Unit 6 - Organisms respond to changes in their internal and external environments Flashcards
Tropism
A plants growth response to an external stimulus
Types of tropisms
Gravitrophism
Hydrotrophism
Phototrophism
How does IAA move through plant
Via phloem (Mass transport)
Then diffusion/ active transport through cells
Two types of responses in animals and definitions
Taxis- An organism moves towards/ away from a directional stimulus
Kinesis- Movement affected by a non-directional stimulus
Nervous response cycle
Stimulus–> Receptors—> CNS —–> Effectors —-> Response
Two parts of nervous system
Central nervous system
Peripheral nervous system
Two types of peripheral nervous system
Somatic
Autonomic
Two types of autonomic nervous system + description
Sympathetic- ‘Fight or flight’
Parasympathetic- ‘Rest + Digest’
Benefits of reflexes
-Rapid
- Prevent tissue damage
- Don’t require conscious thought
What is the resting potential difference
-70 mV
Steps in an action potential being generated
1- Resting potential
2- Depolarisation pt 1 (stimulus)
3- Depolarisation pt 2 (Channels open)
4- Repolarisation
5- Hyperpolarisation then return to resting potential
What is a stimulus
A detectable change in the environment
What is a reflex?
The body responds to a stimulus without mkaing a consious decision to respond.
What are pacinian corpuscles
Pressure recpetors in the skin that detect mechanical stimuli
How does the brain portray light?
- Light enters the eye and hits the photoreceptors
- This is absorbed by light-sensitive optical pigments
- Light bleaches the pigments, causing a chemical change and altering the membrane permability to sodium ions
- A generator potential is created and if it reaches the threshold, a nerve impulse is snet along a bipolar neurone.
Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
What are the two photoreceptors and where are they found?
- Rods - peripheral parts of the retina
- cones - closely packed together in the fovea
Are rods very sensitive?
Yes - Rods are very sensitive to light and work well in dim settings
This is beacuse many rods join to one neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.
Are cones very sensitive?
No - Cones are less sensitive than rods beause one cone joins to one neurone. This means it takes more light to reach the threshold and trigger the action potential.
Do rods have high visual acutity?
No - Rods give low visual acuity because many rods join to the same neurone which means light from two points close together cannot be told apart.
Do cones have high visual acuity?
Yes - Cones give high visual acuity beacuse cones are close together and one cone joins one neurone.
Produces multiple different impulses to brain
Step 1 - Stimulus- Pacinian corpuscle
A stimulus excites the neurone cell membrane causing stretch mediated sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ions electrochemical gradient. This makes the inside of the neurone less negative.
Step 2 - Depolarisation
If the potential difference reaches the threshold (around -55mV), more voltage gated sodium channels open. More sodium ions diffuse rapidly into the neurone.
Step 3 - Repolarisation
At a potential difference of around +33 mV the sodium ion channels close and the voltage gated potassium channels open.
The membrane is more permeable to potassium ions so potassium ions diffuse out of the neuron down their concentration gradient.
Step 4 - Hyperpolarisation
Potassium ion channels are slow to close.
This means there is a slight overshoot where too many potassium ions diffuse out the neurone.
The potential difference becomes more negative than the resting potential (< -70mV)
Step 4 - Hyperpolarisation
Potassium ion channels are slow to close.
This means there is a slight overshoot where too many potassium ions diffuse out the neurone.
The potential difference becomes more negative than the resting potential (< -70mV)
Step 5 - Resting potential
The voltage-gated ion channels are closed.
Sodium potassium pump pumps sodium out and potassium in in the ratio 3:2
Potassium ion channels open so potassium diffuses back out of membrane down electrochemical gradient
Wave of depolarisation steps
- When an action potential occurs, come of the sodium ions that enter the neurone diffuse sideways
- This causes sodium ion channels in the next region to open and sodium ions diffuse into that part
- This causes a wave of depolarisation to travel along the neurone
What is the refactory period?
It is a time delay between action potentials where ion channels are recovering and cant be opened.
Why is the refactory period important?
- Action potenials dont overlap - pass on discrete impulses
- There is a limit to the frequency at which nerve impulses can be transmitted
- Action potentials are unidirectional
All or nothing theory for action potentials
- Once the threshold is reached an action potential will always fire with the same change in voltage, no matter how big the stimulus is.
- A bigger stimulus won’t cause a bigger action potenial but will cause them to fire more frequently.
What 3 factors affect the speed of conducation of action potentials?
- Myelination
- Axon diameter
- Temperature
How does temperature affect the speed of conducation of action potentials?
- Diffusion occurs faster at higher temperatures, increasing the speed of conduction.
- Only increases up to around 40C, as proteins start to denature.
- If plasma membrane proteins and enzymes denature there is NO conduction!
How does myelination affect the speed of conduction of action potenials?
- The myelin sheath acts as an electrical insulator.
- Prevents action potentials in the part of the axon covered by the myelin.
- Action potential jumps from node to node (saltatory conduction). i.e. depolarisation is not required all the way along the axon.
- Increases speed of conduction.
How does the Axon diameter affect the speed of conduction of action potenials?
- The greater the diameter of an axon, the faster the speed of conductance.
- There is less resistance to the flow of ions in the cytoplasm of wider axons.
- Ions therefore reach other parts of the neurone faster.
What does myogenic mean?
It can contract and relax without receiving signals from nerves
What and where is the sinoatrial node?
- It is located in the right atrium and is like a pacemaker.
- It sets the rhythm of the heartbeat by sending out regular waves of electrical actvity.
It causes the right and left atria to contract at the same time.
What and where is the atrioventricular node?
- It is loacted near the border of the right and left ventricle but still within the artia.
- It is reposnsible for passing on waves of electrical activity on to the bundle of His.
(There is a slight delay beore the AVN reacts to make sure the atrira have emptied before the ventricles contract)
What and where is the bundle of His?
- Runs through the septum
- It is a group of muscle fibres responsible for conducting waves of electrical activity between the ventricles to the apex of the heart.