17.1: نظرة عامة على نظام الغدد الصماء
- Page ID
- 203092
أهداف التعلم
- التمييز بين أنواع التواصل بين الخلايا وأهميتها وآلياتها وتأثيراتها
- التعرف على الأعضاء والأنسجة الرئيسية لنظام الغدد الصماء وموقعها في الجسم
الاتصال هو عملية يقوم فيها المرسل بإرسال الإشارات إلى جهاز استقبال واحد أو أكثر للتحكم في الإجراءات وتنسيقها. في جسم الإنسان، يشارك نظامان رئيسيان للأعضاء في الاتصال «البعيد المدى» نسبيًا: الجهاز العصبي ونظام الغدد الصماء. هذان النظامان معًا مسؤولان بشكل أساسي عن الحفاظ على التوازن في الجسم.
الإشارات العصبية والغدد الصماء
يستخدم الجهاز العصبي نوعين من الاتصالات بين الخلايا - الإشارات الكهربائية والكيميائية - إما عن طريق العمل المباشر للجهد الكهربائي، أو في الحالة الأخيرة، من خلال عمل الناقلات العصبية الكيميائية مثل السيروتونين أو النوربينفرين. تعمل الناقلات العصبية محليًا وبسرعة. عندما تصل إشارة كهربائية في شكل جهد فعل إلى الطرف المشبكي، فإنها تنتشر عبر الشق المشبكي (الفجوة بين الخلايا العصبية المرسلة والخلايا العصبية المستقبلة أو الخلية العضلية). بمجرد تفاعل الناقلات العصبية (الارتباط) مع المستقبلات الموجودة على الخلية المستقبلة (ما بعد التشابك)، يتم تحويل تحفيز المستقبل إلى استجابة مثل استمرار الإشارات الكهربائية أو تعديل الاستجابة الخلوية. تستجيب الخلية المستهدفة في غضون أجزاء من الثانية من تلقي «الرسالة» الكيميائية؛ ثم تتوقف هذه الاستجابة بسرعة كبيرة بمجرد انتهاء الإشارة العصبية. بهذه الطريقة، يمكّن الاتصال العصبي وظائف الجسم التي تتضمن إجراءات سريعة وموجزة، مثل الحركة والإحساس والإدراك. في المقابل، يستخدم نظام الغدد الصماء طريقة واحدة فقط للاتصال: الإشارات الكيميائية. يتم إرسال هذه الإشارات من قبل أعضاء الغدد الصماء، التي تفرز المواد الكيميائية - الهرمون - في السائل خارج الخلية. يتم نقل الهرمونات بشكل أساسي عبر مجرى الدم في جميع أنحاء الجسم، حيث ترتبط بمستقبلات الخلايا المستهدفة، مما يؤدي إلى استجابة مميزة. ونتيجة لذلك، تتطلب إشارات الغدد الصماء وقتًا أطول من الإشارات العصبية لتحفيز الاستجابة في الخلايا المستهدفة، على الرغم من أن مقدار الوقت الدقيق يختلف باختلاف الهرمونات. على سبيل المثال، تحدث الهرمونات التي يتم إطلاقها عندما تواجه موقفًا خطيرًا أو مخيفًا، يسمى استجابة القتال أو الهروب، عن طريق إطلاق هرمونات الغدة الكظرية - الإيبينيفرين والنوربينفرين - في غضون ثوانٍ. في المقابل، قد يستغرق الأمر ما يصل إلى 48 ساعة حتى تستجيب الخلايا المستهدفة لهرمونات تناسلية معينة.
بالإضافة إلى ذلك، عادةً ما تكون إشارات الغدد الصماء أقل تحديدًا من الإشارات العصبية. قد يلعب نفس الهرمون دورًا في مجموعة متنوعة من العمليات الفسيولوجية المختلفة اعتمادًا على الخلايا المستهدفة المعنية. على سبيل المثال، يعزز هرمون الأوكسيتوسين تقلصات الرحم لدى النساء أثناء المخاض. كما أنها مهمة في الرضاعة الطبيعية، وقد تشارك في الاستجابة الجنسية ومشاعر الارتباط العاطفي لدى كل من الذكور والإناث.
بشكل عام، يتضمن الجهاز العصبي استجابات سريعة للتغيرات السريعة في البيئة الخارجية، وعادة ما يكون نظام الغدد الصماء أبطأ في العمل - الاهتمام بالبيئة الداخلية للجسم، والحفاظ على التوازن، والتحكم في التكاثر (الجدول\(\PageIndex{1}\)). So how does the fight-or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.
| Endocrine system | Nervous system | |
|---|---|---|
| Signaling mechanism(s) | Chemical | Chemical/electrical |
| Primary chemical signal | Hormones | Neurotransmitters |
| Distance traveled | Long or short | Always short |
| Response time | Fast or slow | Always fast |
| Environment targeted | Internal | Internal and external |
Structures of the Endocrine System
The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. The endocrine gland is the major player in this system. The primary function of these ductless glands is to secrete their hormones directly into the surrounding fluid. The interstitial fluid and the blood vessels then transport the hormones throughout the body. The endocrine system includes the pituitary, thyroid, parathyroid, adrenal, and pineal glands (Figure \(\PageIndex{1}\)). Some of these glands have both endocrine and non-endocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries, and male testes are other organs that contain cells with endocrine function. Moreover, adipose tissue has long been known to produce hormones, and recent research has revealed that even bone tissue has endocrine functions.
The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine.
Other Types of Chemical Signaling
In endocrine signaling, hormones secreted into the extracellular fluid diffuse into the blood or lymph, and can then travel great distances throughout the body. In contrast, autocrine signaling takes place within the same cell. An autocrine (auto- = “self”) is a chemical that elicits a response in the same cell that secreted it. Interleukin-1, or IL-1, is a signaling molecule that plays an important role in inflammatory response. The cells that secrete IL-1 have receptors on their cell surface that bind these molecules, resulting in autocrine signaling.
Local intercellular communication is the province of the paracrine, also called a paracrine factor, which is a chemical that induces a response in neighboring cells. Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues. A familiar example to those with asthma is histamine, a paracrine that is released by immune cells in the bronchial tree. Histamine causes the smooth muscle cells of the bronchi to constrict, narrowing the airways. Another example is the neurotransmitters of the nervous system, which act only locally within the synaptic cleft.
CAREER CONNECTIONS: Endocrinologist
Endocrinology is a specialty in the field of medicine that focuses on the treatment of endocrine system disorders. Endocrinologists—medical doctors who specialize in this field—are experts in treating diseases associated with hormonal systems, ranging from thyroid disease to diabetes mellitus. Endocrine surgeons treat endocrine disease through the removal, or resection, of the affected endocrine gland.
Patients who are referred to endocrinologists may have signs and symptoms or blood test results that suggest excessive or impaired functioning of an endocrine gland or endocrine cells. The endocrinologist may order additional blood tests to determine whether the patient’s hormonal levels are abnormal, or they may stimulate or suppress the function of the suspect endocrine gland and then have blood taken for analysis. Treatment varies according to the diagnosis. Some endocrine disorders, such as type 2 diabetes, may respond to lifestyle changes such as modest weight loss, adoption of a healthy diet, and regular physical activity. Other disorders may require medication, such as hormone replacement, and routine monitoring by the endocrinologist. These include disorders of the pituitary gland that can affect growth and disorders of the thyroid gland that can result in a variety of metabolic problems.
Some patients experience health problems as a result of the normal decline in hormones that can accompany aging. These patients can consult with an endocrinologist to weigh the risks and benefits of hormone replacement therapy intended to boost their natural levels of reproductive hormones.
In addition to treating patients, endocrinologists may be involved in research to improve the understanding of endocrine system disorders and develop new treatments for these diseases.
Chapter Review
The endocrine system consists of cells, tissues, and organs that secrete hormones critical to homeostasis. The body coordinates its functions through two major types of communication: neural and endocrine. Neural communication includes both electrical and chemical signaling between neurons and target cells. Endocrine communication involves chemical signaling via the release of hormones into the extracellular fluid. From there, hormones diffuse into the bloodstream and may travel to distant body regions, where they elicit a response in target cells. Endocrine glands are ductless glands that secrete hormones. Many organs of the body with other primary functions—such as the heart, stomach, and kidneys—also have hormone-secreting cells.
Interactive Link Questions
Visit this link to watch an animation of the events that occur when a hormone binds to a cell membrane receptor. What is the secondary messenger made by adenylyl cyclase during the activation of liver cells by epinephrine?
Answer: cAMP
Review Questions
Q. Endocrine glands ________.
A. secrete hormones that travel through a duct to the target organs
B. release neurotransmitters into the synaptic cleft
C. secrete chemical messengers that travel in the bloodstream
D. include sebaceous glands and sweat glands
Answer: C
Q. Chemical signaling that affects neighboring cells is called ________.
A. autocrine
B. paracrine
C. endocrine
D. neuron
Answer: B
Critical Thinking Questions
Q. Describe several main differences in the communication methods used by the endocrine system and the nervous system.
A. The endocrine system uses chemical signals called hormones to convey information from one part of the body to a distant part of the body. Hormones are released from the endocrine cell into the extracellular environment, but then travel in the bloodstream to target tissues. This communication and response can take seconds to days. In contrast, neurons transmit electrical signals along their axons. At the axon terminal, the electrical signal prompts the release of a chemical signal called a neurotransmitter that carries the message across the synaptic cleft to elicit a response in the neighboring cell. This method of communication is nearly instantaneous, of very brief duration, and is highly specific.
Q. Compare and contrast endocrine and exocrine glands.
A. Endocrine glands are ductless. They release their secretion into the surrounding fluid, from which it enters the bloodstream or lymph to travel to distant cells. Moreover, the secretions of endocrine glands are hormones. Exocrine glands release their secretions through a duct that delivers the secretion to the target location. Moreover, the secretions of exocrine glands are not hormones, but compounds that have an immediate physiologic function. For example, pancreatic juice contains enzymes that help digest food.
Q. True or false: Neurotransmitters are a special class of paracrines. Explain your answer.
A. True. Neurotransmitters can be classified as paracrines because, upon their release from a neuron’s axon terminals, they travel across a microscopically small cleft to exert their effect on a nearby neuron or muscle cell.
Glossary
- autocrine
- chemical signal that elicits a response in the same cell that secreted it
- endocrine gland
- tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion
- endocrine system
- cells, tissues, and organs that secrete hormones as a primary or secondary function and play an integral role in normal bodily processes
- exocrine system
- cells, tissues, and organs that secrete substances directly to target tissues via glandular ducts
- hormone
- secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body
- paracrine
- chemical signal that elicits a response in neighboring cells; also called paracrine factor