Stimuli and responses

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  • Created by: DBaruch
  • Created on: 26-10-16 18:40

Survival and reponse

  • Organisms increase their chances of survival by responding to changes in their external environment. Organisms also respond to changes in their interal environment to make sure that the conditions are always optimal for their metabolism
  • Any chang in the interal or external environment for example light or temperature is called a stimulus
  • Simple organisms like woodlice have simple responses to keep them in a favourable environment and their reponse can either be tactic or kinetic
  • A tactic reponse(taxis) is a directional movement in reponse to a stimulus and the direction of the stimulus affects the reponse for example woodlice show a taxis reponse to light as they move away from the stimulus (light)
  • A kinectic reponse (kinesis) is a non-directional or random movement in response to a stimulus. The intensity of the stimulus affects the response
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Receptors, effectors and the nervous system

  • Receptors detect stimuli and can be cells, or proteins on cell surface membranes. There are loads of different types of receptors that detect different stimuli for example baroreceptors detect a change in blood pressure. Receptors are specfic to a certain stimulus.
  • Effectors are cells that bring about a resposne to a stimulus, to produce and effect. Effectors include muscle cells and cells found in glands. Receptors communicate with effectors via the nervous system or the hormonal system and sometimes using both
  • The nervous system is made up of a complex network of cells called neurones and there are 3 types of neurone
  • Sensory neurones- transmit electrical impulses from receptors to the CNS, the brain and spinal cord
  • Motor neurones- transmit electrical impulses from the CNS to effectors
  • Relay neurones- transmit electrical impulses between sensory neurones and motor neurones
  • A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone. When an electrical impulse reaches the end of a neurone chemicals called neurotransmitters take the information across the synapse to the next neurone, where another electrical impulse is generated. The CNS processes the information and sends impulses along motor neurones to an effector
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Nervous response and simple reflexes

  • When an electrical impulse reaches the end of a neurone, chemical messengers called neurotransmitters are secreted directly onto cells, so the nervous response is localised. Neurotransmitters are quickly removed once they have done their job, so the job is short lived and electrical impulses are fast, so the reponse is rapid
  • A simple relfex is a rapid, involuntary response to a stimulus. The pathway goes through the spinal chord but not through conscious parts of the brain, so the response happens automatically. Because you do not have to spend time deciding how to respond, information travels really fast from receptors to effectors
  • Reflex arc- is a pathway of neurones linking receptors to effectors in a simple reflex is called a reflex arc. 3 neurones are involved called sensory, relay and motor neurones.
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Example of a simple reflex

Image result for example of reflex arc (http://1.bp.blogspot.com/-BzW2zu3NM-4/VeOTqVTeteI/AAAAAAAABto/8oIoOPPaFW4/s1600/knee%2Bjerk.jpg)

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Tropisms

  • A tropism is the response of a plant to a directional stimulus. Plants respond to stimuli by regulating their growth. A positive tropism is growth towards the stimulus, whereas a negative tropism is growth away from a stimulus.
  • Phototropism is the growth of a plant in reponse to light. Shoots are positively phototrophic and grow towards light whereas roots are negatively phototropic and grow away.
  • Gravitropism is the growth of a plant in response to gravity. Shoots are negatively gravitrophic and roots are positively gravitrophic
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Auxins

  • Plants respond to directional stimuli using specific growth factors- these are hormone-like chemicals that speed up or slow down plant growth. Plant growth factors are produced in the growing regions of the plant and they move to where they are needed in the other parts of the plant.
  • Auxins are produced in the tips of shoots and diffuse backwards to stimulate the cell just behind the tips to elongate and this is where cell walls become loose and stretchy, so the cells get longer. If the tip of a shoot is removed the shoot will stop growing as no auxin will be available
  • Indoleacetic acid (IAA) is an important auxin thats produced in the tips of shoots and roots of flowering plants. It is moved around plants to control tropisms and moves via diffusion and active transport over short distances and via the phloem over long distances. This results in different parts of the plants having different concentrations of IAA. 
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How receptors work

  • There are many different types of receptor that each detect a different type of stimulus. Some repceptors are cells for example photoreceptors are receptor cells that connect to the nervous system.
  • Resting potential- When a nervous system receptor is in a resting state, there is a difference in charge between the inside and outside of the cell. The inside is negatively charged compared to the outside. The potential difference when a cell is at rest is called resting potential and it is generated by ion pumps and ion channels
  • Generator potential- When a stimulus is detected, the cell membrane is excited and becomes more permeable meaning that more ions can move in and out of the cell this changes the potential differences. The change in potential difference is called a generator potential. The bigger the stimulus the bigger the movement of ions means there is a bigger change in potential difference so a bigger generator potential is produced
  • Action potential- If there is a big enough genetor potential an action potential will be triggered and is only triggered when the generator potential reaches a certain level called the threshold level. Action potentials are all one size, so the strength of the stimulus is measured by the frequency of action potentials. If the stimulus is too weak the generator potential won't reach the threshold.
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Pacinian corpuscles

  • Pacinian corpuscles are mechanoreceptors and they detect mechanical stimuli like pressure and vibrations and they are found in the skin. Pacinian corpuscles contain the end of a sensory neurone, imaginatively called a sensory nerve ending. The sensory nerve ending is wrapped in lots of lamellae
  • When the pacinian corpuscle is stimulated the lamellae are defromed and press on the sensory nerve ending. This causes the sensory neurones cell membrane to stretch, deforming the stretch-mediated sodium ion channels. The channels open and sodium ions diffuse into the cell, creating a generator potential. If the generator potential reaches a threshold, it triggers an action potential
  • Image result for pacinian corpuscle (http://images.slideplayer.com/25/8002532/slides/slide_3.jpg)
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Photoreceptors- Rods and cones

  • Photoreceptors in your light detect light. Light enters the eye through the pupil and the amount of light is controlled by the iris. Light rays are then focused by the lens onto the retine. The retina contains the photoreceptor cells. The fovea is an area of the retine where are are lots of photoreceptors. Nerve impulses from the photoreceptor cells are carreid from the retine to the brain by the optic nerve.
  • Light enters the eye and hits the photoreceptors and is abosrbed by light-sensitive optical pigments. Light bleaches the pigments, causing a chemical change and altering the membrane permeablity to sodium ions. A generator potential is create and if it reaches a threshold, nerve impulse is sent along a bipolar neurone. Bipolar neurones connect the photoreceptors to the optic nerve.
  • Rods are mainly found in the peripheral parts of the retine and cones are mainly found in the fovea. Rods only give information in black and white (monocromatic) and cones give information in colour (trichromatic or RGB). 
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Photoreceptors- sensitivity and visual acuity

  • Rods are very sensitive to light. This is because many rods join 1 bipolar neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.
  • Cones are less sensitive than rods. This is because 1 cone joines 1 bipolar neurone, so it takes more light to reach the threshold and trigger an action potential.
  • Visual acuity is the ability to tell apart points that are close together. Rods give low visual acuity because many rods join the same bipolar neurone, which means lgith from 2 points close together cant be told apart.
  • Cones give high visual acuity because cones are close together and 1 cone joins 1 bipolar neurone. When light from 2 points hits 2 cones, 2 action potentials go to the brain, this means you can distinguish between 2 points that are close together as 2 separate points.
  • Image result for summary table of rods and cones (http://images.slideplayer.com/24/7060669/slides/slide_67.jpg)
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Structure of the nervous system and control of hea

  • The nervous system is split into 2 different systems, the CNS and the peripheral nervous system. The cNS is made of the brain and spinal chord whereas the peripheral nervous system is made up of neurones that connect the CNS to the rest of the body.
  • The peripheral nervous system also has 2 systems, the somatic and autonomic nervous syste. The somatic nervous system controls conscious activities and the autonomic nervous system controls unconscious activities. The autonomic is split into the sympathetic and parasympathetic nervous system which have opposite effects on the body.
  • The sympathetic nervous system is the "flight or flight" system and the parasympathetic is the "rest and digest" system. The autonomic is involved in the control of heart rate.
  • Cardiace muscle is myogenic which means it can contract and relax without receiving signals from nerves and the pattern of contractions controls the heartbeat.
  • The process starts at the sinoatrial node (SAN) which is a small tissue in the wall of the right atrium. The SAN sets the rhythm of the heartbeat by sending out regular waves of electrical activity and allows the right and left atria to contract at the same time.
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Control of heart beat continued

  • A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles. Instead these waves of electrical activity are transfereed from the SAN to the atrioventricular node (AVN)
  • The AVN is responsible for passing the waves of electrical activity on to the bundle of His. But, there is a slight delay before the AVN reacts, to make sure the atria have emptied before the ventricles contract
  • The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex of the heart. The bundle splits into finer muscle fibres in the right and left ventricle walls called the Purkyne tissue. The Purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up
  • Image result for control of heart beat (http://intensivecarehotline.com/wp-content/uploads/2013/01/Control-of-heart-by-heart-conduction-system_.jpg)
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Communication between the heart and brain

  • The SAN generates electrical impulses that cause the cardiac muscles to contract. The rate at which the SAN fires is unconsciously controlled by a part of the brain called the medulla.
  • Animals need to alter their heart rate to respond to interal stimuli. Internal stimuli are detected by pressure receptors and chemical receptors.
  • Electrical impulses from receptors are sent to the medula along sensory neurones. The medulla processes the information and sends impulses to the SAN along sympathetic and parasympathetic neurones
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Control of heart rate in response to different sti

  • High blood pressure- baroreceptors detect high blood pressure and send impulses along sensory neurones to the medulla, which sends impulses along parasympathetic neurones. These secrete acetylcholine(ACh), which binds to receptors on the SAN. This causes the heart rate to slow down in order to reduce blood pressure back to normal
  • Low blood pressure- Baroreceptors detect low blood pressure and send impulses along sensory neurones to the medulla, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on the SAN. This causes the heart rate to speed up in order to increase blood pressure back to normal.
  • High blood oxygen, low carbon dioxide or high blood pH levels- chemoreceptors detect chemical changes in the blood and send impulses along sensory neurones to the medulla, which sends impulses along parasympathetic neurones. These secrete ACh, which binds to receptors on the SAN. This causes the heart rate to decrease in order to return levels back to normal
  • Low blood oxygen, low carbon dioxide or low blood pH levels- chemoreceptors detect chemical changes in the blood and send impulses along sensory neurones to the medulla, which sends impulses along sympathetic neurones. These secrete noradrenaline, which binds to receptors on the SAN. This causes the heart ratre to increase to return levels back to normal
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Control of heart rate by the medulla

Image result for control of heart rate by the medulla (http://image.slidesharecdn.com/controlofheartrate-120712053535-phpapp01/95/control-of-heart-rate-4-728.jpg?cb=1342071380)

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