ALL

• What is a Stimuli?

  - a change in the internal or external environment
• Why do Organisms need to Respond to Stimuli?

  - for increasing chance of survival (predator/prey awareness, homeostasis, )
• How do Simple Organisms Respond to Stimuli?

  - Taxis

  - Kinesis
• What is Taxis?


  -directional responseto a stimuli (towards or away from)
• What is Kinesis?


  -non-directional movementfrom an unfavourable area to a favourable area

  - organism moves rapidly and randomly in unfavourable area until they reach favourable area where they move slowly and less randomly

  - so spends more time in favourable area, less time in unfavourable area
• Example of Response to Stimuli in Plants?

  -directional growth in plants in response to a stimuli

  - towards = positive, away = negative

  light = photo, water = hydro, gravity = geo

  shoot shows positive phototropism and negative geotropism

  root shows positive geotropism and positive hydrotropism

  controlled by a Plant Growth Factor = Indoleacetic Acid (IAA) - auxin
• Example of Response to Stimuli in Plants?

  - Tropism
• What is Tropism?


  - directional growth in plants in response to a stimuli

  - towards = positive, away = negative

  - light = photo, water = hydro, gravity = geo

  - shoot shows positive phototropism and negative geotropism

  - root shows positive geotropism and positive hydrotropism

  controlled by a Plant Growth Factor = Indoleacetic Acid (IAA) - auxin
• What is a Plant Growth Factor?

  - equivalent to animal hormones

  - difference: made by cells throughout the plant, only affects cells locally, affects growth

  - i.e. IAA
• What are the affects of IAA?

  - promotes growth in the shoot, inhibits growth in the root
• How does positive phototropism in the shoot take place?

  - normally: shoot tip produces IAA, sending it down both sides causing the shoot to grow forwards

  - if light is present on one side, the IAA redistributes to the opposite side (shaded side)

  - this causes the opposite side to grow faster

  - so the shoot bends towards the light
• How does negative geotropism in the shoot take place?

  - if gravity is present on one side, the IAA redistributes to the same side

  - this causes the same side to grow faster

  - so the shoot bends away from gravity towards the light
• How does positive geotropism/hydrotropism in the root take place?

  - if gravity/water is present on one side, the IAA redistributes to the same side

  - this causes the same side to grow slowly, so the opposite side grows faster

  - so the root bends towards the gravity/water
• Evidences for Tropism (positive phototropism in shoot)?


  - removing or covering shoot tip prevents tropism [tip causes tropism]

  - placing micin (prevents movement of chemicals e.g. IAA) across shoot inhibits tropism [tropism caused by movement of chemicals]

  - placing gelatine (prevents movement of electrical signals) across shoot does not affect tropism [tropism not caused by movement of electrical signals]

  - if shoot tip is moved to one side, that side grows faster and the shoot bends the other way [IAA promotes growth in shoot]

  - when in light or darkness the overall levels of IAA remain the same [light does not inhibit or breakdown IAA but rather redistributes it]
• Response to Stimuli in Mammals?

  - uses Nervous System & Hormonal System - systems coordinate response to stimuli
• Job of Nervous System?

  - coordinate response to certain stimuli

  - response is fast, short acting
• Pathway of Nervous System?

  stimuli -> receptor -> sensory neurone -> spinal cord -> brain -> spinal cord -> motor neurone -> effector for response
• What does a Receptor do?

  - detects stimuli & converts stimuli energy into nerve impulse (acts as a transducer - converts one type of energy into another)

  - each type of stimuli has a specific receptor

  - uses stimuli energy to send Na+ ions into the start of the sensory neurone

  - i.e. Pacinian Corpuscle, Retina of Eye
• What does a Pacinian Corpuscle do?


  - touch (mechanical stimuli) receptor

  - found in skin, fingers and toes

  - responds to pressure/touch

  - structure = corpuscle (several layers of tissue) wrapped around the start of a sensory neurone

  Process:

  1. pressure applied, corpuscle compressed

  2. stretch-mediated Na+ channels opened

  3. Na+ ions move into the start of the sensory neurone
• How does the Retina of the Eye work?

  - photoreceptors (detects light) so the brain can generate an image

  - detected by retina (located at back of eye)

  - made of Cone and Rod cells

  -Cone Cellsdetect high light intensity only, produces colour image, with high visual acuity

  -Rod Cellscan detect low light intensity, produces black and white image, with low visual acuity

  - Cone Cells located in centre of retina (fovea) - site of high light intensity

  - Rod Cells located in periphery of retina
• Properties of Cone Cells in Retina?

  - one cone cell connects to one bipolar neurone which connects to one sensory neurone (therefore no summation of light can take place so only detects bright light)

  - because one cone cell connects to one bipolar neurone which connects to one sensory neurone, each stimuli can be distinguished = high visual acuity
• Properties of Rod Cells in Retina?

  - a few rod cells connect to one bipolar neurone which connects to one sensory neurone (therefore summation of light can take place so can detect low light intensity)

  - because a few rod cells connect to one bipolar neurone which connects to one sensory neurone, the stimuli will be merged together = low visual acuity
• What is the Central Nervous System (CNS)?

  - made of brain and spinal cord

  - brain = analyses and coordinates response to stimuli

  - spinal cord = connects brain to sensory and motor neurones
• What is the Peripheral Nervous System (PNS)?

  - made of the sensory and motor neurone

  - a neurone transmits a nerve impulse

  - sensory neurone takes nerve impulse from receptor to CNS

  - motor neurone takes nerve impulse from CNS to effector

  - sensory neurone has its cell body in the middle and has a dendron and axon

  - motor neurone has its cell body at the start and only has a long axon
• What are the 2 different types of Motor Neurone?

  - Voluntary (Somatic) and Involuntary (Autonomic)

  -Somaticsupplies skeletal muscle = under conscious control

  -Autonomicsupplies cardiac muscle, smooth muscle, glands = under subconscious control

  - Autonomic can beSympathetic(flight or fight) andParasympathetic(rest and digest)
• What is a Nerve Impulse?

  - movement of an action potential along a neurone

  - action potential = change in membrane potential

  - charge in one section of the neurone changes from negative (polarised) -> positive (depolarised) -> back to negative (hyperpolarised)
• What is Resting Potential?

  - membrane potential of neurone at rest

  - is -70mV

  - polarised

  - caused by having more positive ions outside neurone compared to inside

  - involves Na+/K+ pump, pumping 3 Na+ ions out, 2 K+ ions in

  - K+ channel allowing K+ ions to diffuse out (K+ ions will eventually stop diffusing out due to a positive potential outside)
• What happens during an Action Potential?

  - stimuli causes Na+ ions to enter the start of the neurone

  - makes membrane potential less negative

  - if it reaches threshold (-55mV), Na+ channels open

  - more Na+ ions diffuse into the neurone, therefore membrane potential becomes positive (depolarised)

  - the membrane potential reaches +40mV, the Na+ channels close, the K+ channels open

  - K+ ions diffuse out, therefore membrane potential becomes negative (repolarised)

  - too many K+ ions move out, so the membrane potential becomes more negative than normal (hyperpolarised)

  
  

  one action potential= resting potential (-70mV) -> threshold (-55mV) -> depolarisation -> repolarisation (+40mV), hyperpolarisation (less than resting) -> resting potential (-70mV).
• How does an Action Potential move along a Neurone?

  - wave of depolarisation

  - if the stimuli energy is large enough and enough Na+ ions enter the start of the neurone, threshold will be reached and an action potential (AP) will occur (the 1st AP is called a Generator Potential)

  - Na+ ions that move in during depolarisation of the generator potential diffuse along the neurone causing the next section to reach threshold and an AP to occur

  - this process continues along the neurone
• Why are action potentials unidirectional?

  - repolarisation causes Na+ channels to close and K+ channels to open (refractory period) and threshold cannot be reached
• How does the Size of Stimuli affect a Nerve Impulse?

  - does not affect size of AP (AP is all or nothing)

  - reaches threshold = AP generated [all]

  - doesn't reach threshold = no AP [nothing]

  - larger stimuli increases the frequency (number) of APs
• What affects Speed of Nerve Impulse?

  -temperature= higher temp, higher kinetic energy, faster rate of diffusion of ions (faster nerve impulse)

  -axon diameter= wider diameter, less friction to flow of ions (faster nerve impulse)

  -myelination:

  - some neurones are myelinated

  - have a myelin sheath made of Schwann cells (electrical insulator)

  - Schwann cells wrap around axon, insulates axon preventing AP

  - therefore AP only occurs in gaps (nodes of Ranvier)

  - AP jumps from node to node = saltatory conduction (faster nerve impulse than non- myelinated neurone)
• What is a Synapse?


  - connection between 2 different neurones

  - sends nerve impulse across the gap (synaptic cleft) using neurotransmitters

  - AP arrives in end of presynaptic neurone

  - Ca2+ channels open

  - Ca2+ ions enter presynaptic neurone (active transport)

  - causes vesicles containing neurotransmitter to move to presynaptic membrane

  - vesicle binds to membrane releasing neurotransmitter into cleft (exocytosis)

  - neurotransmitter diffuses across cleft

  - binds to complementary receptors on postsynaptic membrane

  - Na+ channels open, Na+ ions enter

  - if threshold is reached, AP occurs

  - to return to rest: enzyme breaks down neurotransmitter, products re-absorbed by presynaptic neurone to reform neurotransmitter

  
• What are the Properties of Synapses?

  -unidirectional= AP/nerve impulse travels in one direction (pre -> post) pre has the neurotransmitter, post has the receptors

  -filters out low level stimuli= low level stimuli do not release enough neurotransmitter, therefore not enough Na+ ion channels open, therefore not enough Na+ ions enter postsynaptic neurone for threshold to be reached, no AP produced

  -summation= low level stimuli add together to release enough neurotransmitter to produce an AP in postsynaptic neurone, can be temporal or spatial

  - temporal = low level stimuli present for extended period of time

  - spatial = a low-level stimuli from a few presynaptic neurones add together

  -inhibitory= normal synapses are excitatory (cause AP), some can be inhibitory - prevent action potential from occurring by making postsynaptic neurone hyperpolarised
• What is a Reflex?

  - a rapid involuntary response to a stimuli

  - does not use the brain

  - the sensory neurone connects directly to motor neurone

  - ensures less damage done and does not require learning

  
  

  Pathway = stimuli -> receptor -> sensory neurone -> relay neurone -> motor neurone -> effector for response
• How is Heart Rate controlled?

  - the heart is myogenic, its heart beat is initiated by the SAN (does not require signals from nerves)

  - Medulla Oblongata in the brain can increase or decrease heart rate

  - receives nerve impulse from chemoreceptors (respond to blood pH) in the carotid arteries and pressure receptors (respond to blood pressure) in the carotid arteries and aorta

  - sends impulse in sympathetic nerves to SAN to increase HR and sends impulse in parasympathetic nerves to SAN to decrease HR
• How does Exercise affect Heart Rate?

  - exercise = muscle contraction, which requires respiration so waste product CO2 is released into blood

  - this lower pH of blood (more acidic)

  - detected by chemoreceptors in carotid arteries

  - sends impulses to medulla oblongata

  - medulla oblongata sends impulses to SAN via the sympathetic nerves causing the heart rate to increase

  
  

  benefit= increase blood flow to lungs to remove CO2 and take in O2
• How does Low Blood Pressure affect Heart Rate?


  - if a person moves from lying/sitting to standing, blood pressure falls (reducing blood flow to the brain)

  - detected by pressure receptors in the carotid arteries and aorta

  - sends impulses to medulla oblongata

  - medulla oblongata sends impulses to SAN via the sympathetic nerves causing the heart rate to increase

  
  

  benefit= increasing heart rate leads to an increase in blood pressure (so enough blood can reach the brain)
• What are the different types of Muscles?

  - Skeletal

  - Smooth

  - Cardiac

  
• What is the job of the Skeletal Muscle?


  - moves the body skeleton

  - when the muscle contracts (shortens) the tendon pulls on joints causing movement

  - i.e. biceps contract, triceps relax