Control and co-ordination in plants and animals - class-X

Larynx lies ventral to the thyroid cartilage. The thyroid cartilage is the largest of the laryngeal cartilages. The thyroid cartilage forms the bulk of the front wall of the larynx. It protects the vocal cords, which are located directly behind it.

Growth hormone is released into the bloodstream from the anterior pituitary gland. Growth hormone release is caused by another hormone (growth hormone-releasing hormone), which is released from the hypothalamus and it causes the pituitary gland to release the growth hormone.

Vasopressin/antidiuretic hormone (ADH) is secreted by posterior pituitary gland and targets nephron tubules to increase water reabsorption and thereby decreasing water content in urine while increasing that of blood. Increased water concentration in blood raises blood volume and blood pressure. It makes the urine concentrated. Thus, the correct answer is option A.

Follicle-stimulating hormone (FSH) is a glycoprotein and gonadotropin secreted by the anterior pituitary in response to gonadotropin-releasing hormone (GnRH) released by the hypothalamus. It helps in the development of follicles in female and triggers Sertoli cells to initiate spermatogenesis in the male.

Abnormal secretion of hormones brings about physical changes in the body. The hyposecretion of  ADH showcases as diabetes insipidus with increased urine output and excessive thirst. Thus, the correct answer is option B.

The hormones FSH and LH are together called as gonadotropin hormones. They are called the gonadotropins as they stimulate gonadal activity. The anterior pituitary (pars distalis) releases following gonadotrophic hormones:

The anterior pituitary (also called the adenohypophysis or pars anterior) contains five types of endocrine cell, and they are defined by the hormones they secrete: somatotropes (GH); prolactins (PRL); gonadotropes (LH and FSH); corticotropes (ACTH) and thyrotropes (TSH). Hormones secreted by the anterior pituitary are trophic hormones and tropic hormones. 

Vasopressin also known as ADH (Anti-Diuretic Hormone) is made up by hypothalamus and it is transported to posterior pituitary gland which secretes the hormone into the blood stream. Its functions:

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are called as gonadotropins, because stimulate the gonads - in males, the testes, and in females, the ovaries.

In vertebrates, the pituitary gland is an endocrine gland which is about the size of a pea. The intermediate lobe synthesizes and secretes melanocyte-stimulating hormone (MSH). It stimulates the production and release of melanin by melanocytes in skin and hair. Thus, the correct answer is option D.

Androgens are a group of hormones, that primarily influence the growth and development of the male reproductive system. The predominant and most active androgen is testosterone, which is produced by the male testes. The other androgens, which support the functions of testosterone, are produced mainly by the adrenal cortex, the outer portion of the adrenal glands and only in relatively small quantities.

The placenta connects the developing foetus to the wall of the mother’s uterus during pregnancy.

LH
and FSH are collectively called as gonadotropin. They are called the
gonadotropins as they stimulate gonadal activity. The anterior pituitary (pars
distalis) releases following gonadotrophic hormones:

Spermatogenesis depends on the action of the FSH and LH hormone of the hypophysis on the testicular cells. LH acts on the interstitial cells, stimulating the production of testosterone necessary for the normal development of spermatogenic cells. FSH acts on the sertoli cells, stimulating adenylate cyclase and consequently increasing the presence of cAMP; it also promotes the synthesis and secretion of androgen-binding protein. This protein combines with testosterone and transports it into the lumen of the seminiferous tubules. 

Adrenocorticotropic hormone (ACTH), also known as corticotropin, is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland. It is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological stress. Its principal effects are increased production and release of corticosteroids. 

Gonadotropins (or glycoprotein hormones) are protein hormones secreted by gonadotropes of the anterior pituitary of vertebrates. It includes the mammalian hormones follicle-stimulating hormone (FSH), luteinizing hormone (LH), placental chorionic gonadotropins hCG and chorionic gonadotropin.

The acidophils are the somatotropic cells and the lactotrophic cells. Thus, growth hormone and prolactin are secreted by acidophilic cells. In the anterior pituitary, the term acidophil is used to describe two different types of cells: somatotrophs, which generate somatotropin (also known as growth hormone) and mammotrophs, which generate prolactin.

The adrenal glands are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. Adrenal glands are concerned with salt equilibrium in the body. 

The growth process begins in the hypothalamus. When the hypothalamus releases growth hormone releasing hormone (GHRH), the anterior pituitary is stimulated to release growth hormone (GH). Growth hormone then acts on the liver and other tissues and stimulates them to secrete insulin like growth factor-1 (IGF-1). IGF-1 directly promotes the development of bone and muscle, causing bones to grow in length, and muscles to increase protein synthesis.

Prolactin is a peptide gonadotrophic hormone secreted by the pituitary gland; it stimulates growth of the mammary glands. Progesterone, secreted from the corpus luteum, prolongs the secretion of prolactin. Luteinizing hormone is a hormone produced by gonadotroph cells in the anterior pituitary gland. In females, an acute rise of LH triggers ovulation and development of the corpus luteum. 

If a person drinks enough water, this condition will not have significant effects on the fluid or electrolyte balance of the body. If the person does not drink enough fluids, high urine output may cause dehydration and high levels of sodium in the blood. The primary symptoms of diabetes insipidus are excessive thirst and urination. A person can produce large amounts of urine, usually more than 3 litres up to 15 litres per day.

MSH is important in the regulation of melanin synthesis in cells called melanophores (a type of chromatophore) that are in the skin of amphibians, some fishes, and reptiles. Light reflected from water surface stimulates photoreceptors, which send information to the hypothalamus. Pituitary MSH then causes the pigment synthesis in the melanophores to disperse and the skin to darken.

The anterior pituitary is also known as the adenohypophysis, meaning glandular undergrowth. Gonadotropes (LH and FSH) are secreted from the anterior pituitary. Luteinizing hormone (LH) stimulates the ovulation cycle in female mammals, while in the males, LH stimulates the synthesis of androgen. FSH helps in the growth of the reproductive system.

Diabetes insipidus is a condition caused by hyposecretion of antidiuretic hormone (ADH). 

Excess amounts of androgens result in acne, hirsutism (excess hair growth on the chin or upper lip) and thinning hair. Many women with high levels of a form of testosterone have polycystic ovary syndrome (PCOS), infertility, blood sugar disorders. High levels of androgens are associated with insulin resistance and diabetes, high cholesterol, high blood pressure and heart disease.

Antidiuretic hormone, also known commonly as vasopressin, is a nine amino acid peptide secreted from the posterior pituitary. It has two principal sites of action: the kidney and blood vessels. Its two primary functions are to retain water in the body and to constrict blood vessels. 

Menstruation is the shedding of the lining of the uterus (endometrium) accompanied by bleeding. Bleeding occurs after oestrogen and progesterone levels decrease at the end of the previous cycle. This decrease causes the top layers of thickened lining of the uterus (endometrium) to break down and be shed. 

The adrenal cortex produces a hormone, called as aldosterone, that is involved in regulating the amount of salt and water in the body. Aldosterone is a mineralocorticoid, which acts on the principal cells of the collecting ducts, ducts of sweat and salivary glands.

In the male, luteinizing hormone promotes the development of the interstitial tissue (Leydig cells) of the testes and hence promotes the secretion of the male sex hormone, testosterone. 

Insulin helps control blood glucose levels by signaling the liver and muscle and fat cells to take in glucose from the blood. Insulin therefore helps cells to take in glucose to be used for energy.

Adrenaline is a natural stimulant hormone made in the adrenal gland of the kidney. Adrenaline is carried in the bloodstream and affects the autonomous nervous system, which controls functions such as the heart rate, dilation of the pupils, and secretion of sweat and saliva.

Progesterone is a hormone secreted by ovaries. It maintains the lining of the uterus during the middle part of the menstrual cycle and during pregnancy. If a pregnancy occurs, progesterone is produced in the placenta, and levels remain elevated throughout the pregnancy. One of the progesterone's most important functions is to cause the endometrium to secrete special proteins during the second half of the menstrual cycle, preparing it to receive and nourish an implanted fertilized egg. If implantation does not occur, estrogen and progesterone levels drop, the endometrium breaks down and menstruation occurs.

In females, ovulation of mature follicles on the ovary is induced by LH secretion known as the preovulatory LH surge. Residual cells within ovulated follicles proliferate to form corpus luteum, which secretes the steroid hormones progesterone and estradiol. Progesterone is necessary for the maintenance of pregnancy, and, in most mammals, LH is required for continued development and function of corpora lutea. The name luteinizing hormone derives from this effect of inducing luteinization of ovarian follicles.

Leydig cells, also known as interstitial cells of Leydig, are found adjacent to the seminiferous tubules in the testicle. They produce testosterone in the presence of luteinizing hormone (LH). 

In men, testosterone plays a key role in the development of male reproductive tissues, such as the testis and prostate as well as promoting secondary sexual characteristics, such as increased muscle, bone mass, and the growth of body hair.

The male sex hormone testosterone is produced by Leydig cells. These cells are located in the connective (interstitial) tissue, that holds the tubules together within each lobule. The tissue becomes markedly active at puberty under the influence of the interstitial-cell-stimulating hormone of the anterior lobe of the pituitary gland.

Prolactin is produced both in the anterior pituitary gland. It stimulates the mammary glands to produce milk (lactation); increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and prepare for the production of milk. 

In men, testosterone plays a key role in the development of male reproductive tissues, such as the testis and prostate as well as promoting secondary sexual characteristics, such as increased muscle, bone mass, and the growth of body hair.

In women, relaxin is secreted into the circulation by the corpus luteum in the ovary. During pregnancy it is also released from the placenta, the membranes, which surround the foetus, and the lining of the uterus. During pregnancy, relaxin levels are at their highest in the first trimester. At this time it is believed to promote implantation of the developing foetus into the wall of the uterus and the growth of the placenta. Towards the end of pregnancy, relaxin promotes rupture of the membranes surrounding the foetus and the growth, opening and softening of the cervix and vagina to aid the process of childbirth. Relaxin can relax the ligaments at the front of the cervix to ease delivery of the baby. 

Gonadotropins are secreted from the anterior pituitary gland and act on the gonads (i.e., the ovaries or testes). Gonadotrophs secrete two primary gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). 

Estrogen promotes the development of the ductal system, while progesterone promotes the development of the lobulo-alveolar system in the breasts during pregnancy.

Prolactin was discovered in non-human animals around 1930 by Oscar Riddle. It promotes milk production in the female. It is also known as a lactogenic hormone. It is produced by the pituitary gland.

Androgens include the male hormones testosterone and androsterone and promote development of male sex characteristics. The androgens promote the secondary sex characteristics of muscle growth, lowered voice range, the Adam's apple, and increased body hair.

Glucagon's role in the body is to prevent blood glucose levels dropping too low. It acts on the liver and stimulates the conversion of stored glycogen (stored in the liver) to glucose, which can be released into the bloodstream. This process is called as glycogenolysis.

Gynaecomastia is enlargement of the male breast tissue. Oestrogen stimulates breast tissue growth, while androgens inhibit it. The important factor is the ratio of active androgens to oestrogens. The ratio can be altered as a result of reduced testosterone production/action or enhanced oestrogen production/action or both. 

Follicle-stimulating hormone (FSH) is synthesized and secreted under the influence of GnRH on the anterior pituitary. It regulates the development, growth, maturation of ovarian follicle, pubertal maturation and reproductive processes of the body. FSH and luteinizing hormone (LH) act synergistically in reproduction. Under the influence of FSH ovary secretes estrogen. Thus the correct answer is option C.

The major sex hormone in men is testosterone, which is produced mainly in the testes. They are crucial for male sexual and reproductive function. They are also responsible for the development of secondary sexual characteristics in men, including facial and body hair growth, and bone and muscle development. 

Progesterone is a natural female hormone. It is called as the pregnancy hormone, it is essential before and during pregnancy. After ovulation occurs, the ovaries start to produce progesterone needed by the uterus. Progesterone causes the uterine lining or endometrium to thicken. This helps prepare a supportive environment in the uterus for a fertilized egg. After 8 to 10 weeks of pregnancy, the placenta takes over progesterone production from the ovaries and substantially increases progesterone production.

The secretion of pituitary gonadotrophic hormones is stimulated by gonadotropin hormone-releasing hormone (GnRH) from the hypothalamus and modulated by testicular hormones. At the onset of puberty, the pituitary gland secretes GnRH, or gonadotropin-releasing hormone. In response, the hypothalamus release follicle stimulating hormone (FSH) and luteinizing hormone (LH) into the male system. LH enters the testes, stimulating the Leydig cells to release testosterone. Testosterone hormone is responsible for the secondary sexual characteristics like deepening of the voice, the growth of facial, axillary, and pubic hair, and the beginnings of the sex drive.

Acromegaly is a hormonal disorder that develops when pituitary gland produces too much growth hormone during adulthood. When this happens, bones increase in size, including those of hands, feet and face. Acromegaly usually affects middle-aged adults.

Insulin and glucagon work synergistically to keep blood glucose concentrations normal. An elevated blood glucose concentration results in the secretion of insulin which leads to glucose transportation into body cells. Glucagon is secreted by the alpha cells of the pancreas when blood glucose is low. Glucagon's function is to cause the liver to release stored glucose from its cells into the blood. Corticoids are also responsible for the regulation of glucose metabolism. 

Human chorionic gonadotrophin is the embryonic hormone, which ensures the corpus luteum continues to produce progesterone throughout the first trimester of pregnancy. The corpus luteum continues to produce progesterone until the placenta is established (the placenta then takes over progesterone production).

Alpha cells are endocrine cells in the islets of Langerhans of the pancreas. Thy are responsible for synthesizing and secreting the peptide hormone glucagon, which elevates the glucose levels in the blood.

Most birth control pills are combination pills containing a combination of the hormones oestrogen and progesterone to prevent ovulation. A woman cannot get pregnant if she doesn't ovulate because there is no egg to be fertilized. The pill also works by thickening the mucus around the cervix, which makes it difficult for sperm to enter the uterus and reach any eggs that may have been released. The hormones in the pill can also sometimes affect the lining of the uterus, making it difficult for an egg to attach to the wall of the uterus.

Hyperglycemia, or high blood sugar (glucose), is a serious health problem for those with diabetes. Hyperglycemia develops, when there is too much sugar in the blood.

Intravenous injection of insulin is the injection of insulin directly into the blood. It is used to control blood sugar in people, where the body does not make insulin and therefore cannot control the amount of sugar in the blood. Insulin cannot be taken orally because the oral dose of insulin degenerates in the stomach by digestive enzymes. Thus, the correct answer is option C.

The posterior pituitary is the part of the brain that extends down from hypothalamus; the posterior pituitary and infundibulum are together termed as neurohypophysis. Pars nervosa is the posterior lobe of pituitary associated with neurohypophysis while pars distalis is anterior lobe of the adenohypophysis. Supraoptic and paraventricular nuclei of hypothalamus secrete oxytocin and vasopressin which are then stored and released into blood stream via neurohypophysis. Thus, the correct answer is option C.

In males, luteinizing hormone (LH) is also called as an interstitial cell-stimulating hormone (ICSH). It stimulates the production of testosterone by the Leydig cells. It acts synergistically with FSH.

Antidiuretic hormone (ADH) is secreted by posterior pituitary gland and targets nephrons to increase water reabsorption and thereby decreasing water content in urine. Oxytocin facilitates uterine contractions to push the foetus downward which in turn stretches cervix and leads to childbirth. MSH stimulates melanin production from melanocytes. Adrenocorticotropic hormone (ACTH) is secreted by the anterior pituitary gland and targets adrenal cortex to produce its hormones. Thus, the correct answer is option B.

Oxytocin is released by posterior pituitary during parturition in response to foetus ejection reflex. It stimulates strong uterine muscle contraction forcing the fetus downwards during parturition. Oxytocin helps to initiate release of milk by mammary gland after child birth. The more a baby suckles, the more oxytocin is released leading to milk let down. Thus, the correct answer is D.

MSH stimulates secretion of melanin from pigment cells in the skin and thereby causes skin darkening as required for camouflage in some lower vertebrates. Depending on the amount of melanin and carotene, the colour of skin is determined from pale yellow to dark. Thus, the correct answer is option A.

Rickets is a disorder caused by a lack of vitamin D, calcium, or phosphate. It leads to softening and weakening of the bones. Vitamin D helps the body control calcium and phosphate levels. If the blood levels of these minerals become too low, the body may produce hormones that cause calcium and phosphate to be released from the bones. This leads to weak and soft bones.

A fall in the concentration of calcium in the bloodstream is detected by the parathyroid glands, which then produce parathyroid hormone. 

Hormones of the adrenal cortex and pineal body are under control of tropic hormones of the anterior pituitary gland. The hormonal secretion of anterior pituitary is under control of regulatory hormones of the hypothalamus. The hormones of posterior pituitary are stored in axon ending and are secreted under nervous control. Thus, the correct answer is C.

Aldosterone of the adrenal cortex is life-saving hormone because it serves to retain sodium and water to maintain a sufficient blood volume for circulation. So, it maintains the osmolarity and volume of ECF. The pituitary gland regulates secretion of other endocrine glands. Pineal gland secretes melatonin that regulates body’s daily rhythms. Thyroid hormones regulate the basic metabolic rate. Thus, the correct answer is C.

The main hormone secreted by the testes is testosterone, an androgenic hormone. The placenta is an endocrine gland, that is only present during pregnancy. It secretes human chorionic gonadotrophin hormone (hCG), which is present only during pregnancy.

The adrenal medulla secretes 2 hormones - adrenalin and noradrenalin. 

The hypothalamus is a mass of grey matter present at the base of the cerebrum. The centre for thirst and drinking regulation is present in lateral hypothalamus while that for hunger and satiety is present in the arcuate nucleus, ventral nucleus and lateral hypothalamic area. The anterior hypothalamic region regulates body temperature.

Undersecretion of insulin causes diabetes mellitus type I while myxedema is caused by thyroxine deficiency in adults. Hyposecretion of parathyroid hormone causes increased excitability of muscles and their sustained contraction referred to as parathyroid tetany. Luteinizing hormone deficiency causes failure of ovulation as LH surge during midcycle stimulates rupture of Graffian follicle. 

Puberty begins with a surge in hormone production, which in turn causes a number of physical changes. It is the stage in life in which a child develops secondary sex characteristics as his or her hormonal balance shifts strongly towards an adult state. This is triggered by the pituitary gland which secretes a surge of hormonal agents into the blood stream, initiating a chain reaction. The male and female gonads are subsequently activated, which puts them into a state of rapid growth and development. The testes release testosterone and the ovaries release estrogen. The production of these hormones increases gradually until sexual maturation is met.

The hypothalamus is the control centre for many autonomic functions of the peripheral nervous system. Connections with structures of the endocrine and nervous systems enable the hypothalamus to play a vital role in maintaining homeostasis. The hormones produced by this area of the brain govern body temperature, thirst, hunger, sleep, circadian rhythm, moods, sex drive, and the release of other hormones in the body. Hence, if hypothalamus is damaged in an accident, control of water balance, hunger and body temperature will be lost. 

The synthesis and secretion of estrogen is stimulated by follicle-stimulating hormone (FSH), which is, in turn, controlled by the hypothalamic gonadotropin releasing hormone (GnRH).

Trophoblast refers to outer cells of a blastocyst that serve in placenta formation in the beginning of the second trimester. It secretes human chorionic gonadotropin that ensures persistence of the corpus luteum. The later serves to secrete ovarian hormone estrogen, progesterone, and relaxin that are required to maintain pregnancy. Hence, removal of ovaries after 4th month would not affect the pregnancy. The correct answer is C.

FSH is primarily involved in stimulating the growth of ovarian follicles, while LH induces ovulation. At ovulation, the ovum or egg is released and the ruptured follicle is transformed into a corpus luteum, which secretes progesterone as its main product. 

Leydig cells, also known as interstitial cells of Leydig, are found adjacent to the seminiferous tubules in the testicle. They produce testosterone in the presence of luteinizing hormone (LH). 

Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus and stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. 

The corpus luteum in the ovaries is the major site of progesterone production in humans, progesterone is also produced in smaller quantities by the ovaries themselves, the adrenal glands and, during pregnancy, the placenta.

Progesterone belongs to a group of steroid hormones called as progestogens. It is mainly secreted by the corpus luteum in the ovary during the second half of the menstrual cycle. It plays important roles in the menstrual cycle and in maintaining the early stages of pregnancy. The progesterone prepares the body for pregnancy in the event, that the released egg is fertilised. If the egg is not fertilised, the corpus luteum breaks down, the production of progesterone falls and a new menstrual cycle begins. Hence, progesterone is called as the 'anti abortion' hormone. 

When contraction of the uterus starts, oxytocin is released, which stimulates more contractions and more oxytocin is released. In this way, contractions increase in intensity and frequency. Oxytocin causes contractions during the second and third stages of labor.

The corpus luteum is a temporary endocrine structure in female mammals, that is involved in the production of progesterone and moderate levels of estradiol and inhibin. The corpus luteum develops from an ovarian follicle during the luteal phase of the menstrual cycle or oestrous cycle, following the release of a secondary oocyte from the follicle during ovulation. 

Luteinizing hormone (LH) stimulates the production of sex hormones by gonads. Since LH acts on gonads, it is known as gonadotropic hormones.  

Gynaecomastia refers to the presence of enlarged mammary glands in males like that of females and is caused by the presence of extra X chromosome in males. Gonochorism is the unisexual condition. Feminization refers to the development of female secondary sexual characters caused by hormonal changes at puberty in females. So, the correct answer is option D.

ADH secretion is regulated by plasma osmolality. If osmolality increases it stimulates secretion of ADH which act at the collecting duct of nephron where it causes the reabsorption of only water and produces the concentrated urine.

Progesterone is produced from the corpus luteum, which is the small gland, that forms in the ovary after ovulation has occurred. This hormone is essential for the maintenance of pregnancy and also for the development of many symptoms during early pregnancy.

Vasopressin, also known as antidiuretic hormone (ADH), is a neurohypophysial hormone which has two primary functions- to retain water in the body and to constrict the blood vessels. Vasopressin regulates the body's retention of water by increasing water reabsorption in the collecting ducts of the kidney nephron.

Prolactin (PRL), also known as luteotropic hormone or luteotropin, is a protein, that in humans is best known for its role in enabling female mammals to produce milk. Prolactin is a single-chain protein hormone. It is secreted by the lactotrophs in the anterior pituitary (adenohypophysis).

In women, FSH hormone stimulates the growth of ovarian follicles in the ovary, before the release of an egg from one follicle at ovulation. It also increases oestradiol production. 

Acromegaly is the abnormal growth of the hands, feet, and face. It is caused when there is oversecretion of growth hormone (GH) in adulthood. Similarly, oversecretion of growth hormone in childhood causes Gigantism. 

The adrenal gland is known as 4S gland. 4S stands for the source of energy, sugar metabolism, salt retention and sex hormones. 

All of the above are gastrointestinal hormones,

The pineal gland, also known as the pineal body, conarium or epiphysis cerebri, is a small endocrine gland in the vertebrate brain.

The thyroid gland is a bilobed organ of the endocrine system located in the front of the neck. It secretes hormones that are involved in human development, growth, and metabolism.

The adrenal glands (also known as supra-renal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys.

When the body has extra glucose, it stores it in the liver and muscles. This stored form of glucose is called glycogen.

Homeostasis relates to dynamic physiological processes that help us maintain an internal environment suitable for normal function. Feedback mechanism also helps in maintaining this balance of the hormones in the body.

Myxedema refers to a severe form of hypothyroidism than can occur when the condition is left untreated or is not treated sufficiently. The term also applies to the effects that hypothyroidism can have on the skin, making it appear swollen and puffy.

Testosterone is the primary male sex hormone and an anabolic steroid. In male humans, during puberty, the growth of mustache and beard is seen as a sign of growth.

All the mentioned are the secretions of the adrenal gland.

• (i) Auxin - (D) parthenocarpy in tomato -Auxins are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins also induce parthenocarpy in

A neuron can be defined as a long cytoplasmic process of the cell body, which transmits the impulse from the cell body to other neurons is termed. The axon is covered by a lipid-rich membrane, called myelin sheath. The myelin sheath is formed by Schwann cells and each Schwann cell covers a part of the axon.
Thus, the correct answer is the option (B), 'Axon of neurons'.