Properties of Synapse

1) One way conduction (or law of forward conduction by Sherington)

 Synapse permits transmission of impulse only in one direction from pre synaptic to post synaptic neuron. It is extremely important for orderly organization of neural functions Although in electrical synapse transmission can be in both ways.

 2) Synaptic delay

When an impulse reaches the pre synaptic terminal there is minimal time interval before responses (initial depolarization) is obtained at post synaptic membrane. This time is due to release of synaptic transmitter and its action on postsynaptic receptor. Usual delay at one synapse is about 0.5 msec. Thus this time interval can be used to calculate number of synapses in the neural pathway. Although electrical synapses hardly show any synaptic delay

3) Fatigue

When an excitatory synapse is repetitively stimulated, in first few stimuli response is increased (due to beneficial effect) but eventually response becomes progressively less and less. It is mainly due to depletion of synaptic transmitter and partly due to progressive decrease in post synaptic receptors and abnormal build up of ions inside postsynaptic neuron. Synapse is first seat of fatigue during muscular exercise. Fatigue property has great protective value when neural discharges are abnormally high like in epilepsy,

4) Convergence and Divergence

Post synaptic neuron received multiple presynaptic terminal (may be from same or different sources). This is called convergence, Advantages of convergence are (i) information from multiple sources can be integrated (ii) Special summation is possible i. e. necessary for development of AP.

 While pre synaptic neuron divides into many branches that end on many post synaptic neurons. It is called divergence

 Divergence can be (i) Amplifying type or (ii) may be in to multiple tracks

5) Spatial and Temporal summation

When the presynaptic terminals, which are nearly situated on a membrane, are stimulated simultaneously their effects are summed. It is called spatial summation. Normally when a single presynaptic is stimulated it produces very small EPSP (0.5 to 1mv) far less than threshold but when many terminals are simultaneously, stimulated the summed EPSP crosses the threshold of excitations and AP is generated

 When a single presynaptic terminal is repetitively and rapidly stimulated, the effects produced by impulses are added. It is called temporal summation. When EPSP or IPSP is produced it stays for about 15msecs during this time if next EPSP or IPSP is generated, summation of the effects occurs.

 

6) Subluminal fringe and Occlusion 

When the neurons are separately stimulated, sum of their effects is less than the effect, which is produce when they are simultaneously stimulated. This property is called subluminal fringe.

- E.g. Suppose nerve B' (presynaptic) is supplying 3 motor neurons (postsynaptic). But out of these 3 motor neurons, the nerve is providing threshold stimulus only for 1 neuron. For the other 2 neurons, it is providing sub threshold stimuli. Hence if the nerve 'a' is stimulated, only 1 motor neuron responds and in the other 2 neurons, excitatory changes are produced but not amounting to response Similarly there is another nerve 'C' also supplying 3 motor neurons. Here also, suppose the nerve is threshold for 1 neurons and sub threshold for other 2 neurons. Hence if nerve 'b' is stimulated, 1 neuron will respond and only excitatory changes are produced in other 2 neurons.

 Hence sum of excitation of B' and 'C' when stimulated separately is a+b=1+1=2.

 The 2 neurons in which sub minimal changes are produced lie in common neuronal pool i.e they receive fibers from both 'a' and 'b' nerves. Now, if 'a' and 'b' are stimulated together, a+b= 4 motor neurons respond. This is because two subliminal stimulus from 'a' and 'b' summate and become minimal stimulus for 2 neurons in common neuronal pool and 4 neurons respond

When the neurons are separately stimulated, sum of their effects is more than the effect which is produce when they are simultaneously stimulated. This property is called occlusion. Ex.-

-E.g. when nerve 'B' is excited, 3 motor neurons (postsynaptic) respond. When nerve 'C' is excited, again 3 motor neurons respond. Thus total of motor neurons responding when 'a' and 'b' are stimulated separately is a + b =3+3=6 But when 'a' and 'b' are stimulated together only a + b =4 motor neurons respond. This is because 2 motor neurons are common to both 'a' and 'b' nerves. These common motor neurons are called as 'common neuronal pool' Motor neurons in common neuronal pool have double nerve supply ('a' and 'b').

7)Facilitation and presynaptic facilitation

It is excitatory or easily excitable state of neuron. When a sub threshold stimulus is applied to postsynaptic neuron although AP is not produced but EPSP is generated. In this state the postsynaptic neuron can be stimulated even by a sub threshold stimulus

 When a presynaptic terminal received axo- axonic excitatory discharge from a neuron, it is facilitated before it reaches to the postsynaptic neuron. Now it produces larger EPSP on postsynaptic neuron. In majority of the cases, neurotransmitter involving in presynaptic facilitation is serotonin (this is basis of intermediate long term memory).

8)After Discharge

When postsynaptic neuron continues to discharge even after pre synaptic neuron has stopped sending the impulse. After discharge can be of 3 types. a) Synaptic after discharge: due to long acting synaptic transmitter

 b) Parallel circuit type: due to diversion of impulse into many pathways which have different numbers of synapses

 c) Reverberatory or oscillatory circuit type: due to reverberatory or oscillatory circuit in which a branch of the axon of postsynaptic neuron re excites the same neurons. Ex. oscillatory circuit is possible explanation of short term memory, important circuits as lead to continuous neural discharge (basal ganglia) and rhythmic neural discharge (respiratory centre).

9)Synaptic Inhibition

2 types

a) Direct or postsynaptic inhibition- when Presynaptic neuron secrete inhibitory synaptic transmitter which hyperpolarizes the postsynaptic neuron by producing IPSP. Ex. Reciprocal inhibition of Sherington during Stretch reflex, Renshaw cell inhibition etc.

 b)Presynaptic inhibition- here inhibition occur in the presynaptic neuron by a inhibitory axo-axonic terminal before reaches to the synapse. Most of the time synaptic transmitter is GABA which produces IPSP at Presynaptic neuron and reduces its effect on postsynaptic neuron

 10) Synaptic block

Certain chemicals can compete at receptor sites with synaptic transmitter and can reduce its effect. It is called synaptic block. Ex. - strychnine and tetanus toxoid do abolish IPSP produced by GABA. Strychnine can also block effect of glycin at spinal cord level and can lead to abnormally high neural activity

11) Plasticity ( Learning ) function of synapse

Synaptic function can change due to past experience

 Ex.-a) Habituation-if the stimulus is non- injurious or unimportant and applied repeatedly, the response progressively becomes lesser and lesser and may cease. It is called Habituation

 Ex.-b) Sensitization- if the stimulus is injurious or important and applied repeatedly, the response progressively becomes more and more. It is called Sensitization.

 Ex.-c) Post tetanic potentiation- after a trail of stimuli (tetanic stimulus) Presynaptic neuron respond more to the intensity of stimulus before this and response may last as much as 60 secs. Cause is increase level of Ca++ in the Pre synaptic terminal.

12) Factors affecting synaptic excitability and sensitivity

 Alkalosis, tea, coffee increases synaptic excitability while hypoxia and anesthetic drugs decrease it. In short-term and routinely synaptic sensitivity is regained by rest while long-term change in synaptic sensitivity is due to up regulation or down regulation of postsynaptic receptors