stimuli and response

• respond to stimuli

  • increase survival rate
  • receptors detect stimuli and effectors cause response
  • receptors communicate via nervous system or hormonal system
• stimuli 


  any changes in internal or extrenal environment
• neurones

  • sensory - transit electrical impulses from receptors to CNS ( brain /spine)
  • motor - transmit electrical impulses from CNS to effectors
  • relay - transmit electrical impulses between sensory and motor neurones
• nervous system

  1. made up of neurones
  2. stimulus detected by receptor cells and electrical impulse sent along sensory neurone
  3. electrical impulse reach end of neurone, neurotransmitters (target cells) take info to next neurone (response localised + short lived)
  4. CNS (coordinator) processes info and send impulses along motor neurons to effector
• nervous system over view

  1. Stimulus - see friend waving
  2. receptors - light receptors in eyes detect wave
  3. CNS - process info
  4. effectors - muscle cells stimulated by motor neurons
  5. response - muscles contract to make arm wave
• nervous system split into 2

  1. central - brain and spinal cord
  2. Peripheral
• peripheral nervous system

  • made up of neurones that connect CNS to rest of body
  • somatic - control conscious activities
  • ANS - unconscious activities (sympathetic and parasympatheti)
• Reflexes

  • body respond to stimulus without making conscious decision to respond
  • no time lost deciding how to respond - info travel fast from receptor to effectors
  • protect body
  • reflex arc
• reflex arc 


  pathway if neurones linking receptors to effectors in reflex

  if no relay neuron involved then possible to override reflex - withstand heat
• tropism

  • response of plant to directional stimulus
  • regulate growth
  • photo/gravitropism, positive and negative
• indoleacetic acid (auxin)

  • shoot tips
  • moved around plant to control tropism - diffusion and active transport + phloem
  • different part of plant have differing conc of IAA - uneven distribution means uneven growth
• taxes response

  • organism move towards/away from directional stimulus
  • e.g. light
• kinetic response 


  organisms movement is affect by non-directional stimulus

  • e.g. humidity
  • move faster/vary directions
  • slow but constant direction
• receptors

  • specific
  • cells or proteins
  • resting potential, generator potential and action potential
• generator potential 


  when stimulus detected cell membrane becomes excited + increase permeability- increase potential difference
• resting potential 


  receptor in nervous system in at resting state - not stimulated
• pacinian Corpuscle

  • mechanoreceptors - pressure + vibrations
  • sensory nerve ending wrapped in lamellae - end of sensory neurone
  • lamellae deform and press sensory nerve ending when stimulated
  • deform stretch-mediated sodium ion channels - open and Na diffuse into cell - generator potential trigger action potential
• photoreceptors (light)

  • light enter via pupil
  • light rays focused by lens into retina (photoreceptors)
  • fovea contains most photoreceptors
  • nerve impulses carried from retina to brain by optic nerve - leaves eye via blind spot
• photoreceptors convert light into impulse

  1. light enters eye, hit receptors and absorbed by light sensitive optical pigments
  2. light bleach pigment causing chemical change and alter membrane permeability to Na+
  3. generator potential created and if exceed threshold nerve impulse sent along bipolar neurone
• bipolar neurone


  connect photoreceptors to to optic nerve which takes impulse to brain
• Rods eye receptors

  • peripheral parts of retina
  • black and white
  • light sensitive
  • many rods join one neurone
  • many weak generator potentials combine to reach threshold and trigger action potential. Low visual acuity - light from 2 points close together cant be told apart
• cone eye receptors

  • packed in fovea
  • colour - red/green/blue sensitive
  • less light sensitive
  • one cone joins one neurone
  • takes longer for more light to reach threshold and trigger action potential
  • high visual acuity - cones close together. can distinguish 2 points close together as two separate points
• conduction system

  1. SA node initiate impulse. (pacemaker) in right atria
  2. Cause left and right atria to contract at same time
  3. A band on non-conducting collagen tissue prevent impulse passing to ventricle
  4. impulse transferred to AV node - delay impulse until atrial systole complete
  5. Reach bundle of his split impulse to purkyne fibres
  6. contract at base of ventricle
• heart rate regulation 


  rate of SA release of impulse is unconsciously controlled by medulla oblongata

  -baro/chemo receptors detect changes

  -send impulses to SAN and (para)sympathetic neurones
• neurones polarised at rest

  • resting state - outside membrane positive compared to inside (polarised)
  • -70mV
  • created and maintained by sodium-potassium pump
• neurones membrane become depolarised when stimulated

  • stimulus - excite causing open, increase permeability to sodium (gradient)
  • depolarisation - potential difference exceeds threshold (-55mV), more sodium ion channels open and diffuse in
  • repolarisation - potential difference (+30mV) the Na channels close and K open (K diffuse out and return to resting)
  • Hyper-polarisation - K channels slow to close so slight overshoot ( p.d become more negative)
• refractory period

  • neurone cell can’t be edited again straight after action potential
  • ion channels recovering and can’t be opened
  • Na+ channels closed during repolarisation and K channels are closed during hyper polarisation
• factors affecting speed of conduction action potential

  1. myelination
  2. axon diameter
  3. temperature
• myelination

  • have myelin sheath - electrical insulator
  • sheath made of Schwann cells- between have small bare membrane - Node of Ranvier (Na channels conc)
  • depolarisation only occur at node of ranvier
  • cytoplasm conducts enough impulse to depolarise next node - saltatory conduction
• Axon diameter

  • action potential conducted quickly along axons with bigger diameters because there’s less resistance to flow of ions.
  • less resistance, depolarisation reached other part of neurone cell membrane quicker
• temperature

  • speed of conduction increases the temp increases
  • ions diffuse faster
  • 40°C
• Synapse

  junctions between 2 neurones or between a neurone and an effector
• synaptic cleft 


  tiny gap between cells at synapse
• presynaptic neurone

  • before synapse
  • swelling - synaptic knob
  • contains synaptic vesicles filled with neurotransmitters
• postsynaptic neurones

  • action potential reached end of neurone it causes neurotransmitters to be released into synaptic cleft
  • diffuse across to post synaptic membrane and bind to specific receptors
  • receptors only on post synaptic membrane - unidirectional
• Ach transmits impulse at Cholinergic synapse

  1. action potential reach synaptic knob and stimulate Ca+ channels to open (Ca diffuse in)
  2. influx of Ca ions cause synaptic vesicles to move to presynaptic membrane and fuse together ( vesicle release Ach - exocytosis)
  3. Ach diffuse across cleft and bind to cholinergic receptors on post synaptic
  4. Na+ chanel open (Na diffuse in) - depolarisation
  5. Ach removed so response doesn’t reoccur, broken down by acetylchokinesterase
• excitatory neurotransmitter

  depolarise postsynaptic membrane - create action potential
• inhibitory neurotransmitter


  hyperpolarise postsynaptic membrane preventing action potential
• spatial summation

  • many neurones connect to one neurone
  • small vol. neurotransmitter released from each is enough to reach threshold together in post synaptic and trigger an action potential
  • if release inhibitory then total effect of all neurotransmitters lead to no action potential
• temporal summation 


  2+ nerve impulses arrive in quick succession from same presynaptic neurone

  action potential more likely because more neurotransmitter released into cleft