UNIT 5 BIOLOGY: RESPONSE TO STIMULI

A STIMULUS IS A DETECTABLE CHANGE IN THE INTERNAL OR EXTERNAL OF AN ORGANISM ENVIRONMENT THAT PRODUCES A RESPONSE 

ABILITY TO RESPOND TO STIMULUS INCREASES ORGANISMS CHANCE OF SURVIVAL 

ORGANISMS THAT SURVIVE GREATER CHANCE OF PASSING ALLELES ON TO THEIR NEXT GENERATION -

CAUSING SELECTION PRESSURE THAT FAVOURS MORE APPROPIATE RESPONSES 

DETECTED BY A RECEPTOR ( A CELL/ORGAN)

RECEPOTRS TRANSFORM ENERGY OF STIMULUS INTO SOME FORM OF ENERGY THAT CAN BE PROCESSED BY ORGANISM TO LEAD TO A RESPONSE 

RESPONSE IS CARRIED OUT BY EFFECTORS - MUSCLES AND GLANDS 

RECEPTORS AND EFFECTORS ARE OFTEN FAR APART AND THEREFORE A FORM OF COOMUNICATION IS NEEDED IF THE ORGANISM IS GOING TO RESPOND WELL, ONE WAY OF DOING THIS IS THROUGH HORMONES - SLOW PROCESS

ANOTHER WAY WHICH IS MUCH QUICKER - IS USING THE NERVOUS SYSTEM!!!!

A SIMPLE RESPONSE, WHOSE DIRECTION IS DETERMINED BY THE DIRECTION OF STIMULUS 

A MOTILE ORGANISM EITHER MOVES TOWARDS A FAVOURABLE STIMULUS (POSITIVE TAXIS) OR AWAY FROM AN UNFAVOURABLE STIMULUS (NEGATIVE TAXIS)

E.G. SINGLE CELLED ALGAE - move towards light (postive phototaxis) for photosynthesis

EARTHWORMS MOVE AWAY FROM LIGHT - (negative phototaxis) - this takes them in to the soil and increases chance of survival - food and avoidance of predators 

BACTERIA MOVE TOWARDS AREAS WHERE GLUCOSE IS HIGH - (positive chemotaxis) - increases chance f survival - glucose is their source of food 

MORE UNPLEASANT THE STIMULUS THE MORE RAPIDLY IT MOVES AND CHANGES DIRECTION 

INCREASES IN RANDOM MOVEMENTS 

DESIGNED TO BRING ORGANISM BACK TO FAVOURABLE POSITIONS 

IMPORTANT FOR THINGS LIKE HUMIDTY OR TEMPERATURE WHEN STIMULUS IS LESS DIRECTIONAL 

EXAMPLE - WOODLICE - lose water from their bodies when in dry conditions - when they are in the dry they move rapidly to increase chance of moving into a different area with moisture - where they will slow down and changed direction less - increasing chance of survivial 

GROWTH MOVEMENT OF PART OF A PLANT IN RESPONSE TO A DIRECTIONAL STIMULUS 

THE PLANT PART EITHER GROWS TOWARDS STIMULUS - (POSTIVE) - EXAMPLE - plant shoots grow towards the light (positive phototropism) so leaves are in a more favourable position to catch light for photosynthesis

OR AWAY FROM STIMULUS (NEGATIVE) - EXAMPLE - plant roots grow away from light (negative phototropism) and towards gravity (positive geotropism) - this means the roots will grow down meaning they can absorb more water and minerals

NERVOUS SYTEM 2 MAJOR DIVISIONS; 

• CENTRAL NERVOUS SYSTEM - BRAIN AND SPINAL CORD 
• PERIPHERAL NERVOUS SYTEM - PAIRS OF NERVES THAT ORIGINATE TO EITHER THE BRAIN OR SPINAL CORD 

PNS DIVIDED INTO ;

• SENSORY NEURONES - CARRY NERVE IMPULSES FROM RECEPTORS TO THE CNS 
• MOTOR NEURONES - CARRY NERVE IMPULSES AWAY FROM THE CENTRAL NERVOUS SYSTEM TO EFFECTORS 

MOTOR NERVOUS SYTEM FURTHER DIVIDED INTO; 

• VOLUNTRARY - CARRIES NERVE IMPULSES TO BODY MUSCLES AND IS IN VOLUNTARY CONTROL 
• AUTONOMIC - CARRIES NERVE IMPULSES TO GLANDS, SMOOTH MUSCLE AND CARDIAC MUSCLE INVOLUNTARY 

COLUMN OF NERVOUS TISSUE THAT RUNS ALONG THE BACK AND LIES INSIDE THE VERTEBRAL COLUMN FOR PROTECTION 

EMERGING AT INTERVALS ALONG SPINAL CORD ARE PAIRS OF NERVES 

THIS RESPONSE IS INVOLUNTARY - REFLEX 

PATHWAY OF NEURONES INVOLVED IN A REFLEX IS KNOWN AS REFLEX ARC 

SEQUENCE: 

• stimulus 
• receptor 
• sensory neurone 
• intermediate neurone 
• motor neurone 
• effector
• response 

MAKES SURVIVAL MORE LIKELY 

• INVOLUNTARY - do not require decision making powers of the brain - leaving it free to carry out more complex responses - brain does not overloaded 
• PROTECT - PROTECT BODY FROM HARMFUL STIMULI - effective from birth and dont have to be learnt 
• FAST - neurone pathway is short with very few synapses, helps get away quickly from harmful stimuli 

CONTROLS THE INVOLUNTARY ACTIVITIES OF INTERNAL MUSCLES AND GLANDS 

2 DIVISONS 

• SYMPATHETIC NERVOUS SYTEM - stimulates effectors and speeds up any activity,stimulates effectors when we excersise or experience powerful emotions - helps us cope with stressful situations by heightening awareness - fight or flight repsonse
• PARASYMPATHETIC - inhibits effectors and slows down activity, controls activities under normal resting conditions - helps conserve energy and replenshing bodies reserves 

BOTH PARA AND SYNPATHETIC NERVOUS SYTEMS OPPOSE ONE ANOTHER AND ARE CALLED ANTAGONISITC 

RESTING HEART RATE FOR AVERAGE HUMAN 70BPM - essential this rate can vary to meet varying demand for oxygen e.g. excerise

CHANGES TO HEART RATE CONTROLLED BY MEDULLA OBLONGATA

IT HAS 2 CENTRES;

• CENTRE THAT INCREASES HEART RATE: linked to sinoatrial node by the sympathetic nervous sytem
• CENTRE THAT DECREASES HEART RATE: linked to sinoatrial node by parasympathetic 
• which one of these centres is stimulated depends upon info they recive from types of receptor which respond to one of these.....
• chemical changes in blood
• pressure changes in blood  

FOUND IN THE WALL OF CAROTID ARTERIES (arteries that serve brain) 

SENSITIVE TO CHANGES IN PH OF BLOOD WHICH RESULTS FROM CHANGES IN CO2 CONCENTRATION - CO2 FORMS AN ACID WHICH LOWERS PH 

PROCESS OF CONTROL WORKS AS FOLLOWS;

• when blood has higher co2 concentration ph is lowered 
• chemorecpetors in carotid arteries detect this and increase nerve impulses to the centre of the medulla oblongata that increases heart rate 
• this centre increases frequency of impulses via sympathetic nervous sytstem to the sinoatrial node which in turn increases heart rate 
• this increased blood flow leads to co2 being removed from lungs and so co2 levels return to normal 
• ph of the blood then rises to normal 

OCCUR IN WALLS OF CAROTID ARTERIES AND AORTA 

• WHEN BLOOD PRESSURE IS HIGHER THAN NORMAL - they transmit a nervous impulse to centre of medulla oblongata that decreease heart rate, this centre send an impulse to sinoatrial node through parasympathetic nervous sytem - which decreases heart rate 
• WHEN BLOOD PRESSURE IS LOWER THAN NORMAL - they transmit a nervous impulse to centre of medulla oblongata that increases heart rate, this cente sends an impulse to sinoatrial node via sympathetic nervous sytem - increases heart rate 

PACINIAN CORPUSCLE - RESPOND TO CHANGES IN MECHANICAL PRESSURE 

AS WITH ALL RECEPTORS A CORPUSLCE:

• SPECIFIC TO A SINGLE TYPE OF STIMULUS - in this case only responds to mechanical pressure, it wont respond to any toher like heat 
• PRODUCES A GENERATOR POTENTIAL BY ACTING AS A TRANSDUCER - role of transducer to convert the info provided by stimulus into a form that can be processed by the body, stimulus has energy, the nerve impulse is also energy - receptors therefore convert one form of energy into another - energy of stimulus is converted into a nervous impulse caled a generator potenital.

RESPOND TO MECHANCIAL PRESSURE 

OCCUR DEEP IN THE SKIN - MOST ABUNDANT ON FIONERS AND SOLES OF FEET 

ALSO OCCUR IN JOINTS, LIGAMENTS AND TENDONS WHERE THEY ENABLE ORGANISM TO KNOW WHICH JOINTS ARE CHANGING DIRECTION 

SENSORY NEURONE ENDING AT THE CENTRE HAS A SODIUM CHANNEL IN ITS PLASMA MEMBRANE - STRETCH MEDIATED SODIUM CHANNEL 

• in normal resting state - stretch mediated sodium channel does not let sodium ions to pass - too narrow - in this state the neurone of pacinian corpuscle has a resting potential 
• when pressure is applied - it changes chape and the membrane around becomes stretched 
• this stretching widens sodium channel in membrane and sodium ions diffuse into neurone 
• influx of sodium ions changes the potential of the membrae - depolarised - producing a generator potential 
• generator potential creates an action potential that passes along the neurone and to the CNS 

LIGHT RECEPTORS OF EYE ARE FOUND ON ITS INNERMOST LAYER - RETINA 

MILLIONS OF LIGHT RECEPTORS FOUND IN RETINA ARE OF 2 TYPES 

ROD CELLS

CONE CELLS 

BOTH ACT AS TRANSDUCERS BY CONVERTING LIGHT ENERGY INTO ELECTRICAL ENERGY OF NERVE IMPULSE

CANNOT DISTINGUISH BETWEEN DIFFERENT WAVELENGTHS OF LIGHT AND THEREFORE ONLY PRODUCE IMAGES IN BLACK AND WHITE 

MORE NUMEROUS THAN CONE CELLS (120 MILLION IN EACH EYE) 

• many rod cells share a single sensory neurone, therefore they can respond to light at very low intensity - this is because a certain threshold has to be reached before a generator potential is created and because a number of rod cells are attatched to a single neruone (RETINAL CONVERGENCE) more chance of this threshold being reached 
• in order to create generator potential, pigment in the rod cells RHODOPSIN needs to be broken down - low intensity light is sufficent to cause this, also explaining why rods cells respond to low intensity light 
• negative of retinal convergence - light recieved by rod cells sharing the same neurone will only generate a single impulse regardless of how many of neurones are stimulated - hard to distinguish between seperate sources of light close together - LOW VISUAL ACUITY

6 MILLION CONE CELLS IN EACH EYE 

ONLY COONECTED TO ONE BIOPOLAR NEURONE EACH - ONLY RESPOND TO HIGH INTENSITY LIGHT - TAKES LOTS TO REACH GENERATOR POTENTIAL 

CONTAIN PIGMENT IODOPSIN, REQUIRES HIGHER LIGHT INTENSITY, SO THEREFORE A GENERATOR POTENITAL WILL ONLY BE REACHED IN HIGH INTENSITY LIGHT 

AS ONLY CONE CELLS RESPOND TO DIFFERENT WAVE LENGTHS (COLOURS) OF LIGHT, THIS ALSO EXPLAINS WHY WE CANT SEE COLOURS IN LOW INTENSITY LIGHT (NIGHT) 

HIGHER VISUAL ACUITY - BRAIN CAN DISTINGUISH BETWEEN 2 DIFFERENT SOURCES OF LIGHT BECAUSE EACH CONE CELL HAS OWN CONNECTION TO A BIOPOLAR NERUONE 

CONE CELLS ARE FOUND IN FOVEA MORE THAN ROD, BECAUSE THIS IS WHERE HIGHEST LIGHT INTENSITY IS - ROD CELLS FOUND AT PERIPHERIES OF RETINA