Topic 6 A: Stimuli and Response Flashcards
What is a stimulus?
A detectable change in the internal or external environment of an organism that leads to a response.
- there is a selection pressure favouring organisms with better responses.
Order of response?
Stimulus, receptors, coordinators, effectors, response.
Examples of stimuli?
Light, pressure, touch, temperature, chemicals, and smell.
What is a tactic response (taxis)?
- directional movement
- direction of stimulus affects the response
Types of taxis?
1 - positive taxis = move towards stimulus
2 - negative taxis = move away from stimulus
Example of tactic response?
Woodlice move away from the light to protect themselves from predators. They also favour damp conditions as it prevents water loss.
- therefore, light is the stimulus.
What is the kinetic response (kinesis)?
- random movement
- affected by the intensity of the stimulus
Example of kinetic response?
Flatworms - when stone is removed they move in random directions in response to light. When they are brought back into darkness they stop moving.
What is tropism?
Growth of part of a plant in response to a directional stimulus.
Explain phototropism in roots?
- plant roots grow away from light - negative phototropism
- ensures probability that roots will grow into soil
- can then absorb water and mineral ions for growth
- roots are positively gravitropic
Light tropism in roots and shoots comparison?
- shoots grow towards light, positively phototropic as auxin stimulates growth in the shoot and the stem curves up
- light is needed for photosynthesis
- roots grow away from light, negatively gravitropic as auxin gathers in the lower half of root and inhibits growth, so curves downwards
Gravity tropism in roots and shoots?
- roots are sensitive to gravity, positively gravitropic
- roots need to be firmly anchored in the soil
- shoots move away from gravity, negatively gravitropic
What allows plants to react to stimuli?
- specific growth factors, which are hormone like chemicals.
- auxins are a group of chemical plant growth factors and the main one is IAA (indoleacetic acid) which is made in the meristem of shoots and roots
IAA when light distribution is even? (shoots)
- IAA molecules are evenly distributed
- diffuse downwards
- cells elongate through shoot and plant grows upwards
IAA when light distribution is uneven? (shoots)
- unilateral light source from one direction
- IAA diffuses down shady side of shoot, increasing concentration on that side
- IAA stimulates cell elongation causing shady side to grow at faster rate - the extra weight causes plant to bend towards light
Gravitropism in roots explained?
- amyloplasts sink to the bottom of the root
- amyloplasts = detect gravity and consist of starch
- IAA actively transported to the region in the root tip where the amyloplasts have sunk - lower side grows at slower rate than upper side
- IAA inhibits cell elongation, more elongation on the upper side causes root to bend downwards
Exam Question (generic) : Explain why the student removed the shoot tip from each seedling.
IAA is produced in the tip/meristem which is where cell division and mitosis occurs.
Exam Question (generic) : Explain why the student added glucose solution to each petri dish?
Respiration which provides ATP/energy for growth
What is plasticity?
The ability to stretch cell walls
Plasticity process? (P1)
- IAA binds to the receptor protein on the cell-surface membrane.
- Hydrogen ions are actively transported from the cytoplasm into the spaces of the cell wall, this is performed by ATPase proton pumps
- This activates proteins called expansins which loosens bonds between celullose microfibrils, increasing a plants ability to stretch and elongate - this is known as ACID GROWTH HYPOTHESIS
Plasticity process? (P2)
- Potassium ion channels open, this leads to an increased concentration of potassium ions in the cytoplasm. This decreases water potential in the cytoplasm.
- The cell absorbs water by osmosis, it enters through the aquaporins and stores water in the vacuole
- End result = increased internal pressure and the cell wall is able to stretch and elongate
Role of a receptor?
They are specific, and convert the energy of the stimulus into the electrical energy used by neurones.
What is a reflex arc?
Pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain.
- quicker than any other nervous response
- e.g., blinking
- detection involves a stimulus being detected by a receptor cell
2 main divisions of the nervous system?
- central nervous system (brain and spinal cord)
- peripheral nervous system (multiple neurones)
CNS pathway?
nervous system - cns - brain / spinal cord
peripheral nervous system pathway?
nervous system - pns - sensory / motor nervous system - voluntary / autonomic nervous system
Reflex example: standing on a sharp object then removing foot.
- sensory neurone - recieves electrical impulse from receptors and sends these impulses to the cns
- relay neurone - connects the sensory and motor neurones together and is referred to as ‘intermediate’ neurones
- motor neurone - recieves information from cns and sends electrical impulses to the effectors (e.g., muscles in quads)
The motor nervous system divides into?
- voluntary nervous system = carries nerve impulses to body muscles and is under conscious control
- autonomic nervous system = carries nerve impulses to glands, smooth muscles, and cardiac muscle and is subconscious.
What is the resting potential?
The potential difference when the cell is at rest. (-70mV)
- receptor hasn’t been stimulated
- more positive ions (Na+ and K+) on the outside than inside
- voltage / potential difference across membranes
- maintained by sodium potassium pump (involving active transport and ATP)
What is the generator potential?
The change in potential difference due to a stimulus.
- cell membrane becomes excited and more permeable
- more ions move in and out
- alters potential difference across membrane
What is depolarisation?
When potential difference increases above resting potential and becomes more positive.
What is action potential?
An electrical impulse generated along a neurone.
- only triggered if gen pot reaches threshold
- passing threshold means more energy allow more sodium ions to diffuse in, giving sharp increase in pd and creates action potential
- action potential moves along the nodes of ranvier like a mexican wave
How is stimulus strength measured?
- by the frequency of action potentials (all same size)
- voltage across neurone membrane becomes more positive due to membrane becoming more permeable to Na+ ions
Voltage and time graph explanation?
- stimulus must reach -55mv to cross the threshold
- generator potential and depolarisation
- action potential - the bigger the stimulus, the bigger the movement of ions and bigger change in potential difference (bigger gen pot)
- repolarisation (havent learned)
- refractory period (havent learned)
- resting state (havent learned)
What are Pacinian Corpuscles?
A mechanical receptor that responds to changes in pressure and are found in skin of fingers, soles of feet, and joints.
Features of Pacinian Corpuscle?
- sensory neurone ending
- layers of connective tissue with gel between
- capsule
- blood capillary
- sensory neurone axon
Describe structures in Pacinian Corpuscle?
- sensory neurone ending - found at the ends of sensory neurone axons (not a seperate cell)
- the inside of the membrane is negatively charged in comparison to outside
- the sensory nerve is wrapped in lots of layers of connective tissue called lamellae
- the gel between the layers contain positively charged sodium ions, the gel also contains stretch-mediated sodium ion channels. they open when sufficient pressure is applied
Example: pushing down on a table
- Before - no pressure, there is excess sodium ions outside axon (resting potential)
- Pressure is applied - the layers are distorted causing the stretch-mediated sodium channels to open, sodium ions enter the axon of the sensory neurone
- Generator potential is established, there are excess sodium ions inside the end of the axon which causes depolarisation of membrane. If enough generator potentials are produced, an action potential will be established and a nerve impulse begins along the axon
What are photoreceptors and where are they found?
Receptors that detect light in the eye.
Structure and functions in the eye?
- cornea - transparent lens that refracts (bends) light as it enters the eye
- iris - controls how much light enters the pupil
- lens - transparent disc that can change shape to focus light onto retina
- retina - contains light receptor cells - rods = detect light intensity and cones = detect colour
- optic nerve - sensory neurone that carries impulses between eye and brain
- pupil - hole that allows light to enter eye
How do photoreceptors in eye work?
- pigment absorbs light (light sensitive optical pigments)
- light bleaches pigment, this alters membrane permeability to sodium ions
- nerve impulse sent along bipolar neurone
Retinal convergence?
Ability to detect light.
Rod cell photoreceptor explained?
- sensitive to light intensity
- processes images in black and white (monochromatic vision)
- can’t distinguish between different light wavelengths
- detect light at very low intensities through retinal convergence
- light energy breaks down the pigment rhodopsin
- enough pigment has to be broken down to reach the threshold which links rod cells to sensory neurones
Advantages of rod cells?
- doesnt take alot of light energy to break down rhodopsin - so function at low light intensities
- multiple rod cells connect to one bipolar cell so the pigment will generate big enough generator potential to generate an action potential - called spatial summation
Cone cell photoreceptor explained?
- sensitive to different wavelengths of colour
- processes images in colour (trichromatic) - it takes more light in order to reach threshold
- three types = red, green, and blue sensitive
- different proportions of three colours allow a range of colours to be seen
- light energy is absorbed to break down protein pigment iodopsin
- red, green, and blue version of iodopsin
- only broken down if there is a high light intensity
- no spatial summation
Visual acuity definition?
The ability to tell apart points that are close together.
Compare rod and cone cells?
- R = low visual acuity. C = high visual acuity
- R = less accurate vision due to multiple rod cells connecting to one bipolar neurone. C = accurate and sharper vision because only one cell is connected to one bipolar neurone
- R = light source could be triggered by any three rod cells. C = brain able to interpret two cones at two different spots of light
What is the peripheral nervous system and what can it be divided into?
- consists of neurones
- divides into:
1. somatic / voluntaric nervous system = controls conscious activities e.g., running
2. autonomic nervous system = controls unconscious activities e.g., heart rate
Medulla oblongata?
Specific region of the brain that plays a vital role in controlling heart rate - referred to as the cardioregulatory centre
What does the autonomic NS split into?
- sympathetic NS = ‘fight or flight’, stimulates effectors when we exercise or experience emotions and helps us cope with stressful situations
- parasympathetic NS = ‘rest and digest’ response that calms body down, it inhibits effectors and controls activities under normal resting conditions
Why is heart muscle described as myogenic?
Capacity of the heart to generate its own impulses.
Stage one in controlling heart rate?
- Sinoatrial node ‘the pacemaker’ (SAN)
- beat starts in the heart muscle itself with a signal from SAN
- impulse spreads through the heart as a ‘wave of excitation’
- impulse spreads through walls of the atria causing contractions (atrial systole)
Stage two in controlling heart rate?
- Atrioventricular node (AVN)
- impulse passes through AV node
- impulse is delayed for approximately 0.1 seconds to enable atria to fully contract
- relays the impulse between the upper and lower sections of the heart
Stage three in controlling the heart rate?
- atrioventricular node
- sends out a wave of excitation to the ‘Bundle of His’
Stage four in controlling the heart rate?
- purkyne tissue
- conducts wave of excitation down septum of heart to the apex, before it is carried upwards in the walls of the ventricles, the blood contracts from its base, upwards and outwards
Stage five in controlling the heart rate?
- ventricles contract
How to remember sequence of controlling heart rate?
Sally Always Arrives Before Professor Vicky
s = sinoatrial node
a = atrial systole
a = atrioventricular node
b = bundle of his
p = purkyne tissue
v = ventricles
The sequence of events before controlling the heart rate?
- start at cardioregulatory centre - consists of medulla oblongata, acceleratory centre, and inhibitory centre
- the acceleratory centre increases heart rate and the inhibitory centre decreases heart rate
- both centres connected to SAN
1. impulses travel from acceleratory centre along the sympathetic nerve to the SAN
2. a neurotransmitter called noradrenaline is secreted at the synapses with the SAN. this increases the frequency of electrical waves - increased heart rate
The two types of neurotransmitters involved with heart rate?
- noradrenaline = increase heart rate
- acetylcholine = decrease heart rate
Exam question: describe how a heartbeat is initiated and coordinated?
- SAN sends wave of excitation/electrical activity causing atrial contraction
- non-conducting tissue (AVN) prevents immediate contraction of ventricles
- AVN delays impulse whilst blood leaves atria
- AVN sends wave of electrical activity down Bundle of His
- causing ventricle to contract from base up
What are neural mechanisms and the two types?
- internal stimuli are detected by pressure receptors and chemoreceptors.
1. Baroreceptors
2. Chemoreceptors
Mechanism 1 - Baroreceptors?
- respond to changes in blood pressure
- found in aorta and carotid arteries
- stimulated by high and low blood pressure
- neurotransmitters released depending on the scenario
Mechanism 2 - Chemoreceptors?
- detect chemical changes
- found in aorta, cartoid arteries, medulla
- monitors oxygen, carbon dioxide, ph levels
- neurotransmitter is released depending on the scenario
Sequence of events leading to increased heart rate?
- (stimuli) increased co2 conc in blood, decrease in blood pressure
- (receptor) increased stimulation of chemoreceptors and baroreceptors
- (coordination) acceleratory centre in medulla sends impulses via sympathetic nerves
- (effectors) SAN stimulated by noradrenaline neurotransmitter
- (response) increased frequency of waves of electrical activity spread across atria and ventricles and heart rate increases
Sequence of events leading to a reduced heart rate?
- (stimuli) decreased co2 conc in blood, increased in blood pressure
- (receptor) decreased stimulation of chemoreceptors and baroreceptors
- (coordination) inhibitory centre in medulla sends impulses via parasympathetic nerves
- (effectors) SAN stimulated by acetylcholine neurotransmitter
- (response) reduced frequency of waves of electrical activity spread across atria and ventricles and heart rate decreases
