Coordination and Control

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Article Index
Coordination and Control
Hormones
Menstrual Cycle
Control of Fertility
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The Nervous System

The nervous system enables humans to react to their surroundings and coordinate their behaviour.

Cells called receptors detect stimuli (changes in the environment).  These include:

  • receptors in the eyes which are sensitive to light
  • receptors in the ears which are sensitive to sound
  • receptors in the ears which are sensitive to changes in position and enable us to keep our balance
  • receptors on the tongue and in the nose which are sensitive to chemicals and enable us to taste and to smell
  • receptors in the skin that are sensitive to touch, pressure, pain and to temperature changes.

Information from receptors passes along cells called neurones (nerve cells) to the brain. These are found in bundles of hundreds or thousands of neurones known as nerves.  This information is processed by the brain which coordinates any response.

Reflex Actions

Sometimes your response to a stimulus is so rapid that you do not have time to think about it.  This is called a reflex action.  Relex actions help you to avoid danger or harm. For example, if you touch something hot you rapidly move your arm away from the hot object.  If something comes close to your eyes, you blink.  Shining a light into somebody's eyes causes the pupils to get smaller.

Reflex actions often involve sensory, relay and motor neurones.  How they are linked together is shown in the diagram below.

Impulses from a receptor pass along a sensory neurone to the central nervous system.  The central nervous system is made up of the brain and the spinal cord.

At a junction (called a synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released which causes an impulse to be sent along a relay neurone.

A chemical is then released at the synapse between a relay neurone and a motor neurone in the central nervous system, causing impulses to be sent along a motor neurone to the organ (the effector) which brings about the response.  The effector is either a muscle or a gland.  Glands respond by releasing chemical substances (e.g. adrenaline).

For example if you touch a hot object with your hand, a pain receptor in your skin is stimulated.  A nerve impulse passes along a sensory neurone to the central nervous system (in this case the spinal cord).  The message crosses a synapse to a relay neurone and then crosses a second synapse to a motor neurone.  The motor neurone carries the impulse to a muscle in the arm (the effector).  The muscle will then contract to move your hand away from the hot object.  This action is called the response. This chain of events can be summarised as follows:

Stimulus → Receptor → Coordinator → Effector → Response

The coordinator is the central nervous system (brain or spinal cord).

Can you correctly label the Reflex Arc?

 


 

Control of Internal Conditions

In order for your body to work properly it's internal environment must be kept constant.  Enzymes which control many of the the body's processes work best at 37oC.  Changing amounts of water and sugar can affect chemical reactions that are essential for you body to work properly.  Internal conditions which are controlled include:

  • The water content of the body.  Water leaves the body when we breathe out and through the skin when we sweat.  Any excess water is also lost via the kidneys in urine when we go to the toilet.
  • The ion (salt) content of the body.  Ions (salt) are lost via the skin when we sweat.  Excess ions are also lost via the kidneys in urine.
  • Temperature.  Your body temperature is kept very close to 37oC to maintain the temperature at which enzymes work best.
  • Blood sugar levels.  When you digest carbohydrates lots of sugar (glucose) passes into your blood.  Hormones made in the pancreas keep the blood sugar level at a constant level to provide your cells with a constant supply of energy.  The hormone insulin reduces blood sugar levels if they become too high.  Another hormone causes the blood sugar level to rise if it becomes too low.  People who suffer from diabetes often need to inject themselves with insulin to help control their blood sugar levels.  If your blood sugar levels become too high or too low this causes chaos in your body.

Hormones

Many processes in the body are controlled by chemical substances called hormones.  Hormones are released into the blood (secreted) by glands.  The hormones are transported to their target organs by the bloodstream.  The target organ is where the hormone has an effect.  The diagram below shows some of the glands in the body where hormones are made.  (Glands which release hormones into the blood are called endocrine glands).

endocrine

The diagram shows some of the major glands that release hormones into the blood.  The pancreas releases the hormones insulin and glucagon which control the blood sugar level.  The role of these hormones will be discussed in detail in Module B2.  The pituitary gland releases the hormones FSH and LH.  These are involved in the control of the female menstrual cycle.  Their target organs are the ovaries.  The ovaries also make hormones such as oestrogen.  The role of these hormones in the control of the menstrual cycle is discussed in the next section.

 


 

 The Menstrual Cycle

The monthly release of an egg from a woman's ovaries and the changes in the thickness of the lining of her womb are controlled by hormones secreted by the pituitary gland and by the ovaries.  Several hormones are involved in controlling the menstrual cycle.  These include:

FSH which is secreted by the pituitary gland and causes eggs to mature in the ovaries.  FSH also stimulates the ovaries to produce hormones including Oestrogen.

Oestrogen which is secreted by the ovaries and stops the the production of FSH.  Oestrogen also stimulates the lining of the uterus to build up/thicken ready for pregnancy.  Oestrogen stimulates the pituitary gland to produce another hormone called LH.

LH is secreted by the pituitary gland.  It stimulates the release of a mature egg from one of the ovaries in the middle of the menstrual cycle. 

FSH is produced by the pituitary gland and it's target organs are the ovaries.

Oestrogen is produced by the ovaries and it's target organs are the pituitary gland (blocks production of FSH) and the uterus (causes lining to thicken).

LH is produced by the pituitary gland and it's target organs are the ovaries. 

How the levels of the hormones change during the menstrual cycle are show in the diagram below.  Day one of the menstrual cycle is the first day of menstruation (your period).  Duing the first five days of the cycle the lining of the uterus is gradually shed.  Changes in the thickness of the uterus lining are also shown in the diagram below.

menstrual

 


 

The Artificial Control of Fertility

Hormones can be used to control fertility

Oral contraceptives (the pill) contain hormones which stop the production of FSH so that no eggs mature.  Contraceptive pills often contain oestrogen which blocks the production of FSH.

pill

A pack of contraceptive pills.

FSH may be given to woman as a fertility drug.  A woman's own level of FSH may be too low to stimulate eggs to mature and giving extra FSH may help her to become pregnant.

In vitro Fertilisation (IVF)

IVF ('test-tube' babies) involves giving a mother FSH to stimulate the maturation of several eggs and LH to stimulate the release of these eggs.  The eggs are collected from the mother and mixed with sperm from the father in a glass petri dish.  The fertilised eggs develop into embryos.  At the stage when the are tiny balls of cells, two or three embryos are inserted into the mother's womb.  The woman can then undergo a normal pregnancy.

ivf

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