Synapse

Synapse:

• A junction that mediates information transfer from one neuron to:
  - another neuron
  - or an effector cell

Axodendritic Synapse

• presynaptic neuron that then forms a synapse with the dendrites of the target cell
  - very common

Axo Somatic synapse

• presynaptic neuron and here it synapses onto the soma of the target neuron
  - very common

Axonaxonic synapse

• presynaptic neuron synapses onto the axo hillock
  - very rare

Pre/Post synaptic neuron

• Presynaptic neuron: conducts impulses towards the synapse (brings information towards the junction) and synapse onto post synaptic neuron
  has voltage gated channels all the way down its axon and down to axon terminals, when these channels re activated they create action potential

• Postsynaptic neuron: transmits impulses away from the synapse (receives information)

• talk to each other using neurotransmitters

Electrical synspases

• result of gap junctions in the membranes of two adjoining cells

• not common

• very rapid

• unidirectional or bidirectional

• are important in:
  - ebryonic nervous tissue
  - some brain regions
  - heart

Chemical synapses

• the normal ones you think of in the nervous system

• pre-synapse and post synapse

• space between is called synaptic cleft

• specialized for the release of reception of neurotransmitters

• typically composed of two parts
  - axon terminal - presynaptic neuron
  - Buttons
  - has calcium voltage gated channels and are triggered when membrane potential changes
  - does not physically touch post synaptic neuron
  - has mitochondria, vesicles (store neurotransmitters), channels in the membrane, calcium voltage gated channels
  - receptor region- postsynaptic neuron

How do neurons communicate?

• this occurs at a synapse where button contains ntm and is released into the cleft

• the vesicles with the neurotransmitter they have to dock with membranes via T-snares and V-snares to perform exocytosis which cause release of neurotransmitter into the synaptic cleft

• synaptic cleft is occupied by interstitial fluid

• post synaptic neuron has receptors (ligand chemically gated channels)

• neurotransmitters goal is to create a graded potential on the post synaptic membrane and the magnitude gets determined by the amount of ntm and the frequency of the ntm that binds to these receptors which allows ions to flow across the membrane

• the flow of calcium triggers the vesicles to dock with the membrane, T-snares dock with V-snares, and exocytosis occurs and ntm gets released into synaptic cleft

• NTM diffuse down concentration gradient towards target site

• NTM then bind to these receptors which cause receptors to change shape and when this happens the receptors open and allow ions to flow across membrane

• this causes positive change in membrane potential and graded potential occurs

• ntm communicates with post synaptic neuron by creating a graded potential

• action potential on presynaptic neuron causes graded potential on post- synaptic neuron, if adequate it generates action potential

Why does communication between neurotransmitters take place?

• result of the secretion of neurotransmitters by the presynaptic neuron

• this then activates receptors on past synaptic neuron

Vesicles

• storied within button

• contain neurotransmitter
  - produced in soma

Ligand gated channels

• membrane of postsynaptic

• receptors associated with receptive areas

How can we stop presynaptic neuron signaling to the post-synaptic neuron?

• presynaptic neuron releases ntm when it has action potential but when there is no action potential no ntm’s are realesed

• you have to stop stimulating the presynaptic neuron, the ntm will stop secreting

• stop generating action or greater potentials on the postsynaptic neuron

• when ntm are realesed into synaptic cleft, it can bind a neurotransmitter or bind a receptor and create a greater potential on the target cell

What happens after this?

• can re uptake the ntm into the presynaptic neuron

• enzymtic degradation

• neurons are enzymes present in the synaptic cleft
  - enzymes can break down NTM so that its no longer capable of stimulating the p0stynaptic membrane

• ntm can diffuese out of synaptic cleft and move down concentration gradient, in which case they can go from the cleft to the concnetration gradient and this means they can go from the cleft to the intersistual fluid space outiside of the cleft