COORDINATION

Allergen - a normally harmless substance that causes the immune system to produce an immune response
Neurotransmitter - one of a number of chemicals that are involved in communication between adjacent neurones or between nerve cells and muscles. Two important examples are acetylcholine and noradrenaline

There are two main forms of coordination in mammals:

the nervous system: uses nerve cells to pass electrical impulses along their length. They are able to stimulate their target calls by secreting chemicals known as neurotransmitters directly onto them which results in rapid communication between specific parts of the organism. The responses that are produced are ofter 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 their target cells which they then stimulate. This often results in a slower and less specific form of communication between parts of an organism. The responses are often long lasting and widespread

Both systems work together and with one another.

At a cellular level, these systems are helped by substances called chemical mediators, another form of coordination. They lead to swelling of the affected area and an increase in temperature and so the response is known as the 'inflammatory response'.

Two examples of a chemical mediator are:

histamine: stored in white blood cells and released following injury or as a response to an allergen. It causes the dilation of small arteries and arterioles and increases the permeability of the capillaries which causes localised swelling, redness and itching
prostaglandins: found in the cell membranes and cause the dilation of small arteries and arterioles. Their release following injury increases the permeability of the capillaries. They also affect blood pressure and neurotransmitters and as a result, affect pain sensation

Plants and IAA

Plants have no nervous system however they must be able to respond to things like:

light (stems grow towards light-positively phototrophic-as light is needed for photosynthesis)
gravity (as roots are positively geotropic-grow downwards)
water (plant roots are positively hydrotrophic-grow towards water for use in photosynthesis and other metabolic processes)

Plants therefore respond to external stimuli using plant growth factors which:

exert their influence by affecting growth
unlike animal hormones, are made by cells found throughout the plant instead of just in particular organs
unlike animal hormones, can affect the tissues that release them rather than acting on a distant target organ

Plant growth factors are produced in small amounts

One example of a plant growth factor is indoleacetic acid (IAA) which causes plant cells to grow longer.
IAA works by:

cells in the tip of the shoot produce IAA which is then transported down the shoot
to begin with the IAA is transported to all sides as it begins to move down the shoot
light then 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
because IAA causes cells to elongate and there is a greater concentration of IAA on the shaded side of the root, the cells on this side elongate more
the shaded side of the shoot grows faster, causing the shoot to bend towards the light

IAA also controls the bending of roots in the direction of gravity. However, whereas a high concentration of IAA increases the growth in stem cells, it decreases the growth in root cells

NEURONES
Action potential - change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes
Axon - a process extending from a neurone that conducts action potentials away from the cell body
Depolarisation - temporary reversal of the charges on the cell-surface membrane of a neurone that takes place when a nerve impulse is transmitted
Phagocytosis - mechanism by which cells transport large particles across the cell-surface membrane
Potential difference - the difference in charge between the inside and the outside of the axon
Repolaristion - return to the resting potential in the axon of a neurone after an action potential

Neurones are specialised cells that are adapted for quickly carrying electrochemical changes called nerve impulses from one part of the body to the other.

The structure of neurones

a cell body - contains a nucleus and large amounts of a rough endoplasmic reticulum (associated with the production of proteins and neurotransmitters)
dendrons - small extensions of the cell body which subdivide into smaller branched fibres called dendrites that carry nerve impulses towards the cell body
an axon - a single long fibre that carries nerve impulses away from the cell body
Schwann cells - these surround the axon, protecting and insulating it. They also carry out phagocytosis and play a part in nerve regeneration
a myelin sheath - acts as a kind of covering for the axon and is made up of the membranes of Schwann cells. These membranes have lots of a lipid called myelin. Neurones with a myelin sheath are known as myelinated neurones, but some don't have one and are called unmyelinated neurones. Myelinated neurones can transmit nerve impulses faster than unmyelinated ones
nodes of Ranvier - gaps between adjacent Schwann cells where there is no myelin sheath

Neurones are classified by their function:

sensory neurones: transmit nerve impulses from a receptor to an intermediate or motor neurone. They have a dendron that carries the impulse towards the cell body and an axon that carries it away from the cell body
motor neurones: transmit nerve impulses from an intermediate (such as a relay neurone) or sensory neurone to an effector, like a gland or muscle. They have a long axon and many short dendrites
intermediate neurones: transmit impulses between neurones, they have lots of short processes

The nerve impulse

A nerve impulse is a temporary reversal of the electrical potential difference across an axon membrane (from the resting potential to the action potential)

The movement of Na⁺ and K⁺ ions is what changes the resting potential to the action potential when stimulated. The movement of these ions is controlled in a number of ways:

the phospholipid bilayer of the axon's plasma membrane prevents sodium and potassium ions diffusing across it
intrinsic proteins span the bilayer which contain some gated and some non-gated ion channels to let sodium or potassium ions pass through them
some intrinsic proteins actively transport potassiums ions into the axon and sodium ions out of the axon in a process called the sodium-potassium pump

The inside of the axon is negatively charged compared to the outside, with the inside at a resting potential of about -70 mV (millivolts). The axon is therefore said to be polarised
This potential difference is due to:

sodium ions are being actively transported out of the axon by the sodium-potassium pump while simultaneously, potassium ions are being actively transported into the axon
3 sodium ions are being moved out for every 2 potassium ions that move in so there's a greater positive charge on the outside of the axon
the sodium ions begin to diffuse back into the axon down a high-to-low concentration gradient while potassium ions diffuse back out of the axon along the same principle
however, most of the potassium-gated channels are open whereas the sodium-gated channels are closed
this means that the axon membrane is more permeable to potassium ions and so they diffuse out of the axon faster than sodium ions can diffuse in, increasing the potential difference between the inside and outside of the axon

When a stimulus is received by a receptor, its energy causes a temporary reversal of the charges of the axon membrane-the inside of the membrane goes from -70 mV to +40 mV. This is known as the action potential and the membrane is said to have been depolarised.

How the gated-ion channels open at resting potential:

some potassium-gated channels are open but the sodium ones are closed
the nerve impulse and energy of the stimulus causes some sodium-gated channels in the axon membrane to open and sodium ions start diffusing into the axon down a concentration gradient. They trigger a reversal in the potential difference across the membrane, as the inside slowly becomes less negative
as the sodium ions diffuse into the axon, more sodium-gated channels open, increasing the amount of sodium ions diffusing in and increasing the permeability of the membrane in respect to sodium ions
once the inside of the membrane has reached threshold value (around -40 mV) all the sodium-gated channels open causing a massive influx of sodium ions, making the inside of the membrane more positive
once the action potential of +40 mV is reached, the sodium-gated channels close and the potassium-gated channels begin to open
with some potassium-gated channels open, potassium ions start to diffuse out of the membrane, causing more potassium-gated channels to open, meaning more potassium ions diffuse out etc which causes depolarisation of the membrane
the outward diffusion of the potassium ions causes the inside of the axon to be more negative than usual (hyperpolarisation). The gated channels for potassium now close and the sodium-potassium pump starts to pump sodium ions out of the axon and potassium ions into the axon again
the resting potential of the axon is re-established and the axon has been repolarised

The action potential is caused by diffusion while the resting potential is maintained by active transport

Once the action potential has been created, it moves quickly along the axon. It stays the same size all the way along the axon. It is able to be transferred all the way along the axon because the axon membrane's charge is reversed along the membrane, effectively moving the AP along it.
As one region of the axon is produces an action potential and becomes depolarised, it acts as a stimulus for the depolarisation of the adjacent section of the membrane. Therefore the action potential is regenerated along each small region of the axon membrane while the previous regions are repolarised-return to their resting potential.

At resting potential, the concentration of sodium ions outside the axon membrane is high compared with the inside, whereas the concentration of potassium ions is higher on the inside than on the outside. Overall, the outside of the membrane is more positively charged that the inside which is more negatively charged (due to things like proteins which are negative and too big to move through the membrane). The axon membrane is therefore polarised
A stimulus causes a sudden influx of sodium ions and reverses the charge on the axon membrane-this is the action potential and the membrane has been depolarised
The localised electrical circuits caused by the influx of sodium ions causes the opening of sodium-gated channels further along the axon and the resulting influx of sodium ions in this region causes depolarisation. At the original region of the action potential, the sodium-gated channels close and the potassium ones open, allowing potassium ions to move out by diffusion
There action potential (depolarisation) is transferred in the same way further along the axon. The outward movement of the potassium ions has caused the original section of the membrane to return to its original charged state-it has been repolarised
Repolarisation of the axon membrane allows sodium ions to be actively transported out, again returning the axon to its resting potential, ready for when another stimulus arrives

Action potentials along a myelinated neurone

In myelinated axons, the fatty sheath of myelin surrounding the axon acts as an electrical insulated and prevents action potentials from forming. However, at the nodes of Ranvier there is no myelin sheath and so an action potential can form there. The localised circuits therefore form at the nodes of Ranvier and the action potentials jump from one node to another in what's called saltatory conduction.
Therefore, an action potential can pass along a myelinated neurone faster than it can on an unmyelinated one as it doesn't have to depolarise all of the membrane.

The transmission of the action potential along the axon of a neurone is the nerve impulse

Factors that affect how fast the action potential

the myelin sheath: the speed of the action potential increases if the neurone is myelinated compared with an unmyelinated one due to saltatory conduction
the diameter of the axon: the greater the diameter of the axon, the faster the speed of conduction due to less leakage of ions from a large axon (leakage makes membrane potentials harder to maintain)
temperature: affects the rate of diffusion of ions and therefore the higher the temperature the faster the nerve impulse. The energy for active transport comes from respiration. Respiration, as well as the sodium-potassium pump is controlled by enzymes. Enzymes function more rapidly at higher temperatures up to a point. Above a certain temperature, enzymes and the plasma membrane proteins are denatured and impulses fail to be conducted at all. Therefore temperature is an important factor to response times in cold-blooded (ectothermic) animals, whose body temperature varies according to the environment

The refractory period is the time when it is impossible for further action potentials to be generated because the sodium-gated channels are closed so there can't be any inward movement of sodium ions. The refractory period is useful for three reasons:

in means that action potentials are only generated in one direction only: an action potential can only pass from an active resin to a resting region because action potentials can't be propagated in a region that is refractory, which means it can only move in a forward direction
it produces discrete impulses: due to the refractory period, a new action potential can't be formed immediately behind the first one which 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

Nerve impulses are all-or-nothing responses. There is a certain level of stimulus-the threshold value-which triggers an action potential. Below the threshold value, no action potential and therefore no impulse is generated. Any stimulus above the threshold value will generate an action potential-the same size action potential each time

The size of a stimulus is measured in two ways:

by the number of impulses passing in a given time: the larger the stimulus, the more impulses that are generated in a given time
by having different neurones with different threshold values: the brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size

Structure and function of synapses

Synapse - the point where the axon of one neurone connects with the dendrite of another or with an effector

Synapses are important because they link together different neurones and therefore coordinate activities.
They transmit impulses from one neurone to another using chemicals called neurotransmitters. Neurones are separated by a small gap called the synaptic cleft. The neurone that releases the neurotransmitter is called the pre-synaptic neurone. The axon of this neurone ends in a swollen portion known as the synaptic knob. This possesses many mitochondria and large amounts of endoplasmic reticulum which are required in the manufacture of the neurotransmitter which is stored in the synaptic vesicles.
Once the neurotransmitter is released from the vesicles it diffuses across to the post-synaptic neurone which has receptor molecules on its membrane to receive the neurotransmitter.

Synapses transmit impulses from one neurone to another and act as a junction, allowing:

a single impulse along one neurone to be transmitted to a number of different neurones at a synapse which allows a single stimulus to create a number of simultaneous response
a number of impulses to be combined at a synapse which allows stimuli from different receptors to interact in order to produce one response

To understand the basic function of synapses it is important to understand:

a neurotransmitter is only made in the presynaptic neurone
the neurotransmitter is stored in synaptic vesicles and released into the synapse when an action potential reaches the synaptic knob
when it's released, it diffuses across the synapse to receptor molecules on the postsynaptic neurone
the neurotransmitter binds with the receptor molecules and sets us a new action potential in the postsynaptic neurone

Synapses have a lots of different features:

They are unidirectional: synapses can only pass impulses in one direction
Inhibition: on the post-synaptic membrane of some synapses, the protein channels carrying chloride ions can open leading to an inward diffusion of chloride ions making the inside of the post-synaptic membrane more negative-hyperpolarisation-meaning that it's less likely for threshold value to be reached and therefore less likely for a new action potential to be triggered
Low frequency action potentials often produce not enough neurotransmitter to start a new action potential in the postsynaptic neurone. But an action potential can be brought about by summation which involves a build-up of the neurotransmitter in the synapse by 2 different methods:

spatial summation: different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the post-synpatic neurone and together they can trigger a new action potential
temporal summation: a single pre-synaptic neurone releases neurotransmitter many times over a short time. If the total amount of neurotransmitter exceeds the threshold value of the post-synaptic neurone, a new action potential is triggered

Acetyl choline is one example of the transmitter substances used in synapses in the central nervous system and at neuromuscular junctions.

Drugs can act on synapses by:

stimulating the nervous system by creating more action potentials in post-synaptic membranes - drugs can do this by mimicking the neurotransmitter, stimulating the release of more neurotransmitter or inhibiting the enzyme that breaks down the neurotransmitter.
inhibiting the nervous system by creating fewer action potentials in post-synaptic membranes - drugs can do this by inhibiting the release of neurotransmitter or blocking the receptors on sodium/potassium channels on the post-synaptic neurone

How action potentials cross the synaptic cleft:

The arrival of an action potential at the end of the pre-synaptic neurone causes the calcium ion channels to open and calcium ions to enter the synaptic knob
The influx of calcium ions into the pre-synaptic neurone causes synaptic vesicles to fuse with the pre-synaptic membrane, so releasing acetylcholine into the synaptic cleft
Acetylcholine molecules fuse with receptor sites on the sodium ion channel in the membrane of the post-synaptic neurone. This causes the sodium ion channels to open allowing sodium ions to diffuse rapidly along a concentration gradient
The influx of sodium ions generates a new action potential in the post-synaptic membrane
Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl) which diffuse back across the synaptic cleft into the presynaptic neurone. As well as recycling the choline and acetyl, the breakdown of acetylcholine also prevents it from continuously generating a new action potential in the post synaptic neurone
ATP released by the mitochondria is used to recombine choline and acetyl into acetylcholine which is stored in the synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites

	Creado por meganh-b hace casi 10 años

Apunte sobre COORDINATION, creado por meganh-b el 18/02/2015.