Monday, May 7, 2018

Functions of Bones

Basic Anatomy , Skeleton
Functions of Bones

Functions of Bones


Bones form an important component of the Skeleton System . They perform a wide range of important functions that can be classified into three categories:

Mechanical Functions of bones


  • Protection:
    At numerous places inside the body, bones serve to protect important and delicate organs. The best examples to be quoted here are those of brain (which is protected by the skull) and heart (which is protected by the ribcage).
  • Shape:
    Because of their rigid nature, bones provide a framework around which the body is built. So bones are responsible for the shape and form of human body.
  • Movement:
    Working with skeletal muscles, tendons, ligaments and joints, the bones form the moving machinery of human body. The major role of bones in movement is that they act as levers, which make use of the forces generated by skeletal muscles in a beneficial way.

Synthetic Functions of Bones


  • Synthesis of blood cells:
    The major synthetic role of bones is to produce blood cells. The bones themselves are not capable of doing this. Instead, they house the bone marrow, which contains Hematopoietic stem cells, capable of producing blood cells. In infants, bone marrow of all long bones is capable of this synthesis, however, as a person gets older, the red marrow turns into yellow fatty marrow, which is no more capable of hematopoiesis. The red marrow in adults and older individuals is restricted to vertebrae and heads of tibia and femur.

Metabolic Functions of Bones


  • Mineral Storage:
    Bones serve as an important store house of minerals such as calcium and phosphorus.
  • Fat storage:
    The yellow bone marrow of long bones act as a storage of fats.
  • Role in acid-base balance:
    Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts


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Sunday, May 6, 2018

Deep Fascia

Basic Anatomy , Skin and Fascia
Deep Fascia

Deep Fascia


Deep fascia is the dense fibrous connective tissue that interpenetrates and surrounds the muscles, bones, nerves and blood vessels of the body. It is in the form of a fibrous sheet which invests the body beneath the superficial fascia. It is devoid of fat, and is usually inelastic and tough.

Distribution of deep fascia


  • Deep fascia is best defined in the limbs where it forms tough and tight sleeves and in the neck where it forms a collar.
  • It is ill-defined on the trunk and face

Important Features of Deep Fascia


1. Extensions (prolongations) of the deep fascia form:

  • The intermuscular septa which divide the limb into compartments.
  • The fibroareolar sheaths for the muscles, vessels and nerves.
2. Thickenings of the deep fascia form:

  • Retinacula (retention bands) around certain joints like wrist and ankle.
  • The palmar and plantar aponeuroses for protection of nerves and blood vessels.
3. Deep fascia never crosses a subcutaneous hone. Instead it blends with its periosteum and is bound down to the bone.

Modifications of Deep Fascia


1. Forms the intermuscular septa separating functionally different group of muscles into separate compartments.

2. Covers each muscle as epimysium which sends in the septa to enclose each muscle fasciculus known as perimysium. From the perimysium septa pass to enclose each Muscle fiber. These fine septa are the endomysium. Through all these connective tissue septa, e.g. epimysium, perimysium and endomysium, arterioles, capillaries, venules, lymphatics and nerves traverse to reach each muscle fibred.

3. Deep fascia covers each nerve as epineurium, each nerve fascicle as perineurium and individual nerve fibered as endoneurium. These connective tissue coverings support the nerve fibers and carry capillaries and lymphatics.

4. Forms sheaths around large arteries, e.g. carotid sheath, axillary sheath. The deep fascia is dense around the artery and rather loose around the vein to give an allowance for the vein to distend.

5. Modified to form the capsule, synovial membrane and bursae in relation to the joints.

6. Forms tendon sheaths wherever tendons cross over a joint. This mechanism prevents wear and tear of the tendon. In the region of palm and sole it is modified to form aponeuroses, e.g. palmar and plantar aponeuroses which afford protection to the underlying-structures. It also forms septa between various muscles. These septa are specially well developed in the calf muscles of lower limb. The contraction of calf muscles in the tight sleeve of deep fascia helps in pushing the venous blood and lymph towards the ‘heart. Thus the deep fascia helps in venous and lymphatic return from the lower limb.

7. In the forearm and leg, the deep fascia is modified to form the interosseous membrane, which keeps:

  • The two bones at optimum distance.
  • Increases surface area for attachment of muscles.
  • Transmits weight from one bone to other.

Functions of Deep Fascia


  1. Deep fascia keeps the underlying structures in position an preserves the characteristic surface contour of the limbs and neck.
  2. It provides extra surface for muscular attachment
  3. It helps in venous and lymphatic return.
  4. It assists muscles in their action by the degree of tension and pressure it exerts upon their surfaces.
  5. The retinacula act as pulleys and serve to prevent the loss of power. In such situations the friction is minimized by the synovial sheaths of tendons.


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Superficial Fascia

Basic Anatomy , Skin and Fascia
Superficial Fascia

Superficial Fascia


Superficial fascia is a general coating of the body beneath the skin, made up of loose areolar tissue with varying amounts of fat. It is the layer that primarily determines the shape of the body. In addition to its subcutaneous presence, the supreficial fascia surrounds organs , glands and neurovascular bundles. It is also found at many other locations where it fills otherwise unoccupied space.

Distribution of Fat in Superficial Fascia


  • Fat is abundant in the gluteal region (buttocks), lumbar region (flanks), front of the thighs, anterior abdominal wall below the umbilicus, mammary gland, postdeltoid region, and the cervicothoracic region.
  • In females, fat is more abundant and is more evenly distributed than in males.
  • Fat is absent from the eyelids, external ear, penis, and scrotum.
  • The subcutaneous layer of fat is called the panniculus adiposus.
  • In females fat is in the superficial fascia of the lower abdomen, upper thigh, whereas in males it is inside the abdominal cavity.
  • In general, in women fat forms a thicker and more even layer than in men.
  • Fat (adipose tissue) fills the hollow spaces like axilla and orbits.
  • Fat present around the kidneys in abdomen supports the organs.

Types of fats


There are two types of fat; i.e. yellow and brown fat. Most of the body fat is yellow, only in hibernating animals it is brown. The cells of brown fat are smaller with several small droplets, and multiple mitochondria.

Fat cells are specialized cells and the size of fat cells increases during accumulation of fat, rather than the number of cells. Any attempt to reduce excessive fat (obesity) must be slow and steady and not drastic, as the latter may cause harm to the body.

Important Features of Superficial Fascia


  • Superficial fascia is most distinct in the lower part of the anterior abdominal wall, perineum, and the limbs.
  • It is very thin on the dorsal aspect of the hands and feet, sides of the neck, face, and around the anus.
  • It is very dense in the scalp, palms, and soles.
  • Superficial fascia shows stratification (into two layers) in the lower part of anterior abdominal wall, perineum, and uppermost parts of thighs.
  • It contains:
    (a) Subcutaneous muscles in the face, neck and scrotum
    (b) Mammary gland
    (c) Deeply situated sweat glands
    (d) Localized groups of lymph nodes
    (e) Cutaneous nerves and vessels.

Functions of Superficial Fascia


  • Superficial fascia facilitates movements of the skin.
  • It serves as a soft medium for the passage of nerves and vessels to the skin.
  • It conserves body heat because by nature, fat is a bad conductor of heat.
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Sweat Glands

Basic Anatomy , Skin and Fascia
Sweat Glands

Sweat Glands


Sweat glands, also known as sudoriferous glands, are small tubular structures of skin that produce sweat. They are distributed all over the skin, except lips, glans penis and nail bed. These glands are of two types: Eccrine and Apocrine.

Eccrine Sweat Glands


Eccrine glands are much more abundant and distributed in almost every part of the skin. Each one of these is a single tube, the deep part of which is coiled into a ball. The coiled part is called body of the gland. It lies in the deeper part of corium or in the subcutaneous tissue. The straight part is called duct. It traverses the dermis and epidermis and opens on the surface of the skin.

The glands are large in the axilla and groin. They are most abundant in the regions of palms and soles and least abundant in the neck and back.
The eccrine glands are merocrine in nature, which means that they produce their thin watery secretion without any disintegration of the epithelial cells. They are supplied and controlled by cholinergic sympathetic nerves.The glands help in regulation of body temperature by evaporation of sweat and also help in excreting the body salts.

Apocrine Sweat Glands


The apocrine are confined to axilla, eyelids (where they are known as Moll’s glands), nipple, and areola of the breast, perianal region and the external genitalia. They are larger than the eccrine glands and produce a thicker secretion having a characteristic smell. They develop in close association with hair and their ducts typically open into the distal ends of hair follicles.

The Ceruminous glands of the external auditory meatus are modified apocrine sweat glands. The apocrine glands also are merocirne in nature, but are regulated by a dual autonomic control. Some research workers are not inclined towards calling these glands as sweat glands. The main reason behind this is that they don’t respond sufficiently to temperature changes.

Average secretion of sweat


On average, about one liter of sweat is secreted each day. The total water loss on daily basis for a healthy adult individual is about 1500 ml. This shows that greater part of the water lost from our body is in the form of sweat. In hot climates, the proportion rises to even  a higher value and the secretion of sweat may amount to 3-7 liters per day. As long as the sweat glands are intact, the skin can regenerate, however, if the sweat glands are lost, skin grafting becomes necessary.


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Sebaceous Glands

Basic Anatomy , Skin and Fascia
Sebaceous Glands

Sebaceous Glands


Sebaceous glands are microscopic glands in the skin that secrete an oily secretion known as sebum. They are widely distributed all over the dermis of skin, except for the palms and soles. They are especially abundant in the scalp and face. They are also very numerous around the apertures of ear, nose, mouth and anus.

Structure of Sebaceous Glands


Sebaceous glands are small and sacculated in appearance. They are made up of a cluster of about 2-6 piriform alveoli. Their ducts open into the hair follicles, with the exception of lips, glans penis, inner surface of prepuce, labia minora, nipple and areola of the breast, and tarsal glands of the eyelid, where the ducts open on the surface of the skin.

Nature of Sebaceous Glands


Sebaceous glands are holocrine in nature, which means that they produce their secretion by complete degeneration of the central cells of the alveolus. The degenerated central cells are then replaced by proliferating peripheral cells.

Role of Sebum


The secretion of sebaceous glands is under hormonal control, especially the androgens. The sebum, which is the secretion of the sebaceous glands, lubricates the skin and protects it from moisture, desiccation and harmful sun rays. Sebum also lubricates hair and prevent them from becoming brittle. In addition, the sebum also has some bacterial action. Sebum makes the skin water proof, thus reducing the loss of water from skin surface.


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Anatomy of Hair

Basic Anatomy , Skin and Fascia
Anatomy of Hair

Anatomy of Hair


Hair are keratinous filaments derived from invaginations of the germinative layer of epidermis into the dermis. These are peculiar to mammals (like feathers to the birds), and help in conservation of their body heat. However, in man the heat loss is prevented b the cutaneous sensation of touch

Distribution of Hair


Hair are distributed all over the body, except on the palms, soles, dorsal surface of distal phalanges, umbilicus, glans penis, inner surface of prepuce, the labia minora, and inner surface of labia majora. The length, thickness and color of the hair vary in different part of the body and in different individuals.

Structure of Hair


  • Each hair has an implanted part called the root, and a projecting part called the shaft.
  • The root is surrounded by a hair follicle (a sheath of epidermis and dermis), and is expanded at its proximal end to form the hair bulb.
  • Each hair bulb is invaginated at its end by the hair papilla (vascular connective tissue) which forms the neurovascular haunt of the hair and its sheath.
  • Hair grows at the hair bulb, by proliferation of its cells capping the papilla.
  • The hair follicles, enclosing hair roots lie obliquely to the surface of the skin, which is responsible for characteristic hair strearns in different parts of the body.

Arrectores Pilorum Muscles


The arrectores pilorum muscles (smooth muscles supplied by sympathetic nerves) connect the undersurface of the follicles to the superficial part of the dermis. Contraction of these muscles leads to erection of hair, squeezes out the sebum and produces “goose skin”.


Fetal Hair


The fetal skin is covered by fine hair called lanugo (primary hair). These are mostly sited by birth and are replaces during infancy by another set of fine hair called his secondary hair). These are retained in most parts of the body, but are replaced by the thick and dark terminal hair of the scalp and eyebrows, and other hairy areas of the adult skin.

Growth rate and life span


The hair grow at the rate of about 1.5-2mm per week. Their growth is controlled by hormones. The life span of the hair varies from 4 months (eyelashes, axillary hair) to 4 years (scalp hair)


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Anatomy of Nail

Basic Anatomy , Skin and Fascia
Anatomy of Nail

Anatomy of Nail


Nail is a hardened keratin plates (cornified zone) on the dorsal surface of the tips of fingers and toes, acting as a rigid support for the digital pads of terminal phalanges. They are horn-like envelopes covering the dorsal aspect of terminal phalanges of fingers and toes of human body.

Parts of Nail


Each nail has the following parts.

  • Root: Root is the proximal hidden part which is buried into the nail groove and is overlapped by the nail fold of the skin.
  • Free border: Free border is the distal part free from the skin.
  • Body: Body is the exposed part of the nail which is adherent to the underlying skin. The proximal part of the body presents a white opaque crescent called lunule. Each lateral border of the nail body is overlapped by a fold of a skin, termed the nail wall.
The skin (germinative zone + corium) beneath the root and body of the nail is called nail bed. The germinative zone of the nail bed beneath the root and lunule is thick and proliferative (germinal matrix), and is responsible for the growth of the nail.

The rest of the nail bed is thin (sterile matrix) over which the growing nail glides. Under the translucent body (except lunule) of the nail, the corium is very vascular. This accounts for their pink color.

Clinical anatomy of Nail


  • In anemia the nails are pale and white.
  • In iron deficiency anemia the nails become thin, brittle and spoon-shaped (koilonychia).
  • Hypertrophy of the nail bed (clubbing) occurs in chronic suppurative disease (lung abscess, bronchiectasis, osteomyelitis) and in severe type of cyanosis (Fallot’s tetralogy, chronic congestive cardiac failure).
  • Disturbances of nail growth due to acute illness or trauma give rise to transverse grooves in the nail substance, which move distally with the nail growth. Since the average rate of growth is about 0.1 mm per day or 3 mm per month, the date of the past illness can be estimated.
  • It takes about 90-120 days for the whole nail (body) to grow. Therefore, in fungal diseases of the nails the course of treatment should last for not less than this period. The growth is faster in summer than in winter, in the fingers than in toes, and in the longer fingers than in the shorter ones.


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Lines and Ridges of Skin

Basic Anatomy , Skin and Fascia
Lines and Ridges of Skin

Lines and Ridges of Skin


The skin is marked by three types of surface irregularities: The tension lines, flexure lines and papillary ridges.

Tension lines


Form a network of linear furrows which divided the surface into polygonal or lozenge shaped areas . These lines to some extent correspond to variations in the pattern of fibers in the dermis.

Flexure lines


These are commonly known as skin creases or skin joints. They are certain permanent lines along which the skin folds during habitual movements of the joints. As the folding of skin normally occurs during flexion, they are known as flexure lines.The skin along these lines is thin and firmly bound to the deep fascia. The lines are prominent opposite the flexure of the joints, particularly on the palms, soles and digits.

Papillary ridges


These are also known as friction ridges. They are confined to palms and soles along with the digits. They form narrow ridges separated by fine parallel grooves, arranged in curved arrays. They correspond to patterns of dermal papillae. The major patterns in the human fingerprints include loops, whorls and arches. These patterns and many other minor features are determined genetically by multifactorial inheritance.


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Structure of Skin

Basic Anatomy , Skin and Fascia
Structure of Skin

Structure of Skin


The skin is composed of two distinct layers: epidermis and dermis.

Epidermis


It is the superficial avascular layer of stratified squamous epithelium, It is ectodermal in origin and gives rise to the appendages of the skin, namely hair, nails, sweat glands and sebaceous glands.

Structurally the epidermis is made up of a superficial cornified zone and a deep germinative zone. The cells of the deepest layer proliferate and pass towards the surface to replace the cornified cells lost due to wear and tear. As the cells migrate superficially, they become more and more flattened and lose their nuclei to form flattened dead cells of the stratum corneum. In the germinative zone, there are cells which synthesize melanin.

Dermis


Dermis or corium is the deep, vascular layer of the skin derived from mesoderm. It is made up of connective tissue mixed with blood vessels, lymphatics and nerves. The connective tissue is arranged into a superficial papillary layer and a deep reticular layer. The papillary layer forms conical, blunt projections, called dermal papillae. These papillae fit into reciprocal depressions on the undersurface of the epidermis. The reticular layer is composed chiefly of the white fibrous tissue arranged mostly in parallel bundles.

The direction of the bundles constitutes the flexor lines, also known as cleavage lines or Langer’s lines. These are longitudinal in the limbs and horizontal in the trunk and neck.

At the flexure lines the skin is firmly adherent to the underlying deep fascia. Dermis is the real skin, because when dried it makes green hide and when tanned it makes leather. Its deep surface is continuous with the superficial fascia.


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Pigmentation of Skin

Basic Anatomy , Skin and Fascia
Pigmentation of Skin

Pigmentation of Skin


The color of the skin is determined by at least five pigments present at different levels and places of the skin. These pigments are:

  • Melanin:

It is brown is color and is found in the germinative zone of the epidermis.

  • Melanoid:

It resembles the melanin pigment in structure, however it is present diffusely throughout the epidermis.

  • Carotene:

It is a yellow to orange pigment found in the stratum corneum and the fat cells of dermis and superficial fascia.

  • Hemoglobin:

It is actually a blood pigment but it can make the skin appear purple in color.

  • Oxyhemoglobin:

It is the oxygenated form of hemoglobin and is also found in blood however, it can make the skin look red.

The amounts of first three pigments (Melanin, Melanoid and Carotene) vary with race, age and part of the body. In white races, the color of the skin depends chiefly on the vascularity of the dermis and thickness of the keratin. More vascularity means red color and more thickness means white (pale) color. In the lips, where the keratin is thin, the color is red, while in the palms and soles, the keratin is thick and thus the color is white.


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Skin

Basic Anatomy , Skin and Fascia
Skin

Skin


Skin is the general covering of the entire external surface of the body, including the external auditory meatus and the outer surface of tympanic membrane.


The skin is continuous with the mucous membrane at the orifices of the body.  The skin performs a large number of important functions, and consequently it is considered a very important organ of the body.

Surface area of Skin


In an adult human, the surface area of the skin is 1.5-2 square meters. The proportion of skin covering various parts of human body can be explained in percentages as:

  • Head and Neck = 9%
  • Each upper limb = 9%
  • Front of the trunk = 18%
  • Back of the trunk (including buttocks) = 18%
  • Each lower limb = 18%
  • Perineum =1%
This assessment is very important in cases involving burns, where the patient requires treatment according the magnitude of damaged area of skin.

Functions of Skin

  • Protection:

The skin is the largest organ of human body and guards the underlying muscles, bones, ligaments and other delicate organs. Furthermore, the skin is in contact with the environment all the time and because of this reason, a healthy intact skin is very important for protection against the pathogens and other harmful agents.

  • Prevent dehydration:

The skin forms a water proof layer all around the body thus preventing excessive loss of water, which is vital for people living in deserts and dry places.

  • Sensation:

The skin contains a variety of receptors that react to different types of stimuli. The common sensations originated at the skin include: touch, pressure, hot, cold, vibration, injury etc.

  • Heat regulation:

Skin forms an important organ for regulation of heat. It receives far greater blood supply than is needed. This blood supply is precisely controlled. Whenever the body gets overheated, the blood supply to skin increases, and because the skin is in direct contact with the atmosphere, the heat from the blood is lost quickly and the temperature of the body is regulated. The sweating process further facilitates the cooling of blood.

  • Excretion:

The skin excretes small amount of urea in the form of sweat. The sweating process is primarily meant for regulating the temperature of body, however, excretion of urea is also achieved through it.


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Comparison of Sympathetic and Parasympathetic Nervous Systems

Basic Anatomy , Nervous System
Comparison of Sympathetic and Parasympathetic Nervous Systems

Comparison of Sympathetic and Parasympathetic Nervous Systems


Sympathetic and parasympathetic nervous systems are two divisions of the autonomic nervous system of body. They are closely coordinated with one another to regulate the functions of human body. In the lines below a brief comparison of the two systems is provided.

Sympathetic Nervous System


  1. All neurons forming this system originate from T1 to L2 segment of spinal cord. So it is called thoracolumbar outflow.
  2. Pre-ganglionic fibers are short, relay either in lateral ganglia or collateral ganglia
  3. Post-ganglionic fibers are long Nerve endings are adrenergic in nature except in sweat gland
  4. Functionally, sympathetic nerves are vasomotor, sudomotor and pilomotor to skin. It is seen when subject is in fear, fight and flight position. It dilates skeletal muscle blood vessels
  5. Effect is widely diffused and directed towards mobilization of resources and expenditure of energy during emergency and emotional crisis
  6. It supplies visceral blood vessels, skin. Afferents from viscera and specific area of skin reach the same spinal segment to go to the cerebrum. Since pain is better appreciated from the skin, it appears to be coming from skin rather than the viscera. This is the basis of referred pain.

Parasympathetic Nervous System


  1. All neurons forming this system originate from brain (Ill, VII, IX, X cranial nerves) and S2—S4 segment of spinal cord. So it is called craniosacral outflow.
  2. Pre-ganglionic fibers are very long reaching up to terminal ganglia mostly on viscera. Postganglionic fibers are short
  3. Nerve endings are cholinergic in nature
  4. Functionally, it is seen when subject is fully relaxed. Parasympathetic system has no effect on skin
  5. Effect is discrete, isolated, directed towards conservation and restoration of the resources of
  6. It only supplies viscera
For complete details visit the following links:


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Autonomic Nervous System

Basic Anatomy , Nervous System
Autonomic Nervous System

Autonomic Nervous System


Autonomic nervous system is the part of the Peripheral Nervous System (PNS), which control the unconscious functions of body. Autonomic nervous system controls involuntary activities of the body, like sweating, salivation, peristalsis, etc. It differs fundamentally from the somatic nervous system in having:

  • The preganglionic fibers arising from the CNS
  • The ganglia for relay of the preganglionic fibers
  • The postganglionic fibers arising from the ganglia which supply the effectors (smooth muscles and glands).
In contrast, the somatic nerves after arising from the CNS reach their destination without any interruption.

Autonomic nervous system is divided into two more or less complementary parts: the sympathetic nervous system and parasympathetic nervous system.

The sympathetic activities are widespread and diffuse, and combat the acute emergencies. The parasympathetic activities are usually discrete and isolated, and provide a comfortable environment. Both systems function in absolute coordination and adjust the body involuntarily to the given surroundings.



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Parasympathetic Nervous System

Basic Anatomy , Nervous System
Parasympathetic Nervous System

Parasympathetic Nervous System


Parasympathetic nervous system is one of the two main divisions of the autonomic nervous system.  It is responsible specifically for stimulation of activities that occur when the body is at rest. The main goal of this system is to conserve the resources of the body so that they can last longer. The parasympathetic nervous system is also known as craniosacral outflow because it arises from the brain (mixed with III, VII, IX and X cranial nerves) and sacral 2-4 segments of the spinal cord. Thus it has a cranial and a sacral part.

The preganglionic fibers are very long, reaching right up to the viscera of supply. The ganglia, called terminal ganglia, are situated mostly on the viscera and, therefore, the postganglionic fibers are very short. Parasympathetic nerve endings are cholinergic in nature, similar to the somatic nerves.

Functionally, parasympathetic activity is seen when the subject is fully relaxed. His pupils are constricted, lenses accommodated, face flushed, mouth moist, pulse slow, blood pressure low, bladder and gut contracting, and the perineal sphincters relaxed.
In general the effects of parasympathetic activity are usually discrete and isolated, and directed towards conservation and restoration of the resources of energy in the body.


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Sypmathetic Nervous System

Basic Anatomy , Nervous System
Sypmathetic Nervous System

Sypmathetic Nervous System


Sympathetic nervous system is a part of the autonomic nervous system of body. It is mainly concerned with mobilization of body’s resources under stress to induce the “fight or flight response“.

Sympathetic nervous system is also known as thoracolumbar outflow because it arises from T1 to L2 segments of the spinal cord.

The myelinated preganglionic fibers (white rami communicantes) arise from the lateral column of the spinal cord, emerge through the ventral rami where the white rami are connected to the ganglia of the sympathetic chain.

Preganglionic fibers relay either in the lateral ganglia (sympathetic chain) or in the collateral ganglia, e.g. the celiac ganglion. The non-myelinated postganglionic fibers (grey rami communicantes) run for some distance before reaching the organ of supply. The adrenal medulla is a unique exception in the body; it is supplied by the preganglionic fibers.

Sympathetic nerve endings are adrenergic in nature, meaning thereby that noradrenalin is produced for neurotransmission. The only exception to this general rule is the case of cholinergic sympathetic nerves supplying the sweat glands and skeletal muscle vessels for vasodilatation.

Functionally, sympathetic nerves are vasomotor (vasocons¬trictor), sudomotor (secretomotor to sweat glands), and pilomotor (contract the arrector pili and cause erection of hair) in the skin of limbs and body wall. In addition, sympathetic activity causes dilation of pupil, pale face, dry mouth, tachycardia, rise in blood pressure, inhibition of hollow viscera, and closure of the perineal sphincters.

The blood supply to the skeletal muscles, heart and brain is markedly increased. Thus, sympathetic reactions tend to be ‘mass reactions’, widely diffused in their effect and that they are directed towards mobilization of the resources of the body for expenditure of energy in dealing with the emergencies or emotional crises (fright, fight, flight).



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Myelinated and Non-myelinated Nerve Fibers

Basic Anatomy , Nervous System
Myelinated and Non-myelinated Nerve Fibers

Nerve Fibers


Nerve fiber is a thread like extension of a neuron, which is formed by the axon and its covering. Thus each nerve fiber is an axon with its coverings. Larger axons are covered by a myelin sheath and are termed myelinated or medullated fibers. The fatty nature of myelin is responsible for the glistening whiteness of the peripheral nerve trunks and white matter of the CNS. Thinner axons, of less than one micron diameter, do not have the myelin sheath and are therefore termed non-myelinated or non-medullated.

However, all the fibers whether myelinated or non-myelinated have a neurolemmal sheath, which is uniformly absent in the tracts. In peripheral nerves, both the myelin and neurolemmal sheaths are derived from Schwann cells.

Myelinated Fibers


Myelinated fibers form the bulk of the somatic nerves. Structurally, they are made up of following parts from within outwards.

  1. Axis cylinder forms the central core of the fiber. It consists of axoplasm covered by axolemma.
  2. Myelin sheath, derived from Schwann cells, surrounds the axis cylinder. It is made up of alternate concentric layers of lipids and proteins formed by spiralization of the mesaxon: the lipids include cholesterol, glycolipids and phospholipids.
    Myelin sheath is interrupted at regular intervals called the nodes of Ranvier where: the adjacent Schwann cells meet. Collateral branches of the axon arise at the nodes of Ranvier. Thicker axons possess a thicker coat of myelin and longer internodes.
    Each internode is myelinated by one Schwann cell. Oblique clefts in the myelin, called incisures of Schmidt Lantermann, provide conduction channels for metabolites into the depth of the myelin and to the subjacent axon. Myelin sheath acts as an insulator for the nerve fibers.
  3. Neurolemmal sheath (sheath of Schwann) surrounds the myelin sheath. It represents the plasma membrane (basal lamina) of the Schwann cell. Beneath the membrane there lies a thin layer of cytoplasm with the nucleus of the Schwann cell. The sheaths of two cells interdigitate at the nodes of Ranvier.
  4. Neurolemmal sheath is necessary for regeneration of a damaged nerve. Tracts do not regenerate because of absence of neurolemmal sheath.
  5. Endoneurium is a delicate connective tissue sheath which surrounds the neurolemmal sheath.

Non-Myelinated Fibers


Non-myelinated fibers comprise the smaller axons of the CNS, in addition to peripheral postganglionic autonomic fibers, several types of fine sensory fibers (C fibers of skin, muscle and viscera), olfactory nerves, etc. Structurally, a non-myelinated fiber consists of a group of small axons that have invaginated separately a single Schwann cell (in series) without any spiraling of the mesaxon.
The endoneurium, instead of ensheathing individual axons, surrounds all the neurolemmal sheath by virtue of which the non-myelinated fibers, like the myelinated fibers, can regenerate after damage.



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