Functions and properties of synapse, reflex

Source - Guyton and Hall, Physiology

1. Functions and properties of synapse, Reflex Dr. Anupam Mittal MBBS, MD King George’s Medical University, Lucknow

2. Synapse The  point at which the nerve impulse passes from one to another is the synapse. In the CNS, this other cell is also a neuron In the PNS, the other cell may be either a neuron or an effector cell within a muscle or gland

3. TYPES OF SYNAPSES CHEMICAL ELECTRICAL

4. A. Chemical Synapse Mostly found in CNS Impulse transmission found in CNS, is carried by Neurotransmitter (Chemical) Either excite the neuron, inhibit it, or modify its sensitivity Example of Neurotransmitter : Acetylcholine, norepinephrine, epinephrine and other “One- Way” Conduction : Chemical Synapses

5. B. Electrical Synapse Mostly found in gap junctions of smooth muscle Allow free movement of ions from the one cell to next cell

6. Typical anterior motor neuron in the anterior horn of the spinal cord It is composed of three major parts — T he soma A single axon D endrites

7. Presynaptic neurons release neurotransmitters into the synapse to trigger graded potentials in post-synaptic neurons Some generate  excitatory  post-synaptic potentials (EPSPs) O thers produce  inhibitory  post-synaptic potentials (IPSPs)

8. EPSPs trigger depolarisation in the post-synaptic membrane, IPSPs trigger hyperpolarisation in the post-synaptic membrane If the combination of signals reaches a threshold level, an action potential will be triggered in the post-synaptic neuron

9. Summation The combination of graded potentials in the post-synaptic neuron is known as summation

10. Types of Summation Spatial Temporal

11. “Spatial Summation” in Neurons— Threshold for Firing S umming simultaneous postsynaptic potentials by activating multiple terminals on widely spaced areas of the neuronal membrane is called spatial summation

12. “Temporal Summation” Caused by Successive Discharges of a Presynaptic Terminal The more rapid the rate of stimulation, the greater the postsynaptic potential become Su ccessive discharges from a single presynaptic terminal, if they occur rapidly enough, can add to one another; that is, they can summate. This type of summation is called temporal summation

13. Properties of SYNAPTIC TRANSMISSION

14. 1. One-way conduction : Impulses are transmitted only in one direction in synapse (From pre to post Neuron) Not vice-versa

15. 2. Synaptic Delay During the transmission of impulses via synapses there is short delay in the transmission. Is due to time taken by, Discharge of the transmitter substance by the presynaptic terminal; Diffusion of the transmitter to the postsynaptic neuronal membrane; Action of the transmitter on the membrane receptor; Action of the receptor to increase the membrane permeability ; and Inward diffusion of sodium to raise the EPSP to a high enough level to elicit an action potential

16. 3. Fatiguability When synapses are continuously stimulated, after some time, due to exhaustion of neurotransmitter at presynaptic terminals, impulses fail to get conducted. This results in fatigue occurring at level of synapse. Fatigue is temporary phenomenon.

17. 4. Convergence and Divergence Convergence: Allows a neuron to receive input from many neurons in network. Divergence: Allows one neuron to communicate with many other neurons in a network.

18. 4. Summation The addictive effect of several electrical impulses on neuromuscular junction Individually the stimuli can not evoke a response, but collectively they can generate a response

19. 5. Excitation or Inhibition The impulse conduction across a synapse may either stimulate or inhibit activity of post neuron

20. 6. Synaptic Facilitation It is a form of short-term plasticity that enhances synaptic transmission for less than a second

21. Reflexes Coordinated Involuntary motor response Initiated by stimulus Applied to peripheral receptor

22. Reflex Arc