- Ectodermal thickening in the dorsum of digits - Nail fields, surrounded by nail folds

Module 2B Flashcards by Mary Rose Carvajal

Dual Origins of the Skin

Epidermis(superficial layer) –develops from surface ectoderm

Dermis(deep layer) –develops from the underlying mesenchyme

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Initially, the embryo is covered by a single layer of ectodermal cells. In the beginning of the second month, this epithelium divides, and a layer of flattened cells,__, is laid down on the surface.

the periderm, or epitrichium

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With further proliferation of cells in the basal layer during the second month, a third, ___ is formed

intermediate zone

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Finally, at the end of the fourth month, the epidermis acquires its definitive arrangement, and four layers can be distinguished, namely:

BASAL LAYER, OR GERMINATIVE LAYER - is responsible for production of new cells. This layer later forms ridges and hollows, which are reflected on the surface of the skin in the fingerprint.

SPINOUS LAYER - consists of large polyhedral cells containing fine tonofibrils.

GRANULAR LAYER - contains small keratohyalin granules in its cells.

HORNY LAYER - forming the tough scalelike surface of the epidermis, is made up of closely packed dead cells containing keratin.

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True or False

Cells of the periderm are usually cast off during the second part of intrauterine life and can be found in the amniotic fluid

True

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During the first 3 months of development, the epidermis

is invaded by cells arising from the neural crest. These cells synthesize melanin pigment in __

melanosomes

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Dermis is derived from mesenchyme that has three sources:

Lateral plate mesoderm - supplying cells for dermis in the limbs and body wall
Paraxial mesoderm - supplying cells for dermis in the back
Neural crest cells - supplying cells for dermis in the face and neck

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During the third and fourth months, the corium forms many irregular papillary structures called __ which contain a capillary and sensory nerve organ

dermal papillae

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a deeper layer of the dermis which contains a large amount of fatty tissue

Subcorium

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a whitish paste formed by secretion from sebaceous glands and degenerated epidermal cells and hairs
it protects the skin against the macerating action of amniotic fluid

vernix caseosa

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Hairs begin development as solid epidermal proliferations from the germinative layer that penetrates the underlying dermis. At their terminal ends, ___ invaginate.

hair buds

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are rapidly filled with mesoderm in which vessels and nerve endings develop

hair papillae

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cells in the center of the hair buds become spindle-shaped and keratinized, forming the hair shaft, while peripheral cells become cuboidal, giving rise to the __

epithelial hair sheath

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The ___ is formed by the surrounding mesenchyme. A small smooth muscle (arrector pili), also derived from mesenchyme, is usually attached to the dermal root sheath.

dermal root sheath

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Continuous proliferation of epithelial cells at the base of the
shaft pushes the hair upward, and by the end of the third month, the first hairs appear on the surface in the region of the __.

eyebrow and upper lip

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The first hair that appears, __, is shed at about the time of birth and is later replaced by coarser hairs arising from new hair follicles.

lanugo hair

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Cells from small bud at epithelial wall of hair follicle form the __

sebaceous glands

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Cells from the central region of the gland degenerate, forming a fat-like substance __ secreted into the hair
follicle, and from there, it reaches the skin.

sebum

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Two types of Sweat Glands

Eccrine - Buds from germinative layer of the epidermis

2. Apocrine - From same epidermal buds that produce hair follicles

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Sweat glands and mammary glands develop from __

epidermal proliferations

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Mammary glands are modified sweat glands and first appear as bilateral bands of thickened epidermis called __

mammary lines or mammary ridges

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Development of Nails

Ectodermal thickening in the dorsum of digits

- Nail fields, surrounded by nail folds

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Largest organ
15-20%
Integument, cutaneous covering

Skin

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Functions of the Skin

Protection
Excretion
Sensory
Thermoregulation
Metabolic
Sexual signaling

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the skin is composed of:

epidermis - an epithelial layer of ectodermal origin | dermis - a layer of mesodermal connective tissue

At the irregular junction between the dermis and epidermis, projections called __ interdigitate with invaginating epidermal ridges to strengthen adhesion of the two layers.

dermal papillae

Beneath the dermis lies the ___ a loose connective tissue layer usually containing pads of adipocytes.

subcutaneous tissue or hypodermis

- consists mainly of a stratified squamous keratinized epithelium composed of cells called keratinocytes. - forms the major distinction between thick skin and thin skin - Germinal basal layer mitosis

Epidermis

There are also three much less abundant epidermal cell | types:

pigment-producing melanocytes antigen-presenting Langerhans cells tactile epithelial cells called Merkel cells

Rate of mitosis/ Rate of desquamation

15-30 days, 25 to 50 days

epidermis consists of four layers of keratinocytes or five layers in thick skin namely

1. Stratum Basale 2. Stratum Spinosum 3. Stratum granulosum 4. Stratum lucidum 5. Stratum corneum

- is a single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction - Hemidesmosomes in the basal cell membranes join these cells to the basal lamina, and desmosomes bind the cells of this layer together in their lateral and upper surfaces - is characterized by intense mitotic activity and contains, along with the deepest part of the next layer, progenitor cells for all the epidermal layers - basal stem cells for keratinocytes found here - Nucleus: large; Cytoplasm: basophilic

basal layer (stratum basale)

An important feature of all keratinocytes in the stratum basale is the cytoskeletal __, intermediate filaments about 10 nm in diameter

keratins

- is normally the thickest layer, especially in the epidermal ridges - Large, cuboidal to polygonal, some flattened cells - Numerous cytoplasmic prickles bound by desmosomes to adjacent cells - Central nucleus - Cytoplasmic basophilia - Cytokeratin (synthetic fibrillarprotein) aggregates to form tonofibrils (intracellular fibrils) - Tonofibrils converge upon desmosomes

Stratum spinosum

- 2-3 rows of flattened cells - Nucleus: central - Cytoplasm filled with numerous “keratohyalin”, coarse basophilic granules

Stratum granulosum

- found only in thick skin - consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes - Nuclei and organelles have been lost, and the cytoplasm consists almost exclusively of packed keratin filaments embedded in an electron-dense matrix - Few layers of compacted highly refractile eosinophiliccells - Wavy clear strip - Cytoplasm contain dense packed filaments in a matrix called “Eleidin” - Desmosomes are still seen

stratum lucidum

- consists of 15 to 20 layers of squamous, keratinized cells filled with birefringent filamentous keratins. Keratin filaments contain at least six different polypeptides with molecular - Dead and dying cells - Flat and cornified cells; anucleated - Filled with mature keratin - Squamous plates or scales fused together - Plates are remnants of cells that contain keratin

stratum corneum

- Deeper layers contain cornified cells with desmosomal junctions, orderly pattern of intracellular keratin - Most superficial layer slough off or “desquamates” - Desquamation: preceded by disruption of desmososmes and internal structure of cells

stratum corneum

Structural changes in keratinization involve the following:

* Aggregation and arrangement of filaments * Formation of keratohyaline granules * Loss of organelles as a result of accumulation of granules

Cells of the Epidermis

Keratinocytes Melanocytes Langerhans cells Merkel cells

- Principal cells/ parenchyma | - Synthesize keratin: a family of polypeptides with 40,000 to 70,000 m.w.

Keratinocytes

- Synthesis and release of the brown pigment “melanin” - exists in various forms from yellowish brown to black and has a protective function against ultraviolet light - Located scattered infrequently in the basal layer and junction of dermis - Round cells with pale-staining cytoplasm - From cell body –long cytoplasmic processes, run in spaces between keratinocytes of stratum spinosum - Contain tyrosinase for melanin synthesis

Melanocytes

- antigen-presenting cells (APCs) which are usually most clearly seen in the spinous layer - represent 2% to 8% of the epidermal cells - Cytoplasmic processes extend from these dendritic cells between keratinocytes of all the layers, forming a fairly dense network in the epidermis - bind, process, and present antigens to T lymphocytes in the same manner as immune dendritic cells in other organs - Birbeck granules: rod-like with regular cross-striations, one end distends in a vesicle (tennis racket)

Langerhans cells

- are sensitive mechanoreceptors essential for light touch sensation - Joined by desmosomes to keratinocytes of the basal epidermal layer - resemble the surrounding cells but with few, if any, melanosomes. - are abundant in highly sensitive skin like that of fingertips and at the bases of some hair follicles - originate from the same stem cells as keratinocytes - Presence of dense cored vesicles in the cytoplasm - Considered paraneuronsinvolved in sensory reception

Merkel Cells

- is the layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis) - thickness varies with the region of the body and reaches its maximum of 4 mm on the back - is very irregular and has many projections (dermal papillae) that interdigitate with projections of the epidermis

dermis

- - an important constituent of both zones - Black against red stained collagen - Long and thick in reticular, and follow course of collagen fibers - Very fine, scanty, scarcely stained in papillary

Elastin

Cellular components of Dermis

- Fibroblasts - Lymphocytes - Mast cells - Tissue macrophages

The dermis contains two sublayers with indistinct boundaries

papillary layer | reticular layer

- thin layer which includes the dermal papillae - consists of loose connective tissue, with types I and III collagen fibers, fibroblasts and scattered mast cells, macrophages, and other leukocytes - From this layer, anchoring fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis - Contains venules, Arterioles, Capillary loops, Lymphatics and Fine nerve twigs from Meissner’s corpuscles (tactile)

papillary layer

- is much thicker, consists of dense irregular connective tissue (mainly bundles of type I collagen), with more fibers and fewer cells than the papillary layer - A network of elastic fibers is also present, providing elasticity to the skin - between the collagen and elastic fibers are abundant proteoglycans rich in dermatan sulfate - Blood vessels are larger and deeper - Cells are mainly fibroblasts and histiocytes

reticular layer

- consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them -also called the hypodermis or superficial fascia, contains adipocytes that vary in number in different body regions and vary in size according to nutritional state - has extensive vascular supply which promotes rapid uptake of insulin or drugs injected into this tissue

subcutaneous layer

Skin Appendages

- Hairs - Sebaceous glands - Sweat glands - Nails

- Highly modified keratinized structures - Produced by hair follicles - Hair bulb: a terminal expansion of the follicle where hair growth takes place - Dermal papilla: vascular tissue - Arrector pili muscle

Hairs

- One or more associated with a hair follicle - Secrete an oily substance called sebum - Embedded in the dermis except in areas lacking hairs - Holocrine - Mucous secretion - Closely applied to hair follicles into which they drain - Lubricates the epidermis and hair. - Simple branched alveolar glands

Sebaceous glands

Relationship between hair follicle, sebaceous gland, and arrector pili:

- Smooth muscle form arrectorpilimuscle; obliquely placed in relation to epidermis - Contraction of muscle = erection of hair shaft (and follicle), producing depressions in the skin (orange peel appearance) known as gooseflesh - Compression of sebaceousglandsaids in emptying glands into hair follicle, oiling the hair shaft - Sebum (mixture of triglycerides, waxes, squalene, and cholesterol and its esters and remnants of degenerating/dead cells).

- Act as an excretory organ by eliminating metabolic waste products. - Secrete watery fluid, ammonia, sodium, chloride, urea, and uric acid by merocrine process - Discharge directly into the skin surface - Sudoriferous and simple, coiled tubular glands - Important in thermoregulation - React by secreting in stressful situations

Sweat glands

- Secretory portions: Single layer of large cuboidal or columnar cells - Excretory ducts: Two layers of smaller cuboidal cells; Narrower lumen - Myoepithelial cells: Between secretory cells and basement membrane Contraction expels sweat into ducts

Eccrine sweat glands

- Activated by adrenergic nerves; secretion increased by emotional stress. - Secretory portion Coiled tubular type Widely dilated lumen Low cuboidal cells Eosinophilic cytoplasm

Apocrine sweat glands

- A highly specialized appendage - Consists of dense keratinized plate: Nail plate - Rests on a stratified squamous epithelium: Nail bed - Proximal end of the nail: Nail root

Nails

Composition of Nails

Nail matrix:underlying nail root; nail growth occurs Lunula:white crescent shape at base of nail Nail fold:skin overlying root of nail Eponychium:highly keratinized free edge Hyponychium:skin beneath free end of nail

- keratocytes are typically produced and differentiate at accelerated rates, causing at least slight thickening of the epidermal layers and increased keratinization and desquamation - caused by overactive T lymphocytes that trigger an autoimmune reaction in the skin, which can also lead to inflammation with redness, irritation, itching, and scaling, with a defective skin barrier

psoriasis

- involves skin depigmentation, often only in affected patches, due to the loss or decreased activity of melanocytes

vitiligo

Melanocytes can normally proliferate in skin to produce | __

moles or benign melanocytic nevi

Central nervous system (CNS) is formed in week 3 of development, during which time the neural plate develops. The neural plate, consisting of neuroectoderm, becomes the ____, which gives rise to the brain and spinal cord. -- closure in craniocaudal function

neural tube

Peripheral nervous system (PNS) is derived from three sources:

1. NEURAL CREST CELLS 2. NEURAL TUBE, which gives rise to all preganglionic autonomic nerves (sympathetic and parasympathetic) and all nerves (-motoneurons and -motoneurons) that innervate skeletal muscles 3. MESODERM, which gives rise to the dura mater and to connective tissue investments of peripheral nerve fibers (endoneurium, perineurium, and epineurium)

refers to the formation and closure of the neural tube

Neurulation

The three primary brain vesicles and two associated flexures develop during __

week 4

Three Primary brain vesicles

a. PROSENCEPHALON (FOREBRAIN) is associated with the appearance of the optic vesicles and gives rise to the telencephalon and diencephalon. b. MESENCEPHALON (MIDBRAIN) remains as the mesencephalon. c. RHOMBENCEPHALON (HINDBRAIN) GIVES rise to the metencephalon and myelencephalon.

Two Associated Flexures

a. CEPHALIC FLEXURE (MIDBRAIN FLEXURE) is located between the prosencephalon and the rhombencephalon. b. CERVICAL FLEXURE is located between the rhombencephalon and the future spinal cord

Five secondary brain vesicles become visible in week 6 of development and form various adult derivatives of the brain.

1. Telencephalon 2. Diencephalon 3. Mesencephalon 4. Metencephalon 5. Myelencephalon

gives rise to the cerebral hemispheres, caudate, and putamen

Telencephalon

gives rise to the epithalamus, subthalamus, thalamus, hypothalamus, mammillary bodies, neurohypophysis, pineal gland, globus pallidus, retina, iris, ciliary body, optic nerve (CN II), optic chiasm, and optic tract

Diencephalon

gives rise to the midbrain

Mesencephalon

gives rise to the pons and cerebellum

Metencephalon

gives rise to the medulla

Myelencephalon

Failure of the anterior neuropore to close results in upper neural tube defects like __

anencephaly

The posterior neuropore closes during week 4 (day 27). Failure of the posterior neuropore to close results in lower NTDs like

spina bifida with myeloschisis

As the neural plate folds, some cells differentiate into ___

neural crest cells

The rostral part of the neural tube becomes the adult __

brain

The caudal part of the neural tube becomes the adult __

spinal cord

- Fast-acting control system - Responds to internal and external change - Activates muscles and glands

Nervous system

Functions of the Nervous System

SENSORY INPUT - gathering information INTEGRATION - To process and interpret sensory input and decide if action is needed MOTOR OUTPUT - A response to integrated stimuli; The response activates muscles or glands

Functional Classification of the Peripheral Nervous System

Sensory (afferent) division - nerve fibers that carry information to the central nervous system Motor (efferent) division - nerve fibers that carry impulses away from the central nervous system

Motor (efferent) division: Two subdivisions

Somatic nervous system - voluntary | Autonomic nervous system - involuntary

- Also known as the soma and as the perikaryon - Trophic (nutritive) and genetic center of a neuron - The nucleus is spherical, usually large, pale staining, centrally located with a prominent nucleolus. - It is described to have a “fish eye” appearance.

Cell Body

- The cytoplasm contains Nissl’s granules, which are basophilic, due to the abundance of granular reticulum and ribosomes. - It also contains mitochondria, centrosomes, neurofibrils and inclusions of glycogen, fat and lipofuschin.

Cell Body

- are usually numerous in number - Short cytoplasmic processes that are specialized in receiving stimuli - are found close to the soma. - divide into branches and their surfaces are covered with minute, tiny spines called gemmules, which serve as sites of synaptic contact - have Nissl’s bodies, neurofibrils , mitochondria, centrosomes and inclusions.

Dendrites

- Also known as the axis cylinder. - Usually, there is one axon per neuron. - It is a long cytoplasmic process that is specialized in conduction of action potentials - It contains neurofibrils and mitochondria. - It does not contain Nissl’s substance.

Axon

Regions of Axon

1. Axon hillock 2. Initial segment 3. Axon proper 4. Terminal arborization 5. Terminal end bulb

- Part of the perikaryon that leads directly into the axonal process - first portion of the axon

Axon hillock

- Part of the myelinated axon between the apex of the axon hillock and the beginning of the myelin sheath

Initial segment

- Main trunk of the axon, excluding the initial segment

Axon proper

- Also known as telodendroglia | - Make synaptic contact

Terminal arborization

- Also known as terminal bouton - Contains abundant mitochondria and neurosecretory vesicles. - Forms part of a synapse, the pre-synaptic membrane - contacts another neuron or non-nerve cell at a synapse to initiate impilse

Terminal end bulb

regions with concentrated RER and other polysomes appear as clumps of basophilic material called __

chromatophilic substance or Nissl substance, Nissl bodies

Classification of Neurons

1. Structure (morphology) 2. Function 3. Size 4. Neurotransmitter released

Types of Neurons According to Morphology

1. Multipolar 2. Bipolar 3. Pseudounipolar 4. Unipolar

- More than 2 processes, usually with multiple dendrites and a single axon - Found in motor neurons of the CNS and peripheral autonomic ganglia

Multipolar

- Has 2 processes, consisting of a single axon and a single dendrite - Found in the retina, spiral (cochlear) and vestibular ganglia

Bipolar

- Single process close to perikaryon but divides into 2 branches, forming a T shape - Found in cerebrospinal ganglia or sensory ganglia located in the dorsal root of the spinal cord

Pseudounipolar

- Contains a single process | - Usually not seen in adult man

Unipolar

Types of Neuron according to function

1. Sensory neuron 2. Motor neuron 3. Association neuron

- Also known as afferent neuron - Carries nerves impulses toward the center - Example: cerebro-spinal ganglia

Sensory neuron

- Also known as efferent neuron - Carries impulses away from the center - Example: anterior horn cell of the spinal cord

Motor neuron

- Serves as linkage between afferent and efferent neurons - transmit impulses from one part of the CNS to another - Their processes do not leave the CNS - Example: interneurons of the spinal cord

Association neuron

Type of Neuron: According to size

1. Golgi type l - Long axon and large soma - Example: motor neurons of the spinal cord 2. Golgi type ll - Short axon and small soma - Example: interneurons of the spinal cord

Type of Neuron: According to neurotransmitter released

1. Cholinergic neuron Example: parasympathetic postganglionic neuron 2.Adrenergic neuron Example: sympathetic postganglionic neuron

- Contact area of one axon with the dendrites or perikaryon of another - They possess polarity - artificially stimulated axons can propagate a wave of depolarization in either direction, but the signal can travel in only one direction across a synapse, which functions as a unidirectional valve

Synapse

Each synapse has three major structural components, namely:

1. Pre-synaptic membrane - Portion of the terminal bouton membrane closest to the target cell 2. Synaptic cleft - Synaptic gap; Fluid filled space that guide the neurotransmitter across the gap 3. Post-synaptic membrane - Portion of the plasma membrane of the next neuron or target cell

Synapses are named according to the structures they connect, namely:

Axo-dendritic Axo-somatic Axo-axonic Dendro-dendritic

Direction of Signal Transmission

1. Orthodromic spread 2. Antidromic spread 3. Saltatory conduction 4. Blocking signal transmission

- Sequence of depolarization or impulse transmission travels along the axon away from the cell body

Orthodromic spread

Impulse travels toward the cell body

Antidromic spread

Depolarization of myelinated axons occur only at the nodes of Ranvier

Saltatory conduction

Cold, heat and pressure on a nerve fiber can block impulse conduction

Blocking signal transmission

- Consists of: Brain and Spinal cord - is characterized by cells which are closely packed with little extracellular substance and are connected by frequent cell to cell junction - Foremost of it’s properties is a special type of intercellular junction, the chemical synapse, which is important for survival

Central Nervous System

- Contains the perikaryons of mostly unmyelinated and some myelinated nerves and neuroglial cells such as protoplasmic astrocytes, oligodendrocytes and microglia

Gray mater

- Contains myelinated nerve fibers and neuroglial cells like the fibrous astrocytes, oligodendrocytes and microglia

White mater

- Also known as glial cells - Supporting cells of the CNS - These include: Astrocytes; Oligodendrocytes; Microglia and Ependymal cells - Do not synapse with other cells - On H&E stain, only the nuclei are seen while cytoplasm and processes of the neuroglia are not visible

Neuroglial Cells

- Most important supporting element of the CNS - Nucleus is spherical and centrally located - It is pale staining - Processes have expanded pedicles at their ends

Astrocytes

There are two types of Astrocytes

1. Protoplasmic astrocytes - Found in gray mater of the CNS - Abundant granular cytoplasm - Processes have many branches but shorter and relatively thick 2. Fibrous astrocytes - Found in white mater of the CNS - Cytoplasm shows fibrillar material - Processes are long, slender and smooth that branch infrequently

- Smaller cells - Nucleus is round, small and dense - The cytoplasm is also electron dense - Processes are less numerous and shorter - In the white mater of the CNS, this is precursor of myelin sheath

Oligodendrocytes

- Spider-like phagocytes - Dispose of debris - The cell bodies are smaller, denser and elongated - The nucleus is made up of of condensed chromatin material - The processes have a thorny appearance - are found in both grey and white mater

Microglia

- Supporting cells that line the central canal of the spinal cord and ventricular cavities of the brain - The cells are flattened to cuboidal to columnar shaped with wide bases - Their surfaces are provided with cilia during embryonic life - The nucleus is elongated

Ependymal Cells

- Composed of an outer (grey mater) cortex and an inner medulla - The cortical layer follow the involutions of the cerebellar folia - The medullary white mater is composed of myelinated nerve fibers and glial cells

Cerebellum

The cortex is composed of 3 layers (Cerebellum)

1. Molecular layer - Composed of dendritic arborizations with densely packed axons coursing parallel to the long axis of the folia - Contains basket cells and outer satellite cells 2. Purkinje cell layer - Sheet of large flask shaped cells, the Purkinje cells, which are uniformly arranged along the upper margin of granular layers 3. Granular layer - Consists of granule cells, which are small and numerous and numerous stellate cells

- is made up of an inner core of grey mater and an outer core of white mater - The grey mater is composed of anterior (ventral) horns and posterior (dorsal) horns joined at the center by a thin grey mater called the central commissure, surrounding the central canal - The neurons and their neuroglial cells are found in the grey mater - The white mater is primarily composed of myelinated nerve fibers and neuroglial cells

spinal cord

Coverings of the Central Nervous System

1. Dura mater 2. Arachnoid mater 3. Pia mater

- Pachymeninx - Dense connective tissue - Separated from the periosteum by the epidural space, which has loose connective tissue with thin walled veins and fat cells - Separated from the arachnoid by the subdural space

Dura mater

- Delicate impermeable membrane made up of loose connective tissue devoid of blood vessels - Has a cobbed web like appearance - A system of trabeculae extends into the pia mater and the cavities which forms the subarachnoid space, filled with cerebro-spinal fluid

Arachnoid mater

- Vascular membrane closely investing the brain and spinal cord - Composed of loose connective tissue with fine elastic fibers

Pia mater

- Composed of nerves and ganglia - The nerves are bundles of nerve fibers outside the CNS while the ganglia are groups of nerve cell bodies outside the CNS or found in peripheral nervous system

Peripheral Nervous System

- Nerves are composed of nerve fibers which either can be an axon or a dendrite - The conducting core of a nerve fiber is called the axis cylinder - They can either be myelinated or unmyelinated - Unmyelinated nerve fiber is devoid of myelin sheath

Peripheral Nerves

Myelination of a nerve fiber is derived from the __ where in they become spirally disposed along the membrane and becomes compacted to form the lamellae of myelin sheath

Schwann cells

-- is a bimolecular lipoprotein complex

myelin sheath

myelin sheath shows cone-shaped clefts called the __

cleft or incissures of Schmidt-Lantermann

Between 2 nodes of Ranvier is the neurolemma or __

sheath of Schwann cells

__ have elongated nuclei that lie parallel to the axon

Schwann cells

- is a fibrous connective tissue that covers a nerve, either myelinated or unmyelinated

sheath of Henle (sheath of Key and Retzius)

Types of Nerve Fibers According to Diameter

1. Group A fibers 2. Group B fibers 3. Group C fibers

- Myelinated large axons with long internodes - Rapid impulse conduction - Include motor and some sensory fibers

Group A fibers

- Myelinated intermediate sized axons with short internodes - Moderate rate of impulse conduction - Mainly visceral sensory fibers

Group B fibers

- Unmyelinated small axons - Slow rate of impulse conduction - Include autonomic and some sensory fibers

Group C fibers

Connective Tissue Coverings of Nerves

1. Endoneurium - Loose connective tissue surrounding the individual nerve fiber 2. Perineurium - Dense irregular connective tissue enclosing a bundle or fascicle of nerve fibers 3. Epineurium - Dense irregular connective surrounding a peripheral nerve

Neuroglial cells of the PNS

Schwann cells | Satellite cells

- Also known a neurolemma | - Capable of secreting myelin sheath in the PNS

Schwann cells

- gaps in myelin sheath along the axon

Nodes of Ranvier

- Specialized Schwann cells found in the dorsal root ganglia and the autonomic ganglia of the PNS

Satellite cells

- Aggregation of nerve cell bodies outside the CNS. - Two types of ganglia: Cranio-spinal ganglia Autonomic ganglia

Ganglion

- Also known as dorsal root ganglia - Found in the dorsal roots of all spinal nerves and some cranial nerves - Sensory in function - Pseudo-unipolar neuron - There are no synapses

Cranio-spinal Ganglia

- The satellite cells are more numerous with spherical nuclei and completely surround the soma of each ganglion cell. - The ganglion cells are concentrated peripherally. The cytoplasm of the ganglion cells contain small and uniformly distributed Nissl’s granules. - The nucleus is large, oval and centrally located.

Cranio-spinal Ganglia

- Consists of sympathetic and parasympathetic ganglia - These contain synapses - Visceromotor in function - Multipolar neuron

Autonomic ganglia

- The satellite cells are less numerous with ovoid nuclei and form a discontinuous sheath around the ganglion cell bodies. - The ganglion cells are smaller and do not show definite groupings. The cytoplasm of the ganglion cells contain Nissl’s granules that are intermediate in size and are massed toward the periphery. Lipofuschin granules are more frequently found in its cytoplasm. The nucleus is large, oval and eccentrically located.

Autonomic ganglia

- The involuntary branch of the nervous system - Consists of only motor nerves - Divided into two divisions: Sympathetic division and Parasympathetic division

Autonomic Nervous System

Differences Between Somatic and Autonomic Nervous Systems

1. Nerves Somatic – one motor neuron Autonomic – preganglionic and postganglionic nerves 2. Effector organs Somatic – skeletal muscle Autonomic – smooth muscle, cardiac muscle,and glands 3. Nerurotransmitters Somatic – always use acetylcholine Autonomic – use acetylcholine, epinephrine, or norepinephrine

- Originates from T1 through L2 - Ganglia are at the sympathetic trunk (near the spinal cord) - Short pre-ganglionic neuron and long postganglionic neuron transmit impulse from CNS to the effector - Norepinephrine and epinephrine are neurotransmitters to the effector organs

Anatomy of the Sympathetic Division

- Originates from the brain stem and S1 through S4 - Terminal ganglia are at the effector organs - Always uses acetylcholine as a neurotransmitter

Anatomy of the Parasympathetic Division

- “fight-or-flight” - Response to unusual stimulus - Takes over to increase activities - Remember as the “E” division = exercise, excitement, emergency, and embarrassment

Sympathetic

- housekeeping activities - Conserves energy - Maintains daily necessary body functions - Remember as the “D” division - digestion, defecation, and diuresis

Parasympathetic

Responses of Nerve Tissue to Injury

Damage to the nerve cell body - May result in autolysis, a process termed “transneural degeneration” Damage to the axon - Primary, ascendant or retrograde degeneration - Secondary, descendant or Wallerian degeneration

- Proximal to the site of injury - Incomplete degeneration - Perikaryon enlarges - Chromatolysis or dispersal of Nissl’s substance occur - Nucleus moves to eccentric position - Change takes about two weeks

Primary degeneration

- Distal to the site of injury - Schwann cells proliferate and phagocytose the degenerated tissues and invade the remaining endoneural sheath - Changes take 2 to 3 days

Secondary degeneration

- This usually begins in the third week after an injury. The proximal stump of the axon gives rise to a protrusion of smaller processes termed neurites. - Neurites grow 3–4 mm/day distally, guided and then myelinated by the Schwann cells.

Nerve Regeneration

Layers of the Cerebral Cortex (from the outermost to the innermost)

1. Molecular layer 2. External granular layer 3. External pyramidal layer 4. Internal granular layer 5. Internal pyramidal layer 6. Multiform layer

- Also known as plexiform layer - Consists of horizontally directed nerve fibers or dendrites In the deeper portion, lies the HORIZONTAL CELLS OF CAJAL and spindle shaped cells

Molecular layer

- Composed of small pyramidal cells, some stellate cells and granule cells

External granular layer

- Composed of medium sized pyramidal cells, some stellate cells and granule cells

External pyramidal layer

- Composed of large pyramidal cells, numerous closely packed stellate cells (forming the BAND OF BALLERGER) and granule cells

Internal granular layer

- Composed of large pyramidal cells, some stellate cells and granule cells

Internal pyramidal layer

- Also known as polymorphic layer - Consists of predominantly fusiform cells - Granule cells, stellate cells and INVERTED CELLS OF MARTINOTTI are also found here

Multiform layer

- Muscular tissue is the primary tissue of motion wherein the fundamental protoplasmic property of contractility is highly developed. - responsible for locomotion and for the movements of the different parts of the body - develops from the mesoderm, except for the muscles of the iris which are neuroectodermal in origin

Muscular System

Skeletal muscle is derived from __, which | forms somites from the occipital to the sacral regions and somitomeres in the head

paraxial mesoderm

Smooth muscle differentiates from visceral __ surrounding the gut and its derivatives and from ectoderm (pupillary, mammary gland, and sweat gland muscles).

splanchnic mesoderm

Cardiac muscle is derived from visceral __ surrounding the heart tube.

splanchnic mesoderm

The myoblasts that form the skeletal muscles of the trunk are derived from __

mesoderm in the myotome regions of the somites

The limb muscles develop from __ in limb buds

myogenic precursor cells

This frontier separates two mesodermal domains in the embryo:

1. The PRIMAXIAL DOMAIN that comprises the region around the neural tube and contains only somite-derived (paraxial mesoderm) cells. 2. The ABAXIAL DOMAIN that consists of the parietal layer of lateral plate mesoderm together with somite cells that have migrated across the lateral somitic frontier.

– first indication is elongation of nuclei and cell bodies of mesenchymal cells as they differentiate to myoblasts

Myogenesis

- is a thick plate of mesoderm on each side of the midline. | - becomes organized into segments known as somitomeres, which form in a craniocaudal sequence.

Paraxial mesoderm

__ do not form somites but contribute mesoderm to the head and neck region (pharyngeal arches).

Somitomeres 1–7

The remaining somitomeres further condense in a craniocaudal sequence to form __ of the trunk region.

42–44 pairs of somites

- further differentiate into the sclerotome (cartilage and bone component), myotome (muscle component), and dermatome (dermis of skin component).

Somites

- is derived from somitomeres 1–7 of the head and neck region, which participate in the formation of the pharyngeal arches.

Head and neck musculature

- is derived from myotomes in the trunk region. Each myotome partitions into a dorsal epimere and a ventral hypomere.

Trunk musculature

- develops into the extensor muscles of the neck and vertebral column (e.g., erector spinae). - is innervated by dorsal rami of spinal nerves.

Epimere

- develops into the scalene, prevertebral, geniohyoid, infra hyoid, intercostal, abdominal muscles, lateral and ventral flexors of the vertebral column, quadratus lumborum, and pelvic diaphragm. - is innervated by ventral rami of spinal nerves.

Hypomere

Migration of myoblasts from the pharyngeal arches forms the muscles of branchiomeric in origin

Pharyngeal Arch Muscle

Muscles: Muscles of mastication, anterior belly of digastric muscle, mylohyoid, tensor tympani and tensor palatine Cartilage: Mandible, malleus, incus Nerve: CN V (Trigeminal Nerve)

1st Pharyngeal arch: MANDIBULAR ARCH

Muscles: Muscles of facial expression, stapedius, stylohyoid, posterior belly of the digastrics and auricular muscle Cartilage: Stapes, styloid process, stylohyoid ligament, lesser horn and upper part of the body of hyoid Nerve: CN VII (Facial Nerve)

2nd Pharyngeal arch: HYOID ARCH

Muscle: Stylopharyngeal muscle Cartilage: Lower part of the body and greater horn of the hyoid bone Nerve : Glossopharyngeal nerve

3rd Pharyngeal arch

Muscle: Cricothyroid levator and palatinepharyngeal constrictor (4th) Intrinsic muscle of the larynx (6th) Cartilage: Thyroid, cricoid, arytenoids, corniculate, cuneiform cartilages of the pharynx Nerve: Superior laryngeal branch of vagus nerve (4th) Recurrent laryngeal branch of the vagus nerve (6th)

4th and 6th Pharyngeal Arches

The smooth muscle in the walls of many blood and lymphatic vessels arises from __

somatic mesoderm

The muscles of the iris (sphincter and dilator pupillae) and myoepitheial cells in mammary and sweat glands are derived from the __

ectoderm

Cardiac myoblasts differentiate from the __

primordial myocardium

Myoblasts adhere to each other but the intervening cell membranes do not disintegrate. These areas of adhesion give rise to __

intercalated discs

- Absence of the pectoralis muscle, often its sternal part, is usually its sternal associated with

syndactyly (fusion of digits)

- Failure of normal muscle development may be widespread leading to immobility of multiple joints - involved muscles are replaced partially or completely by fat and fibrous tissue

Congenital arthrogryposis

- Fixed rotation and tilting of the head due to fibrosis and shortening of the sternocleidomastoid muscleon one side - result from tearing of fibers of the sternocleidomastoid muscle during childbirth

Congenital torticollis

- Composed of elongated cells in the axis of contraction and organized in long units of structure which are often referred to as Muscle fiber and Intercellular substance

Muscle Tissue

- a form of loose connective tissue found between the smooth muscle cells but it is more abundant between cardiac and skeletal muscle - a rich network of capillaries and nerves are found in this connective tissue which also serve to bind together the muscle fibers

Intercellular substance

General Characteristics of Muscular Tissue

- unit of structure is elongated in shape which is an adaptation to its function of contraction - sarcoplasm appears fibrillar due its contents of myofibrils - staining is acidophilic or pinkish because it has a distinct affinity to acid stains - cells and fibers are bound together by varying amounts of areolar connective tissue containing blood vessels and nerves

Cytoplasm of muscle cells and fibers which are acid staining

Sarcoplasm

Cell membrane complex consisting of an external coating of protein polysaccharide over sarcoplasm

Sarcolemma

Granules in the sarcoplasm which under the elerctron microscope is actually a mitochondrion

Sarcosome

- Fine threadlike structures in the sarcoplasm which are responsible for muscle contraction - may appear either striated or not - is composed of finer myofilaments

Myofibrils

- A linear unit which are membrane bound structures containing granules and sarcoplasmic reticulum, within the cytoplasm

Sarcomere

- Refers to the smooth endoplasmic reticulum which is responsible for the release or uptake of calcium ions during contraction and relaxation of muscle respectively

Sarcoplasmic reticulum

There are three types of muscle tissues based on structure and function. These are:

Skeletal muscle Cardiac muscle Smooth muscle

- Striated, voluntary muscle - Composed of cylindrical muscle cells, which are referred to as “fibers” because of their shape in the light microscope - Large numbers of parallel muscle fibers are grouped into fascicles or bundles

Skeletal muscle

Organization of Skeletal Muscle

EPIMYSIUM - external sheath of dense connective tissue, surrounds the entire muscle PERIMYSIUM - thin connective tissue layer that immediately surrounds each bundle of muscle fibers (fascicle) ENDOMYSIUM - surrounds the external lamina of individual muscle fibers

- Multinucleated - Nuclei are found in the periphery or eccentrically located - In transverse sections, they are resolved as fine dots either uniformly distributed or grouped in polygonal areas called the fields of Cohnheim.

Skeletal Muscle Cell

- are interpreted as shrinkage artifacts and have no functional significance

cohnheim’s fields

- Functional and structural unit of contraction in skeletal muscle - In longitudinal sections of muscle, it shows the cross striations, due to alternating dark and light bands - is defined as the segment between two successive Z lines and therefore includes the A band and half of the two contiguous I bands.

Sarcomere

The dark bands are called the __ because they are doubly refractile or birefringent under polarized light.

A bands or anisotrophic bands

The light bands are called the __ because they do not alter polarized light.

I bands or isotrophic bands

Each I band is bisected by a dark transverse line called

Z line

At the center of the A band is a pale zone termed the __

H band (intermediate disc of Hensen)

In the center of this H band is a narrow dark line, known as __

M band (mesophragma)

- is specialized for Ca2+ sequestration

sarcoplasmic reticulum

- To trigger Ca2+ release from sarcoplasmic reticulum throughout the fiber simultaneously and cause uniform contraction of all myofibrils, the sarcolemma is folded into a __ - long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle every myofibril near the aligned A- and I-band boundaries of sarcomere

system of transverse or T tubules

This specialized complex, consisting of SR-T tubule – SR components, is known as the __

triad of skeletal muscle

- occupy the interior of the muscle fiber - In transverse sections, they are uniformly distributed in polygonal areas called CONHEIM’S AREA OR FIELDS OF CONHEIM, believed to be a shrinkage artifact - are grouped into parallel bundles longitudinally arranged along the muscle fiber - These bundles constitutes the KOLLICKER’S COLUMNS which are separated from the adjacent bundles by a narrow area of granular sarcoplasm

Myofibrils

The myofibrils, under electron microscope, are found to be composed of smaller units called __

myofilaments

- thicker; the principal constituent of the A band.

myosin filaments

- thinner; constitutes the the I band, that extends on either sides of the Z lines

actin filament

The mechanism of contraction is due to __ with each other.

sliding or overlapping of the myofilaments

Each axonal branch forms a dilated termination situated within a trough on the muscle cell surface. This synaptic structure is called __ - are located in narrow zones in a given muscle - always lies in the mid-portion of the fiber it supplies - Larger muscle fibers have larger end plates

Motor End Plate

This specialized junctional region at the termination of a motor nerve on skeletal muscle fibers is also termed as __

myo-neural junction

- Red muscle - Small fibers - Slowest ATPase activity - Many mitochondria - Abundant myoglobin (red) - Glycogen – minimal - Energy is more from aerobic O2 phosphorylation - Slow twitch, continuous, prolonged contractions - Fatigue resistant / less tension - Postural back muscles - High endurance athletes

Red (slow; Type I)

- medium fibers - fast ATPase activity - Many mitochondria - Many myoglobin (pink) - Glycogen abundant - O2 phosphorylation + Anaerobic glycolysis - Fast twitch / rapid contractions/short burst - Fatigue resistant / hi tension - Leg muscles - Middle distance athletes

Intermediate (Type IIa)

- White muscle - Large fibers - High ATPase activity - fast glycolytic (high anaerobic enzyme activity) - Less mitochondria - Less myoglobin (light pink) - More Glycogen - Energy is less from glycolysis - rapid contractions/ fatigue – prone - small muscles w/ large NMJ – fine movements (hands ands eyes)

White (fast; Type IIb)

Regeneration of Striated Muscle

Skeletal muscles increase in volume during intense activity by enlargement of the existing fibers through an increase in the amount of sarcoplasm but not in the number of fibrils After destruction of muscle fibers, regeneration always starts from existing fibers which is most successful when the nuclei with the surrounding sarcoplasm remain alive. A large defect in the muscle tissue is replaced by a connective tissue scar.

Distinguishing Features of Skeletal Muscles

- Consists of elongated muscle fibers that do not branch - Big diameter of the muscle fibers - Nuclei are peripherally located and multinucleated in appearance - Striated myofibrils arranged in characteristic KOLLICKERS COLUMNS - Presence of two types of myofilaments (thick myosin and thin actin) - More abundant areolar tissue containing blood vessels intervening between sections of the muscle fibers - May contain more striations - Absence of the intercalated discs

- A unique and distinguishing characteristic of this muscle is the presence of darkly staining transverse lines that cross the chains of cardiac cells at irregular intervals, the intercalated discs.

Cardiac Muscle

There are three main junctional specialization within the discs, namely: (Cardiac Muscle)

Fascia adherens Desmosomes Gap junction

- Also known as fascia junction - Is the most prominent membrane specialization in the transverse portion of the disc - Serves as anchoring sites for actin filaments of the terminal sarcomeres

Fascia Adherens

- Also known as maculae adherens - together these serve to bind cardiac muscle cells firmly together to prevent their pulling apart under constant contractile activity

Desmosomes

- On the lateral portion of the discs, which has ionic continuity between adjacent cells

Gap Junction

- The pattern of cross striations of cariac muscle is similar to that of skeletal muscle. - Its contractile substance is composed of actin and myosin filaments in the same interdigating relationship as that of the skeletal muscle.

Cardiac Muscle

Cardiac Muscle: Sarcoplasmic Reticulum and Transverse Tubule System

- The T tubule system of cardiac muscles are more numerous and larger than that of skeletal muscle. - T tubules are located at the level of the Z lines rather than at the A-I junction. - sarcoplasmic reticulum is composed of a simple plexiform arrangement of tubular elements occupying slender clefts within the mass of myofilaments. - The association is called “DIADS” because T tubules are associated with only one cisternae at any point

Distinguishing Features of Cardiac Muscles

- elongated muscle fibers that branch - Mono or bi-nucleated and located centrally - Myofibrils appear striated, composed of thin and thick bands when viewed in electron micrographs - Presence of intercalated discs - Sarcoplasmic reticulum is not as well developed - Absence of terminal cisternae - T tubes appear larger and siuated at the Z line - Presence of bigger and numerous mitochondria containing closely-placed cristae with angulations resulting in a characteristic “zig-zag” appearance

- Can be found dispersed in the connective tissue of certain organs - May be grouped to form a small bundle, like the arrector pili muscle in the skin - May be the predominant tissue, like the uterus

Smooth Muscle

- is composed of spindle or fusiform shaped cells - Contains one, centrally located, nucleus - Arranged into layers or sheets - Covered by connective tissue sheaths - myofilaments criss cross obliquely through the cell, forming a latticelike network - They do not form a sarcomere - T tubule system is not regularly arranged as that seen in striated muscle

Smooth Muscle Cell

True or False Contraction of smooth muscle cells is relatively slow and the cells may remain contracted for long periods without fatigue

True

- Smooth muscle cells that lie between the glandular epithelial cells and the basal lamina of the secretory alveoli (acini) of salivary glands, lacrimal glands and mammary glands. - referred to as “basket” cells because of their branching, stellate shape and their position near the base of the alveolus - expel the secretion from the acini to the ducts of the glands by contraction

Myoepithelial Cells

Regeneration of Smooth Muscle

Capacity for regeneration is small, and great defects in smooth muscle heal by scar formation. It is practically certain that they can develop from perivascular mesenchymal cells The smooth muscle, especially of the uterus, is capable of undergoing both hypertrophy and hyperplasia in the event of pregnancy. A smooth muscle cell may divide by mitosis to form additional smooth muscle cells but this capacity is limited

Distinguishing Features of Smooth Muscles

- spindle or fusiform shaped muscles which do not branch - Myofibrils appear non-striated, myofilaments when viewed under electron microscopy are not divided distinctly into two types - Nucleus is single and situated in the middle bulging portion of the cells - Presence of scanty loose connective tissue between the smooth muscles - Electron micrographs reveal presence of dense bodies - Sarcolemma show pinocytic vesicles - Few, long and slender mitochondria

``` Shape: Elongated with blunt ends Arrangement: Bundles or fascicles Number of nucleus: Multi-nucleated Location of nucleus: Peripheral Cross-striations: Present SR-T tubule system: Triadic pattern Contraction: Quick, forceful and voluntary ```

Skeletal Muscle

``` Shape: Elongated with branching ends Arrangement: Bundles or fascicles Number of nucleus: Mono- or bi-nucleated Location of nucleus: Central Cross-striations: Present SR-T tubule system: Diadic pattern Contraction: Rhythmic and involuntary ```

Cardiac Muscle

``` Shape: Elongated with tapering ends Arrangement: Layers or sheets Number of nucleus: Mono-nucleated Location of nucleus: Central Cross-striations: Absent SR-T tubule system: Absent Contraction: Slow and involuntary ```

Smooth Muscle

- X linked recessive disease affecting boys - Associated with total absence of dystrophin, a protein of the membrane cytoskeleton that is necessary for stabilizing the cell membrane of all three types of muscle - Results to progressive muscular weakness which ultimately becomes fatal

Duchene’s Muscular Dystrophy