Biology 1a-Human Biology


The Nervous System

Sense organs detect stimuli

  • A stimulus is a change in your environment which you may need to react to
  • You have five different sense organs- eyes, ears, nose, tongue and skin
  • They all contain different receptors
    • Receptors are groups of cells which are sensitive to a stimulus--they change stimulus energy into electrical impulses
  • A stimulus can be light, sound, touch, pressure, chemical or a change in temp. or position

Central Nervous System

  • The CNS is where all the information from the sense organs is sent and where reflexes and actions are coordinated. The CNS consists of the brain and spinal cord only
  • Neurones (nerve cells) transmit the information very quickly to and from the CNS
  • 'Instructions' from the CNS are sent to the effectors (muscles and glands) which respond accordingly 
    • The eye is a sense organ. It contains light receptors
    • The ear is a sense organ. It contains sound receptors

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The Nervous System 2

The Five Sense Organs and the receptors that each contains:

  • Eyes-Light receptors-sensitive to light. Cells contain nucleus, cytoplasm, cell membrane
  • Ears-Sound receptors-sensitive to sound-sensitive to changes in position
  • Nose-Smell receptors-sensitive to chemical stimuli
  • Tongue-Taste receptors-sensitive to bitter,salt,sweet,sour and chemical stimuli
  • Skin-Sensitive to touch, pressure, pain and temperature change

Sensory Neurones- The nerve cells that carry signals as electrical impulses from the receptors in the sense organs to the central nervous system

Relay Neurones- The nerve cells that carry signals from sensory neurones to motor neurones

Motor Neurones- The nerve cells that carry signals from the CNS to the effector (muscles or glands)

Effectors- Muscles and glands-they respond in different ways- Muscles contract in response to a nervous impulse whereas glands secrete hormones

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Synapses and Reflexes

Synapses connect Neurones

  • The connection between two neurones is called a synapse
  • The nerve signal is transferred by chemicals which diffuse across the gap
  • These chemicals then set off a new electrical signal in the next neurone

Reflexes help prevent injury

  • Reflexes are automatic responses to certain stimuli- they can reduce chance of injury e.g. if someone gets a bright light in their eyes your pupils automatically get smaller so that less light gets into the eye- this stops it getting damaged
  • The passage of information in a reflex (from receptor to effector) is called a reflex arc

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Synapses and Reflexes 2

The Reflex Arc goes through the Central Nervous System

  • When a stimulus (e.g. a bee sting) is detected by receptors, impulses are sent along a sensory neurone to the CNS
  • When the impulses reach a synapse between the sensory neurone and a relay neurone, they trigger chemicals to be released. These chemicals cause impulses to be sent along the relay neurone
  • When the impulses reach a synapse between the relay neutrone and motor neurone, the same thing happens. Chemicals are released and cause impulses to be sent along the motor neurone
  • The impulses then travel along the motor neurone to the effector (e.g. muscle)
  • The muscle then contracts and moves your hand away from the bee

stimulus-receptor-sensory neurone-relay neurone-motor neurone-effector-response

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Hormones are chemical messengers sent in the blood

  • Hormones are chemicals released directly into the blood. They are carried in the blood plasmas to other parts of the body, nut only affect target cells in particular places
  • Hormones control things in organs and cells that need constant adjustment
  • Hormones are produced and secreted by various glands. They travel in your body at the speed of the blood
  • Hormones tend to have relatively long lasting effects

Hormones are chemical messengers which travel in the blood to activate target cells

The Pituitary Gland

  • They produce many important hormones such as FSH and LH which are involved in the menstrual cycle


  • Produce oestrogen which is involved in the menstrual cycle
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Hormones 2

Hormones and Nerves do similar jobs but there are differences

  • Nerves
    • very fast action
    • act for a very short time
    • act on a very precise area
  • Hormones
    • slower action
    • act for a long time
    • act in a more general way
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The Menstrual Cycle

The Menstrual Cycle has four stages

  • Stage 1
    • day 1 is when the bleeding starts. The uterus lining breaks down for about 4 days
  • Stage 2
    • the lining of the uterus builds up again, from day 4 to 14, into a thick spongy layer full of blood vessels, ready to receive a fertilised egg
  • Stage 3
    • an egg is released from the ovary at day 14
  • Stage 4
    • the wall is then maintained for about 14 days, until day 28. If no fertilised egg has landed on the uterus wall by day 28, then the spongy lining starts to break down again and the whole cycle starts again

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The Menstrual Cycle 2

Hormones control the different stages

  • Three main hormones involved
  • FSH (Follicle Stimulating Hormone)
    • Produced by the pituitary gland
    • causes an egg to mature in one of the ovaries
    • stimulates the ovaries to produce oestrogen
  • Oestrogen
    • Produced in the ovaries
    • Causes pituitary to produce LH
    • Inhibits the further release of FSH
  • LH (Luteinising Hormone)
    • Produced by the pituitary gland
    • Stimulates the release of an egg at around the middle of the menstrual cycle
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Controlling Fertility

Hormones can be used to reduce fertility

  • Oestrogen can be used to prevent the release of an egg
  • If oestrogen is taken everyday to keep the level of it permanently high it inhibits the production of FSH and after a while egg development and production stop
  • Progesterone (produced in the ovaries-menstual cycle) stimulates the production of thick cervical mucus which prevents any sperm getting through and reaching an egg
  • The pill-the first version was made in the 1950s and contained oestrogen and progesterone
  • There were concerns about the link between oestrogen in the pill and side effects e.g. blood clots. The pill now contains lower doses of oestrogen and has fewer side effects
    • Pros
    • The pill is 99% effective at preventing pregnancies
    • It reduces the risk of getting some types of cancer
    • Cons
    • Its not 100% effective-still a slight chance of getting pregnant
    • Can cause side effects like headaches, nausea, irregular menstrual bleeding
    • It doesn't protect against STD's
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Controlling Fertility 2

Hormones can be used to increase fertility

  • Some women have levels of FSH that are too low to cause their eggs to mature. This means that no eggs are released and the women can't get pregnant
  • The hormones FSH and LH can be injected by the women to stimulate egg release in their ovaries
    • Pros
    • It helps a lot of women to get pregnant when previously they couldn't
    • Cons
    • It doesn't always work-some women may have to do it many times, which can be expensive
    • Too many eggs could be stimulated, resulting in unexpected multiple pregnancies
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Controlling Fertility 3


  • In Vitro Fertilisation involves collecting eggs from the woman's ovaries and fertilising them in a lab using the man's sperm. These are then grown into embryos
  • Once the embryos are tiny balls of cells, one or two of them are transferred to the women's uterus to improve the chance of pregnancy
  • FSH and LH are given before egg collection to stimulate egg production
    • Pro
    • Fertility treatment can give an infertile couple a child
    • Cons
    • Some women have a strong reaction to the hormones e.g. abdominal pain, vomiting
    • Some reports of an increased risk of cancer due to the hormonal treatment
    • Multiple births can happen if more than one embryo grows into a baby- this is risky for the mother and babies- higher risk of miscarriage or stillbirth

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