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Two main forms of communication:

  • Nervous System: uses nerve cells to pass electrical impulses along their length, which stimulate their target cells by secreting neurotransmitters onto them. This results in rapid communication between specific parts of an organism. The responses produced are often short-lived and restircted to a localised region of the body.
  • Hormonal System: produces chemicals (hormones) that are transported in the blood plasma to their target cells, which they then stimulate. This results in a slower, less specific form of communication between parts of an organism. The responses are often long-lasting and widespread.

Both systems work together and interact with one another.

Chemical Mediators: chemicals that are released from certain cells and have an effect on cells in their immidiate vicinity. They are released by infected or injured cells that cause small arteries and arterioles to dilate. This leads to a rise in temperature and swelling of the affected area. 

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Chemical Mediators


stored in certain white blood cells and released following injury or in response to an allergen, e.g pollen. It causes dilation of small arteries and arterioles and increased permeability of capillaries, leading to localised swelling, redness and itching.


found in cell membranes and also cause dilation of small arteries and arterioles. Their release following injury increases the permeability of capillaries. They also affect blood pressure and neurotransmitters and so affect pain sensation.

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Plant Growth Factors

Plants have no nervous system and so respond to external stimuli by means of plant hormones (plant growth factors).

  • they exert their influence by affecting growth
  • they are made by cells located throughout the plant rather than in particular organs
  • some affect the tissues that release them rather than acting on a distant target organ

They are produced in small quantities and have their effects close to the tissue that produces them. They have their effects close to the tissue that produces them.

Indoleacetic Acid (IAA) is a plant growth factor.

  • cells in the tip of the shoot produce IAA which is transported down the shoot
  • the IAA is initially transported to all sides as it begins to move down the shoot
  • light causes the movement of IAA from the light side to the shaded side of the shoot
  • a greater concentration of IAA builds up on the shaded side of the shoot than on the light side
  • as IAA causes elongation of cells, the shaded side of the shoot elongate more
  • the shaded side of the shoot grows faster, causes the shoot to bend towards the light
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  • Cell Body: contains a nucleus and large amounts of rough ER. This is associated with the production of proteins and neurotransmitters
  • Dendrons: small extenstions of the cell body which subdivide into smaller branched fibres, called dendrites, that carry nerve impulses towards the cell body
  • Axon: a single, long fibre that carries nerve impulses away from the cell body
  • Schwann Cells: surround the axon, protecting it and providing electrical insulation. They also carry out phagocytosis and play a part in nerve regeneration. They wrap themselves around the axon many times so that layers of their membranes build up around it
  • Myelin Sheath: forms a covering to the axon and is made up of the membranes of the Schwann cells. These membranes are rich in a lipid: myelin. Myelinated neurones transmit nerve impulses faster than unmyelinated neurones
  • nodes of Ranvier: gaps between adjacent Schwann cells where there is no myelin sheath.
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Nerve Impulse

Resting Potential:

The movement of ions across the axon membrane is controlled in a number of ways:

  • the phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions diffusing across it
  • ion channels span this bilayer which can allow sodium or potassium ions to move through them, depending on whether they are open or closed
  • the sodium-potassium pump actively transports potassium ions into the axon and sodium ions out of the axon

Due to this, the inside of the axon is negatively charged relative to the outside. This is the resting potential.

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Resting Potential

  • sodium ions are actively transported out of the axon by the sodium-potassium pumps
  • potassium ions are actively transported into the axon by the sodium-potassium pumps
  • active transport of s-p pump: three sodium ions move out for every two potassium ions that move in
  • there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm, and more potassium ions in the cytoplasm than in the tissue fluid, creating a chemical gradient
  • the sodium ions begin to diffuse back naturally into the axon, while the potassium ions begin to diffuse back out of the axon
  • most potassium gates are open, whereas most sodium gates are closed so the axon membrane is 100 times more permeable to potassium ions, which diffuse back out of the axon faster than the sodium ions diffuse back in. This increases the potential difference between the negative inside and the postive outside of the axon
  • more potassium ions diffuse out the axon, causing the outside of the axon to become more positive. Further outward movement of the potassium ions becomes difficult because, being positively charged, they are attracted by the overall negative state inside the axon, which compels them to move into the axon, and repelled by the overall positive state of the surrounding tissue fluid, which prevents them from moving out of the axon.
  • an equilibrium is established, where chemical and electrical gradients are balenced and there is no net movement of ions
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Action Potential

When a stimulus is recieved by a receptor, its energy causes a temporary reversal of the charges on the axon membrane. This causes the negative charge of -65mV inside the membrane to become a positive charge of +40mV.

  • at resting potential, some potassium channels are open but sodium channels are closed
  • the energy of the stimulus causes some sodium channels in the axon membrane to open so sodium ions diffuse back into the axon along their electrochemical gradient. Being positively charged, they trigger a reversal in the potential difference across the membrane
  • as the sodium ions diffuse into the axon, more sodium channels open, causing an even greater influx of sodium ions by diffusion
  • once an action potential of +40mV is established, the sodium channels close and the potassium channels begin to open
  • with some potassium channels now open, the electrical gradient that was preventing further outward movement of potassium ions is now reversed, causing more potassium ion channels to open. Potassium can diffuse out causing repolarisation of the axon
  • the outward diffusion of potassium ions causes an overshoot of the electrical gradient, with the inside of the axon being more negative than usual (hyperpolarisation). The potassium channels close and the s-p pump again causes sodium ions to be pumped in and potassium out. The resting potential of -65mV is re-established and the axon is repolarised
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Action Potential


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Passage of an Action Potential

Action potentials 'move' rapidly along the axon. As one region of the axon produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the next region of the axon. The previous region of the membrane returns to its resting potential 

  • at resting potential, the conc of sodium ions outside the axon membrane is higher than the inside, whereas the conc of potassium ions is higher on the inside than the outside. The overall conc of positive ions is greater on the outside, making it positive compared to the inside. The axon membrane is polarised
  • a stimulus causes a sudden influx of sodium ions, causing a reversal of change on the axon membrane. This is the action potential and the membrane is depolarised
  • the electrical circuits caused by the influx of sodium ions cause the opening of sodium channels further along the axon. The resting influx of sodium ions in this region causes depolarisation. Behind this new region of depolarisation, the sodium channels close and the potassium ones open. Potassium ions begin to leave the axon along their electrochemical gradient
  • the action potential is propagated in the same way further along the neurone. This outward movement of potassium ions causes the membrane behind the action potential to repolarise
  • repolarisation of the neurone allows sodium ions to be actively transported out, once again returning the neurone to its resting potential.
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Myelinated Axon

The fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming. 

There are breaks in this myelin, nodes of Ranvier, where action potentials can occur at these points. The action potential 'jumps' from node to node in a process known as saltatory conduction.

As a result, an action potential passes a myelinated neurone faster than along an unmyelinated neurone.


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Nerve Impulse

Factors affecting the speed if an action potential:

  • myelin sheath
  • diameter of the axon: the greater the diameter, the faster the speed of conductance
  • temperature: this affects the rate of diffusion. The higher the temprerature the faster the nerve impulse. Energy for active transport comes from respiration, which involves enzymes. Temperature affects enzymes.

Refractory Period:

Once an action potential has been created, there is a period of time where it is impossible for a further action potential to be generated because inward movement of sodium ions has been prevented as the sodiun channels are closed. This has 3 purposes:

  • ensures that an action potential is propagated in one direction only
  • produces discrete/seperate impulses
  • limits the number of action potentials
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All or Nothing Principle

Nerve impulses are 'all or nothing' responses. There is a certain level of stimulus, called the threshold value, which triggers an action potential. Below the threshold value, no action potential and therefore no impulse is generated. Any stimulus, whatever strength, that is below the threshold value will fail to generate an action potential: nothing. Any stimulus, whatever strength, abov the threshold value will suceed in generating an action potential. It does not matter how much above the threshold it is, it will still only generate one action potential: all.

The strength of an stimulus is perceived in two ways:

  • by the number of impulses passing in a given time. The larger the stimulus, the more impulses generated.
  • by having different neurones with different threshold values.
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  • allow a single impulse along one neurone to be transmitted to a number of different neurones at a synapse. So a single stimulus can create a number of responses
  • allow a number of impulses to be combined at a synapse, allowing stimuli from different receptors to interact in order to produce a single response


  • unidirectionality: synapses can only pass impulses in one direction
  • summation: low frequency action potentials that on their own do not trigger a new action potential can combine to reach the threshold:
    • spatial summation: a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone
    • temporal summation: a single presynaptic neurone releases neurotransmitter many times over a short period. 
  • inhibition: chloride protein channels open, causing an inward diffusion of Cl- ions, making the inside more negative (hyperpolarisation), which makes it less likely that another action potential will be created.
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Choligernic Synapse

Cholinergic synapses are ones that transmit the neurotransmitter: acetylcholine.

  • the arrival of an action potential at the end of the presynaptic neurone causes calcium ion channels to open and calcium ions enter the synaptic knob
  • the influx of calcium ions causes synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft
  • acetylcholine fuses with the receptor sites on the sodium ion channel in the membrane of the postsyaptic neruone. This causes sodium ion channels to open, allowing sodium ions to diffuse in rapidly along a concentration gradient
  • the influx of sodium ions generates a new action potential in the postsynaptic neurone
  • acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), which diffuses back across the synaptic cleft into the presynaptic neurone (recycling). The breakdown of acetylcholine also prevents it from continuously generating a new action potential
  • ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine, which is stored in the synaptic vesicles for future use. Sodium channels close int he absense of acetylcholine in the receptor sites
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Effect of Drugs

  • stimulate the nervous system by creating more action potentials in the postsynaptic neurones
    • by mimicking a neurotransmitter, stimulating the release of more neurotransmitter
    • by inhibiting the enzyme that breaks down the neurotransmitter
  • inhibit the nervous system by creating fewer action potentials in the postsynaptic neurones
    • inhibiting the release of neurotransmitter
    • blocking the receptors on the sodium or potassium ion channels on the postsynaptic neurone
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