Synaptic Transmission

Types of synapses

• chemical

  • in CNS most synapses are chemical

  • >99%

  • slower, modulatable, dominant in cognition

• electrical

  • <1%

  • fast, synchronous, rare

Synapse

• point of contact between cells where signals are passed on

• typical neuron of the cortex creates thousands of synapses with other neurons

• dendrites and cell body are covered with them

• axons usually have synapses only at the axon terminal

Chemical synapse

consist of 3 parts:

• presynaptic cell

  • contains vesicles - store neurotransmitters (chemical substances)

  • presynaptic membrane of axon terminal

• synaptic cleft

• postsynaptic cell

  • membrane of target cell

Process of synaptic signal transmission

• Action potential arrives at axon terminal

→ causes voltage change in the presynaptic membrane

→ calcium channels open: calcium ions rush in

→ calcium influx causes vesicles to dock at presynaptic membrane

→ vesicles release neurotransmitters into synaptic cleft

→ neurotransmitters bind to specific receptors on postsynaptic membrane → causes voltage changes in postsynaptic membrane (De- or Repolarization)

• depending on the receptor, transmitters have excitatory or inhibitory effect

Excitatory post-synaptic potential (EPSP) 1

excitatory effect - AP can be evoked

• neurotransmitters bind to specific receptors on postsynaptic membrane

  • sodium (Na+ ) channels open in the postsynaptic membrane

    → Na+ ions rush in

    → positivity causes a small depolarization of the cell membrane = EPSP

• ! Excitation of neuron caused by depolarization of the membrane

Excitatory post-synaptic potential (EPSP) 2

• mechanism of triggering an Action potential at Axon hillock same as the propagation of an AP along the axon

• transmitter bind to receptor of postsynaptic membrane → causes Na+ channels to open: influx of Na+ ions • Na+ ions move to Axon hillock

  • once Na+ ions reach Axon hillock, membrane becomes less negative

  • when threshold of -55mV is reached Na+ channels open

    → Na+ influx, i.e. an action potential occurs & spreads along the axon

Excitatory post-synaptic potential (EPSP) 3

To trigger an action potential:

• numerous synapses need to be activated to have an impact at the axon hillock

• 2 ways

  • spatial summation - combined effect of different neurons all reaching postsynaptic neuron together

  • temporal summation - same synapses activated repeatedly

• location of contact: synapses closer to axon hillock have stronger influence on target cell

Inhibitory postsynaptic potential (IPSP)

Inhibitory effect - less likely to generate an AP

What happens when there is an inhibitory effect?

• Action potential reaches presynaptic membrane

  → causes calcium channels to open: influx of calcium

• → transmitter bind to specific receptors on postsynaptic membrane

• → !! chloride (Cl- ) channels open → negative ions rush in

• → causes hyperpolarisation (inside of membrane becomes more negative) = IPSP

• ! Inhibition of neuron caused by hyperpolarisation of the membrane

IPSP Function

Balance between excitatory and inhibitory neurons

• preventing excessive firing of neurons

• e.g. Epilepsy - this balance doesn’t exist = neurons fire uncontrollably

Neurotransmitters

• maintain signals in the nervous system by binding to receptors on post-synaptic neurons

• all serve a different purpose in the brain and body

Glutamate

• involved in learning & memory

  • regulates development & creation of new nerve pathways

GABA

(gamma-aminobutyric acid)

major inhibitory neurotransmitter

Serotonin

involved in regulating body temperature, sleep, mood, appetite, & pain

• reduced → e.g. depression, aggressiveness

Dopamine

• involved in controlling movement & posture

  • reduced → e.g. Parkinson’s Disease

• affects cognition

  • reduced → e.g. poor concentration

• affects behaviour

  • increased → e.g. schizophrenia

Acetylcholine (ACh)

PNS

• neurotransmitter at neuromuscular junctions

• triggers muscle contraction

• Myasthenia Gravis (muscle weakness) = blocked/attired/destroyed ACh receptors

• Botox - prevents release of ACh from axon terminals at neuromuscular junction

CNS
- neuromodulatory fxn - can be inhibitory or excitatory

Electrical synapses (gap junctions)

• less common

• pre- & postsynaptic membranes are connected

  → presynaptic cell affects postsynaptic cell directly & electrically → electrical signal travels straight over to the other cell, evoking depolarization

• → postsynaptic reaction arises almost at the same time as the presynaptic potential

  • evokes a more synchronized effect; faster (~0.1-0.5ms) than chemical synapses (~1-5ms)

• Present in the heart, reflex pathways, retina, and some medulla connections