6.5 NEURONS & SYNAPES

Neurons: Nervous System cells

Neurons carry messages in the form of rapid electrical impulses

Myelinated nerve cell fibres have a myelin sheath and the small gaps are called Nodes of Ranvier

The Nodes of Ranvier allow nerve impulses to jump from node to node

Saltatory Conduction: Nerve impulses jump from node to node which speeds up transmission

Nerve impulses are conducted along the Neuron

Nerve impulse results because of the change in Na ions and K ions across the membrane of an neuron

These changes are called Depolarization and Repolarization which make up Action Potential

Resting Potential: Voltage (electrial potential) across the plasma membrane of a neuron when it is NOT conducting a nerve impulse

Resting Potential: -70 millivolts 

• Using Sodium-Potassium pumps in the membrane so ions are pumped by Active Transport
• Sodium ions are pumped out
• Potassium ions are pumped in
• Concentration gradients of both ions are established
• Some Potassium ions diffuse back OUT - outside membrane potential more positive
• The inside of the neuron develops a net-negative charge compared to the outside because the presence of chloride and other negatively charged ions - THEREFORE THERE IS A POTENTIAL ACROSS THE MEMBRANE - Resting Potential

Once a current reaches a certain threshold of -50 millivolts Action Potential takes place

Action Potential: Facilitated diffusion of ions through voltage gated ion channels

2 parts Depolarization and Repolarization

Depolarization: Membrane potential rises from -70 to -50 millivolts

Voltage gated sodium channels open and ions goes in the membrane (down concentration gradient)

• Inside of neuron now has a net positive charge compared to the outside

Depolarization: Potential across membrane is reversed

Repolarization

Voltage gated potassium channels open and ions diffuses out of the membrane

• Inside of neuron to develop and net negative charge compared to the outside

Repolarization: Potential across membrane is restored

• Resting Potential: Voltage (electrical potential) across a membrane of a neuron when it is not conducting an impulse

• Action Potential: Depolarization of electrical potential as nerve impulse passes along

• Action Potential: Repolarization of electrial potential as nerve impulse passes along

Resting Potential: Voltage (electrical potential) across a membrane that is not conducting an impulse

• Using sodium-potassium pumps in the membrane - pumping happens by Active Transport
  

• Sodium ions are pumped out
• Potassium ions are pumped in

• Concentration gradients of both ions are established

• Some potassium ions diffuse back out, leaving the outside membrane potential more positive

• The inside of the neuron develops a net negative charge compared to the outside  because some Chlorides and other negatively charged particles.
  

Depolarization of electrial potential as nerve impulses passes along

• Membrane potential reaches -50 millivolts
• Voltage gated sodium ion channels open and Na diffuses out/goes down concentration gradient. This causes the inside of the neuron to have a net positive charge compared to the outside

Repolarization of electrical potential as nerve impulse passes along

• Voltage gated potassium ion channels open and K ions diffuse out/goes down concentration gradient
• This causes the neuron to develop a net negative charge

Propogation of the nerve impulse: An action potential in one part of the axon triggers another action potential in the next part

Propogate: Spread

This is due to the diffusion of sodium ions between a region of action potential and the next region which is still at resting potential

Local currents: Diffusion of sodium ions along the axon both inside and outside of the membrane

Local currents change the voltage across the membranes from resting potential of -70 mv to the threshold potential of -50 mv. This causes an Action Potential because voltage gated sodium channels open

Synapse: Junction between two neurons or a neuron and effector cell (muscle)

Synaptic Cleft is the space between the neurons or the neuron and effector cell

Messages are passed across the synapse in the form of chemicals called neurotransmitters

Neurotransmitters move from the pre-synaptic neuron to the post synaptic neuron

At the dendric end end of a nerve cell each dendrite collects the nerve impulse from the terminal end of a different nerve cell. The impulse needs to jump across a small gap (synapse) by the process of synaptic transmission

The electrical impulse is converted to a chemical neurotransmitter

• A nerve impulse reaches the end of the pre-synaptic neuron

• Depolarization of the pre-synaptic membrane causes vesicles of neurotransmitters to move to the pre-synaptic membrane and fuse with it, releasing the neurotransmitter into the synaptic cleft by exocytosis

• The neurotransmitter diffuses across the synaptic cleft and binds to receptors in the post-synaptic membrane

• The receptors are transmitter gated sodium channels - opens when the transmitter binds. Sodium ions diffuse into the post-synaptic neuron. This causes depolarization of the post-synaptic membrane

• The depolarization passes down the post-synaptic neuron as an Action Potential

• Neurotransmitter in the synaptic cleft is rapidly broken down to prevent continuous synaptic transmission

• Nerve impulse (AP) travels to the end of the pre-synaptic neuron and triggers the influx of Calcium

• This triggers the synaptic vesicles to fuse with the membrane

• Neurotransmitters are releseased into the Synaptic cleft by diffusion

• The neurotransmitters bind to the postsynaptic neuron

• Sodium diffuses into the postsynaptic neuron leading to depolarization

• Calcium is pumped back into the synaptic cleft by Active Transport

• Acetylecholine, Serotonin and Dopamine examples of neurotransmitters

Active Transport

• Sodium-potassium pump resets resting potential in the axon following a nerve impulse
• Re-uptake of neurotransmitters to the pre-synaptic neuron following synaptic transmission
• Removal of Calcium from the pre-synaptic neuron following synaptic transmission

Simple Diffusion

• Diffusion of neurotransmitters across the synaptic cleft
• Diffusion of K ions out of the axon in resting potential

Facilitated Diffusion

• Opening of voltage gated Na and K channels in Action Potential
• Opening of the voltage gated Ca channels at the pre-synaptic terminal
• Na channels are activated at the post-synaptic terminal to propogate AP

Vesicle Transport

• Influx of Calcium activates vesicles of neurotransmitters
• Exocytosis of neurotransmitters from the pre-synaptic neuron to the synaptic cleft

• Action Potential activates voltage-gated sodium channels
• Sodium ions rush into axon
• Potential increases
• If potential increases beyond threshold more sodium voltage gates open
• The axon depolarizes stimultating adjacent sections
• Potassium channels open and potassium rushes out
• Potential is reduced (repolarization)
• Refractory period ensure a one way conduction of action potential
• Sodium-potassium pump returns  axon section from action potential to resting potential

Synapes do not all use the same neurotransmitters

Many do use Actylecholine which activates mucles

• Pre-synaptic neuron secretes acytlecholine into the synaptic cleft
• Actylecholine diffuses across synapse
• Binds to receptors in the post-synaptic neuron
• Acytlecholine is broken down into acytle and choline in the cleft by an enzyme Cholinesterase in the cleft
• Choline is reabsorbed by the pre-synaptic neuron

• A type of insectide similiar to nicotine

Bind to post-synaptic receptors in insects that normally accept the neurotransmitter Actylecholine

It is not broken down by Cholinerstrase

Therefore the receptor becomes permenantly blocked and actylecholine cannot bind - this causes the paralysis of the insect and can possibly lead to death.