Synaptic and Junctional Transmission

Transmission of impulses between nerves or nerves and muscle or gland

Types of transmission:

1. Electrical synapse or Gap junction - action potential passes directly from cell to cell, connexon proteins form tunnel that connects cells' cytoplasm, 2-way transmission, allows for synchronization of activity, faster than chemical, occurs in smooth and cardiac muscle
2. Chemical synapse - action potential produces a chemical signal that crosses the space and produces a new action potential, 1-way, allows for modification

Chemical Synapse

Transmission is not jumping of action potential but a complex chemical process permitting grading and modulation (frequency change) of neural activity

Components of chemical synapse:

1. Presynaptic neuron - neuron sending the impulse
   1. Axon of presynaptic neuron terminates on the soma or dendritic region of the postsynaptic neuron
   2. Axon ends in terminal branches with synaptic knobs that contain many mitochondria and vesicles of a chemical neurotransmitter
   3. Number of knobs per cell varies, (1-40,000)
2. Synaptic cleft - space between cells across which an impulse must be transmitted
   1. No direct connection, about 20-50 nm space between presynaptic and postsynaptic
3. Postsynaptic neuron - neuron receiving impulse
   1. Neurotransmitter produces the action potential
   2. Divergence - axon divides into many terminal branches and projects to many postsynaptic neurons
   3. Convergence - neuron may receive input from thousands of other neurons
      1. Oscillating circuit - neuron provides positive feedback to presynaptic neurons, prolongs response to stimulus

Transmission process

1. Presynaptic action potential causes voltage-gated Ca⁺² channels to open in synaptic knobs
   1. Ca⁺²   diffuses in along its concentration gradient
   2. Ca⁺² stimulates exocytosis of vesicles and releases neurotransmitter into the cleft
2. Transmitter diffuses across cleft
   1. Synaptic delay (0.2-0.5 msec) - time between presynaptic action potential and PSP
   2. Fewer synapses produce shorter delay
3. Transmitter binds to receptor sites on postsynaptic and causes Na⁺ channels to open
   1. Ligand-activated receptors specific for neurotransmitter
   2. Influx of Na⁺ produces a graded postsynaptic potential (PSP)
   3. PSP can be depolarizing (excitatory) or hyperpolarizing (inhibitory)
4. Transmitter removed from cleft
   1. By diffusion, enzymatic degradation (e.g. cholinesterase) or cellular uptake (monoamines by synaptic knobs)
   2. Na⁺ channels close
5. Synaptic fatigue - no neurotransmitter remaining in presynaptic knobs

Integration and modulation at synapse

1. Presynaptic output - by modifying the quantity of neurotransmitter released
   1. Facilitation - axon of a 2^nd neuron synapses with the presynaptic axon
      1. 2^nd neuron releases excitatory neurotransmitter that increases number of presynaptic vesicles of 1^st neuron to exocytose
      2. enhances and prolongs effects on postsynaptic neuron, ex. Serotonin
   2. Inhibition - axon of a 2^nd neuron synapses with the presynaptic axon
      1. 2^nd neuron releases inhibitory neurotransmitter that decreases transmitter released by 1^st neuron
2. Postsynaptic input: by summation of PSPs, additive effect
   1. Neurotransmitter released from 1 synaptic knob produces a small PSP at one location, insufficient to produce action potential and gradually decays
      1. Depolarizing PSP is called an Excitatory Postsynaptic Potential or EPSP, result of chemical-gated Na⁺ channels opening, lasts about 20 msec
      2. Hyperpolarizing PSP is called an Inhibitory Postsynaptic Potential or IPSP, result of chemical-gated K⁺ or Cl ^-channels
   2. Summation effects
      1. If the sum of EPSPs minus the sum of IPSPs exceeds stimulus threshold then action potentials will be generated at the initial segment of the axon as long as the sum is above threshold
      2. Subthreshold EPSPs and IPSPs decay
      3. If IPSPs are greater, neuron is unable to generate any action potentials
   3. Types of summation
      1. Spatial - large number presynaptic terminals fire at same time
      2. Temporal - same presynaptic terminals fire in rapid succession
3. Types of neurotransmitters
   1. Acetylcholine - ACh, is excitatory at skeletal neuromuscular junction, is inhibitory in vagus nerve to the cardiac muscle
   2. Catecholamines\monoamines - epinephrine, norepinephrine and dopamine can be excitatory or inhibitory depending on the receptors
   3. Amino acids - GABA and glycine are inhibitory in the brain. Glutamate and aspartate are excitatory