Endocrine System

The endocrine system These chemicals are known as hormones. A hormone is a specific messenger molecule synthesized and secreted by a group of specialized cells called an endocrine gland. These glands are ductless, which means that their secretions (hormones) are released directly into the bloodstream and travel to elsewhere in the body to target organs, upon which they act.

There are three general classes (groups) of hormones. These are classified by chemical structure, not function.

• steroid hormones including prostaglandins which function especially in a variety of female functions (aspirin inhibits synthesis of prostaglandins, some of which cause “cramps”) and the sex hormones all of which are lipids made from cholesterol,
• amino acid derivatives (like epinephrine) which are derived from amino acids, especially tyrosine, and
• peptide hormones (like insulin) which is the most numerous/diverse group of hormones.

The major human endocrine glands include:

Hypothalamus and pituitary gland
The pituitary gland is called the “master gland” but it is under the control of the hypothalamus. Together, they control many other endocrine functions. They secrete a number of hormones, especially several which are important to the female menstural cycle, pregnancy, birth, and lactation (milk production). These include follicle-stimulating hormone (FSH), which stimulates development and maturation of a follicle in one of a woman’s ovaries, and leutinizing hormone (LH), which causes the bursting of that follicle (= ovulation) and the formation of acorpus luteum from the remains of the follicle.

Thyroid hormones regulate metabolism, therefore body temperature and weight. The thyroid hormones contain iodine, which the thyroid needs in order to manufacture these hormones. If a person lacks iodine in his/her diet, the thyroid cannot make the hormones, causing a deficiency. In response to the body’s feedback loops calling for more thyroid hormones, the thyroid gland then enlarges to attempt to compensate (The body’s plan here is if it’s bigger it can make more, but that doesn’t help if there isn’t enough iodine.). This disorder is called goiter. Dietary sources of iodine include any “ocean foods” because ocean-dwelling organisms tend to accumulate iodine from the seawater, and would include foods like ocean fish (tuna) and seaweeds like kelp. Because of this, people who live near the ocean do not have as much of a problem with goiter as people who live inland and don’t have access to these foods. To help alleviate this problem in our country, our government began a program encouraging salt refiners to add iodine to salt, and encouraging people to choose to consume this iodized salt.

pancreas
This organ has two functions. It serves as a ducted gland, secreting digestive enzymes into the small intestine. The pancreas also serves as a ductless gland in that the islets of Langerhans secrete insulin andglucagon to regulate the blood sugar level. The glucagon, which tells the liver to take carbohydrate out of storage to raise a low blood sugar level. The insulin to tell the liver to take excess glucose out of circulation to lower a blood sugar level that’s too high.

adrenal glands
These sit on top of the kidneys. They consist of two parts, the outer cortex and the inner medulla. The medulla secretes epinephrine and other similar hormones in response to stressors such as fright, anger, caffeine, or low blood sugar. The cortex secretes corticosteroids such as cortisone. Corticosteroids are well-known as being anti-inflammatory, thus are prescribed for a number of conditions. However, these are powerful regulators that should be used with caution. Medicinal doses are typically higher than what your body would produce naturally, thus the person’s normal feedback loops suppress natural secretion, and it is necessary to gradually taper off the dosage to trigger the adrenal glands to begin producing on their own again. Because the corticosteroids suppress the immune system, their use can lead to increased susceptibility to infections, yet physicians typically prescribe them for people whose immune systems are hard at work trying to fight off some pathogen. For example, back when I was in grad school, I was diagnosed with mono, and the campus doctor prescribed penicillin and cortisone.

gonads or sex organs
In addition to producing gametes, the female ovaries and male testes (singular = testis) also secrete hormones. Therefore, these hormones are called sex hormones. The secretion of sex hormones by the gonads is controlled by pituitary gland hormones such as FSH and LH. While both sexes make some of each of the hormones, typically male testes secrete primarily androgens including testosterone. Female ovaries make estrogen and progesterone in varying amounts depending on where in her cycle a woman is. In a pregnant woman, the baby’s placenta also secretes hormones to maintain the pregnancy.

pineal gland
This gland is located near the center of the brain in humans, and is stimulated by nerves from the eyes. In some other animals, the pineal gland is closer to the skin and directly stimulated by light (some lizards even have a third eye). The pineal gland secreted melatonin at night when it’s dark, thus secretes more in winter when the nights are longer. Melatonin promotes sleep (makes you feel sleepy). It also affects reproductive functions by depressing the activity of the gonads.

Brain

The brain is the center of the nervous system.

The brain is made of three main parts: the forebrain, midbrain, and hindbrain. The forebrain consists of the cerebrum, thalamus, and hypothalamus (part of the limbic system). The midbrain consists of the tectum and tegmentum. The hindbrain is made of the cerebellum, pons and medulla. Often the midbrain, pons, and medulla are referred to together as the brainstem.

The cerebrum or cortex is the largest part of the human brain, associated with higher brain function such as thought and action. The cerebral cortex is divided into four sections, called "lobes": the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.

Frontal Lobe- associated with reasoning, planning, parts of speech, movement, emotions, and problem solving

Parietal Lobe- associated with movement, orientation, recognition, perception of stimuli

Occipital Lobe- associated with visual processing

Temporal Lobe- associated with perception and recognition of auditory stimuli, memory, and

The cerebellum, or "little brain", is similar to the cerebrum in that it has two hemispheres and has a highly folded surface or cortex. This structure is associated with regulation and coordination of movement, posture, and balance.

Brain stem is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. Scientists say that this is the "simplest" part of human brains because animals' entire brains, such as reptiles (who appear early on the evolutionary scale) resemble our brain stem.

The brain stem is made of the midbrain, pons, and medulla.

Midbrain It is involved in functions such as vision, hearing, eyemovement, and body movement

Pons It is involved in motor control and sensory analysis.

Medulla It is responsible for maintaining vital body functions, such as breathing and heartrate

Nervous System

The human nervous system has two main divisions: the central nervous system (CNS), and the peripheral nervous system (PNS), which includes the somatic motor nervous system, and the sensory nervous system. The CNS consists of the brain and spinal cord. It acts as the central control region of the human nervous system, processing information and issuing commands. The autonomic nervous system (ANS) is the command network the CNS uses to maintain the body's homeostasis. It automatically regulates heartbeat and controls muscle contractions in the walls of blood vessels, digestive, urinary, and reproductive tracts. It also carries messages that help stimulate glands to secrete tears, mucus, and digestive enzymes.

The nerves that are easily visible to the unaided eye are not single cells. Rather, they are bundles of nerve fibers (neurons) each of which is itself a portion of a cell. The fibers are all traveling in the same direction and are bound together for the sake of convenience, though the individual fibers of the bundle may have widely differing functions. There are no cell bodies in nerves; cell bodies are found only in the CNS or in the ganglia. Ganglia are collections of cell bodies within the PNS.

The main portion of the neuron, the cell body, is not too different from other cells. It contains a nucleus and cytoplasm. Where it is most distinct from cells of other types is that out of the cell body, long threadlike projections emerge. Over most of the cell there are numerous projections that branch out into still finer extensions. These branching threads are called dendrites ("tree" in Greek). At one point of the cell, however, there is a particularly long extension that usually does not branch throughout most of its sometimes enormous length. This is the axon (the axis). Figure 01b shows the three parts of the neurons: dentrite(s), cell body, and axon. A dendrites conducts nerve impulses toward the cell body, the part of a neuron that contains the nucleus and other organelles. An axon conducts nerve implses away from the cell body. There are three types of neurons: sensory neuron, motor neuron, and interneuron. A sensory neuron takes a message from the recptors in the sense organ to the CNS. A motor neuron sends a message away from the CNS to an effector, a muscle fiber or a gland. An interneuron is always found completely within the CNS and conveys messages between parts of the system. In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with sheath. These cells maintain the tissue by supporting and protecing the neurons. They also provide nutrients to neurons and help to keep the tissue free of debris. The neurons require a great deal of energy for the maintenance of the ionic imbalance between themselves and their surrounding fluids, which is constantly in flux as a result of the opening and closing of channels through the neuronal membranes. Neurotransmitters are chemicals that take a nerve signal across the synaptic gap between a sending neuron, and a receiving one.

Diencephalon

The diencephalon is the region of the brain that includes the thalamus, metathalamus, hypothalamus, epithalamus, prethalamus or subthalamus and pretectum. The diencephalon and the telencephalon both derive from the prosencephalon.The diencephalon is located near the midline of the brain, above the midbrain.

Thalamus is the largest structure in the diencephalon, the part of the brain situated between the midbrain (mesencephalon) and forebrain (telencephalon).

Hypothalamus is an extremely complex region in the brain of humans, and even small nuclei within the hypothalamus are involved in many different functions.

Epithalamus is a dorsal posterior segment of the diencephalon (a segment in the middle of the brain also containing the hypothalamus and thethalamus) which includes the habenula, the stria medullaris and the pineal body. Its function is the connection between the limbic system to other parts of the brain.

Metathalamus is a composite structure of the thalamus, consisting of the medial geniculate nucleus and the lateral geniculate nucleus.

Prethalamus is patterned by Sonic hedgehog signalling from the ZLI. Anatomically, it develops efferent (output) connections to the striatum (caudate nucleus and putamen) in the telencephalon, to the thalamus (medial and lateral nuclear groups) in the diencephalon, and to the red nucleus and substantia nigra in the mesencephalon. It receives afferent (input) connections from the substantia nigra and striatum.

Pretectum, also known as the pretectal area, is a region of neurons found between the thalamus and midbrain. It receives binocular sensory input from retinal ganglion cells of the eyes, and is the region responsible for maintaining the pupillary light reflex.