Synapse Physiology Notes

Synapse Definition

Synapse is the junction between the two neurons. It is not the anatomical continuation. But, it is only a physiological continuity between two nerve cells.

Classification Of Synapse

Synapse is classified by two methods, anatomical classification, and functional classification.

Anatomical Classification: Usually synapse is formed by axon of one neuron ending on the cell body, dendrite, or axon of the next neuron. Depending upon the ending of the axon, the synapse is classified into three types:

Read And Learn More: Medical Physiology Notes

1. Axoaxonic Synapse: It is the synapse in which axon of one neuron terminates on axon of another neuron.
2. Axodendritic Synapse: This is the synapse in which the axon of one neuron terminates on the dendrite of another neuron.
3. Axosomatic Synapse: Axosomatic synapse is the type of synapse in which axon of one neuron ends on soma (cell body) of another neuron.

Functional Classification: Function classification of the synapse is on the basis of mode of impulse transmission. Accordingly, the synapse is classified into two categories:

1. Electrical synapse
2. Chemical synapse.

However, generally, the word synapse refers to a chemical synapse.

1. Electrical Synapse
   • An electrical synapse is a synapse in which the physiological continuity between the presynaptic and the postsynaptic neurons is provided by the gap junction between the two neurons.
   • There is a cereal exchange of ions between the two neurons though the gap junction. Because of this reason, the action potential reaching the terminal portion of the presynaptic neuron directly enters the postsynaptic neuron.
   • The important feature of the electrical synapse is that the synaptic delay is very less because of the direct flow of current.
   • Moreover, unlike the chemical synapse, the impulse is transmitted in either direction through the electrical synapse. It is also seen in some tissues like the cardiac muscle fibers, smooth muscle fibers of the intestine, and the epithelial cells of the lens in the eye.
2. Chemical Synapse
   • A chemical synapse is the junction between a nerve fiber and a muscle fiber or between two nerve fibers, through which the signals are transmitted by the release of chemical transmitter.
   • In the chemical synapse, there is no continuity between the presynaptic and postsynaptic neurons because of the presence of a space called synaptic cleft between the two neurons.
   • The action potential reaching the presynaptic terminal causes release of neurotransmitter substance from the vesicles of this terminal. The neurotransmitter reaches the postsynaptic neuron through synaptic cleft and causes the production of potential change. The structure and functions of the chemical synapse are given here.

Functional Anatomy Of Chemical Synapse

• The functional anatomy of a chemical synapse is shown. The neuron from which the axon arises is called the presynaptic neuron and the neuron on which the axon ends is called the postsynaptic neuron.
• The axon of the presynaptic neuron divides into many small branches before forming the synapse. The branches are Mown as presynaptic axon terminals. Anatomically, the axon terminals are of two types:

1. Germinal knobs: Some of the terminals are enlarged slightly like knobs called terminal knobs. The terminal knobs are concerned with the excitatory function of the synapse.
2. Terminal coils or free endings: The other terminals are wavy or coiled with free ending without the knob. These terminals are concerned with inhibitory function.

The presynaptic axon terminal has a definite intact membrane known as a presynaptic membrane. The presynaptic terminal has two important structures:

1. Mitochondria, which help in the synthesis of neurotransmitter substances
2. Synaptic vesicles, which store neurotransmitter substances.

The membrane of the postsynaptic neuron is called the postsynaptic membrane. It contains some receptor proteins. The small space in between the presynaptic membrane and the postsynaptic membrane is called synaptic cleft. The basal lamina of this cleft contains cholinesterase, which destroys acetylcholine.

Functions Of Synapse

The main function of the synapse is to transmit the impulses, i.e. action potential from one neuron to another. However, some of the synapses inhibit these impulses. So the impulses are not transmitted to the postsynaptic neuron.

Thus, the synapses are of two types:

1. Excitatory synapses, which transmit the impulse’s excitatory function
2. Inhibitory synapses, which inhibit the transmission of impulses – inhibitory function.

Excitatory Function

• Excitatory Postsynaptic Potential
  • Excitatory postsynaptic potential (EPSP) is the nonpro- paginated electrical potential that develops during the process of synaptic transmission.
  • When the action potential reaches the presynaptic axon terminal, the voltage-gated calcium channels at the presynaptic membrane are opened. Now, the calcium ions enter the axon terminal from ECF.
  • The calcium ions cause the release of neurotransmitter substances from the vesicles by means of exocytosis.
  • The neurotransmitter, which is excitatory in function (excitatory neurotransmitter) passes through the presynaptic membrane and synaptic cleft and reaches the postsynaptic membrane.
  • Now, the neurotransmitter binds with the receptor protein present in the postsynaptic membrane to form the neurotransmitter receptor complex. The neurotransmitter-receptor complex causes the production of a non propagated EPSP. The most common excitatory neurotransmitter in a synapse is acetylcholine.
• Mechanism of Development of EPSP
  • The neurotransmitter-receptor complex causes the opening of ligand-gated sodium channels.
  • Now, the sodium ions from ECF enter the cell body of the postsynaptic neuron. As the sodium ions are positively charged, the resting membrane potential inside the cell body is altered and mild depolarization develops. This type of mild depolarization is called EPSP. It is a local response in the synapse.
• Properties of EPSP: EPSP is confined only to the synapse. It is a graded potential. It is similar to receptor potential and endplate potential. EPSP has two properties.
  1. It is nonpropagated
  2. It does not obey all or no law.
• Significance of EPSP
  • The EPSP is not transmitted into the axon of the postsynaptic neuron. However, it causes the development of action potential in the axon.
  • When the EPSP is strong enough, it causes the opening of voltage-gated sodium channels in the initial segment of the axon.
  • Now, due to the entrance of sodium ions, depolarization occurs in the initial segment of axon and thus, the action potential develops. From here, the action potential spreads to another segment of the axon.

Inhibitory Function: Inhibition of synaptic transmission is classified into three types

1. Postsynaptic inhibition
2. Presynaptic inhibition
3. Renshaw cell inhibition.

 

1. Postsynaptic Inhibition
   • Postsynaptic inhibition is the type of synaptic inhibition that occurs due to the release of an inhibitory neurotransmitter from the presynaptic terminal instead of an excitatory neurotransmitter substance.
   • It is also called direct inhibition. The most important inhibitory neurotransmitter is gamma amino butyric acid (GABA). The other inhibitory neurotransmitter substances are dopamine and glycine.
   • Action of GABA – Development of inhibitory postsynaptic potential
     • Inhibitory postsynaptic potential (IPSP) is the electrical potential in the form of hyperpolarization that develops during postsynaptic inhibition.
     • The inhibitory neurotransmitter substance acts on the postsynaptic membrane by binding with the receptor. The transmitter-receptor complex opens the ligand-gated potassium channels instead of sodium channels.
     • Now, the potassium ions which are available in plenty in the cell body of postsynaptic neurons move to ECF. Simultaneously, chloride channels also open, and chloride ions (which are more in ECF) move inside the cell body of the postsynaptic neuron.
     • The exit of potassium ions and the influx of chloride ions cause more negativity inside, leading to hyperpolarization. The hyperpolarized state of the synapse inhibits synaptic transmission.
2. Presynaptic Inhibition: It is the synaptic inhibition that occurs because of the failure of the presynaptic axon terminal to release the excitatory neurotransmitter substance. It is also called indirect inhibition.
3. Renshaw Cell Inhibition
   • It is the type of synaptic inhibition that is caused by Renshaw cells in the spinal cord. Renshaw cells are small motor neurons present in the anterior gray horn of the spinal cord.
   • The anterior nerve root consists of nerve fibers that leave the spinal cord. These nerve fibers arise from motor neurons in the anterior gray horn of the spinal cord and reach the effector organ, muscles. Some of the fibers called collaterals fibers terminate on Renshaw cells instead of leaving the spinal cord.
   • When motor neurons send motor impulses, some of the impulses reach the Renshaw cell by passing through collaterals. Now, the Renshaw cell is stimulated.
   • In turn, if sends inhibitory impulses to motor neurons so that, the discharge from motor neurons is reduced.
   • Significance of synaptic inhibition
     • The synaptic inhibition in CNS limits the number of impulses going to muscles and enables the muscles to act properly and appropriately.
     • Thus, inhibition helps to select an exact number of impulses and to omit or block the excess ones. When a poison like strychnine is introduced into the body, it destroys the inhibitory function at the synaptic level resulting in continuous and convulsive contraction even with slight stimulation.
     • In nervous disorders like Parkinsonism, the inhibitory system is impaired resulting in rigidity.

Properties Of Synapse

1. One-Way Conduction Bell Magendie Law: According to Bell-Magendie law, the impulses are transmitted only in one direction in the synapse, i.e. from the presynaptic neuron to the postsynaptic neuron.
2. The Synaptic Delay: Synaptic delay is a short delay that occurs during the transmission of impulses through the synapse, it is due to the time taken for
   1. Release of neurotransmitter
   2. Passage of neurotransmitters from the axon terminal to the postsynaptic membrane
   3. The action of the neurotransmitter opens the ionic channels in the postsynaptic membrane.
      • The normal duration of synaptic delay is 0.3-0.5 msec. The synaptic delay is one of the causes of the reaction time of the reflex activity.
      • Significance of determining synaptic delay: Determination of synaptic delay helps to find out whether the pathway for a reflex is monosynaptic or polysynaptic.
3. Fatigue
   • During continuous muscular activity, the synapse forms the seat of fatigue along with the Betz cells present in the motor area of the frontal lobe of the cerebral cortex.
   • The fatigue at the synapse is due to the depletion of neurotransmitter substance, acetylcholine.
   • Depletion of acetylcholine occurs by two factors:
     1. Soon after the action, acetylcholine is destroyed by acetylcholinesterase
     2. Due to continuous action, new acetylcholine is not synthesized.
     3. These two factors lead to the depletion of acetylcholine resulting in fatigue.
4. Summation
   • It is the fusion of effects or progressive increase in the excitatory postsynaptic potential (EPSP) in postsynaptic neurons when many presynaptic excitatory terminals are stimulated simultaneously or when the single presynaptic terminal is stimulated repeatedly.
   • The increased EPSP triggers the axon potential in the initial segment of the axon of the postsynaptic neuron.
   • Summation is of two types:
     1. Spatial Summation: It occurs when many presynaptic terminals are stimulated simultaneously.
     2. Temporal Summation: It occurs when one presynaptic terminal is stimulated repeatedly.
        • Thus, both spatial summation and temporal summation play an important role in the facilitation of response.
5. Electrical Property: The electrical properties of the synapse are the EPSP and IPSP, which are already described in this chapter.

Convergence And Divergence

• Convergence: Convergence is the process by which many presynaptic neurons terminate on a single postsynaptic neuron.
• Divergence: Divergence is the process by which one presynaptic neuron terminates on many postsynaptic neurons.