Coordination

Coordination, Neurones, The Nerve Impulse, Passage of an Action Potential, Speed of a Nerve Impulse, Structure and Function of Synapses and Transmission Across a Synapse

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  • Coordination
    • How Coordination is Achieved
      • Principles:
        • Nervous System
          • Electrical impulses pass along nerve cells and stimulate target cells by secreting neurotransmitters directly onto them
            • Results in rapid communication between the specific parts of an organism
          • Responses are rapid, short-lived and restricted to one region of the body (e.g. reflex arcs)
        • Hormonal System
          • Hormones are transported through the blood plasma to their target cells, which they then stimulate
            • Results in slower, less specific communication between different parts of an organism
          • Responses are slower, long-lasting and widespread (e.g. control of blood glucose)
      • Chemical Mediators
        • Chemicals that are released from certain mammalian cells and have an effect on cells in their immediate vicinity
        • Typically released by infected or injured cells and cause arteries and arterioles to dilate leading to an inflammatory response (rise in temp. and swelling of affected area)
        • Histamine - is stored in certain white blood cells and is released following an injury or in response to an allergen (e.g. pollen). Causes dilation of arteries and arterioles , and increased permeability of capillaries leading to localised swelling, redness and itching
        • Prostaglandins - are found in cell membranes and also cause dilation of arteries and arterioles an increase in permeability of capillaries, however, they also affect blood pressure and neurotransmitters. Due to this they affect pain sensation
      • Plant Growth Factors
        • Though they have no nervous system, plants must respond to changes in their internal or external environment such as:
          • Water - plant roots are positively hydrotropic ( grow towards water) because it needs to be absorbed for use in photosynthesis, for support, and other metabolic processes
          • Gravity - plant roots are positively geotropic (they grow in the direction of gravity's pull) because plants need to be firmly anchored in the soil
          • Light - plant stems are positively phototropic ( they grow towards light)  because it is needed for photosynthesis
        • Plant Growth Factors
          • Plants use these to respond to external stimuli
          • Also called hormones, however 'plant growth factors' is more accurate because...
            • They are unlike animal hormones in that they are made by cells located throughout the plant , not in particular organs
            • Their influence is exerted by affecting growth
            • Unlike animal hormones, some plant growth factors affect the tissues that release them rather than a distant target organ
          • IAA (indoleacetic acid) is a plant growth factor that causes plant cells to elongate
      • Control of Tropisms by IAA (indoleacetic acid)
        • A tropism is a growth movement of a plant in response to a directional stimulus
        • If light is directed at a young shoot from one side the shoot will bend towards the light due to these events...
          • 5. IAA causes elongation of cells so cells on the shaded side of the shoot elongate more
          • 3. Light causes IAA to move from the light side to the shaded side of the shoot
          • 6. Shaded side of the shoot grows faster, causing it to bend towards the light
          • 2. Initially, IAA is transported to all sides as it moves down the shoot
          • 4. A greater conc. of IAA is on the shaded side than the light side
          • 1. Cells in the tip of the shoot produce  IAA which is then transported down the shoot
    • Neurones
      • Sensory - transmit nerve impulses from a receptor to an intermediate or motor neurone. They have one Dendron that carries the impulse towards the cell body and one axon that carries it away
      • Motor - transmit nerve impulses from an intermediate or sensory neurone to an effector (e.g. gland or muscle). They have a long axon and many short dendrites
      • Intermediate - transmit nerve impulses between neurones. (e.g. from a sensory to a motor).
      • Structure of a  Mammalian Neurone
        • Axon - single long fibre that carries nerve impulses away from the cell body
        • Schwann cells - surround the axon providing protection and electrical insulation. Also carry out phagocytosis and are involved in nerve regeneration. They wrap themselves many times around the axon, building up  layers of their membranes around it
        • Dendrons - small extensions of the cell body that sub-divide into dendrites (smaller branched fibres) that carry nerve impulses towards the cell body
        • Myelin sheath - made up of membranes of Schwann cells that are rich in the lipid myelin, it forms a covering to the axon. Not all neurones have a myelin sheath
        • Cell body -  contains nucleus and lots of  rough ER which are both needed for the manufacture of proteins and neurotransmitters
        • Nodes of Ranvier - small gaps between adjacent Schwann cells that occur every 1-3mm (in humans)
    • The Nerve Impulse
      • A self-propagating wave of electrical disturbance that travels along the surface of the axon membrane. It is a temporary reversal of the electrical potential difference between two states - resting potential and action potential - across the axon membrane
      • Resting Potential
        • How movement of ions across the axon membrane is controlled:
          • The phospholipid bilayer of the axon plasma membrane prevents Na+ and K+ diffusing across it
          • Intrinsic proteins spanning the phospholipid bilayer have ion channels. Some channels are 'gated' and can be opened or closed to allow ions through at particular times. Some channels remain open all the time
          • Sodium-potassium pumps actively transport Na+ ions out of and K+ ions into the axon
        • The establishment of resting potential ( a potential difference where the charge inside is -ve compared to the outside. Ranges between 50-90 mV. The axon is polarised due to these events...
          • 2. K+ ions are actively transported into the axon by the Na+/K+ pumps
          • 3. Three Na+ ions move out for every two K+ ions that move in
          • 6. Most of the K+ ion channel gates are open while most of the Na+ ion channel gates are closed
          • 5. Na+ ions naturally diffuse back into the axon and K+ ions diffuse back out
          • 8. As well as the chemical gradient there is an electrical gradient. As the outward diffusion of K+ ions causes the outside of the axon to become more and more +ve, they begin to repel against the +ve state of the tissue fluid and become attracted to the negative inside of the axon. K+ ions then are compelled to move into the axon
          • 9. This establishes an equilibrium where there is no net movement of ions and both the electrical and chemical gradients are balanced
          • 1. Na+ ions are actively transported out of the axon by Na+/K+ pumps
          • 7. This means the axon membrane is 100 times more permeable to K+ ions than Na+ ions. K+ ions therefore diffuse out of the axon faster than Na+ ions diffuse in so the potential difference between the +ve outside and -ve inside of the axon is increased
          • 4. A chemical gradient is created as the outward movement of Na+ ions is greater than the inward movement of K+ ions. There are more Na+ ions in the tissue fluid surrounding the axon than in the cytoplasm, and more K+ ions in the cytoplasm than in the tissue fluid
      • Action Potential
        • Stages of an Action Potential
          • 1. Resting potential. Some voltage-gated channels but the Na+ ones are closed
          • 4. When an action potential of around +40 mV has been established the voltage-gated sodium channels close and the voltage gates on the potassium channels open
          • 3. More Na+ channels open causing a greater influx of Na+ ions into the axon by diffusion
          • 5. The electrical gradient preventing outward movement of K+ ions is reversed due to the potassium voltage gated channels now being open so more K+ ions diffuse out of the axon. This repolarises the axon
          • 2. Stimulus. Energy from the stimulus causes some Na+ voltage gated channels in axon membrane to open. Na+ ions diffuse into axon along their electrochemical gradient and trigger a reversal in potential difference across the membrane due to them being positively charged
          • 6. Hyperpolarisation occurs ( a slight overshoot of the electrical gradient where the inside is more -ve than the outside) due to K+ ions diffusing out. The potassium ion channels voltage gates close. Na+/K+ pumps establish a resting potential ( -65mV )  again by pumping Na+ ions out and K+ ions in, therefore repolarising the axon
    • Action Potentials
      • Action Potentials Across Myelinated Axons
        • Nodes of Ranvier - breaks in the myelin insulation that occur at intervals of 1-3mm
          • Action potentials occur at the nodes of Ranvier
        • The fatty sheath of myelin around the axon acts as an electrical insulator and prevents action potentials from occuring
        • Saltatory Conduction - localised circuits arise between adjacent nodes of Ranvier. The action potentials 'jump' from node to node
          • Due to this, action potentials travel down myelinated neurones faster than unmyelinated ones
      • Action Potentials Across Unmyelinated Axons
        • 1. Resting Potential. Axon membrane is polarised (overall conc. of +ve ions is greater outside than inside axon). Na+ ion conc. is higher outside, although K+ ion conc. is higher inside.
        • 2. Stimulus causes sudden influx of Na+ ions and a reversal of charge on the axon membrane (an action potential). Membrane is depolarised
        • 3. Depolarisation. The localised electrical circuits established by the influx of Na+ ions cause the sodium voltage-gated channels to open a little further along the axon. This causes an influx of Na+ ions and therefore, depolarisation in this region. Behind this new region, the sodium voltage-gated channels close and potassium ones open. Potassium ions begin to leave the axon along their electrochemical gradient
        • 4. Repolarisation. The action potential (depolarisation) is propagated along the neurone, and as K+ ions move out of the axon, the axon membrane behind the action potential returns to its original charged state (+ve outside, -ve inside). It has been repolarised.
        • 5. Return to resting potential. Repolarisation of the neurone allows the active transport of Na+ ions out of the axon membrane. This returns the neurone to its resting potential and ready for a new stimulus
    • Speed of Nerve Impulses
      • Factors Affecting
        • The myelin sheath
          • Saltatory conduction increases the spped of conductance from 30 m s-1 in an unmyelinated neurone to 90 m s-1 in an unmyelinated neurone
        • The axon diameter
          • Greater the diameter the faster the conductance speed due to less leakage of ions. (Larger axon means more leakage and leakage makes membrane potentials harder to maintain)
        • Temperature
          • Affects rate of ion diffusion (higher the temp. faster the nerve impulse). Energy from respiration is also required for active transport, which like the Na+/K+ pump, is controlled by enzymes. Although enzymes function more rapidly at higher temps, above a certain temp. the enzymes and intrinsic proteins in the plasma membrane will be denatured and no nevre impulses will be conducted. Temp. is a particularly important factor in response times for ectotherms (cold-blooded animlas) as their body temp. changes with the environment
      • Refractory Period
        • Purpose of the refractory period
          • To ensure the action potential is propagated in one direction only
            • An action potential cannot be propagated in a region that is refractory, so it only moves in a forward direction. Action potentials can therefore only pass from an active region to a resting region, and are prevented from spreading out in both directions
          • To produce discrete impulses
            • Action potentials are separated from one another because the refractory period means a new action potential cannot be formed immediately behind the first one
          • To limit the number of action potentials
            • The number of action potentials that can pass along an axon in a given time is limited due to them being separated from one another
        • The period after an action potential has been created in any region of an axon when the inward movement of Na+ ions is prevented due to the sodium voltage-gated channels being closed. Generation of any further action potential Is impossible at this point
      • All-or-Nothing Principle
        • How organisms perceive the size of a stimulus
          • The number of impulses passing in a certain time. (The larger the stimulus, the more impulses are generated in a given time)
          • Different neurones have different threshold values. The brain can interpret the number and type of neurones passing impulses as a result of a given stimulus, and therefore determine the stimulus size
        • Any stimulus below the threshold value will fail to produce an action potential, yet any stimulus above the threshold will successfully produce one action potential regardless of its strength
        • Nerve impulses are described as 'all-or-nothing' responses
    • Synapses
      • Structure
        • The synaptic cleft is a 20-30nm wide gap  that separates two neurones
        • The presynaptic neurone releases the neurotransmitter
          • The axon of the presynaptic neurone ends in a swollen portion called the synaptic knob which has many mitochondria and endoplasmic reticulum (ER) as they are required in the manufacture of neurotransmitter
      • Function
        • To act as junctions. This allows...
          • A number of impulses to be combined at a synapse, allowing stimuli from different receptors to interact producing a single response
          • A single impulse along one neurone to be transmitted to a number of different neurones at a synapse, allowing a single stimulus to create a number of simultaneous responses
        • Brief Summary of Synapse Transmission
          • 2. Neurotransmitter diffuses across the synapse to receptor molecules on the postsynaptic neurone
          • 1. Neurotransmitter stored in synaptic vesicles is released into the synapse when an action potential reaches the synaptic knob
          • 3. Neurotransmitter binds with receptor molecules and sets up a new action potential in the postsynaptic neurone
      • Features
        • Unidirectional. Impulses only pass in one direction - from the presynaptic neurone to the postsynaptic neurone. Similar to valves
          • Neurotransmitter is made only in the presynaptic membrane and not in the postsynaptic membrane
          • Only the postsynaptic membrane has receptor molecules
        • Can undergo summation - when action potentials that are low freq. are made to produce enough neurotransmitter to trigger an action potential in the postsynaptic neurone. Without summation the amount of neurotransmitter produced would be insufficient to do this.
          • Spatial Summation - a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold value of a postsynaptic neurone. Therefore, together they produce an action potential
          • Temporal Summation - a 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 then an action potential is triggered
        • Inhibition - on the postsynaptic membrane of inhibitory synapses the protein channels carrying Cl- chloride ions can be made to open, causing an inward diffusion of Cl- ions and making the inside of the postsynaptic membrane more negative than resting potential. This is hyperpolarisation and makes it less likely that an action potential will be triggered
      • A synapse is  the point where the axon of one neurone connects with the dendrite of another or an effector
    • Transmission Across a Synapse
      • Cholinergic Synapses
        • The neurotransmitter is acetylcholine
        • Common in vertebrates, where they occur in the CNS  and at neuromuscular junctions
      • Process of Transmission Across a Cholinergic Synapse
        • 1. An action potential arrives at the end of the presynaptic neurone causing Ca2+ ion channels to open and Ca2+ ions to enter the synaptic knob
        • 2. The influx of Ca2+ ions into the presynaptic neurone causes the synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft
        • 3. Acetylcholine molecules fuse with receptor sites on Na+ ion channels in the membrane of the postsynaptic neurone, causing the Na+ ion channels to open and allow Na+ ions to diffuse in rapidly along a conc. gradient
        • 4. The influx of Na+ ions generates a new action potential in the postsynaptic neurone
        • 5. Enzyme Acetyl cholinesterase hydrolyses acetylcholine back into choline and ethanoic acid (acetyl) which then diffuse back across the synaptic cleft into the presynaptic neurone. (Recycling). This breakdown of acetylcholine also prevents it from continuously generating a new action potential in the postsynaptic neurone
        • 6. Choline and ethanoic acid are recombined into acetylcholine using ATP released from mitochondria. The acetylcholine is then stored in synaptic vesicles for future use and Na+ ion channels close due to the absence of acetylcholine in receptor sites

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