Response to stimuli

How inernal and external stimuli are detected by organisms and how they lead to response.

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Sensory Reception

Stimulus and response: Stimulus - detectable change in the interal or external environment, produces a response in an organism.  Ability to respond to stimuli increases the chances of survival, those that survive have a better chance of raising offsrping and passing their alleles to the next generation, therefore a selection pressure favours organisms with appropritate responses.  Stimuli, detected by receptors (transform energy of stimulus into a form of energy that can be processed to create a response.) Response is carried out by cells called effectors. Effectors and response are far away from one another an a form of communication is needed between them: hormones (very slow)  Another way of communication is the nervous system. Each receptor is linked to a central coordinator, this acts as a 'switchboard' connecting information from each receptor to appropriate effector.  Taxes: a resonse determined by direction of a stimulus, responds directly to environmental changes by moving towards a favourable or unfavourable stimulus. Kinesis: organism does not move towards or away from stimulus, instead the more unpleasant the stimulus, the more rapidly is moves to try and get to a more favourable stimulus. Tropisms: a growth movement of a plant in response to a directional stimulus.

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Nervous control

Nervous organisation: central nervous system made up of the brain and spinal cord, the peripheral system, made up of a pair of nerves either side of the brain or the spinal cord. Peripheral nervous system is divided into 2: sensory neurones (carry nerve impulses from receptors -> CNS) motor neurones (carry nerve impulses away from CNS -> effectors) Motor nervous system divided into 2: voluntary nervous system (nerve impulses --> mucsles under conscious control) Autonomic (carry nerve impulses to glands/cardiac muscle, under subconscious control)

The Spinal cord: a column of tissue runs alone the backs, emerging at intervals are nerves.

A reflex arc: Response to pulling hand away from fire= involuntary, by the time the brain has recieved the message, the muscles have pulled the hand away called a reflex: 1) Stimulus 2) receptor 3) sensory neurone 4) intermediate neurone 5) motor neurone 6) effector 7) response.

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Control of heart rate

Autonomic nervous system: sypathetic nervous system = stimulates effectors and speeds activity, Stimulates effecotrs when we exercise strenuously, it helps us cope with stressful situations by heightening our awareness and preparing us for acitvity. Parasympathetic nervous system = inhibits effectors, slows down activity, conserves energy and replenishes body reserves. These actions are antagonistic and so oppose one another.

Control of heart rate:

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