Synapses Flashcards
Features of synapses
- One way conduction
- Synaptic delay
- Summation
- Fatigue
- Occlusion
- Subliminal fringe
- Facilitation
- Divergence
- Convergence
- Reverberatory circuits
Anatomical classification of synapses
- Axoaxonic
- Axodendritic
- Axosomatic
Chemical classification of synapses
- Electrical
- Chemical
- Conjoint
Electrical synapses found in
cardiac muscle fibres
smooth muscle fibres of intestine
epithelial cells of lens of eye
Synaptic delay time
0.5msec for 1 synapse
Define summation
Facilitated response to repeated stimuli applied simultaneously or one after another
Properties of EPSP/IPSP
- can be summated
- local response
- does not get propagated
- does not follow all-or-none law
- due to opening of ligand gated Na+ or Ca+2 influx or closure of K+ channels for EPSP
- due to Ca+2 influx or K+ efflux or closure of Na+ channels for IPSP
Divergence is found in
- Reticular Activating System
- SNS
- Focal epileptic seizures
Convergence is found in
- Anterior horn cells
- Sensory tracts
- Retinal to visual cortex
Define reverberatory circuits and examples
Activity in neuronal circuits continues for longer periods even after cessation of stimulus
- Short-term memory
- Smooth decline and termination of movement
Define plasticity
- Changes that occur in synaptic function after repeated stimulus
- may be excitatory or inhibitory
- only by chemical syn.
- due to functional or anatomical modifications
- pre/post synaptic changes
Mechanisms of synaptic plasticity
- PTP
- LTP
- Habituation/Fatigue
- Sensitization
- Long term depression
PTP
When an excitatory presynaptic neuron is stimulated for a short time by tetanizing current, the synapse becomes more excitable after cessation of stimulus.
- inc. in post synaptic potentials
- due to accumulation of lots of Ca+2 in presynaptic endings
- helps in short term memory
LTP
Appears due to prolonged increase in excitability of synapse after brief tetanic stimulation of presynaptic afferents
- inc. glutamate release
- inc. Ca+2 influx into post syn. terminal
- NO produced in post syn. diffuses into pre syn. terminal to stimulate glutamate release
- permanent memory (hippocampal neurons)
Habituation
Repeated application of stimulus causes response to disappear.
Due to gradual inactivation of Ca+2 channels and reduction in NT release
Sensitization
When habituated stimulus is coupled with noxious stimulus there is increase in post synaptic response
Due to inc. in cAMP via inc. in Ca+2 levels
Long term depression
Appears with repeated stimulation of inhibitory afferent inputs
Due to partial or total inactivation of Ca+2 channels
Caused by NO in cerebellum
