principles of coordination
There are two main forms of coordination in mamals: The nervous and the hormonal system-
The nervous system- uses nerve cells to pass electrical impulses along their length.
They stimulate their target cells by secreting chemicals-known as neurotransmitters- directly towards them
Rapid communication between specific parts of an organism
The respnse is short lived and restricted to a localised region of the body
The hormonal system- produces chemicals- hormones- that are transported in the blood plasma to target cells
Slower less specific from of communication between parts of an organism
Responses are long-lasting and widespread
They are released by infected or injured cells and cause small arteries and arteroiles to dilate
Leads to a rise in temperature and swelling of the affected area- The Infammatorty Response
Two examplesof chemical mediators are:
Histamine- stored in certain white blood cells and released after injury or in response to an allergen
It causes dilation of small arteries and arterioles and increased permeablilty of the capillaries
leading to localised swelling readness and itching
Prostaglandins- found in cell membranes and causes dilation of small arteries and arterioles and increases permeablity of capillaries
They also affect blood pressure and neurotransmitters- so affect pain sensation
plant growth factors
Plants have no nervous system
Plants must respond to:
Light- stems grow towards light because it is needed for photosynthesis
Gravity- plants need to be firmly anchored. Roots are sensitive to gravity
Water- most plants grow toward water because it is needed for photosynthesis
Plants respond to external stimuli by hormones- Plant Growth Factors:
- They exert their influence by affecting growth
- They are made by cells throughout the plant rather than in particular organs
- Some plant growth factors affect the tissues that released them rather than acting on a target cell
- They are produced in small quatities
Control of tropism by Indoleacetic acid (IAA)
1- Cells in the tip of the shoot produce IAA, which is then transported down the shoot
2- The IAA is initially transported to all sides as it begins to move down the shoot
3- Light causes the movement of IAA from the light side to the shaded side of the shoot
4- A greater concentration of IAA builds up on the shaded side of the shoot than on the light side
5- As IAA causes elongation of cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more
6- The shaded side of the shoot grows faster causing the shoot to bend towards the light
The IAA also controls the bending of roots in the direction of gravity
A high concentation of IAA increases growth in the stem cells but decreases growth in root cells.
Structure of neurones
A cell body- containg a nucleus and large amounts of rough endoplasmic reticulum. This is associated with the production of proteins and neurotransmitters
Dendrons- small extensions of the cell body which subdivide into smaller branched fibres- called dentrites- 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 providing electrical insulation. They carry out phagocytis and play a part in nerve regeneration. They wrap themselves around the axon many times.
Myelin shealth- forms a covering to the axon and is made up of the membranes of the schwann cells. The membrane contains the lipid myelin. Neurones with myelin sheath are called mylenated neurones and transmit nerve impulses faster than unmylenated neurones.
Nodes of Ranvier- gaps between adjacent schwann cells where there is no myelin sheath. The gaps are 2-3 um long and occur every 1-3mm in humans
Neurones can be classified by their function
Sensory neurones- Transmit nerve impulses from one receptor to an intermediate or motor neurone. They have one dendron that carries the impulses towards the cell body and one axon that carries it away from the body
Motor neurone- Transmit nerve impulses from an intermediate or sensory neurone to an effector. They have a long axon and many short dendrites
Intermediate neurones- Transmit impulses between neurones. They have short processes.
The resting potential of a nerve impulse
The movement of ions such as sodium ions (Na+) and potassium ions (K+) 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
Molecules of proteins- known as intrinsic proteins -span the phospholipid bilayer. These proteins contain channels called ion channels which pass through them. Some channels are gated
Some intrinsic proteins actively transport potassium ions into the axon and sodium ions out of the axon. This process is called the sodium-potassium pump
As a result the inside of the axon is negatively charged relative to the outside. This is known as the resting potential and ranges from 50-90 millivolts (mV)
The axon is polarised
potential difference in the axon is due to:
Sodium are actively transported out of the axon by the sodium-potassium pumps
Potassium ions are actively transported into the axon by the soduim-potassium pumps
The transport of sodium is 3 for every 2 potassium ions
This means that there are more sodium ions in the tissue fluid surrounding the axon and more potassium in the cytoplasm so creating a chemical gradient
The sodium ions begin to diffuse back into the axon while the potassium ions begin to diffuse out
most of the potassium gates are open and most of the sodium gates are closed
The axon membrane is 100X more permeable to potasium ions so they diffuse back faster. This inceases the potential difference. Inside of the axon becomes negative
As more potassium ions diffuse out the more negative the axon becomes.
Eventually the potassium ions cannot move out. Thery are attracted to the negative charge
The action potential
- At resting potential some potassium voltage-gated channels are open but the sodium voltage- gated channels are closed
- The energy of the stimulus causes some sodium voltage-gated channels in the axon membrane to open and sodium diffuses into the axon along their electrochemical gradient. They trigger a reversal in the potential difference across the membrane
- As the sodium ions diffuse into the axon so more sodium channels open causing an even greater influx of sodium ions by diffusion
- Once the action potential has been established the sodium voltage gates close and the potassium voltage gates open
- The electrical gradient that was preventing further outward movement of potassium ions is not reversed causing more potassium channels to open and potassium to move out causing repolarisation of the axon
- The outward diffusion of potassium ions causes a temporary overshoot of the electrical gradient with the inside of the axon being more negative. The gates on the potassium ion channels close. potassium in pumped in and sodium in pumped out. The axon is repolarised
passage of an impulse along an umyelinated neurone
At resting potential- concentrtation of soduim ions outside the axon membrane is high. The potassium concentration inside the membrane is high. More positive ions on the outside so the inside is negative relative to the outside. The membrane is polarised.
A stimulus causes an influx of sodium ions and the reversal of charge in the membrane- The Action Potential. The membrane is depolarised
Localised electrical circuits are established and cause sodium gated channels to open and sodium to move in. This causes depolarisation of the axon. The sodium gates close and potassium open so potassium leaves the axon along the electrochemical gradient.
The action potential (depolarisation) is propagated further along the membrane. The movement of potassium ions has caused the axon to move back to its original state. It has been repolarised
Repolarisation of the neurone allows sodium ions to be actively transported out. Returning the axon back to is resting potential
passage of an action potential along a myelinated
In myelinated axons the fatty sheath of myelin around the axon acts as an electrical insulator preventing action potentials from forming.
At intervals of 1-3mm there are breaks in the myelin insulation- Nodes Of Ranvier
The localised circuits arise between adjacent nodes of ranvier and the action potentials jump from one node to another - Saltatory Conduction
The action potential travels faster in the myelinated neurone
Factors affecting the speed of an action potential
The Myelin sheath- The speed of conductance in salatory conduction in a myelinated axon increases from 30ms-90ms
The Diameter Of The Axon- The greater the diameter the faster the speed of conductance. This is due less leakage of ions from a large axon
Temperature- This affects the rate of diffusion of ions. The higher the temperature the faster the nerve impulse. The engergy comes from repsiration is controlled by enzymes, which work faster at higher temperature but become denatured if the temperature is too high.
The Refractory Period
After an action potential has been created there is a period when the sodium gated channels close, so no further action potential potential can be created- The Rafractory Period
It Serves Three Purposes:
It Ensures That An Action Potential Is Propagated In One Direction- An action potential can only pass from an active region to a resting region. So that the action potential flows in one direction otherwise it would go in both directions.
It Produces Discrete Impulses- Due to the refractory period a new action potential cannot be formed immediately behind the first on. This ensures that action potentials are separated from one another.
It Limits The Number Of Action Potentials- As action potentials are separated from one another this limits the number of action potentials that can pass along an axon in a given time.
Structure of a synapse
Neurones are separated by a small gap called the synaptic cleft which is 20-30nm wide
This neurone that releases the neurotransmitter is called the presynaptic neurone
The axon of this neurone ends in a swollen portion- known as the synaptic knob
This contains many mitochondria and large amounts of endoplasmic reticulum
These required in the manufacture of the neurotransmitter
Once made the neurotransmitter is stored the synaptic vesicles
Once the neurotransmitter is released from the vesicles it diffuses across ot the postsynaptic neurone, which contains receptor molecules on its membrane to recieve it
Functions OIf Synapses
Synapses transmit impulses from one neurone to another. They act as junctions allowing:
A single impulse along one neurone to be transmitted to a number of different neurones at a synpse. This allows a single stimulus of different number of stimultaneous responses
A number of impulses to be combined at a synapse. This allows stimuli from different receptors to interact in order to produce a single response
Features Of Synapses
Undirectionality- Synapses can only pass impulses in one direction: from the presynaptic neurone to the postsynaptic neurone.
Summation- low frequency action potentials do not produce enough neurotransmitters to trigger an action potential in the postsynaptic neurone - by a process called summation -A build-up of the neurotransmitter in the synapse by the one of TWO METHODS:
Spatial summation- Presynaptic neurones release enough neurotransmitters to exceed the threshold value of the postsynaptic neurone. triggering a new action potential
Temporal summation- Single presynaptic neurone releases neurotransmitter many times over a short period. If the total amount of neurotransmitter exceeds the threshold value of the postsynaptic neurone. Triggering a new potential
Inhibition- On the postsynaptic membrane of some synapses, the protein channels carrying chloride ions can be made to open. leads to an inward diffusion of Cl- ions, making the inside of the postsynaptic membrane even more negative than its resting potential- called hyperpolarisation and makes it less likely that a new action potential will be created. These synapses are called inhibitory synapses
Neurone A releases neurotransmitter but quantity is below threshold to trigger action potential in postsynaptic neurone
Neurone B releases neurotransmitter but quantity is below threshold to trigger action potential in postsynaptic neurone
Neurone A and B release neurotransmitter. Quantity is above threshold and so an actionpotential is triggered in the post synaptic neurone
Low frequency action potentials lead to release of small amount of neurotransmitter. Quantity is below the threshold to trigger an action potential in the postsynaptic neurone.
High-frequency action potentials lead to relaease of large amount of neurotransmitter. Quanitty is above the threshold to trigger an action potential in the postsynaptic neurone.
Transmission Across A Cholinergic Synapse
1-The action potential at the end of the presynaptic neurone causes calcium ion channels to open and calcium ions enter the synaptic knob
2- The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylchlorine into the synaptic cleft
3- Acetylchlorine molecules fuse with receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone. This causes the sodium ion channels to open, allowing sodium ions to diffuse in rapidly along a concentration gradient.
4- The influx of sodium ions generate a new action potential in the postsynaptic neurone.
5- Acetylchlorinesterase hydrolyses acetylchlorine into chlorine and ethanoic acid, which diffuse back across the synaptic cleft into the presynaptic neurone. The break down of acetylchlorine also prevents it from continuously generating a new action potential
6- ATP released by mitchochondria is used to recombine chlroine and ethanoic acid into acetychlorine. This is stored in synaptic vesicle for future use. Sodium ion channels close in the absence of acetychlorine in the receptor sites.