T14 - Processes + features Flashcards
How is light converted into an electrocal impulse using photoreceptors?
Light hits photopigments - light-sensitive proteins in photoreceptor cells.
Light bleaches and causes a chemical change in the photopigments.
This increases the permeability of photoreceptor cells to sodium ions.
An influx of sodium ions generates a receptor potential.
If this reaches threshold, a bipolar sensory neurone carries a signal to the optic nerve and then the brain.
How does the eye focus objects?
The ciliary muscles and suspensory ligaments work antagonistically to change the shape of the lens, altering the way light refracts onto the retina where the image is focused
Features of rod cells
Mostly in the periphery of the retina, not in blind spot of fovea
Highly sensitive to light
Low visual acuity
Highly numerous
Evenly distributed on the retina but absent in the fovea
Use a pigment called Rhodopsin - detects light + dark
Monochromatic - only detect one wavelength of light
3 rod cells (summative) connected to a single neurone - Retinal convergence
Features of cone cells
In the fovea
Less sensitive to light
High visual acuity
Fewer cells than rod cells
Distributed mainly in the fovea
Use a pigment called Iodopsin - detects colour
Trichromatic - divided into 3 types and each responds to a different wavelength of light - red, blue or green
1 cone cells connected to a single neurone - No retinal convergence - separate impulses are sent to the brain
How do circular and radial muscles work antagonistically?
In bright light:
- Circular muslces contract
- Radial muslces relax
- Pupils constricted/narrow
(CCC - Circular, contract, constrict)
In dim light:
- Circular muslces relax
- Radial mucles contract
- Pupil dialates/widens
How is positive phototropism controlled in shoots?
IAA is produced in cells in the tip of the plant shoot.
IAA is transported down the plant shoot.
Light stimulates IAA to move from the light side of the shoot to the shaded side.
IAA becomes concentrated and stimulates more cell elongation on the shaded side of the shoot.
The shoot bends towards the light.
How is negative phototropism and positive gravitropism controlled in plant roots?
IAA is produced in cells in the tip of the plant root.
IAA is transported along the plant root.
Any light available stimulates IAA to move from the light side of the root to the shaded side.
Gravity also pulls IAA from the upper side of the root to the lower side of the root.
IAA becomes concentrated and inhibits cell elongation in the lower, shaded side of the root.
The root bends away from any available light and downwards towards the pull of gravity.
How do auxins (e.g. IAA) cause cell elongation?
Auxin binds to the cell-surface membrane.
Hydrogen ions are actively transported from the cytoplasm into the cell wall.
The cell wall becomes more plastic.
Cells elongate and the plant grows.
What are the stages of a reflex arc?
Stimulus - This triggers the reflex, such as heat from a hot object.
Receptor - These are specialised cells, like temperature receptors in the skin, that detect the stimulus and generate nerve impulses.
Sensory neurone - This transmits the nerve impulses to relay neurones.
Relay neurone (intermediate neurone) - Usually located in the spinal cord or more rarely in the brain, these connect sensory neurones to motor neurones.
Motor neurone - This transfers nerve impulses from the relay neurones to effectors
Effector - This is the muscle or gland that receives the signal and carries out a response, like muscles in the arm receiving a signal to contract.
Response - This is the final action taken, such as quickly moving the hand away from the hot object.
What is the importance of reflex arcs?
Involuntary - They allow the brain to concentrate on complex processes.
Rapid - They ensure a swift response.
Protective - They safeguard the body from potential injuries.
Innate - They are intrinsic mechanisms present from birth, eliminating the need for learning.
What are the stages of receptor cell function?
At rest, the receptor cell-surface membrane has a voltage across it due to differences in ion concentration inside and outside the cell, known as the resting potential.
When a stimulus is detected, the cell-surface membrane becomes more permeable, allowing more ions to flow in and out.
This alters the membrane’s voltage, creating what is called the generator potential (or receptor potential).
A larger stimulus results in a bigger change in voltage, producing a larger generator potential.
If the generator potential reaches a threshold level, it triggers an action potential, which is an electrical signal sent along a neurone.
More action potentials indicate a stronger stimulus. If the stimulus is too weak, the threshold isn’t reached and no action potential is generated.
What happens when the Pacinian corpuscle is stimulated?
The lamellae deform, pressing on the sensory neurone ending.
This stretches the neurone’s membrane, causing it to change shape.
This opens stretch-mediated sodium ion (Na+) channels in the membrane, increasing its permeability to Na+.
Na+ diffuses into the neurone, depolarising it and resulting in a generator potential.
If this signal reaches the threshold, an action potential is triggered.
How does a wave of electrical excitation travel through the heart?
Sinoatrial node (SAN) - Initiates the heartbeat by stimulating the atria to contract.
A layer of collagen fibres - Prevents direct electrical flow from atria to ventricles.
Atrioventricular node (AVN) - Picks up the electrical activity from the SAN and imposes a slight delay.
Bundle of His - Receives electrical activity from the AVN and conducts the wave of excitation to the apex (base) of the heart.
Purkyne fibres - These branch off the bundle of His, causing the right and left ventricles to contract from the bottom upwards.
How does the autonomic nervous system respond during exercise?
For example, during exercise:
Blood has a higher concentration of carbon dioxide, and so a lower pH.
Chemoreceptors in the carotid arteries and aorta detect this and increase the frequency of nervous impulses sent to the medulla oblongata.
The medulla oblongata increases the rate of impulses to the sinoatrial node (SAN) via the sympathetic nervous system.
Heart rate increases.
This provides the extra oxygen required for increased respiration in active muscles.
The increased blood flow also helps remove excess carbon dioxide via the lungs.
Carbon dioxide concentration reduces.
Hormones, such as adrenaline and noradrenaline, can also influence heart rate. In times of stress, these hormones are released and cause the SAN in the heart to increase the heart rate as part of the ‘fight or flight’ response
