SNAB Topic 8:Grey Matter

• Eyes in bright light:

  1. Photoreceptors detect change in amount of light (brighter).
  2. Circular muscles contract.
  3. Radial muscles relax.
  4. Pupil constricts (diameter narrows). 
  5. Less light enters the eye. 

  • Prevents damage to the retina.
• Eyes in dim light:

  1. Photoreceptors detect change in amount of light (dimmer).
  2. Circular muscles relax.
  3. Radial muscles contract.
  4. Pupil dilates (diameter widens).
  5. More light enters the eye.

  • Maximises amount of light entering the eye, improving vision.  
• • Na-K pumps present in csm.
  • Use ATP to actively transport Na+ out and K+  in. 
  • Every 3Na+ pumped out : 2K+ pumped in
  • Creates a conc grad across the membrane for both ions
  • Both ions will diffuse back across the membrane.
  • Sodium and potassium ions to move across the membrane by facilitated diffusion.
  • Neurone membrane is ↓permeable to Na+ than K+  so K+  diffuse out faster than Na+ diffuse back in.
  • Results in more pos. ions outside of cell so inside has -ve resting membrane potential.
• • SOME ion channels in the neuron membrane are voltage gated. (closed when at rest).
  • When a neuron is stimulated, the membrane becomes more permeable to Na+ so Na+ move into the axon DOWN their concentration gradient.
  • This reduces the electrical potential across the membrane as inside becomes ↑+ve.
  • When enough Na+ enter the membrane to make the pd -55mV (THRESHOLD POTENTIAL), more sodium channels open, leading to influx. 
  • An action potential can only be generated if the threshold value is reached.
  • Once +30mV is reached, action potential is generated. 
• • 1ms after AP, VGSC close.
  • VGPC open, so K+ diffuses out of axon, causing inside to become negatively charged again (REPOLARISATION).
  • Short period where MP is lower than RP (HYPERPOLARISED), this is the refractory period. 
  • During this time, action potentials CANNOT be generated - makes sure impulses travel in only one direction. 
  • VGPC close and Na-K pumps work to restore RP.
  • Membrane can be stimulated again.
• • Once an AP has been generated, it can be propagated along the length of the axon.
  • Sodium ions diffuse along the cytoplasm into the next axon section, causing that region to depolarise and trigger another AP.
  • Process repeats along axon. 
  • AP moves in a wave of depolarisation.
• • In unmyelinated axons, transmission speed is low as AP must travel across entire axon. 
  • Insulating axon increases transmission speed. 
  • Depolarisation cannot occur at sections with a myelin sheath. 
  • AP can only occur at nodes of ranvier (gaps between schwann cells that make up the myelin sheath). 
  • Diffusion of Na ions like this = local currents/local circuits.
  • AP ‘jumps’ = saltatory conduction.
• 1. Incoming action potential causes depolarisation in the presynaptic knob.
  2. Calcium channels open.
  3. Ca2+  floods into the presynaptic knob. 
  4. Calcium ions cause release of the neurotransmitter (acetylcholine)
  5. Influx of Ca2+ causes synaptic vesicles to fuse with the presynaptic membrane.
  6. Neurotransmitter (acetylcholine) released into the synaptic cleft. 
  7. NT (acetylcholine) binds to receptor sites on Na+ channels, causing them to open.
  8. Sodium ions diffuse into the postsynaptic knob and depolarise the synapse, repeating the process again. 
• Features of a Synapse:

  1. Unidirectionality - message can only be transmitted in one direction from pre-post synaptic neurone due to the position of NT vesicles and sodium channels.
  2. Summation - allows action potentials to be generated. Low freq. AP’s release insufficient NT amounts to exceed threshold in postSN. Summation establishes a build up of NT in the synapse.

  Spatial summation - many preSN share same synaptic cleft. Together, they release enough NT to create an action potential.
• 1. Light that hits the retina passes through layers of ganglion and bipolar cells before the pigment located within the photoreceptors. No impulse has been triggered yet. 
  2. When light reaches rhodopsin, it breaks it down into retinal and opsin via photolysis. (bleaching). 
  3. Results in a chemical change within the photoreceptor that generates a nerve impulse. 
  4. Hyperpolarisation occurs and continues through the bipolar cells to the ganglion cells that begin an action potential through the optic nerve and to the brain. 
  5. The neurotransmitter involved in the synaptic transmission is glutamate.