the lacunae, Their processes lie in the canaliculi, Processes communicate with one another through gap junctions, Substances (nutrients, waste products) are passed on from cell to cell

Osteoclasts

 Very large,  Multinucleate (up to 50),  On inner and outer surface of bone,  Lie in depressions on the surface called Howships lacunae,  The cell surface facing the bone has short irregular processes

Acidophylic

 Has many lysosomes, polyribosomes and rough EPR

 Lysosomal enzymes are secreted to digest the bone

 Resorbs the organic part of bone

Histogenesis

Two types of bone development.

- intramembranous ossification

- endochondral ossification

In both these types of bone development temporary primary bone is deposited which is soon replaced by secondary bone. Primary bone has more osteocytes and the mineral content is lower.

->The sides and base of the skull are formed partly by these bones.
->Each bone consists of four morphologically distinct parts that fuse during development (squamous, petromastoid, and tympanic parts and the styloid process).
->The flat squamous part is external to the lateral surface of the temporal lobe of the brain.
->The petromastoid part encloses the internal ear and mastoid cells and forms part of the base of the skull.
->The tympanic part contains the bony passage from the auricle (external ear), called the external acoustic meatus. The petromastoid part also forms a portion of the bony wall of the tympanic cavity (middle ear). The meatus and tympanic cavity are concerned with the transmission of sound waves.
->The slender, pointed styloid process of the temporal bone gives attachment to certain ligaments and muscles (e.g., the stylohyoid muscle that elevates the hyoid bone).
->The temporal bone articulates at sutures with the parietal, occipital, sphenoid, and zygomatic bones.
->The zygomatic process of the temporal bone unites with the temporal process of the zygomatic bone to form the zygomatic arch. The zygomatic arches form the widest part of the face.
->The head of the mandible articulates with the mandibular fossa on the inferior surface of the zygomatic process of the temporal bone.
->Anterior to the mandibular fossa is the articular tubercle.
->Because the zygomatic arches are the widest parts of the face and are such prominent facial features, they are commonly fractured and depressed. A fracture of the temporal process of the zygomatic bone would likely involve the lateral wall of the orbit and could injure the eye.