topic 6: - response, muscles, synapses & homeostasis

• Taxes (taxis)


  a directional movement towards or away from a stimulus
• Kineses (kinesis)


  a random movement due to an

  adverse stimulus. Rate and speed of movement of

  directional change is proportional to the strength of

  the stimulus
• Tropisms


  growth of a plant in response to a

  directional stimulus

  
  

  Phototropism = growth of shoots towards light as light is needed for the LDR in photosynthesis

  
  

  Hydrotropism = growth of roots towards

  water

  
  

  Negative Phototropism = growth of roots

  away from light

  
  

  Positive Gravitropism = growth of roots

  downwards
• what is a stimulus 


  detectable change in the environment
• receptors 


  changes can be detected by cells which are receptor
• what is IAA

  • type of auxin and can control cell elongation in shoots and Inhibit growth of cells in the roots
  • made in the tips, roots or shoots but can diffuse to other cells
• Phototropism


  positive phototropism

  • shoot tip cells produce IAA causing cell elongation
  • the IAA diffuses to other cells
  • if there is unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there
  • the cells on the shaded side to elongate more and result I the plant bending towards the light source

  negative phototropism

  • roots do not photosythensis so do not need light
  • a high concentration of IAA inhibits cell elongation, causing root cells to elongate more on the lighter side and so the roots bending away from the light
• gravitropism 


  shoots - negative gravitropism

  • IAA will diffuse from the upper side to the lower side of a shoot
  • if a plant is vertical this causes the plant cells to elongate and the plant grows upwards
  • if a plant is on its side it will cause the shoot to bend upwards

  roots - postive gravitropism

  • IAA moves to the lower side of the roots so that the upper side elongates and the root bends down towards gravity and anchors the plant in
• what is a reflex

  • a rapid automatic response to protect you from danger
  • a reflex arc is made up of sensory neurones, relay neurones and motor neurones
• simple responses 


  taxes and kinesis are simple responses which keep organisms within the favourable conditions of their environment

  1. taxes - an organism will move its entire body towards a favourable stimulus or away from an unfavourable stimulus
  2. kinesis - an organism changes the speed of movement and the rate it changes direction
• receptors

  • detect stimuli
  • each receptor responds only to specific stimuli and this stimulation of a receptor leads to the establishment of a generator potential which can cause a response
  • 3 receptors we must know: pacinian corpuscle, rods, cones
• pacinian corpuscle

  • there are pressure receptors located deep in the skin, mainly in fingers and feet
  • the sensory neurone has special channel proteins in its plasma membrane
  • the membrane have stretch-mediated sodium channels
  • these open and allow sodium to enter only when they are stretched and deformed
  • when a threshold of pressure has been met it deforms the neurone plasma membrane, stretches and widens the na+ channels so sodium can diffuse in which leads to the establishment of a generator potential
• the human retina -rods

  • rods process imagines in black and white
  • to create a generator potential the pigment of rod cells which is rhodopsin must be broken down by light energy
  • they can detect light of very low intensity as many rods cells connect to one sensory neurone which is called retinal convergence
  • this means the brain cannot distinguish between the separate sources of light that stimulate it - low visual acuity
• the human retina - cones

  • cones process imagines in colour
  • there are three types that contain different types of iodopsin pigment - red, green and blue. which absorb different wavelengths of light
  • iodopsin is only broken down if there is a high light intensity so action potential can be only generated if there's enough light
  • one cone cell connects to a bipolar cell therefore cones can only respond to high light intensity
  • as each cone is only connect to one bipolar cell the brain can distinguish between separate sources of light therefore cone cells give high visual acuity
• distribution of rods and cones

  • the distribution in the retina is uneven
  • light is focused by the lens on the fovea which will receive the highest light intensity
  • most cones are located near the fovea
  • rod cells are further away
• control of the heart

  • cardiac muscle is myogenic it contracts on its own Accord, but the rate of contraction is controlled by a wave of electrical activity
  • the sinoatrial node (SAN) is located in the right atrium and is known as the pacemaker
  • the atrialventricular node (AVN) is located near the border of the right and left ventricle within the atria
  • the bundle of his is the tissue that runs through the septum
  • purkyne fibres in the walls of the ventricles
• control of the heart - process p1

  • SAN releases a wave of depolarisation across the atria causing it to contract
  • AVN releases another wave of depolarisation when the first reaches it, a non-conductive layer between the atria and ventricles prevents the wave of depolarisation travelling to the ventricles
• • control of the heart - process p2
  • the bundle of his conducts the wave down the septum and the purine fibres
  • as a result the apex and walls of the ventricles contract. there is a short delay before this happens, whilst the AVN transmits the second wave
  • allows enough time for the atria to pump all the blood into the ventricles so the cells can repolarise and the cardiac muscle can relax
• what triggers at what rate the SAN releases waves of depolarisation

  • the medulla oblongata in the brain controls the heart rate via the autonomic nervous system

  how?

  1. a centre linked to the sinoatrial node to increase heart rate via the sympathetic nervous system
  2. another that decreases heart rate via the parasympathetic nervous system
• how does PH change heart rate

  • the PH of blood will decrease during high respiratory rate e.g exercise
  • due to the production of carbon dioxide or lactic acids
  • excess must be removed as the acids can cause enzymes to denature
  • this is achieved by increasing the heart rate through more impulses via sympathetic nervous system to the san
  • so carbon dioxide can diffuse out into the alveoli more rapidly
• how does pressure change the rate of the heart

  • if pressure is too high this can cause damage to the walls of the arteries and it is important to put mechanisms to reduce the pressure, this results in more impulses via parasympathetic nervous system to decrease heart rate
  • if blood pressure too low there is a insufficient amount of oxygen to respiring cells. this results in more impulses via parasympathetic nervous system to increase heart rate
• structure of a myelinated motor neurone

  1. the cell body - contains organelles. proteins and neurotransmitter chemicals are made here
  2. dendrites - carry action potential to surrounding cells
  3. axon - conductive, long fibres that carry the nervous impulses along the motor neurone
  4. Schwann cells - wrap around the axon to form myelin sheath which is a lipid therefore charged ions cannot pass through
  5. nodes of ranvier - the gaps between myelin sheath
• resting potential

  • when a neurone is not conducting an impulse as there isn't a stimulus present
  • there is no difference between the electrical charges inside and outside of the neurone
  • there are more positive ions sodium and potassium on the outside compared to the inside therefore the inside is more negative at -70mV
• establishing a resting potential

  • the resting potential is maintained by a sodium potassium pump involving active transport and ATP
  • the pumps moves 2 k+ ions in and 3 Na+ ions out
  • this creates an electrochemical gradient causing k+ to diffuse out and Na+ to diffuse in
  • the membrane is more permeable to k+ so more are moved in so results in a charge of 70mV
• action potential

  • this generates a nervous impulse
  • depolarisation is due to the neurone membrane becoming more permeable to NA+
  • once action potential is generated it moves along the axon
• action potential process - p1

  1. Stimulus meeting the threshold value causes Na+ channels to open
  2. Na+ diffuses into the axon
  3. More Na+ channels open – greater influx of Na+
  4. Net charges are reversed leading to action potential of +40mV
  5. A nerve impulse is generated
  6. Once the +40mV is reached, Na+ channels close
  7. K+ channels remain open and those that were closed start to open
• action potential process - p2

  1. No more influx of Na+ but K+ still leave the axon
  2. K+ leaving the axon start the REPOLARISATION of the membrane potential
  3. A time delay between K+ leaving, the closure of some K+ channels and all the Na+ channels closing causes HYPERPOLARISATION
  4. Hyperpolarisation ends restoring the resting potential to -65mV
• all or nothing principal

  • if depolarisation does not exceed -55mV an act potential and impulses are not produced
  • any stimulus that does trigger depolarisation to -55mV will always peak at the same maximum voltage, but bigger stimulus increase the frequency of action potentials
  • this is important as it makes sure organisms only response to large enough stimuli rather then responding to every change in the environment
• refractory period

  • after an action potential has been generated the membrane enters refractory period when it can' t stimulated because sodium channels are recovering
  • its important because:
  • makes sure each action potential is separate, and ensures discrete impulses
  • it ensures action potential travels in one direction so means a response is generate
  • limits the amount of responses
• factors that affect speed of conductance -myelin sheath

  • saltatory conduction -action potential jumps from node to node which means action potential travels along the axon faster
  • Myelin sheath acts as an insulator therefore cannot conduct electrical impulses
  • Only exposed nodes of Ranvier can conduct AP
• factors that affect the speed of conductance -axon diameter

  • the wider the axon diameter the quicker the speed of conductance
  • a wider a diameter means there is less leakage of ions and therefore action potential travels faster
• factors that affect the speed of conductance -temperature

  1. a higher temperature increases the speed of conductance for 2 reasons
  2. the ions diffuse faster and the enzymes involved in respiration work faster therefore the more ATP there is for active transport in the na+/k+ pump
• what are synapses

  • the gaps at the end of the axon of one neurone and the dendrite of another
  • action potential is transmitted as neurotransmitters that diffuse across the synapse
• function of a synapse - part 1

  • an action potential arrives at synaptic knob
  • depolarisation of knob leads to opening of ca2+ channels and it diffuses in
  • vesicles containing neurotransmitters move towards and fuse with the presynaptic membrane
  • neurotransmitter is released to the synaptic cleft
  • NT diffuses down the concentration gradient across synaptic cleft to the post synaptic membrane
  • NT binds by complimentary of shape to the receptors on the surface of the post-synaptic membrane
• function of a synapse - part 2

  • na+ channels on the post synaptic membrane open
  • if enough na+ diffuses in above the threshold the post synaptic neurone becomes depolarised
  • NT is degraded and released from receptor, the na+ channel closes and resting potential can be reestablished
  • NT is transported back into the pre-synaptic neurone to be recycled
• summation

  • the rapid build up of neurotransmitters in the synapses to help generate action potential either by spacial or temporal summation

  1. spacial - many different neurones collectively trigger an action potential by combining the neurotransmitter they release to exceed the threshold value
  2. temporal - one neurone releases neurotransmitters repeatedly over a short time to add up enough to exceed the threshold value
• inhibitory synapses

  • causes chloride ions to move into the postsynaptic neurone and potassium ions to move out
  • this makes the membrane's potential to decrease to -80mV to hyperpolarisation.
  • therefore unlikely for a potential to be generated
• features of a cholinergic synapse

  • unidirectional
  • could be excitatory or inhibitory
  • connects two neurones which could be sensory, relay or motor
  • a new action potential is generated in the next neurone
  • acetylcholine binds to the receptors on post synaptic membrane of a neurone
• features of a neuromuscular junction

  • unidirectional
  • only excitatory
  • connects motor neurones to muscles
  • this is the end point for action potential
  • acetylcholine binds to receptors on muscle fibre membranes
• features of a muscle

  • work in antagonist pairs
  • can be automatic as part of a reflex or controlled by conscious thought
  • myofibrils are made up of fused cells that share nuclei and cytoplasm known as sarcoplasm
  • high number of mitochondria