Synapses

What is a synapse:

A synapse is a gap where information is transmitted from one neuron to another



Presynaptic Neuron: Neuron transmitting signal (**Most neurons are both presynaptic and postsynaptic)

Postsynaptic Neuron: Neuron Receiving signal (**Most neurons are both presynaptic and postsynaptic)


Charles Scott Sherrington (1857-1952) Reflex arc: circuit from sensory neuron to muscle response 


Sherrington found the circuit arc by:

Pinching a dogs foot which would make the dog extend its foot


Temporal summation: Repeated stimuli in the same location in rapid succession


Spatial summation: Stimuli in several close locations at one time



















Circuit arcs diagram

A single circuit that is responsible for reflexes


How does a reflex circuit work:

Skin sensory neurons feel the stimulus, Which then sends this to the spinal cord. Activating the motor neurons responsible for making the reflex.


Knowing this, Sherrington knew he could calculate how long the synapse takes to pass the through the synapses.






Sherringtons observations


1. (Sherringtons observations) Reflexes are slower than conduction along an axon: The fact that the reflex arc is slower than conduction along the axon would've been, proves the synapses.




2.(Sherringtons observations) Several weak stimuli present at slightly different times OR locations produce a stronger reflex than a single stimulus: Graded potentials can stack and multiple small stimuli (like light tickling) can create a stronger reflex then a single stimulus.




Graded potential: a small stimulus that changes the voltage in the membrane, but does not activate the threshold of excitation. (Small amounts of sodium are released into the axon, but not enough to reach the threshold of excitation)



  • Multiple graded potentials can add up which can cause an action potential (by reaching the threshold of excitation)



3. (Sherringtons observations) When one set of muscles becomes excited…: another set relaxes.


  • Excitatory synapses: Make a neuron more likely to fire

  • Inhibitory Synapses: Make the neuron less likely to fire


  • Excitatory postsynaptic potential (EPSP): A graded depolarization

    • Caused by positively charged sodium entering the neuron


  • Inhibitory postsynaptic potential (IPSP): A graded hyperpolarization

    • Caused by negatively charged chloride ions entering the neuron

  • Spontaneous Firing Rate: Neurons are always firing at different rates even without synaptic input





Otto Loewi (1873-1961) Frogs were used in research due to:

Well studied anatomy, Low ethical concern, Lots of them

How did Otto Loewi (1873-1961) use frogs in his research, to prove that nerves send messages through chemicals: He took the fluid from one frogs heart, which he then successfully used to stimulate another frogs heart



To test the idea of chemical synapses, Loewi:

He stimulated a frogs heart, then took the fluid from that first frogs heart, transmitting it into another frogs heart, which then stimulated that one.

  • From this he concluded that nerves send messages by releasing chemicals


Vagus nerves: slow the heart rate down

Accelerator nerve: accelerates the heart rate


(Where can synapses occur)

  1. (Where can synapses occur) On dendrites: Axodendritic synapses (Most important)

  2. (Where can synapses occur) On the soma: Axosomatic synapses

  3. (Where can synapses occur) On other axons: Axoaxonic synapses


Components of the synapses

Presynaptic membrane: The membrane of axon

terminal (Where neurotransmitters are released.)

Postsynaptic membrane: The membrane opposite the axon terminal button in a

Synapse (Receives the message.)



Synaptic cleft: Space between presynaptic and postsynaptic membrane.


Synaptic vesicle: A small, spherical hollow organelle;

contain molecules of a neurotransmitter




Neurotransmitter: Chemical messengers that carry chemical signals


Ligand: A neurotransmitter that binds.


 Ligand-Gated Ion Channels: Transmembrane ion-channel

proteins which open to allow (e.g., Na⁺, K⁺, Cl⁻) to pass through membrane in response to the binding of a chemical messenger



Steps of what happens at a synapse - REMEMBER THE DIAGRAM FOR EXAM

  1. Step #1 what happens in the synapse: Action potential arrives at axon terminal

  2. Step #2 what happens in the synapse: Voltage gated calcium channels open and calcium(Ca) enters the axon terminal

  3. Step #3 what happens in the synapse: Entry of the Ca causes neurotransmitter-containing synaptic vesicles to released contents by the process of Exocytosis (the process of the bubble breaking)

  4. Step #4 what happens in the synapse: Neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels on the postsynaptic membrane

  5. Step #5 what happens in the synapse: Binding of neurotransmitter opens ligand- gated ion channels, resulting in graded potentials

  6. Step #6 what happens in the synapse: Reuptake by the presynaptic neuron, enzymatic degradation and diffusion reduce neurotransmitter levels, terminating the signal