THE FUNCTION OF NERVOUS SYSTEM AND ENDOCRINE GLANDS:Other Glands

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Health Psychology PSY408

LESSON 08

THE FUNCTION OF NERVOUS SYSTEM AND ENDOCRINE GLANDS

The Peripheral Nervous System

As you can see in the diagram, the peripheral nervous system has two parts:

a- Somatic Nervous System

b- Autonomic Nervous System.

The somatic nervous system is involved in both sensory and motor functions, serving mainly the skin

and skeletal muscles. The autonomic nervous system activates internal organs, such as the lungs and

intestines, and reports to the brain the current state of activity of these organs.

In the somatic nervous system, afferent neurons carry messages from sense organs to the spinal cord, as

you can see the diagram. Efferent neurons carry messages to, and activate, striated (grooved) skeletal

muscles, such as those in the face, arms, and legs, that we can move voluntarily. A disorder called myasthenia

gravis can develop at the junction of these muscles and neurons, weakening muscle function of the head and

neck. This produces characteristic symptoms-- such as drooping eyelids, blurred vision, and difficulty

swallowing and breathing--and can lead to paralysis and death. Although medical treatment is effective in

restoring muscle function, some symptoms may recur when the person is under stress (AMA, 1989).

The diagram also shows that in the autonomic nervous system, neurons carry messages between the spinal

cord and the smooth muscles of the internal organs, such as the heart, stomach, lungs, blood vessels, and

glands. This system itself has two divisions, the sympathetic and parasympathetic, which often act in

opposite ways, as you will see in this next diagram.

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The sympathetic nervous system helps us mobilize and expend energy in responding to emergencies,

expressing strong emotions, and performing strenuous activity. For instance, suppose you are crossing a

street, notice a speeding car barreling toward you, and hear its brakes start to squeal. The sympathetic

nervous system instantly moves into action, producing several simultaneous changes--for example, it

speeds up the heart, dilates certain arteries to increase blood flow to the heart and skeletal muscles,

constricts other arteries to decrease blood flow to the skin and digestive organs, decreases salivation, and

increases perspiration. These changes, in general, enable you to mobilize energy, and you leap to safety out

of the cars path. This system is called "sympathetic" because it acts in agreement with your current

emotional state.

What does the parasympathetic division do? The prefix "Para" means "alongside of"--this division acts

alongside of, and often in opposition to the sympathetic division. The parasympathetic nervous system

regulates "quiet" or calming processes, helping our individual organ systems conserve and store energy. One

example of parasympathetic activity can be seen in the digestion of food. When you eat a meal, the

parasympathetic nervous system carries messages to regulate each step in the digestive process, such as by

increasing salivation and stomach contractions. Another example can be seen in the course of emotional or

Health Psychology PSY408

emergency reactions--when an emergency has passed, the parasympathetic division helps restore your

normal body state.

Communication within the peripheral nervous system is handled by 12 sets of cranial nerves, most of which

originate in the brainstem. The Vagus nerve extends from there to muscles of most major body organs, such

as the airways, lungs, heart, and intestines, and is directly involved in the regulation of sympathetic and

parasympathetic activity. Efferent messages from the brain can target specific organs to increase or decrease

their function. As you now realize, the nervous system is connected to and regulates all of our other body

systems; and the brain is the control center.

THE ENDOCRINE SYSTEM

The endocrine system consists of a set of glands that often work in close association with the autonomic

nervous system. These systems share an important function: they communicate with various parts of the

body. But they do this in somewhat different ways. Whereas the nervous system uses both electrical and

chemical messages, the endocrine system communicates only with chemical substances, which are called

hormones. Each endocrine gland secretes specific hormones directly into the bloodstream, which carries

these chemicals to various parts of the body.

The diagram shows where several important endocrine glands are located. Certain chemicals are produced

by both the endocrine and nervous systems and function as both hormones and neurotransmitters.

THE ENDOCRINE AND NERVOUS SYSTEMS WORKING TOGETHER

How are the endocrine and nervous systems associated? The nervous system is linked to the endocrine

system by connections between the hypothalamus (in the forebrain) and a gland that lies just below it-- the

pituitary gland. The hypothalamus sends chemical messages directly to the pituitary gland, causing it to

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release pituitary hormones into the blood. In turn, most of these hormones selectively stimulate the other

endocrine glands to secrete chemicals. Because the pituitary gland controls the secretion of other endocrine

glands, it is called the "master gland".

Researchers have identified dozens of different hormones that course through our veins and arteries. Each

hormone has its own specific effects on cells and organs of the body, thereby directly or indirectly affecting

psychological and physical functions.

Some hormones, such as estrogens and testosterone, are produced mainly in females' ovaries (where egg

cells develop) and males' testes (where sperm develop). These hormones are especially important in the

development and functioning of female and male reproductive systems. Other hormones affect blood

pressure, general body growth, and the balance of various chemicals, such as calcium, in the body. Still other

hormones help us react to specific situations we encounter in our lives.

We saw earlier that the autonomic nervous system plays an important role in our reaction to an emergency.

So does the endocrine system through a process called the hypothalamus--pituitary--adrenal axis

(Sternberg & Gold. 1997).

Let's see how by returning to the incident in which you leaped out of the path of a speeding car. When the

sympathetic nervous system reacts to your emergency, the hypothalamus immediately sends a hormone

called Cortico-tropin-releasing factor to the pituitary gland. This causes the pituitary to release ACTH

(adreno-cortico-tropic hormone) into the blood. The ACTH then travels throughout the body and

stimulates the release of a variety of hormones-- especially those of the adrenal glands--that affect your

reaction to the emergency.

Adrenal Glands

The adrenal glands are located on top of the kidneys (see the diagram). These glands release several

important hormones in response to emergencies and stress. One of these hormones, Cortisol, helps control

swelling when we are injured. If when you leaped to avoid being hit by the car you sprained your ankle, this

hormone would help reduce swelling. But continued high levels of cortisol and similar hormones over a

long time can be harmful to the body. They can lead to high blood pressure and the formation of ulcers, for

example.

Two other important adrenal hormones are epinephrine and nor-epinephrine (also called adrenalin and

noradrenalin). These hormones work in conjunction with the sympathetic nervous system to produce such

bodily reactions as speeding up heart and respiration rates and increasing the livers sugar output for quick

energy. After the emergency has passed and sympathetic activity has subsided, some impact of the

hormones may continue for a while because they are still in the bloodstream.

The impact of the nervous and endocrine systems' activities in emergency situations differs in the speed and

persistence of their effect. The nervous system responds by sending messages that move instantly to specific

locations: once they reach their destination, they become deactivated or dissipated. For example, the

nervous system also produces and uses epinephrine and nor-epinephrine, but these chemicals function as

neurotransmitters, relaying their commands from neuron to neuron and having a localized effect. The

impact of the message stops quickly, and persists only if additional messages are sent. Hormones from the

endocrine system move more slowly and broadly through the bloodstream, and their effects can be delayed

and long-lasting.

Other Glands

Several other endocrine glands are also important. The thyroid gland, located in the neck, produces

hormones, such as thyroxine, that regulate the body's general activity level and growth. Disorders in

thyroid production are of two types: hypothyroidism, or insufficient secretion of thyroid hormones, and

hyperthyroidism, or excessive thyroid secretion (AMA. 1989).

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Hypothyroidism leads to low activity levels and to weight gain. If the condition is congenital and untreated,

dwarfism and mental retardation often result. The condition can be treated medically by having the person

take hormone supplements orally. Hyperthyroidism leads to high activity levels, short attention spans,

tremors, insomnia, and weight loss. Untreated people with a common form of this condition, called Graves'

disease, act in a highly restless, irritable, and confused manner.

The thymus gland, which is located in the chest, is quite large in infancy and childhood but diminishes in

size and efficiency after puberty. The thymus plays an important role early in life in the development of

antibodies and immunities against diseases. We will discuss the immune system in one of our later lectures.

Another endocrine gland is the pancreas, which is located below the stomach. Its main function is to

regulate the level of blood sugar, or glucose. The pancreas does this by producing two hormones, glucagon

and insulin, that act in opposition. Glucagon raises the concentration of glucose in the blood, and insulin

lowers it. The disorder called diabetes mellitus results when the pancreas does not produce sufficient

insulin to balance the action of glucagon. This imbalance produces excess blood sugar levels--a condition

called hyperglycemia. If this condition persists and is untreated, it may cause coma and death. Diabetes

can be medically controlled, generally through diet and either medication or daily insulin inspections.

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