Saltatory conduction and synapses

What is Myelin sheath?

Myelin sheath made of Schwann cells.

Sections covered by myelin sheath depolarization

cannot occur as it prevents the movement of sodium

and potassium ions

What are nodes of ranvier?

• small sections which are not insulated

• These are sections of membrane with cluster s of ion pumps and channels.

• These pumps and channels allow action potentials to occur

• Action potential jumps from one node to the next

structure of synapse

• A gap between the neurones known as the synaptic cleft 

• The neurone before the synapse is known as the presynaptic neurone and has a rounded end known as the synaptic knob

• The neurone after the synapse is known as the postsynaptic neurone

• Nerve impulses are passed across the synaptic cleft by the diffusion of chemicals known as neurotransmitters e.g. acetylcholine

  • Neurotransmitters are contained within vesicles in the synaptic knob

Roles of synapses?

• unidirectionality of impulse transmission

• Divergence of nerve impulse

• Amplification of nerve signals by summation

Role of synapses - uni directionality of transmission

• synapses enable uni-directionality of impulse transmission

• Ensure one -way transmission of impulses

• Impulses can only pass in one direction at synapses because neurotransmitter is released on one side and its receptors are on the other, chemical transmission cannot occur in the opposite direction

divergence of nerve impulse at synapse

• divergence of nerve impulses

• One neurons can connect to several other neurons at a synapse, allowing nerve signals to be sent in several directions from a single presynaptic neurone

How synapses cause amplification of nerve signals by summation

• Impulse arrives at a synapse it does not always cause an impulse to be generated in the next neurone; a single impulse that arrives at a synaptic knob may be insufficient to generate an action potential in the post-synaptic neurone

• Only a small amount of acetylcholine may release into the synaptic

cleft

• A small number of sodium ion channels are opened in the

postsynaptic axon membrane

• An insufficient number of sodium ions pass through the membrane

• The threshold potential is not reached

• multiple impulses added together to over come this-known as summation

How can summation be achieved

• Several presynaptic neurones converging to meet a

single postsynaptic neurone

• This is known as synaptic convergence

• Many action potentials arriving at a postsynaptic

knob in quick successio

synaptic transmission-diffusion to presynaptic neurone

• Electrical impulses cannot ‘jump’ across the synaptic cleft

• When an action potential arrives at the end of the axon of the presynaptic neurone the membrane becomes depolarised, causing voltage gated calcium ion channels to open

• Calcium ions diffuse into the synaptic knob via calcium ion channels in the membrane

• The calcium ions cause vesicles in the synaptic knob to move towards the presynaptic membrane where they fuse with it and release chemical messengers called neurotransmitters into the synaptic cleft by exocytosis

  • A common neurotransmitter is acetylcholine, or ACh

excitatory post synaptic potential

• Neurotransmitter diffuses across cleft

• Attached to specific receptor site on the sodium channel

• If binding results in opening of sodium ion channels sodium enters causing depolarization.

• This is known as an excitatory post synaptic potential.

• A number of EPSPs may be needed for the membrane to reach threshold potential.

• If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane, then an action potential is generated, which then travels down the axon of the postsynaptic neurone

how likelyhood of action potential being generated is increased

depends on the number of action potentials arriving at the presynaptic knob

• Many action potentials will cause more neurotransmitter to be released by exocytosis 

• A large amount of neurotransmitter will cause many sodium ion channels to open

• Many sodium ion channels opening will allow a large influx of sodium ions, increasing the likelihood of threshold being reached

what is inhibitory post synaptic potential

Neurotransmitters can also have opposite effect.

They can bind to different ion channels and cause the opening of ion channels which allow influx of negative ions.

In this case the membrane becomes hyperpolarized and this makes an action potential less likely to occur.

what are cholinergic nerves?

• Actelycholine is a common neurotransmitter and it is found at the majority of synapses.

• It is made in the synaptic knob (inside presynaptic neurone)

• Many mitochondria present as it requires ATP to make it.

• Nerves that use acetylcholine as a neurotransmitter are known as cholinergic nerves.

• Break down of acetylcholine by acetylcholinesterase, hydrolysis reaction.

how acetycholinesterase hydrolysis reaction

• Breakdown of acetylcholine ensures that no longer affects the

postsynaptic membrane.

• Acetate and choline can then move by diffusion back to presynaptic knob to be reabsorbed into the neurone.

• Resynthesizes acetyl choline.

• Acetylcholine is the neurotransmitter for all nerves of the parasympathetic nervous system and all motor neurons.

what nerves use noradrenaline

Some nerves use noradrenaline in as their neurotransmitter. These are known as adrenergic nerves.

how noradrenalin works

• Noradrenaline will only bind to receptors on the postsynaptic

neurone when its in high concentration.

• Once the presynaptic knob stops releasing noradrenaline the levels will fall and then as the concetration has decreased it is released from the receptors.

• It is then reabsorbed by the presynaptic knob and repackaged into vesicles.