BioDigital Systems: Availble for free (individual) or for a fee (groups/businesses), this interactive system will first require you to sign in via your Facebook or Google account to gain access. Once inside the system, you can zoom and rotate your virtual skeleton. Eleven systems in total are able to view and examine. Note: this site allows you to repeatedly quiz yourself on all eleven systems that are covered within the site.
The AK Lectures are a series of lectures from a (external) educational platform designed to "promote collaboration between our users and help spread knowledge to every part of the world."
These lectures vary in length, and will open in a new window when you click on the provided link.
Introduction to the Endocrine System: In order for multicellular organisms to survive, the individual cells within that organism must be able to communicate with one another. This is known as intracellular communication. Cells communicate with one another using special types of molecules. The nervous system is one type of system that uses a chemical called the neurotransmitter in intracellular communication. The neurotransmitter travels only very short distances, is very specific to the type of cell it binds to and creates a very rapid but short-lived response. Another system that our body uses to communicate is the endocrine system. The endocrine system uses glands to create and release chemicals called hormones. The endocrine glands release the hormone into the blood stream or lymph system, which means the hormone circulates throughout the body and travels a long distance before it locates its target cell. Unlike neurotransmitters, hormones do not only bind to specific cells but rather bind to a wide range of different types of cells. Hormones travel a long distance, are slow-acting and can affect the organism in the long-term. Endocrine glands should not be confused with exocrine glands, which are glands that release chemicals through a duct and into some external environment. The sudoriferous gland is an example of an exocrine gland. The endocrine system uses hormones and hormones come in three different types. Peptide hormones are synthesized in the rough ER, packaged and modified in the Golgi complex and released into the blood stream. They dissolve in the blood because they are peptide-based and do not need any carrier proteins. Once they arrive at the target cell, they cannot pass across the cell membrane because the membrane is mostly hydrophobic. Therefore peptide hormones bind to receptor proteins found in the membrane of the target cell. Once bound, they usually use some sort of secondary messenger system to induce a change in the cell. Steroid hormones are made from cholesterol or other lipids in the smooth ER or the mitochondria. They are released into the blood stream and use a protein carrier because they are lipid-soluble. Once they arrive at the target cell, they can easily pass across the cell membrane and bind to the receptor protein in the cytosol of the cell. They then enter the nucleus, where the steroid hormone will induce some sort of transcriptional change. Tyrosine-derivative hormones come in two types - water-soluble and lipid-soluble. Water-soluble tyrosine hormones bind to receptor proteins on the plasma membrane while fat-soluble go directly to the nucleus of the target cell to induce a response.
Anterior Pituitary Gland: The pituitary gland is one of the many glands of the endocrine system and it contains two divisions - the anterior pituitary and posterior pituitary division. The hypothalamus found in the forebrain is responsible for controlling and stimulating the anterior pituitary gland. The hypothalamus is connected to the anterior pituitary through a network of blood vessels called the hypophyseal portal system. The hypothalamus releases its own set of hormones that travel down the hypophyseal portal system and to the anterior pituitary gland, where they stimulate the release of six different hormones. The six different hormones of the anterior pituitary gland are human growth hormone (HGH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), prolactin, follicle stimulating hormone (FSH) and luteinizing hormone. All these hormones are peptides, which means they are water-soluble and can travel in the blood without a carrier protein. It also means that they bond onto target cells at receptors found on the membrane.
Posterior Pituitary Gland: The backside of the pituitary gland is called the posterior pituitary gland. It is connected to the hypothalamus via a network of neurons. Unlike the anterior pituitary gland, the posterior pituitary gland does not actually produce any hormones of its own. Instead two hormones are produced within the cell bodies of the neurons in the hypothalamus and then travels down to the posterior pituitary gland for storage. These two hormones are the antidiuretic hormone (ADH) and oxytocin. The antidiuretic hormone, also called vasopressin, is synthesized in the supraoptic neurons and then travel down the axons in secretory vesicles. They are stored along the axon and at the axon terminal is special vesicles called Herring bodies. ADH is stimulated by a high blood osmolarity (a high concentration of solute in the blood) or a low amount of water volume in the blood (low blood pressure). ADH acts on the collecting duct of the kidneys and forces them to reabsorb more water back in the body. This concentrates the urine and decreases the volume of urine while at the same time increasing the blood water volume. ADH also constricts the blood vessels and increases the blood pressure of the organism. Oxytocin is produced in the supraoptic and paraventricular nuclei of the hypothalamus and stored in the axons of the posterior pituitary gland. They are released during childbirth and stimulates the contraction of the smooth muscle in the uterus. It is also released post-childbirth and is responsible for secreting milk.
The Thyroid Gland: The thyroid gland is located along the front portion of the windpipe, positioned right below the Adam's apple. It contains specialized cells that are responsible for synthesizing and releasing three important hormones. The two lipid-soluble hormones are triiodothyronine (T3) and thyroxine (T4) while the water-soluble polypeptide hormone is called calcitonin. The T3 and T4 hormones are both tyrosine derivatives and require carriers within the blood. They can easily travel across the membrane of the cell and enter the nucleus of the target cell, where they act at the transcriptional level. T3 and T4 hormones act in very similar ways and are responsible for resetting the basal metabolic rate of the human body. This means they can affect processes such as cellular respiration, the contraction of the heart, protein synthesis and degradation, and many more. They are also crucial in human growth and development. T3 and T4 hormones are produced in thyroid cells called follicular cells. Hypothyroidism and hyperthyroidism are two abnormalities of the thyroid that can affect the human in different ways. Calcitonin is produced by parafollicular cells (C-cells) and is a water-soluble large polypeptide. This means that calcitonin can travel within the blood without any protein carrier and it binds onto protein receptors found on the membrane of the cell. Calcitonin is stimulated and released when the blood calcium concentration is high. It can decrease the plasma concentration in three ways - by inhibiting the kidneys from absorbing calcium into the body, it can decrease the amount of calcium absorbed in the intestines and it can also cause the bone to absorb more calcium from the blood by decreasing the activity of osteoclasts and increasing the activity of osteoblasts.