Response to Stimuli Flashcards
Stimulus
A detectable change in the internal or external environment of an organism that produces a response in the organism
How does the ability to respond to stimuli increase the chances of survival?
There is a selection pressure favouring organisms with more appropriate responses.
A Taxis
A simple response whose direction is determined by the direction of the stimulus. They can either move towards or away from stimulus, positive or negative.
Kinesis
Organism doesn’t move towards or away from stimulus but moves more rapidly with changing direction the more unfavourable the stimulus is.
What are kinesis designed for?
To bring the organism back into favourable conditions. Important when a stimulus is less directional eg. humidity and temperature.
Tropism
A growth movement of part of a plant in response to a directional stimulus.
What are the components of the peripheral nervous system?
Sensory neurones and motor neurones.
What are the components of the motor nervous system?
Voluntary nervous system and autonomic nervous system.
The spinal cord
Column of nervous tissue that runs along the back and lies inside the vertebral column for protection. Pairs of nerves emerge at intervals along the spinal cord.
Reflex
Involuntary response to a sensory stimulus.
Advantage of reflex arcs
- Involuntary -> don’t require decision making powers of the brain, leaving it free to carry out more complex responses
- Protect the body from harmful stimuli (from birth)
- Fast, neurone pathway is short with very few synapses. Important in withdrawal reflexes.
What are the components of the autonomic nervous system?
The sympathetic nervous system -> stimulates effectors. The parasympathetic nervous system -> inhibits effectors.
What part of the brain regulates changes to heart rate?
Medulla Oblongata
What are the two connecting centres of the medulla oblongata?
- Centre that increases heart rate -> SAN
2. Centre that decreases heart rate -> SAN
Where are chemoreceptors found?
The wall of the carotid arteries.
Chemoreceptors
Sensitive to changes in the pH of the blood that result from changes in CO2 conc.
Outline the process of control by chemoreceptors.
- When blood has a higher than normal conc. of CO2, pH is lowered
- Chemoreceptors in the wall of the carotid arteries and the aorta detect this and increase the frequency of nervous impulses to the centre in medulla oblongata that increases heart rate
- Centre increases frequency of impulses with the sympathetic nervous system to SAN which increases heart rate
- Increased blood flow causes more CO2 to be removed by lungs so CO2 levels return to normal
- pH rises to normal and chemoreceptors in wall of carotid arteries and aorta reduce frequency of nerve impulses to medulla oblongata
- Medulla reduces the frequency of impulses to SAN, decreasing heart rate
How do pressure receptors react when blood pressure is higher than normal?
They transmit a nervous impulse to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses through the parasympathetic nervous system to the SAN of the heart, which decreases the heart rate.
How do pressure receptors react when blood pressure is lower than normal?
They transmit a nervous impulse to the centre in the medulla oblongata that increases heart rate. This centre sends impulses through the sympathetic nervous system to SAN, increasing heart rate.
Features of the Pacinian corpuscle
- Specific to mechanical pressure
2. Produces a generator potential by acting as a transducer
Receptors
Convert the energy of the stimulus into a nervous impulse known as a generator potential
What type of sodium channel does the Pacinian corpuscle have?
Stretch-mediated sodium channel
Outline the function in a Pacinian Corpuscle
- At resting potential, the stretch-mediated sodium channels of the membrane are closed
- When pressure is applied to the Pacinian corpuscle, it changes shape and the membrane around its neurone becomes stretched
- This allows the sodium channels to open and sodium ion diffuse into the neurone
- The influx of sodium ions causes depolarisation, producing a generator potential
- The generator potential creates an action potential that passes along the neurone to the the CNS.
How do rod and cone cells transduce?
Convert light energy into the electrical energy of a nerve impulse.
Why do rod cells only produce black and white images?
Can’t distinguish different wavelengths of light.
How do rod cells respond to light of very low intensity?
Many rod cells share a single sensory neurone. A certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are attached. Greater chance that threshold value will be exceeded as a number of rod cells are attached to a single bipolar cell.
How do rod cells create a generator potential?
Rhodopsin, the pigment in the rod cells must be broken down. Low-intensity light is sufficient to cause this breakdown.
Why do rod cells have low visual acuity?
Many rod cells are linked to a single bipolar cell so light received rod cells sharing the same neurone will only generate a single impulse regardless of how many neurones are stimulated.
How do cone cells allow us to perceive images in full colour?
3 different types of cone cells each responding to a different wavelength of light. Proportion of each type that’s stimulated allows us to see in full colour.
Why do cone cells only respond to high light intensity?
Each cone cell often has their own separate bipolar cell connected to a sensory neurone so stimulation of several cone cells can’t combine to help exceed threshold value.
How do cone cells create a generator potential?
Iodopsin, requires a higher light intensity for its breakdown to create a generator potential.
Why do cone cells have a high visual acuity?
Each cone cell has its own connection a single bipolar cell so if two adjacent cone cells are stimulated the brain receives 2 separate impulses and can distinguish between 2 separate sources of light.
How are rod cells and cone cells distributed on the retina?
Cone cells -> fovea, retina opposite the pupil
Conc. of cone cells diminishes further away from fovea. Rod cells are found at the peripheries of retina.
Are there more rod cells or cone cells?
More rod cells.
