section 6

• Simple response: Taxis -> directional movement in response to a stimulus. The direction of the stimulus affects the response.
• Simple responses: Kinesis -> non-directional (random) movement in response to a stimulus. The intensity of the stimulus affects the response.
• Receptors - detect stimuli. Can be cells or proteins on cell surface membrane.
• Effectors - cells that bring about a response to a stimulus, to produce an effect. Include muscle cells and cells found in glands.
• Receptors communicate with effectors via the nervous system or the hormonal system.
• The nervous system is made up of many complex network of cells called neurones.
• Sensory neurones -> transmit electrical impulses from receptors to the central nervous system.
• Motor neurons -> transmit electrical impulses from the central nervous system to effectors.
• Relay neurones -> transmit electrical impulses between sensory neurones and motor neurones.
• Nervous communication - 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 gap (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.
• Stimulus -> Receptors -> CNS -> Effectors -> Response
• The nervous response - When an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto cells, so the nervous response is localised. Neurotransmitters are quickly removed once they’ve done their job, so the response is short-lived. Electrical impulses are really fast, so the response is usually rapid - allows animals to react quickly to stimuli.
• Simple reflexes - a rapid and involuntary response to a stimulus in order to help organisms avoid damage to the body, due to response happening quickly.
• The reflex arc - the pathway of neurones linking receptors to effectors in a simple reflex.
• Three neurones involved in reflex arc - sensory neurone, relay neurone, motor neurone.
• Tropism - the response of a plant to a directional stimulus. A positive tropism is growth towards the stimulus whereas a negative tropism is growth away from the stimulus.
• Phototropism - the growth of a plant in response to light. Shoots are positively phototrophic and grow towards light. Roots are negatively phototrophic and grow away from light.
• Gravitropism -> growth of a plant in response to gravity. Shoots are negatively gravitropic and grow upwards. Roots are positively gravitropic and grow downwards.
• Auxins - growth factors that are produced in tips and shoots and diffuse backwards to stimulate the cell just behind the tips to elongate. If the tip of a shoot is removed, no auxin will be available and the shoot stops growing.
• Auxins stimulate growth in shoots but high concentrations inhibit growth in roots.
• Indoleacetic acid (IAA) - important auxin thats produced in the tips of shoots and roots in flowering plants. Its moved around the plant to control tropisms - moves by diffusion and active transport over short distances, and via the phloem over long distances. This results in different parts of the plant having different concentrations of IAA. The uneven distribution of IAA means there’s uneven growth of the plant.
• Receptors are specific - they only detect one particular stimulus. There are many different types of receptor that each detect a different type of stimulus.
• Receptors in the nervous system convert the energy of the stimulus into the electrical energy used by the neurones.
• The resting potential - when a nervous system is in its resting state, there's a difference in charge between the inside and the outside of the cell - the inside is negatively charged relative to the outside. This means there's a voltage across the membrane. The potential difference when a cell is at rest is called its resting potential. The resting potential is generated by ion pumps and ion channels.
• The generator potential - when a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell - altering the potential difference. The change in potential different due to a stimulus is called the generator potential. A bigger stimulus excited the membrane more, causing a bigger change in potential difference - so a bigger generator potential is produced.
• The action potential - If the generator potential is big enough it'll trigger an action potential - an electrical impulse along a neurone. An action potential is only triggered if 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, so there's no action potential.
• Pacinian corpuscles - mechanoreceptors, detect mechanical stimuli. They're found in your skin. Contain the end of a sensory neurone - sensory nerve ending. Sensory nerve ending is wrapped in loads of layers of connective tissue called lamellae
• When a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the sensory nerve ending. This causes the sensory neurone's 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 the threshold, it triggers an action potential.
• Photoreceptors - receptors in your eye that detect light. Light enters eye through the pupil, and the amount of light that enters is controlled by the muscles of the iris.
• Light rays are focused by the lens onto the retina, which lines the inside of the eye.
• The retina contains photoreceptor cells.
• The fovea is an area of the retina where there are lots of photoreceptors.
• Nerve impulses from the photoreceptor cells are carried from the brain by the optic nerve, which is a bundle of neurones.
• Where the optic nerve leaves the eye is called the blind spot - there aren't any photoreceptor cells, so it's not sensitive to light.
• How photoreceptors work:

  • light enters eye, hits photoreceptors and is absorbed by light-sensitive optical pigments.
  • light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions.
  • a generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.
  • bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.
• The human eye has two types of photoreceptor - rods and cones
• Rods are mainly found in the peripheral parts of the retina.
• Cones are mainly found packed together in the fovea.
• Rods and cones contain different optical pigments making them sensitive to different wavelengths of light.