09 Control and Coordination - part 04 - Synapse

 

09 Control and Coordination - part 04 - Synapse


Synapse :
  • Junction between two nerve cells with a minute gap (synaptic cleft) in between them.
  • allows transmission of impulse by a neurotransmitter bridge.
Properties of nerve fibres :

1. Excitability/Irritability - 
  • Presence of a polarised membrane
  • Have the ability to perceive stimulus and enter into a state of activity.
2. Conductivity - 
  • Ability to transmit the excitation.
3. Stimulus - 
  • Any detectable, physical, chemical, electrical change in the external or internal environment which brings about excitation in a nerve/muscle/organ/organism.
  • Must have a minimum intensity called threshold stimulus.
  • Subliminal (weak) stimulus will have no effect .
  • Supraliminal (strong) stimulus will produce the same degree of impulse as the threshold stimulus.
4. Summation effect -
  • A single subliminal stimulus will have no effect 
  • If many such weak stimuli are given in quick succession, they may produce an impulse due to addition or summation of stimuli.
5. All or none law - 
  • The nerve will either conduct the impulse along its entire length or will not at all conduct the impulse, as in case of subliminal or weak stimulus.
6. Refractory period - 

  • Time interval (about millisecond) during which a nerve fails to respond to a second stimulus however strong it is.
7. Synaptic delay - 
  • Impulse requires about 0.3 to 0.5 milliseconds to cross a synapse. 
  • This time is required for release of neurotransmitter from the axon terminal and excitation in the dendron of the next neuron.
8. Synaptic fatigue - 
  • Transmission of nerve impulse across the synapse halts temporarily due to exhaustion of its neurotransmitter.
9. Velocity - 
  • Rate of transmission of impulse is higher in long and thick nerves. 
  • Higher in homeotherms than in poikilotherms. 
  • Velocity of transmission is higher in voluntary fibres (100 - 120 m/second in man) as opposed to autonomic or involuntary nerves (10-20 m/ second). 
  • Faster in medullated nerve, as the impulse has to jump from one node of Ranvier to the next. 
  • At the synapse where the neurons communicate with one another. 
  • Neuron carrying an impulse to the synapse is the pre-synaptic neuron. 
  • Neuron receiving input at the synapse is the post synaptic neuron or generator region (gland or muscle). 
  • A synaptic cleft or a small intercellular space lies in between two cells having a width about 20- 30 nm between them. .
  • The process by which the impulse from the pre-synaptic neuron is conducted to the post-synaptic neuron or cell is called synaptic transmission. 
  • It is a one way process carried out by neurotransmission.
Types of synapses :
  1. Electrical synapse
  2. Chemical synapse
1. Electrical synapse : 
  • Gap between the neighbouring neurons is very narrow. 
  • Synapse between such closed neurons is mechanical. 
  • The electrical conductive link is formed between the pre and post synaptic neurons. 
  • At the gap junction, the two cells are within almost 3.8 nm distance of each other. 
  • Transmission across the gap is faster but depends on the connection located at the gap junctions between the two neurons.
  • Found in those places of the body requiring fastest response as in the defence reflexes. 
  • Bidirectional, allowing transmission in either direction or may be unidirectional.
2. Chemical synapse : 
  • Specialized junctions through which cells of the neural system send chemical signal to the other neurons and to non-neuronal cells, such as gland and muscle. 
  • Synaptic gap is larger than that in electrical synapse.
  • It is 20-40 nm.
  • A chemical synapse between a motor neuron and a muscle cell is called a neuromuscular junction. 
  • There are three components of a typical chemical synapse - 
  1. The presynaptic terminal (mostly axonic terminal)
  2. The synaptic membrane of the post synaptic cell (usually on the dendrite of the next neuron/ gland cell/ muscle) and
  3. The post synaptic neuron.
  • Impulse travels along the axon of the pre-synaptic neuron to the axon terminal. 
  • Most presynaptic neurons or axons have several synaptic knobs at their ends or terminals. 
  • These knobs have arrays of membranous sacs, called synaptic vesicles, that contain neurotransmitter molecules.
  • When an impulse reaches a synaptic knob, voltage sensitive Ca++ channels open and calcium (Ca++) diffuses inward from the extracellular fluid.
  • The increased calcium concentration inside the cells, initiates a series of events that fuse the synaptic vesicles with the cell membrane of presynaptic neuron, where they release their neurotransmitters by exocytosis. 
  • Once the neurotransmitters bind to the receptors of the post-synaptic cell, the action is either excitatory (turning a process on) or inhibitory (turning a process off). 
  • This is dependent on the nature of the neurotransmitter involved.
  • Once the impulse has been transferred across the synapse, the enzyme like cholinesterase destroys the neurotransmitter and the synapse is ready to receive a new impulse.

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