NEURAL COMMUNICATION - L2,P1

NEURAL COMMUNICATION - L2,P1

1. ELECTRICAL SIGNALS

   • WHAT DOES A NEURON LOOK LIKE?
     • At rest is a balance of electrochemical forces.
     • Ions - electrically charged molecules: anions (-ve) and cations (+tve)
     • Ions are dissolved in intracellular fluid (filled with this on the inside. separated from extracellular fluid (outside of the neuron) by the cell membrane
   • RESTING POTENTIAL
     • When you impale the neuron and measure the intracellular space, it goes down from 0 to 60mV. The charge of neurons is negative due to the make up (proteins) being naturally negative. Range from -50 to -80mV. A store of negative energy on intracellular side relative to the extracellular side. Electrical charge across the cell membrane in the absence of stimulation.
       • summary: intracellular protein (-ve), Na+/K+ pump, permeability to K+, lack of permeability to Na+, diffusion, electrostatic forces
     • IONIC BASIS OF RESTING POTENTIAL
       • K+ reaches equilibrium when movement is balanced by the movement in.
       • Corresponds to the resting membrane potential of about -60mV (values may range between -50 and 80mV)
     • HOW DO YOU CHANGE AT RESTING POTENTIAL?
       • Input from pre-synaptic neurons cause changes in electrical potential (polarisation) across membrane of post-synaptic neuron
         • HYPERPOLIRISATION – increase in membrane potential – the interior of the membrane becomes even more negative, relative to the outside Produces an immediate response that passively follows the stimulus The greater the stimulus the greater the response – the change in potential is called a graded response LOCAL POTENTIAL – as the potential spreads across the membrane, it diminishes as it moves away from the point of stimulation Not great for delivering signal
           • DEPOLIRISATION – decrease in membrane (-60 to -50 to -40) potential – the interior of the cell becomes less negative. Inside of neuron is getting more positive and the switch is flicked – jump up to 40mV and then comes down and regulates. The same as a hyperpolarising one If the membrane reaches the threshold (about 40mV), it triggers an action potential The membrane potential reverses and the inside of the cell becomes positive
   • CELL MEMBRANE
     • Lipid bilayer with two layers of lipid molecules. Ion channels are proteins that span the membrane and allow ions to pass.
     • Gated ion channels open and close in response to voltage changes, chemicals or mechanical action.
     • Certain channels allow certain things to enter the neuron such as calcium
   • ION CHANNELS
     • Some channels are open all the time to allow only K+ ions to cross
     • The neuron shows selective permeability to K+ it can enter or leave the cell freely
     • Usually only open if something is present such as a voltage or chemical or mechanical so channels found on the fingertips which respond to pressure - open and allow neurotransmitter/chemical to open and the sensory input makes you aware that you are touching something
     • SELECTIVE PERMEABILITY TO POTASSIUM - ENTER AND LEAVE FREELY
     • HOW DO IONS MOVE?
       • Diffusion causes ions to flow from areas of high to low concentration along their concentration gradient
       • Two opposing forces drive ion movement:
       • Once it is evened out, it will stop
       • ELECTROSTATIC PRESSURE - causes ions to flow towards oppositely charged areas, similar to how Thomas trains attract/repel, +tve and -tve will repel
       • Cl- --- potassium starts to attract itself to it
       • Positive ions are more likely to swamp the cell once it finds a window to enter
       • There are lots of negatively charged proteins inside the cell that cannot get past the membrane, making the inside of the cell negative
       • At rest K+ ions move into the negative interior of the cell because of electrostatic pressure. As K+ ions build up inside the cell, they also diffuse out the membrane, along the concentration gradient - inflow and outflow of potassium at rest. Sodium - little inside, some outside cannot get in
     • SODIUM POTASSIUM PUMP
       • Specialised ion channel
       • It pumps three sodium ions (Na+) out for every two K+ ions pumped in
       • At rest, it does very little, mostly activated during an action potential, uses a lot of energy
       • Not acctive at rest but things change dramatically at action potential
   • ACTION POTENTIALS
     • Brief but large changes in membrane potential, originate in the axon hillock and propagated along the axon
       • ALL OR NONE
         • Self sustaining, brief (1.5msec), voltage gated channels
     • Patterns of action potentials carry information to postsynaptic targets
     • All or none property of action potentials: the neuron fires at full amplitude or not at all. Does not reflect increased stimulus strength
     • Increases in frequency with increased stimulus strength
       • Afterpotentials are changes in membrane potential after action potentials
     • HOW IS AN ACTION POTENTIAL PRODUCED?
       • Due to depolarising stimulus - flips from negative to positive
       • Produced by the movement of Na+ ions into the cell through channels in the membrane. At the peak of an action potential, the concentration gradient pushing Na+ ions in equals the positive charge driving them out. Membrane shifts briefly from resting state to an active state and back. Peak of AP we have a voltage gated channel that is selective to sodium and will only activate when there is a presence of voltage, as the membrane starts to get more positive the channels opens and sodium rushes in and sweeps into the neuron – does it so fast that potassium will start exiting but not at a fast-enough rate. Peak of activity because of the sodium flow. When potassium starts to exit, goes back to normal resting potential. Sodium potassium pump enforces the concentrations of ions that maintain resting potential.
         • POST SYNAPTIC POTENTIALS
           • Brief changes in the resting potential at the post synaptic terminal
           • (1) Excitatory postsynaptic potential (EPSP) (2) Inhibitory postsynaptic potential (IPSP
             • Produces a small hyperpolarisation, pushing the cell further away from threshold. IPSPs result from chloride ions (Cl=) entering the cell, making the inside more negative. Both sending action potentials but with different messages. Don’t fire
           • Graded, long lasting (10-100msec), proportional to stimulus, chemically gated channels
       • REFACTORY PERIOD
         • Time when only some stimuli can produce an action potential. Absolute refractory phase = time when no action potentials are produced. Relative refractory phase = time when only strong stimulation can produce an action potential.
     • Action potential are regenerated along the axon – each adjacent section is depolarised, and a new action potential occurs. Travels in one direction because of the refractory state of the membrane after a depolarisation – analogy (one direction the fuse can go once ignited).
       • When unmyelinated it is a slow process. CONDUCTION VELOCITY – speed of propagation of action potentials – varies with diameter. MYELIN – the insulating sheath formed by glia cells around axons. NODES OF RANVIER – small gaps in the insulating myelin sheath. SALTATORY CONDUCTION – the axon potential travels inside the axon and jumps from node to node.
         • WHAT IS MAKING THE ACTION POTENTIALS HAPPEN?
           • Post synaptic neuron all input from pre-synaptic neurons is synthesised. Determines where, when and how many action potentials to produce. Pre-synpatic vs post-synaptic terminals (boutons).

2. MULTIPLE SCLEROSIS

   • - Immune system attacks the myelin produced by the oligodendrocytes. Autoimmune disease (probably). Inflammation of CNS nerves. Affects insulating layer of axons.

3. SYNAPTIC INPUT

   • Neurons perform information processing to integrate synaptic inputs. A postsynaptic neuron will fire an action potential if a depolarisation that exceeds threshold reaches its axon hillock. Enough depolarising stimuli from EPSP to trigger an action potential. Depends which are firing because more inhibitory ones are firing here it means that hyperpolarisation occurs and no action potential.

4. SPATIAL SUMMATION

   • The summing of potentials the come from different spatial parts of the cell. If the overall sum of EPSPs and IPSPs can depolarise the cell at the axon hillock, then an action potential will occur. Messages degrade over distance. How far away the input is coming from.
   • TEMPORAL SUMMATION
     • - The summing of potentials that arrive at the axon hillock at different times. - The closer together in time that they arrive, the greater the summation and possibility of an action potential. - More likely to reach the threshold – piggy back on each other.