Grey Matter

• What is resting potential

  When an axon is not stimulated / firing, it is at rest.

  The resting potential of an axon = -70mv
• Resting potential: Step 2

  As ATP binds to the sodium-potassium pump, it is hydrolysed.

  For each ATP hydrolysed:

  3 Na+ sodium ions are pumped out

  2 K+ potassium ions are pumped inside
• What is an action potential?

  Electrical impulses that send signals around the body. Action potentials are triggered by a stimulus and received by receptor cells
• Stages of an action potential

  Depolarisation

  Repolarisation

  Hyperpolarisation

  Resting potential
• Transmission of action potential - Step 1

  In response to a signal, the axon becomes depolarised. The depolarisation at the site causes sodium to diffuse along the axon
• Transmission action potential - Step 2

  This causes depolarisation at the next site of the neurone and an action potential is generated
• Transmission of action potential - Step 3

  The previous section of membrane is temporarily unresponsive to depolarisation because it is in phase of hyper-polarisation
• Transmission of action potential - step 4

  This process repeats along the length of the axon = wave of depolarisation
• Saltatory conduction

  1. The nodes of Ranvier get depolarised by the impulse / when sodium ions arrive
  2. The action potential causes the sodium ion channels to open and the diffusion of sodium ions creates local circuits. 
  3. The action potential jumps from one node to the next 
  4. This is saltatory conduction 
• Synaptic transmission: Step 1

  An action potential arrives at the end of an axon of the presynaptic neurone
• Synaptic Transmission: Step 2

  The nerve impulse triggers the voltage dependent gated calcium ion channels to open and calcium ions diffuse into the presynaptic knob
• Synaptic Transmission: Step 3

  The release of the calcium ions causes the vesicles containing neurotransmitters to fuse with the presynaptic membrane
• Synaptic transmission - Step 4

  The neurotransmitters are released into the synaptic cleft by exocytosis. The neurotransmitters diffuse across the gap to the post-synoptic neurone
• Synaptic Transmission - Step 5

  The neurotransmitters bind to receptors on the post-synaptic neurone causing sodium ion channels in the dendrites of the post-synaptic neurone to open and cause depolarisation
• Synaptic Transmission - Step 6

  The neurotransmitters remaining in the synapse can be taken back up by the pre-synaptic neurone or enzymes such as acetylcholinesterase break down the neurotransmitters to prevent action potentials continuously forming
• Rod Cells in the dark: Step 1

  Sodium ions are pumped out of the rod cell, generating a concentration gradient. The Na+ diffuse back into the axon via the open sodium ion channels. The influx of Na+ causes slight depolarisation - 40 / - 50mv.
• Rod cells in the dark: Step 2

  The slight depolarisation of rod cell causes the neurotransmitter glutamate to be released, which diffuses down the synapse to the bipolar neurone. Glutamate binds to the receptors on the bipolar neurone.
• Rod cells in the dark: Step 3

  The binding of glutamate inhibits the generation of an action potential in the bipolar neurone = glutamate is an inhibitory neurotransmitter.
• Rod cells in light: Step 1

  Rod cells contain a light-sensitive pigment rhodopsin. Rhodopsin is made up of retinal and opsin. When light hits rhodopsin, it breaks into its trans isomer and opsin. This is known as bleaching.
• Rod cells in light: Step 2

  Opsin causes the sodium ion channels to close so Na+ cannot diffuse back into the rod cell.

  The active transport of Na+ ions continues while Na+ ions cannot diffuse inside. This causes the inside of the rod cell to become negative until it reaches a hyperpolarised state.
• Rod cells in light: Step 3

  The hyperpolarised rod cell stops releasing the neurotransmitter glutamate. This means glutamate is no longer present to inhibit the generation of an action potential in the bipolar cell.

  The lack of glutamate results in depolarisation of the bipolar cell, which results in an action potential.
• Visual cortex

  Region of the cerebral cortex, in which visual information is processed.

  Light enters the eye and stimulates nerve cells. This generates action potential in the axons. Impulse is transmitted from optic nerve to thalamus and carried to the visual cortex
• Monocular deprivation

  When one eye is deprived of light, the axons in the eye cells are not stimulated so action potential is not generated. Synapses do not fire and no neurotransmitters are released.

  Only the axons receiving light will be stimulated and fire.

  the synapses from the light-deprived eye are weakened and will eventually be cut back
• Habituation definition

  Process of habituation = if a stimulus is repeated many times or there is prolonged exposure with no negative outcome, the organism learns not to respond to it
• Process of habitaution

  1.When habituation takes place, fewer calcium ions are released from the presynaptic neurone when a nerve impulse arrives

  2. Fewer neurotransmitters are released

  3. Fewer neurotransmitters bind to receptors on the postsynaptic membrane

  4. Fewer sodium ion channels open so the charge inside axon remains negative

  5.Threshold potential not reached and action potential not generated
• dopamine and Parkinson's 


  Loss of dopamine-producing neurons

  Low levels of neurotransmitter dopamine in the basal ganglia 

  Less dopamine is released into the synaptic cleft so less dopamine can bind to receptors in postsynaptic neuron

  Fewer sodium ion channels open so depolarisation does not occur

  This means fewer action potentials are generated so nerve impulses do not reach the effector muscles leading to tremors and slow movement
• Dopamine Agonists

  • Drugs that bind to and activate the dopamine receptors on the postsynaptic membrane, triggering an action potential
  • Mimic the role of dopamine in the brain 
• L-dopa

  • Chemicals that can be converted into dopamine in the neurons
  • L-dopa is a precursor in the manufacture of dopamine.
  • L-dopa can be given to patients with Parkinson's. It is converted into dopamine in the brain, thus increasing the concentration of dopamine. 
• MAOB

  • MAOB inhibitors inhibit the activity of the enzyme, monoamine oxidase, which would normally be responsible for the breakdown of dopamine in the synaptic cleft
  • This increases the levels of dopamine present in the brain 
  • there is more dopamine to bind to receptors on post-synaptic neurone
  • More voltage-gated sodium ion channels open so post-synaptic neurone more likely to be depolarised.
• 1. MDMA / ecstasy 

  • Affects multiple neurotransmitters, most notably serotonin
  • Inhibits reuptake of serotonin by binding to the specific proteins that enable serotonin reuptake on the presynaptic membrane 
  • This increases the concentration of serotonin in the synaptic cleft and the brain. There is repeated stimulation of the postsynaptic neuron so more action potentials are generated. 
  • Also triggers the release of further serotonin
• How are impulses transmitted to the visual cortex?

  Light enters the eye -> stimulates rod cells in the retina -> impulses in the optic nerve -> carried to the thalamus -> impulses carried to the visual cortex 
• ligase

  DNA ligase joins two genes by joining phosphodiester bonds by condensation reactions
• Describe how flies could be genetically modified to produce one form of the human TAU protein

  extract mRNA for one form of TAU protein

  copy mRNA into DNA

  use restriction enzymes to create sticky ends

  insert the TAU DNA into vector DNA using enzyme ligase, forming phosphodiester bonds between the two DNA

  introduce vector into fertilised egg / embryonic stem cell
• advantages of selective breeding

  increased yield and better quality products because organisms with advantageous alleles are chosen

  disease resistance

  drought resistance

  crops have higher nutrition content
• Disadvantages of selective breeding

  genetic drift occurs which reduces genetic diversity

  leading to loss of useful alleles

  reducing ability of crops to adapt to changes
• How does MDMA cause depression

  MDMA results in serotonin depletion

  post synaptic membrane becomes less responsive to serotonin

  lack of serotonin associated with depression.
• what is the critical period

  period of time during early development

  when the nervous system must obtain specific experiences to develop properly

  so that synapses are strengthened / unstimulated synapses are removed
• how do CT scans work?

  Beam of x-rays aimed at patient from all angles  

  Digital X-ray detectors used to pick up the x-rays as they exit the body 

  Denser tissue absorbs more x-ray so they show up as lighter regions on the scan

  Produces cross-section images of brain
• Describe what happens to the pupils in dim lights

  • Radial muscles and circular muscles in the iris work in antagonistic pairs
  • photoreceptors detect changes in the environment
  • Radial muscles contract and circular muscles relax
  • pupil dilates = diameter of pupil widens
  • more light enters the eye
• repolarisation


  1. At +30 mv, the Na+ ion channels close and voltage dependent gated potassium, K+ channels open
  2. The K+ ions diffuse out of the axon down their concentration gradient. This causes the inside of the axon to become negatively charged again.
  3. This process is repolarisation.