Neurotransmitters versus Secretions
The synapse between two neurons, like the neuromuscular junction between a motor neuron and a muscle fiber, employs a special type of chemical communication (see Figure 15.1, A and B). Specifically, vesicles within the axon terminals of an excited neuron rupture and release neurotransmitter molecules (such as acetylcholine). The neurotransmitter molecules then diffuse and bind to the membrane of either another neuron (in the case of a synapse) or a skeletal muscle fiber (in the case of a neuromuscular junction). In both cases, however, there is a type of chemical communication between cells that occurs with the release of neurotransmitter molecules.
Glands And Secretion
But there is another mode or way of chemical communication between cells. This mode is called secretion (sih- KREE -shun). Secretion is literally the “process of separating” – the separation of certain substances from the bloodstream, followed by their release.
The secreted (see- KREE -ted) substance usually comes from epithelial cells. Remember that epithelial tissue is the body’s main covering and lining tissue. But it also occurs within glands . A gland is a “little acorn” – a rounded, somewhat “acorn”-shaped mass of one or more epithelial cells that have become specialized for the function of secretion.
Exocrine ( EK -suh-krin) glands are glands of “external” ( exo -) secretion that release some useful product into a duct, which then carries the secretion to some body surface (study Figure 15.1, C). A good example of exocrine glands are the sweat glands of the skin, which secrete sweat into numerous sweat ducts. Sweat, as you no doubt remember, is a useful product because it helps cool the body and prevent hyperthermia (excessive body temperature).
Endocrine ( EN -doh-krin) glands , in contrast, are glands of “internal” ( endo -) secretion that release a hormone into the bloodstream, right within the gland itself. A hormone is literally “an arouser” ( hormon ). A hormone is a chemical messenger secreted into the bloodstream by an endocrine gland. It gets its name from the fact that the hormone often “arouses” (stimulates) certain target cells in the body to increase their activity. The target cells of the hormone may be located far downstream, but the blood will eventually circulate to bring the hormone molecules to them (see Figure 15.1, D).
The endocrine glands, like the nervous system, are vitally important in both communication and control of the internal environment within the bodies of vertebrates. In fact, it is sometimes very hard to separate the nervous and endocrine systems at all! In these cases, we use the term neuroendocrine ( NUR -oh- en -doh-krin) system to describe them. The neuroendocrine system is an organ system that contains parts of the nervous system, as well as parts of the endocrine gland system. Thus, communication can occur via release of neurotransmitters (the nervous component), and also via secretion of particular hormones into the bloodstream (the endocrine component). We will now describe some specific examples.
Fig. 15.1 Two ways for cells to communicate: Neurotransmitters versus secretions
The Hypothalamus–pituitary Connection
Chapter 14 described the hypothalamus as an area deep within the cerebrum that contains a number of control centers (such as the temperature control center) that are vitally important for maintaining various aspects of homeostasis. Located just below the hypothalamus is a narrow, funnel-shaped pituitary (pih- TWO -eh- tear -ee) stalk. This hollow stalk is attached to the top of the pituitary body. The front of the pituitary body is called the anterior pituitary gland, while the back is called the posterior pituitary gland. The exact Latin translation of the word pituitary becomes evident from a look at Figure 15.2. It literally “pertains to mucus ( MYOO -kus) or phlegm (flem).” This term derives from the fact that the pituitary body is located just above and behind the nose. Hence, it was only logical for the ancient anatomists to think that the pituitary was the source of the “slime” (mucus) that sometimes leaks from your nose when its interior is “inflamed” (phlegm)!
Fig. 15.2 The pituitary stalk: A neuroendocrine connection.
Even though we dwellers of the 21st century realize that the pituitary body is not some kind of channel that carries mucus from the brain down into the nose, we do recognize that there is definitely a hypothalamus–pituitary connection . The main connection is through the pituitary stalk. But instead of carrying mucus or phlegm, the pituitary stalk contains a network of tiny blood vessels that run down from the hypothalamus into the pituitary body.
In addition to vital control centers, the hypothalamus also contains an amazing collection of secretory ( SEE -kreh- tor -ee) neurons – neurons that secrete hormones or hormone-like substances. These hormones (or hormone-like substances) are secreted into the tiny blood vessels flowing down through the pituitary stalk. Thus, the pituitary stalk (and its collection of blood vessels) creates an important neuroendocrine connection. This is because the hypothalamus is generally considered part of the nervous system, while the pituitary body and its two glands (anterior pituitary and posterior pituitary) are classified as members of the large endocrine gland system.
One group of secretory neurons produce antidiuretic ( an -tee- die -yuh- RET -ik) hormone . Antidiuretic hormone, abbreviated as ADH , is named for its primary function. ADH helps the kidney retain water due to its effect “against” ( anti -) too much “urine passing through” ( diuret ) and out of the body. Although ADH is actually secreted by the neurons in the hypothalamus, the hormone molecules travel down through the blood vessels in the pituitary stalk, and are then stored by the posterior pituitary gland and later released into the general bloodstream. [ Study suggestion: Look at the “kidney faucet” model in Figure 15.2. How does the definition of ADH help explain the turning off of the faucet from full blast to a slow drip?]
Releasing Hormones And Trophic Hormones
The largest group of secretions coming from the hypothalamus have no direct connection to ADH. These other secretions are called the releasing hormones ( RHs ). The releasing hormones (RHs) are secreted into the network of tiny brain blood vessels coursing down through the pituitary stalk. The transported RHs finally enter the anterior pituitary gland (rather than the posterior pituitary gland). The releasing hormones get their name from their primary function. They literally stimulate the release of a variety of trophic ( TROHF -ik) hormones from the epithelial cells of the anterior pituitary gland.
Remember (Chapter 14) that troph means “nourishment” in the sense of stimulating something. Trophic hormones, therefore, are hormones secreted by the anterior pituitary gland that have individual endocrine glands as their main targets for “nourishment” or stimulation. Figure 15.2 outlined the highly orderly three-step process:
1. A particular releasing hormone (RH) is put out by the hypothalamus.
2. Each RH passes down through the pituitary stalk and triggers the release of a certain trophic hormone.
3. Each trophic hormone in turn enters the general bloodstream and stimulates a particular individual endocrine gland target to secrete its own hormone.
Figure 15.3 shows pictures of the major endocrine gland targets being stimulated by various trophic hormones from the anterior pituitary. Take, for instance, the thyroid ( THIGH -royd) gland in the front of the neck. The thyroid gland is named for its “resemblance” (-oid) to a broad “shield” (thyr). The thyroid is stimulated or “nourished” by thyroid-stimulating hormone, abbreviated as TSH. The thyroid gland is stimulated by TSH to increase the rate of secretion of its own individual hormone, thyroxine ( thigh-ROCKS -in). Thyroxine, in turn, circulates throughout the bloodstream to affect most of the body cells. Thyroxine, for example, increases the basal ( BAY -sal) metabolic rate or BMR : that is, the rate at which body cells burn calories during their metabolism under resting or “basal” conditions. [ Study suggestion: From its effect upon BMR, how would you expect thyroxine to influence oral body temperature? Why?]
Fig. 15.3 Some major trophic hormones and their endocrine targets.
The name of a second important trophic hormone is a real tongue-twister! Its name is adrenocorticotrophic (uh- dree -noh- kor -tuh-koh- TROHF -ik) hormone , simply abbreviated as ACTH . The adrenocortico - part of the hormone’s name comes from its endocrine gland target – the adrenal (uh- DREE -nal) cortex . Just as the cerebral cortex (Chapter 14) literally forms a thin “bark” over the surface of the cerebrum, the adrenal cortex is an endocrine gland forming a thin “bark” ( cortex or cortico -) over the surface of the adrenal body . This body is a curved, stocking cap-shaped structure that lies “toward” ( ad -) the top of each “kidney” ( renal ).
The adrenal cortex or thin outer bark of the adrenal body secretes the hormone cortisol ( KOR -tih- sol ). Cortisol raises the blood glucose level whenever it is low, and it also acts to relieve the symptoms of tissue inflammation.
A third trophic hormone is called growth hormone (GH) . Growth hormone, as its name states, circulates to most of the body cells and stimulates their growth by promoting such processes as protein synthesis and cell division. But the specific target gland of its trophic influence are the beta ( BAY -tuh) cells of the pancreas . The beta cells are well known because they secrete the hormone, insulin ( IN -suh- lin ). Insulin is absolutely critical for human survival, because it helps to transport glucose out of the bloodstream, thereby feeding the tissue cells.
A fourth trophic hormone is follicle-stimulating hormone ( FSH ). You may bring back to mind (Chapter 13) the “little bags” or hair follicles within the skin. There is another type of “little bag” or follicle within the ovaries ( OH -var- ees ) or “eggs” ( ova ) of females. The ovarian ( oh - VAIR -ee-an) follicles are tiny bags or sacs within the ovaries. Under the stimulating effect of FSH, they increase their secretion of the two hormones, estrogen ( ES -troh-jen) and progesterone (proh- JES -ter- own ). Estrogen stimulates the development of so-called secondary sex characteristics in the female, such as a higher voice and softer skin. Progesterone prepares the female body for a possible pregnancy.
A fifth trophic hormone, luteinizing ( LEW -tuh- neye -zing) hormone or LH , also acts upon the female ovary. It triggers a rupture of the mature ovarian follicle, thereby causing ovulation ( ahv -you- LAY -shun) – the release of a “little egg” ( ovul ) into the abdominal cavity. But luteinizing hormone derives its name from the corpus ( KOR -pus) luteum ( LEW -tee-um) or “yellow body” that is still left within the ovary after ovulation. LH thus indirectly has a luteinizing or “yellowing” effect, creating a yellowish body (corpus luteum) that secretes lots of progesterone (and some estrogen).
The Sympathetic Nerve–adrenal Connection
In addition to the hypothalamus–pituitary connection, there are other examples of neuroendocrine relationships. Prominent among these are the partners in the “Fight-or-Flight” Stress Response. Whenever a human or other vertebrate is under severe stress, then this “Fight-or-Flight” Response automatically engages. The vertebrate gets ready to either stand its ground and fight for survival, or else take flight and simply run away from encroaching danger.
The sympathetic nerves are critical elements of the “Fight-or-Flight” Response, as are both the adrenal cortex and the adrenal medulla (meh- DEW -lah). The adrenal medulla lies in the “middle” ( medull ) of the adrenal body, whereas the adrenal cortex forms a thin bark around it. Figure 15.4 illustrates how the sympathetic nerves leave the hypothalamus area and carry nerve impulses (action potentials) down into the adrenal medulla. The sympathetic nerves are literally the “suffering” ( path ) “with” ( sym -) nerves, reflecting their central role in carrying out the body’s response to alarm or stress.
The sympathetic nerve endings stimulate epithelial cells within the adrenal medulla to increase their secretion of the two related hormones, epinephrine ( ep -ih- NEF -rin) and norepinephrine ( NOR -ep-ih- nef -rin). These two hormones are given the alternate names of adrenaline (ah- DREN -uh- lin ) and noradrenaline ( NOR -ah- dren -ah-lin). Epinephrine (adrenaline) is secreted from the “adrenal” (hence the adrenaline name), which is located “upon” ( epi -) the “kidney” ( nephr ). Norepinephrine (noradrenaline) is “normally” ( nor -) secreted by the adrenal medulla, along with its close chemical cousin epinephrine (adrenaline), but in much smaller amounts.
Epinephrine circulates widely throughout the bloodstream and generally stimulates the heart to beat faster and harder, and the blood vessels to constrict (narrow), thereby creating a rise in blood pressure. Faster and stronger heart contractions, and a higher blood pressure to push the blood through the vessels more rapidly, help the body under stress to either flight or flee from danger. This danger is usually recognized first within the brain, which then engages the “Fight-or-Flight” Response involving the sympathetic nerves and adrenal hormones. This close functional inter-relationship may thus be called the sympathetic nerve–adrenal connection .
Fig. 15.4 “Fight-or-Flight”: The sympathetic nerve-adrenal connection.
Practice problems for these concepts can be found at: Glands and Hormones Test
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