NEURONES

• what is the state of the neurone when it is not stimulated/resting?

  membrane polarised- outside positively charged vs inside negatively charged
• movement of Na+ & K+:

  • resting potential maintained by Sodium and potassium pumps and potassium ion channels
  • Na+ & K+ pumps actively transport to move 3 Na+ out of neurone for 2 K+ in - ATP recq
  • K+ channel allows for faciliated diffusio of K+ out of neurone- down conc grad
• Movement of Sodium ions and Potassium ions:

  1. Na+ pump moves 3 Na+ out of neurone- membrane not permeable to Sodium, thus Na+ cannot diffuse back in - creates electrochemical gradient- + outside
  2. Na+&K+ pump moves 2 K+ into cell
  3. when cell at rest- most K+ channel open- membrane permeable to K+ - some diffuse out
• what is an action potential?

  the process in which the cell membrane becomes depolarised
• A.P: i

  STIMULUS:

  • excites neurone cell membrane- Na2+ voltage gated channel open
  • membrane more permeable to Na2+, Na2+ diffuse into neurone down Na2+ electrochemical gradient
  • inside membrane less negative
• A.P: ii

  DEPOLARISATION:

  • if potential difference reaches threshold (-55mv) more Na2+ channels open=more Na2+ diffuse into neurone
• A.P: iii

  REPOLARISATION:

  • at potential difference (+30mv) Na2+ channels close & K+ channels open- membrane permeable to K+
  • K+ moves out of neurone - returns membrane to resting potential
• A.P: iv

  HYPERPOLARISATION:

  • K+ channels slow to close- slight overshoot- too many K+ diffuse out of neurone
  • potential difference more negative resting potential
• A.P: v

  RESTING POTENTIAL:

  • ion channels reset
  • Na2+ K+ pump returns membrane to resting potential- sodium diffuses out, Potassium in
• A.P: vi

  REFRACTORY PERIOD:

  • ion channels recovering- neurone membrane cannot be excited
  • Sodium ions closed during REPOLARISATION
  • Potassium ions closed during HYPERPOLARISATION
  • acts as time delay- action potential doesnt overlay & unidrectional
• waves of Depolarisation:

  • some Na2+ enter neurone by diffusing in sideways- causes Na2+ channel to open in next region
  • wave of depolarisation travels along neurone - wave moves away from parts of Neurone cell membrane that is in refractory period
• All or Nothing principle:

  • once threshold reached (-55mV) - A.P fires same change in voltage- no matter size of stimulus
  • if threshold is not reached- A.P not fired
  • bigger stimulus will not cause a greater A.P but will cause A.P to fire more frequently
• Speed of CONDUCTION:

  • myelination
  • saltatory conduction
  • axon diameter
  • temperature
• S.O.C - Myelination:

  • neurones, e,g Motor neurones are myelinate- have myelin sheath
  • Myelin Sheath is an electrical insulator
  • in peripheral nervous system- sheath made of Schwann Cells
  • Between Schwann cells- tiny patches of bare membrane - Nodes of Ranvier
  • Nodes of Ranvier- where most Sodium ion channels are concentrated
• S.O.C - Saltatory Conduction:

  • in myelinated neurone- depolarisation occurs at N.O.R (Sodium ions can pass through membrane)
  • Cyptoplasm conducts enough electrical charge to depolarise next node - impulse jumps from node to node
  • in non- myelinated- impulse travels along the whole axon membrane - depolarisation across whole membrane - slower
• S.O.C - Axon Diameter:

  • Action potential quicker along axon with bigger diameter- less resistance to flow of ions than in cyptoplasm of smaller axon
  • wave of depolarisation reaches parts of neurone cell membrane quicker with less resistance
• S.O.C - Temperature:

  • speed of conduction increases as temperature increases- ions diffuse faster
  • speed increases only until 40*c - after which protein denatures e.g channels
• synapse: junction between neurones or between neurone and effector cell
• tiny gaps between cells at synapes- Synaptic Cleft
• pre-synaptic neurone swelling - synaptic knob- containing synaptic vesicles filled with neurotransmitters
• effect of A.P on neurone:

  1. A.P reaches end of neurone - causes N.T to be released into synaptic cleft
  2. diffuse across to post synaptic membrane and bind to specific receptors
  3. when N.T binded to receptors, trigger A.P in neurone- cause muscle contraction, Hormone secretion

  • receptors only on post synaptic membrane- synapse ensures unidirectional
  • N.T removed from cleft - response not repeated - taken back into pre-synaptic neurone or broken down by enzyme
• Acetylcholine: a neurotransmitter which binds to cholinergic receptors on Cholinergic Synapses
• Cholinergic Synapse-

  1. Arrival of Action Potential
  2. Fusion of Vesicles
  3. Diffusion of Acetylcholine
• Cholinergic Synapse: Arrival of Action Potential

  • Action potential arrives at synaptic knob
  • Action potential stimulates voltage-gates calcium ion channels to open
  • Calcium ions diffuse into synaptic knob (later pumped out by active transport)
• Cholinergic Synapse: Fusion of Vesicles

  • influx of calcium ions into synaptic knob causes synaptic vesicles to fuse with pre-synaptic membrane
  • Vesicles releases Acetylcholine into synapse cleft by exocytosis
• Cholinergic Synapse: Diffusion of Acetylcholine

  • Acetylcholine diffuses acros synaptic cleft and binds to cholinergic receptors on post-synaptic membrane- causes sodium ion channels to open
  • influx of sodium ions into post-synaptic membrane causes depolarisation
  • Action Potential on post-synaptic membrane generated if threshold reached
  • Acetylcholine then removed from synaptic cleft - response stopped
  • ACh broken down by ACETYLCHOLINESTERASE - products reabsorbed by pre-synoptic neuroneuted to make ACh
• Excitatory & inhibitory neurotransmitters:

  • Excitatory- depolarise (increase positivity) post-synaptic membrane if threshold is reached - Acetylcholine in CNS
  • Inhibitory- hyperpolarise (increase negative potential difference) post-synaptic membrane preventing firing of A.P - ACh in Heart cause potassium ion channels to open
• Summation at Synapses:

  • if stimulus is weak- small amount N.T released from neurone into synaptic cleft
  • not enough to excite post-synaptic membrane to threshold level for A.P
  • Summation is where the effect of N.T released from many neurones is added together , or one neurone stimulated a lot in a short amount of time
  • synapse accurately process information - tuning response
• Spatial Summation:

  • 2 or more pre-synaptic neurones release their N.T at same time onto same post-synaptic neurone
  • small amount small amount of N.T released from each neurone can amount to reach threshold level in post synaptic neurone to trigger A.P
  • if Inhibitory - total effect of N.T might not be A.P
• Temporal summation:

  • two or more nerve impulses arrive in quick succession from same pre-synaptic neurone - A.P more likely, N.T more in synaptic cleft
• Neuromuscular junction : specialised cholinergic synapse between a motor neuron and muscle cell
• Neuromuscular junction:

  • use ACh- bind to nicotinic cholingeric receptors
  • Post-synaptic membrane lots of folds forming clefts- stores AChE
  • Post-synaptic membrane more receptors than other receptors
  • ACh always excitatory where M.N fires A.P -> triggers response in muscle cell