A2 Biology unit 14

A stimulus is a dectectable responce to a change in the internal or external environment of an organism that leads to a responce in the organism. 

Example: To be able to detect danger and improve the chances of survival for the organism is desirable and passed off to the opsring of surviving generations. 

The stimulus is detected by receptors which are specific to one type of stimulus. A coordinator formulates a appropriate responce to a stimulus. A responce is produced by an effector.

Stimulus -> Receptor -> Coordinator -> Effector -> Responce

Taxes

Taxes is a simple responce which the direction is determined by the onset of the stimulus.

This is a responce caused directly by environmental changes either towards a favourable stimulus or away from a unfavourable stimulus.

Examples:

Single-celled algae will move towards light (posative phototaxis). Increases their chances of surviving as they are photosynthetic they requir light to produce food.

Earthworms move away from light (negatvie phototaxis) as they move into soil away from preditiors and towards food source. 

Kineses

A kenesis is a form of movement where the organism changes the speed of at which it moves and the rate at which it changes direction. In an unfsvouralbe envirnment the rate of turning decreases 

Example: Woodlouse move into a dry area from a damp one, they move more rapidly and change direction until they're back into a dry area. 

Tropisms

A tropism is the growth part of a plant in responce to a directional stimulus. 

Plant shoots grow away from light (phototropism) and away from gravity (negative gravitropism) this means their leaves are in ghe most favourable position for the leaves for photosynthasis. 

Plant roots grow away from light (negative phototropism) and with gravity (gravitropism). This allow the roots to have the best chance for survival where they're better to absorb water and minerals. 

Light- Plants grow towards light to ensure that optimum conditions for photosynthasis are there.

Gravity- To ensure that plants are firmly anchored into the ground- They are posatively gravitropic.

Water- Almost all plants grow towards water (posatively hydrotropic) for photosynthasis and other mtabolic process. 

Nervous Organisation 

• cental nervous system(CNS)- Made up of tyhe brain and spinal chord. 
• Peripheral neurons(PNS)- Made up of pairs of nerves that are from the brain or spinal chorde. 
• Sensory neurons- Carry nerve impulses from receptors to centra nervous system 
• Motor neurons- Carry nerve impulses away from CNS to effectors
• Volantary nervous system- Carries nerve impulses to body muscles- Under volantery control 
• Autonomic nervous system- Carries nerve impulse to glands- Smooth muscle and cardiac muscle is involuntary (subconcious)
  

Any type of involantery stimulus is called a reflex. Reflex arcs involve just three nuerons  

• 1. Stimulus- Heat from hot object
• 2. Receptor- Temperature receptors in the skin on the back of the hand are stimulated causing nerve impulse. 
• 3. Sensory neuron- Passes nerve impulses to the spinal chorde
• 4. Coordinator- Links sensory neuron to motor neuron on the spinal chorde
• 5. Motor neuron- Carries nerve impulse from spinal chord to the muscle in the upper arm.
• 6.Effector- Muscle in the upper arm is stimulated to contract.
• 7. Responce- Pulling hand away from hot object.

Importance of reflex arcs

• They are involantary therefore do not requir a decision. Leaving the brain to control more complex responces. 
• They prevent the body from pain
• They are fast as the neuron pathway is short with typically one or two synapses

Receptors are specific to a single type of stimulus. In the casde of the pacinian corpuscle it responds only to mechanical pressure.

Produces a genorator potential by acting as a transducer. The transducer converts a stimulus into a form of energy-Nerve impulses known as the Genorator potential. 

The sensory neurone ending at the centre of the pacinian corpuscle has sodium channels in its plasma membranes forming a strech-mediated sodium channel. These deform when under pressure which allows sodium ions to enter the cell. 

This causes depolarisation- Therefore creating a genorator potential

Cone cells

• Cone cells are of three different types, each responding to different wavelengths of light. 
• Dependent of how many of each type are stimulated we can see images in fuller colour. 
• Each cone cells has its own bipolar cells connected to a sensory neurone and cannot be combined to exceed the threshold value. Can only meet threshold for low intencity light.
• Iodopsin is broken down by only high light intensities 
• Each cone cell has its own connection to a bipolar cell allowing the brane to distingiush if two adjacent cells have been stimulated. 
• Good visual acuiety 

Sympatic nervous system- Stimulates effectors so it stimulates activity. Helps us deal with stressful situations by hightening our awareness.

Parasympathetic nervous system- Inhibits nerbvous system and slows down acticity. Concered with conserving energy and replenishing bodies reserves. 

Control of heart rate:

• A wave of electrical spreads out from the sinoatrial node across (SAN) obth atria- Contraction 
• Layer of non-conductive tissue(atrioventricular septum) prevents wave corssing into ventricals
• Wave of excitation enters second group of cells atrioventicualr node (AVN).
• After a short delay the wave of electrical excitation goes down the Purkyne tissue into the Bundle is His
• Bundle of His conducts the wave conducts the wave through atrioventricular septum to the base of the ventricals.
• The wave of excitation is released from Purkyne tissue causing ventricals to contract quickly

Modifying the resting heart rate

Changes to the heart rate are controlled by the medulla oblongata 

• I has centre which increases heart rate - Linked to SAN by the sympathetic nervous system
• A centre which decreases heart rate by the parasympathetic nervous system.