Action Potential; threshold

• A passive ion channel allows passive diffusion of a substance down the chemical gradient. If they are open, they are called voltage gated ion channels.
• Neurons and skeletal fibres have electrical membranes containing voltage gated ion channels that are activated by changes in the membrane potential.
• Changes in the membrane potential can temporarily reverse the distribution of electrical charges across the plasma membrane.
• Chemically gated ion channels are open when they bind specific chemicals. They have receptors that bind ACh at the neuromuscular junction.
• Chemically gated ion channels are the most abundant on the dendrites, areas where most synaptic communication occurs.
• Mechanically gated ion channels open in response to a mechanical stimuli that physically distort the neurons membrane surface. It is when the membrane returns to its original shape, the channel closes.
• Label 1

  A) dendrites

  B) cell body

  C) axon hillock

  D) axon

  E) axon terminals

  
• Dendrites and cell body contain chemically gated Na+ and K+ channels.
• Axon Hillock contains voltage gated Na+ and K+ channels.
• Axon terminals contain Voltage gated Ca2+ channels.
• Local potentials are graded potentials. They are a change in membrane potential voltage at a localised area of the dendrite or the cell body membrane.
• A local potential occurs by when a neurotransmitter binds to and opens chemically gated ion channels on dendrites and/or cell body, allowing ions (Na+) to move in or allowing ions (K+) to move out.
• The loss of K+ ions makes the inside negative, thus local potentials can be excitatory (EPSP) or inhibitory (IPSP).
• EPSP forms when a presynaptic neuron releases an excitatory neurotransmitter ACh. When the neurotransmitter binds, it opens chemically gated Na+ channels. These Na+ enters the post-synaptic cell, which causes depolarisation (membrane becomes more positive).
• IPSP form when a neuron releases inhiibitory neurotransmitters (GABA). When the neurotransmitter binds, it opens chemically gated K+ channels, which exit the post-synaptic cell, causing hyperpolarisation. The membrane becomes more negative.
• Usually a post-synaptic cells neuron receives input from multiple pre-synaptic neurons.
• The summed effect of all EPSP and IPSP determines if the post-synaptic neuron activates.
• Local potentials are summed in two ways, spatial summation and temporal summation.
• Spacial summation is the summed input from multiple pre-synaptic neurons.
• Temporal summation is the summed input from repeated firing of one pre-synaptic neuron.
• (|) Neurons - Functional components (zones)


  Input zone contains dendrites and cell body and receives chemical signals from other neurons. The summation zone structure is the axon hillock and it is the summation of inputs.
• Pre-synaptic inputs are summed at the axon hillock because it has a high density of voltage-gated channels.
• The threshold potential -(60mV) is the key that opens voltage gated channels. Thus, if summation occurs to or above (-60mV), voltage-gated Na+ channels open at the axon hillock.
• Action potential Step 1

  Voltage-gated Na+ channels open when membrane depolarises to -60mV.
• Action Potential Step 2

  A massive influx of Na+ causes 'rapid depolarisation phase' of the action potential.
• Action Potential Step 3.5

  As the membrane gets closer to the resting membrane potential, voltage-gated Na+ channels begin shifting from inactive to closed state.
• Action potential Step 4

  Voltage-gated K+ channels begin to close slowly. This permits excess K+ to exit, causing hyperpolarisation phase of the action potential.
• Action potential Step 5

  When all voltage-gated K+ channels close, the membrane returns to -70mV.
• Action potential Step 3

  Voltage-gated Na+ channels inactivate. Then voltage-gated K+ channels opens, K+ exits. This causes the repolarisation phase of the action potential.