Unit 5: Section 3- Responding to the Environment

A summary of section 3 of Grey Matter

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Nervous and Hormonal Communication

  • Animals and plants increase their chances of survival by responding to changes in their external (eg. avoiding places which are too hot) and internal environments. A change to either environment is a stimulus.
  • Receptors detect stimuli, and effectors bring about a response to a stimulus. Receptors and effectors communicate via the nervous and hormonal systems.
  • The nervous system is a network of three types of neurone
  • 1. Sensory neurones- impulses from receptors to CNS
  • 2. Motor neurones- from CNS to effectors
  • 3. Relay neurones- between sensory and motor neurones
  • When an impulse reaches the end of a neurone, neurotransmitters transmit the impulse to the next neurone
  • The hormonal system is made up of glands and hormones. Glands= a group of cells specialised to secrete hormones. Hormones= chemical messengers
  • Hormones are secreted when a gland is stimulated by a change in concentration of a substance or by an electrical impulse
  • Hormones diffuse into the blood and are carried to cells, where they bind to specific receptors
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The Nervous System- Receptors 1

  • Receptors are specific, and will only detect one stimulus. Some are cells and some are proteins
  • When a receptor is in its rest state, the membrane is polarised, as there is a potential difference between the inside and outside of the cell. This pd is maintained by ion pumps and channels
  • When a stimulus is detected, the permeability of the cell to ions changes, changing the potential difference. If the pd is big enough, it triggers an action potential.
  • Light enters the eye through the pupil, the size of which is controlled by the iris muscles. Light rays are focused by the lens onto the retina. The retina contains photoreceptor cells to detect light. The fovea is an area of high photoreceptor concentration. Impulses are carried between the eye and the brain by the optic nerve
  • When light hits photoreceptors, it bleaches the light sensitive pigments in it, causing an electrical change. There are two types of photoreceptors- rods (black and white) and cones (colour)
  • Bipolar cells carry an impulse from the photoreceptors to the CNS
  • Dark. Na+ is pumped out of the cell via active transport, but diffuse back in via open channels. This depolarises the membrane, as the inside is slightly negative compared to outside. This triggers the release of neurotransmitters which inhibit bipolar neurones from firing action potential
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The Nervous System- Receptors 2

  • Light- Photons cause rhodopsin to break down into retinal and opsin. This is bleaching
  • Na channels are caused to close, so Na+ cannot move back into the cell after being actively moved out.
  • Na+ builds up outside the cell, and the membrane becomes hyperpolarised, as the inside is very -ve
  • Hyperpolarisation stops the release of neurotransmitter, so there is no inhibition of the bipolar neurones. The bipolar neurone can depolarise and transmit an action potential to the brain
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The Nervous System- Neurones 1

  • Motor neurones- Have many short dendrites to carry impulses from CNS to cell body. One long axon for impulses from CNS to effector
  • Sensory neurones- Have one long dendrite to carry impulse from receptor to cell body, and an axon to carry from cell body to CNS
  • Relay neurones- Many short dendrites to link to sensory, many short axons to link to motor
  • The resting potential of neurones is created and maintained by sodium-potassium pumps and potassium ion channels
  • Na-K pumps move Na+ out, but can't move them back in. Creates a Na+ electrochemical gradient, more +ve Na+ outside the cell.
  • Na-K pumps move K in, but the membrane is permeable to K+, so they can diffuse back out via K+ channels. The outside of the cell becomes +vely charged compared to the inside
  • Action potential sequence of events-
  • 1. Stimulus- causes Na+ channels to open, Na+ diffuses in down the electrochemical gradient and the inside becomes less negative
  • 2. Depolarisation- if the pd reaches threshold (-55 mV), more Na+ channels open so more Na+ diffuses in
  • 3. Repolarisation- at a pd of +30 mV, Na+ channels close and K+ channels open. K+ moves out of the cell down the conc gradient. Membrane starts to return to resting potential
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The Nervous System- Neurones 2

  • 4. Hyperpolarisation- K+ channels are slow to close, so there is an 'overshoot', where too many K+ move out, and the pd becomes more -ve than the resting potential
  • 5. Resting potential- Ion channels are reset, and the Na-K pump returns the membrane to the resting potential
  • When an action potential happens, some Na+ enters the neurone sideways, causing Na+ channels in the next region to open. The causes a wave of depolarisation.
  • The refractory period (where channels are recovering and cannot be opened) acts as a time delay between action potentials, making sure the impulses are discrete and unidirectional
  • A big stimulus causes more frequent impulses
  • Some neurones have a myelin sheath, an electrical insulator made of Schwann cells.
  • Between Schwann cells are nodes of Ranvier- areas of bare membrane with high Na+ ion channels.
  • Depolarisation only happens at the nodes. The cytoplasm conducts enough charge to depolarise the next node, so the impulse jumps quickly through saltatory conduction.
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The Nervous System- Synapses

  • A synapse is a junction between neurones. The gap between cells at a synapse is the synaptic cleft.
  • The presynaptic neurone has a bulge called a synaptic knob. Contains vesicles filled with neurotransmitter
  • 1. An action potential arrives at the presynaptic knob, and stimulates Ca2+ channels to open. Ca2+ ions diffuse into the synaptic knob
  • 2. The influx of Ca2+ causes the vesicles to move to and fuse with the presynaptic membrane. They release neurotransmitter into the synaptic cleft- this is exocytosis.
  • 3. The neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane. This causes Na+ channels to open in the postsynaptic neurone, causing depolarisation as Na+ moves in. An action potential is generated, and the neurotransmitter is removed so the response doesn't keep happening
  • Synapses allow information to be dispersed (synaptic divergence- one neurone connected to many more) or amplified (synaptic convergence- many neurones connect to one)
  • Summation= effect of neurotransmitter from many neurones is added together. Finely tunes the nervous response
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Responses in Plants

  • Phototropism- growth of a plant in response to light. Shoots are positively phototropic (towards light) and roots are negatively phototropic
  • Plants respond to stimuli using growth factors. They are produced in growing regions of the plants and move to where needed.
  • Auxins stimulate shoot growth by cell elongation. Indoleacetic acid (IAA) is an auxin produced by shoot cells. It moves down the stem via diffusion and active transport.
  • It diffuses down the stem unevenly so there is uneven growth. IAA moves to more shaded parts, so they grow more to bend the stem towards the light.
  • Plants detect light using phytochromes. They exist in two states- Pr absorbs red light, and Pfr absorbs far-red light
  • In red light Pr -> Pfr (quick)
  • In far-red light Pfr -> Pr (quick)
  • In darkness Pfr -> Pr (slow)
  • Daylight contains more red light than far-red light, so Pr is converted to Pfr
  • In some plants, high Pfr levels stimulate flowering. In summer when nights are short, there is not much time for Pfr to be converted back to Pr, so Pfr builds up. This means plants flower in summer
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