Biology 1- The Nervous System & Homestasis

nervous system, reflexes etc.

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Sense Organs & Receptors

  • Sense Organs 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, pain, chemical or a change in position or temperature.

The Five Sense Organs and The Receptors They Contain

  • Eyes- Light receptors
  • Ears- sound and balance receptors
  • Nose- smell receptors (sensitive to chemical stimuli)
  • Tongue-Taste receptors
  • Skin- Sensitive to touch pressure and temperature change
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CNS-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 transmit the information as electrical impuluses very quickly to and from the CNS.
  • 'Instructions' from the CNS are sent to effectors (muscles and glands) which respond
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The Reflex Arc

Stimulus --> receptor --> sensory neurone --> CNS --> motor neurone --> effector (muscles & organs)

  • Reflex actions are automatic, they do not require conscious thought and happen very fast. Their job is to protect you from harm.


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  • The connection between two neurones is called a synapse
  • A electrical impulse from the first neurone causes a chemical called a neurotransmitter to pass across the synapse into the next neurone
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  • Blood vessel/Capillary: get vasodilates or vasoconstricts changing blood flow to the skin
  • Erector muscle: makes the hairs on the skin lie flat or stand on end
  • Sweat Gland: produces water from the blood
  • Sweat Pore: a break in the skin which releases water (sweat)
  • Hair: stands up or lays flat: trap a layer of air or not
  • Epidermis: the dead, waterproof cells that are filled with protein
  • Dermis: the living cells that will replace the dead cells above
  • Fat Layer: acts as a thermal insulator

Water loss

  • Water is lost from the body through the skin as sweat, the lungs as breath and via the kidneys as urine.
  • On a cold day you don't sweat much so you'll produce more urine which will be pale (since the waste carried in the urine will be more diluted)
  • On a warm day, you sweat a lot so you produce less urine (more concentrated). You will also lose more water through your breath (exercise)
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Glucose Level is Too High

1)Insulin injected by pancreas

2)Insulin makes liver turn glucose into glycogen

3)Glucose removed by liver

4)Blood glucose removed

Glucose Level Is Too Low

1)Blood with too little glucose

2)insulin not injected

3)glucose added by liver

4)Blood glucose increased

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Insulin and Diabetes

  • Insulin controls blood sugar levels
  • Eating food containing carbohydrates puts glucose into the blood from the gut, Vigorous exercise and normal respiration removes a lot of glucose from the blood
  • Type 1 diabetes is a condition where the pancreas doesn't produce enough insulin
  • The result is a person's blood sugar can rise to a level that can kill them

How to control it

  • Avoiding foods rich in carbohydrates
  • Injecting insulin into the blood at mealtimes
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Keywords For Nervous System

  • Stimulus- A change in your environment which you may need to react to.
  • Receptor Cells- Groups of cells which are sensitive to a stimulus , they change stimulus energy into electrical impulses
  • Nerve Impulse- A signal transmitted along a nerve fiber.
  • Withdrawal reflex- actions that are automatic and protect you from harm
  • CNS- Central Nervous System
  • Spinal Cord- Part of the CNS
  • Synapse- a gap between two neurones
  • Effector-muscles and glands which respond with the instructions from the CNS
  • Motor Nerve Cell- The nerve cells that carry signals to the effectors
  • Sensory Nerve Cell- The nerve cells that carry signals as electrical impulses  from the receptors in the sense organs to the CNS
  • Connector nerve cells
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Homostasis: Too Cold

When we get too cold:

  • Muscles contract rapidly causing shivering which generates heat in the muscles.
  • Vasoconstriction occures: blood vessels near the skin surface become narrower letting less blood flow through the skin and conserving heat in the body
  • The hair erector muscle contracts and make the hair stand more upright. This traps warm, still air close to the surface of the skin which mean less heat is lost by radiation.
  • Sweat glands stop producing sweat.
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Homeostasis: Too Hot

When you body is too hot:

  • Sweat glands in the skins release more sweat. This evaporates, removing heat energy from the skin
  • Vasodilation occurs: blood vessels near the skin surfaceee become wider allowing more blood to flow through the skin, and more heat to be lost via radiation.
  • The hair erector muscles release and make the hairs lie flat against the skin. This means that less air is trapped near the skin surface and therefore more heat is lost by radiation
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