9- nervous transmission Flashcards
polarisation
• Neurons are polarised in their resting state.
• The resting potential across the neuron membrane is -70mV.
• This polarisation is caused by the net movement of cations out of the cell.
Maintaining polarisation (sodium-potassium pump and facilitated diffusion)
• A sodium-potassium pump moves Na+ ions out of and K+ ions into the neuron.
• 3 Na+ ions are moved out for every 2 K+ ions moved in.
• The membrane is impermeable to Na+ ions. Na+ ions remain outside the cell.
• The membrane has K+ ion channels. K+ ions diffuse back out of the cell.
• The imbalance of cations leaves the outside of the cell positively charged.
• This creates an electrochemical gradient.
forming an action potential
- Neuron excitation:
A. Triggered by a stimulus.
B. Causes the opening of Na+ ion channels.
C. This increases permeability to Na+ ions. - Sodium influx:
A. Na+ ions diffuse into the neuron, reducing internal negativity. - Threshold achievement:
A. At -55mV, more Na+ ion channels open.
B. The potential difference rises to +30mV.
C. This marks the end of the depolarisation and start of repolarisation. - Repolarisation:
A. Na+ ion channels close, and K+ ion channels open.
B. This allows K+ ions to exit the neuron. - Hyperpolarization:
A. The closing of K+ ion channels is delayed.
B. This causes the potential difference to exceed the resting potential. - Resting potential restoration:
A. The sodium-potassium pump re-establishes the -70mV potential difference.
propagating an action potential- wave of depolarisation
The action potential travels along the neuron as a wave of depolarisation - Na+ ions move to adjacent resting regions, where they trigger a change in potential difference, thus stimulating another action potential.
propagating an action potential- absolute refractory period
A. Applies to the neuron segment where the action potential was just generated.
B. Na+ ion channels are blocked, preventing another action potential.
C. Ensures unidirectional propagation and distinct separation of impulses.
propagating an action potential- relative refractory period
A. Follows the absolute refractory period in the same neuron segment.
B. Na+ ion channels are unblocked, but
K+ ion channels remain open.
C. Higher stimulus required to trigger another action potential.
D. Continues the one-way flow and orderly transmission of nerve impulses.
myelin sheath
• Produced by Schwann cells.
• Insulates axons and dendrons.
saltatory conduction
• The sheath is impermeable.
• This forces action potentials to jump between gaps in cells of the sheath (nodes of Ranvier).
• This enhances the rate of electrical potential transmission.
synapse
• Synapses are junctions between two neurons.
• They transmit nerve impulses between two neurons through the release of neurotransmitters.
synaptic transmission
• Action potentials cannot cross the synaptic cleft (gap).
• They cause the release of a neurotransmitter from the presynaptic knob by exocytosis.
• Neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic membrane.
stages in synaptic transmission- opening of Ca+ ion channels
An action potential arrives. The presynaptic membrane depolarises, therefore causing the Ca+ ion channels to open, which subsequently allow Ca+ ions to enter the neuron.
stages in synaptic transmission- release of neurotransmitters
The presence of Ca+ ions in the neuron causes the fusion of synaptic vesicles filled with a neurotransmitter with the presynaptic membrane, thus causing the release of the neurotransmitter into the synaptic cleft.
stages in synaptic transmission- stimulation
The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. It either:
A. Excitatory postsynaptic potential:
1. Stimulates the opening of ligand-gated Na+ ion channels.
Il. Na+ ions diffuse in. If enough Na+ ions diffuse in to reach the threshold potential, voltage-gated
Na+ ion channels open.
Ill. The postsynaptic membrane is now depolarised. An action potential is triggered.
B. Inhibitory postsynaptic potential:
I. Stimulates the opening of ligand-gated CI- ion channels.
Il. Cl- ions diffuse in.
III. The postsynaptic membrane is now hyperpolarized. This makes it more difficult to generate a new action potential.
stages in synaptic transmission- break down
Neurotransmitters are broken down, so the postsynaptic membrane can repolarise in order to generate another action potential.
stages in synaptic transmission- reabsorption
Broken down neurotransmitters are reabsorbed into the presynaptic knob for resynthesis.
