Synapses and drugs

Charles Sherrington major discoveries

• Reflexes are slower than conduction along the axon

• Temporal/Spatial summation

• Inhibitory synapse

Polysynaptic

sensory neuron excites an intrinsic neuron, which excites a motor neuron, which excites a muscle

Reflexes are slower than conduction along an axon

• difference in the speed conduction in a reflex arc from previously measured action potentials

• believed the difference must be accounted for by the time it took for communication between neurons

• evidence validated this

• synaptic delay

  • myelination influences speed

  • thicker axons = faster conduction

Temporal summation

Repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimulus is too weak

Presynaptic neuron

neuron that delivers the message

Postsynaptic neuron

neuron that receives the message

Excitatory postsynaptic potential

graded potential that decays over time and space relate to the cumulative effects of temporal and spatial summation (depolarisation)

makes postsynaptic cell more likely to fire and action potential

Spatial summation

• several weak stimuli in a similar location produce a reflex response when a single stimulus does not

• synaptic input from several locations can have a cumulative effect and trigger a nerve impulse

both summations are critical to brain function

• neurons conversing there info to one

• Each neuron receives many incoming axons that frequently produce synchronised responses

• Temporal summation and spatial summation ordinarily occur together

• The order of a series of axons influences the results

Inhibitory postsynaptic potential

• the temporary hyperpolarisation of a membrane

• servers as an active brake that supresses excitations

• break being put on the muscle message to inhibit the go message to a muscle

EPSP, IPSP and action potentials

• Sherrington assumed that synapses produce on and off responses

• synapses vary enormously in the duration of effects

  • The effect of two synapses is not simply double the effect of either one alone

  • Allows for the complexity of neural networks

Sequence of events at a synapse

• the neuron synthesises chemicals that serve as neurotransmitters

• Action potentials travel down the axon

• Released molecules diffuse across the cleft, attach to receptors and alter the activity of the postsynaptic neuron

• The neurotransmitter molecules separate from their receptors

• the neurotransmitters may be taken back into the presynaptic neuron recycling or diffuse away

• some postsynaptic cells may send reverse message to say they got the message

Neurotransmitters

• chemicals that travel across the synapse and allow communication between neurons

• chemical transmission predominates throughout the nervous system

• Amino Acids, modified amino acid monoamines, neuropeptides, purines, gases

Vesicles

tiny spherical packets located at the presynaptic terminal where neurotransmitters are held for release

snare proteins

anchor the vesical that holds the neurotransmitters

v-SNARES

Snare proteins on the vesical

t-SNARES

Snare proteins on the membrane

Exocytosis

• release of neurotransmitters from the presynaptic terminal into the synaptic cleft

  • Triggered by an action potential

Contransmission

• transmission across the synaptic cleft by a neurotransmitter takes fewer than 0.01 microseconds

  • most individual neurons release at least two or more different kinds of neurotransmitters

Postsynaptic receptors

• The effect of a neurotransmitter depend on its receptor on the postsynaptic cell

• transmitter gated or ligand gated channels are controlled by a neurotransmitter

• Ionotropic effects

• Metabotropic effects

Ionotropic effects

• occur when a neurotransmitter attaches to a receptor and immediately opens ion channels

• effects Occur very quickly (sometimes less than a millisecond after attaching) and are
  very short lasting

  • Rely on glutamate or GABA

  • sodium channel is waiting for a neurotransmitter to open and then closes

Metabotropic effects

• occur when neurotransmitters attach to a receptor and initiate a sequence of slower and longer lasting metabolic

• when attaches it bends the receptor protein and goes through the membrane of the cell

  • bending allows a portion of the protein inside the neuron to react with other molecules

• attach bend move dislodge than becomes a second messenger that produces some sort of change in the cell

• Metabotropic events include such behaviours as taste, smell, and pain

• long term effects

Synapses to the Axon

• EPSP and IPSP dissipate over distance and time

  • no voltage gated channels

• Action potential is only possible along the axon

  • if EPSP is strong enough at the axon then action potential will occur

  • temporal and spatial summation increase chance of exceeding threshold of excitation

• calcium gate voltage

  • calcium helps vesicles

Electrical synapse

• few special purpose synapses operate electrically

• faster than chemical

• gap junction

  • the direct contact of the membrane of one neuron with the membrane of another

  • depolarisation occurs in both cells, resulting in the two neurons acting as if they were one

Chemical synapse v electric

electric membranes come into direct contact. Ion channels remain open

Whereas chemical: Membranes are separated by the synaptic cleft. Ion channels are transmitter-gated