3.6.1 stimuli

• What’s a stimulus

  Change in internal or external environment of an organism, leads to a response
• Why do organisms respond to stimuli

  Increases chance of survival
• What’s a receptor

  Detects stimuli, each receptor is specific to a type of stimulus
• What’s a coordinator

  Links receptors and effectors, formulates a suitable response to a stimulus
• What’s an effector

  Produces a response
• What’s are two types of response

  Hormonal

  Nervous system
• What’s the sequence of a response to stimuli

  Stimulus, receptor, coordinator, effector, response
• Three types of simple responses to stimuli

  Taxis Kinesis Tropism (plants)
• What’s taxis

  Organism responds directly to stimulus by moving towards (a favourable) or away from (an unfavourable) stimulus eg earthworks move away from light
• What’s kinesis

  Organism changes the rate at which it moves/ rate it changes direction In favourable conditions organisms turn lots to stay In that environment In unfavourable conditions organisms turn less to help them move into a new favourable environment
• What’s the central nervous system (CNS)

  Brain and spinal cord
• What’s the peripheral nervous system (PNS) and which nerves do they contain

  Pairs of nerves that originate from the brain or spinal cord

  Sensory neurones, carry nerve impulses from receptors towards the central nervous system

  Motor neurones, carry nerve impulses from CNS to effectors
• Two types of motor nervous system

  Voluntary nervous system, nerve impulses are carried to body muscle, under conscious control Autonomic nervous system, nerve impulses carried to glands/smooth muscle/Cardiac muscle, subconscious
• What’s a reflex arc

  The pathway of neurones involved in a reflex
• What’s a reflex

  A rapid and automatic response to a stimulus
• Stages of a reflex arc

  Stimulus eg heat from an object

  Receptor eg Receptors on skin generates a nerve impulse in the sensory neurone

  Sensory neurone, passes nerve impulse to spinal cord

  Coordinator (relay neurone), connects sensory neurone to motor neurone in spinal cord

  Motor neurone, carries nerve impulses from spinal cord to an effector eg muscle

  Effector eg muscle in upper arm contracts

  Response eg hand pulled away from hot object
• Why is a reflex arc important

  Involuntary, response is always the same

  Protect body from harm, effective from birth

  Fast, neurone pathway has one or two synapses, response is also automatic
• What’s phototropism

  Response of shoots to light
• What’s gravitropism

  The response of roots to gravity
• What are specific growth factors

  Regulate growth of shoots/roots in response to directional stimuli
• Explain phototropism in plants

  IAA evenly distributed through plant, Light causes movement of IAA to shaded side of plant, higher conc of IAA on dark side of shoot, cells on this side elongate more than the other side, shoot too bends towards light
• Explain gravitropism in plants

  IAA evenly distributed through roots, greater conc of IAA on lower side of root, IAA in roots inhibits elongation of cells on lower side, roots bend downwards towards gravity
• What types of tropisms effect shoots and roots

  Both tropisms effect both shoots and roots, IAA always inhibits cell elongation in roots
• How does IAA elongate cells

  H+ ions are actively transported from the cytoplasm into the cell wall, cells become more plastic allowing it to elongate
• Two types of light receptors in the eye

  Cone cells Rod cells
• How are rod cells connected to a sensory neurone and it’s impact on light sensitivity colour sensitivity and visual acuity

  Many rod cells are connected to a single sensory neurone this is called retinal convergence, this allows for spatial summation, threshold value is reached, more sensitivity to light

  
  

  Due to many rod cells sharing a single impulse the brain can’t differentiate between the separate sources of light, low visual acuity

  
  

  Cannot distinguish differences in the wavelength of light
• How are cone cells connected to a sensory neurone and it’s impact on visual acuity colour sensitivity and light sensitivity

  Each cone cells connects to its own sensory neurone, no spatial summation, not very sensitive to light

  
  

  Each cell makes its own connection, where each sends its own SEPERATE impulse to the brain, brain can distinguish between two different light sources, high visual acuity

  
  

  Each cell contains one of three different iodopsin pigments, each cell absorbs a different wavelength of light, colours depend on the proportion of each type that is stimulated
• What pigment do rod cells contain

  Rhodopsin
• What pigment do come cells contain

  Iodopsin
• How do the photoreceptors work

  Rhodopsin in rod cells or iodopsin in cone cells absorb light, the pigment breaks down, if threshold value is reached (depends on light intensity) a generator potential is created,
• Structure of the retina and its effect on visual acuity, sensitivity to light

  Lens, focuses light onto retina

  Retina, back of eye opposite pupil

  Fovea- light focused by lens, highest light intensity, cone cells found here, NO rod cells, low sensitivity to light but high visual acuity

  Rest of retina, low light intensity, only rod cells found, very sensitive to light, low visual acuity

  Blind spot, no cells
• What’s a pacinian corpuscle

  A receptor
• How does a pacinian corpuscle work

  Specific to stimulus of mechanical pressure Acts as a transducer to produce a generator potential, transducers mechanical energy into a generator potential (one for of energy converted into another)
• Structure and function of a pacinian corpuscle

  Single sensory neurone at centre of tissue layers, contain stretch mediated sodium channels Capsule surrounds layers of tissue and gel Tissue layers separated by gel
• How is a generator potential established in a pacinian corpuscle

  Resting state, stretch mediated Na+ channels are too narrow for Na+ to pass through Pressure applied to corpuscle, deformed, membrane around neurone becomes stretched, widens Na+ channel, Na+ diffuses in Membrane becomes depolarised, produces generator potential, which produces an action potential
• What’s a property of a the heart

  Myogenic, contraction is initiated from within the muscle itself
• What cells are found in the right atrium of the heart

  SAN, known as pacemaker, sets the Rhythm of stimulation
• Location of chemoreceptors

  Found in wall of carotid arteries,
• What’s the location of pressure receptors

  Found in walls of carotid artery and aorta
• Role of pressure receptors

  Higher than normal blood pressure Pressure receptors transmit more nervous impulses to centre in medulla oblongata, decreases heart rate, centre sends impulses via parasympathetic nervous system to SAN, decreased heart rate Lower blood pressure than normal Pressure receptors transmit more nervous impulses to centre in medulla oblongata to increase heart rate, centre sends impulses via sympathetic nervous system to SAN increases heart rate