Stimulus & Response

• simple response: a response that is not complex and does not involve a lot of processing, to remain in favourable environment - tactic or kinetic
• Tactic response: directional movement in response to stimulus - direction of stimulus affects response

  • woodlice move away from light to avoid predator
• Kinetic response: non-directional movement- intensity of stimulus affects the response

  • woodlice move into humid area to avoid water loss
• reflex arc: simple neural pathway between sensory receptors and effector organs (muscles/glands)
• Receptors: detect stimuli and send signals to the CNS

  • cells or proteins on cell surface membrane
  • specific to one stimulus
• Motor neurone: carries impulse from CNS to effector organ
• Sensory neurones: carry impulses from receptor cells to the CNS
• Effector: muscle or gland cell that responds to a stimulus and produces a response

  • receptor to effector communicates through nervous or hormonal system
• Neurones: the basic units of the nervous system, which carry electrical impulses around the body
• Neurones:

  • sensory neurone - transmit E.I from receptors to CNS - brain & spinal cord
  • Motor neurone - transmit E.I from CNS to Effector
  • Relay neurone - transmit E.I between sensory and Motor neurone

  Stimulus- Receptors- CNS- Effectors- Response
• Nervous response:

  • neurotransmitter secreted directly onto cell - localised
  • E.I + response rapid -> quick reaction
• Simple Reflexes - Rapid, involuntary response to a stimulus

  • goes through spinal cord not conscious part of brain
• Reflex arc: Pathway of neurones linking receptors to effectors

  • Sensory- Relay- Motor neurone
• Tropism: the growth response of a plant to an external stimulus, such as gravity or light.

  • phototropism
  • Gravitropism
• Phototropism: The growth of a plant in response to light

  • positive - growth towards plant - shoot
  • negative- growth away from plant- roots
• Gravitropism: The growth of a plant in response to gravity

  • positive- grow downwards - roots
  • negative- grow upwards - shoots
• Auxins: A plant hormone that promotes the growth of shoots and roots by cell elongation

  • produced in tips of shoots and move backwards to stimulate cell behind tips of shoots to elongate
• Indoleactic Acid (IAA): type of plant hormone that stimulates plant growth

  • moves throughout plant through diffusion and active transport via Phloem over long distance
  • different concentration of IAA across plants-> uneven growth of plant
• The Pacinian corpuscle is a mechanoreceptor found in skin and other tissues. It consists of concentric layers of connective tissue surrounding a fluid filled cavity. When pressure is applied to the outer layer, it compresses the inner layers causing deformation of the nerve endings within the capsule resulting in action potential being generated.
• Pacinian Corpuscle: mechanoreceptor that responds to pressure and vibrations

  • found in skin
  • contain end of sensory neurone/ sensory nerve ending - wrapped in lamellae
• when Pacinian Corpuscle stimulated:

  • Lamellae deformed and press on sensory nerve ending
  • Causing stretch-mediated sodium channels to deform and open
  • Sodium ion diffuses into cell creating generator potential, if reaching threshold, A.P PRODUCED
• Resting potential: The potential difference across the membrane of a neuron when it is not stimulated
• Generator potential: The change in the electrical potential difference across the membrane of a neuron when it is depolarized, due to stimulus

  • membrane becomes more permeable
• Action potential: the electrical impulse that travels along the axon of a neuron

  • only reached if generator potential reaches threshold level
  • one size
• Photoreceptors: Light-sensitive cells in the retina that convert light energy into nerve impulses
• photoreceptors interaction with eye:

  • light enters through pupil- amount entering controlled by muscles of iris
  • light rays focused by lens into retina-> lines inside of eye
  • Retina contains photoreceptors cells
  • Fovea has high concentration of Photoreceptor cells
  • Nerve impulse from retina to brain by optic nerve ->bundle of neurones
  • where optic nerve leave eye is the blind spot -> not sensitive to light
• How photoreceptors work:

  • light enters eye, hits photoreceptors and is absorbed by light sensitive opitc pigments
  • light bleaches pigment= chemical change that alters permeability of membrane to sodium ions
  • Generator potential created & if threshold reached -> nerve impulse sent along bipolar neurone
  • B.N- connect photoreceptor to optic nerve that sends nerve impulse to brain
• Rods:

  • mainly found in peripheral parts of retina
  • only give info in black and white (monochromatic vision)
  • Very sensitive to light - work in dim light
  • Many rod cell join to 1 bipolar neurone -> weak generator potential to reach threshold to trigger A.P
  • Low visual Acuity -> many rod cells to one bipolar neurone - harder to distinguish light sources
• Cone cells:

  • found closely packed in Fovea
  • give information in colour (trichomatic)
  • 3 types of cone cells
  • less sensitive to light than rod cells (work in bright light)
  • high visual acuity - cone cells closely packed & 1 cone cell to 1 bipolar neurone