The connection points between a neuron and another neuron or a target organ are called synapses.
Synapses are found in three locations:
- Between two neurons
- Between a sensory neuron and a receptor organ
- Between a motor neuron and an effector organ
EXAMPLE: Synapses occur between the following structure pairs:
- Between the receptor and the dendrite of the sensory neuron
- Between the axon of the sensory neuron and the dendrite of the interneuron
- Between the axon of the interneuron and the dendrite of the motor neuron
- Between the axon of the motor neuron and the effector
- Between the axon of the sensory neuron and the dendrite of the motor neuron
In the central nervous system, synapses can be observed between the dendrites of certain neurons.
Here are some examples of structure pairs where synapses are not observed:
- Between the dendrite of the sensory neuron and the axon of the motor neuron
- Between the axon of the interneuron and the effector organ
Synapses are of two types: electrical synapses and chemical synapses. However, since electrical synapses are not discussed in this program, only chemical synapses will be addressed here.
Chemical Synapses: Most synapses in the nervous system of vertebrates are of the chemical type.
- In chemical synapses, there is a narrow gap known as the synaptic cleft. This gap separates the synaptic terminal of the transmitting neuron from the receiving cell. When an impulse reaches the axon terminal of the transmitting neuron, it is converted into a chemical signal composed of neurotransmitter molecules.
- Neurotransmitter: In short, neurotransmitters are chemical substances that transmit impulses from one neuron to another.
- To elaborate, neurotransmitter substances are chemical messenger molecules that convert the electrical signal (impulse) into a chemical signal, generating an action potential (impulse) in the receiving cell and transferring the impulse from one neuron to another. They are located in the synaptic vesicles at the axon terminal.
- Substances such as dopamine, histamine, serotonin, acetylcholine, adrenaline, and noradrenaline function as neurotransmitters.
Sequence of Events During the Transmission of a Stimulus from the Neuron to the Receiving Cell
- When an action potential depolarizes the synaptic terminal membrane, it triggers the entry of Ca²⁺ into the cell.
- This leads to the fusion of synaptic vesicles with the plasma membrane of the transmitting neuron.
- The vesicles open, and neurotransmitter substances are released into the synaptic cleft via exocytosis (using ATP).
- Neurotransmitters diffuse across the synaptic cleft and bind to appropriate receptors located on ion channel proteins in the plasma membrane of the receiving cell.
- The binding of neurotransmitters to the receptors causes the opening of ion channels in the cell membrane at the dendrite end. This allows Na⁺ to enter the cell, depolarizing it. As a result, the incoming impulse is transferred to the next neuron with the same intensity and characteristics.
- After transmission, the neurotransmitter substances in the synaptic cleft are broken down by enzymes or reabsorbed by the nerve cell (reuptake). This causes the Na⁺ channels at the dendrite end to close, and impulse transmission stops.
- Sometimes, neurotransmitters prevent the formation of an action potential (impulse), meaning that all impulses reaching the axon terminal do not pass to the next nerve cell. This is called selective resistance.
- Selective resistance ensures that signals in synapses do not spread throughout the body. It ensures that the impulse follows a specific path and only reaches the target organ.
- If the impulse continues to be transmitted to the dendrite of the neighboring cell, it is called a facilitative synapse. If it is blocked and not transmitted, it is called an inhibitory synapse.
- In facilitative synapses, the neurotransmitter substances released from the axon terminal cause depolarization in the neighboring cell and the impulse is transmitted to the next cell.
- In inhibitory synapses, a neurotransmitter released from the axon terminal increases the polarization of the membrane, halting the passage of the impulse through the neuron.
- Blocking and facilitation occur only in synapses.
In inhibitory synapses, neurotransmitters increase chloride (Cl⁻) permeability in the next neuron. As more chloride (Cl⁻) enters the cell, the inside of the membrane becomes even more negative (-). In this state, no impulse is generated, and transmission stops.
Morphine temporarily inhibits the release of neurotransmitter substances, thereby disrupting synaptic transmission. For example, when a tooth needs to be extracted, morphine is used to block neural transmission in the affected area. This prevents pain perception during the extraction.
Chemical synapses can process highly complex information. Each transmitting neuron can release different amounts and types of neurotransmitters. These factors enable the nervous system to process highly complex stimuli and generate appropriate responses.
In synapses, the transmission of a stimulus from one neuron to another occurs from the axon terminal of the transmitting neuron to the dendrite of the receiving neuron.
IMPORTANT REMINDERS
- The energy required for impulse transmission is provided by the neuron, not the stimulus.
- The primary energy-providing molecule used by neurons for ATP production is typically glucose.
- No matter how much the stimulus intensity increases, the impulse speed remains constant; only the number of impulses increases. An increase in impulse count enhances the intensity of the response (e.g., pulling our hand away quickly from a very hot object).
- The intensity, frequency, and duration of a stimulus affect the number of impulses generated.
- Saltatory conduction at the nodes of Ranvier saves both time and energy.
- A neuron cannot respond to a second stimulus until the depolarized region returns to its original state.
- If an impulse passes through a synapse, the passage of subsequent impulses through the same synapse becomes easier.
- The transmission of an impulse within a neuron is faster than its transmission across a chemical synapse.
- The all-or-nothing principle applies only to a single nerve cell (a nerve fiber) or a single muscle fiber. It does not apply to nerve bundles or muscle bundles.
- Although impulses are transmitted in the same manner across all neurons, the formation of different sensations results from the involvement of different centers in the central nervous system.