Response to stimuli

Includes the topics:

  • Sensory reception - stimulus and response, Taxes, Kinesis, Tropisms
  • Nervous control - nervous organisation, the spinal cord, a reflex arc, importance of reflex arcs
  • Control of heart rate - autonomic nervous system, control of heart rate, control by chemoreceptors, control by pressure receptors
  • Role of receptors - features of sensory reception by the pacinian corpuscle, structure and function of pacinian corpuscle, receptors working together in the eye, rod cells, cone cells
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What is a reflex?

  • A rapid response to a stimulus
  • Which is autonomic
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Importance of reflex actions

  • To protect body from harmful stimuli and do not have to be learned
  • Fast because neurone pathway is short
  • Involuntary and do not require decision-making powers of the brain
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Control of hear rate

  • Respiration produces an increase in carbon dioxide
  • Decreases pH of the blood
  • Which is detected by chemoreceptors
  • In the carotid/ aortic bodies
  • More impulses sent from the medulla along the synaptic nerve
  • To the SAN
  • Increasing heart rate
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Functions of the pacinian corpuscle

  • When pressure is applied, the membrane around the neurone becomes attached and this widens the Sodium  channels
  • Na+ ions diffuse into the neurone
  • This changes the potential of membranes - it becomes depolarised and generates an action potential
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How connections of rod cells to neurones makes it

  • Several rods have connection with one neurone
  • Uses summation (build up of neurotransmitter)
  • To exceed threshold for action potential
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Taxes

A simple response whose direction is determined by direction of stimulus.

Responds directly to environmental changes by moving towards favourable stimulus or away from unfavourable stimulus.

E.g. Single-celled algae will move towards light (positive phototaxis). This increases their chances of survival since, being photosynthetic , they need light to manufacture their food.

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Kinesis

A form of response in which organsim does not move towards or away from stimulus. Instead, the more unpleasant the stimulus, the more rapidly it moves and the more rapidly it changes direction.

Results in an increase in more random movements. Designed to bring organism back to favourable conditions.

E.g. woodlice lose water from their body in dry conditions. When they are in a dry area they move more rapidly and change direction more often. Increases their chances of moving into a different aea. If the different area happens to be moist they slow their movement if not, they continue to rapidly move.

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Tropisms

A growth movement of part of a plant in response to a directional stimulus.

  • Plant shoots grow towards light so that their leaves are in the most favourable position to capture light for photosynthesis.
  • Plant roots grow away from light and towards gravity. Response increases probability roots will grow into soil, where they are better able to absorb water and minerals.
  • Plant roots grow towards water so that, within the soil, root systems will develop where there is most water.
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Nervous organisation

Nervous system has two divisions:

  • CNS - made up of brain and spinal cord
  • PNS - made ip of pairs of nerves whcih originate from brain or spinal cord

PNS divided into:

  • Sensory neurones - carry nerve impulses from receptor towards CNS
  • Motor neurones - carry nerve impulses away from CNS to effectors

Motor nervous system divided into:

  • Voluntary nervous system - carries nerve impulses to body muscles and is under voluntary control.
  • Autonomic nervous system - carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control.
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A reflex arc

Involve three neurones. One of the neurones is the spinal cord, main stages of a spinal reflex arc such as withdrawing hand from a hot object include:

  • Stimulus - heat from hot object
  • Receptor - temperature receptors in skin on back of hand, create a nerve impulse in a sensory neurone
  • Sensory neurone - passes nerve impulse to spinal cord
  • Intermediate neurone - links snesory neurone to motor neruone in spinal cord
  • Motor neurone - carries nerve impulse from spinal cord to muscle in upper arm
  • Effector - muscle in upper arm, which is stimulated to contract
  • Response - pulling hand away from hot object
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Important of reflex arcs

  • Involuntary therefore, do not require decision making powers of the brain, thus leaving us to carry out more complex responses.
  • Protect the body from harmful stimuli, are effective from birth and do not need to be learned
  • Fast, becauses neurone pathway is short with very few synapses
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Autonomic nervous system

Sympathetic nervous system

  • Stimulates effectors so speed up any activity

Parasympathetic nervous system

  • Inhibits effectors and so slows down any activity

Both act antagonistically so as one system contracts a muscle the other relaxes it.

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Control by chemoreceptors

  • When blood has a higher than normal concentration of carbon dioxide, its pH is lowered
  • Chemoreceptors in wall of carotid arteries and aorta detect this and increase frequency of nervous impulses to cnetre in medulla oblongata that increases heart rate
  • Centre increases frequency of impulses via sympathetic nervous system to SAN which in turn increases heart rate
  • Increased blood flow leads to more carbon dioxide being removed by lungs
  • pH of blood rises to nromal and chemoreceptors in wall or carotid artery and aorta reduce frequency of nerve impulses to medulla oblongata
  • Medulla oblongata reduces frequency of impulses to SAN, and therefore, decreases heart rate to normal.
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Control by pressure receptors

  • When blood pressure higher than normal - transmit a nervous impulse to centre in medulla oblongata that decreases heart rate. Sends impulses via parasympathetic nervous system to SAN of heat, whcih decreases rate of heart beat.
  • When blood pressure is lower than normal - transmit a nervous impulse to centre in medulla oblongata that increases heart rate. Sends impulses via sympathetic nervous system to SAN, which increases rate at which heart beats.
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