Topic 6 - Nervous System

• The PNS includes all other nerves outside of the CNS, including cranial nerves that connect to the brain and spinal nerves that connect to the spinal cord
• The CNS consists of the brain and spinal cord
• What are the two types of tropism?

  • Tropism is growth in response to a stimuli
  • Phototropism - growth response to light
  • Gravitropism - growth response to gravity
• Phototropism and IAA

  When shoots grow towards the light it’s an example of positive phototropism

  • IAA moves to the shaded side of the plant
  • The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation there
  • Causing the shoot to grow/bend towards the light
• Gravitropism and IAA


  When plants roots grow down (with gravity) it’s an example of positive gravitropism.

  IAA inhibits root growth

  • IAA is actively transported to the region in the root tip where the amyloplasts have sunk
  • The larger concentration of IAA at the lower side of the root inhibits cell elongation
  • As a result, the lower side grows at a slower rate than the upper side of the root
  • This causes the root to bend downwards
• What is kinesis?

  • Kinesis is a non-directional response to a stimulus
  • The rate of movement of an organism is affected by the intensity of the stimulus
• What is taxis?

  • Taxis is a directional response to a stimulus
  • The organism moves directly away from or towards the stimulus
• What are the pacinian corpuscles? and what do they do?

  Receptors found in skin,tendons, ligaments which respond to changes in pressure.

  • Has stretch mediated sodium channels in its membrane
  • When the membrane is stretched and opens allowing Na+ diffuse through causing a generator potential
  • Size of generator potential is proportional to size of stimulus
  • If generator potential is higher than threshold value and action potential is generated (large numbers of Na+ channels open)
• Rod cells

  • Rod cells contain a pigment called rhodopsin (embedded into discs in the membrane called lamellar)
  • When light hits it rhodopsin becomes bleached
  • The breakdown of the pigment is what triggers a generator potential which can result in an action potential
  • Rhodopsin is then regenerated
• Rod cells - sensitivity and visual acuity

  • Rhodopsin is broken down after in bright light so is used for seeing at low light intensities.
  • As only one bipolar cell joins onto many Rod cells it means that even though individually they may only breakdown a small amount of rhodopsin together they can stimulate a generator potential
  • Poor visual acuity - many Rod cells converge on one bipolar cell. Meaning many Rod cells can be stimulated but only one action potential would occur
• Cone cells

  Similar structure to Rod cells

  • Contain a pigment called iodopsin
  • Iodopsin is only broken down by high light intensities
  • Highly concentrated around the fovea
• Cone cells - sensitivity and visual acuity

  • Each cone cell is connected to a single bipolar cell so have little sensitivity
  • Each cone cell can stimulate an action potential so have high visual acuity
  • Colour vision is a property of cone cells
• Cone cells and colour vision

  Intensity of colour is determined by the frequency of impulses generated

  Different colours are seen due to the combination and stimulation of cones
• Reflex arc

  1. Receptor
  2. Sensory neurone
  3. Spinal cord
  4. Relay neurone
  5. Motor neurone
  6. Effector (muscle or gland)
• What is a reflex action?

  Unconscious, rapid response to a harmful stimulus
• Explain how a resting potential is maintained across the axon membrane in a neurone.

  • 3Na+ are actively transported out the neurone for every 2K+ that are actively transported in
  • The membrane is permeable to K+ ions so K+ can diffuse back out the membrane
  • Therefor the inside of the neurone is more negative than outside of the cell (-70mv)
• Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon

  • Action potential is conducted faster along the myelinated axon it ‘jumps’ from one axon to the next as it conducts enough charge to depolarise the next node.
  • In non-myelinated it moves as a wave through the whole axon making it slower
• Describe how a heartbeat is initiated and coordinated

  • SAN sends a wave of electrical activity causing the atria to contract
  • Non-conducting tissue prevents immediate contraction of the ventricles
  • AVN delays impulse whilst blood leaves atria
  • AVN sends a wave of electrical activity down Bundle of HIS
  • Causing ventricles to contract from bottom up
• Factors effecting the speed of conduction

  • Myelination
  • Axon diameter - quicker with larger diameter ( less resistance to the flow of ions so depolarisation reaches other parts of the neurone quicker)
  • Temperature - increases in speed as temperature increases ( ions diffuse faster - until 40°c when they denature so speed decreases )
• What is a wave of depolarisation? And what causes it?

  When an action potential occurs some Na+ diffuse sideways . Causes Na+ channels in the next part of axon to open.

  So a wave of depolarisation moves along the membrane
• The refractory period.

  After an action potential another one cannot occur straight away / cannot be excited straight away.

  This is as the ion channels are recovering.
• Describe the role of receptors and of the nervous system in the increase in heart rate during exercise

  • Chemoreceptors detect the lowering of the pH caused by a rise in CO2 concentration
  • This increases the frequency of impulses sent along the sensory neurone to the medulla oblongata
  • This causes an increase in frequency of impulses sent to SAN via the synthetic node
  • Increasing heart rate
• Explain how a rise in blood pressure results in a decrease in heart rate

  • Baroreceptors in atria detect the change in blood pressure
  • Impulse sent to medulla oblongata
  • medulla oblongata send impulse via parasympathetic neurone to SAN
  • Acetylcholine binds to receptors on SAN
  • Decrease in impulses sent to AVN
  • Slowing heart rate
  • Preventing high blood pressure
• Describe the sequence of events involved in transmission across a cholinergic synapse

  • an action potential reaches presynaptic membrane
  • causes voltage-gated Ca2+ channels to open so Ca2+ moves into cytoplasm ove pre-synaptic nerve
  • this stimulates ACh containing vesicles to fuse to pre-syn membrane. ACh moves into synaptic cleft by exocytosis
  • ACh diffuse across cleft and bind to cholinergic receptors on post-syn membrane.
  • causes Na+ channels to open and Na+ causes depolarisation in post-syn nerve
• Difference between excitatory and inhibitory neurotransmitters

  Excitatory - depolarise post-syn membrane so an action potential is reached.

  Eg. Acetylcholine

  Inhibitory - binds to receptors causing K+ channels to open causing post-syn membrane to become hyperpolarised

  Eg. GABA