Neurons (1)

Neurons (1)

1. Resting membrane Potential

   • Electrical potential difference across the cell membrane which results from separation of charge
   • There is more negative charges inside the cell in comparision to the extracellular fluid
   • Unequal conc. of Na+ and K+
     • Outsde: Na+ higher
     • Inside: K+ higher
     • Ca2+, Cl- and large ions don't affact RMP
   • Unequal permeability
     • Non-gated (Leak) open at rest
     • Gated (voltage, ligand) closed at rest
     • Permeability K:Na is 40:1
   • Equilibrium Potential
     • An intracellular potential at which the net flow of ions is zero, inspite of a concentration gradient and permeabiltiy.
     • The higher the permeability of the cell membrane to a particular ion, the greater the ability of this ion to shift to RMP towards its equilibrium potential
     • At rest the RMP is closer to the equilibrium potential for K+   
     • Nernst Equation
       • When cell membrane is permeable to only one ion
     • Goldmans Equation
       • Takes account of both concentration gradient and relative permeability

2. Action Potentials

   • Hyperpolarisation
     • Potential inside neurons becomes more negative, move closer to K+
   • Depolarisation
     • Potential inside neurons becomes less negative, move closer to Na+
   • Absolute refractory
     • 1. MP reaches threshold: fast depolarisation (overshoot) 2. Repolarisation
     • 1. Threshold: fast depolarisation (overshoot)
       • Sudden activation of voltage gated K+
     • 2. Repolarisation
       • Inactivation of Na
       • Acitvation of K+
     • Collision: APs transmitted opposite cancel each other
       • Orthodromic: from cell body
       • Antidromic: by Electrical stimulation of axon
   • Relative refractory
     • 3. After Hyperpolarisation
     • After Hyperpolarsation
   • Evoked by current
     • RULE #2: When the current generated by an outside source flows through the cell membrane from outside to inside= hyperpolarisation. And when current flows from inside to outside= depolarisation.   
     • outside from + to -
     • Across the membrane and inside axon
   • Generated in axon initial segment (hillock)
     • Depolarisation to threshold evoke by EPSP
     • APs transmitted along axon, away from cell body
   • Axons
     • Myelinated: large, fast
       • Myelin Sheath
         • CNS: Oligodendrocytes
         • PNS: Schwann Cells
       • Myelination
         • Increases passive spread of current due to the insulating properties of myelin, there is less current dissipation as it flows along the axon.
         • Interrupted by nodes of Ranvier
         • Saltatory Conduction
     • Unmyelinated: small slow, continuous
       • AP must be regenerated at every point on the membrane. This takes time and therefore conduction velocity is slow.
   • Passive Spread
     • 1. Depolarisaiton at one point
     • 2. Passive curent flow
     • 3. Depolarisation of adjacent parts
   • Receptor Potential (PNS)
     • Information about the strength of the stimulus is coded in the amplitude of the receptor potential and the frequency of APs   
     • 1. Evokes graded depolarisation
     • 2. RP speads passively to trigger zone
     • APs generated spread along axon

3. Synaptic Transmission

   • Messages are transmitted from one excitable cell to another
   • Chemical synapses
     • Excitatory Synapses (EPSP)
       • Depolarisation of postsynaptic mebrane
       • Opening of Na, K and Ca
     • Inhibitory Synapses (IPSP)
       • Hyperpolarisation of postsynaptic mebrane
       • Opening of K and Cl
     • Neurotransmitters
       • Small molecule (Direct)
         • Ach
         • Amino acids
         • Amines
       • Neuropeptides (indirect)
       • Inactivation
         • 1. Diffusion away from synapse
         • 2. Enzymatic degradation in synaptic celft
         • 3. Re-uptake and Recycling
   • Electrical Synapses