Stimuli and responses

• what is taxes?


  a directional movement in response to a stimulus, either movement towards or away from the stimulus
• what is kinesis?


  a random, non directional movement in response to a stimulus

  - the intensity of the stimulus affects the response with greater movement and a greater rate of movement in less favourable conditions
• what is tropism?


  plant growth in response to a stimulus
• what is phototropism?


  - growth of a plant in response to light

  - shoots are positively phototropic (grow towards light)

  - roots are negatively phototropic (grow away from light)
• what is gravitropism?


  - growth of plant in response to gravity

  - shoots are negatively gravitropic (grow upwards)

  - roots are positively gravitropic (grow downwards)
• what is IAA (idoleacetic acid)?

  an important auxin produced in the tips of shoots and roots of flowering plants

  - group of growth factors
• how does IAA cause growth?

  - synthesised in the tip cap

  - diffuse backwards to stimulate the cell just behind the tips to elongate - cell walls become loose and stretchy

  - stimulate growth ib shoots but high concentrations inhibit growth in roots

  - uneven distribution means that there's uneven growth of the plant
• how does IAA cause phototropism?

  - IAA moves to the more shaded parts of the shoots and roots, so there's uneven growth

  - shoots : IAA concentration increases on the shaded side - cells elongated and the shoot bends towards the light
• how does IAA cause gravitropism?

  - IAA moves to the underside of shoots and roots, so there's uneven growth

  - shoots : IAA concentration increases on the lower side - cells elongate so the shoot grows upwards

  - roots : IAA concentration increases on the lower side - growth is inhibited so the root grows downwards
• what is the structure of a motor neurone?

  - cell body at beginning of axon

  - axon

  - dendrites

  - myelin sheath

  - nodes of ranvier

  - muscle / gland
• what is the structure of a sensory neurone?

  - receptor cell

  - nodes of ranvier

  - myelin sheath

  - cell body part way along axon
• what is the structure of relay neurone?

  - axon

  - dendrites

  - cell body at beginning of axon
• why is motor and sensory neurone white matter?


  - has a myelin sheath
• where are the types of neuroes found?

  - relay - only in central nervous system

  - motor and sensory - in peripheral and central nervous system
• what is the cell body part of a neurone?

  - contains a nucleus and large amounts of rough ER

  - this is associated with the production of proteins and neurotransmitters
• what is the dendrites part of a neurone?

  - small extensions of the cell body which subdivide into smaller branched fibres that carry nerve impulses towards cell body
• what is the axon part of a neurone?

  a single long fibre that carries nerve impulses away from the cell body
• what is the Schwann cell part of a neurone?

  - surround the axon, protecting it and providing electrical insulation

  - they also carry out phagocytosis (the removal of cell debris) and play a part in nerve regeneration

  - the cells wrap themselves around the axon many times, so that layers of their membrane build up around it
• what is the myelin sheath part of a neuroen?

  - forms a covering to the axon and is made up of the membranes of the Schwann cells

  - these membranes are rich in a lipid known as myelin

  - neurones with it are called myelinated neurones and they transmit nerve impulses faster than unmyelinated neurones
• what is nodes of ranvier part of a neurone?

  - gaps between adjacent schwann cells where there is no myelin sheath
• what is a reflex action?

  a reflex action is a rapid, automatic response to a stimulus
• what is the function of reflex action?

  prevent harm and damage to tissues- homeostasis- balance
• where are synapses located?

  between motor and relay neurones and between sensory and relay neurones
• why is a reflex action fast?

  - because it does go to the brain only to CNS so has a shorter distance and theres fewer synapses involved
• which root does the sensory neurone travel through?

  - through the dorsal root

  - motor neurone goes out through the ventral root
• how can the dorsal root be identified?

  - theres a bulge around the cell body on the sensory neurone in the dorsal root
• how is a resting potential established in an axon?

  - sodium potassium pump actively transports 3 sodium ions out of the cell and 2 potassium opus into the cell

  - this leads to a higher concentration of sodium ions outside of the cell than inside and a higher concentration of potassium ions inside the cell compared to the outside

  - potassium ions diffuse out of the cell down the concentration gradient whereas odium ions diffuse in

  - the membrane is more permeable to potassium ions than to sodium ions because there are more open complementary channels for potassium ions. The voltage gated sodium ion channels are closed

  - therefore, more potassium ions diffuse out and little sodium ions diffuse ion

  - this leads to more positively charged ions on the outside of the cell compared to the inside

  - the inside of the cell/ axon is more negative than the outside with a potential difference of -70mV
• how does an action potential occur in an axon?

  - some (voltage gated) sodium ion channels open

  - sodium ions move into the cell/ axon via facilitated diffusion, and this makes the potential difference less negative

  - if the change in potential difference reaches threshold (-55mV), all the voltage gated sodium ion channels open

  - sodium ions diffuse in and the membrane depolarises

  - a potential difference of +40mV occurs
• how does repolarisation occur in an axon?

  - at +40mV the voltage gated sodium ion channels close and additional potassium ion channels open

  - more potassium ions diffuse out of the cell to give more positively charged ions outside the cell than inside

  - the sodium potassium pump also actively transports 3 sodium ions out of the cell (and two potassium ions in)

  - the potential difference falls, and the inside come more negative than the outside of the cell

  - the membrane repolarises

  - hyperpolarisation occurs with the potential difference reaching -80mV

  - the additional voltage gated potassium ion channels close

  - the sodium potassium pump continues to actively transport 3 sodium ions out of the cell and two potassium ions in and this re-established resting potential
• how does an action potential travel through a pre-synaptic neurone into the synaptic cleft?

  - an action potential occurring in the pre-synaptic neurone leads to voltage gated calcium ion channels opening

  - calcium ions diffuse into the pre-synaptic neurone

  - this triggers the vesicles to move to the pre-synaptic neurone and fuse with it

  - this releases the neurotransmitter (acetylcholine is the neurotransmitter if it a cholinergic synapse) into the synaptic cleft?
• how does an action potential travel through the synaptic cleft to a post-synaptic neurone?

  - the neurotrasnmitter diffuses across the gap via simple diffusion

  - the neurotransmitter binds to complementary shaped receptors in the post-synaptic neurone membrane

  - the binding triggers chemical gated sodium ion channels in the membrane to open

  - sodium ions diffuse into the post synaptic neurone

  - this increases the potential difference, so the post-synaptic membrane depolarises

  - an action potential is generated, and transmission of a nerve impulse occurs in the post synaptic neurone
• how does the synapse go back to how it was before an action potential?

  - within the synaptic cleft are enzymes (acetylcholinesterase in a cholinergic synapse) that hydrolase the bonds in the neurotransmitter to break it down (into acetyl and choline)

  - the products of the breakdown diffuse back across the synaptic cleft and across the pre-synaptic neurone via facilitated diffusion

  - within the pre synaptic neurone the neurotransmitter is re-synthesised and this required the energy from ATP

  - the re-synthesis maintains the gradient for the uptake of the neurotransmitter components

  - the removal of the neurotransmitter from the synaptic cleft prevents the continuous generating of action potentials in the post-synaptic neurone

  - this is because the removal of the neurotransmitter means that sodium ion channels in the post synaptic membrane close
• how is a receptor a transduces?

  it converts energy from a stimulus to electrical energy
• what is a pacinian corpuscle?

  is a sensory receptor that responds to changes in mechanical pressure
• what do all receptors have in common?

  - specific to a single type of stimulus

  - produces a generator potential by acting as a transducer
• what is the pacinian corpuscle in its resting state?

  - the stretch mediated sodium ion channels of the membrane around the neurone of a corpuscle are too narrow too allow sodium ions to pass along them. In this state, the neurone of the pacinian corpuscle has a resting potential

  - at the resting potential the inside of the cell is more negative than the outside (-70mV) and the membrane is polarised
• how does the pacinian corpuscle generate a generator potential?

  - when pressure is applied to the pacinian corpuscle, it changes shape and the lamella becomes deformed which alters the shape of stretch mediated sodium ion channels therefore making them wider

  - so sodium ions diffuse into the neurone

  - the influx of sodium ions changes the potential of the membrane (ie becomes depolarised) thereby producing a generator potential

  - at the generator potential the inside of the cell is more positive than the outside

  - the generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones to the central nervous system
• what is temporal summation?

  - a high frequency of impulses travel down a single neurone

  - each impulse causes some neurotransmitter (such as acetylcholine) to be released from the post synaptic neurones

  - the higher concentration of acetylcholine leads to enough sodium ions moving into the pos synaptic neurone which causes the potential difference to become less negative

  - this means threshold is reached and therefore depolarisation of the post synaptic neurones membrane occurs
• what is spatial summation?


  - impulses travel down multiple neurones

  - each impulse causes some neurotransmitter (such as acetylcholine) to be released from the pre-synaptic neurones

  - the higher concentration of acetylcholine leads to tough sodium ions moving into the post synaptic neurone which causes the potential difference to become less negatve

  - this means threshold is reached and therefore depolarisation of the post synaptic neurones membrane occurs
• what pigments do rods and cones contain?

  - rods have the pigment Rhodopsin

  - cons have the pigment Iodopsin